Power converters

IS 2 390 133 T3

DESCRIPTION

Power converters

Technical field

The present invention relates to power converters and, in particular, to power converters that can be used to connect generators providing a variable voltage with a variable frequency to an electrical network or supply network at a fixed nominal voltage and frequency. The present invention also includes features that allow power converters to remain connected to the utility grid and maintain control during grid failure and transient conditions. Power converters are particularly useful for unrestricted use with generators that are driven by wind turbines.

Background technique

It is possible to convert wind energy into electrical energy by using a wind turbine to drive the rotor of a generator, either directly or by means of a gearbox. The ac frequency that develops across the generator stator terminals (the “stator voltage”) is directly proportional to the speed of rotation of the rotor. The voltage across the generator terminals also varies as a function of speed and, depending on the particular type of generator, the level of flux. For optimal energy capture, the rotation speed of the wind turbine output shaft will vary according to the speed of the wind driving the turbine blades. To limit energy capture at high wind speeds, the output shaft rotation speed is controlled by altering the pitch angle of the turbine blades. Matching the variable voltage and frequency of the generator to the nominally constant voltage and frequency of the utility grid can be achieved through the use of a power converter.

US Patent 5083039 describes a variable speed wind turbine in which the rotating shaft of the wind turbine is used to drive the rotor of an ac induction generator. A power converter is used to connect the generator output to an electrical network. The power converter includes active power switching semiconductor devices that control the electrical amounts of stator in each phase of the generator. A torque control device is used to derive a torque demand signal indicative of a desired torque. A generator controller operates under the control of field orientation and is responsive to the torque demand signal to define a desired transverse current representing the torque in rotating field coordinates normal to the rotor flux field. The active power switching semiconductor devices are then controlled by the generator controller using a pulse width modulation circuit to produce electrical amounts of stator corresponding to the desired cross current. An inverter controller regulates the output current to supply polyphase AC power that has forward and lag currents at an angle specified by a power factor control signal. In this arrangement, a loss of voltage in the network during a drop in supply leads to a loss of control of the DC link voltage. Consequently, the reactive current control capacity, which is essential for the voltage support functions demanded by the codes of the network, is also lost.

US Patent 5225712 extends the above principle to include reactive power control or power factor angle control as a function of a mode switch. Similarly, the inverter bridge controller scheme of US patent 5225712 is solely responsible for regulating the dc link voltage. Therefore, both schemes suffer from the disadvantage that during the situation where the mains voltage is lost, control of the dc link voltage and the ability to control the reactive current are also lost during the drop. tensile.

Summary of the invention

The present invention is defined by a power converter and a power converter operating method with the technical characteristics of independent claims 1 and 39. Other alternatives are defined by the characteristics of the dependent claims.

The present invention aims to reduce, at least, the above problems and disadvantages by providing a power converter that can be used to connect a generator providing variable voltage at a variable frequency with a supply network operating at a fixed nominal voltage and a fixed nominal frequency, the power converter comprising:

a first active rectifier / inverter electrically connectable to the generator stator and including a plurality of power switching semiconductor devices;

a second active rectifier / inverter including a plurality of power switching semiconductor devices;

a DC link connected between the first active rectifier / inverter and the second active rectifier / inverter;

ES 2 390 133 T3 a filter connected between the second active rectifier / inverter and the supply network, the filter including network terminals;

a first controller for the first active rectifier / inverter; and a second controller for the second active rectifier / inverter;

wherein the first controller uses a dc link voltage demand signal, indicative of a desired dc link voltage, to control the power switching semiconductor devices of the first active rectifier / inverter to achieve the desired voltage level of the dc link that corresponds to the voltage demand signal of the dc link; and wherein the second controller uses a power demand signal indicative of the power level that must be transmitted from the dc link to the utility grid through the second active rectifier / inverter, and a voltage demand signal indicative of the voltage that must be achieved at the filter mains terminals to control the semiconductor power switching devices of the second active rectifier / inverter to achieve the desired levels of power and voltage that correspond to the signals of power and voltage demand.

The power converter can be used to connect the generator to the utility grid during normal operating conditions, but also includes features that allow it to operate in situations where the utility grid voltage varies due to grid failures or transitory situations in the supply network. More particularly, the second controller can use a utility grid voltage measurement to determine limits to the power that can be exported from the second active rectifier / inverter when the utility grid voltage deviates from its nominal condition. , and you can also use a utility grid voltage measurement to determine the level of current that needs to be supplied from the second active rectifier / inverter to provide voltage support to the utility grid when the utility grid voltage drops. it deviates from its nominal condition.

The generator can be a linear or rotary generator of any suitable type. Examples would include an induction generator or a synchronous generator driven by any suitable means, such as permanent magnets or conventional or superconducting field windings. In the case of a rotating generator, the rotor may be connected to, or driven by, the output shaft of a turbine or prime mover, such as a wind turbine, a tidal turbine, a hydro turbine, a steam turbine engine, a diesel engine or a gas turbine engine, for example. A linear generator could be used in applications that inherently benefit from reciprocating, for example wave generators.

Preferably, the first controller uses a flow demand signal indicative of a desired level of flow to be achieved in the generator and converts this to a longitudinal current demand signal for the first active rectifier / inverter. The first controller can then use a longitudinal current demand signal to control the operation of the semiconductor power switching devices of the first active rectifier / inverter to produce electrical amounts of stator that achieve the desired longitudinal current for the first rectifier / inverter. active. In this document, the term "electrical stator quantities" is used to refer to any of the magnitude of voltage of the individual phases, the magnitude of the current of the individual phases, the phase and the frequency, and all of them. , in a polyphase generator.

The operation of the semiconductor power switching devices in the first active rectifier / inverter can be controlled using gate drive control signals derived in accordance with a conventional pulse width modulation strategy. It will be immediately appreciated that various types of pulse width modulation strategies can be considered. In a preferred aspect of the present invention with a two-level voltage source inverter, a fixed frequency pulse width modulation strategy may be implemented as follows. In a digital processor, the output voltage requirement, determined from the combination of longitudinal and transverse voltage signals, is multiplied by the value of an enhanced sinusoidal waveform in Odd multiples of the third harmonic by the value of the angle with that the output voltage is to be applied for a given phase in the active rectifier / inverter. An enhanced sine waveform at Odd multiples of the third harmonic is used to maximize the output voltage that can be achieved at the AC terminals of the active rectifier / inverter block for a given DC link voltage. The resulting signal is compared to a triangular waveform running at a fixed frequency to determine the specific switching times of the upper and lower power switching semiconductor devices in that phase of the active rectifier / inverter. To overcome known switching delays and prevent simultaneous conduction of the upper and lower power switching semiconductor devices, blanking periods can be interposed at specific switching times between the upper power switching semiconductor device being disconnected and the power switching device. power switching lower semiconductor device being connected. Corresponding blanking periods are imposed between the lower power switching semiconductor device being turned off and the upper power switching semiconductor device being turned on. The same procedure is repeated for each phase of the active rectifier / inverter:

Preferably, the first controller converts the flow demand signal into the longitudinal current demand signal with reference to one or more characteristics of the generator. Features could

ES 2 390 133 T3 include the parameters of the generator equivalent circuit and / or data from the nameplate, such as nominal current, voltage, speed, power and frequency and data such as the magnetization curve. The magnetization curve provides the relationship between the stator flux for the generator and the longitudinal current needed to achieve it. Typically, the magnetization curve for a generator will show a linear relationship between living flux and longitudinal current up to a certain flux level. However, beyond this certain level, small increases in flow will require large increases in longitudinal flow. This non-linear region is associated with the saturation of the iron used to form the generator's magnetic circuit. The magnetization curve can be derived from the test procedure for the generator during its manufacture or from current injection tests carried out during the generator start-up stage. Such current injection tests may be arranged to be automatic as part of an automatic start-up routine for the power converter / generator combination.

Preferably, the first controller compares the dc link voltage demand signal, indicative of a desired dc link voltage, with a dc link voltage feedback signal to determine a cross current demand signal for the first active rectifier / inverter. The first controller can then control the power switching semiconductor devices of the first active rectifier / inverter to produce electrical amounts of stator that achieve the desired cross current for the first active rectifier / inverter.

The second controller can supply a control signal that varies according to prevailing mains voltage conditions to the first controller. This allows the first controller to compare the DC link voltage demand signal, indicative of a desired DC link voltage, with the DC link voltage feedback signal to determine a DC link current demand signal. DC link, and then limit the DC link current demand signal using the control signal from the second controller to determine a DC link current demand limited signal. The current demand limited signal from the dc link can then be used by the first controller to determine the cross current demand signal for the first active rectifier / inverter.

Alternatively, the second controller may supply a control signal that varies according to prevailing supply network voltage conditions and / or the power demand signal to the first controller. This allows a first controller dc link voltage controller to compare the dc link voltage demand signal, indicative of a desired dc link voltage, with the dc link voltage feedback signal to provide a output signal that is added to the control signal to determine a DC link current demand signal. The dc current demand signal can then be used to determine the cross current demand signal for the first active rectifier / inverter.

Preferably, the second controller converts the power demand signal, indicative of the power level to be transferred from the dc link to the supply network through the second active rectifier / inverter, into a cross current demand signal for the second active rectifier / inverter. The second controller can then control the power switching semiconductor devices of the second active rectifier / inverter to produce electrical filter / supply grid quantities that achieve the desired cross current for the second active rectifier / inverter. In this document, the term "supply network / filter electrical quantities" is used to refer to any of the magnitude of the voltage of the individual phases, the magnitude of the current of the individual phases, the phase and the frequency. , and all of them, in a polyphase active rectifier / inverter. The term "polyphase" typically refers to triphasic, but can include other numbers of phases. The operation of the semiconductor power switching devices in the second active rectifier / inverter can also be controlled using gate drive control signals derived according to a conventional pulse width modulation strategy.

The power demand signal can be converted into the cross current demand signal by dividing the power demand signal by a signal derived from the voltage at the filter mains terminals. Preferably, this signal is the cross-sectional component of the ac voltage that is derived from the measurement of the three-phase voltage on the mains side of the filter. Alternatively, the power demand signal can be converted into the cross current demand signal by dividing the power demand signal by a filtered version of the signal that is derived from the voltage at the filter mains terminals.

Preferably, the second controller uses an additional DC link voltage demand signal indicative of a desired DC link voltage and compares the additional DC link voltage demand signal with the DC link voltage feedback signal. to determine an unlimited cross current demand signal. The unlimited cross current demand signal can then be limited to determine the cross current demand signal for the second active rectifier / inverter. The unlimited cross current demand signal can be limited to a value that is determined by a limiting signal which, in turn, is preferably derived from the power demand signal.

IS 2 390 133 T3

The unlimited cross current demand signal can be added to a cross current direct feed signal derived from the following signals: a signal indicative of the generator power, a voltage feedback signal measured at the filter mains terminals and a gain signal that varies according to the prevailing voltage conditions of the supply network.

The signal indicative of the generator power can be supplied to the second controller from the first controller. Alternatively, the signal indicative of generator power minus the output of a PI controller from a DC link voltage controller of the first controller can be supplied to the second controller and is used by the second controller only during a voltage drop situation. of the supply network.

The second controller can modify the limiting signal derived from the power demand signal according to the prevailing voltage conditions of the supply network. The limiting signal can be modified by the second controller in response to deviations in the supply network from nominal voltage conditions, for example during a supply network failure or transient conditions. This will result in changes in the transfer of power to the supply network to satisfy supply network service requirements such as voltage and / or frequency support.

The dc link can include a capacitor. In this case, the power converter may further include a current sensor to measure the current flowing in the capacitor and provide an output signal. The current sensor output signal can be subtracted from a signal derived from a signal indicative of generator power to provide an inferred signal that is added to the output of a DC link voltage controller from the first controller to determine a DC link current demand signal for the first active rectifier / inverter. Alternatively, the current sensor output signal can be subtracted from a signal derived from a signal indicative of generator power to provide a signal that is filtered and added to the output of a first DC link voltage controller. controller to determine a DC link current demand signal for the first active rectifier / inverter.

Alternatively, the power converter may further include a voltage sensor to measure the dc link voltage and provide a dc link voltage feedback signal. Means may also be provided for measuring the rate of change of the dc link voltage feedback signal. The integral value of a PI controller of a first controller dc link voltage controller can then be modified by a predetermined factor when the dc link voltage feedback signal is greater than a first threshold and the rate of change of the dc link voltage feedback signal is greater than a second threshold.

During a mains voltage drop situation, a cross current demand signal for the second active rectifier / inverter can be derived from a limited version of the response speed of a signal that is derived from the power limit rating of the second active rectifier / inverter, which is modified according to the prevailing voltage conditions of the supply network.

Preferably, the second controller compares the voltage demand signal, indicative of the voltage level to be achieved at the filter mains terminals, with a voltage feedback signal measured at the filter mains terminals to determine a signal of Longitudinal current demand for the second active rectifier / inverter. The second controller can then control the power switching semiconductor devices of the second active rectifier / inverter to produce electrical supply filter / grid quantities that achieve the desired longitudinal current for the second active rectifier / inverter.

The second controller can modify the longitudinal current demand signal according to the prevailing voltage conditions of the supply network.

The second controller can modify an error signal that arises from the difference between the voltage demand signal, indicative of the voltage level to be achieved at the filter mains terminals, and the voltage feedback signal measured at the terminals. filter network according to a signal derived from the longitudinal current demand signal. The purpose of modifying the error signal according to a signal derived from the longitudinal current demand signal is that a characteristic can be realized that can contribute to current sharing among multiple generators that are connected to a particular supply network.

Preferably, the power converter further comprises a speed sensor to derive a speed signal indicative of the speed of the moving part of the generator (that is, the rotor in the case of a rotating generator and the translator in the case of the linear generator). However, in some cases the speed sensor can be substituted for a speed observing system that uses internal signals to the first active rectifier / inverter to derive a speed signal. The speed signal (derived from the speed sensor or speed observing system) can then be used to derive the power demand signal by reference to a look-up table of power demand signal as a function of speed. The lookup table can be combined with a PI controller. Preferably, the speed signal is modified by a filter function. The speed signal can also be modified by a second filter function and multiplied by a gain to provide an attenuation term that is added to the power demand signal.

ES 2 390 133 T3 derived with reference to the look-up table to give a total power demand signal. The filter functions can be used independently or together to attenuate any shaft or transmission resonance if applicable.

The present invention also provides an arrangement comprising a plurality of power converters, as described above, connected together in parallel to a supply network operating at a fixed nominal voltage and at a fixed nominal frequency by means of a parallel connection. The voltage demand signal, indicative of the voltage to be achieved at the filter mains terminals of each power converter, is preferably derived from a comparison of a higher voltage demand signal and a higher voltage feedback signal. which is measured at the point where the parallel connection connects to the utility grid.

Preferably, each individual power converter includes a step-up transformer electrically connected between the associated filter and the parallel connection. The arrangement may also include a step-up transformer electrically connected between the parallel connection and the supply network. The upper voltage feedback signal can be measured either on the supply network side or on the side of the parallel connection of the step-up transformer electrically connected between the parallel connection and the supply network. The advantage of measuring the voltage feedback upper signal on the mains side of the step-up transformer is that the measurement on the side of the parallel connection is subject to regulation between the ends of the step-up transformer. Therefore, this regulation effect is eliminated if the measurement is performed on the mains side.

The power converter is suitable for use in a wind turbine. Therefore, the present invention also provides a wind turbine comprising a generator having a stator and a rotor, a turbine assembly including a turbine blade or turbine blades for rotating the rotor of the generator, and a speed converter. power as described above. The turbine assembly can be integral with the generator rotor. Alternatively, the turbine blade (s) (three blades could be typical) are mounted on a rotating shaft and the generator rotor is coupled to the rotating shaft. The rotor of the generator can be coupled directly to the rotating shaft or indirectly by means of a gearbox.

Several wind turbines can be connected to each other forming a wind farm. Therefore, the present invention further provides a wind farm comprising a supply network operating at a fixed nominal voltage and a fixed nominal frequency, and a plurality of wind turbines as described above. The respective power converters of the plurality of wind turbines are connected to each other in parallel to the supply network by means of a parallel connection, and the voltage demand signal, indicative of the voltage to be achieved at the network terminals of the filter of each power converter, It is derived from a comparison of an upper voltage demand signal and a higher voltage feedback signal that is measured at the point where the parallel connection connects to the supply network.

Preferably, each wind turbine includes a step-up transformer electrically connected between the associated power converter filter and the parallel connection. The wind farm may further comprise a step-up transformer electrically connected between the parallel connection and the supply network. The upper voltage feedback signal can be measured either on the supply network side or on the side of the parallel connection of the step-up transformer electrically connected between the parallel connection and the supply network.

The present invention further provides a method of operating a power converter that can be used to connect a generator providing a variable voltage with a variable frequency to a supply network operating at a fixed nominal voltage and at a fixed nominal frequency. , comprising the power converter:

a first active rectifier / inverter electrically connectable to the generator stator and including a plurality of power switching semiconductor devices;

a second active rectifier / inverter including a plurality of power switching semiconductor devices;

a DC link connected between the first active rectifier / inverter and the second active rectifier / inverter;

a filter connected between the second active rectifier / inverter and the supply network, the filter including the network terminals;

a first controller for the first active rectifier / inverter; and a second controller for the second active rectifier / inverter; the procedure comprising the stages of:

The first controller uses a DC link voltage demand signal, indicative of a desired DC link voltage, to control the power switching semiconductor devices of the first active rectifier / inverter to achieve the desired DC link voltage level. dc corresponding to the voltage demand signal of the dc link; Y

ES 2 390 133 T3 the second controller uses a power demand signal Indicative of the power level that must be transferred from the dc link to the supply network through the second active rectifier / inverter, and a voltage demand signal indicative of the voltage that must be achieved at the filter mains terminals to control the semiconductor power switching devices of the second active rectifier / inverter to achieve the desired levels of power and voltage that correspond to the signals of power and voltage demand.

The procedure may include additional steps, as outlined below.

The second controller can use a utility grid voltage measurement to determine limits to the power that can be exported from the second active rectifier / inverter when the utility grid voltage deviates from its nominal condition.

The second controller may also use a utility grid voltage measurement to determine the level of current to be provided from the second active rectifier / inverter to provide voltage support to the utility grid when the utility grid voltage supply deviates from its nominal condition.

The first controller may use a flow demand signal indicative of a desired level of flow to be achieved in the generator, convert the flow demand signal into a longitudinal current demand signal for the first active rectifier / inverter and control the power switching semiconductor devices of the first active rectifier / inverter to produce electrical amounts of stator that achieve the desired longitudinal current to the first active rectifier / inverter. The step of converting the flow demand signal into the longitudinal current demand signal can be carried out with reference to one or more characteristics of the generator.

The first controller can compare the DC link voltage demand signal, indicative of a desired DC link voltage, with a DC link voltage feedback signal to determine a cross current demand signal for the first active rectifier / inverter and control the power switching semiconductor devices of the first active rectifier / inverter to produce electrical amounts of stator that achieve the desired cross current for the first active rectifier / inverter.

The second controller may supply the first controller with a control signal that varies according to prevailing supply network voltage conditions during a supply network voltage drop situation. The first controller can compare the dc link voltage demand signal, indicative of a desired dc link voltage, with the dc link voltage feedback signal to determine a dc link current demand signal and limiting the dc link current demand signal by using the control signal from the second controller to determine a dc link current demand limited signal. The first controller can then use the current demand limited signal from the dc link to determine the cross current demand signal for the first active rectifier / inverter so that no power is drawn from the utility grid during the outage situation. supply network voltage.

Alternatively, the second controller may supply a control signal that varies according to prevailing supply network voltage conditions and / or the power demand signal to the first controller. A first controller dc link voltage controller can then compare the dc link voltage demand signal, indicative of a desired dc link voltage, with the dc link voltage feedback signal to provide a signal. output that is added to the control signal to determine a DC link current demand signal. The current demand signal from the dc link can then be used to determine the cross current demand signal for the first active rectifier / inverter.

The second controller can convert the power demand signal, indicative of the power level to be transferred from the DC link to the utility grid through the second active rectifier / inverter, on a cross current demand signal for the second active rectifier / inverter and control the semiconductor power switching devices of the second active rectifier / inverter to produce electrical supply filter / grid quantities that achieve the desired cross current for the second rectifier / active investor.

The above step of converting the power demand signal into the transverse current demand signal can be carried out by dividing the power demand signal by a signal derived from the voltage at the filter mains terminals. Alternatively, the power demand signal can be converted into the cross current demand signal by dividing the power demand signal by a filtered version of the signal that is derived from the voltage at the filter mains terminals.

The second controller can use an additional DC link voltage demand signal indicative of a desired DC link voltage, compare the additional DC link voltage demand signal with the DC link voltage feedback signal to determine an unlimited current demand signal

ES 2 390 133 T3 cross and limit the cross current demand limited signal to a value determined by a limiting signal derived from the power demand signal to determine the cross current demand signal for the second rectifier / inverter active during start-up and normal operating condition of the power converter.

The method may further comprise the step of adding the unlimited cross current demand signal to a cross current direct feed signal derived from the following signals: a signal indicative of the generator power a feedback signal of voltage measured at the filter network terminals and a gain signal that varies according to the prevailing voltage conditions of the supply network.

The signal indicative of the generator power can be supplied to the second controller from the first controller. Alternatively, the signal indicative of generator power minus the output of a PI controller from a DC link voltage controller of the first controller can be supplied to the second controller and is used by the second controller only during a voltage drop situation. of the supply network.

The second controller can modify the limiting signal derived from the power demand signal according to the prevailing supply network voltage conditions in a supply network voltage drop situation. The use of the word "drop" in this description in relation to grid failure situations refers to a situation in which the supply grid voltage drops below its nominal value as a consequence of power supply conditions. network failure, whether symmetric or asymmetric.

The dc link may include a capacitor and the power converter may further include a current sensor to measure the current flowing in the capacitor and provide an output signal. In this case, the method may further comprise the steps of subtracting the current sensor output signal from a signal derived from a signal indicative of generator power to provide an inferred signal that is added to the output of a voltage controller. from the first controller dc link to determine a dc link current demand signal for the first active rectifier / inverter. Alternatively, the method may further comprise the steps of subtracting the current sensor output signal from a signal derived from a signal indicative of generator power to provide a signal that is filtered and added to the output of a controller. of the first controller dc link voltage to determine a dc link current demand signal for the first active rectifier / inverter.

Alternatively, the power converter may further include a voltage sensor for measuring the dc link voltage and providing a dc link voltage feedback signal and means for measuring the rate of change of the voltage feedback signal. of the cc link. In this case, the method may further comprise the steps of modifying the integral value of a PI controller of a voltage controller of the dc link of the first controller by a predetermined factor when the voltage feedback signal of the dc link is greater than a first threshold and the rate of change of the dc link voltage feedback signal is greater than a second threshold.

In a mains voltage drop situation, a cross current demand signal for the second active rectifier / inverter can be derived from a limited version of the response speed of a signal derived from the rated power limit of the second Active rectifier / inverter, which is modified according to the prevailing voltage conditions of the supply network.

The second controller can compare the voltage demand signal, indicative of the voltage level to be achieved at the filter mains terminals, with a voltage feedback signal measured at the filter mains terminals to determine a longitudinal current demand signal for the second active rectifier / inverter and control the power switching semiconductor devices of the second active rectifier / inverter to produce electrical quantities filter / supply network that achieve the desired longitudinal current for the second active rectifier / inverter.

The second controller can modify the longitudinal current demand signal according to the prevailing voltage conditions of the supply network.

The second controller can modify an error signal that arises from the difference between the voltage demand signal, indicative of the voltage level to be achieved at the filter mains terminals, and the voltage feedback signal measured at the terminals. filter network according to a signal derived from the longitudinal current demand signal.

A speed signal indicative of the speed of the moving part of the generator can be derived and used to derive the power demand signal. The power demand signal can be modified by one or more filter functions, which can also be used to provide attenuation of any shaft or transmission resonance.

The power demand signal can be derived from a look-up table or a mathematical function in which the modified speed signal forms a pointer to the look-up table or a value for which the function is calculated.

ES 2 390 133 T3 mathematics. The power demand signal can also be summed with a signal derived from a filtered version of the speed signal.

The present invention also provides a method of operating a plurality of power converters, as described above, connected together in parallel to a supply network operating at a fixed nominal voltage and at a fixed nominal frequency by means of a parallel connection, the method comprising the step of deriving the voltage demand signal indicative of the voltage to be achieved at the network terminals of the filter of each power converter, from a comparison of a higher voltage demand signal and a higher signal of voltage feedback that is measured at the point where the parallel connection connects to the utility grid. The method may also comprise the step of measuring the upper voltage feedback signal either on the supply network side or on the side of the parallel connection of the step-up transformer electrically connected between the parallel connection and the supply network.

The present invention also provides an unclaimed method of operating a wind turbine comprising a generator providing variable voltage at variable frequency and having a stator and rotor, a turbine assembly including at least one blade for rotating the rotor of the generator and a power converter that connects the generator to a supply network operating with a fixed nominal voltage and a fixed nominal frequency, comprising the power converter:

a first active rectifier / inverter electrically connectable to the generator stator and including a plurality of power switching semiconductor devices;

a second active rectifier / inverter including a plurality of power switching semiconductor devices;

a DC link connected between the first active rectifier / inverter and the second active rectifier / inverter;

a filter connected between the second active rectifier / inverter and the supply network, the filter including the network terminals;

a first controller for the first active rectifier / inverter; and a second controller for the second active rectifier / inverter;

wherein, in response to a change in wind speed, the procedure comprises the steps of:

controlling the second active rectifier / inverter to change the level of power exported from the dc link such that the dc link voltage changes from a desired level; and controlling the first active rectifier / inverter to import sufficient current to the dc link through the generator bridge from the generator to restore the dc link voltage to the desired level.

Drawings

Figure 1 is a schematic drawing showing how a power converter according to the present invention is used to connect a wind turbine driving a variable speed generator and a fixed frequency electrical network;

Figure 2 is a schematic drawing showing more detail of the control dc link for the generator bridge (active rectifier) ​​of Figure 1;

Figure 3 is a schematic drawing showing more detail of the current control for the generator bridge (active rectifier) ​​of Figure 1;

Figure 4 is a schematic drawing showing more detail of the power control for the grid bridge (inverter) of Figure 1;

Figure 5 is a schematic drawing showing more detail of the current control of the grid bridge (inverter) of Figure 1;

Figure 6 is a schematic drawing showing how several power converters according to the present invention can be connected to each other in parallel to the supply network, forming a wind farm; Figure 7 is a schematic drawing showing full control of wind farm voltage; Figure 8 is a schematic drawing showing how a first reciprocating power converter according to the present invention is used to connect a wind turbine driving a variable speed generator and a fixed frequency electrical network;

Figure 9 is a schematic drawing showing more detail of the DC link voltage and lower current controls for the generator bridge (active rectifier) ​​of Figure 8; Figure 10 is a schematic drawing showing more detail of the power control, the grid voltage control and the lower current controls for the grid bridge (inverter) of Figure 8;

Figure 11 is a schematic drawing showing how a second reciprocating power converter according to the present invention is used to connect a wind turbine driving a variable speed generator and a fixed frequency electrical network;

Figure 12 is a schematic drawing showing more detail of a first option for dc link control for the generator bridge (active rectifier) ​​of Figure 11;

Figure 13 is a schematic drawing showing more detail of the power control for the grid bridge (inverter) of Figure 11; and Figure 14 is a schematic drawing showing more detail of a second option for dc link control for the generator bridge (active rectifier) ​​of Figure 11.

Power converter topology

The basic topology of the power converter will be outlined with reference to Figure 1.

The power converter is used to connect a wind turbine 2 driving a variable speed ac induction generator 4 and a fixed rated frequency power grid (marked RED). Typically, the wind turbine includes three turbine blades (one turbine blade or two turbine blades or more than three turbine blades are also possible) mounted on a rotating shaft and whose pitch angle can be controlled by means of an actuator of the pitch angle to optimize and / or limit the capture of wind energy in the generator 4. A gearbox 8 is used to connect the rotating shaft to the rotor of the variable speed generator 4. In some cases, the rotating shaft can be directly connected to the rotor of the variable speed generator 4. This means that the rotational speed of the rotor varies as a function of the wind speed and, therefore, that the frequency of the voltage developed in the stator of generator 4 (the “stator frequency”) can vary over a wide range. . Several wind turbines, as represented by the entirety of Figure 1, may be connected to each other, defining a wind farm.

The terminals of the generator 4 are connected to the ac terminals of a bridge 10 of a three-phase generator, which in normal operation operates as an active rectifier to supply power to a DC link 12. The generator bridge 10 has a conventional three-phase two-level topology with a series of fully controlled and regulated power switching semiconductor devices using a pulse width modulation strategy. However, in practice, the generator bridge 10 can have any suitable topology, such as a fixed three-level neutral point topology or a multilevel topology (eg, the Foch-Maynard arrangement). The derivation of gate drive command signals that are used to control semiconductor power switching devices is described in more detail below.

The DC output voltage of generator bridge 10 is supplied to the DC terminals of a grid bridge 14 which, in normal operation, operates as an inverter. The main control for the DC output voltage is achieved by controlling the generator bridge 10. The network bridge 14 has a two-level, three-phase topology similar to that of the generator bridge 10, with a series of fully controlled and regulated power switching semiconductor devices using a pulse width modulation strategy. However, in practice, the network bridge 14 may have any suitable topology, as discussed above for the generator bridge 10. The grid bridge 14 is controlled to satisfy two main objectives: namely, active power and grid voltage. A detailed description of how this control is achieved is provided below. Also described in greater detail below is the derivation of gate drive command signals that are used to control power switching semiconductor devices.

As described herein, active rectification (as the fundamental mode of operation of the generator bridge 10) is the conversion of energy from the ac terminals of the bridge of the three-phase generator to the dc link, and reversal (as a mode fundamental operation of the network bridge 14) is the conversion of energy from the dc link of the three-phase network bridge to its ac terminals. However, it will be readily appreciated that there may be occasions when it might be necessary or desirable to operate the generator bridge 10 as an inverter and to operate the grid bridge 14 as an active rectifier. For example, during startup, grid bridge 14 will operate as an active rectifier to supply power from the utility grid to DC link 12. In situations where a drop in grid voltage occurs, the generator bridge 10 can operate in either the active rectifier mode or an inverter mode, as required to control the voltage of the dc link 12. The action of the controllers for generator bridge 10 and network bridge 14 (i.e. generator bridge controller 18 and network bridge controller 46, described in more detail below) are coordinated in case from a grid voltage drop, so that no power is drawn from the supply grid, but subject to parameterization and the level of the voltage drop, the power converter can still supply power to the grid of supply.

It can also be advantageous for maintenance purposes and when the wind turbine is operating at very low speeds to operate the generator 4 in a drive mode. In this case, power can be supplied from the utility grid to the generator 4 through the grid bridge 14, which operates as an active rectifier, and the generator bridge 10, which operates as an inverter.

The ac output voltage of the grid bridge 14 is filtered by inductors 16 (and other possible filters) and supplied to the fixed rated frequency power grid through a step-up transformer 6. Protective control mechanisms (not shown) may be included. to provide a reliable connection to the utility grid and to isolate the generator system from the utility grid for various operational and non-operational requirements.

IS 2 390 133 T3

Wind farm topology

As briefly mentioned above, several wind turbines as represented by the entirety of Figure 1 may be connected to each other, defining a wind farm. This is shown schematically in Figure 6, in which several power converters 1a to 1d are connected to a supply network (marked RED) of fixed nominal frequency by means of a parallel connection 72. Each power converter 1a to 1d includes a filter 16a to 16d and a step-up transformer 6a to 6d. And an additional step-up transformer 74 of the wind farm is also provided between the parallel connection 72 and the supply network. Figure 6 shows how the wind farm voltage feedback signal, which is described in more detail below with reference to Figure 7, can be measured on the side of the parallel connection (marked POWER SUPPLY A VOLTAGE FEEDBACK A). WIND FARM) or on the supply network side (marked WIND FARM VOLTAGE FEEDBACK B) of the step-up transformer 74 of the wind farm. The advantage of measuring the upper voltage feedback signal on the supply network side of the wind farm step-up transformer 74 is that the measurement on the parallel connection side is subject to regulation between the ends of the step-up transformer. Therefore, this regulation effect is eliminated if the measurement is performed on the mains side. Alternatively, the measurement of the voltage feedback signal from the wind farm on the supply network side can be calculated using the measurement of the voltage feedback signal from the wind farm on the side of the parallel connection, the characteristics of the wind farm step-up transformer 74 and the amplitude and angle of the current through the wind farm step-up transformer.

Generator Bridge Control

The control of generator bridge 10 will now be explained with reference to Figures 1 to 3.

A generator bridge controller 18 receives a DC link voltage demand signal VDC_GEN * and a voltage feedback signal VDC_FB indicative of the DC link voltage. VDC_FB is subtracted from VDC_GEN * and the difference is supplied to a PI controller 20 with the inputs of the variable integral gain Ki and the proportional gain Kp to provide a DC link current demand signal IDC_GEN *, which is the effective current which is required to flow on dc link 12 to satisfy prevailing operating conditions. This DC link current demand signal IDC_GEN * is then limited during network fault conditions by an IDC_ LIM signal supplied from the network bridge controller 46 (see below) to form an IDC_GEN * _LIM signal. To convert the DC link current demand limited signal IDC_GEN * _LIM into a cross current demand signal IQ_GEN * relative to the generator phase current, the link current demand limited signal IDC_GEN * _LIM is first multiplied by the voltage feedback signal VDC_FB to provide a power signal POWER_GEN. The power signal POWER_GEN is then converted in block 92 of the IQ_GEN * calculator function into the transverse current demand signal IQ_GEN * applying the following formula:

IQ _ GEN * = (POWER GEN \ - (VD _ FF x ID _ GEN *) VQ FF in which VD_FF is the direct supply component of the direct voltage within a current controller 26 of Figure 2, ID_GEN * is the longitudinal demand current supplied from a saturation characteristic function block 32 and VQ_FF is the feedforward component of the transverse voltage within the current controller 26.

The cross current demand signal IQ_GEN * is limited by a limiting function to stay within the non-breakdown region of the generator characteristic and the voltage and current ratings of the generator and mains bridges. This limit is determined by means of an off-line calculation to create a look-up table integrated into functional block 22 based on machine equivalent circuit parameters, drive rate parameters, and the required operating speed range. The resulting look-up table is used during converter operation by accessing it with a rotor speed feedback signal N (or an observed rotor speed signal) and taking the resulting signal as the limit value for block 24 limit function of IQ_GEN *. The resulting limited cross current demand signal IQ_GEN * _LIM is then supplied to a current controller 26 (described in more detail below). The limited cross current demand signal IQ_GEN * _LIM is also used to determine the slip frequency WS to be applied to the generator 4 to achieve the necessary power flow from the generator to the DC link 12. Slip frequency WS is determined using the following function:

IQ GEN * LIM x RRx LM WS = - = - = -,

Φ * x LR

ES 2 390 133 T3 in which RR is the rotor resistance, LM is the magnetization inductance, Φ * is the generator flux demand signal and LR is the rotor leakage inductance.

Integrating the slip WS frequency provides an OS output, which is the slip angle. Integrating the output of a speed observer 28 provides OR, which is the observed angle of the rotor. (The observer function 28 can be substituted for direct measurement of rotor position using an incremental coder or similar device). A rotor flux angle 00 can then be determined by adding the slip angle OS and the rotor angle 0R. The rotor flux angle 00 is the angle at which the combination of the longitudinal voltage VD and the transverse voltage VQ is to be applied at the terminals of the stator of the generator 4 by means of a pulse width modulation generator 30 . This is described in more detail below. It should be noted that for synchronous generators the slip frequency definition and integration step is not required.

The flow demand signal Φ * from the generator (which can be constant or variable, depending on the required characteristics of the system) is applied to a function block 32 that contains the saturation characteristic of the magnetizing inductance of the generator. The saturation characteristic is determined either by direct measurement when the generator is started or by extracting data from the factory test results for the generator. The output of the saturation characteristics function block 32 is a magnetizing current signal and is converted to the longitudinal current demand signal ID_GEN * applied to the current controller 26. For synchronous generators, the longitudinal current demand signal is determined by the voltage requirements of the generator terminals for each speed and load condition. By adjusting the longitudinal current demand signal to synchronous generation, excitation can be modified by action of generator bridge 10 to optimize terminal voltage and overall generator efficiency for each operating condition.

The current controller 26 for the generator bridge 10 includes two regulators, one operating on the longitudinal current axis and one operating on the transverse current axis. In total, the current controller 26 operates in a synchronous reference frame aligned with the rotor flux angle. Figure 3 shows the total longitudinal and transverse current regulators of the generator bridge 10.

In addition to the limited cross current demand signal IQ_GEN * _LIM and the longitudinal current demand signal ID_GEN *, a cross current feedback signal IQ_ GEN and a current feedback signal ID_GEN are also supplied to the current controller 26. longitudinal, derived from the measurement of the generator phase currents IU, IV and IW. Conversion of triphasic components in a stationary reference frame to longitudinal / transverse components in a synchronous reference frame is accomplished using a combined block 34 of the Clarke / Park transform. The transform uses the angle 00 of the rotor flux for the conversion. It can be seen from Figure 3 that the current controller 26 also receives the following additional signals: the flow demand signal Φ * from the generator (which can be constant or variable, depending on the required characteristics of the system) and the frequency W0 of the generator. generator stator. The stator frequency W0 is calculated from the sum of the slip frequency and the rotor frequency. The rotor frequency is derived from the observed rotor speed and the number of poles in the generator.

The current controller 26 operates by comparing the longitudinal current demand signal ID_GEN * with the longitudinal current feedback signal ID_GEN, and the limited transverse current demand signal IQ_GEN * _LIM with the transverse current feedback signal IQ_GEN and applying the errors resulting to independent PI controllers. The outputs of the PI controllers are then summed with cross-coupling signals derived from the product of current demands and machine parameters to produce a total output voltage for the longitudinal and transverse axes, VD_GEN * and VQ_GEN *, respectively. The cross-coupling terms are shown in Figure 3 and emulate the standard voltage equations for generator 4 at steady state. Referring to cross-coupling terms, ÓLS is the generator stator leakage inductance and RS is the generator stator resistance.

The final voltage outputs of the current controller 26, VD_GEN * and VQ_GEN *, are converted from Cartesian to polar coordinates using a coordinate converter 38. The magnitude of the total voltage V_GEN * is calculated according to the equation:

V _ GEN * = y¡ (VD _ GEN *² + VQ _ GEN *²) and is supplied to the door drive command signal controller 36. The angle between the magnitude of the total stress V_GEN * and the transverse stress VQ_GEN * is 0_GEN and is calculated from the tangent arc of VD_GEN * / VQ_GEN * as follows:

IS 2 390 133 T3

GEN = arctan

VD _ GEN * 3 VQ GEN *)

The angle 0_GEN between the magnitude of the total voltage V_GEN * and the transverse voltage VQ_GEN * is added to the angle Θ0 of the rotor flux to determine the angle at which the total voltage is to be printed on the generator 4 stator terminals.

The individual upper (U) and lower (L) gate drive command signals for the three phases U, V, and W resulting in the individual signals UU, UL, VU, VL, WU, and WL from generator bridge 10 are calculated in the pulse width modulation (PWM) generator 30 using the magnitude of the total voltage V_GEN *, the sum of the angles Θ_GEN and Θ0 and the frequency of the pulse width modulation. The DC link voltage feedback signal VDC_FB is also taken into account in these PWM calculations. The DC link voltage feedback signal VDC_FB can be independently derived when using separate controllers for generator bridge 10 and network bridge 14, respectively. This is particularly necessary when generator bridge 10 and network bridge 14 are physically separated from each other and there is significant inductance between the DC link capacitance of each bridge. In situations where an independently derived DC link voltage feedback signal is provided for each bridge, it will be readily appreciated that the following substitution should be made:

For generator bridge 10: VDC_FB = VDC_FB_GEN For network bridge 14: VDC_FB = VDC_FB_NET

The current controller 26 also produces a power forward signal indicative of the generator power POWER_FF, which is calculated as follows:

POWER_ FF = yÍ3 (VQ _ GEN * x IQ _ GEN + VD _ GEN * x ID _ GEN)

This is used as a feedback signal to the network bridge controller 46.

Network bridge control

The control of the network bridge 14 will now be explained with reference to Figures 1 and 4 to 7. The control is based on a voltage control scheme and is different from the power factor angle control scheme and the power factor scheme. reactive power control used in the conventional power converters described above.

The voltage control scheme includes two levels of control. With reference to Figure 7, the former is defined at the wind farm level and is responsive to a voltage demand signal from the wind farm that is typically set by the utility company that controls the wind farm. This voltage demand signal from the wind farm is compared to a voltage feedback signal from the wind farm and the error between the two signals is applied to a proportional and integral controller 40 to define a voltage demand signal VTURB * of turbine that is transmitted to all wind turbines T1 to TN of the wind farm. A second level of control is then applied to each of the individual wind turbines to regulate its own output voltage in response to the turbine voltage demand signal VTURB *.

With reference to Figure 4, in each of the wind turbines of the wind farm, the turbine voltage demand signal VTURB * is compared at an adder node 42 with a cross voltage subordinate feedback signal VQ_NET (see below ) which is derived from the three-phase voltage measurement on the mains side of inductors 16. The difference between the two signals is supplied to a PI controller 44 to form a reactive current demand signal ID_NET * which is supplied by a limiting block 66 to a current controller 58 described in more detail below.

The reactive current demand signal ID_NET * is also re-supplied through a proportional gain controller 48 to the summing node 42 to further modify the voltage difference signal. This serves to provide an attenuation characteristic, such that when multiple wind turbines are connected to each other in parallel to a wind farm transformer through different connection impedances, the reactive current sharing between each wind turbine is more balanced. The attenuation gain can be regulated, depending on the site network configurations, to give a proper balance of the current between the wind turbines and to respect the nominal limitations. Limits apply to the longitudinal and transverse current demand signals ID_NET * and IQ_NET *, respectively, as described below, for grid voltage drop situations.

The voltage control scheme is integrated into the network bridge controller 46 as follows. The network bridge controller 46 has five main input signals and seven main feedback signals and 13

ES 2 390 133 T3 uses these to derive gate drive command signals to control the operation of the power switching semiconductor devices in the network bridge 14.

The input signals include a DC link voltage demand signal VDC_NET * for the network bridge, a power export demand signal P *, the turbine voltage demand signal VTURB *, a REGIME parameter DRIVE that defines the drive current regime and the POWER_FF signal of direct power supply supplied from the generator bridge controller 18 and which is indicative of the generator power. Feedback signals include three-phase voltage measurements VRY, VYB, and VBR (i.e., voltage measurements taken between the so-called red (R), yellow (Y), and blue (B) output lines supplying power from bridge 14 from mains to mains), three-phase current measurements IR, IY, and IB, and the voltage feedback signal VDC_FB indicative of the dc link voltage. The feedback signals are used to derive the following voltage and current subordinate feedback signals for the network bridge 14 on the longitudinal and transverse axes: VD_NET, VQ_ NET, ID_NET, and IQ_NET. In addition, a control signal IDC_LIM is passed from the grid bridge controller 46 to the generator bridge controller 18 to allow rapid derating and coordinated control between the controllers during grid fault conditions. During such grid failure conditions, the control dc link voltage is distributed between the grid and generator bridges so that no active power is drawn from the supply grid and the required voltage support is achieved. supply network and power export requirements.

Function block 68 incorporates a phase locked loop (PLL) system to derive the 0MAINS signal, which is a measure of the angle of the mains voltage.

The DC link voltage demand signal VDC_NET * is only required to satisfy starting requirements, to maintain connection to the grid during no-wind conditions, and to allow fast coordinated control of the DC link voltage between the controller 18 the generator bridge and the grid bridge controller 46 during grid fault conditions. In operation, the voltage feedback signal VDC_FB is subtracted from the DC link voltage demand signal VDC_NET * and the result is applied to a PI controller 50 to determine the VDC_PIJQ_NET * signal. In function block 71, a signal IQ_FF, indicative of the cross network current required to export the instantaneous power of the generator, is calculated from the direct power supply signal POWER_FF, of a signal that represents the voltage VQ_NET of the network and a gain signal PFF_GAIN which is an output from limiting block 66. This is then added to the VDC_PI_IQ_NET * token to create an unlimited IQ_NET * token. The resulting signal is limited by a limiting function (block 52 of the limiting function) controlled by the lower of P * / VQ_NET or by the limit derived from the requirements of the mains voltage drop.

Referring to Figure 1, the rotor speed feedback signal N is derived from a speed sensor 54 (or, alternatively, from an observed rotor speed signal) and is then filtered to provide a first filtered signal. N 'of the speed and a second filtered signal N'2 of the speed. The second N'2 filtered velocity signal provides attenuation for any shaft resonance via an attenuation gain KD. The first filtered speed signal N 'provides a pointer to a pre-calculated look-up table 56 of the power demand as a function of the filtered speed. The lookup table can be combined with a PI controller. The resulting power export demand signal P *, which is the sum of the attenuation and power demand signals from the look-up table, is applied to the network bridge controller 46, as shown in Figure 1. More particularly, the power export demand signal P * is divided by the cross-voltage background feedback signal VQ_NET to become the limiting signal for the cross-current demand signal IQ_NET * under normal operating conditions. Alternatively, the power export demand signal P * can be converted into the cross current demand signal IQ_NET * by dividing the power export demand signal P * by a filtered version of the cross voltage subordinate feedback signal VQ_NET which is derived from the voltage at the network terminals of the inductors 16.

The cross current requirements limited signal IQ_NET * _LIM (ie, the output of the limiting function block 52) is the input of a current controller 58. The current controller 58 for the network bridge 14 includes two regulators, one operating on the longitudinal axis and the other operating on the transverse axis. In total, the current controller 58 operates in a reference synchronous frame aligned with the network cross voltage VQ_NET. Figure 5 shows the global longitudinal and transverse current regulators of the network bridge 14.

In addition to the limited transverse current demand signal IQ_NET * _LIM and a limited signal ID_NET * _LIM of longitudinal current demand (i.e., the output of limiting block 66), an IQ_NET signal is also supplied to the current controller 58 cross current feedback signal and a longitudinal current feedback signal ID_NET, which are derived from the measurement of the phase currents IR, IY and IB of the network bridge. Conversion of triphasic components in a stationary reference frame to longitudinal / transverse components in a synchronous reference frame is accomplished using a combined block 70 of the Clarke / Park transform. The transform uses the 0MAINS angle of the mains voltage for the conversion.

IS 2 390 133 T3

The current controller 58 operates by comparing the longitudinal current demand limited signal ID_NET * _LIM with the longitudinal current feedback signal ID_NET, and the transverse current demand limited signal IQ_NET * _LIM with the transverse current feedback signal IQ_NET and applying the resulting errors to independent PI controllers. The outputs of the PI controllers are then summed with cross-coupled signals derived from the product of the current demands and impedance values ​​of the grid-side circuit to produce a total output voltage for the longitudinal and transverse axes, VD_NET * and VQ_NET *, respectively. The cross-coupling terms are shown in Figure 5 and emulate the standard voltage equations for the entire network circuit at steady state. With reference to the cross-coupling terms, LN is the inductance of the line filter and WN is the frequency of the line voltage waveform.

The final voltage outputs of the current controller 58, VD_NET * and VQ_NET *, are converted from Cartesian to polar coordinates using a coordinate converter 64. The magnitude of the total voltage V_NET * is calculated according to the equation:

V _ NET * = ^ ¡(vD ^ ÑeT ^^^ Vq ^ NeT * ^) and is supplied to the gate drive command signal controller 62. The angle between the magnitude of the total stress V_NET * and the transverse stress VQ_NET * is 8_NET and is calculated from the tangent arc of VD_NET * / VQ_NET * as follows:

NET = arctan

VD _ NET * VQ NET *

The angle θ_ NET between the magnitude of the total voltage V_ NET * and the transverse voltage VQ_ NET * is added to the angle θMAINS of the network voltage to determine the angle at which the total voltage is to be printed by the bridge 14 of network in the total circuit of the network side.

The individual upper (U) and lower (L) gate drive command signals for the three phases R, Y and B resulting in the individual signals RU, RL, YU, YL, BU and BL of the network bridge 14 are calculated in the pulse width modulation generator 60 using the magnitude of the total voltage V_NET *, the sum of the angles θ_NET and θMAINS and the frequency of the pulse width modulation. The DC link voltage feedback signal VDC_FB is also taken into account in these PWM calculations. The signal

DC link voltage feedback VDC_FB can be independently derived when using separate drivers for grid bridge 14 and generator bridge 10, respectively. This is particularly necessary when generator bridge 10 and network bridge 14 are physically separated from each other and there is significant inductance between the DC link capacitance of each bridge. In situations where an independently derived DC link voltage feedback signal is provided for each bridge, it will be readily appreciated that the following substitution should be made:

For network bridge 14: VDC_FB = VDC_FB_NET For generator bridge 10: VDC_FB = VDC_FB_GEN

In situations where a grid voltage drop occurs, the limiting block 66 calculates the respective current allocation available from the grid bridge 14, based on its thermal limits, with respect to the transverse and longitudinal axes and also calculates the maximum current IDC_LIM of the generator dc link. The IDC_LIM signal, supplied from the network bridge controller 46 to the generator bridge controller 18, is used to quickly establish the level of current that can be provided by the generator bridge 10 to the intermediate DC link 12.

There are different requirements within the various network codes that prioritize active or reactive current output and the percentages of reactive current required based on the magnitude of the dip. In other words, the behavior of the power converter depends on how it is parameterized for operation in different countries or regions.

Overall, the operation of the power converter is fundamentally different from the operation of conventional power converters described above, because it maintains control of the dc link 12 during mains voltage drops by directly controlling the power flow. from generator 4. By maintaining control of the dc link voltage during a mains voltage drop, it is possible to maintain the required reactive current output from the mains bridge 14 to satisfy the voltage support requirements of the electrical mains.

During a mains voltage drop, the mains bridge dc link voltage controller (a combination of the PI 50 controller and the preceding summing node) becomes the master of the converter system

ES 2 390 133 T3 and assigns signals for both the power limit and the DC link current to both the grid bridge 14 and the generator bridge 10, respectively, based on the magnitude of the voltage drop of the net.

The export of power from the dc link 12 is determined by referencing the power applied to the network bridge 14. As more power is exported from the dc link 12 (to discharge it), the generator bridge 10 will then react to this by taking more power from the generator 4 to fill the dc link. This is in stark contrast to conventional four-quadrant power converters, where power is loaded into the dc link to increase the dc link voltage as a result of the torque demand applied to the generator bridge. Power export to the grid is then determined by the action of the grid bridge controller when the DC link voltage exceeds the grid bridge voltage demand.

Power converter operation

A possible operational implementation of the above power converter topology is as follows. At startup, the DC link voltage demand signal VDC_NET * is set to 1050 volts. The power-switching semiconductor devices of the network bridge 14 are enabled and, under the control of the network bridge controller 46, the dc link voltage is brought up to 1050 volts. This will almost always require an import of power from the supply network to the dc link 12, so that the cross current demand output signal IQ_NET * results in the power flow entering the dc link in this condition. Boot.

At the same time, the DC link voltage demand signal VDC_GEN * applied to the generator bridge power controller 18 is set to 1100 volts.

Assuming that the wind is blowing and that the wind turbine 2 is turning, when the generator bridge 10 is enabled it will control the longitudinal current ID_GEN to achieve that the necessary magnetic flux enters the generator 4 for the prevailing speed conditions, and the current IQ_GEN will be regulated under the control of generator bridge 10 to achieve the target of a dc link voltage of 1100 volts.

As the dc link voltage increases to meet the 1100 volt target, it will exceed the dc link voltage demand signal VDC_NET * for network bridge 14. Consequently, the error signal derived by the network bridge controller 46 when the DC link voltage demand signal VDC_NET * is subtracted from the voltage feedback signal VDC_FB will act in such a way as to transfer power from the link 12 DC to the supply network, the magnitude of this power transfer being limited (block 52 of the limiting function) by a signal derived from the power export demand signal P *. The speed sensor signal N is filtered to provide a first N 'filtered speed signal and a second N'2 filtered speed signal. The attenuation gain KD applied to the second filtered speed signal N'2 provides attenuation of shaft resonance in the turbine transmission. The first filtered speed signal N 'is used as a pointer to a precomputed lookup table 56 of P * as a function of N'. The power export demand signal P * derived from lookup table 56 is applied to controller 46 of network bridge 14. The applied power export demand signal P * is divided by the prevailing network cross voltage VQ_NET to obtain a limit signal to be applied to the cross current demand output signal IQ_NET * derived from the voltage demand signal VDC_NET * DC link for network bridge 14.

In the event of a mains voltage drop, the assignment of a nominal output power (VA) to the active and reactive axes of the network bridge controller 46 will be determined in line with the requirements of the specific network code for the one that the wind turbine is parameterized. The apparent power limit is calculated from the prevailing voltage V_NET as measured by the mains voltage feedback circuits and the overload current rating I_OVERLOAD. More in particular:

Apparent power limit = 4Ϊ (V _ NET x I _ OVERLOAD)

In Figure 4, DRIVE RATE is equivalent to I_OVERLOAD in the above equation.

The power converter operates dynamically to accommodate changes in wind speed. For example, for increasing wind speed, the rotational speed of the wind turbine 2 will also increase, thereby providing an increasing power export demand signal P * to the network bridge controller 46. The network bridge controller 46 causes the network bridge 14 to export more power from the dc link 12 to the supply network. Increasing the amount of power that is exported to the utility grid leads to a drop in the DC link voltage. The generator bridge controller 18 responds to this drop in the dc link voltage by action of the dc link voltage controller 76 (comprising the PI controller 20 and the preceding summing node) to cause the bridge 10 of the generator draws more power from generator 4 to provide more current to dc link 12 until a new steady state is achieved (i.e. wherein the amount of power being supplied to the supply network from the network bridge 14 is equal to the

ES 2 390 133 T3 amount of power supplied to generator bridge 10 from generator 4). In this steady state, the dc link voltage has coincided with the dc link voltage demand signal VDC_GEN *. For a reduction in wind speed the opposite control actions take place.

During the same conditions of increasing wind speed, the power converter described in US Patent 5083039 modifies a torque demand signal to the generator bridge controller to cause increasing generator torque and hence increasing flow of power. power coming from the generator through the generator bridge to the dc link. This causes an increase in the DC link voltage. The network bridge controller then responds to the increase in dc link voltage by acting on its dc link voltage controller to increase the amount of power that is exported to the supply network and thereby return to power. reduce the dc link voltage to its reference value.

Thus, US Patent 5083039 describes a situation where more power flow from the generator to the dc link is "pushed" through the power converter in response to an increase in wind speed, and the secondary response is to export power from the dc link to the supply network through the network bridge. However, the power converter of the present invention operates in the opposite manner, such that, in response to an increase in wind speed, more power is "pulled" from dc link 12 by network bridge 14, and the secondary answer is to import power to the dc link from generator 4 through generator bridge 10 to achieve more current on the dc link.

Alternative power converter topologies

The basic topologies of two different arrangements of alternative power converters will be outlined with reference to Figures 8 to 14. Alternative power converters are very similar to the power converter of Figure 1 and similar parts have been given the same numbers. reference. The purpose of the alternative power converter topologies is to eliminate one or up to three characteristics of the power converter in the Figure, specifically (i) the DC link voltage demand signal VDC_NET * for the network bridge 14 and its associated voltage feedback signal VDC_FB and the PI controller 50, (ii) the iDc_LIM signal that is supplied from the network bridge controller 46 and is used to limit the DC link current demand signal IDC_GEN * during grid failure conditions, and (iii) the signal POWER_FF is a direct feed of power that is produced by the current controller 26.

The first alternative power converter topology removes only the characteristics of the DC link voltage demand signal VDC_ NET * for the network bridge 14, its associated voltage feedback signal VDC_FB and the PI controller 50. It also modifies the action of the IDC_LIM signal by replacing it with a new feedforward IDC_FF signal. A modified power direct feed signal POWER_FF 'is still active, but is only used by controller 46 of the grid bridge during a situation. In this case only, the modified power forward signal POWER_FF 'is calculated from the standard power forward signal POWER_FF described elsewhere minus the output of the PI controller 20 of the dc link voltage controller 76. This is shown in Figure 9. The modified power direct supply signal POWER_FF 'is used in the grid bridge controller 46 together with a signal (marked IQ_CAPACITY) relative to the IQ capacity of the grid bridge 14 during a supply grid voltage drop situation. , the power limit POWER_LIMIT and a signal relative to the amplitude of the prevailing voltage VQ_NET of the network to calculate a limited signal IQ_NET * _LIM of cross current demand that is used during a situation of voltage drop of the supply network. The nominal source for the IQ_NET * _LIM signal is ignored in this situation.

In topologies where the DC link voltage demand signal VDC_NET * is removed for the network bridge, the network bridge 14 can be powered using the network voltage. The dc link voltage is determined by the rectified value of the mains voltage, which is nominally \ 2 χ VLL (that is, the line-to-line voltage at the ac terminals of the mains bridge 14). This establishes a DC power supply from which auxiliary circuitry can be derived, such as the microprocessor (s) for generator bridge controller 18 and utility bridge controller 46, and gate drive power for generator bridge 10. generator and network bridge 14. The DC link voltage is then available to flow from generator 4 and bring it under control.

Assuming the wind is blowing and the wind turbine 2 is turning, then the generator 4 can start supplying power to the dc link 12 and achieve a dc link voltage that is equal to the voltage demand signal VDC_GEN * from the cc link.

The basic topology of the first alternative power converter arrangement will now be described with reference to Figures 8 to 10. In this arrangement, the DC link voltage controller 76 of the generator bridge controller 18 remains active under all conditions. operational. Under steady state conditions, the action of the integral term within the PI controller 20 of the dc link voltage controller 76 is minimized by the inclusion of the feedforward signal IDC_FF from the network bridge controller 46. In a utility voltage drop situation, the feedforward signal IDC_FF provides information on the amount of DC current to be supplied by the bridge 14 17

ES 2 390 133 T3 of the generator in response to changes in the supply network voltage. The IDC_FF signal is calculated in function block 90. By including these features, the variation in DC link voltage during a utility voltage drop situation is minimized. Furthermore, the action required by the integral term within the PI controller 20 of the dc link voltage controller 76 is minimized and thus much smaller deviations in the actual dc link voltage are required to increase or decrease the value. integral to the correct value to achieve steady state operation.

The basic topology of the second alternative power converter arrangement will now be described with reference to Figures 11 to 14. In this arrangement, the DC link voltage is removed from the network bridge controller 46 in the same manner as has been done. described above.

A first option for the control dc link for the generator bridge 10 of the second alternative power controller arrangement will now be described with reference to Figures 11 and 12. The purpose of the IDC_LIM signal in the power converter of Figure 1 is to pass critical information about prevailing grid voltage conditions and power flow levels to the generator bridge controller 18. This is particularly important during a mains voltage drop situation when power throughput capacity is severely limited. In the alternative power converter, the signal IDC_LIM is replaced by an inferred signal IDC_NET 'which is used only by the generator bridge controller 18.

The inferred signal IDC_NET 'is calculated from information that is available to the generator bridge controller 18 using the following equation:

IDC NET '=

POWER _ FF 'VDC FB.

-1 CAP

In the second alternate power converter, the generator power signal POWER_FF is derived from the current controller 26 of the generator bridge controller 18, as illustrated in Figure 3, using the equation:

POWER_FF = / 3 (VQ_ GEN * xIQ_ GEN + VD_ GEN * xID_ GEN)

However, the generator power signal POWER_FF is not supplied to the network bridge controller 46, but is used solely by the generator bridge controller 18 in the derivation of the inferred signal IDC_NET '. (POWER_FF has the same derivation here as POWER_FF in the first arrangement shown in Figure 2. In this memory, the POWER_FF flag is kept the same for consistency; however, in this case, the signal is not a literal power direct feed signal). The inferred signal IDC_NET 'is used to indicate an effective dc current that the network bridge 14 exports to the supply network, but is calculated from the conditions of the generator bridge 10.

Dividing the generator power signal POWER_FF by the voltage feedback signal VDC_FB gives the effective dc current that is supplied to dc link 12 from generator bridge 10.

Measurement of the current charge (or discharge) of the volumetric capacitor 82 at dc link 12 is accomplished by adding a small capacitor 78 in parallel with the volumetric capacitor, measuring the current in the small capacitor using a current sensor 80, and rescaling the measured current by a factor related to the ratio between the capacitance of the small capacitor and the total capacitance of the DC link 12. The sign of the current signal I_CAP is positive when the volumetric capacitor 82 is charging and negative when it is discharging. Since the current flowing in volumetric capacitor 82 is a switched waveform, it is necessary to integrate the current over a full period of pulse width modulation (PWM).

The inferred signal IDC_NET 'is added to the output of the DC link voltage controller 76 at the summing node shown in Figure 12.

When a supply network voltage drop situation occurs, in the first case the generator bridge 10 is not aware that the network bridge 14 can no longer export power to the supply network at the previous rate. The current surplus between that imported from generator 4 and that exported to the supply network charges the volumetric capacitor 82 and is seen as an increase signal on the load current of the volumetric capacitor derived from the current signal I_CAP. The signal IDC_NET 'is then recalculated and the signal that is added to the output of the DC link voltage controller 76 is modified, thereby modifying the real power that is imported from the generator 4 during the voltage drop situation of the supply network.

In situations where the volumetric capacitances of the dc link of the generator bridge 10 and the network bridge 14 are separated by a significant distance, there may be an inductance between bridges causing a resonance between the two decoupled volumetric capacitors. In this case, the small capacitor 78

ES 2 390 133 T3 can be replaced by a network of two capacitors and an inductor that are selected to achieve the same resonant frequency as the combination of the capacitance of the decoupled volumetric capacitors and the inductance between bridges. The current is then measured as the current flowing in both small capacitors, so that any resonance between them is canceled by the measurement procedure.

Power control for network bridge 14 will now be described in greater detail with reference to Figure 13.

IQ_CAPACITY is a signal that is related to the parameters of the drive regime and the prevailing voltage VQ_ NET of the network. It will be appreciated that the generator bridge 10 and the generator bridge controller 18 together have a finite response time to changes in operating conditions. To account for this within the network bridge controller 46, the IQ_CAPACITY signal is the limited speed of response to produce a limited cross current demand signal IQ_NET * _LIM that is applied only during a grid voltage drop situation. of supply. The speed of response limit is regulated in such a way that the limited cross current demand IQ_NET * _LIM is reduced at the same rate as the power in the generator bridge 10 is reduced. The speed of response limit is properly regulated when the DC link voltage disturbance that occurs during a utility voltage drop situation is minimized.

The switch 84 takes the output of a response speed limiting function when DIP_DETECT = 1 (that is, when the limiting block 86 determines that a mains voltage drop situation exists with reference to specific operating conditions). mains voltage and the parameterization of the controller 46 of the mains bridge). If not, in normal operating situations, when DIP_DeTECt = 0, then the cross current demand signal IQ_NET * is derived from the prevailing voltage VQ_NET of the network, and the final power limit POWER_LIMIT, determined by the drive parameters, such as shown in Figure 12. The transverse current demand signal IQ_NET * is limited by a clamping function determined by the power export demand signal P * and the prevailing voltage VQ_NET of the network. When DIP_dEtECT = 0, the output of the clamp function is applied to the current controller 58 as a cross current demand signal IQ NET * _LIM.

Limiting block 86 provides a longitudinal current demand limited signal ID_NET * _LIM to current controller 58 in a mains voltage drop situation. In normal operating situations, the longitudinal current demand signal ID_NET * is supplied directly to the current controller 58 as the longitudinal current demand limited signal ID_NET * _LIM.

A second option for the control dc link for generator bridge 10 of the second alternative power controller arrangement will now be described with reference to Figures 11 and 14.

If the power converter is operating, for example, at full capacity, then the integral of the PI controller 20 of the dc link voltage controller 76 will have a significant value. In the absence of any other control feature, in the event of a mains supply voltage drop, an error would have to occur in the dc link voltage to discharge or reset the integral value. Such an error in the dc link voltage would be a transient positive voltage value, with the risk of overvoltage disconnection of the dc link occurring due to finite voltage limitations of the hardware.

During a mains voltage drop situation, the rate of change of the voltage feedback signal VDC_FB, indicative of the DC link voltage (as represented by dVDC_FB / dt), is significantly greater than it would be experienced during normal power converter operation. If dVDC_FB / dt is greater than a threshold, it can be inferred that something has affected the ability of the network bridge 14 to export power and it is probably that the network voltage has been reduced.

The second option for the control dc link shown in Figure 14 is based on the determination that if the voltage feedback signal VDC_FB is greater than a first threshold (VDC_FB_THRESHOLD) and dVDC_FB / dt is greater than a second threshold (dVDC_FB / dt_THRESHOLD), then the integral value in the PI controller 20 is multiplied by a value less than 1, the value being determined by parameterization of the controller 18 of the generator bridge.

If these threshold requirements continue to be exceeded, then the same action of the control dc link will be applied on consecutive PWM sweeps (i.e., a single PWM sweep representing an iteration of the control program), such that the integral value in the PI controller 20 is stepped down sequentially.

The two threshold parameters are based on knowledge of the characteristics of the wind turbine, on the expected maximum of dVDC_FB / dt during normal operation and the expected value of dVDC_FB / dt in the event of a power grid failure. The predicted maximum of dVDC_FB / dt during normal operation can be calculated with knowledge of the DC link capacitance and drive parameters.

IS 2 390 133 T3

Alternative Power Converter Operation

A possible operational implementation of the alternative power converter topology shown in Figures 11 to 14 is as follows. At startup, the dc link voltage is established by means of suitable pre-charge circuits (not shown) from the transformer 6 shown in Figure 11. At this point, the semiconductor power switching devices of the network bridge 14 follow disabled.

The DC link voltage demand signal VDC_GEN * applied to the generator bridge power controller 18 is fixed at 1100 volts.

Assuming that the wind is blowing and that the wind turbine 2 is turning, when the generator bridge 10 is enabled it will control the longitudinal current ID_GEN to achieve that the necessary magnetic flux enters the generator 4 for the prevailing speed conditions, and the current IQ_GEN will be regulated under the control of generator bridge 10 to achieve the target of a dc link voltage of 1100 volts.

The power export demand signal P * is set to zero and the output of the turbine mains voltage controller 88 (and, more particularly, the PI controller 44) is set to zero. At this point, the power switching semiconductor devices on the network bridge 14 are enabled.

In the normal mode of operation, in which the mains voltage seen at the ac terminals of the mains bridge 14 is within normal limits, the following control action is implemented. The speed sensor signal N is filtered to provide a first N 'filtered speed signal and a second N'2 filtered speed signal. The attenuation gain KD applied to the second filtered speed signal N'2 provides attenuation of the shaft resonance in the turbine transmission. The first filtered speed signal N 'is used as a pointer to a precomputed lookup table 56 of P * as a function of N'. The power export demand signal P * derived from lookup table 56 is applied to controller 46 of network bridge 14. The applied power export demand signal P * is divided by the prevailing network cross voltage VQ_NET to obtain a limit signal. This limit signal is applied by means of a clamping function to the cross current demand signal IQ_NET * to form the cross current demand signal IQ_NET * _LIM.

In this mode of operation, the cross current demand signal IQ_NET * is set to a value greater than the maximum value that can be derived from the power export demand signal P *, so that the attenuation function described in the foregoing.

In the event of a supply network voltage drop, the assignment of a nominal output power (VA) to the active and reactive axes of the network bridge controller 46 will be determined in line with the requirements of the specific network code. for which the wind turbine is parameterized.

Practical implications of the power converter topology

The power converter topology arrangements can be implemented as follows. The generator bridge 10 and grid bridge 14 can each be implemented using a liquid-cooled DELTA MV3000 inverter module with a suitable power rating. This is an IGBT-based voltage source inverter suitable for operation on a 690 V ac network with a resulting voltage from the dc link of 1100 V. The generator bridge controller 18 and the grid bridge controller 46 can each be implemented using a DELTA MV3000 controller. This is an electronic controller based on a microprocessor, whose firmware incorporates the necessary functionality to implement the previous power control schemes. The microprocessor operates on a fixed time base, sometimes referred to as "sweep time," related to the frequency of the controller's pulse width modulation (PWM). All of these products are supplied by Converteam Ltd, of Boughton Road, Rugby, Warwickshire, CV21 1BU, UK.

Possible modifications to the power converter topology

The proposed power converters described above can be similarly arranged if the induction generator 4 is replaced by a permanent magnet or field winding synchronous generator. In situations where a field winding synchronous generator is employed, the additional field drive input to the generator will typically be used to provide the main flux, with the longitudinal current demand signal from the stator being zeroed. For situations of high dynamics and / or weakening of the field, the longitudinal current demand signal of the stator can be set at non-zero values ​​to more quickly regulate the flow in the generator. Typically the generator will be a three phase machine, but other phase numbers may be used. The power converter can also be arranged to operate with multi-level inverters instead of the two-level inverter arrangement described above.

IS 2 390 133 T3

The controller arrangement described above proposes two independent controllers that are coordinated by means of control signals that are sent from generator bridge controller 18 to network bridge controller 46 and vice versa. It would be equally suitable to integrate the functionality of the controllers into a single physical controller. Similarly, the functionality could be extended to more than two controllers if this is convenient for the practical implementation of the power converter.