Circuit arrangement for use in a variable speed wind turbine system comprising a double-fed induction generator and a back-to-back converter

0001CIRCUIT ARRANGEMENT FOR USE IN A VARIABLE SPEED WIND TURBINE SYSTEM COMPRISING A DOUBLE-FED INDUCTION GENERATOR AND A BACK-TO-BACK CONVERTER

FIELD OF THE INVENTION

0003The present invention relates to a circuit arrangement for use in a variable speed wind turbine system comprising a double-fed induction generator and a back-to- back converter of the kind set forth in the preamble of claim 1 , a method of controlling such a circuit arrangement, and a control system for a wind turbine comprising such a circuit arrangement and for performing said method.

BACKGROUND ART

0005In wind turbine systems with variable rotational speed, it is known to use a back-to- back converter for transferring electrical energy between the rotor of the double-fed induction generator and the grid. The back-to-back converter comprises a rotor- connected converter (in the following rotor converter), an intermediate DC-circuit (in the following DC-link) and a grid-connected converter (in the following grid converter). The back-to-back converter can be controlled in such a way that the stator windings of the generator can be connected directly to the grid due to the fact that the currents in the rotor windings are controlled in such a way that the frequency of the stator windings corresponds to the grid frequency. Furthermore, the current in the rotor windings are controlled in such a way that desired values of active and reactive power delivered by the generator can be obtained. Such wind turbine systems are e.g. known from US-5,083,039.

0006In connection with voltage drops on the grid, the generator is demagnetized, and when the voltage returns the generator is remagnetized. During remagnetization, the generator consumes a large amount of reactive power. During demagnetization and remagnetization, the generator will overload the back-to-back converter providing an uncontrolled increase in the voltage on the DC-circuit. Due to the fact that the voltage on the grid is low, the grid converter cannot deliver active power to the grid. The normal procedure during voltage drops on the grid has been to disconnect the generator from the grid and short-circuit the generator rotor windings using a controlled rectifier bridge connected across the rotor windings. Reconnection of the wind turbine after re-establishment of the grid voltage would normally be performed without the wind turbine taking active part in the re-establishment of the grid voltage.

DISCLOSURE OF THE INVENTION

0008It is the object of the present invention to provide a circuit arrangement for use in a variable speed wind turbine system comprising a double-fed induction generator and a back-to-back converter connected for transferring energy between the rotor of the double-fed induction generator and the grid, with which it is possible to keep the stator windings of the generator connected to the grid during grid faults, such as low grid voltage or short circuits on the grid, and this object is achieved with a circuit arrangement of said kind, which according to the present invention also comprises the features set forth in the characterising clause of claim 1. With this arrangement, the wind turbine generator can actively take part in the re-establishment of the grid voltage and the generator can contribute to the short-circuit current during the fault conditions. Furthermore, it is possible to control the active and reactive power contribution from the generator during fault conditions and re-establishment of the grid voltage. During the fault conditions, the controlling of the currents of the rotor provides the possibility of reducing speed oscillations and avoiding over-speed conditions for the wind turbine, due to the fact that active power from the wind turbine can be dissipated in the impedances during low grid voltage conditions. During a fast recovery of the grid voltage the controlled switches in the rotor converter can temporarily be turned off. Thereby the freewheeling diodes transfer power to the DC-link, when the rotor voltages are higher than the DC-link voltage. This provides a soft and fast re-establishment of the power production and lead to low stresses on the wind turbine and the grid.

0009In order to reduce the power dissipation during normal operation, the coupling means for connecting the resistance and disconnecting the power-switching element from the grid comprises contactors, but in such cases where the connections and disconnections have to be made in a speedy way, the coupling means may comprise power-electronic switches, which will however introduce additional losses during normal operation.

BRIEF DESCRIPTION OF THE DRAWINGS

0011In the following detailed part of the present description, the invention will be explained in more detail with reference to the exemplary embodiments of a circuit arrangement in accordance with the invention shown in the drawings, in which Figure 1 shows a circuit arrangement in accordance with the present invention,

0012Figure 2 shows an alternative circuit arrangement in accordance with the present invention,

0013Figure 3 shows a different connection possibility for the impedances for dissipating the power transferred from the DC-link, and Figure 4 shows a more complete schematic diagram of the control system for a wind turbine in which the circuit arrangement in accordance with the present invention is implemented.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0015The circuit arrangement shown in Figure 1 comprises a double-fed induction generator G. The stator windings of the double-fed induction generator G are directly connected to the grid, as indicated by the transformer connection T. It is naturally possible, if not even necessary, to have appropriate protection devices in the connection between the stator of the double-fed induction generator G and the grid, but in connection with the present invention and the following description thereof, such protection devices are of minor importance.

0016The rotor windings of the double-fed induction generator G are connected to a back- to-back converter comprising a rotor converter RC, a DC-link and a grid converter GC, said grid converter GC being connected to the grid, as indicated by the transformer connection T. As is well known within this technique, each of the converters comprises six power switching elements connected in a bridge configuration and connected to the DC- link in order to provide the possibility of controlling the currents i in the rotor windings in such a way, that desired values of active and reactive power P,Q delivered by the generator stator windings can be obtained. Furthermore, each of the power switching elements are associated with a free-wheeling diode, as well known in this technique. Figure 1 shows rotor inductances L_rot0r connected between the rotor converter and the rotor terminals on the generator G and corresponding grid inductances Lg_rj_d connected between the grid converter GC and the grid. In accordance with the present invention a switch S2 is provided to enable a disconnection of at least one of the phases of the grid from the grid converter GC, and a corresponding, switch S1 is provided, enabling the connection of an impedance between the power switching elements of the grid converter GC, which are thus disconnected from the grid and the DC-link. In this way the power switching elements of the grid converter GC are able to provide a controlled dissipation of power from the DC-link to the impedance connected by means of the switch S1. In the embodiment shown in Fig. 1 , all three phases are disconnected from the grid by means of S2 and correspondingly three individual impedances are connected between the power switching elements and the DC-link by means of the switch S1. In Figure 1 the impedances are connected to the positive terminal of the DC-link, the power switching elements of the grid converter GC connected to the positive and negative terminal of the DC-link, controlling the power dissipation in the corresponding impedances. An alternative connection possibility is shown in Figure 3, indicating the possibility of connecting the impedances in a star configuration, with the star point connected to the midpoint of the DC-link. Other possibilities are a delta configuration, a star configuration without any connection to the star point, etc., where these different configurations each have their own advantages, as well known for a man skilled in the art.

0017Figure 1 further shows an over voltage protection circuit OVP in the form of a controlled rectifier bridge short circuited by a resistor. This over voltage protection circuit OVP is brought, into action, when the grid voltage is reduced at a high rate, leading to fast demagnetisation of the generator G which would, if this over voltage protection circuit OVP was not present, result in an over voltage on the DC-link being generated via the free-wheeling diodes of the rotor converter RC. Figure 2 shows an alternative circuit arrangement in accordance with the present invention, in which the switches S1 and S2 in Figure 1 are substituted by power electronic switches, whereby the switching times for these switches can be reduced, leading to a faster reaction of the circuit arrangement.

0018During normal operation the wind turbine system is controlled to optimise generated power P, e.g. by controlling the pitch θ of the wind turbine wings, and the switching of the rotor converter RC and grid converter GC, respectively. At the occurrence of a grid fault, such as a short circuit, the grid voltage U_g detected will typically be reduced and if the DC over voltage is too high, the over voltage protection OVP is activated. After a reconfiguration of the grid converter GC by opening the switch S2 and closing the switch S1 , the over voltage protection OVP - if triggered - can be disabled and the rotor converter RC can take over the control of the currents i in the rotor windings. The controlling of the currents i in the rotor windings provides the possibility of keeping the stator windings connected to the grid during a fault and providing a high short circuit current contribution during the fault. The initial short circuiting of the rotor windings by means of the over voltage protection circuit OVP provides a high short circuit contribution to the grid, helping to activate protection relays in the grid in order to clear the grid faults. In the following period until the grid fault has been cleared and the grid voltage U_g rises to rated voltage, the rotor converter RC controls the currents i in the rotor windings in order to increase the short circuit contribution from the wind turbine. This control is performed in such a way that the DC-voltage is controlled close to its rated voltage by setting the active power reference positive, when the speed of the generator G is above synchronous speed, and setting said reference negative, when said speed is below synchronous speed. Furthermore, the power reference is set in such a way, that possible oscillations in speed are dampened and over-speed conditions are avoided. During this control, the pitch θ is controlled in order to keep below maximum speed, i.e. avoiding over-speed conditions, but controlling the speed at a high speed above synchronous speed, which makes it possible to deliver active power P to the grid and charge the DC-link at the same time. Furthermore, the kinetic energy stored in the high rotational speed can be used to ramp up the power production fast after the grid has been restored. When a normal grid voltage U_g is detected, the rotor converter RC takes over the control of the DC-link voltage, while the grid converter GC is reconnected by disconnecting the switch S1 and connecting the switch S2, whereafter normal operation can be resumed.

0019As can be seen from the above, the present invention provides the possibility of avoiding disconnection of the stator windings from the grid and provides a high short circuit current during fault conditions. When the grid fault is removed and the grid voltage U_g is restored, the wind turbine is able to provide a controlled active/reactive power P,Q contribution for stabilising the grid, and large reactive power draw is avoided during reestablishment of the grid voltage. Furthermore, the present invention provides a soft and short response, when the grid fault is removed and the grid voltage U_g is restored, thus actively limiting the stress on the wind turbine and the grid, during restoration of the grid voltage U_g.

0020Due to the sudden removal of the electrical torque in connection with a grid fault, an over-speed condition for the turbine may arise due to the slow response of the pitch system to remove the torque input from the wind. Furthermore, oscillations in the transmission drive train can be initiated with the sudden torque change. In this respect, the rotor converter RC can be controlled to damp the mechanical oscillations and avoid over-speed conditions at speeds above synchronous speed. This can be achieved by increasing the active power P delivery to the grid during the fault, even though the grid voltage U_g is reduced. Active power transferred to the DC-link during this control method is dissipated in the grid resistors. When the grid fault is removed, a soft response is achieved by suitable control of the rotor converter RC. When the grid voltage U_g is restored, the generator G has to be remagnetised, and if the rise of the grid voltage U_g is very fast, the current i through the rotor converter RC will be correspondingly high. The rotor converter RC is not designed for these high currents and accordingly, the active switches in the rotor converter RC are switched off and the free wheeling diodes, which are able to handle high pulse-currents, conduct current i from the rotor windings to the DC-link, when the rotor voltages are higher than the actual DC-link voltage. In this way the currents i in the rotor and hence the electrical torque generation is limited. This situation corresponds to a situation, in which a high rotor resistance is inserted in the rotor, whereby the remagnetising time is actively reduced and the rotor currents i decay fast and the rotor converter RC can be activated to regain full control of active and reactive power P,Q. During the remagnetisation of the generator G, the DC-link will be charged, and the grid converter GC will dissipate the excessive energy in the resistors connected via the switch S1. Due to the fact that the resistors are used to dissipate the power instead of transferring the power back to the grid, the response time can be implemented very low and the active switches in the grid converter can fully be utilised to dissipate the maximum amount of energy delivered from the rotor. As soon as the rotor converter RC has regained the power control, the grid converter GC is reconnected to the grid by switching S1 off and S2 on. During this switching, the DC-link voltage is controlled by the rotor converter RC by actively altering the power reference according to the DC-link voltage level.

0021In Figure 4 is shown a more complete schematic diagram of the control system for the wind turbine, in which the different control units and measuring functions are indicated, said control units and measurings functions providing the possibility of performing the desired control in accordance with the present invention.

0022In the system shown in Figure 4, a grid-connected measuring and control unit 1 is connected to measure the grid voltages U_g and currents l_g in each phase of the grid, and to detect possible discontinuities in the voltages in order to initiate appropriate control in accordance with the present invention, when a grid fault situation is detected, and to resume normal operation, when the grid fault has been cleared, as detected by this measuring and control unit 1. The measuring and control unit 1 is also connected to control the overvoltage protection circuit OVP and the switches S1 and S2.

0023A power and speed controller 2 is connected to receive information regarding the grid status R and active and reactive power P,Q delivered to the grid, said information being delivered from the grid-connected measuring and control unit 1. Furthermore, the power and speed controller 2 is connected to receive a reference speed ω_r for the rotational speed of the generator G. In accordance with this received information, the power and speed controller 2 controls the pitch θ of the rotor blades and setting of current references i_r for the rotor converter controller 3.

0024The rotor converter controller 3 receives information regarding the rotor currents i and the rotor current references i_r and, based on this information, the rotor converter controller 3 delivers switching information to the drive circuit 4 for the rotor converter RC. Furthermore, the rotor converter controller 3 receives information regarding the DC voltage on the DC link in order to be able to control the DC voltage of the DC link during reconfiguration of the grid converter GC.

0025The grid converter controller 5 receives information regarding the grid voltages U_g and the currents delivered from the grid converter i_g, and based on this information and the DC link voltage, the grid converter controller 5 delivers switching information to the grid converter drive circuit 6 controlling the switches of the grid converter GC.

0026The connection between the rotor 8 of the wind turbine and the generator G is established through a gear mechanism 7, in order to convert the slow rotational speed of the wind turbine rotor 8 to the higher rotational speed of the generator G.

0027Above, the invention has been described in connection with preferred embodiments and several modifications can be envisaged by a man skilled in the art without departing from the following claims.