Insulation monitoring system for secured electric power system

ES 2 585 225 T3

DESCRIPTION

Insulation control system for safe electrical network

Technical field

The invention relates to the identification of the presence of an insulation fault for an electrical distribution network with an isolated neutral comprising some derivations, associated in particular with the location and measurement of insulation defects. The invention relates more particularly to the redundancy of the insulation control in applications that require it in order to increase operational safety.

State of the art

The insulation control of a distribution network allows identifying if a fault occurs: parameters are measured continuously, and the decrease in the insulation resistance of the network allows detecting the presence of a fault in order to react, for example by giving the alarm, and sometimes cutting off a network or part of an electrical network. In particular, a device known with the acronym CPA, or permanent isolation controller, is traditionally connected to the grid transformer in such a way that it determines the isolation impedance and evaluates an "abnormality" characteristic of the presence of a fault: see for example document FR 2 647 220 or document EP 0 593 007.

For a network provided with bypasses, it may also be important to locate the defect detected at the central level through the CPA, and even to fully understand its characteristics. Different methods have been developed, in which measurement means have been installed in the leads, associated with treatment and calculation means, thus forming insulation fault "locators", which can be mobile or permanent: see FR 2 963 679, FR 2 676 821 and FR 2 917 838.

Thus, an insulation control architecture is constituted by an insulation controller at the transformer level, alone or associated with fault location devices at the level of the outputs. However, in the event of a failure of the insulation controller, even if the relays have a fault signal, a failure to detect a fault or, conversely, an unexpected trip may occur. Now, the redundancy of the measurement equipment cannot be advocated for each of the devices and / or functions, either for technical reasons, cost, size or available area of ​​the electronic card in the processing means.

Description of the invention

Among other advantages, the invention aims to offer a simple and economical solution to increase the operational safety of existing insulation control systems, in particular to comply with the IEC 61508 and IEC 61557-15 standards.

Thus, the invention relates to a method for controlling the insulation of an electrical network that has several branches downstream of a transformer, comprising the injection of a predetermined voltage current and a frequency different from that of the network, for example 2.5 Hz for a three-phase network at 50 Hz, and two independent determinations of the insulation impedance, the first by direct measurement of the injected current, the second downstream of the transformer, in particular by means of one or more cores. Each of the two impedance values ​​is compared with the same threshold (for example a hundred ohms) that, when it is not reached (that is, the impedance is lower, the current exceeds a predefined value, allows detecting if it occurs an insulation defect. To guarantee maximum reliability, preventing an injection defect from causing non-detection, the procedure also includes verifying the reality of the injection, comparing the current measurement carried out downstream of the transformer at a fixed value that must be exceeded; This comparison of the current measurement is preferably carried out by means of the insulation impedance determined by another party, said impedance having to be kept below a second threshold, for example of the order of 10 ΜΩ. The method may comprise the lighting of a pilot light or the activation of a relay; preferably, the indication is differentiated in a way that identifies which comparison result is problematic.

The procedure also comprises the control of the insulation for the branches, by means of a similar comparison of the insulation impedance of the branches, determined based on the measurement of the current at the injection frequency that circulates in them, with a threshold that is you can set for each branch, in particular between 10 kD and 1 ΜΩ. The results of the various comparisons can be indicated in a differentiated or common way.

The measurement of the current downstream of the transformer is independent and is carried out by specific means.

The invention also relates to an insulation control system that allows the implementation of the above procedure. In particular, the control system comprises means for injecting a voltage alternating current signal fixed in the part of the network upstream of the transformer, at the level of the secondary or preferably at the level of the neutral of the transformer. The control system comprises a first permanent controller

ES 2 585 225 T3 insulation, with means for measuring the intensity of the injected current and for determining whether or not the impedance calculated from this intensity exceeds a first threshold.

The control system comprises, on the other hand, means for measuring the current at the injection frequency downstream of the transformer, and means for verifying that the measured intensity is greater than a threshold value, which allows to validate the correct operation of the injection means, traditionally integrated into the permanent isolation controller. The control system also comprises means for processing and calculating the measured signal to calculate the impedance from this downstream intensity and means for determining whether or not the calculated impedance exceeds the first threshold; the result of this comparison is thus redundant, which allows to increase the security guaranteed by the control system, in particular in applications of the nuclear power plant or computer data center type that require it. Verification of the intensity of the current can be carried out by means of a comparison of the impedance calculated at a second threshold.

Preferably, the control system comprises means for measuring the voltage at the injection frequency downstream of the transformer, and the means for determining the impedances use this measurement.

The control system further comprises means for measuring the current at the injection frequency at the level of the branches, preferably each one, of the signal processing and calculation means to calculate the impedance from this intensity. branch and means for determining whether or not the calculated impedance exceeds a threshold set according to the load characteristics of the branch, in order to locate a possible insulation fault.

The locator thus formed by the measurement means, the treatment and calculation means and the determination means associated with each branch, is adapted to perform the second measurement of the current at the injection frequency, downstream of the transformer, and the treatment that it experiences. Advantageously, the means for determining and comparing the locator, an element of the system downstream of the transformer, are grouped together on a single calculation card, for example a twelve-way card, which allows checking the isolation of eleven leads in addition to global check on twelfth.

Lastly, the control system comprises means for indicating a suspected insulation fault or injection fault, for example pilot lights. According to one embodiment, the indications are differentiated, with one output for each type of detected faulty comparison result; In an alternative, the same means can be triggered regardless of the type of fault, for example with a single pilot lighting for the two comparisons from the measurement of the current downstream of the transformer.

The invention also relates to a network protected by the above system.

Brief description of the figures

Other advantages and characteristics will be shown more clearly from the description that follows of some particular embodiments of the invention, given by way of illustration and in no way limiting, represented in the attached figures.

Figure 1 illustrates a network provided with an insulation control system according to a preferred embodiment of the invention.

Figures 2A and 2B represent the upstream and downstream elements of a control system according to an embodiment of the invention.

Detailed description of a preferred embodiment

As illustrated in figure 1, the three distribution lines of a three-phase network 1 are supplied with alternative electrical energy by means of a transformer 2; downstream of the transformer 2, the main network 1 supplies electrical energy at several use impedances Zj each connected in a three-phase branch Bj (j = 1 ^ p). In particular, in an apparatus according to the invention, the outputs Bj are concentrated in a defined place, for example inside a cabinet or a distribution station. The illustrated network 1 is called "with isolated neutral", that is to say that the neutral N of the transformer 2 is connected to earth by means of an isolation resistance (or impedance) of the network 1; In this way, a global resistance Rf can be determined, mainly constituted by insulation resistances of the cables, by some equipment, etc.

When one of the load or use impedances Zj presents an insulation fault 4 with respect to ground, this results in the harmful presence of a fault impedance Zd between at least one of the three phase wires or the neutral and the land. A fault impedance Zd is usually schematized by an additional circuit comprising a resistor Rd in parallel with a capacitance Cd; this fault impedance Zd alters (in particular decreases) the value of the “normal” insulation impedance Zf of network 1 between the neutral N terminal of the CPA 10 and the earth.

ES 2 585 225 T3

To detect and measure the presence of this type of leakage 4, a permanent insulation controller 10, or CPA, is for example wired between the neutral N of the transformer 2 and the earth. The CPA 10 comprises means 12 for generating and injecting into the supply network 1 an alternative voltage U0 with a different frequency f and usually lower than the frequency F0 specific to the electrical supply network 1, in particular a submultiple. The injection into the network 1 causes the circulation of a leakage current If that can be measured at the level of the CPA by means of measurement means 14, in particular a measurement resistor.

In the presence of the insulation fault 4, the leakage current If flows in the fault impedance Zd and loops towards the CPA 10 through the earth and the measurement means 14; When it exceeds a threshold, this current is then called the fault current Id and indicates the presence of a fault 4.

Thus, a CPA 10 traditionally comprises means 14 adapted to determine the values ​​of the resistance Rf and the insulation capacity Cf, and means 16 to determine whether these values ​​do not correspond to the "normal" impedance of the network 1 , in order to give an alarm (figure 2A). In particular, in the event of an insulation fault 4, the overall insulation impedance decreases: when it goes below a first threshold D, which can traditionally be set at one or several hundred ohms, an audible and / or visual signal warns the user of the possibility of a defect 4. In a supplementary manner, means can be installed for displaying the general isolation impedance values ​​Zf of the network 1, as well as means for transmitting the result to a central unit for an action on the network 1 (not illustrated).

On the other hand, for each of the branches Bj, an insulation fault locating device 20 is installed (see also in Figure 2B). In particular, means 22j for measuring the local fault current transmits the representative signal to means 24 for processing and calculating, preferably common to all means 22 for measuring; transmission can be done by any means, but to protect the system and as the application preferably refers to a compact network 2, wired connections are preferred. The processing and calculation means 24 and / or the measurement means 22 operate, independently of the CPA 10, continuously or intermittently.

Depending on the option selected, the locator 20 can simply indicate the presence of a defect in the output, for example by comparing by means of adapted means 26 the impedances Zfj calculated at each output Bj at a threshold Dj determined as a function of the characteristics of the output Bj , traditionally between 10 ΚΩ and 1 ΜΩ, and that must be exceeded when the Bj output is good; In an alternative, the locator gives a measurement of the insulation impedance Zfj by an adapted means known from the prior art.

According to the invention, network 1 is secure; in particular, network 1 feeds critical Z loads that should not be interrupted, such as a computer center or a nuclear power plant. Thus, the insulation control system 10, 20 comprises a second insulation fault alarm circuit 4, independent of the CPA 10 and which provides redundant information so as to identify a fault at the level of the CPA 10 and avoid a delay. in dealing with a problem reported by this device.

As the injection functions 12 and detection 14 of a CPA 10 are closely overlapping, double measurement 14 of the injected current without double injection 12 is technically unrealistic. According to the invention, a second measurement is therefore carried out by means of the locator 20, which has an extensive function so that it can also carry out a measurement of the insulation resistance of the entire network 1, this being second redundant measurement independent of the one carried out by the CPA 10.

It also matters, therefore, that the locator 20, and in particular its processing and calculation means 24, are independent of the CPA 10; for this, in the embodiment according to the invention, the locating device 20 comprises means 28 for measuring the voltage downstream of the transformer 2 so that it simply calculates the impedances Zfj at each output Bj.

Furthermore, in the preferred embodiment, to minimize the risk of failure, independent measuring means of the injected current 30 are installed downstream of the transformer 2 and give a signal representative of the current flowing at the frequency f of injection, in order to redundantly evaluate the reality of the injection using the CPA 10. For example, when all the branches Bj are provided with means 22j for measuring an insulation fault locator 20 (in the absence of loads Z illustrated with a dotted line in Figure 1), the means 30 for measuring the injected current in the network 1 they comprise means for making the sum of the currents measured by the measurement means 22j associated with each branch Bj (not illustrated).

However, to overcome the accumulation of measurement inaccuracies, the measurement means comprises a core 30 around three conductors at the output of the transformer 2. This embodiment allows complete independence from redundant measurement, and increases the accuracy in the value. of the measured parameter in order to later evaluate the reality of the operation of the CPA.

This information relative to the injected current measured downstream of the transformer 2 is transmitted to calculation and processing means 24 'to determine the impedance Zf0 relative to it. In particular, the same calculation card is used for the treatment of the two types of signals, that is to say that the card comprises an input

ES 2 585 225 T3 specific for the current coming from the global measurement means 30 and an input for the measurement 22j carried out in each branch Bj; the calculation of the overall leakage impedance Zf0 is also carried out in the same way as for the other measurement means 22, in particular by means of the voltage measurement 28.

The result Zf0 of this second measurement, which corresponds to the redundant measurement of the insulation impedance by the measurement means located downstream of the transformer 2, is here also compared by means 26 'adapted to a threshold D, which is identical to that of used for the first measurement by the CPA 10: If the threshold D is not reached, that is, Zf0 <D <about 100 Ω and, therefore, the detected current is higher than a “normal” leakage current, warning means are implemented, similar to what is described above. described previously for the CPA 10. In this way, redundancy is completed, while these second means 24 ', 26', 30 for identifying the presence of a defect 4 have as the only common element with the CPA 10 the use of the injected measuring current.

In order, in addition, to overcome a problem caused by a potential fault of the current injection at the measurement frequency f by the CPA 10, the second measurement means 30 are also used to verify the presence of the injected current, it is tell the correct operation of the specific means 12. In particular, if the overall current detected by the current measurement means 30 is less than a threshold value, an alarm is triggered. The comparison can be made directly on the signal measured by the specific means (not illustrated); in an alternative, as indicated in figure 2B, the result Zf0 of the treatment and calculation of the global fault impedance is compared with a second threshold D "by means of adapted means 26", indicating the exceeding of this second threshold D " due to the impedance (Zf0> D ”> about 100 ΜΩ) that the injected current is insufficient. The alarm can be common to the result of the signal processing from the second measurement means 30, or differentiated, with two alarms that are triggered depending on whether the problem detected is insufficient current or the presence of a fault.

In fact, none of the existing devices takes into account the option that the CPA 10 does not inject current: in the existing devices, in the event of a CPA failure, it is considered that there is no defect, the calculated impedance being infinite, and therefore always higher than the first threshold D.

Thus, the network 1 is equipped with an insulation fault identification and location system 10, 20 that preferably comprises:

- a current injector 12, which operates permanently;

- a central isolation controller 10 connected to the neutral of transformer 2 and operating permanently;

- means 22, 30 for measuring the current at the frequency f injected downstream of the transformer 2, to permanently measure the global current of the network 1 and the current in each branch Bj of said network 1;

- means 28 for measuring the voltage at the injection frequency f downstream of the transformer 2;

- means 24 for processing and calculating the impedances downstream of the transformer 2, based on the current measurements and on the voltage measurement;

- means 26 '' for permanently determining the reality of the current injection by means of the downstream measuring means 30;

- means 26 'for permanently controlling the isolation of the network 1 downstream of the transformer 2;

- means 26 for locating insulation defects in the branches Bj.

The treatment and calculation means 24, 26 downstream of the transformer 2 are grouped together, and are totally independent from the upstream controller 10. The system formed by the CPA 10 and the localization means 20 modified according to the invention, makes it possible to improve the level of SIL (“Safety Integrity Leve /) as defined in the standards IEC 61508 and IEC 61557-15 increasing the tolerance of HFT defects. (“Hard Fault Tolerance) of the functions of local insulation warning LIW (“ Local Insulation Warning) and remote insulation warning RIW (“Remote Insulation Warning). In particular, the characteristics of the control system 10, 20 are defined to comply with the SIL-2 criterion (“Safety Integrity Level 2), that is, to reduce by a factor between 100 and 1,000 the risk of a hazard, and even SIL-3 (reduction by a factor between 1,000 and 10,000), economically and not very consuming of computing resources and / or treatment means, without adding material (the appearance of a hazard that corresponds here to the non-detection of a first fault in the network 1 which has the potential consequence of an electric shock or a critical loss of power supply if a second fault occurs when the first fault has not yet has been removed).

Although the invention has been described with reference to a three-phase network 1 in neutral N to which the injection 12 of the permanent insulation control system 10, 20 is connected, it is not limited to this: The proposed solution can be applied to different power supplies, for example with a frequency other than 50 Hz or single-phase, or emergency groups of the generator or inverter type or DC voltage sources, and / or the injection device 12 can inject its signal in a phase of the network. The different identification, localization and calculation methods can be used: for example, the injection means 12 can be adapted for simultaneous or consecutive injections at various frequencies and the measurement and treatment means 22, 30 also ...