METHOD AND DEVICE FOR MONITORING A COMPENSATED THREE-VOLTAGE NETWORK FOR A CHANGE IN THE TUNING OF THE EARTH FAILURE EXTINGUISHING COIL

Description

[0001] The invention relates to a method and a device for monitoring a compensated three-phase network for a tuning change of the earth fault suppression coil.

[0002] In compensated networks, an earth fault suppression coil (Petersen coil) is used to compensate capacitive earth fault currents from outgoing circuits with an inductive current.

[0003] If the inductive current through the earth fault quenching coil exactly corresponds to the capacitive zero currents of the outgoing circuits (resonance tuning), the remaining mains frequency fault current is minimal and is determined by the active losses in the network.

[0004] In practice, a slight under- or overcompensation is often set, but it must be ensured that, given the earthing conditions and the remaining fault current at the fault location, the occurring step voltage does not exceed the permissible limits and the permissible duration.

[0005] It is therefore very important to automatically and reliably detect changes in the network configuration or changes in the detuning so that a new determination of the network parameters is initiated and a new tuning point for the earth fault suppression coil can be determined and the earth fault suppression coil can be tuned accordingly in a tuning process.

[0006] Known methods exploit the fact that in real three-phase networks there is always a certain asymmetry of the individual phases, so that a certain displacement voltage occurs at the star point, i.e. also at the earth fault suppression coil. However, even in very symmetrical networks, a sufficiently high displacement voltage can be achieved at the tuning point of the earth fault suppression coil by connecting an artificial asymmetry in at least one phase or by feeding a mains frequency current into the star point of the transformer. A change in the network detuning can then be detected by means of a significant change in the star point displacement voltage.

[0007] New tuning processes require a new determination of network parameters, which is usually complex and the number of these should be kept as small as possible.

[0008] Network parameters for a three-phase network can be determined by determining the network parameters using mains frequency current injection, or using multi-frequency methods or non-mains frequency current injection, or by adjusting the coil (as with plunger coils) or changing the degree of detuning (with stepped coils).

[0009] It is known that rapid changes in the residual voltages can occur even without changes in the grid configuration, for example due to strong load current fluctuations, capacitive coupling or even short-term fluctuations in the grid frequency.

[0010] In particular, when determining the network parameters by adjusting the coil or changing the degree of detuning (step coils), too frequent tuning processes should be avoided, as this can have a negative impact on the service life of the earth fault suppression coil.

[0011] Previously known methods secure the residual voltage as a reference value immediately after tuning the earth-fault suppression coil. The residual voltage is then monitored for changes, and additional filters may be applied. One such method is described, for example, in DE 36 43 391 A1.

[0012] If the tripping circuit (vector measurement of the residual voltage) is left due to a change in the residual voltage, a tripping delay is started. Fig. 2 A tripping circuit N2 for a "vector trip" is identified. If the residual voltage returns to within the tripping circuit, the tripping delay is reset. After the tripping delay has elapsed, a new tuning process can be performed.

[0013] Excessively frequent triggering of tuning processes due to changes in the residual voltage due to network frequency fluctuations can only be avoided in currently known systems by correspondingly long triggering delays or large permissible changes in the residual voltage. This may increase the time period during which the compensation level of the earth fault quenching coil does not correspond to the desired setting or smaller changes in the detuning level of the network can no longer be reliably detected.

[0014] When the earth fault suppression coil is used in combination with the active residual current compensation, a compensation current is fed in in the event of an earth fault, which counteracts the fault current.

[0015] A fault active current can be caused by grid losses and a fault reactive current, whereby the fault reactive current is caused by a grid detuning at the current grid frequency.

[0016] To ensure complete compensation of the fault current in the event of an earth fault, rapid response to changes in the network's detuning, as well as to changes in attenuation or asymmetry, is necessary. The network parameters must be recalculated, and the earth-fault suppression coil must be retuned relative to the nominal frequency, at least in the event of significant changes in detuning, because the converters used for residual current compensation only have a limited power and cannot compensate for arbitrarily large reactive currents. The majority of the reactive current component in the fault current is compensated by the earth-fault suppression coil, even during residual current compensation. On the other hand, frequent tuning triggering is undesirable, because sensitivity must be reduced during the calculation of the network parameters, and the response of the residual current compensation may be delayed.

[0017] It is therefore an object of the invention to detect changes in the network configuration and thus changes in the network detuning and also in the case of network frequency changes in a robust and reliable manner by means of a short triggering delay and small triggering thresholds.

[0018] The object is achieved by a method and a device of the type mentioned at the outset in that reference network parameters and a reference network frequency are determined for a tuned state, and a current network frequency is determined for a current state, and a reference parameter which is proportional to a displacement voltage is determined using the reference network parameters for the current network frequency, and a current parameter which is proportional to a displacement voltage is determined at the current network frequency, and a difference value is determined from the reference parameter and the current parameter, from which difference value a tuning change is detected when a predetermined limit value is exceeded and changed network parameters are determined.

[0019] The invention makes it possible to apply a triggering criterion with a short triggering delay to carry out a new compensation, which is very robust against false triggering due to frequency fluctuations.

[0020] In a further development of the invention, it is provided that the reference characteristic and the current characteristic are each a displacement voltage.

[0021] This makes it particularly easy to implement the method according to the invention.

[0022] In a further development of the invention, it is provided that the amount of a vectorial difference is used for the difference size.

[0023] This makes it particularly easy to implement the method according to the invention.

[0024] In a further development of the invention, it is provided that the determination of the changed network parameters takes place with a time delay if the difference value continuously exceeds the predetermined limit value over a predetermined period of time.

[0025] As a result, the method according to the invention can be carried out in a particularly robust and fault-independent manner.

[0026] In a further development of the invention, it is provided that the network parameters comprise at least one inductance of the earth fault quenching coil and at least one line capacitance.

[0027] This makes it particularly easy to implement the method according to the invention.

[0028] The device according to the invention for monitoring a compensated three-phase network for a tuning change of the earth fault quenching coil comprises a measuring device for detecting a displacement voltage and a network frequency, a computing device for determining network parameters, wherein the device is designed to carry out the method according to the invention.

[0029] The invention is explained in more detail below using an exemplary embodiment shown in the accompanying drawings. In the drawings: Fig. 1 a simplified zero equivalent circuit diagram of a three-phase system, Fig. 2 examples of locus curves of displacement voltages and tripping thresholds when monitoring a star point voltage, Fig. 3-4 examples of the frequency curve of a three-phase network over time, Fig. 5-7 examples of locus curves of a three-phase network.

[0030] Fig. 1 shows an example of a simplified zero equivalent circuit of a three-phase network.

[0031] A network asymmetry is shown, which is mainly caused by different conductor-to-earth capacitances of the three phases. In the zero-equivalent circuit, the asymmetry current representing this asymmetry is I u via the driving unbalance voltage E u and the asymmetry impedance Z u shown.

[0032] To compensate for the capacitive fault current I f Due to the earth fault, the star point of the network is protected by means of an earth fault suppression coil L ASC (English "arc suppression coil") (Petersen coil) grounded.

[0033] The network impedance shown in the equivalent circuit diagram Z 0 is achieved by connecting an ohmic equivalent resistor in parallel R 0 for the total network losses, including the losses of the earth fault suppression coil, the network capacity C(line capacitance) and inductance L ASC the earth fault quenching coil.

[0034] The equivalent resistance R 0 determines a damping current I D .

[0035] The impedance of the earth fault quenching coil L ASC and thus the level of an inductive compensation current can be changed directly by adjusting the air gap in the iron circuit of the earth fault quenching coil, or the secondary side of the earth fault quenching coil L ASC is connected to an inductor, a capacitor, or a defined current input, and / or the star point or one of the three phases can be connected to an ohmic resistor, an inductor, a capacitor, or a defined current input. Such measures or devices are known to those skilled in the art, so they need not be explained or illustrated in detail.

[0036] The coil position can be determined by a position measurement using a potentiometer. This results in a coil current I pos .

[0037] The tuning of the earth fault quenching coil L ASC occurs during normal operation of the network (fault-free network state), whereby the earth fault quenching coil L ASC is set so that the inductive current through the earth fault quenching coil L ASC the capacitive current across the network capacitance (resonance current I res ) corresponds.

[0038] In practice, a slight overcompensation or undercompensation can be set for a resonant network, but without active residual current compensation in normal operation. This can be achieved by setting a current I pos which depends on the position of the plunger earth fault quenching coil L ASC depends.

[0039] With active residual current compensation, it can be useful to precisely target a resonance ( I pos = I res ) to vote.

[0040] With exact tuning, the network impedance Z 0 = R 0 , which means that there is a maximum of the impedance of the parallel resonant circuit ( L ASC , R o , C) occurs, and the current across the fault location becomes minimal without residual current compensation.

[0041] Even with precise tuning, complete compensation of the fault current with the quenching coil alone is not possible, since ohmic losses cannot be compensated by the arrangement.

[0042] These losses can be compensated by active residual current compensation, i.e. active current injection with a compensation current I eci ( t ), take place.

[0043] At the earth fault extinguishing impedance Z 0 a complex displacement stress occurs U 0 to and through the network impedance Z 0 a displacement current flows I 0 .

[0044] The asymmetry current I u , the current through network impedance Z 0 and the compensation or displacement current I eci flow together at the star point of the three-phase network.

[0045] Fig. 2 shows examples of a locus N3 of the displacement stress U 0 and tripping thresholds N1, N2 when monitoring a star point voltage.

[0046] Threshold N1 is a trigger criterion based on changes in the absolute value of the residual voltage U 0 and N2 indicates a tripping criterion based on the vectorial change of the zero sequence voltage U 0 for a recompensation of the network according to the state of the art.

[0047] Fig. 3 shows a first example of the frequency response of a three-phase network over time.

[0048] It shows a snapshot of the generally very stable European UCTE network, where the frequency change is about 30 mHz within five minutes.

[0049] Fig. 4 shows a second example of the frequency response of a three-phase network over time.

[0050] It shows a snapshot of an Australian network in which the frequency change is approximately 200 mHz within five minutes.

[0051] Compared to Fig. 3 It is evident that the Australian network fluctuates significantly more in frequency.

[0052] Fig. 5 shows a first example of a locus curve of a three-phase network in a 200.0A network with a circle drawn in as a tripping criterion.

[0053] With a damping current I D of 5 A, a frequency change of approximately 130 mHz can cause a vectorial voltage change of more than 20% and thus, for example, the tripping criterion in the form of the corresponding circle radius in the figure is met.

[0054] Calculation of network admittance Y o depending on the frequency: Y o = 1 Z o R o = V nom I d Y o f = 1 R o ︸ Verluste + j ⋅ w f ⋅ C − 1 w f L ASC ︸ Verstimmung U nom . Y o f = U nom . 1 R o + j ⋅ U nom . w f ⋅ C − U nom w f L ASC U nom . Y o f = I d + j ⋅ U nom . w f w f 50 . w f 50 ⋅ C − U nom w f w f 50 . w f 50 . L ASC k f = w f w f 50 Y o f = 1 U nom . I d + j ⋅ k f . I res f 50 − 1 k f . I pos f 50 ︸ I v f

[0055] Where a nominal voltage U nom is the phase voltage.

[0056] An angular frequency w ( f ) refers to the current mains frequency, which may show a deviation from the nominal mains frequency of 50 Hz, while an angular frequency w ( f 50) refers to the nominal mains frequency of 50 Hz. w f = 2 ⋅ π ⋅ f

[0057] A weight factor k f sets the current frequency f in relation to the nominal frequency f50.

[0058] The relevant frequency change corresponds to the difference between the current frequency and the frequency during the tuning process.

[0059] In the above contexts, the mains frequency during a tuning process was assumed to be 50 Hz, which is indicated by the index " f 50" should be expressed.

[0060] Considerations are also valid for networks with other nominal frequencies, such as 60 Hz.

[0061] After the network parameters are calculated for each tuning process, including the unbalance impedance Z u , are the parameters of the parallel resonant circuit Z o , formed from the inductance of the earth fault quenching coil L ASC , the line capacities C and the total network losses R o more or less precisely known.

[0062] Additional external inductances, such as fixed coils or decentralized earth fault suppression coils, can be detected using the well-known multi-frequency method.

[0063] Furthermore, a parametric consideration of additional inductances is possible, such as a fixed coil with a value at a nominal frequency and a switching state.

[0064] When calculating the network parameters via a coil adjustment or 50Hz current feed, only the detuning I v be determined, the calculation back to the network size at nominal or resonance frequency I res is done via the measured coil position I pos with I res = I pos - I v .

[0065] The grid frequency is measured repeatedly, for example, using a reference voltage measurement between two phases. The grid parameters are related to the measured frequency.

[0066] After the tuning process, the zero sequence voltage U 0 ,ref secured, and used as a reference value for the vectorial difference formation at the current frequency.

[0067] This measured value can be recorded with or without additional filters, or parametrically based on the determined network parameters.

[0068] The reference displacement voltage U 0,ref can now also be related to the currently measured grid frequency via the grid parameters in order to achieve frequency-dependent triggering and compensation.

[0069] Due to the described relationships, the method according to the invention for monitoring a compensated three-phase network for a tuning change of the earth fault quenching coil can be summarized by the following method steps: For a tuned state, reference grid parameters and a reference grid frequency are determined. For a current state, a current grid frequency is determined. A reference displacement voltage is determined using the reference grid parameters for the current grid frequency. A current displacement voltage is determined at the current grid frequency. A difference value is determined from the reference displacement voltage and the current displacement voltage using the amount of a vectorial difference. If a predetermined limit is exceeded, a tuning change is detected and changed grid parameters are determined.

[0070] The previous steps can be carried out repeatedly, whereby the determination of the changed network parameters is carried out with a time delay if the difference value continuously exceeds the predetermined limit value over a predetermined period of time.

[0071] The calculation of the reference parameter at the current frequency f can be calculated, for example, from the network parameters of the simplified zero equivalent circuit according to the Fig. 1 take place.

[0072] The network impedance Z o ( f ) and the asymmetry impedance Z u ( f ) may be frequency dependent. Z o f = 1 Y o f U o f REFERENZ = E u Z o f + Z u f + I eci . Z u f Z o f + Z u f . Z o f

[0073] The determined reference parameter can thus be used to calculate the difference with the measured displacement voltage.

[0074] The difference value can optionally be additionally filtered, for example by averaging or linear or non-linear filters.

[0075] Fig. 6 shows a second example of locus curves of a three-phase network in a 100.0 A, 200.0 A and 300.0 A network with a circle drawn in as a tripping criterion for the latter network.

[0076] With a damping current I D of 5 A, a frequency change of approximately 90 mHz can cause a vectorial voltage change of more than 20% and thus, for example, the tripping criterion in the form of the corresponding circle radius in the figure is met.

[0077] Fig. 7 shows a third example of a locus curve of a three-phase network in a 200.0 A network.

[0078] Two circles are visible, one circle with the center at 50.0 Hz and one circle with the center at 50.2 Hz, which allow a frequency-dependent adjustment of the trigger circuit.

[0079] In other words, the trigger criterion for a new determination of the network parameters is defined as frequency-dependent.

[0080] As long as the tripping criterion is not exceeded by the current displacement voltage, the inductance of the earth quenching coil L ASC kept constant. Bezugszeichenliste:

[0081] C Network capacity E u driving unbalance voltage I d Damping current I eci Compensation current (ECI, “enhanced current injection”) I pos Current with respect to coil core position I res Resonance current I u Unbalance current I v Detuning current (Iv=Ipos-Ires) L ASC Inductance of the arc suppression coil (ASC) N1, N2 Limit value N3 Locus of the residual voltage U 0 NWNetz, English "network" R 0 equivalent resistance U 0 , U 0, res zero-sequence voltage Z 0 Impedance earth quenching coil Z u Impedance asymmetry

Claims

1. A method for monitoring a three-phase power network operated in a compensated manner for a tuning change of the arc suppression coil, wherein reference network parameters and a reference network frequency are ascertained for a tuned state, and a current network frequency for a current state is ascertained, and a reference characteristic variable is determined, which is proportional to a displacement voltage, using the reference network parameters for the current network frequency, and a current characteristic variable is determined, which is proportional to a displacement voltage, at the current network frequency, and a differential variable is determined from the reference characteristic variable and the current characteristic variable, from which a tuning change is identified when a predetermined threshold value is exceeded, and modified network parameters are ascertained.

2. The method according to the preceding claim, wherein each of the reference characteristic variable and the current characteristic variable is a displacement voltage.

3. The method according to any one of the preceding claims, wherein the magnitude of a vectorial difference is used for the differential variable.

4. The method according to any one of the preceding claims, wherein the determination of the modified network parameters is time-delayed if the differential variable continuously exceeds the predetermined threshold value over a predetermined period of time.

5. The method according to any one of the preceding claims, wherein the network parameters comprise at least one inductance of the arc suppression coil and at least one line capacitance.

6. A device for monitoring a three-phase network operated in a compensated manner for a tuning change of an arc suppression coil, comprising a measuring device for ascertaining a displacement voltage and a network frequency, a computing device for determining network parameters, wherein the device is configured to carry out the method according to one of the preceding claims.