Patent ID: 12206238

DETAILED DESCRIPTION OF THE INVENTION

FIG.1shows an exemplary embodiment of the invention with a simplified null equivalent circuit diagram of a three-phase network in the presence of a ground fault present on one phase,FIG.2shows a corresponding equivalent circuit diagram of a three-phase system with an output circuit.

Of a three-phase transformer T with the phases in a star connection, only the secondary side is shown in the figures.

The ground fault can be described by a driving fault voltage Efand a fault resistance Rf.

Furthermore, a network asymmetry is illustrated, which is formed mainly by different conductor—earth capacitances of the three phases. In the null equivalent circuit diagram, the unbalance current Iurepresenting this unbalance is represented via the driving unbalance voltage Euand the unbalance impedance Zu.

The driving fault voltage Efgenerates a fault current Ifthrough a fault resistance, which can be formed by the ground fault.

To compensate for the capacitive fault current Ifdue to the ground fault, the neutral point of the network is earthed by means of an arc suppression coil LASC(Petersen coil).

The network impedance Z0shown in the equivalent circuit diagram is formed by the parallel connection of an ohmic equivalent resistance R0for the entire network losses, including the losses of the arc suppression coil LASC, the network capacitance C (line capacitance) and the inductance of the arc suppression coil.

The equivalent resistance R0determines a damping current ID.

The impedance of the arc suppression coil LASC and thus the level of an inductive compensation current can be changed directly by adjusting the air gap in the iron core of the arc suppression coil, or the secondary side of the arc suppression coil LASC is wired to an inductance, a capacitor or a defined current supply, and/or the neutral point or one of the three phases can be wired with an ohmic resistor, an inductor, a capacitor or a defined current supply. Such measures and/or devices are known to the person skilled in the art, so they do not need to be explained or described in more detail.

The disadvantage of a “static” compensation in the prior art can be seen here.

The coil position can be determined by measuring the position using a potentiometer. This results in a coil current Ipos.

The tuning of the arc suppression coil LASCtakes place during the normal operation of the network (fault-free network condition), wherein the arc suppression coil LASCis adjusted such that the inductive current through the arc suppression coil LASCis the same as the capacitive current through the line capacitance (resonance current Ires).

In practice, however, for an arc-suppressed network without active residual current compensation in normal operation, a slight overcompensation or undercompensation can be set. This can be achieved by setting a current Ipos, which depends on the position of the plunger of the arc suppression coil.

With active residual current compensation, it may be appropriate to tune exactly to a resonance (Ipos=Ires).

With an exact tuning, the network impedance Z0=R0, which means that a maximum of the impedance of the parallel resonant circuit (LASC, R0, C) is present, and the current through the fault location is minimal without residual current compensation.

Even with exact tuning, a complete compensation of the fault current is not possible with the arc suppression coil alone, since ohmic losses cannot be compensated by the arrangement.

These losses can occur by means of active residual current compensation, i.e. active current infeed with a compensation current Icomp(t).

The compensation current Icomp(t) is composed of a static current IStat(t) and a boost current IBoost(t):
IComp(t)=IStart(t)+IBoost(t)

The static current IStat(t) is determined during the tuning process in normal operation. All parameters of the simplified null equivalent circuit are determined.

In this way, in the event of a ground fault in phase L1, L2or L3, the infeed current can be determined for complete compensation of the fault current.

A complex displacement voltage U0occurs across the arc suppression impedance Z0and a displacement current I0flows through the network impedance Z0.

The unbalance current Iu, the fault current If and the compensation or displacement current I0flow together at the neutral point of the three-phase network.

In addition, the boost current IBoost(t) is fed in at the star point by a converter circuit, as inFIG.6, for example, which can be determined by the following relationship:

IBoost(t)=ULXE(t)RBoost(t)=ULxe(t)·YBoost(t)

For example, the boost conductance YBoostcan be set such that it limits a maximum current that the converter circuit used can still support.

The boost current IBoost(t) can be a current that is proportional to the faulty phase—ground voltage and therefore also decreases with decreasing voltage ULxe(t), with x=1, 2, 3.

Alternatively, the compensation current could be temporarily doubled, for example.

The boost conductance YBoostcan be constant for a selected time range, such as 20 ms, 40 ms or 120 ms. In general, however, this is not a requirement and the boost conductance YBoost(t) can therefore also be time-variant.

The compensation according to the invention takes place in three steps:1. Basic compensation in the arc-suppressed network: capacitive conductor currents are compensated in the arc-suppressed network by an inductive current using a Petersen coil.The network is tuned in a fault-free state, wherein the inductive current corresponds to the capacitive zero current of the network at the optimal setting of the Petersen coil.In the case of a single-pole ground fault, with this measure only an active current would now flow through the fault location, which in many cases is sufficient to extinguish a ground fault, for example due to an arc.In the event of overcompensation (Ipos>Ires) or undercompensation (Ipos<Ires), an inductive or capacitive reactive current would occur in the fault current Ifin addition to the active component.

The active component of the fault current Ifcannot be compensated by the “passive” Petersen coil, but by means of an “active” residual current compensation.2. Feeding in the static current IStat(t): the active residual current compensation feeds this current into the neutral point of the three-phase transformer, for example via a power auxiliary winding of the Petersen coil.The static current Istat(t) counteracts the fault current, which reduces the active component in the fault current and, in the case of over- or undercompensation, also a reactive component, or in the optimum case even completely compensates for it.The infeed of the static current IStat(t) can completely discharge the fault location.The three-phase system reacts to this compensation in such a way that the displacement voltage U0increases and the phase-ground voltage (UL1efor a ground fault in phase L1, UL2efor a ground fault in phase L2, UL3efora ground fault in phase L3) of the phase affected by the ground fault decreases.For ground faults that are located very close to the bus bar, i.e. close to the transformer and the longitudinal impedance from the bus bar to the fault location is negligible, the voltage UL1e(or UL2e, UL3e) corresponds to the voltage at the fault location.If the voltage at the fault location, i.e. the voltage of the phase with respect to ground in the case of a unipolar ground fault, is close to 0 V in the optimum case, the fault current is also reduced to 0 A.However, the three-phase system cannot react abruptly to the static current IStat(t), as a recharging of the conductor-earth capacitances must take place.This recharging depends on the network time constant of the network in the fault-free state, and on the ground fault resistance.The recharging is very fast for very low-resistance ground faults, and correspondingly slower for ground faults with a higher resistance.3. Feeding in the boost current IBoost(t): the active residual current compensation feeds this current into the neutral point of the three-phase transformer.In order to accelerate the recharging process, the boost current IBoost(t) is fed in at the neutral point in addition to the static current IStat(t).This results in a faster reduction of the voltage UL1e(or UL2e, UL3e) and thus a correspondingly faster reduction of the voltage at the fault location.

The three-phase transformer T converts the voltages of a generator or source UL1, UL2, UL3to the network voltages UL1e, UL2eand UL3e, which are distributed via a bus bar BB for the phases L1-L3.

To model a real transformer, the internal impedances of the transformer are taken into account in the form of line resistances RT1-RT3and inductances LT1-LT3.

For each of the network conductors of the individual phases L1-L3, an equivalent circuit diagram is shown in the form of Π-elements Π1-Π3for line impedances with line inductances, line capacitances and line resistances, as well as a terminating load network RF1.

In addition, an unbalance impedance Zuis intended to represent the network unbalance, as shown in the drawing by way of example on phase L3.

A fault current Ifflows through a fault resistance Rf.

In the fault-free network, all parameters of the simplified null equivalent circuit diagram are determined by tuning the arc suppression coil. This allows the static infeed current IStat(t) to be determined for complete compensation of the fault current in the event of a ground fault in phase L1, L2or L3.

A measurement and control device1is provided and designed to detect the network voltage UL1e, UL2e, UL3eand on that basis to identify a line fault Ifoccurring, for example by determining a fault resistance Rf, and to determine a control variable to compensate for the line fault.

A converter circuit2is also provided, which is designed to generate a control current ICompfrom the control variable and to feed in the control current at the neutral point of the transformer T to compensate for the line fault If.

The measurement and control device1is also designed to determine a boost conductance YBoostfrom which to determine a boost function, to generate a corresponding boost current IBoostand to feed in the boost current at the neutral point of the transformer T.

The faulty phase and the corresponding fault resistance Rf can be determined by the position of the neutral point in the three-phase medium- or high-voltage network, as is known to the person skilled in the art.

The measurement and control device1is designed to detect the network voltages UL1e, UL2e, UL3ewith a sampling rate of, for example, 10,000 samples per second or even greater.

The converter circuit2is designed to generate the control variable with a sampling rate of, for example, 10,000 samples per second or even greater.

The control current ICompor the boost current IBoostis fed in at the neutral point of the power supply network.

It is possible to limit the boost current IBoostby the size of the maximum current intensity that results from the performance of the electronic components used in the converter circuit.

The converter circuit2corresponds to a frequency converter, i.e. a converter that generates an alternating voltage or an alternating current that is variable in frequency and amplitude for the compensation, and which is fed into the neutral point of the three-phase network, as explained previously. Such converters with high temporal resolution, such as those with more than 10,000 samples per second, are known to the person skilled in the art.

The measurement and control device is used to detect a fault current on the phases L1-L3and calculates the amplitude and phase of the required compensation current.

Such microprocessor-based measurement and control devices with high temporal resolution (10 k samples per second) are known to the person skilled in the art.

Together with the converter circuit, the measurement and control device connects the network voltage side with the network voltages UL1e, UL2e, UL3eof the transformer T to the source voltage side with the source voltages UL1, UL2, UL3by applying the corresponding compensation current, that is, the control current ICompor boost current IBoost.

A method for compensating a line fault If occurring on a three-phase power supply network can be specified accordingly.

In the method, a network voltage UL1e, UL2e, UL3eis detected, a line fault Ifthat occurs on this basis is detected, e.g. by determining a fault resistance Rf, a control variable is determined to compensate the line fault, and from the control variable a control current Icompis generated and fed in at the neutral point of the transformer T of the power supply network to compensate for the line fault If.

The boost conductance YBoostis determined, from which a boost function is determined, a corresponding boost current IBoostis generated and fed in at the neutral point of the transformer T.

The boost function has a maximum of 20%, preferably 50%, particularly preferably 100% or 200% of the magnitude of the maximum of the RMS value of the control variable.

The maximum of the RMS value of the boost function occurs within a maximum of 25 ms, preferably a maximum of 20 ms, and the boost function decays to a maximum of 10% of the magnitude of the maximum of the RMS value of the control variable, that is, the static current IStat(t), within a maximum of 120 ms, preferably a maximum of 35 ms.

FIG.3toFIG.5show representations of currents and voltages without using the boost function.

The index “eff” is used to designate the RMS value of each variable.

FIG.4andFIG.5are enlarged representations ofFIG.3for improved clarity.

The network voltage UL1ewith a peak value of 18 kV and an RMS value of approximately 12,700 V can be identified.

At the time of 0 ms, the power supply network experiences a fault current Ifand the network voltage UL1ebreaks down. The figure also shows the RMS value of the fault current Ifwith a 100-fold scaling.

At the time of approximately 40 ms, the infeed current IStat(t) is calculated and applied to determine an arc suppression coil LASCin accordance with the conventional method, as is apparent from the RMS value of the infeed current Ieciwith a 100-fold scaling.

It is apparent that it takes approximately 113 ms for the voltage to drop below 1,900 V, as required by network operators.

However, the requirement is that this voltage target should be reached within 85 ms. Therefore, the compensation shown is not fast enough. Network operators define, for example, a low impedance fault (LIF) of e.g. 400 Ohms, at which the remaining phase—ground voltage of the faulty phase in a 22 kV network must have decayed below 1900V within 85 ms.

The actual network time constant is higher,the lower the losses in the network, i.e. the higher the resistance R0, andthe higher the fault resistance Rfin a unipolar ground fault, andthe greater the capacitive current of the network.

By supplying the “static” compensation current IStat(t), the fault current Ifcan be reduced to 0 A.

In this example, it is assumed that the compensation current is only fed in after 40 ms.

FIG.6toFIG.8show representations of currents and voltages using the boost function.

FIG.7andFIG.8are enlarged representations ofFIG.6for improved clarity.

By temporarily increasing the infeed current, i.e. by means of the boost function, the voltage UL1e, and thus the voltage at the fault location, is reduced more quickly.

It is apparent that the infeed current IComp(t) is greater than the pure static current IStat(t), and then subsequently decreases, referenced to the RMS value in each case.

In other words, the boost current IBoost(t) drops to zero because the voltage UL1ealso reduces to zero, and the infeed current IStat(t) remains. The description is also the same as for the previous figure.

However, the waveform of the infeed current IStat(t) is increased by the boost function IBoost(t), so that a faster compensation of the fault is achieved.

It can therefore be seen that after only 73 ms the voltage UL1has dropped so quickly below the voltage of 1900 V that the above-mentioned requirements can be met by network operators.

FIG.9shows a block diagram of a circuit for generating the compensation or infeed current IStat(t) and/or IBoost(t), based onFIG.2with the network voltages UL1e, UL2e, UL3e, wherein in this example the phase L1has a low-resistance ground fault.

With such an arrangement, the compensation or infeed current IStat(t) and/or IBoost(t) can also be generated according toFIG.1.

LIST OF REFERENCE SIGNS

1measurement and control device2converterBB bus barC grid capacityEfdriving fault voltageEUdriving unbalance voltageIASCcurrent through the arc suppression coil impedanceIddamping currentIcompinfeed currentIfcurrent through the fault locationI01zero current for output circuit1Iposcoil positionIresresonance point of the networkIuunbalance currentL1, L2, L3phaseLASCinductivity of the arc suppression coil (ASC)LT1, LT2, LT3,RT1, RT2, RT3internal impedance of the transformerΠ1, Π2, Π3Π-elements as phase-internal impedance of the transformerR0equivalent resistanceRASCequivalent resistance of the arc suppression coilRffault resistance, ground fault resistanceT transformerU0displacement voltage, zero voltageUL1, UL2, UL3generator voltage of phase to groundUL1e, UL2e, UL3enetwork voltage of phase to groundZuimpedance unbalance