DE202012101041U1 - Test system for the synthetic testing of the breaking capacity of high-voltage tap changers - Google Patents

Abstract

Circuit for a test system for synthetic switching performance testing of a device under test with a low voltage source and a medium voltage source, characterized in that a first semiconductor switch switches the load current, wherein a resistor limits the load current in the first semiconductor switch, that the medium voltage source is a transformer, wherein a second semiconductor switch with the DUT in the medium voltage circuit is unipolar connectable to this or separable from this.

Description

The invention relates to a circuit for a test system for synthetic switching performance testing of a device under test with a low-voltage source and a medium-voltage source.

In the prior art, the switching performance tests are performed by means of a resonant circuit test facility. As a result, almost no power is taken from the mains between two load switches, since the currents in the two test pieces compensate each other. During load switching, however, energies of up to several 100 kJ are recorded within a few 10 ms. As a result, the flicker limits of the 110 kV grid are almost exhausted.

However, the construction of a larger test facility based on the same principle is not possible without reducing the network feedback by means of rapid compensation.

The existing switching capacity test system is not designed for the testing of switches with a higher switching capacity than those sold by the applicant under the name VACUTAP ^® VR tap changers at full step voltage. In the event that such a switch is to be developed, a test system for higher performance is to develop, which does not stress the network intermittently.

Test circuits for synthetic switching performance tests are z. B. off EP 0 342 321 known. Due to the teaching of EP 0 342 321 it is possible to construct both voltage superposition and current superposition test circuits economically. In addition, the space required for these test circuits is much smaller or the available space can be used for testing higher breaking capacities. In the conventional synthetic test circuits, a high-performance capacitor bank is charged to high voltage in the high-voltage circuit. By means of a spark gap, this capacitor battery is switched on an RLC resonant circuit of the high voltage circuit and this at the same time on the test switch on it. The RLC resonant circuit is dimensioned so that its transient response to the charging voltage of the capacitor bank simulates the prescribed by international test regulations transient recurring voltage across the test switch. However, a prerequisite for this is an approximately ten times greater capacity of the capacitor bank than the capacitance of the capacitor of the RLC resonant circuit. As a result, only about 10% of the energy stored in the capacitor battery is actually used to test the turn-off capability.

From the DE-AS 11 80 841 a test circuit is known. In this test circuit, which operates on the known voltage superposition principle, a charged, comparatively large capacitor bank is used as an energy source, which provides the essential part of the recurrent voltage after a shutdown of the test switch. However, as is well known, the majority of the energy stored in this capacitor bank can not be used for the test. Even with test circuits that operate according to the known current superposition principle, the stored energy of the corresponding capacitor bank is mainly not used. However, the turn-off performances to be tested continue to increase, so that comparatively larger and more powerful capacitor banks are needed if these conventional synthetic test circuits continue to be used. However, the space required and also the financial outlay for such an enlargement of the capacitor banks go beyond the economically justifiable framework.

In principle, a combination of a high-current circuit and a high-voltage circuit is used to test a switching path.

The prior art test circuits all use spark gaps as switching elements for the high voltage circuit and capacitors as source for the high voltage circuit. Because of this, only a slow switching sequence is possible, since always the charging time of the capacitor has to be waited before a new circuit can take place. In addition, only the first increase in voltage of the high voltage can be emulated correctly over the capacitor and not the further course. EP 0 342 321 For this purpose, an additional sinusoidal voltage is used in the high-voltage circuit.

In the proposed arrangement is dispensed spark gaps and capacitors. Instead, semiconductor switches and sinusoidal voltage sources are used.

For this purpose, a test system is proposed, which consists of the combination of a current source for the simulation of the load current and a voltage source for the step voltage, both from a common DC buffer. The supply of this storage from the grid must be made evenly with an upward limited current.

The operation of the switching sequence of the tap-changer OILTAP ^® M, which is also distributed by the applicant, is shown below, and the current and voltage values of the two sources are indicated. The power output of the DC buffer is temporarily smaller than the sum of the individual power and voltage source powers, as it is partially fed back from the sources to the memory.

Both the ignition of the vacuum interrupter under pulse stress as well as a re-ignition may only occur with negligible probability for use in the tap changer for reasons mentioned above. The experimental proof of such small probabilities can not be made by increasing the number of test events. Instead, the test must be performed in a condition where there is a greater chance of failure due to higher stress.

The protection of the pulse voltage strength is based on the fact that the probability of a breakdown increases with increasing voltage. Since the functional relationship is relatively well known, the parameters of the test can be specified.

The likelihood of ignition of a vacuum interrupter after the arc extinguishes decreases with increasing recovery voltage and with increasing shock, for example by the mechanical drive, to and with increasing stroke. However, the exact functional relationship is not known, especially for the range of very small occurrence probabilities. It is therefore necessary to carry out the tests by varying the parameters stroke, return voltage and vibration and to determine the influence on the probability of occurrence. Subsequently, by extrapolation, statements about the values of the parameters can be derived at very low occurrence probabilities. When varying the parameters, however, care must be taken to ensure that excessively overstressing the transferability of the results may no longer be possible since other physical effects are then effective.

For a small tap-changer with 300 A load current, 1000 V step voltage and a guaranteed electrical life of 150,000 circuits, a vacuum interrupter should be qualified. Depending on the circuit used, the maximum recovery voltage of the vacuum interrupter is approx. 2000-3000 V. The stage is protected by a varistor with approx. 56 kV residual voltage, built up of the series connection of 4 ZnO elements. The required withstand voltage values of the vacuum interrupter are 20 kV _50Hz and 100 kV _{1,2 / 50μs} .

• – Die Anzahl der eingesetzten ZnO-Elemente im Ableiter bestimmt die Ableiter-Restspannung.
• – Die vorgesehene maximale Stufenspannung bestimmt die Prüfspannung für a0 Beanspruchung, zusätzlich muss die auftretende Wiederkehrspannung berücksichtigt werden. Die Ableiter-Restspannung bestimmt die Höhe der Stoßspannungsbeanspruchung (1,1 × vergleichende LI Durchschlagspannung an M-Funkenstrecke)

Anzahl Abschaltungen mit 300 A Stromstoß von ca. 2000 A auf Varistorstapel mit 4 ZnO-Elementen zur Erzeugung der Ableiterrestspannung Wechselspannungs-Prüfung mit 50 Hz (5·U_stufe bzw. 5·max. U_w) Blitzstoßspannung 1,2/50 μs auf Varistorstapel mit 4 ZnO-Elementen mit 270 kV (begrenzt durch Stoßgenerator in HS3) Ladespannung (1,1 × maximal Blitzstoßspannu ng 1,2/50 μs mit 100 kV ohne Varistor 0 (Neuzustand) 100 Stöße 20 kV/5 min 30 kV/1 min 20 pos./20 neg. 20 pos./20 neg. 10'000 100 Stöße 20 kV/1 min 20 pos./20 neg 20 pos./20 neg 30'000 100 Stöße 20 kV/1 min 20 pos./20 neg 20 pos./20 neg 50'000 100 Stöße 20 kV/1 min 20 pos./20 neg 20 pos./20 neg 100'000 100 Stöße 20 kV/1 min 20 pos./20 neg 20 pos./20 neg 150'000 100 Stöße 20 kV/5 min 30 kV/1 min 20 pos./20 neg. 20 pos./20 neg. The test program of the voltage tests includes the testing of 100 vacuum interrupters with the following requirements:

• - The number of ZnO elements used in the arrester determines the arrester residual voltage.
• - The intended maximum step voltage determines the test voltage for a0 load, additionally the occurring return voltage must be considered. The arrester residual voltage determines the magnitude of the surge voltage stress (1.1 × comparative LI breakdown voltage at M spark gap)

Number of shutdowns with 300 A Surge current of approx. 2000 A on varistor stacks with 4 ZnO elements to generate the arrester residual voltage AC voltage test with 50 Hz (5 · U _stage or 5 · max U _w ) Lightning impulse voltage 1.2 / 50 μs on varistor stack with 4 ZnO elements with 270 kV (limited by impulse generator in HS3) Charging voltage (1.1 × max Lightning impulse voltage 1,2 / 50 μs with 100 kV without varistor 0 (new condition) 100 shocks 20 kV / 5 min 30 kV / 1 min 20 pos./20 neg. 20 pos./20 neg. 10,000 100 shocks 20 kV / 1 min 20 pos./20 neg 20 pos./20 neg 30,000 100 shocks 20 kV / 1 min 20 pos./20 neg 20 pos./20 neg 50,000 100 shocks 20 kV / 1 min 20 pos./20 neg 20 pos./20 neg 100,000 100 shocks 20 kV / 1 min 20 pos./20 neg 20 pos./20 neg 150,000 100 shocks 20 kV / 5 min 30 kV / 1 min 20 pos./20 neg. 20 pos./20 neg.

The test is passed if no rollover occurs on all 100 test pieces. For the voltage test, the intended opening stroke must be adjustable (presumably> 6 mm instead of 3.5 mm for the power circuits). In addition, it may be necessary to consider a difference in vapor deposition at 3.5 and 6 mm opening stroke. (possibly increased number of gears ...)

To test the Rückzündverhaltens a test system is required, which can generate the test current and the recurrent voltage after switching off the switching element.

The classical approach uses for this purpose a test voltage source in the amount of the required recovery voltage and an impedance for adjusting the load current. To test ohmic switching conditions ohmic resistances are required as impedance. The power loss is the product of test voltage and load current.

In order to avoid the large power losses, a synthetic test system is to be used in which the load current is generated by a relatively small voltage and a correspondingly small resistance with low power loss. After the test object has been switched off, the low-voltage circuit is disconnected from the test object by a semiconductor switch and a medium-voltage circuit is connected to the test object via a second semiconductor switch.

• 1. Versuchsreihe: 12 Vakuumschaltröhren über 150'000 Schaltungen prüfen mit 12 kV Wiederkehrspannung bei 3,5 mm Hub und der 10-fachen Beschleunigung, die bei der Betätigung im Stufenschalter auftritt.
• 2. Versuchsreihe: 12 Vakuumschaltröhren über 150'000 Schaltungen prüfen mit geänderten Parametern in Abhängigkeit von den Ergebnissen der 1. Versuchsreihe.

As there is still no experience with such a synthetic test system, an alternating voltage of 12 kV should be used in a first step for the medium voltage circuit with the option to increase this later to 24 kV. Below is the description for 12 kV.

• 1st test series: Check 12 vacuum interrupters over 150,000 circuits with a 12 kV recovery voltage at a 3.5 mm stroke and 10 times the acceleration that occurs when operating in the tap changer.
• 2nd test series: 12 vacuum interrupters over 150,000 circuits check with changed parameters depending on the results of the first series of experiments.

Objective: Determination of a functional dependence of the flashback probability on the parameters and extrapolation on the nominal values of the parameters in the tap changer. An occurrence probability of 10 ^-9 appears tolerable.

It is therefore an object of the invention to make a test circuit for synthetic switching power tests of a fast switching sequence possible

This object is achieved by a circuit for a test system for synthetic switching performance testing according to protection claim 1.

In the following the invention will be explained in more detail with reference to embodiments with reference to the accompanying drawings. Show it:

1 a schematic switching sequence of the OILTAP ^® M of the prior art, the switching elements are copper-tungsten contacts that shut off with arc in transformer oil;

2 a schematic representation of an embodiment of a circuit for a test system;

3 a schematic representation of another embodiment of a circuit for a test system; and

4 an oscillogram of a test procedure.

1 shows a schematic switching sequence of the OILTAP ^® M of the prior art, the switching elements are copper-tungsten contacts that shut off with arc in transformer oil.

If, in this switching principle, the existing oil switching sections are replaced by vacuum interrupters without introducing additional series switching sections, then the vacuum interrupters apply increased requirements compared to the use in a contactor or a circuit breaker. A pulse overvoltage due to lightning in a line or switching action in the network coupled depending on the design of the transformer in any way on the step voltage of the tap changer. If surge arresters in the form of spark gaps or varistors are used above the stage, then they are limited to their residual voltage. The resulting overvoltage is in the switching steps 1 to 4 directly to the switching paths b and b1 and in the switching steps 9 and 10 to the switching paths a and a1. A continuation of switching element b or a in these switching steps leads to a step short circuit and thus to the destruction of the tap changer. Igniting b1 or a1 is harmless to the tap changer if the current limited by the resistor is removed from the vacuum interrupter at the next zero crossing. However, if the arc occurs in the vacuum interrupter along the steamed ceramic, then this leads to damage to the vacuum interrupter. A re-ignition of a after deletion of the arc shown in switching step 4 also leads to the step short circuit and thus the destruction of the tap changer, if it occurs in the switching steps 9 or 10.

2 shows an embodiment of the test circuit of a synthetic test system. As a low voltage source in the above-mentioned synthetic test system, a commercial transformer 20 kV / 400 V (50 Hz) is used. The voltage driving the load current is thus 400 V. The load current is preferably 300 A. If the semiconductor switch used in the circuit permits this, the possibility of increasing the load current to 600 A is desirable. The load current is limited by an oil-cooled wire resistor. The low voltage source is connected to the grounded side of the device under test. By a first semiconductor switch, the low-voltage circuit can be unipolar separated from the DUT.

As a medium voltage source, a transformer 400 V / 2-12 kV is used, the of o. G. Low voltage transformer is fed. In the medium-voltage circuit normally no current flows, but only with a re-ignition of the test object. In this case, the current is limited to 50 A by an oil-insulated wire resistor. The medium-voltage source is also connected on the ground side to the test object. By a second semiconductor switch, the medium-voltage circuit can be connected in unipolar with the device under test or separated from it.

For the time in which the first semiconductor switch conducts and the second blocks, the latter is approximately the voltage of the medium voltage source, d. H. approx. 12 kV alternating voltage. For the time in which the first semiconductor switch blocks and the second conducts, the voltage of the medium-voltage source, that is, the voltage of the medium-voltage source, is approximately equal to the first one. H. approx. 12 kV alternating voltage, as long as no current flows.

The entire above-described power test section can be disconnected from the test object by a first disconnect switch. Via a second disconnect switch, a pulse voltage test system can be connected to the test object. Alternatively, the DUT receiving the vacuum interrupters may be portable and alternately installed in a power test facility and a pulse voltage test facility.

The test object consists of six parallel branches (not shown), each consisting of the series connection of two vacuum interrupters, one of which is open and the other is closed. It is operated as follows: First, the vacuum interrupter closes in a branch and thereby switches on the load current. After approx. 50 ms, the other vacuum interrupter opens and switches off the load current. After a further approx. 800 ms, the above procedure is repeated in the next parallel branch. This will continue without further pause in turn for each branch. The total number of tests for a set of test items is 1,800,000.

Control:

The test specimen is driven in such a way that a vacuum interrupter closes in a parallel branch, the other one opens after approx. 50 ms and this occurs in a cycle of approx. 850 ms in succession for all parallel branches. An auxiliary switch of the drive gives about 20 ms before closing the vacuum interrupter, at time -1- in the oscillogram of the 4 , a trigger signal to the controller of the semiconductor switch. Then opens the semiconductor switch 2 in the voltage branch and the semiconductor switch 1 in the current branch closes. This normally happens without power; Current only flows if there is a reignition in the device under test. From this time, the test object has the low voltage of 400 V _AC .

At time -2- closes the first vacuum interrupter. The voltage across the test specimen becomes zero and current flows.

At time -3- opens the second vacuum interrupter. The current continues to flow until it drops to a few amps just before the zero crossing; then it is squeezed off by the vacuum interrupter. As long as current flows with the vacuum interrupter open, the arc voltage in the amount of approx. 20 V appears above the test object.

At time -4- the power is extinguished and over the test specimen occurs again low voltage in the amount of 400 V _AC .

In order to decide when the semiconductor switch 1 should open in the circuit and the semiconductor switch 2 in the voltage circuit should close, the timing -3- and -4- must be evaluated by the controller. The switching of the voltages may take place after the time -4-, but at the earliest an (adjustable) time interval after the time -3-. This ensures that she does not immediately re-ignite. The switchover of the semiconductor switch is normally de-energized.

In the above oscillogram re-ignition occurs in the test specimen at time -5-. Current flows and over the test specimen occurs the arc voltage in the amount of about 20 volts.

The current flow ends when it drops to a few amperes shortly before the zero crossing, at the time -6-.

Shortly before the closing of the next vacuum interrupter comes again a signal from the auxiliary switch of the drive. This time is also designated -1-.

If both semiconductor switches are closed at the same time by an error, then a tension of about 464 V arises with the test part opened by the voltage divider formed on the test object, and a current of about 2.5 m driven by the medium voltage source in the reverse direction through the low-voltage source. 48 A. If the DUT closes in this state, the voltage across the DUT will be zero. The low voltage source then supplies 300 A and the medium voltage source 50 A, so that the current through the DUT is 350 A. This can be detected and switched off by the controller without causing any damage.

However, the resistance in the medium-voltage circuit should be designed so that a failure can be excluded. Because in this case would occur at the low voltage transformer, a voltage which is determined by the voltage divider between the ohmic resistance in the low-voltage circuit and the impedance of the low-voltage transformer. This can damage components in the low-voltage circuit.

Measurement:

The voltage across the test object is measured with a voltage transformer.

The arc voltage is measured via an ohmic divider and cut off in order to be able to resolve the range up to approx. 50 V more accurately.

The current is measured with a Hall transducer.

In order to be able to decide which parallel branch of the test object is currently being actuated, the angle of rotation of the test object is detected with a resistive angle sensor.

In order to be able to assign a re-ignition during the power test or a breakdown during the voltage test to a parallel branch, very low-burdened current transformers are used as current sensors in each parallel branch. The signals of the current sensors can each be reduced via a Schmitt trigger to a digital signal and then measured together in a channel.

3 shows a further embodiment of a circuit for a test system for synthetic switching performance testing of a DUT. The medium voltage source in this embodiment is a sinusoidal voltage source instead of the transformer (400 V / 2-12 kV). In the medium-voltage circuit, normally no current flows, but only with a re-ignition of the test object. In this case, the current is limited to 50 A by an oil-insulated wire resistor. The medium voltage source is also earth side with the DUT connected. By a second semiconductor switch, the medium-voltage circuit can be connected in unipolar with the device under test or separated from it.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0342321 [0005, 0008]
• DE 1180841 [0006]

Claims (5)

Circuit for a test system for synthetic switching performance testing of a device under test with a low voltage source and a medium voltage source, characterized in that a first semiconductor switch switches the load current, wherein a resistor limits the load current in the first semiconductor switch, that the medium voltage source is a transformer, wherein a second semiconductor switch with the DUT in the medium voltage circuit is unipolar connectable to this or separable from this. The circuit of claim 1 wherein the low voltage source is a sinusoidal voltage source. A circuit according to claim 1, wherein the low voltage source is a commercially available transformer. The circuit of claims 1 to 3, wherein the resistor that limits the load current in the first semiconductor switch is an oil-cooled wire resistor. A circuit according to claims 1 to 3, wherein the resistance in the medium voltage circuit is oil insulated wire resistor.

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