Determining impulse generator settings for various test cases with the help of a www-based simulation program

Wolfgang Schufft1*, Wolfgang Hauschild2, Ralf Pietsch2

1Chemnitz University of Technology, Faculty ET/IT, D-09107 Chemnitz; Germany 2 HIGHVOLT Prüftechnik Dresden GmbH, Marie-Curie-Str. 10, D-01139 Dresden, Germany

*E-mail : wolfgang.schufft@etit.tu-chemnitz.de Abstract：The determination of the impulse generator settings is desirable to support the test operation in a test laboratory as well as for teaching purposes. Especially the question whether the standard LI or SI parameters can be reached in a certain test case or not has to be answered in advance. As well the time for changing the resistors in a multi-stage impulse can be shortened essentially by knowing the optimum circuitry of the impulse generator. Commercially available network simulation software is not adapted to the special problem of the determination of the impulse generator settings. Thus a software tool has been developed now, that combines the adaptation of a network simulation program to the problem of determining the impulse generator settings, using www-technology to enable world-wide access to this solution.

INTRODUCTION

A technical solution for determining impulse generator settings has been described before [1]. The one-stage equivalent impulse circuit has been transferred to an virtual user surface on a computer screen.

Settings for each element of the impulse circuit can be made by potentiometers or by corresponding windows. After the input has been finished the data set is sent via www to a server and the test voltage shape is calculated and immediately displayed on the computer screen. The peak value and the time parameters of the calculated impulse are evaluated according to IEC 60060-1 [2]. There are two versions of the so-called “Virtual Impulse Generator - VIG”. The VIG 2002, see fig. 1, is meant for teaching purposes mainly. It contains the main elements of a one-stage equivalent impulse circuit. The overall circuit inductances is concentrated in one element, i.e. in series with the front resistor. Stray capacitances are neglected in this simplified circuit. A chopping circuit enables the calculation and display of chopped impulses, as typically required for transformer testing. The access to the VIG 2002 is possible without any pre-condition, password, etc.:

http://vig-simulator.etit.tu-chemnitz.de/VIG2002/

the VIG 2002 is sufficient for the determining of the impulse generator settings for circuits with relatively small dimensions, i.e. a few metres, which correspond to test voltages of some 100 kV

Fig.1: Virtual Impulse Generator VIG 2002

Fig. 2: User surface of the Virtual Impulse Generator VIG 2003 (input values exemplary, not realistic) C1 - Impulse capacitor; R1 - Front resistor; L1 - Inductance of the front resistor; R2 - Tail resistor; L2 - Inductance of the Glaninger-coil (part of the Glaninger unit); C2 - Capacitance of the HV-electrode; L3 - Impulse circuit inductance; R3, L4, C3 - Series-overshoot-compensation; L5, R4, C4 - Parallel-overshoot-compensation / Damped capacitive voltage divider with an inductance; R5, C5 - Separate damped capacitive voltage divider / Load capacitance; C6 - Capacitive load / Support capacitor / Capacitance of the HV electrode of the separate damped capacitive voltage divider; R6 - Ohmic load / Resistance of the Glaninger-unit; L6, C7 - Transformer; R7 - Termination resistor; C8 - Earth capacitance of the transformer; R8 - Connection resistor; R9, C9 - Separate chopping gap; C10 - Capacitance of the HV electrode of the separate chopping gap Impulse generator circuits in the MV-range have dimensions in the order of some 10 m. Thus a circuitry for extended impulse circuits, which considers diverse stray inductances and capacitances as well as accessories for transformer testing and testing of high-capacitance test objects, must be applied. This circuitry is realised as the VIG 2003, circuitry see user surface fig. 2. The VIG 2003 is accessible by:

http://vig-simulator.etit.tu-chemnitz.de/VIG2003/

With the help of the save-button, see fig. 2, it is possible to download the calculated curve on the computer hard drive. It can be used for import into EXCEL or even for the input of an digital-analogue-converter to generate an adequate electric impulse.

EXAMPLES FOR THE APPLICATION OF THE VIG

Single stage generator: In a first step the VIG 2003 shall be used for the pre-determination of the impulse shape for a single stage HV modular system, which is typically used for teaching purposes, total view see fig. 3, circuitry see fig 4.

Fig.3: Impulse generator arrangement based on a HV modular system

impulse circuit

dividermeas.

rectifier

C1 R2 5

5L1 RR1

CC6

Fig. 4: Circuitry of an impulse generator arrangement based on a HV modular system, element designation see fig. 2.

The input data for the impulse calculation were: C1 = 10 nF, 34,4kV impulse capacitor R1 = 298 Ohm front resistor R2 = 5874 Ohm tail resistor L1 = 2 µH circuit inductance R5 = 250 Ohm divider resistor C5.= 0,3 nF divider/basic load C6 = 0,6 nF test object For the calculation a circuit inductance of 2 µH has been estimated. A comparison between the calculated and measured impulse is given in table 1. The deviation between the calculated and measured impulse data is in the range of a few percent. i.e. not far away from the measuring accuracy of the impulse test system. Table 1: Comparison between calculated and measured impulses for a one-stage HV modular system impulse data calculated measured deviation/%

Up/kV 30,8 30,1 2,19 T1/µs 0,863 0,87 -0,80 T2/µs 46,2 47,5 -2,80

The calculated and measured impulse curves are shown in fig. 5. A variation of the impulse circuit inductance between 0,1 and 10 µH has shown, that this inductance has no essential influence on the impulse shape, caused by a minimal load capacitance of 0,3 nF + 0,6 nF = 0,9 nF and a front resistor of R1 = 298 Ohm.

-5

0

5

10

15

20

25

30

35

-0,5 0,0 0,5 1,0 1,5

Time in µs

Volta

ge in

kV

measured

calculated

Fig. 5: Calculated and measured impulse curves for a HV modular system Large multistage generator: A second example is an impulse voltage test system 420 kJ/4200 kV. It consists of a 21-stage impulse generator with a stage capacitance of 1 µF and a stage charging voltage of 200 kV and a slightly damped capacitive voltage divider, photograph see fig. 6, sketch see fig. 7. Even the large dimensions of the impulse circuit require the separate consideration of the capacitances of the generator and the divider top electrode and the consideration of a long HV lead between these electrodes.

Fig. 6: Impulse voltage test system 420 kJ / 4200 kV

Fig. 7: Impulse voltage test system 420 kJ / 4200 kV

For calculation such a complete multi-stage impulse generator is reduced to a one-stage equivalent circuit, see fig 8. Two options are possible, i.e. to relate the output quantities to one stage or to relate the stage quantities to the output voltage. The first option was chosen in this case leading to the following input data for the one stage equivalent circuit:

C1 = 1000 nF, 200 kV impulse capacitor R1 = 36 Ohm front resistor R2 = 76 Ohm tail resistor L1 = 3,37 µH front resistor inductance C2.= 76,2 pF . 21 = 1600 pF HV-electrode capacitance L3 = 52,5 µH / 21 = 2,5 µH HV lead inductance L5 = 16,8 µH / 21 = 0,8 µH divider inductance R4 = 315 Ohm /21 = 15 Ohm divider resistance C4.= 381 pF . 21 = 8 nF divider capacitance C6 = 76,2 pF . 21 = 1,6 nF divider HV electrode

capacitance

1R L1 L3 L5V(t)C1 R2 C2

C4

R4 6C

Fig. 8: One-stage equivalent circuit of a multi-stage MARX generator

The comparison between calculated and measured impulse data for the 21-stage impulse voltage test system is given in table 2. It is remarkable, that the deviation of calculated and measured impulses is in the range of the measuring accuracy. However the shapes of both curves show visible differences, see fig. 9.

impulse data calculated measured deviation/% Up/kV

4066,7

(193,649 . 21) 4070 -0,08 T1/µs 1,083 1,089 -0,54 T2/µs 55,13 56,54 -2,48

Table 2 Comparison between calculated and measured impulse data for a 21-stage Impulse voltage test system

Impulse testing of transformers: This is a very important application of impulse test systems. There are different problems to be mastered: - to guarantee an impulse front according to IEC

standard for the impulse testing of HV coils considering the capacitive coupling between the HV coils

- to guarantee a time-to-half-value for impulse testing of low-inductive low-voltage coils

- limitation of the over-shoot for large, extended test arrangements

The VIG 2003 includes a simple transformer model consisting of the transformer inductance L6 and the transformer capacitance C7, see fig. 10. A separate chopping gap is realised together with R9 and C9. There are different measures to extend the tail for the testing of low-inductive low-voltage coils, i.e. termination resistor R7 and Glaninger unit with L2 and R6. As a simple example an impulse tests shall be performed for the three HV coils of a power transformer 30 MVA, 110/25 kV. To select the correct front (R1) and tail (R2) resistors the test has been simulated by the VIG2003 using the correct C7/L6 parameters of the coils to be tested. As a result of simulation R1 and R2 are selected from available set of resistors. Then the HV test has been performed with those resistors and the result is compared in tab. 3. The deviation between the calculated and measured front time T1 is about 5 %, the deviation for the time-to-half-value T2 is below 1 %. However the

pre-determination of the generator settings for transformer tests is a challenge because of the needed knowledge of the precise values for winding inductance (L6) and capacitance (C7).

-500

0

500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

4.500

-1 0 1 2 3 4 5

Time in µs

Volta

ge in

kV

measured

calculated

-500

0

500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

4.500

-10 0 10 20 30 40 50

Time in µs

Volta

ge in

kV

measured

calculated

Fig. 9: Calculated and measured impulse curves for a 21-stage impulse test system, impulse front above, impulse tail below

Fig. 10: Input data set for transformer testing: C1 - Impulse capacitor; R1 - Front resistor; L1 - Inductance of the front resistor; R2 - Tail resistor; L3 - Impulse circuit inductance; R5, C5 - Separate damped capacitive voltage divider / Load capacitance; R6 - External tail resistor (ohmic load); L6, C7 – Transformer (stray) inductance, input capacitance

phase 1 phase 2 phase 3 Test object parameters

C7/nF 1,16 1,13 1,05 L6/mH 161 110 72

calc. T1/µs 1,36 1,34 1,28 meas. T1/µs 1,43 1,41 1,35

deviation T1/% -4,90 -4,96 -5,19 calc. T2/µs 57,20 51,60 44,80 meas. T2/µs 57,00 51,70 45,30

deviation T2/% 0,35 -0,19 -1,10 Table 3: Impulse test of the HV coils of a power transformer 30 MVA, 110/25 kV

CONCLUSIONS

(1) The determination of the impulse generator settings, i.e. the values for the front and tail resistors, in order to generate impulses in accordance to the IEC standards is desirable to support the test operation in a test laboratory as well as for teaching purposes. (2) There is a free-accessible www-based solution to determine the impulse curve and data depending on the input data of a one-stage equivalent circuit (3) Impulse circuit arrangements for some 100 kV have small dimensions only and a relatively small test object

capacitance in most of the cases. In general there is no need to consider different stray inductances and stray capacitances for the calculation, i.e. the so-called VIG 2002 is sufficient for the calculation. Whereas impulse circuit arrangements in the MV range require more detailed one-stage equivalent circuits as realised with the VIG 2003. (4) Transformer impulse testing is one important field in the HV test practice. The VIG 2003 contains some elements to reach a standard conform impulse shape. (5) For some test cases calculated and measured impulses were compared to each other. Under the assumption of realistic stray capacitances and inductances there is a remarkably small deviation between calculated and measured impulses.

REFERENCES

[1] Leister, N., Schufft W.: Virtual ASP-based impulse generator. Proceedings of the XIIIth International Symposium on High Voltage Engineering, Delft, the Netherlands August 25 – 29, 2003, p. 272

[2] IEC 60060-1, High-voltage test techniques. Part 1:.General definitions and test requirements

[3] R. M. Del Vecchio, R. Ahuja, R. D. Frenette, “Determining ideal impulse generator settings from a generator-transformer circuit model”. IEEE Trans. on Power Delivery, 17(2002)1