The influence of a compact test circuit in the puncture test

Darcy Ramalho de Mello Lines and Equipment Department

CEPEL Rio de Janeiro, Brazil

darcy @cepel.br

José Antonio D’Affonseca Cardoso High Voltage Department

CEPEL Rio de Janeiro, Brazil

cardoso@cepel.br

José Antonio P. Rodrigues High Voltage Department

CEPEL Rio de Janeiro, Brazil

japr@cepel.br

Francisco de Assis A. F. Vieira High Voltage Department

CEPEL Rio de Janeiro, Brazil

japr@cepel.br

Abstract— The objective of this paper is to present the experience of CEPEL’s laboratory in the puncture test (since 1984) showing the advantages to use a compact test circuit with short dimensions very fast resistive divider, the use of an encapsulated sphere-gap to guarantee the linearity of the spark, reducing the charging voltage and the wearing of the impulse capacitor. This experience is important because several test laboratories around the world had stopped to perform the puncture test due to problems in their impulse generators or problems to obtain the real test voltage due to wrong measuring systems. This paper also presents the results obtained when performing a puncture test on a short standard string.

I. INTRODUCTION

The use of the puncture test, to evaluate ceramic, glass or polymeric insulators, was saw with suspicious during long time due to the test reproducibility. After several studies made by CIGRE working groups the obtained conclusions indicated that if the test is correctly specified (use the voltage amplitude of the test impulse instead of steepness) and the test circuit adopts a suitable measuring technique, the results are reproducible and repeatable. These conclusions were used in a standard published by IEC, in 2004, with the following number: IEC 61211 [1].

This IEC standard indicates that the tests are feasible in most laboratories and the only precaution is the use of adequate measuring techniques. Nothing is said about the necessity to have a compact test circuit in order to reduce the inductance due the connection lengths, the ground length or how to mount the test arrangement.

CEPEL had participated on the studies of this test procedure, from 1984 until 1994, and developed a compact kit

for puncture test with a very fast resistive divider [2]. The use of this test circuit has reduced the impulse generator damages.

This experience is important because several test laboratories around the world have stopped to perform the puncture test due to problems in their impulse generators or problems to obtain the test voltage due to wrong measuring systems.

With the standard evolution, IEC 60383.1 [3] has established the concept of short standard string and CEPEL verified the possibility to perform a puncture test on this string with good results.

II. EVALUATIONS WITH THE PUNCTURE KIT

A photo of the puncture kit is shown in Fig 1 and the equivalent test circuit is shown in Fig 2.

The main component of the puncture test circuit is the measuring system as can be seen in IEC 61211. The resistive divider of the kit is a special project. Their arms are made of one layer of Wenner winding type, using Manganin wire, put inside a tubular insulator filled with gas SF6. Under this condition the divider is designed to withstand up to 500 kV of steep and short duration impulse as used in puncture tests. The puncture kit structure is such that the divider is permanent and very close connected to the insulator under test through a very short lead and it is grounded through a wide and short cooper sheet. These conditions are desirable to improve the dynamic performance of the divider and to keep the lay out unchangeable. Its step response, measured with the divider already assembled in the puncture kit, is shown in Fig. 3. The objective of this arrangement is to guarantee the required uncertainty presents in IEC 61211 and to allow the

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measurement of a very fast impulse, as can be seen in Fig. 4, with the positive and negative waveshapes obtained in the test performed on a 120 kN glass insulator.

Figure 1: Puncture Kit

where

a fi encapsulated sphere gap

b fi resistive divider

c fi ground connection

d fi insulator under test

e fi mechanical structure

Figure 2: Puncture Test Circuit

where

CG fi encapsulated sphere gap

RD fi resistive divider

OT fi object under test (insulator)

O fi oscilloscope

Figure 3 - Step response of the resistive divider

Figure 4 – Positive and negative test waveshapes

A comparison between two dividers, one a fast resistive divider and the other a RC divider, was made during a puncture test on a glass insulator (120 kN). This test was made using an open air gap (10 mm gap) with the same charging voltage of the impulse generator. The results can be seen in Fig. 5.

Figure 5 – Comparison between dividers - blue waveshape: test made with a RC divider;

- red waveshape: test made with fast resistive divider.

b

a

c

d

e

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The sphere gap electrodes are made of graphite with diameter of 180 mm. The gap distance range is from 10 mm to 700 mm. It was assembled inside a porcelain insulator and filled with dry air at ambient pressure. If necessary the insulator can be filled with compressed air (until 3.0 atm). The advantage to use an encapsulated gap is to reduce the influence of air humidity that can affect the gap fire, as can be seen in Fig. 6, and guarantee the same impulse wave shape, as can be seen in Fig. 7.

Figure 6 – Comparison between sphere gaps

- blue waveshape: test made with an encapsulated sphere gap (10 cm gap);

- red waveshape: test made with a non encapsulated sphere gap (10 cm gap).

Figure 7 – Comparison between successive shots using a encapsulated sphere gap

The waveshapes obtained with the length gap variation of the encapsulated gap can be seen in Fig. 8, from 10 cm (the first waveshape, in the left) to 70 cm (the last waveshape, in the right). Each wave shape was obtained varying the gap length in 10 cm between the limits. The test was performed on a 120 kN glass insulator with a constant impulse generator charging voltage (50 kV). The results obtained showed the importance to know the relation among

charging voltage and gap length to obtain a clean waveshape (without oscillations), according to IEC 61211.

The necessity of using a compact test circuit can be seen in Fig. 9, where distance between the encapsulated gap and the insulator under test was varied from 30 cm to zero. It can be seen that when a small cooper sheet is used as a connection, oscillations appeared on the waveshapes.

Figure 8 – Waveshapes varying gap length

Figure 9 – Waveshapes varying the distance between the encapsulated gap and the insulator under test.

- green waveshape: with a cooper sheet with 30 cm;

- blue waveshape: with a cooper sheet with 15 cm;

- red waveshape: without a cooper sheet.

III. PUNCTURE TEST ON A SHORT STANDARD STRING

When the IEC 60383-1 [3] presented a definition of short standard string and established that the dielectric test should be performed preferentially on that string a question appeared:. What is the behaviour of that string under a fast impulse?

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The first thing is verify if it is possible to perform a puncture test on a short standard string, which is the second objective of this paper.

Initially, was verified that the encapsulated gap cannot be used then it was changed by a multiple chopping gap. A coil (named L in Fig. 10) was placed between the impulse generator and the gap. The objective of this coil is to generate an overshoot to obtain a higher test voltage without overload the impulse generator. The main problem of impulse circuits, with a small value of inductance in the test circuit, lies in raising the impulse waveshape tail. As the spark occurs before or over the impulse crest, this problem is unimportant for a puncture test. The use of a coil, as can be seen in Fig.10, allowed performing the puncture test on a string with 5 insulators (standard 120 kN glass insulators) as shown in Fig. 11. The small 500 kV resistive divider was changed for a 2 MV fast resistive divider, with same step response of the small one. The waveshape obtained can be seen in Fig. 12.

Figure 10 – Puncture test circuit on a short standard string.

Figure 11 – Test arrangement for a puncture test circuit on a short standard string.

IV. CONCLUSIONS

Test facilities for performing puncture tests on insulators shall have some special features related to the generation of the steep HV impulse required, to the

measuring system used and to practical aspects of the circuit arrangement. The obtained results showed that the use of a compact puncture test circuit is the best choice.

A compact test circuit, mounted as a Kit by CEPEL, has the following advantages:

• easy to handle during test circuit assembling;

• good reproducibility with an encapsulated sphere gap that also provides a fast chopped impulse as required for this type of test;

• measurement capacity guarantee by a fast resistive divider, connected directly and very close to the insulator under test;

• perform the puncture test on all types of line insulators, glass or porcelain, that can be easily connected to the bottom of the encapsulated sphere gap.

The test results showed that is possible to perform a puncture test on a short standard string, in order to verify the behaviour under fast impulses, considering that this type of string will substitute the single insulator in dielectric tests.

Figure 12 – Waveshape of a puncture test on a short standard string.

V. REFERENCES [1] IEC 61211, “Insulators for overhead lines with a nominal voltage above

1000 V – Impulse puncture testing in air”, 2004.

[2] Oliveira O. B., Mello D. R., Cerqueira W. R., Alvarenga E.: Kit for Performing Puncture Test on HV Insulators. Paper ISH 97, Montreal August 25 – 29, 1997, 4 p.

[3] IEC 60383-1, “Insulators for overhead lines with a nominal voltage above 1000 V – Part 1: Ceramic or glass insulator units for a.c. systems – Definitions, test methods and acceptance criteria”, 1993.

[4] IEC SC 36B, “Puncture withstand testing of insulators,” ELECTRA paper, number ELT-199-1, 2001.

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1) CH1: 4 Volt 500 ns

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