Dielectric Tests

The purpose of dielectric tests is to demonstrate that the transformer has been designed and constructed to withstand the specified insulation levels. The insulation requirements for the transformers and the corresponding dielectric tests are given in IS 2026 Part-3 and IEC Publication 60076-3 with reference to specific windings and their terminals. For oil immersed transformers, the requirements apply to the internal insulation only.

The dielectric tests shall generally be made at the test tab of manufacturer with the transformers preferably at ambient temperature.

Transformers, including bushings and terminal compartments when necessary to verify air clearances, shall be assembled prior to making dielectric tests, but assembly of items, such as radiators and cabinets, which do not affect dielectric tests is not necessary. Bushing shall, unless otherwise authorised by the purchaser, be those to be supplied with the t ransformer.

If a transformer fails to meet its test requirements and the fault is in a bushing, it is permissible to replace this bushing temporarily with another bushing and continue the tests on the transformer to completion without delay. A particular case arises for tests with partial discharge measurements, where certain types of commonly used high-voltage bushings create difficulty because of their relatively high level of partial discharge. When such bushings are specified for the transformer, it is permitted to exchange them for bushings of a partial discharge free type during the testing of transformer.

Test levels and other test parameters shall be as per IEC Publication 60076-3 and the corresponding IS 2026 Part-3.

It is recommended to measure voltage at the high voltage terminal of its transformer. The measuring system shall be in accordance with IEC Publication 60060-2. In case of non- availability of suitable high voltage divider, it is recommended to establish the ratio of HV to applied LV voltage upto 50% of the HV test level and then LV voltage shall be maintained to indicate the proper test voltage.

4.7.1 Rules for Some Particular Transformers

In transformers where uniformly insulated windings having different Um values are connected together within the transformer, the separate source AC withstand test voltages shall be determined by the insulation of the common neutral and its assigned Um.

In transformers which have one or more non uniformly insulated windings, the test voltages for the induced withstand voltage test, and for the switching impulse test, are determined by the winding with highest Um value, and the windings with lower Um values may not receive their appropriate test voltages.

During switching impulse tests, the voltages developed across different windings are approximately proportional to the ratio of turns. Rated switching impulse withstand voltages shall only be assigned to the winding with the highest U_m. Test stresses in other windings are also proportional to the ratio of numbers of turns and are adjusted by selecting appropriate tappings to come as close as possible to the assigned value.

4.7.2 Insulation Requirements and Dielectric Tests

The basic rules for insulation requirements and dielectric tests for different categories of windings are described in Table 1(Refer IEC Publication 60076-3).

Table 1

Routine Not applicable Special Routine Routine

170 < Um < 300 Routine Routine* Routine Special* Routine Uniform and

non-uniform insulation

Um 300 Routine Routine Routine Special Routine

* If ACSD test is specified, the SI test is not required.

The standard dielectric requirements are verified by dielectric tests. They shall, where applicable and not otherwise agreed upon, be performed in the sequence as given below :

Switching impulse test (SI) for the line terminal Lightning impulse test (LI) for the line terminals

Lightning impulse test (LI) for neutral terminal

Separate source AC withstand voltage test (applied potential test) Short-duration induced AC withstand voltage test (ACSD) Long-duration induced AC voltage test (ACLD)

4.7.3 Switching Impulse Withstand Voltage Test, Transformer Winding Um > 170 kV This test is intended to verify the switching impulse withstand strength of the line terminals and its connected windings to earth and other windings, the withstand strength between phases and along the winding under test.

The impulses are applied either directly from the impulse voltage source to a line terminal of the winding under test, or to a lower voltage winding so that the test voltage is inductively transferred to the winding under test.

The detailed test procedures and specific test requirements are addressed in IEC Publication 60076-3.

4.7.3.1 SWITCHING IMPULSE WAVES (a) Polarity

The polarity of test voltage shall be negative because this reduces the risk of erratic external flashovers in the test circuit.

(b) Wave Shape

The voltage impulse shall have a virtual front time of at least 100 ìs, a time above 90% of the specified amplitude of at least 200 ìs, and a total duration from the virtual origin to the first zero passage of at least 500 ìs but preferably 1000 ìs.

(c) Test Sequence and Records

The test sequence shall consist of one impulse of a voltage between 50% and 75 % of the full test voltage and three subsequent impulses of full voltage. If the oscillographic or digital recording should fail, that application shall be disregarded and a further application made.

Oscillographic or digital records shall be obtained of at least the impulse wave-shape on the line terminal under test and preferably the neutral current.

(d) Test Connections

During the test the transformer shall be in a no-load condition. Windings not used for the test shall be solidly earthed at one-point but not short -circuited. For a single phase transformer, the neutral terminal of the tested winding shall be solidly earthed.

A three-phase winding shall be tested phase by phase with the neutral terminal earthed and with the transformer so connected that a voltage of opposite polarity and about half amplitude appears on the two remaining line terminals which may be connected together.

To limit the voltage of opposite polarity to approximately 50% of the applied level, it is recommended to connect high ohmic damping resistors (10 k to 20 k ) to earth at the non tested phase terminals.

(e) Failure Detection

The test is successful if there is no sudden collapse of voltage or discontinuity of the neutral current if recorded on the oscillographic or digital records.

Additional observation during the test (abnormal sound effect etc.) may be used to confirm the oscillographic records, but they do not constitute evidence in themselves.

4.7.4 Lightning Impulse Withstand Voltage Test

This test is intended to verify the impulse withstand strength of the transformer under test.

This test shall only be made on windings that have terminals brought out through the transformer tank or cover.

When non-linear elements or surge diverters are installed for the limitation of transferred over-voltage transients, the evaluation of test records may be different compared to the normal impulse test. These non-linear protective devices connected across the windings may cause difference between the reduced full wave and the full-wave impulse oscillograms. To prove that these differences are indeed caused by operation of these devices, this should be demonstrated by making two or more reduced full-wave tests at different voltage levels to show the trend in their operation.

The detailed test procedure and specific test requirements are addressed in IEC 60076-3.

4.7.4.1 IMPULSE WAVE

The test impulse shall be a full standard lightning impulse: 1.2 µs ± 30% / 50 µs ± 20%.

But in some cases this standard impulse shape cannot reasonably be obtained, because of low winding inductance or high capacitance to earth. In such cases wider tolerance may be accepted by the agreement between purchaser and customer. It is recommended to use IEC Publication 60722 as a guide for non-standard wave shapes.

4.7.4.2 TEST SEQUENCE

The test sequence shall consists one impulse of a voltage between 50% to 75% of full test voltage, and three subsequent impulses at full voltage. If, during any of these applications, an external flashover in the circuit or across a bushing spark gap should occur, or if the oscillographic recording should fail on any of the specified measuring channels, that application shall be disregarded and a further application made.

4.7.4.3 TEST CONNECTIONS

(a) During Test on Line Terminals

The impulse test sequence is applied to each of the line terminals of the tested winding in succession. In the case of a three-phase transformer, the other line terminals of the winding shall be earthed directly or through a low impedance, not exceeding the surge impedance of the connected line. If the winding has neutral terminal, it shall be earthed directly or through a low impedance such as a current measuring shunt.

In the case of separate-winding transformer, terminals of windings not under test are earthed directly or through impedances, so that in all circumstances, the voltage appearing at the terminals is limited to not more than 75% of their rated lightning impulse withstand voltage for star connected windings, and 50% for delta- connected windings.

In case of auto transformer, when testing the line terminal of the high voltage winding the non-tested line terminal shall be earthed through resistors not exceeding 400 Ω to get the impulse waveform as needed.

(b) Impulse Test on a Neutral Terminal

Impulse withstand capability of neutral may be verified by : (i) Indirect application

Test impulses are applied to any one of line terminals or to all three line terminals connected together. The neutral is connected to earth through an impedance or is left open. Then standard lightning impulse is applied to the line terminal which shall not exceed 75% of the rated LI withstand voltage of the line terminal.

(ii) Direct application

Test impulse corresponding to the rated withstand voltage of the neutral is applied directly to the neutral with all line terminals earthed. In this case, however a longer duration of front time is allowed, upto 13 µs.

4.7.4.4 RECORDS OF TEST

The oscillographic or digital records obtained during calibrations and tests shall clearly show the applied voltage impulse shape (front time, time to half value and amplitude). The oscillograms of the current flowing to earth from the tested winding shall also be recorded.

4.7.4.5 TEST SEQUENCE

The test sequence shall consist of one impulse of a voltage between 50% to 75% of full test voltage, and three subsequent impulses at full voltage. If, during any of these applications, an external flashover in the circuit or across a bushing spark gap should occur, or if the oscillographic recording should fail on any of the specified measuring channels, that application shall be disregarded and a further application made.

4.7.4.6 FAILURE DETECTION

(i) Grounded Current Oscillograms

In this method of failure detection, the impulse current in the grounded end of the winding tested is measured by means of an oscilloscope or by a suitable digital transient recorder connected across a suitable shunt inserted between the normally grounded end of the winding and ground. Any significant differences in the wave shape between the reduced full-wave and final full-wave detected by comparison of the two current oscillograms, may be indication of failure or deviations due to non-injurious causes. They should be fully investigated and explained by a new reduced wave and full-wave test. Examples of probable causes of different wave shapes are operation of protective devices, core saturation, conditions in the test circuit external to the transformer.

The ground current method of detection is not suitable for use with chopped-wave tests.

(ii) Other Methods of Failure Detection

Voltage Oscillograms: Any unexplained difference between the reduced full-wave and final full-wave detected by comparison of the two voltage oscillograms, or any such differences observed by comparing the chopped-waves to each other and to the full-wave up to the time of flashover, are indications of failure.

Noise : Unusual noise within the transformer at the instant of applying impulse is an indication of trouble. Such noise should be investigated.

Measurement : Measurement of voltage and current induced in another winding may also be used for failure detection.

4.7.5 Separate Source Voltage Withstand Test 4.7.5.1 DURATION, FREQUENCY AND CONNECTIONS

A normal power frequency, such as 50 Hz, shall be used and the duration of the test shall be one minute.

The winding being tested shall have all its parts joined together and connected to the terminal of the testing transformer.

All other terminals and parts (including core and tank) shall be connected to ground and to the other terminal of the testing transformer.

4.7.5.2 APPLICATION OF VOLTAGE FOR SEPARATE SOURCE WITHSTAND TEST

The test shall be commenced at a voltage not greater than one-third of the full value and be brought up gradually to full value in not more than 15 s. After being held for the specified time of 60 seconds, it should be reduced (in not more than 5s) to one third or less of the maximum value and the circuit opened.

4.7.5.3 FAILURE DETECTION

The test is successful if no collapse of the voltage occurs. Careful attention should be given for evidence of possible failure that could include items, such as an indication of smoke and bubbles rising in the oil, an audible sound such as a thump, or a sudden increase in test circuit current. Any such indication should be carefully investigated by observation, by repeating the test, or by other test to determine if a failure has occurred.

4.7.6 Induced AC Voltage Withstand Tests

ACSD test is intended to verify the AC withstand strength of each line terminal and its connected winding(s) to earth and other windings, the withstand strength between phases and along the winding(s) under test.

As per IS 2026 Part 3-1981 and IEC Pub. 60076-3 of 1981, the test is normally performed with partial discharge measurement (Method 2) for transformers with highest voltage winding of 300 kV. For transformer with highest voltage winding of < 300 kV, the test is performed without partial discharge measurement (Method 1). However, with the latest revision of IEC 60076-3 in the year 2000, the methods for induced over-voltage withstand test are referred as AC short duration test (ACSD) and AC long duration t est (ACLD).

For U_m < 72.5 kV, ACSD test is carried out without partial discharge measurement for 60 seconds.

For Um >72.5 kV, the test is normally performed with partial discharge measurements to verify partial discharge free operation of the transformer under operating condition.

However, the requirements for partial discharge measurement during the ACSD test may be omitted. This shall be clearly stated at the enquiry and order stages.

ACLD test is always performed with the measurement of partial discharge during the whole application of test. This test is not a design proving test, but a quality control test and is intended to cover temporary over voltages and continuous service stress. It verifies PD free operation of the transformers under operating conditions.

An alternating voltage shall be applied to the terminals of one winding of the transformer.

The voltage shall be as nearly as possible sinusoidal and its frequency is sufficiently above the rated frequency to avoid excessive magnetizing current during the test.

The test voltage is the peak value of voltage divided by 2 .The test time at full test voltage shall be 60 sec for test frequency up to and including twice the rated frequency. For frequency above twice the rated frequency the time duration of test shall be:

frequency

Table 1 shows the different conditions of induced AC voltage test as defined in IEC publication 60076-3. The time duration for the application of test voltage with respect to earth is shown in Fig. 2.

Table 1 Induced AC voltage test Type of

Fig. 2 Time sequence for the application of test voltage with respect to earth

Here

U_m=Highest voltage for equipment U1= Test voltage

U2= Partial discharge evaluation level

The detailed procedure and specific test requirements are addressed in IEC-60076-3 5.0 TYPE TESTS

5.1 Lightning Impulse Voltage Withstand Test, Transformer Winding Um 72.5 kV