C57.152 draft Section 6.1.7

Page 1 of 5

6.1.7 Induced voltage test

6.1.7.1 General

This test shall be applied for power transformers as installation tests when assembled on-site on-site test after repair test for condition assessment test for failure identification

The test shall be performed as a withstand test with parallel PD measurement (PD monitored withstand test). Both, a withstand test without PD measurement or only a PD test with voltages near the rated voltage of the transformer under test, can only be recommended when the PD monitored withstand is not applicable.

The three and / or single phase induced test voltages should be within a frequency range of 80 Hz to 240 Hz. This voltage is generated by a suitable generator and adapted to the test object by a step-up transformer.

NOTE: The generator is often also used for the loss-measurements at rated frequencies. This increases the required frequency range from 50 Hz to 240 Hz.

The partial discharges shall be measured according to IEEE Std. C57.113 (2010). Achieving a sufficiently low PD background noise level is difficult on-site. Therefore the PD measuring circuit shall be as compact as possible, also grounding loops shall be avoided and all well known measures for PD noise reduction shall be applied.

The mentioned generator for the induced voltage test may be a motor-generator (M/G) set or nowadays also a static frequency converter (SFC) set.

Transportable M/G sets for three-phase voltage are usually designed for one or two fixed test frequencies. In case of a single-phase test the test power delivered by the M/G set is reduced by a factor 1/3 and the wave shape is more distorted (THD increases) due to a negative sequence component of the test voltage.

Transportable three-phase SFC sets supply test voltages of continuously variable frequencies between 80 Hz and 240 Hz, with respect to other tests (loss measurements) even between 40 Hz and 240 Hz. In single-phase operation the test power is also reduced by the factor 1/3, but the wave shape remains sinusoidal.

Suitable measures for the reduction of PD noise signals shall be applied for both types of generators. Then in both cases more or less identical PD background noise levels can be reached on-site.

C57.152 draft Section 6.1.7

Page 2 of 5

6.1.7.2 Preliminary test procedures

Low-voltage tests (insulation resistance, power factor, ratio, oil dielectric, etc.) should be performed to determine if the insulation of the transformer under test is suitable for energization. The insulating fluid should be sampled following prescribed procedures (refer to ASTM D 923-9 1) and its total dissolved gas-in-oil level analyzed to ensure that it is acceptable. A moisture content test of the oil should be made to ensure that excessive amounts do not exist. A turns ratio test should be performed to confirm that the transformer's tap changer for de-energized operation is properly positioned and that shorted winding turns do not exist. For the interpretation see table 1.

Table 1Recommended diagnostic characteristics

Procedure New transformer Service-aged transformer Power factor < 0.5 % < 2.0 % Total dissolved gasa < 0.5 % < 0.8 %

Moisture content < 10 ppm < 15 ppm

Turns ratio Within 0.5 % of nameplate

Within 0.5 % of nameplate

a) If units are equipped with nitrogen blankets, total dissolved gas should not exceed 1.0 %.

6.1.7.3 Special precautions before test

Corona discharges at the tank of the transformer under test or at nearby grounded or energized objects may not only increase the PD background noise level, but also influence the results of withstand tests. Therefore in preparation for the test, all high-voltage bushings should be fitted with corona rings of sufficient size, so as to eliminate all possibilities of air corona discharges. To prevent corona on the ground side, all sharp edges and points on top of and close to the transformer tank should be masked by covering them with corona rings galvanically connected to the tank. All high-voltage bushings should be carefully cleaned and dried. Immediately prior to the test, they should again be wiped dry. No conductive or semiconductive objects should be left ungrounded on the transformer or close to it, as this would produce discharges from floating objects. Therefore, they should either be taken away, when this is possible, or carefully grounded. All current-carrying connections should be very carefully made to ensure good electrical contact as contact arcing may produce unacceptably high PD background noise levels.

Any transformer-mounted surge arresters should be disconnected before energizing the transformer in order to avoid arrester damage and limitation of the test voltages due to arrester operation.

C57.152 draft Section 6.1.7

Page 3 of 5

6.1.7.4 Power factor pre-test for excitation by M/G sets

After connecting the test set to the transformer under test, a pre-test to determine the power factor at the source should first be performed to ensure that the amount of inductive compensation is sufficient to guarantee that the load on the generator is not capacitive as this could lead to dangerous overvoltages due to generator self excitation. To perform this pre-test, a high-voltage bushing of the test transformer should first be temporarily fitted with an external spark gap adjusted to operate at about 50% of the transformer nominal voltage. During this pre-test, the voltage should not be raised above 30% or preferably only at a level high enough to allow fairly accurate power factor measurement. Note that the generator is capable of driving a slightly capacitive load provided that the power margin is sufficient; it is when this margin is exceeded that generator runaway occurs. To be on the safe side, higher than required inductive compensation should first be used and it should be adjusted to a value that will allow the test to be done at full test voltage without exceeding the generator limits as this would cause generator protection tripping. The user should therefore be absolutely sure that the generator has a sufficient power margin to reach the maximum test level without danger of tripping. It is also important that the ratio of the step-up transformer used to match the voltage output of the generator to that of the transformer under test be as close as possible to the required value (optimum adaption). This will ensure maximum power transfer from the generator to the transformer under test. After the pre-test has been performed and the reactive compensation has been adjusted properly, the temporary spark gap should be removed and the voltage can then be taken to the test level.

6.1.7.5 Pre-tests for excitation by Static Frequency Converters (SFC) sets

As there is no danger of self-excitation when using static frequency converters, a special pre-test for checking the power factor is not necessary. In case of small test objects, the test sequence can simply be started after assembly of the temporary test field on-site. But in case of huge test objects the ratio between the available test power and the needed test power should be checked in advance. It has to be taken into account that the transformer under test is a capacitive test object at high frequencies and an inductive one at lower frequencies. At the self-compensation frequency in between the power demand is at its minimum. The SFC set shall be adjusted to that frequency.

6.1.7.6 Test procedure

The induced voltage test shall be performed as a HV withstand test in combination with a PD measurement (according to IEEE Std. C57.113). Such a withstand test shall be called a PD monitored withstand test. To keep the PD background noise level low enough, the test should be performed on a clear day. All outside interferences such as operating cranes or motorized vehicles should be kept clear of the test side and all precautions before the test (see 6.1.7.3) shall be carefully considered.

C57.152 draft Section 6.1.7

Page 4 of 5

The sequence of this test is characterized by steps of four voltage levels including the agreed voltage withstand level UW and the agreed PD measuring voltage level UPD (Fig. 1). The rate of voltage raise or of voltage reduction between the voltage levels should be about 2% of UW per second. The test sequence is as follows:

Fig.1: Sequence of a PD monitored withstand voltage test (explanations in the text)

1) The voltage is switched on or raised to a voltage U1 = 0.5 U0 = 0.5 Ur / 3 (with Ur rated voltage, U0 - line-to-ground voltage) at which the PD background level is measured and recorded. The voltage is held there for a time t1 3 min which shall allow the determination of a stable PD measuring value.

2) The voltage is raised to U2 U0 and held there for long enough to measure a stable PD level which is recorded (t2 3 min).

3) The voltage is raised to U3 = UPD and held there for a minimum duration of t3 5 min until a stable PD level is reached and recorded.

4) The voltage is raised to U4 = UW and held there for the withstand voltage test of a duration

t4 / s = 120 (rated frequency/test frequency), but not less than t4 = 15 s. The PD level is recorded for information.

5) The voltage is reduced to the agreed PD measuring level U5 = UPD = U3 with an agreed duration of t5 = tPD. The PD level should be continuously recorded but in minimum every 2 min.

6) The voltage shall be reduced to U6 = U2 U0 and held there for long enough to measure and record a stable PD level (t6 t2 3 min).

7) At a voltage level U7 = U1 0.5 U0 the PD background level shall be determined and recorded for a duration t7 t1 3 min. After that the voltage is switched off.

C57.152 draft Section 6.1.7

Page 5 of 5

NOTE: It is recommended that the PD level should be continuously observed on at least one measuring channel during the whole test sequence. Inception and extinction of significant PD activity should be noted.

If no PD measurement can be performed (e.g. if it is not agreed or if the PD background level is too high), the induced voltage test should be performed as a withstand voltage test at the level U4 = UW of a duration

t4 /s = 120(rated frequency/test frequency), but not less than t4 = 15 s. The test remains a step test, and the levels U1 to U3 and U5 to U7 shall be considered as conditioning levels of a duration t 1 min each.

6.1.7.7 Test parameters and test acceptance criteria

The withstand test voltage level U4 = UW (and sometimes its duration tW) as well as the PD measuring voltage level U5 = UPD and its duration t5 = tPD are usually subject to negotiation. The level may vary from initial factory test voltage level for new transformers to lower levels depending on the age and history of the transformer (UW 1.2 U0 cannot be recommended.). If no PD measurement is performed, a test duration tW t4 may be considered. The PD measuring voltage level should be agreed between 80% and 90% of the withstand voltage level (UPD = 0.8.0.9 UW) and its duration t5 = tPD = 20.60 min. The PD limit value shall also bee agreed, a value of the apparent charge between 500 and 1000 pC may be appropriate.

The transformer has passed the PD monitored withstand test, if all the following criteria are fulfilled:

1) No breakdown of the test voltage occurs during the whole cycle. 2) None of the PD levels recorded at the test voltage UPD exceeds the agreed PD

limit. 3) The PD level measured during the PD test does not exhibit any steady raising

trend or sudden sustained increase by more than 100 pC. 4) The PD levels at identical voltage levels before and after the withstand test (Fig.

1): U2 and U6: U3 and U5) shall not increase by more than 100 pC.

NOTE: If no PD measurement is performed, only the first criterion is applied.