A78 The Transformer: Transformer Testing: IEC and IEEE Approaches for Dielectric Test Requirements – In-Depth with IEEE Examples (Part-10)

Dielectric integrity and its verification with Practical Examples

Example 1?

For a three-phase 25 MVA Class I power transformer according to IEEE C57.12.00-2015 and IEEE C57.12.90-2015:?

Rated voltage: 65/15 kV, 60 Hz, YNyn connected, non-graded insulation.?

Insulation Levels:?

According to IEEE C57.12.00-2015, Table 3, HV BIL is 350 kV; LV BIL is 110 kV.?

– Low Frequency Insulation Level: HV: 140 kV

– Low Frequency Insulation Level: LV: 34 kV?

Routine Tests:?

? Applied Voltage Test for HV and LV:?

– HV and HV-neutral: 140 kV

– LV and LV-neutral: 34 kV?

The separate-source AC withstand test, also known as the applied-voltage test, is a routine test as per IEEE standards. In this test, a separate AC source is applied to the transformer, hence the name 'applied-voltage test.' It's important to note that the transformer is not magnetized during this test.?

The purpose of the separate source AC withstand test or applied-voltage test according to IEEE C57 is to verify the integrity of the main insulation. The main insulation does not only mean the insulation system between the two windings (major insulation), but also – more generally - the insulation between the winding and earth (end insulation) and all connections to earth and to each other.?

Test voltage Up relates to the insulation level of transformer winding for transformers with uniform insulation. Every part of the winding is exposed to the full test voltage, Up, between the winding and earth.?

With non-uniform insulation (graded insulation) the test voltage,Up, refers to insulation requirements of the winding end with the lowest requirements, generally the neutral.?

Test voltage Up in kV r.m.s.-value according to IEEE is given in Table 3, and 4 of IEEE C57.12.00-2015 for Class I and Class II power transformers respectively.

Application of the Test Voltage?

The winding under test must have all its components interconnected and linked to the line terminal of the testing transformer. All remaining terminals and parts, including the core and tank, should be connected to ground.?

The voltage should be raised rapidly from 25% or less to the test voltage Up but remain consistent with measurement requirements. IEEE standard cites 15 seconds. At the end of the test the voltage should be reduced rapidly i.e. in about 5 seconds.?

Duration of Test: The test duration is 60 seconds.?

Test Frequency: The test frequency is the normal power frequency.?

Interpretation of the Test?

The test is successful if the test voltage does not collapse or if there are no other fault indications such as smoke, bubbles and thumps or a sudden test circuit current increase.?

If the transformer fails to meet the test requirements and the fault is in a bushing, the bushing may be replaced by a temporary bushing and the test continued. This is also valid when the specified bushing is not PD-free.?

? Induced Voltage Test (phase-to-phase)?

The induced voltage test is intended to verify the AC withstand strength of each line terminal and its connected winding(s) to earth and other windings; it also verifies the withstand strength between phases and along the winding(s) under test (turn-to-turn insulation).?

IEEE distinguishes between Class I (69 kV and below) and Class II (115 kV and higher) transformers. For Class II transformers, a long duration test (“one hour level”) in combination with a partial discharge measurement is always required. The test values for IEEE are given in Table 3 for Class I transformers and in Table 4 for Class II transformers. They are always based on BIL figures.?

IEEE recommends conducting the test at 2.0 times the nominal system voltage, within the frequency range of 100 to 250 Hz, for 7200 cycles for both distribution transformers and Class I power transformers.?

During these tests, the voltage is applied to the low-voltage winding while the high-voltage winding is kept open-circuited. As the frequency increases, the iron core can ensure twice the induced voltage when it is unsaturated. Therefore, the insulation performance between turns, layers, and phases of the winding is evaluated to assess the longitudinal insulation level of the transformer.?

System nominal voltage for 65 kV rated voltage, according to C57.12.00-2015 for Class I power transformer is 69 kV.?

Therefore, 2.0 × nominal system voltage = 2 × 69 = 138 kV?

Alternatively according to C57.12.00-2015, Table 3, Col 6:?

Induced voltage test, (phase to ground) kV rms for 69 kV nominal voltage = 80 kV. So, phase to phase voltage = √3 × 80 kV (138.5 kV)?

Therefore, Three-phase LV supply applied = √3 × 80 (15 / 65) = 31.9 kV for 7200 cycles.?

? Lighting Impulse Test?

The lightning impulse test for Class I transformers not a routine test; rather, it is conducted as “Design Test” or “Other Test”.?

Example 2?

For a three-phase 80 MVA, Class II power transformer according to IEEE C7.12.00-2015 and IEEE C7.12.90-2015:?

Rated voltage 130/15 kV; 60 Hz, YNyn connected, graded insulation (means non-uniform insulation).

Insulation levels:?

According to IEEE C57.12.00-2015, Table 4:?

– HV: Nominal System Voltage: 138 kV, BIL 550 kV

– HV: Neutral: Nominal System Voltage: 25 kV

– LV: Nominal System Voltage: 15 kV, BIL 110 kV?

Routine Tests:?

? Lightning Impulse Test?

Test voltage: 550 kV, 1.2 × 50 μs?

The test sequence depends on the test code, IEEE C57.12.90-2015, and customer requirements. Generally the test sequence starts with reduced level impulses and ends with full wave test impulses at specified amplitude.?

IEEE Specifies:?

? One reduced full wave impulse (calibration impulse)

? Two front-of-wave impulses at specified amplitude (only if specifically requested)

? Two chopped wave impulses at specified amplitude

? One full wave impulse at rated amplitude (BIL)?

The chopped wave should be at 110% of the BIL value so in this case = 1.1 × 550 = 605 kV, 3.0 μs (minimum time to flashover).?

The front-of-wave should have an amplitude according to Table 3 of IEEE C57.12.00-2015 (only for Class I transformers).?

The chopped wave calibration impulse for the applicable wave shape may immediately precede the chopped wave test itself, and it need not be carried out as the initial part of the test sequence.?

The integrity of the transformer is confirmed when there is a close similarity between the voltage traces for the applied calibration impulse voltage and all of the applied full test voltages.?

? Applied Voltage Test?

– HV and HV-neutral: 34 kV

– LV and LV-neutral: 34 kV?

Duration of Test: The test duration is 60 seconds.?

Test Frequency: The test frequency is the normal power frequency.?

Interpretation of the Test:?

The test is successful if the test voltage does not collapse or if there are no other fault indications such as smoke, bubbles and thumps or a sudden test circuit current increase.?

? Induced Voltage Test with PD Measurement?

Long Duration phase-to-phase Test with Symmetrical three-phase LV Supply:?

Enhancement level of 145 kV (phase to earth) and an “one-hour-level” of 125 kV (phase to earth), according to column 6 and 7 of Table 4 of IEEE C57.12.00-2015.?

Every Class II power transformer must undergo an induced-voltage test, with the prescribed test levels induced in the high-voltage winding. Whenever feasible, the tap connections should be selected so that the test levels induced in the other windings during the 1-hour test are x times their respective maximum operating voltages, as outlined in ANSI C84.1, where x represents the ratio of the test voltage applied to the high-voltage winding to the maximum operating voltage.?

Test procedure

The voltage should initially be increased to the 1-hour level and maintained for a minimum of 1 minute or until a stable partial discharge level is achieved to confirm the absence of partial discharge issues. The level of partial discharges should be recorded just before elevating the voltage to the enhancement level. Subsequently, the voltage should be elevated to the enhancement level and sustained for 7200 cycles. Following this, the voltage should be reduced to the 1-hour level and maintained for another hour.

Throughout this 1-hour duration, partial discharge measurements should be conducted at 5-minute intervals. Partial discharge acceptance criteria should be based on each line terminal rated 115 kV and above. These measurements should adhere to the guidelines outlined in IEEE C57.12.90.?

BIBLEOGRAPHY

[1] IEEE Std C57.12.00-2015, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers

[2] IEEE Std C57.12.90-2015, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers

[3] IEEE Std 4-2013, IEEE Standard for High-Voltage Testing Techniques

[4] P Ramachandran, Rated Power of Transformers, 2021.

[5] www.scotech-electrical.com, Three Common Test Methods of Transformer Withstand Voltage Test, 2022.