The Transformer: Transformer Testing: IEC and IEEE Approaches for Dielectric Test Requirements – In-Depth with IEC Examples (Part-9)

This article examines deeply into the IEC's approach, shedding light on its globally recognized standards through practical examples. By offering a comprehensive analysis, this article aims to equip professionals and enthusiasts alike with the knowledge to navigate the complex landscape of dielectric testing.

Dielectric Integrity and its Verification with Practical Examples with Brief Explanation according to IEC 60076-1 and IEC 60076-3:

Example 1

For a three-phase transformer according to IEC 60076-1 (2011) Power Transformers - Part 1: General, and IEC 60076-3 (2013) Power Transformers - Part 3: Insulation levels, dielectric test and external clearances:

1000 kVA, 11 kV/400 V; 50 Hz, Dyn11 connected distribution transformer

HV Um = 12 kV and LV Um = 0.6 kV

Insulation Levels:?

– HV: LI / AC 95 / 34 kV

– LV and LV-neutral: LI / AC - / 3 kV?

Routine Tests:?

? Applied Voltage Test:?

– HV: 34 kV for one minute

– LV and LV-neutral: 3 kV for one minute?

Test Requirement:?

Neither breakdowns nor flashovers may occur.?

Brief Explanation:

This test demonstrates the capability of the insulation between windings, and between windings and structural (core) parts, to withstand the stresses from specified, sinusoidal power frequency test voltages.?

The single phase test voltage from a suitable power source is applied on winding sets on each side of the insulation. The voltage is quickly raised to the specified level and kept there for one minute. This procedure does not excite the transformer core electromagnetically. During the test one winding will be connected to power source and another winding will be grounded.?

The test is monitored by measurement of the voltages.

Passing Criteria: The test is successful if no collapse of the test voltage occurs.?

? Induced Voltage Test:?

All three-phase transformers with uniform insulation should be ACSD tested using a symmetrical three-phase power supply (phase-to-phase test) of 100~250Hz to the low-voltage. If the transformer has a neutral, it should be earthed during the test.?

Three-phase (phase-to-phase) test, three-phase LV-supply with:?

Generator used for Testing (Assume): 160 Hz, 105 kVA, 38-550 V

Testing Transformer used: 160 kVA, 230/100-3000 V?

Test Voltage: Two times of 3-phase rated voltage at 160 Hz applied (2xVr = 800 V to obtain 2 times turn-to-turn voltage).?

Test Time: T = 120 x (fr / ft), but not less than 15 s

= 120 x (50 / 160) = 37.5 sec?

Where, fr = rated frequency and ft = test frequency

Start the test at a voltage no higher than one-third of the required level, then quickly increase it to the full test value while still being able to measure it. After the test, quickly lower the voltage to under one-third of the test level before turning it off.?

Passing Criteria: The test is successful if no collapse of the test voltage occurs.???????????

? LI Test?

Only as Type Test.?

Example 2

For a three-phase 25 MVA, 33/13.8 kV, 60 Hz, Dyn11 connected power transformer according to IEC 60076-1 and IEC 60076-3:?

HV Um = 36 kV and LV Um = 17.5 kV?

Insulation Levels:?

– HV: LI / AC 170 / 70 kV

– LV and LV-neutral: LI / AC 95 / 38 kV?

Routine Tests:?

? Applied Voltage Test:?

– HV: 70 kV for one minute

– LV and LV-neutral: 38 kV for one minute?

? Induced Voltage Test:?

Three-phase (phase-to-phase) test, three-phase LV-supply with:?

Test Voltage: Two times of 3-phase rated voltage at 160 Hz applied (2 x Vr = 2 x 13.8 = 27.6 kV to obtain 2 times turn-to-turn voltage).?

Test Time: T = 120 x (fr / ft)

= 120 x (60 / 160) = 45 sec.?

? LI Test?

Only as Type Test.?

Example 3

For a three-phase 31.5/40 MVA, 132/11.5 kV; 50 Hz, Dyn11 connected power transformer according to IEC 60076-1 and IEC 60076-3:?

HV Um = 145 kV; LV Um = 12 kV?

Insulation Levels:

– HV: LI / AC 650 / 275 kV

– LV and LV-neutral: LI / AC 95 / 34 kV?

Routine Tests:?

? Applied Voltage Test:?

– HV: 275 kV for one minute

– LV and LV-neutral: 34 kV for one minute?

? Induced Voltage Test:?

Three-phase (phase-to-phase) test, three-phase LV-supply with:

23.92 kV = 2.08 x 11.5 to obtain 275 kV phase-to-phase; corresponding 2.08 times turn-to-turn voltage.?

? LI Test:

650 kV, negative polarity, 1,2 ± 30 % / 50 μs ± 20 %?

Example 4?

For a three-phase 250 MVA Auto‐Transformer, 220/132 /11 kV, 50 Hz, YNyn d1 connected, according to IEC 60076-1 and IEC 60076-3:?

– HV Line Um = 245 kV

– HV Neutral Um = 145 kV

– MV Line and Neutral Um = 145 kV uniform insulation

– LV Line Um = 12 kV d-connected?

Insulation Levels:?

– HV Line Terminals: SI / LI 850 / 1050 kV

– HV Neutral: LI / AC 650 / 275 kV

– MV Line Terminals: LI / AC 650 / 275 kV

– MV Neutral: LI / AC 650 / 275 kV

– LV Line Terminals: LI / AC 95 / 34 kV?

Routine Tests:?

? SI Test with 850 kV?

? LI Test with 1050 kV?

? LI Test with 650 kV?

? Applied Voltage Test?

– HV and HV neutral with 275 kV

– MV with 275 kV

– LV with 34 kV?

? Induced Voltage Test with PD Measurement?(IVPD)

– Three-phase LV-supply to obtain UP = 1.7 Um / √3 = 1.7 x 245 / √3 = 240 kV to earth or 415 kV phase-to-phase; corresponding to about 1.73 times turn-to-turn voltage.

PD measurement at 1.5 Um / √3 = 212 kV to ground during 30 minutes.

PD measurement should also be performed at 1.1 Um.

Abbreviations:

The abbreviations have the following meaning:

HV High voltage

LV Low voltage

MV Medium voltage

Um Highest r.m.s. phase-to-phase voltage in a three-phase system for which a transformer winding is designed in respect of its insulation

SI The rated switching impulse withstand voltage level for the line terminals of the winding with the highest Um

LI The rated lightning impulse withstand voltage level for the terminal of each individual winding

AC The highest rated AC withstand voltage level to earth designed for the terminals of each winding

PD Partial Discharge

REFERENCES

[1] IEC 60076-1:2011, Power Transformers - Part 1: General

[2] IEC 60076-3:2018, Power Transformers - Part 3: Insulation levels, dielectric test and external clearances.

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