Various Routine Test of Power Transformer-(Part-1)

VARIOUS ROUTINE TEST OF POWER TRANSFORMER-(PART-1)

INTRODUCTION:

There are various Test required on Transformer to conform performance of Transformer. * Mainly two types of transformer are done by manufacturer before dispatching the transformer mainly (1) Type test of transformer and (2) Routine test. * In addition some other tests are also carried out by the consumer at site before commissioning and also periodically in regular & emergency basis throughout its life. * Transformer Testing mainly classified in * Transformer Tests done by Manufacturer * (A) Routine Tests * (B)Type Tests * (C) Special Tests * Transformer Tests done at Site * (D) Pre Commissioning Tests * (E) Periodic/Condition Monitoring Tests * (F) Emergency Tests

(A) Routine tests:

A Routine test of transformer is mainly for confirming operational performance of individual unit in a production lot. Routine tests are carried out on every unit manufactured. * All transformers are subjected to the following Routine tests: * Insulation resistance Test. * Winding resistance Test. * Turns Ration / Voltage ratio Test * Polarity / Vector group Test. * No-load losses and current Test. * Short-circuit impedance and load loss Test. * Continuity Test * Magnetizing Current Test * Magnetic Balance Test * High Voltage Test. * Dielectric tests * Separate source AC voltage. * Induced overvoltage. * Lightning impulse tests. * Test on On-load tap changers, where appropriate.

(B) Type tests

Type tests are tests made on a transformer which is representative of other transformers to demonstrate that they comply with specified requirements not covered by routine tests: * Temperature rise test (IEC 60076-2). * Dielectric type tests (IEC 60076-3).

Special tests are tests, other than routine or type tests, agreed between manufacturer and purchaser. * Dielectric special tests. * Zero-sequence impedance on three-phase transformers. * Short-circuit test. * Harmonics on the no-load current. * Power taken by fan and oil-pump motors. * Determination of sound levels. * Determination of capacitances between windings and earth, and between windings. * Determination of transient voltage transfer between windings. * Tests intended to be repeated in the field to confirm no damage during shipment, for example frequency response analysis (FRA).

(D) Pre commissioning Tests

The Test performed before commissioning the transformer at site is called pre commissioning test of transformer. These tests are done to assess the condition of transformer after installation and compare the test results of all the low voltage tests with the factory test reports. * All transformers are subjected to the following Pre commissioning tests: * IR value of transformer and cables * Winding Resistance * Transformer Turns Ratio * Polarity Test * Magnetizing Current * Vector Group * Magnetic Balance * Bushing & Winding Tan Delta (HV ) * Protective relay testing * Transformer oil testing * Hipot test

(A) ROUTINE TESTS OF TRANSFORMER

(1) INSULATION RESISTANCE TEST:

Test Purpose:

Insulation resistance test of transformer is essential to ensure the healthiness of overall insulation of an electrical power transformer.

Test Instruments:

For LT System: Use 500V or 1000V Megger. * For MV / HV System: Use 2500V or 5000V Megger.

Test Procedure:

First disconnect all the line and neutral terminals of the transformer. * Megger leads to be connected to LV and HV bushing studs to measure Insulation Resistance (IR) value in between the LV and HV windings. * Megger leads to be connected to HV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the HV windings and earth. * Megger leads to be connected to LV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the LV windings and earth. * NB: It is unnecessary to perform insulation resistance test of transformer per phase wise in three phase transformer. IR values are taken between the windings collectively as because all the windings on HV side are internally connected together to form either star or delta and also all the windings on LV side are internally connected together to form either star or delta. * Measurements are to be taken as follows:

Type of Transformer Testing-1 Testing-2 Testing-3 Auto Transformer HV-LV to LV HV-IV to E LV to E Two Winding Transformer HV to LV HV to E LV to E Three Winding Transformers HV to LV LV to LV HV to E & LV to E

Oil temperature should be noted at the time of insulation resistance test of transformer. Since the IR value of transformer insulating oil may vary with temperature. * IR values to be recorded at intervals of 15 seconds, 1 minute and 10 minutes. * With the duration of application of voltage, IR value increases. The increase in IR is an indication of dryness of in…