Different Transformer Tests for Power and Distribution Transformer

Introduction: 

To confirm the performance of the transformer , different tests are necessary. Before delivering transformers, manufacturers generally perform two types of tests: (A) Type tests and (B) Routine tests. 

Furthermore, the customer performs additional testing on-site prior to commissioning and on a periodic, emergency, and regular basis moving forward. 

Transformer Testing is mainly categorized as 

Transformer manufacturer tests are done by: 

(1) Routine Tests 

(2) Type Tests 

(3) Special Tests 

Transformer tests performed on-site 

(4) pre-commissioning tests 

(5) periodic condition monitoring tests 

(6) Emergency Tests 

(1) Routine tests: 

A transformer’s routine test mostly serves to confirm the operational performance of every unit in a production lot. 

Every manufactured unit undergoes routine testing. 

The following standard tests are performed on every transformer: 

(a)Insulation resistance testing. 

(b)Test for winding resistance 

(C) Turns Ratio / Voltage Ratio Test / TTR Test

(D) Test of the vector group’s polarity / Phase Displacement Test  

(E) No load-current test and no-load losses 

(F) Test for short-circuit impedance and load loss. / Copper Losses Test

(G)Test for Continuity 

(H)Magnetic Current Test 

(I)Test for Magnetic Balance 

(J)High Voltage Examination Test

(K)Dielectric testing 

(L)Separate source AC voltage/ Applied Voltage /High Voltage AC Test. 

(M) Induced overvoltage test 

(N) Lightning Impulse Tests /Switching Impulse Tests 

(O) When necessary, perform tests on on-load tap changers

(P) Radiators and tank vacuum test

(Q) Transformer oil pressure test to check for leakage of tank  joints and gaskets

(2) Type Tests. 

Type tests are performed on a transformer that serves as a representative of other transformers to ensure that they meet specific standards that are not covered by routine tests. 

Dielectric type test/ Lighting Impulse Test (IEC-60076-3) 

Temperature rise test (winding and oil temperature rise) IEC-60076-2

(3) Special tests 

Special tests are tests, other than routine or type tests, agreed between the manufacturer and purchaser. 

(a) Dielectric special tests. 

(b) Zero-sequence impedance on three-phase transformers. 

(c) Short-circuit test. 

(d) Harmonics on the no-load current. 

(e) The power taken by fan and oil-pump motors 

(f) Determination of sound levels 

(g) Determination of capacitances between windings and earth, and between windings.

(h) Determination of transient voltage transfer between windings 

(i) Tests intended to be repeated in the field to confirm no damage during shipment, for example, frequency response analysis (FRA). 

(j) Tests on newly purchased parts and accessories, including buchhloz relays, temperature gauges, pressure relief valves, oil preservation systems, etc.

(4) Pre-commissioning test 

The pre-commissioning test of a transformer is the process carried performed before the transformer is put into operation. These tests are carried out to evaluate the state of the transformer following installation and to compare the results of all the low-voltage tests with the test reports from the factory. 

All transformers tests are put through the subsequent pre-commissioning tests: 

(a) Magnetizing Current test 

(b) Vector Group 

(c) Magnetic Balance 

(d) IR value of transformer test

(e) Winding Resistance test 

(f) Transformer Turns Ratio test 

(g) Polarity Test 

(h) Bushing & Winding Tan Delta (HV ) 

(i) Protective relay testing test 

(j) Transformer oil testing test 

(k) Hipot test

(1) Transformer Routine Tests 

(a) Insulation Resistance Test 

Test purpose: 

A transformer’s insulation resistance test is essential in determining whether the overall insulation of an electrical power/distribution transformer is in good condition. 

Reference Standard with Clause: 

IEC-60076-1 Ref. clause 11.12 

IEC-60076-3 

Test Equipment:

● Use a 500V or 2500V Megger for the LT System. 

● Use a 2500V or 5000V Megger for an MV or HV system. 

● High Voltage Insulation Tester

How to Test a Transformer

Test Procedure for Transformer Test: 

Single-Phase Transformer tests

A single-phase transformer will be thoroughly tested using the five tests listed below and the related wiring schematics. Each test should be given for at least a minute, or until the reading settles down. 

❖ Primary/HV side, winding to secondary/LV side winding and ground

❖ Secondary /LV side winding to primary/ HV side winding and ground

❖ Primary/HV side winding to secondary/ LV side winding 

❖ Primary winding to ground 

❖ Secondary winding to ground 

Three Phase Transformer 

❖ Disconnect the transformer’s line and neutral terminals properly first. 

❖ To measure the Insulation Resistance (IR) value between the LV and HV windings, megger leads must be connected to the LV and HV bushing Brass connectors. 

❖ To measure the Insulation Resistance (IR) value between the HV windings and earth, Megger test set leads must be connected to the HV bushing brass connectors and the transformer tank earth point. 

❖ To measure the Insulation Resistance (IR) value between the LV windings and earth, megger leads must be connected to the LV bushing brass connectors and the transformer tank earth point.

❖ NB: In a three phase transformer, it is not essential to do a phase-by-phase insulation resistance test on the transformer. Because all of the windings on the HV side are internally connected to one another to form either a star or a delta, and all of the windings on the LV side are internally connected to one another to form either a star or a delta, IR values are taken between the windings collectively. 

❖ When the transformer’s insulation resistance is tested, the oil temperature should be documented. Because transformer insulating oil’s IR value can change with temperature.

 ❖ IR readings will be taken every 15 seconds, 1 minute, and 10 minutes

 ❖ The length of the voltage application raises the IR value. The increase in IR is an indication that the insulation is dry and has good condition. 

❖ Absorption Coefficient: is calculated as 1 Minute/ 15 Seconds. 

❖ Polarization Index: 10 minutes of value divided by 1 minute of value 

Test Precautions: 

❖ Verify the connections before the test starts. 

❖ Avoid touching the leads during an immediate discharge.