Transformer Testing Methods:

A transformer has to go through different testing methods to prove that the transformer meets the customer’s specifications and design expectations.

Before connecting the transformer, the following tests should be necessarily carried out,

1. Measurement of Insulation Resistance

2. Measurement of Winding Resistance

3. Measurement of Winding Ratio

4. Vector Group Check

5. Magnetic Balance

6. Dielectric Strength of Transformer Oil

Transformer Testing Procedure:

1. Measurement of Insulation Resistance

Insulation resistance measurement reveals the quality of the transformer’s insulation and the degree of dryness.

The insulation resistance of a transformer is measured as follows.

1. Between the HV winding and tank, the LV winding is being grounded.
2. Between the LV winding and tank, the HV winding is being grounded.
3. Between the LV/HV windings collectively, and the tank grounded.

Insulation Value of the transformer winding (ohms)

Between HV~E	Between LV~E	Between HV~LV

2. Measurement of Winding Resistance

Winding resistance measurement shall be performed when the windings are at ambient
temperature without supply for a time long enough to achieve this condition. The measurements shall be carried out in Direct Current between terminals according to the sequence U-V; V-W; W-U.

The ambient temperature shall also be measured. It shall result in the average value of three
measurements performed by apposite thermal sensors.

 Measure the DC resistance of all windings using a Whetstone Bridge or Kelvin bridge as follows.

HV Winding Resistance Measurement:

HV winding resistance measurement shall be performed by measuring simultaneously voltage and current. The voltmeter and ammeter must be connected as follows:

• Voltmeter terminals must be connected beyond current cables;
• The current shall not exceed 10% of winding rated current;
• The measurement shall be carried out after voltage and current are stable.
Unless otherwise agreed, the HV winding shall be connected on principal tapping.

LV Winding Resistance Measurement:

LV winding resistance measurement shall be performed by measuring simultaneously voltage and current. The voltmeter and ammeter shall be connected as follows:

• Voltmeter terminals shall be connected beyond current cables;
• The current shall not exceed 5% of winding rated current;
• The measurement shall be carried out after voltage and current are stable.

Winding Resistance at a standard temperature of 75 deg C:

The resistance measurement is carried out at an ambient temperature and then converted to resistance at 75 deg C for all practical purposes of comparison with specified design values.

For Copper Windings:

Note down the oil/winding temperature. If the measurements are done after hot oil circulation; the oil/winding temperature may be different from the ambient temperature.

• Convert the resistances to 75 deg C.

• Resistance at 75 deg centigrade = R (measured)X (234.5 + 75)/ (234.5 + Oil temperature at the time of measurement)

3. Measurement of Winding Ratio

A ratio test should be conducted on every transformer for the position of every tap.

1. First, keep the tap changer of the transformer in the lowest position and LV terminals open.
2. Then 3-phase 415 V supply is applied to HV terminals. Measure the voltages applied on each phase (Phase-phase) on HV and induced voltages at LV terminals simultaneously.
3. After the voltages at HV and LV terminals are measured, raise the tap changer of the transformer by one position and repeat the test.
4. Repeat the same for each of the tap positions separately.

4. Transformer Vector Group Check Testing

To carry out the Vector Group test one should establish the electrical connection between Primary and Secondary since the separate winding transformers are inductively coupled.

After establishing the electrical connection between two windings under test, the voltage may be applied in either of the winding preferably on the HV side as it is safe and the voltage measured between the various terminals will be indicative to ensure the phase shift.

Testing Engineer should derive at least three conditions to compare, add and or equate the particular set of voltages to be measured. These conditions should be in such a way as to ensure the phase shift and phase sequence.

If the derived conditions are satisfied the vector shall be confirmed.

Let us see the test results for the YNd1 Vector group transformer

Vector Group Test Procedure:

1. Connect 1U and 2u
2. Keep the Neutral floating
3. Apply 3Φ Voltage on the HV side
4. Measure Voltage between
   • 1U and 2v (V_1U2v )
   •  2v and 1N (V_2v1N )
   •  1U and 1N (V_1U1N )
5. Here We have to check (Condition-01) V_1U2v + V_2v1N = V_1U1N
6. Measure Voltage between
   • 1W and 2w ( V_1W2w )
   • 1V and 2w (V_1V2w )
7. Here We have to check (Condition-02) V_1W2w < V_1V2w
8. Measure Voltage between
   • 1W and 2v (V_1W2v )
   • 1V and 2v (V_1V2v )
9. Here We have to check (Condition-03) V_1W2v = V_1V2v

Vector Group Test Results Example:

Other Vector group’s test conditions are read below.

Transformer Vector Group Test conditions

5. Magnetic Balance Testing of the Transformer

Magnetic Balance Test as it is called is performed to detect faults/defects in the magnetic core structure, shifting and/or de-shaping of windings, failure in inter-turn insulation, or fault in Tap Changer. The faults described above change the effective reluctance of the magnetic circuit which then affects the magnetizing current required to establish flux in the core.

When a fault occurs in your system, checking whether your transformer is in good condition or not, is the simplest test, which can be carried out. Any changes/faults/dimensional instability inside the transformer will be visible in the readings.

Magnetic Balance Test Procedure:

1. On the transformer, you have a primary high-voltage terminal (11/22/33kV, etc.) & Secondary low-voltage terminal (415/433/575/750 V, etc.),

2. Disconnect cables from both sides of the transformer.

3. Connect single phase 230 Volts supply between the first and second phase of the primary side of the transformer (between 1U & 1V)

4. Measure voltages between 1U~1V, 1V~1W, and between 1W~1U,

5. Similarly measure voltages between 2U~2N, 2V~2N & 2W~2N

6. Then connect the single-phase supply between the second and third phases on the primary side (between 1V~1W), and again record readings as mentioned in 5 & 6 above.

7. Then connect the single-phase supply between the third and first phase on the primary side (between 1V~1W), and again record readings as mentioned in 5 & 6 above.

8. Record the readings in the table given below:

9. A transformer will be deemed in good condition if the readings are in the above order. In case of fault, there will be a deviation, or maybe zero voltage may be recorded in any particular phase depending on the type of fault.

6. Dielectric Strength Testing of Transformer Oil

Oil BDV Test:

Verification of the condition of the insulation oil also known as transformer oil, tests for water content, breakdown voltage (BDV), and acidity. Water content that is less than 30 mg/kg is desired. The presence of water reduces the dielectric strength of the insulating oil.

The BDV should be greater than 30 kV in order to ensure that the Avalanche Effect is not imposed
on the insulation resulting in flashovers when applied with voltages lower than BDV. The acidity should be less than 0.15. The presence of sulfur or its compounds may corrode the metal parts and increase the production of sludge.

DGA Test:

The dissolved gas analysis DGA test extracts gases in the oil sample and analyses the gases as per the number of gases in a specific amount of oil. Observing the percentage of the different gases present in the oil can enable the prediction of the state of the transformer.

This test was done to verify the internal condition. A DGA test interprets the percentage of combustible gases within the transformer oil.