(i) Insulation Resistance Test
The Insulation-resistance shall be taken with all the winding earthed, expect, the one being tested. While checking the value, external line, cable and lighting arresters shall be isolated from the transformer. The entire terminal shall be cleared thoroughly, with clean cotton cloth. A 5000/2500V Megger should be used in the measurement. Care should be taken that the lead wires of the megger do not have joints or come in contact with each other or with transformer. It is known that the value of Insulation Resistance is continuous to increases initially, with the duration of measurement and hence for the purpose of comparison, the reading is normally taken at the end of 1 minute
Minimum Insulation Resistance after 1 minute is Rated Voltage + 1 Mega Ohms
(ii) Polarization Index Test
The polarization index is a specialized application of the dielectric absorption test. The index is the ratio of insulation resistance at two different times after voltage application, usually the insulation resistance at 10 minutes to the insulation resistance at 1 minute. The use of polarization-index testing is usually confined to rotating machines, cables, and transformers. Apolarization index less than 1.0 indicates that the equipment needs maintenance before being placed in service.
PI = R10/R1 (dimensionless)
Where:
PI = polarization index
R = resistance.
For polarization indexes in transformers an acceptable value would be 3 or higher, values between 3 and 2 indicate marginal condition, and values below 1 indicate poor condition. After insulation resistance readings have been made, the test voltage is returned to zero and the insulation is discharged
(i) Magnetic Balance Test
This test is done to find out the balancing of the core present in the transformer. The HV and LV sides are isolated by removing the external connections. A 3 phase supply of nearly 415V is applied between two phases of the primary winding and the induced voltages in other two phases are measured. The voltage may preferably be applied on the HV winding, as applying voltage to LV winding may induce very high voltage in the HV winding.
When the voltage is applied to the middle phase, the induced voltage measured on the two other phases should be approximately equal. Where the voltage is applied to an extreme phase, the induced voltage on the middle phase should be substantially high when compared to the voltage induced in the other extreme phase. In each test, the sum of the induced voltages in two phases should be nearly equal to the applied voltage. For measuring the voltages, high impedance voltmeter like digital multimeter should be used. The test may be repeated by applying voltage to the other phases and measuring the induced voltages in remaining phases. When the magnetic circuit is balanced, there would be symmetry in the value of measured induced voltages.
(ii) Magnetizing Current Test
The magnetizing current is measured to test any fault in the magnetic circuit and winding. The measured values are compared with the factory test values. A balanced three phase 415V ac supply is applied to the HV winding and the simultaneous current readings of the three phases are taken using low range A.C ammeters. For a core type transformer, the middle phase magnetizing current will be approximately half that in other
windings. In YyO, Dy1 and Dy11 connections, the currents in „u‟ and „w‟ phases will be nearly double the current in „v‟ phase. In an Yd1 connected transformer, currents in „v‟ and „w‟ phases will be nearly equal and the current in „u‟ phase more than that in „v‟ and „w‟ phases. In an Yd11 connection, currents in „u‟ and „v‟ phases will be nearly equal and the current in „w‟ phase more than that in „u‟ and „v‟ phases. If the measured values widely differ from the above values or from the factory test values, there is reason to suspect some defect in the transformer core and the manufacturer may be consulted. The measured values of magnetizing current may be used as bench marks for the service life of the transformer
(iii) Transformer turns-ratio
Ratio between all the corresponding HV & LV phase is to be measured on all taps. It is desirable to do the test with a Ratio�Meter. If it is not available, a simple test of measuring voltage and current can also serve the purpose. Referring to rating & terminal marking plate, identify the H.V and L.V terminal, which corresponding to one phase. E.g. For a vector Group of Dyn-11, H.V Terminal 1U, 1V &L.V terminal 2U,2N correspond to U phase. Apply 415V AC, 50Hz supply to H.V side and measure the voltage on the L.V side. Measure these voltages on all taps and note them in the „pre-commissioning report‟. Repeat for the other two phase. These observations should indicate a consistent trend of variation. Numerical values should approximately tally with voltage ratio.
(iv) Vector Group Test
The 3 phase voltage (415V/433V) will be applied to the transformer primary with the two phases (Normally 1U &2U) short.And the below condition will be checked for the proper vector group.
Condition for the Dyn11 transformer vector group
1U2U=0
1U1V=1U2N + 1V2N
1W2W< 1W2V
1V2W=1V2V
If the result found satisfactory the transformer vector group was correct
(v) Winding Resistance Test
Winding resistance shall be measured on all winding and tap winding, preferably with a Micro Ohm Meter. The Micro Ohm Meter is connected in between the HV terminal it is between Two Phase say (U-V, V-W, and W-U) and the corresponding resistance will be measured. The same is conducted on the secondary terminal. The value is verified with the manufacturer‟s data sheet.
(vi) Short Circuit Test
For the rated HV voltage and the % Impedance, it would possible to calculate the current, which would flow to the LV-side with 415V applied to HV side, while keeping the LV-side shorted. If the 415V source can feed the current, then shot- circuit test can be carried out. This test would conform proper conduct engagement at all tap position. Apply a 3-phase 415V, 50Hz to HV-side, keeping LV-side shorted. Measure the 3-line currents at all tap position. If the tap-switch is an Off-Circuit tap-switch, the supply has to be disconnected before changing the tap. A consistent trend in the increase or decrease of current, as the case may be,confirms the healthiness of the transformer