Transformers and equipment in underground distribution systems

Transformer Tests

Insulation Resistance and Polarization Index

The purpose of the transformer insulation resistance test is to measure the condition of a “major” insulation system – i.e., the insulation between a winding and ground (core) or between two windings. IEEE C57.125 recommends 500 V, 1000 V, or 2500 V DC to be applied to the transformer winding.

The resistance of each measurement should not be smaller than R = 1.5×U_W/√KVA, where R is in MΩ measured at 20 °C, and U_W is the winding voltage in kV. If the winding is Y-connected, then U_W is the phase-to-ground voltage. If it is Delta-connected, then U_W is equal to phase-to-phase voltage. KVA is the rated power of the winding under the test.

Megaohm meter test results below this minimum value would indicate probable insulation breakdown. If a transformer passes the insulation resistance test, before applying any overvoltage test, it is recommended to do a Polarization Index (PI) test.

Table 1 is a guide to the interpretation of the PI test results.

Table 1 – Test Interpretation

Power Factor

In general, power factor measurement equipment comes with three basic modes of operation: grounded specimen test, a grounded specimen with the guard, and ungrounded. The three measurement modes allow measurement of the current leaking back to the test set on each lead, individually and together.

In general, a power factor of less than 1% is considered good; 1-2% is questionable; and if it exceeds 2%, action should be taken. Practically, the evaluation is not only based on a single power factor data point but is also based on the history of the change in power factor. Values obtained at the time of the original tests are used as benchmarks to determine the amount of insulation deterioration on subsequent tests.

Another value of the power factor measurement is that it will detect voids in the insulation system that may be causing high partial discharges.

Table 2 is a guideline to interpret the insulation power factor test. The tests can be done, respectively, on high-voltage winding to ground, high- to low-voltage winding, and low-voltage winding to ground.

Table 2 – Power Factor Test Interpretation

Turns Ratio

The purpose of a turn-ratio test basically is to diagnose a problem in the winding turn-to-turn or shorted multiturn insulation system in a transformer. This test detects primarily inner winding short circuits. A very low voltage ac source is used to determine the turn ratio.

Two windings on one phase of a transformer are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector, with exciting current also being measured in most cases.

Measured ratios should compare with ratios calculated from nameplate voltage to within 0.5%, but should compare even closer to actual benchmark values. Out-of-tolerance readings should be compared with prior tests.

The turn-ratio test may also detect high-resistance connections in the lead circuitry or high contact resistance in tap changers by higher excitation current and difficulty in balancing the bridge.

Winding Resistance

Winding resistance is used to indicate the winding conductor and tap changer contact condition. The test requires an ohmmeter capable of accurately measuring resistance in the range of 20Ω down to fractions of an ohm.

Resistance measurements can be used to check for proper connections and to determine if an open-circuit condition or a high-resistance connection exists in parallel conductor windings.

Winding resistance varies with oil temperature. Because the resistance of copper varies with temperature, all test
readings must be converted to a common temperature to give meaningful results. Most factory test data is converted to 85°C. This has become the most commonly used temperature. Variations of more than 5% may indicate a damaged conductor in a winding.

Partial Discharge

For large power transformers, the partial discharge (PD) test is performed in the laboratory as a routine test, although a PD test is not required for quality control of distribution transformers. However, the PD test is well known as a diagnostics tool and can be employed to detect minor and progressing problems leading to a catastrophic fault inside a transformer.

The two commonly used PD detection methods are:

1. Detection of the acoustic signals, and
2. Measurement of the electrical signals produced by the PD.

The acceptable PD limits for new transformers are dependent on the voltage and size of the transformers and range from 100 to 500 pC. PD pulses generate mechanical stress waves that propagate through the surrounding oil. To detect these waves, acoustic emission sensors are mounted on the transformer tank wall.

If multiple sensors are used, the PD can be located based on the arrival time of the pulses at the sensors.

In the field, the test can be done on-line or off-line. For the off-line test, a three-phase source is required to apply the voltage. On-line PD measurement can be employed using acoustic sensors, via busing tap, or through high-frequency current transformers (HFCT) located either on bushing tap or in the neutral of the transformer.

Figure 1 shows a PD resolved pattern on the left, recorded using an HFCT sensor via neutral cable. A classification technique is employed to separate the contributions of PD from those generated by disturbances. Each PD pulse waveform is acquired, and the so-called equivalent time-length and bandwidth are evaluated and plotted on the TF map, as shown in Figure 1 (b).