DIELECTRIC DISCHARGE TEST

Partial discharges are electrical sparks that occur in gas voids within the insulation when voltage is high enough. The spark generates a fast current pulse which travels through the stator winding. The larger the Pd pulse, the higher the current pulse that reaches the terminals of the winding. A high voltage capacitor can block the power frequency voltage while allowing the high-frequency pulse signals to reach a Pd detector. The pulse signals after further filtering are displayed on an oscilloscope screen. Partial discharge tests are probably the best means for assessing the insulation condition without a visual inspection. By measuring the Pd activity of a stator an indication of the health of the winding can be

obtained. IEEE std for this test: IEEE – Std. 436

Dielectric Discharge (DD) Test is a diagnostic insulation test that allows aging and deterioration of insulation to be assessed. The result is dependent on the discharge characteristic so the internal state of the insulation is tested, largely independent of any surface contamination. The charge that is stored in the sample is measured during the discharge phase. The DD test is automatic on some MEGGER® insulation testers making it very simple to carry out.

Charging  Insulation

During an insulation test the charging rate depends on the capacitance of the sample, the absorption rate, the leakage current plus the output current of the insulation tester.

Capacitive Charging Current

The insulation material becomes charged in the same way as a dielectric in a capacitor. This charging is dependent on the capacitance value and the resistance of the source. This effect usually decays in a few seconds.

 

Absorption Current

In addition to the capacitive charging, dipoles align in the electric field within the material and some charges are able to move slightly. These charging effects cause absorption current which has a long time constant. They will return to their natural, random state slowly when the test voltage has been removed (causing re-absorption current). These effects are lengthened when the insulation has been contaminated by dirt or moisture. Depending on the type of insulation this may take as long as 30 minutes.

The leakage current

Leakage Current is dominated by surface leakage but includes leakage through the insulation. This is the resistive current that would be measured when the insulation is fully charged and full absorption has taken place.

Surface leakage can be removed by use of the 'Guard' terminal.

Total Current

The total current is measured by an insulation tester to calculate the insulation resistance. To analyse  the insulation,

the time - resistance characteristics can be used to assess how the charging currents are varying.

The charging characteristics can be used to assess the quality of the insulation in the polarization index (10minute:1 minute) or dielectric absorption (60seconds:30 seconds) tests. These ratios can be used to evaluate the level of contamination of the sample because the difference between the absorption and leakage current is measured. Excessive leakage current will swamp absorption current, leading to a flatter time-resistance curve.

 

 

 

Insulation Discharge

The charge that is stored during the insulation test is automatically discharged at the end of the test when the insulation tester's discharge resistors are switched across the terminals. The rate of the discharge depends only on the discharge resistors and the amount of stored charge from the insulation. When the voltage across the insulation has reduced to almost zero the effect of surface leakage will be very small.

The discharge currents change in a similar way to the charging currents; the capacitive current decays quickly due to the time constant of the capacitance and the discharge resistors, the absorption current decays slowly as re absorption effects take place.

Re absorption Current

The capacitive current quickly decays from a high value with a relatively short time constant (a few seconds). The absorption (or re-absorption during a discharge) current starts from a lower level but has a much longer time constant (up to several minutes). It is caused by ions and dipoles re-aligning themselves within the insulation. When an electric field is applied some ions are able to move and some dipoles align themselves within the field.

These effects reverse themselves slowly when the test voltage is removed caused by particles returning to their natural, random state. This can have the effect of a current flowing if the discharge circuit is still connected, or a voltage reappearing on the sample if it is left open circuit. The DD test measures the discharge currents 1 minute after an insulation test has been completed. At this time the capacitive current has usually become insignificant compared with the re absorption current. The level of re absorption after this time shows the state of the insulation material, providing the insulation has been fully charged for full absorption to take place (typically 10 to 30 minutes). A high re absorption current shows that the insulation has been contaminated,  usually by moisture. A low current usually shows that the insulation is clean and has not absorbed much water.

Dielectric Discharge Definition

The dielectric discharge test measures the discharge current 60 seconds after the insulation test is finished. This is converted to a figure of merit which gives a figure for the quality of the insulation, independent of the test voltage. This is temperature dependent so it is important to test at a reference temperature or record the value. The DD value is defined as (in mAV-1F-1):

Multi-layer  Insulation

Insulation in high voltage equipment often consists of layers, each having its own capacitance and associated leakage resistance. When insulation is built up in this way, the aim is to make each layer such that the voltage stress is shared equally between layers. When the insulator is discharged each layer's charge will decrease equally until there is no voltage remaining. The DD test result can also show how similar the layers of insulation are. In the case of insulation failure in a single layer of insulation the leakage resistance will decrease but the capacitance is likely to remain the same. This type of fault is not possible to detect from a standard insulation test because the overall resistance will remain high due to the other, high resistance, layers. Similarly, other tests such as time-resistance measurements, step voltage tests or capacitance measurement will not necessarily show any particular problem. Measuring the discharge current can show when the resistance-capacitance characteristic is incorrect. This effect is smaller than the absorption effects mentioned previously.

DD  Test  Result  Analysis

A low DD value shows that the re-absorption current is decaying quickly and the time constant of each layer of insulation is similar. A high value of DD shows that the re-absorption current exhibits long relaxation times which may point to a problem with the insulation. Typical conditions from practical research, primarily carried out on generators by a major utility, arrived at the figures of merit in the table below. This technique was developed for HV generators but may have applications where insulation condition needs to be determined for other electrical equipment.

 

CRITERIA

 

PROCEDURE:

GENERATOR BUS BAR ßMEGGAR

R-PHASE APPLY VOLTAGEàEARTH

Y-B PHASE SHORT AND EARTH

 

RESULT :

Temp: 38°C

Phase	DD Value
R-Phase	3.83
Y-Phase	3.32
B-Phase	4.18