Partial discharge test of transformer
Partial discharge test of transformer is very important test… That helps us to detect any defect in transformer insulation … Partial discharge occurs as a result of the presence of moisture in liquid insulators or the presence of cavity in insulation due to defects in the manufacture of solid insulators.
To see full details about transformer testing here >>> transformer testing.
Firstly … What is partial discharge?
Partial discharge is in general a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation.
Generally, such discharges appear as pulses having duration of much less than 1 micro sec.
More continuous forms can, however, occur, such as the so-called pulse-less discharges in gaseous dielectrics. This kind of discharge will normally be detected by the measurement methods described in this test.
Purpose of this test:
- Detect the presence of any partial discharge in the insulators in the transformer this could turn into a complete breakdown.
- Verify that the transformer doesn’t exhibit partial discharge greater than specified magnitude.
- Determine the voltage amplitude at which partial discharges of a specified low magnitude commence with increasing and crease with decreasing voltage.
- To determine the magnitude of the specified discharge quantity at a specified voltage.
- Detect the presence of any partial discharge in the cooling oil transformer this is difficult to detect except by conducting that test.
Precautions before does this test:
- Disconnect the electrical current from the transformer.
- Clean the terminals of the transformer.
Steps of partial discharge test of transformer:
- Assume that we have transformer with some defects in its insulation.
- These defects such as a gap in bushing insulation due to defects in manufacturing or degradation of insulation.
- Apply the excitation voltage to the terminals of the low voltage winding.
- (line-to-earth pre stress voltage of (1.8Vm/√3) shall be induced for 30 sec).
- Increase the voltage on the transformer gradually until the partial discharge begins to occur (Follow without interruption by line-to-earth voltage of (1.3Vm/√3) for 3 minutes).
- The voltage value in this case (the case of partial discharge) is called inception voltage.
- We will notice an increase in the value of the leakage current as described in the following diagram.
- At this value of the inception voltage the electric field will break the insulation value and there is a bridge over the gap and then the value of the total insulation will decrease and the value of current will increase.
- We begin to gradually reduce the value of the voltage until the partial discharge stops and this is called the extinction
- If the extinction voltage is less than the operating voltage of the transformer, it means that there is a real danger to the transformer, which means that the partial discharge if it happens to the transformer will never stop.
- If the extinction voltage is higher than the transformer’s operating voltage, the partial discharge is not dangerous.