Chapter 09 - Measurement of Partial Discharge
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Partial discharge is a discharge event that does not bridge the entire insulation system between electrodes. It occurs within cavities in insulation materials under high electric fields. During partial discharge, a plasma channel briefly forms within the cavity, conducting electricity from one side to the other without crossing the entire material. Measurement setups use coupling devices and detectors to monitor the short voltage pulses caused by partial discharge, in order to evaluate insulation condition and detect defects.
Chapter 09 - Measurement of Partial Discharge
- 1. Measurement of Partial Discharge Dated : 04-12-2017
- 2. Partial Discharge • Partial discharge (PD) is a discharge event that does not bridge the electrodes within an electrical insulation system under high field stress. • When PD happens, discharge starts from one end of the cavity surface, bridging through the gas-filled cavity and reaches the other end of the cavity surface. • Thus, PD only bridges the cavity and does not bridge • the whole insulation between electrodes.
- 3. Partial Discharge • PD normally happens in the presence of defect within an insulation under a high electric field. Examples of defects that may exist in polymeric insulation are voids, cracks, cavities. • Partial discharge in a solid dielectric material usually occurs in gas-filled cavities within the material. Since the permittivity of the gas is less than the permittivity of the surrounding material, the electric field in the cavity is higher than the material.
- 4. Partial Discharge • The electric field in the cavity is sufficiently high and the breakdown strength of the gas in the cavity is exceeded, PD can occur. • During the PD event, the gas changes property from a non-conducting to a conducting condition. • Figure 1 shows a basic diagram of PD within a cavity in a dielectric material which is stressed under a high electric field.
- 5. Figure 1: Basic diagram of PD within a cavity in a dielectric material.
- 6. Partial Discharge • The effect of PD within a cavity in high voltage insulation can be very serious because it can ultimately lead to complete failure of the whole system. Repetition of PD causes progressive chemical deterioration of the material. • There are several types of discharge other than partial discharge, including surface discharge and corona discharge. Surface discharge is a discharge event that occurs on an insulation surface where the tangential field across the surface is high. • This discharge can bridge the potential difference between the high voltage source and the ground electrode through cracks or contaminated paths on the insulation surface.
- 7. Partial Discharge • Examples of surface discharge in the field are on the insulation surface of a high voltage cables or at the end-windings of stator windings of large generators. • Corona discharge is discharge in gas due to a locally enhanced field from a sharp point of an electrode which ionizes the surrounding gas molecules.
- 9. Partial Discharge • Examples of surface discharge in the field are on the insulation surface of a high voltage cables or at the end-windings of stator windings of large generators. • When a discharge occurs in the test sample, charges from the coupling capacitor are transferred to the test sample in order to balance the voltage drop across the test sample. As a result, a current pulse of short time duration of nanoseconds range flows in the circuit and a voltage pulse is generated across the coupling device.
- 10. Partial Discharge • The output from this device is connected to PD detector that detects the magnitude of discharge pulse and applied voltage amplitude through its PD impulse input and voltage input respectively. • Finally, the output from PD detector is connected to USB controller through fibre optic cables and the all the data is sent to the PC for user to display, save and analyse PD activities
- 11. Partial Discharge • When a discharge occurs in the test sample, charges from the coupling capacitor are transferred to the test sample in order to balance the voltage drop across the test sample. As a result, a current pulse of short time duration of nanoseconds range flows in the circuit and a voltage pulse is generated across the coupling device. • The measuring impedance (a coupling device) measures the short duration voltage pulse, when a PD occurs.
- 22. Thank You… Dated : 04-12-2017