Partial discharge
Whenever partial discharge is initiated, high frequency transient current pulses will appear and persist for nanoseconds to a microsecond, then disappear and reappear repeatedly as the voltage sinewave goes through the zero crossing.
Another method of measuring these currents is to put a small current-measuring resistor in series with the sample and then view the generated voltage on an oscilloscope via a matched coaxial cable.
When PD, arcing or sparking occurs, electromagnetic waves propagate away from the fault site in all directions which contact the transformer tank and travel to earth (ground cable) where the HFCT is located to capture any EMI or EMP within the transformer, breaker, PT, CT, HV Cable, MCSG, LTC, LA, generator, large hv motors, etc.
Partial discharge degradation can be caused by various factors, including inadequate stress regulation, or the presence of voids or delamination in the ground wall insulation.
[1] With the partial discharge measurement, the dielectric condition of high voltage equipment can be evaluated, and electrical treeing in the insulation can be detected and located.
Data collected during partial discharge testing is compared to measurement values of the same cable gathered during the acceptance-test or to factory quality control standards.
This allows simple and quick classification of the dielectric condition (new, strongly aged, faulty) of the device under test and appropriate maintenance and repair measures may be planned and organized in advance.
Partial discharge measurement is routinely carried out to assess the condition of the insulation system of rotating machines (motors and generators), transformers, and gas-insulated switchgear.
An automatic analysis of the reflectograms collected during the partial discharge measurement – using a method referred to as time domain reflectometry (TDR) – allows the location of insulation irregularities.
The calibration unit is quite simple in operation and merely comprises a square wave generator in series with a capacitor connected across the sample.
By necessity field measurements have to be quick, safe and simple if they are to be widely applied by owners and operators of MV and HV assets.
Dr John Reeves established that TEV signals are directly proportional to the condition of the insulation for all switchgear of the same type measured at the same point.
TEV pulses are full of high frequency components and hence the earthed metalwork presents a considerable impedance to ground.
The frequency for emissions is "white" noise in nature and therefore produces ultrasonic structure waves through the solid or liquid filled electrical component.
In paper-insulated high-voltage cables, partial discharges begin as small pinholes penetrating the paper windings that are adjacent to the electrical conductor or outer sheath.
As PD activity progresses, the repetitive discharges eventually cause permanent chemical changes within the affected paper layers and impregnating dielectric fluid.
This places greater stress on the remaining insulation, leading to further growth of the damaged region, resistive heating along the tree, and further charring (sometimes called tracking).
Although the level of PD heating is generally low for DC and power line frequencies, it can accelerate failures within high voltage high-frequency equipment.
To ensure supply reliability and long-term operational sustainability, PD in high-voltage electrical equipment should be monitored closely with early warning signals for inspection and maintenance.
PD prevention and detection are essential to ensure reliable, long-term operation of high voltage equipment used by electric power utilities.
Utilizing UHF couplers and sensors, partial discharge signals are detected and carried to a master control unit where a filtering process is applied to reject interference.
Depending on the provider of the system, the partial discharge outputs are accessible through either a local area network, via modem or even a via web-based viewer.
