Partial discharge

The equivalent circuit of a dielectric incorporating a cavity can be modeled as a capacitive voltage divider in parallel with another capacitor. The upper capacitor of the divider represents the parallel combination of the capacitances in series with the void and the lower capacitor represents the capacitance of the void. The parallel capacitor represents the remaining unvoided capacitance of the sample.

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. The PD happens near the peak voltage both positive and negative. PD pulses are easy to measure using the high frequency current transducer (HFCT) method. The current transducer is clamped around the case ground of the component being tested. The severity of the PD is measured by measuring the burst interval between the end of a burst and the beginning of the next burst. As the insulation breakdown worsens, the burst interval will shorten due to the breakdown happening at lower voltages. This burst interval will continue to shorten until a critical 2 millisecond point is reached. At this 2 ms point, the discharge is very close to the zero crossing and will fail with a full blown discharge and major failure. The HFCT method needs to be used because of the small magnitude and short duration of these PD events. The HFCT method is done while the component being tested stays energized and loaded. It is completely non-intrusive. 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. Detection of the high-frequency pulses will identify the existence of partial discharge, arcing or sparking. After PD or arcing is detected, the next step is to locate the fault area. Using the acoustic emission method (AE), 4 or more AE sensors are placed on the transformer shell where the AE and HFCT wavedata is collected at the same time. Bandpass filtering is used to eliminate interference from system noises.

A partial discharge measurement system basically consists of:

• a cable or other object being tested
• a coupling capacitor of low inductance design
• a high-voltage supply with low background noise
• high-voltage connections
• a high voltage filter to reduce background noise from the power supply
• a partial discharge detector
• PC software for analysis

A partial discharge detection system for in-service, energized electric power equipment:

• a cable, transformer, or any MV/HV power equipment
• Ultra High Frequency Sensor (UHF) Detection Bandwidth 300 MHz-1.5GHz
• High Frequency Current Transformer (HFCT) Bandwidth 500 kHz-50 MHz
• Ultrasonic microphone with center frequency 40 kHz
• Acoustic Contact Sensor with detection bandwidth 20 kHz - 300 kHz
• TEV sensor or coupling capacitor 3 MHz-100 MHz
• Phase-resolved analysis system to compare pulse timing to AC frequency

A number of discharge detection schemes and partial discharge measurement methods have been invented since the importance of PD was realized in the late 20th century. Partial discharge currents tend to be of short duration and have rise times in the nanosecond realm. On an oscilloscope, the discharges appear as evenly spaced burst events that occur at the peak of the sinewave. Random events are arcing or sparking. The usual way of quantifying partial discharge magnitude is in picocoulombs. The intensity of partial discharge is displayed versus time.

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. They are displayed in a partial discharge mapping format.

A phase-related depiction of the partial discharges provides additional information, useful for the evaluation of the device under test.

The actual charge change that occurs due to a PD event is not directly measurable, therefore, apparent charge is used instead. The apparent charge (q) of a PD event is the charge that, if injected between the terminals of the device under test, would change the voltage across the terminals by an amount equivalent to the PD event. This can be modeled by the equation:

${\displaystyle q=C_{b}\Delta (V_{c})}$

Apparent charge is not equal to the actual amount of changing charge at the PD site, but can be directly measured and calibrated. 'Apparent charge' is usually expressed in picocoulombs.

This is measured by calibrating the voltage of the spikes against the voltages obtained from a calibration unit discharged into the measuring instrument. The calibration unit is quite simple in operation and merely comprises a square wave generator in series with a capacitor connected across the sample. Usually these are triggered optically to enable calibration without entering a dangerous high voltage area. Calibrators are usually disconnected during the discharge testing.

• Wideband PD detection circuits

  In wideband detection, the impedance usually comprises a low Q parallel-resonant RLC circuit. This circuit tends to attenuate the exciting voltage (usually between 50 and 60 Hz) and amplify the voltage generated due to the discharges.

• Tuned (narrow band) detection circuits
• Differential discharge bridge methods
• Acoustic and Ultrasonic methods

Field measurements preclude the use of a Faraday cage and the energising supply can also be a compromise from the ideal. Field measurements are therefore prone to noise and may be consequently less sensitive.[1][2]

Factory quality PD tests in the field require equipment that may not be readily available, therefore other methods have been developed for field measurement which, while not as sensitive or accurate as standardized measurements, are substantially more convenient. 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.

Transient Earth Voltages (TEVs) are induced voltage spikes on the surface of the surrounding metalwork. TEVs were first discovered in 1974 by Dr John Reeves[3] of EA Technology. TEVs occur because the partial discharge creates current spikes in the conductor and hence also in the earthed metal surrounding the conductor. 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 readings are measured in dBmV. TEV pulses are full of high frequency components and hence the earthed metalwork presents a considerable impedance to ground. Therefore, voltage spikes are generated. These will stay on the inner surface of surrounding metalwork (to a depth of approximately 0.5 µm in mild steel at 100 MHz) and loop around to the outer surface wherever there is an electrical discontinuity in the metalwork. There is a secondary effect whereby electromagnetic waves generated by the partial discharge also generate TEVs on the surrounding metalwork – the surrounding metalwork acting like an antenna. TEVs are a very convenient phenomenon for measuring and detecting partial discharges as they can be detected without making an electrical connection or removing any panels. While this method may be useful to detect some issues in switchgear and surface tracking on internal components, the sensitivity is not likely to be sufficient to detect issues within solid dielectric cable systems.

Ultrasonic measurement relies on fact that the partial discharge will emit sound waves. The frequency for emissions is "white" noise in nature and therefore produces ultrasonic structure waves through the solid or liquid filled electrical component. Using a structure borne ultrasonic sensor on the exterior of the item under examination, internal partial discharge can be detected and located when the sensor is placed closest to the source.

HFCT Method This method is ideal for detecting and determining the severity of the PD by burst interval measurement. The closer the bursts get to "zero voltage crossing" the more severe and critical the PD fault is. Location of the fault area is accomplished using AE described above.

Electro Magnetic Field detection picks up the radio waves generated by the partial discharge. As noted before the radio waves can generate TEVs on the surrounding metalwork. More sensitive measurement, particularly at higher voltages, can be achieved using in built UHF antennas or external antenna mounted on insulating spacers in the surrounding metalwork.

Directional Coupler detection picks up the signals emanating from a partial discharge. This method is ideal for joints and accessories, with the sensors being located on the semicon layers at the joint or accessory.[4]