Think you know PD? Test your PD knowledge!
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Start Quiz14 March 2023
Brad Monaghan
Firstly, what is a phase plot - also known as phase resolved partial discharge (PRPD) pattern?
A PRPD pattern is a visual representation of partial discharge (PD) activity relative to the 360 degrees of an AC cycle. PD activity will be caused by some sort of defect within, or on the surface of, a high voltage (HV) insulation system. Defects can include voids, gaps, splits, physical sharp points, imperfections etc on/within HV assets. The defect itself discharges as it is not able to insulate the voltage applied across it.
The above example is caused by an ‘internal void’ defect within a current transformer where the cast resin insulation system contains a gap – sparks are constantly jumping across that gap. Each ‘dot’ within the visual pattern is an event (in this instance a spark) which was caused by the discharging defect.
How to interpret a PRPD pattern
The PRPD plot shows the amplitude of each discharge event (vertical axis) plotted against their phase angle (horizontal axis). In this instance, the amplitude is measured in dBmV using a Transient Earth Voltage (TEV) sensor and 10 seconds of activity is displayed relative to the 50Hz system frequency.
As the primary voltage on a power system rises and falls over time, the voltage applied across each defect also rises and falls over time – causing the defect to discharge only at certain times and amplitudes. This is what creates the PRPD pattern.
Each PRPD pattern will be unique as each discharging defect will be of a certain type and physical shape. That being said, common trends can still be seen in each defect type.
Common defect types
These defect types include corona, internal void, surface, poor contact, floating metal work etc. Through PRPD patterns, we are able to identify the severity and type of discharge, also whether it is single phase, two phase or three phase activity.
Here are some commonly seen PRPD patterns and their defect types:
Electromagnetic testing
Ultrasonic testing
Through my role in EA Technology's HV technical services team, PRPD patterns are the most useful tool in analysing and diagnosing PD activity within HV insulation systems.