What is a PD Phase Plot Telling You?

A phase plot is a critical tool for understanding partial discharge (PD) in high voltage (HV) insulation systems. In this article, we'll explore how phase plots, also known as phase-resolved partial discharge (PRPD) patterns, are created and used to diagnose potential defects in HV assets.

What is a Phase Plot?

A phase plot or PRPD pattern is a visual representation of partial discharge activity over the 360 degrees of an AC cycle. These plots are essential for identifying and understanding defects within or on the surface of HV insulation systems. Common defects include voids, gaps, splits, sharp points, and other imperfections in HV assets. When such a defect is present, it discharges because it cannot insulate the voltage applied across it.

For example, an internal void defect within a current transformer’s cast resin insulation can create a gap where sparks continuously jump across. Each ‘dot’ in the phase plot represents a discharge event, such as a spark caused by the defect.

How to Interpret a Phase Plot

The PRPD plot illustrates the amplitude of each discharge event (vertical axis) against its phase angle (horizontal axis). In this context, amplitude is typically measured in dBmV using a Transient Earth Voltage (TEV) sensor, and the data displayed reflects 10 seconds of activity at the 50Hz system frequency.

As the primary voltage in a power system fluctuates, the voltage across each defect also changes, causing the defect to discharge at specific times and amplitudes. This variation is what creates the distinct patterns seen in a phase plot.

Each PRPD pattern is unique to the specific defect's type and physical characteristics. However, there are common trends that can help identify the nature of the defect.

Common Defect Types Identified by Phase Plots

Phase plots can reveal various types of defects, including:

• Corona Discharge
• Internal Voids
• Surface Discharges
• Poor Contact Issues
• Floating Metal Work

By analysing PRPD patterns, it's possible to determine the severity and type of discharge and whether the activity is single-phase, two-phase, or three-phase.

Here are some commonly seen PRPD patterns and their defect types:

Electromagnetic testing

Ultrasonic testing

Practical Applications of Phase Plots

In my experience with EA Technology's HV technical services team, phase plots are invaluable for analysing and diagnosing partial discharge activity within HV insulation systems. Understanding these patterns helps in maintaining the reliability of HV assets and preventing failures.

For ongoing insights and monitoring, explore our partial discharge monitoring services to ensure your HV assets are consistently maintained and protected.

Enhance Your Analysis with Wireless Phase Reference

EA Technology's newly launched Wireless Phase Reference accessory allows you to achieve an accurate phase lock in any environment, making partial discharge data analysis more straightforward and precise. Learn more here.