Partial Discharge Testing in HV Equipment: A Practical Guide

Partial discharge testing in HV equipment is one of the most critical diagnostic tools in modern power systems. It helps identify insulation defects that could lead to catastrophic failures. As the demand for reliable high-voltage (HV) power infrastructure grows, understanding how to conduct effective partial discharge (PD) testing is no longer optional—it’s essential.

This guide provides a practical, engineer-friendly explanation of how partial discharge testing works, why it’s important, how it’s done, and what results mean. Whether you’re an asset manager, field engineer, or testing contractor, this guide will help you master the essentials.

What is Partial Discharge Testing in HV Equipment?

Partial discharge is a localized electrical discharge that does not completely bridge the insulation between conductors. In simple terms, it’s a tiny spark inside insulation material. Over time, this spark erodes the insulation, leading to equipment failure. This can happen in transformers, switchgear, cables, bushings, and even rotating machines.

Partial discharge testing in HV equipment is the process of detecting, measuring, and analyzing these discharges. By detecting PD early, engineers can prevent unexpected breakdowns, extend equipment life, and reduce maintenance costs.

Why is Partial Discharge Testing in HV Equipment So Important?

Partial discharges are silent killers. They rarely trip protection relays or show up in routine visual inspections. However, they cause cumulative damage that leads to insulation breakdown. This can trigger unplanned outages, expensive replacements, and even dangerous fires or explosions.

Here are some key benefits of PD testing in HV equipment:

• Early Fault Detection: Identifies insulation defects long before failure.
• Improved Asset Life: Helps plan timely maintenance or refurbishment.
• Operational Safety: Reduces risk of catastrophic failures in critical HV assets.
• Cost Optimization: Avoids unnecessary shutdowns and emergency repairs.

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How Partial Discharge Develops in High Voltage Systems

Partial discharges occur due to defects in insulation. These defects can be internal (within solid insulation), surface-based (tracking along insulation surfaces), or external (caused by bad connections or contamination).

Common causes include:

• Voids in insulation during manufacturing
• Improper cable terminations
• Moisture ingress in bushings or joints
• Mechanical damage during installation

As discharges continue, they generate heat, ozone, and even sound or light. Eventually, they create carbonized paths through insulation, reducing its dielectric strength and leading to full breakdown.

Types of Partial Discharge in HV Equipment

There are several types of partial discharges, each associated with different insulation defects. Here’s a table summarizing the key types:

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Partial Discharge Testing Methods in HV Equipment

There are multiple ways to test for partial discharge depending on the equipment type and application. Each method offers different insights.

Offline PD Testing

This method requires removing the equipment from service. A test voltage is applied, and PD is measured under controlled conditions.

Pros:

• Accurate and detailed results
• Suitable for factory acceptance tests

Cons:

• Requires shutdown
• Time-consuming and costly for field testing

Online PD Testing

In this method, PD is measured while the equipment remains energized. It’s ideal for condition monitoring in live substations or power plants.

Pros:

• No shutdown required
• Continuous or periodic monitoring possible

Cons:

• Requires signal separation from background noise
• Less precise than offline testing

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Common Testing Techniques

Steps for Conducting Partial Discharge Testing in HV Equipment

Here’s a simplified step-by-step approach to PD testing:

1. Equipment Preparation

• Isolate equipment if doing offline testing
• Clean and inspect all terminals
• Ensure proper grounding and safety measures

2. Sensor Installation

• Install ultrasonic, TEV, or HFCT sensors as applicable
• For UHF tests, use appropriate antenna or couplers

3. Signal Acquisition

• Connect sensors to a data acquisition unit
• Record background noise level before energizing
• Apply test voltage if offline, or record during live operation

4. Data Analysis

• Identify the PD pulse patterns
• Compare pulse magnitude, repetition rate, and phase-resolved patterns
• Use time-domain or frequency-domain analysis tools

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5. Reporting and Recommendations

• Classify PD activity levels (Low, Medium, High)
• Suggest corrective action: immediate repair, scheduled maintenance, or continued monitoring

PD Interpretation: What Do The Results Mean?

Interpreting PD results isn’t always straightforward. A high PD level doesn’t always mean failure is imminent, but it does warrant attention.

Here’s a typical severity classification:

Modern PD analyzers also offer PRPD (Phase Resolved Partial Discharge) plots, helping to pinpoint the discharge location and type.

Partial Discharge Testing in HV Cables

XLPE cables are particularly susceptible to PD due to manufacturing voids, poor terminations, or thermal aging. PD testing can be done during commissioning or periodically during operation.

Technicians typically use VLF (Very Low Frequency) testing combined with PD measurements. This approach is safer for cable insulation and effective at detecting early-stage defects.

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Partial Discharge Testing in Transformers

Transformers are critical HV assets. PD in windings or bushings can cause sudden insulation failure. UHF sensors and acoustic emission methods are commonly used for PD detection.

Online PD monitoring is often installed permanently in large power transformers. It offers real-time alerts and trending, which helps asset managers plan timely interventions.

Challenges in Partial Discharge Testing in HV Equipment

While PD testing is powerful, it has some challenges:

• Noise Interference: In substations, external noise can mask actual PD signals.
• Sensor Placement: Improper sensor location may result in missed signals.
• Skill Requirements: Data interpretation requires expertise in PD patterns.

Despite these challenges, partial discharge testing remains a valuable tool when integrated with other diagnostic methods like tan delta testing, insulation resistance, and thermal imaging.

Future of Partial Discharge Testing in HV Equipment

The future of PD testing lies in AI-powered diagnostic systems. These systems will use machine learning algorithms to analyze PD signals and recommend actions automatically. Integration with SCADA and asset management systems is already happening.

Portable PD testing kits are also becoming more user-friendly, allowing even smaller utilities and industrial facilities to adopt them easily.

Conclusion

Partial discharge testing in HV equipment is more than just a test—it’s a vital part of condition-based maintenance. With the growing reliance on high-voltage assets in energy systems, the need for accurate, timely, and non-invasive diagnostics will only increase.

By understanding how partial discharge works, selecting the right test method, and interpreting the data correctly, electrical engineers and technicians can drastically improve equipment reliability, reduce costs, and enhance safety.

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