Online Partial Discharge Measurement Service: The Ultimate Engineering Guide

Online Partial Discharge (PD) measurement is the cornerstone of modern predictive maintenance for power systems. It is a specialized, non-intrusive service performed on high-voltage (HV) and medium-voltage (MV) electrical assets—such as switchgear, transformers, and cables—while they remain fully energized and under normal operating load.

Unlike traditional testing methods that require costly shutdowns, this technique detects, locates, and analyzes insulation degradation in real-time. For facility managers looking to move beyond simple visual inspections to a comprehensive Electrical System Assessment, online PD monitoring is the most effective tool available.

In the realm of electrical engineering, particularly within industrial plants and utility substations, the ability to assess the reliability of the power grid without interrupting production is a game-changer. This service utilizes advanced sensor technologies to capture electromagnetic and acoustic emissions associated with insulation defects, allowing Certified Level II/III Engineers to identify specific fault types before they escalate into catastrophic equipment failures.

Why Is Online PD Measurement Critical?

Drawing from over 20 years of field experience, I have observed that Partial Discharge (PD) is the leading cause of long-term insulation failure in HV equipment, accounting for approximately 85% of disruptive switchgear failures. PD is a localized electrical discharge that only partially bridges the insulation between conductors. It is a “cancer” within your electrical system—often silent, invisible, and progressively destructive.

By the time PD becomes audible to the naked ear (crackling) or visible (ozone smell/corrosion), the damage is often irreversible, necessitating emergency replacement. Online PD testing services provide early warning signs, often months or years in advance, shifting your asset management strategy from reactive (fixing after failure) to predictive (fixing before failure).

Strategic Benefits for Facility Management

• Zero Downtime (Business Continuity): The primary advantage is the ability to perform a complete health check without isolating the equipment. Testing is conducted during normal operating hours under actual load conditions. This is crucial because PD activity often fluctuates with voltage and thermal stress, which typically mirrors the findings in Motor Monitoring Solutions where load plays a key role.
• Enhanced Personnel Safety: Catastrophic arc flashes often start as undetected PD. By detecting hazardous insulation defects—such as loose connections or degraded cable terminations—without requiring physical contact with live conductors, we significantly reduce the risk of workplace accidents.
• Data-Driven Maintenance Decisions: The service provides quantitative data (amplitude in dB/pC, pulse count per cycle) and qualitative analysis (PRPD patterns). This allows asset managers to prioritize budgets effectively, focusing resources on “Red” status assets while delaying intervention on “Green” assets. This approach is similar to how we manage Energy Monitoring Solutions to optimize efficiency.
• Compliance and Insurance: Regular non-intrusive testing helps meet requirements for Condition-Based Monitoring (CBM) standards, such as IEEE 1434 and guidance from IEC 60270. Many insurance providers now mandate or offer premiums reductions for facilities that implement robust Insulation Monitoring Solutions alongside PD surveys.

Core Measurement Technologies and Sensor Application

A professional online partial discharge measurement service does not rely on a “magic wand.” It utilizes a multi-physics approach, combining different detection technologies to identify specific types of discharge phenomena. A robust survey must distinguish between internal voids, surface tracking, and corona.

1. Transient Earth Voltage (TEV) – For Metal-Clad Switchgear

TEV sensors are the industry standard for detecting internal discharge activity within metal-clad switchgear panels. This is essential for maintaining the integrity of Medium Voltage Cabinet Maintenance Services. The physics behind this is based on the skin effect and electromagnetic wave propagation.

When a discharge occurs in a void within the solid insulation (e.g., epoxy insulators or busbar supports), it generates a rapid electromagnetic pulse. This pulse travels to the inside surface of the metal cladding, inducing a voltage spike that couples capacitively to the outside surface through openings like gaskets or bolt holes. This is the Transient Earth Voltage.

• Target Defects: Internal insulation voids, dry termination faults, breakdown of solid dielectrics in RMUs and switchgear.
• Methodology: The engineer places a magnetic TEV sensor on the outside of the switchgear panel. The magnitude is measured in dBmV.
• Interpretation: A rise of 10dB over the background noise, or readings exceeding 20-29dB, typically indicates developing discharge activity requiring investigation.

2. Airborne Ultrasonic Detection – For Surface Discharge

While TEV detects internal faults, it is effectively blind to surface issues. Surface tracking, arcing, and Corona discharge emit acoustic energy. This technology is often paired with Thermal Scan Electrical Cabinet Services to provide a complete picture of connection health.

Using highly sensitive airborne ultrasonic microphones and parabolic dishes, engineers can “hear” the electrical system leaking. These emissions are in the ultrasonic range (above 20 kHz—typically centered around 40 kHz), which are inaudible to humans.

• Target Defects: Surface tracking on polluted insulators, loose mechanical connections causing arcing, and corona discharge on HV bushings.
• Methodology: Scanning air vents, door gaps (louvers), and cable boxes. The signal is heterodyned (frequency shifted) into the audible range for the engineer to listen via headphones.
• Interpretation:
  • Continuous buzzing: Likely Corona (often humidity-dependent).
  • Crackling/Popping: Likely destructive Surface Tracking (carbonization path).
  • Rhythmic clicking: Mechanical looseness or vibration.

3. High Frequency Current Transformer (HFCT) – For Cables and Grid Assets

The HFCT (also known as RFCT) is the most reliable method for assessing the condition of XLPE (Cross-linked Polyethylene) and PILC cables. It is also a vital technique when assessing grid components like Reclosers and their associated cabling.

Partial discharge pulses act as high-frequency current pulses that travel along the conductor and shield. By clamping a split-core HFCT around the earth strap (ground wire) of the cable termination, we can capture these transient currents.

• Target Defects: Water treeing evolving into electrical trees in XLPE, void discharges in cable joints, and poor workmanship in cable terminations (stress cones).
• Methodology: Non-intrusive clamping on ground wires without disconnecting the ground.
• Value: This is critical for detecting faults in underground cable networks that are otherwise invisible until failure.

The Service Workflow: A Strict Engineering Protocol

Executing a safe and effective Online PD survey requires more than just handheld equipment; it requires a rigorous procedure to ensure data integrity and personnel safety. Below is the standard workflow adopted by top-tier service providers.

Deep Dive: Interpreting the Data (PRPD & Severity)

The raw value (e.g., 25dB or 500pC) is meaningless without context. The core expertise of an Online Partial Discharge Measurement Service lies in the interpretation of the PRPD (Phase Resolved Partial Discharge) pattern.

The PRPD plot visualizes the PD events relative to the 360-degree sine wave of the system voltage. Different defects produce unique “fingerprints”:

• Internal Voids: Typically show symmetrical “rabbit ear” patterns on the rising edges of the positive and negative half-cycles.
• Corona: Appears as a dense cluster of pulses around the peak of the negative half-cycle.
• Surface Tracking: Often presents as asymmetrical, erratic patterns.
• Noise: Usually appears as a flat line across all phases or distinct vertical bars not locked to the sine wave.

Risk Assessment Levels

Based on the amplitude, repetition rate, and defect type, assets are categorized:

• 🟢 Acceptable (Green): No significant PD detected. Readings are within background noise levels. Action: Continue routine annual monitoring.
• 🟡 Investigate (Amber): PD activity detected with stable amplitude. The insulation system is degraded but not in immediate danger. Action: Increase monitoring frequency (e.g., every 3 months) to track the trend. Verify humidity control.
• 🔴 Critical (Red): High-amplitude PD detected with steep repetition rates or clear evidence of arcing. Action: Immediate intervention required. Schedule a planned outage to inspect, clean, or replace the component to prevent an in-service fault.

Comparing Online vs. Offline Testing

A common question from facility managers is: “We already do VLF Tan Delta testing every 3 years; why do we need Online PD?”

Online and Offline testing are complementary, not mutually exclusive. While offline testing is often part of a major Transformer Station Maintenance Service shutdown, online testing fills the gap between these major events.

When Should You Schedule a PD Survey?

While installing permanent monitoring systems is the gold standard for critical assets, periodic service visits are a highly cost-effective alternative for the balance of the plant. Based on industry best practices and the bathtub curve of equipment reliability, we recommend the following schedules:

1. Baseline Survey (Pre-Commissioning): Conduct a survey 24 hours after energizing new equipment. This validates the installation quality, checking for issues like crushed cables or loose busbar bolts before the contractor leaves the site.
2. Before Warranty Expiry: A strategic scan 1-2 months before the manufacturer’s warranty ends allows you to claim replacements for defective assets (like bubbling voltage transformers) at no cost.
3. Annual Health Check: Integrate into your standard Preventive Maintenance program. This is vital for complex systems like Busway Systems where visual inspection is difficult.
4. Aging Assets (>15 Years): Equipment approaching the end of its design life should be tested more frequently (every 6 months) as the insulation breakdown rate accelerates. This applies heavily to Transformer Maintenance cycles.
5. After Transient Events: Following a nearby lightning strike, short circuit fault, or return of power after a long blackout, a PD scan ensures the insulation withstood the stress.

Next Steps: Securing Your Infrastructure

If you are managing a facility with aging switchgear, or if you are experiencing unexplained tripping, flickering lights, or hearing faint crackling sounds in your substation, do not wait for a scheduled shutdown. These are classic symptoms of advanced insulation degradation.

Online partial discharge measurement is the most prudent, non-invasive first step to assessing your risk. It provides the “medical scan” your electrical system needs without the surgery of a shutdown.

Would you like us to recommend a certified PD service provider or help you define the technical scope for your upcoming maintenance tender?