Partial Discharge in MV Switchgear: Causes & Detection

Partial discharge (PD) is one of the most important indicators of insulation health in medium-voltage (MV) switchgear. If undetected, PD can lead to insulation breakdown, internal arc faults, prolonged outages and costly equipment replacement. As networks modernize, and as utilities adopt SF₆-free technologies, understanding PD behaviour is crucial for long-term asset performance. This article explains what partial discharge is, what causes it, how to detect it, and which design features help prevent PD in modern MV GIS.

What Is Partial Discharge (PD)?

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Partial discharge is a localized electrical breakdown occurring within or across insulation that does not completely bridge the electrodes. It happens when the electric field exceeds the dielectric strength of a defect or void. Typical characteristics:

• Occurs in microscopic voids, cracks or surface defects

• Can be intermittent or continuous

• Generates electromagnetic emissions, heat, light, ozone and other chemical by-products

• Weakens insulation over time

There are different types of PD. Internal PD develops inside voids in solid insulation, surface PD tracks across an interface, and corona forms at sharp points in air or gas. Left untreated, any of these electrical discharge mechanisms can evolve into complete insulation failure, often resulting in an internal arc.

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Why Partial Discharge Matters in MV Switchgear

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PD is responsible for a significant share of MV equipment failures. It erodes insulation layer by layer, gradually reducing dielectric strength, and over time creates permanent carbonized tracking paths. Moisture, contaminants or voltage spikes accelerate PD activity, so discharge activity tends to rise under high humidity or heavy load. The resulting failures are unpredictable and often catastrophic, causing outages, unplanned downtime and reduced asset lifetime. Early PD in new installations is one of the biggest challenges in MV networks.

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Main Causes of Partial Discharge in MV Switchgear

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PD can originate from several internal and external factors:

• Voids and imperfections in solid insulation: manufacturing defects, ageing or thermal cycling create air pockets that become PD hotspots.

• Contamination (dust, humidity, metallic particles): contaminants on insulators or bushings lower surface resistance and encourage discharge.

• Sharp edges or protrusions on conductors: these create local electric-field concentrations, the classic corona points.

• Poor cable terminations: improperly installed heat-shrink or separable connectors are among the most common PD sources.

• Defects in gas insulation: in gas-insulated switchgear (GIS), PD may occur due to low gas pressure, high moisture content, gas contamination, surface defects or misalignment of contacts and insulators.

• Thermal and mechanical ageing: temperature cycles and vibrations gradually deteriorate insulation.

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Partial Discharge in SF₆ vs SF₆-Free GIS

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Gas behaviour under PD differs significantly. With SF₆ partial discharge can generate corrosive and toxic decomposition products (such as HF or SO₂F₂) that damage internal components, require strict PPE and decontamination procedures, and are sensitive to leakage and gas density changes. With dry-air insulated GIS, PD creates only non-toxic by-products, with no HF or SO₂F₂ formation, which is safer for personnel and gives more predictable gas behaviour and lower environmental impact. Tightness still matters for dielectric strength, but any leakage is non-hazardous. Modern SF₆-free GIS achieves very high PD withstand performance thanks to optimized solid insulation.

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How PD Is Detected: Methods and Technologies

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PD detection falls into two categories: offline (equipment de-energized) and online (equipment energized).

Offline PD Testing (Factory or Commissioning)

Offline methods include the PD measurement according to IEC 60270, the industry-standard test using coupling capacitors and calibrators, AC withstand voltage tests that stress the insulation at elevated voltage, and routine PD tests performed during type and routine testing for GIS to verify uniform quality. Offline tests and PD measurements ensure that equipment leaves the factory PD-free, but they cannot detect issues introduced by installation errors.

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Online PD Detection (During Operation)

Online monitoring is increasingly used by utilities for condition-based maintenance. Common partial discharge sensors and techniques include:

• UHF sensors (ultrahigh frequency): detect the electromagnetic pulses from a PD signal, ideal for GIS.

• TEV sensors (transient earth voltage): a non-intrusive method used especially in metal-enclosed switchgear, applicable on-site during a routine PD survey.

• HFCT (high-frequency current transformers): clamped around cable earth shields to detect PD in terminations.

• Acoustic and ultrasonic sensors: an airborne ultrasonic microphone captures the sound waves generated by PD events, another non-intrusive option.

• Optical sensors: useful for visual PD signatures in GIS.

• Continuous and permanent monitoring: IoT PD sensors, often combined with temperature sensors, feed dashboards with real-time data analysis, software alarms and fault detection.

Online PD monitoring is critical for GIS installed in hard-to-access locations, such as basements, offshore substations, tunnels and data centres.

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How to Prevent Partial Discharge in MV Switchgear

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Prevention combines manufacturing quality, clean installation and ongoing monitoring:

• High-quality manufacturing with tight tolerances, avoiding insulation imperfections.

• Clean assembly and installation practices, since dust, moisture or metal particles are leading causes of PD in the field.

• Proper cable termination: use certified jointers, follow torque specifications, check insulation stress cones and verify seating and alignment.

• Monitoring humidity and contamination, particularly important in GIS based on air or clean gases.

• Maintaining correct gas pressure, because gas density directly influences dielectric performance.

• Regular online PD monitoring as part of a preventive maintenance program, especially for high-value substations, primary GIS, industrial or data centre installations, and areas with vibration or temperature cycling.

• Thoughtful design of insulation and electric-field distribution, using advanced epoxy and silicone composites, optimized electrode geometry, field-grading structures and minimized protrusions to reduce the risk of corona inception.

Together, proactive maintenance and good design turn PD from a hidden risk into a managed one, and routine maintenance intervals can be planned around real condition data rather than fixed schedules.

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Why SF₆-Free GIS Performs Well Against PD

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Modern dry-air insulated GIS has several inherent advantages. PD events remain non-hazardous for operators because there are no toxic decomposition by-products. Advanced solid insulation technology, relying more on epoxy and silicone insulators, improves PD withstand. Maintenance is more straightforward, with no specialized gas-handling equipment, no moisture traps and no SF₆ analysis. And because dry air and clean gases do not degrade into harmful compounds, long-term stability is better.

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The Role of PD Control in Reliable MV Switchgear

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Partial discharge is one of the most critical factors influencing the safety, reliability and lifetime of MV switchgear. Understanding its causes, detecting it early with the right sensors, and designing systems that prevent PD helps avoid costly equipment failure and unplanned outages. Modern SF₆-free GIS is designed with robust insulation systems and optimized conductor geometry, enabling excellent PD performance while eliminating the environmental and safety risks of SF₆. By combining strong design principles with proper installation and ongoing monitoring, utilities and EPCs can build substations that are safe, modern and future-ready.