Partial Discharge in MV Switchgear — Causes, Detection, and Prevention

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.

1. What Is Partial Discharge (PD)?

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, and chemical by-products
• Weakens insulation over time

Left untreated, PD evolves into complete insulation failure, often resulting in an internal arc.

2. Why Partial Discharge Matters in MV Switchgear

PD is responsible for a significant share of MV equipment failures. Key reasons include:

• Progressive insulation degradation

PD erodes insulation layer by layer, gradually reducing dielectric strength.

• Creation of carbonized paths

Over time, PD creates permanent conductive “tracking paths.”

• Increased failure risk under high humidity or load

Moisture, contaminants, or voltage spikes accelerate PD activity.

• Higher outage and maintenance risk

Failures caused by PD are unpredictable and often catastrophic.

• Reduced asset lifetime

Early PD in new installations is one of the biggest challenges in MV networks.

3. Main Causes of Partial Discharge in MV Switchgear

PD can originate from several internal and external factors:

3.1 Voids and imperfections in solid insulation

Manufacturing defects, ageing, or thermal cycling create air pockets that become PD hotspots.

3.2 Contamination (dust, humidity, metallic particles)

Contaminants on insulators or bushings lower surface resistance and encourage discharge.

3.3 Sharp edges or protrusions on conductors

These create local electric-field concentrations (“corona points”).

3.4 Poor cable terminations

Improperly installed heat-shrink or separable connectors are among the most common PD sources.

3.5 Defects in gas insulation

In gas-insulated switchgear (GIS), PD may occur due to:

• Low gas pressure
• High moisture content
• Gas contamination
• Surface defects
• Misalignment of contacts or insulators

3.6 Thermal and mechanical ageing

Temperature cycles and vibrations gradually weaken insulation.

4. Partial Discharge in SF₆ vs SF₆-Free GIS

Understanding differences in gas behaviour is important.

With SF₆

• PD can generate corrosive and toxic decomposition products
• By-products can damage internal components
• Requires strict PPE and decontamination procedures
• Sensitive to leakage and gas density changes

With dry-air insulated GIS

• PD creates only non-toxic by-products
• No risk of HF or SO₂F₂ formation
• Sensitive to leakage and gas density changes
• Safer for personnel
• More predictable gas behaviour
• Lower environmental impact

Modern SF₆-free GIS achieve very high PD withstand performance due to optimized solid insulation.

5. How PD Is Detected: Methods and Technologies

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

5.1 Offline PD Testing (Factory or Commissioning)

• PD measurement according to IEC 60270

Industry-standard test using coupling capacitors and calibrators.

• AC withstand voltage tests

Stress-testing insulation at elevated voltage.

• Routine PD tests

Performed during type and routine testing for GIS to verify uniform quality.

Offline tests ensure that equipment leaves the factory PD-free but cannot detect issues from installation errors.

5.2 Online PD Detection (During Operation)

Increasingly used by utilities for condition-based monitoring.

• UHF sensors (Ultrahigh Frequency)

Detect electromagnetic pulses from PD, ideal for GIS.

• TEV sensors (Transient Earth Voltage)

Used especially in metal-enclosed switchgear.

• HFCT (High-Frequency Current Transformers)

Clamped around cable earth shields to detect PD in terminations.

• Acoustic sensors

Capture sound waves generated by PD events.

• Optical sensors

Useful for visual PD signatures in GIS.

• Continuous monitoring / IoT PD sensors

Provide real-time dashboards and alarm notifications.

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

6. How to Prevent Partial Discharge in MV Switchgear

6.1 High-quality manufacturing

Avoiding insulation imperfections and ensuring tight tolerances reduce PD risk dramatically.

6.2 Clean assembly and installation practices

Dust, moisture, or metal particles during installation are leading causes of PD in field installations.

6.3 Proper cable termination

Most PD failures originate from incorrectly installed terminations:

• Use certified jointers
• Follow torque specifications
• Check insulation stress cones
• Verify seating and alignment

6.4 Monitoring humidity and contamination

Particularly important in GIS based on air or clean gases.

6.5 Maintaining correct gas pressure

Gas density directly influences dielectric performance.

6.6 Regular online PD monitoring

Especially for:

• High-value substations
• Primary GIS
• Industrial or data centre installations
• Areas with vibration or temperature cycling

6.7 Thoughtful design of insulation and electric-field distribution

Modern SF₆-free GIS relies on:

• Advanced epoxy / silicone composites
• Optimized electrode geometry
• Field-grading structures
• Minimized protrusions

These design features greatly reduce the risk of corona inception.

7. Why SF₆-Free GIS Performs Well Against PD

Modern dry-air insulated GIS has several inherent advantages:

• No toxic decomposition by-products

PD events remain non-hazardous for operators.

• Advanced solid insulation technology

GIS designs today rely much more on epoxy/silicone insulators, improving PD withstand.

• More straightforward maintenance

No specialized gas-handling equipment, no moisture traps, no SF₆ analysis.

• Better long-term stability

Dry air and clean gases do not degrade into harmful compounds.

8. Conclusion

Partial discharge is one of the most critical factors influencing the safety, reliability, and lifetime of MV switchgear. Understanding its causes, detecting it early, and designing systems that prevent PD helps avoid costly failures and unplanned outages.

Modern SF₆-free GIS is designed with robust insulation systems, 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 ensure long-term reliability and build substations that are safe, modern, and future-ready.