Partial discharge (PD) is one of the most insidious failure mechanisms in medium-voltage (MV) switchgear, responsible for up to 85% of substation equipment failures according to industry surveys. Unlike catastrophic faults that announce themselves with dramatic arcs and tripped breakers, partial discharge is a silent, progressive degradation process that can persist undetected for months or years before culminating in a complete insulation breakdown. For utilities, industrial facilities, and data center operators who depend on uninterrupted power delivery, understanding and detecting PD is no longer optional — it is a fundamental requirement of modern asset management.
What Is Partial Discharge?
Partial discharge is a localized dielectric breakdown of a small portion of a solid or liquid electrical insulation system under high-voltage stress. Unlike a complete breakdown that bridges the entire gap between conductors, PD only partially bridges the insulation. This distinction is critical: PD does not trip protective relays, yet each discharge event incrementally erodes the insulation material, creating carbonized tracks — known as electrical trees — that grow over time until they eventually bridge the full insulation gap and trigger a catastrophic flashover.
The physical mechanism is straightforward. Within every insulation system — whether gas (SF₆, air), liquid (transformer oil), or solid (epoxy resin, XLPE, mica) — microscopic voids, cavities, or contaminants inevitably exist due to manufacturing imperfections, installation stress, thermal cycling, or mechanical vibration. When the electric field intensity across such a void exceeds the breakdown strength of the gas trapped inside (approximately 3 kV/mm for air at atmospheric pressure), a spark occurs within the cavity. This spark typically lasts only nanoseconds to microseconds, with apparent charge magnitudes ranging from less than 1 pC (picocoulomb) to several thousand pC.
PD Types in Switchgear
Internal Discharges: These occur within cavities or voids inside solid insulation materials such as epoxy bushings, CT/PT encapsulation, and cable terminations. Internal PD is particularly dangerous because it is completely invisible from the outside and produces no audible noise. The discharge magnitude tends to be relatively stable but increases gradually as the cavity walls carbonize, reducing the effective insulation thickness.
Surface Discharges: Occurring along the interface between insulation and air — typically on busbar support insulators, cable terminations, and spout insulators — surface discharges result from contamination, moisture ingress, or inadequate creepage distance. The presence of humidity dramatically accelerates surface PD by reducing the surface resistivity of insulation materials, sometimes by orders of magnitude.
Corona Discharges: Corona occurs at sharp metallic points or edges where the local electric field is intensified beyond the dielectric strength of the surrounding gas. In switchgear, common corona sites include bolt heads, busbar edges, and poorly finished conductor surfaces. Corona is often the earliest warning sign of assembly defects and can be detected optically (visible glow in darkness) in addition to electromagnetic methods.
Floating Electrode Discharges: When a metal component loses its intended ground connection — for example, a loose shield, an improperly bonded instrument transformer housing, or a disconnected grading ring — it assumes a floating potential determined by capacitive coupling to adjacent energized conductors. The voltage across the floating gap can reach several kilovolts, resulting in discharges of very high apparent charge (often >10,000 pC) that are among the most destructive PD types.
TEV and Acoustic Emission: The Dual-Detection Approach
Modern field-deployable PD detection for MV switchgear relies on two complementary physical phenomena: Transient Earth Voltage (TEV) and Acoustic Emission (AE). Neither method alone provides complete coverage; together they form a robust detection system capable of identifying all four PD types described above.
Transient Earth Voltage (TEV): When a partial discharge occurs inside switchgear, the electromagnetic wave propagates outward from the discharge site. As this wave encounters the metal enclosure, a tiny voltage pulse — typically in the millivolt range — appears on the outer surface of the grounded metalwork. This is the TEV signal. Because the switchgear enclosure acts as a Faraday cage, the TEV signal on the exterior surface carries information about discharge activity inside without requiring any invasive sensors. TEV sensors are non-intrusive, magnetically attached to the switchgear panel surface, and respond to the rapid voltage transients in the frequency band of 3-100 MHz. TEV is particularly effective at detecting internal discharges and corona, which generate strong electromagnetic pulses.
Acoustic Emission (AE): Every partial discharge event is accompanied by a rapid release of energy that generates an acoustic pressure wave in the surrounding medium. In air-insulated switchgear, this manifests as an ultrasonic signal in the 20-100 kHz range — well above human hearing but easily detected by piezoelectric AE sensors. The key advantage of AE is that it detects surface discharges and floating electrode discharges with high sensitivity, as these PD types produce stronger acoustic signatures than internal discharges which are acoustically damped by the solid insulation. AE sensors are typically mounted inside the switchgear compartment or on cable compartment walls.
The integrated PD monitoring systems offered through Qingdao Britop’s power grid monitoring solutions combine both TEV and AE sensing in a single platform, processing data through edge-computing algorithms that correlate TEV amplitude with AE intensity to classify PD type, assess severity, and filter out external noise sources such as radio interference or mechanical vibration.
Why Continuous Online Monitoring Matters
Traditional PD testing — using handheld TEV detectors or ultrasonic probes during scheduled maintenance outages — suffers from a fundamental limitation: it provides a single snapshot of PD activity at one point in time. PD, however, is a dynamic phenomenon. Discharge activity can fluctuate dramatically with temperature, humidity, and load current. A switchgear panel that tests clean during a cool morning inspection may become dangerously active during afternoon peak load when conductor heating causes thermal expansion that opens cavities in aged epoxy insulation.
PD Severity Assessment and Risk Classification
Not all PD is created equal. A small, stable internal discharge of 50 pC in a well-designed epoxy bushing may be acceptable for continued operation with periodic monitoring, while a rapidly growing 200 pC surface discharge on a contaminated busbar insulator demands immediate attention. Modern PD monitoring systems incorporate multi-parameter severity assessment algorithms that consider:
- Apparent charge magnitude — the primary indicator of discharge energy
- Pulse repetition rate — how frequently discharges occur (pulses per second or per cycle)
- Phase-resolved patterns — the relationship between discharge occurrence and the 50/60 Hz power frequency cycle
- Trend analysis — rate of change over days, weeks, and months
- Environmental correlation — relationship between PD activity and temperature/humidity
The DT801 intelligent monitoring device exemplifies this approach for surge arrester applications, employing zero-flux current sensors to monitor both total leakage current and resistive current with precision down to 5 μA — comparable sensitivity to the best laboratory-grade PD measurement systems.
Conclusion
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