Direct Engineering Answer: Partial Discharge (PD) is the earliest warning indicator of dielectric breakdown in 11kV/33kV switchgear and cables. Non-intrusive online testing with Transient Earth Voltage (TEV) sensors (≥ 30 dBmV alerts) and 40kHz airborne ultrasonic detectors allows 100% live substation inspection with zero production shutdown. TransfoLine conducts certified online PD surveys nationwide.
What is Partial Discharge? (IEC 60270)
A partial discharge is a localized electrical spark that bridges a small portion of the insulation between two conducting electrodes without completely breaking down the entire gap. It occurs in microscopic gas voids, delaminations in solid epoxy, or sharp metallic burrs inside high-voltage equipment.
Each micro-discharge releases a tiny pulse of electrical charge measured in picoCoulombs (pC), along with high-frequency electromagnetic waves, acoustic sound (40 kHz ultrasound), and nitrous gases that corrode surrounding copper and epoxy.
1. The 3 Non-Intrusive Online PD Detection Technologies
- Transient Earth Voltage (TEV) Sensors: Measures high-frequency voltage pulses induced on the exterior steel switchgear enclosure in decibels (dBmV). Levels ≥ 30 dBmV signal severe internal void discharge.
- Airborne Ultrasonic Sensors: Captures 40 kHz acoustic sound generated by surface tracking across dirty bushing insulators and cable terminations without opening panel doors.
- High-Frequency Current Transformers (HFCT): Clamped around 11kV/33kV cable ground shields to detect discharge pulses travelling from defective joints.
Partial Discharge Severity & Action Protocol
| PD Level (TEV / Ultrasonic) | Apparent Charge (pC) | Insulation Condition | Action Protocol |
|---|---|---|---|
| < 15 dBmV / 0 dBμV | < 100 pC | Normal baseline | Standard annual survey. |
| 20 – 29 dBmV / 10–20 dBμV | 500 – 2,500 pC | Early void erosion / surface contamination | Clean insulators, inspect cable heat-shrink boots during next shutdown. |
| ≥ 30 dBmV / > 25 dBμV | > 5,000 pC | Active electrical treeing; imminent puncture | Emergency outage; replace damaged cable termination / CT / insulator. |
How TransfoLine Prevents Switchgear Arc-Flash Explosions
TransfoLine provides non-intrusive online PD diagnostic testing across Pakistan using EA Technology UltraTEV Plus2 and OMICRON MPD systems, distinguishing background electrical noise from genuine void discharges and providing turnkey busbar and termination repairs.
Need Certified Diagnostic Testing or Emergency Support?
TransfoLine's PEC-registered engineers mobilize nationwide within 24 hours with mobile testing vans and complete diagnostic instrumentation.
Consult with a Testing Engineer →Frequently Asked Questions
Can Partial Discharge (PD) testing be performed without shutting down plant power?
Yes. Non-intrusive online PD testing utilizes Transient Earth Voltage (TEV) capacitive sensors and airborne ultrasonic microphones placed against external switchgear panels while the electrical system remains live and fully operational, requiring zero plant downtime.
What is the danger of ignoring partial discharge in 11kV switchgear?
Partial discharge generates localized ozone and nitric acid, chemically corroding solid epoxy insulators. Over months, the microscopic electrical tree bridges the insulation barrier, resulting in a sudden explosive arc-flash that destroys the switchboard.