Partial Discharge in Switchgear and Substations: Hearing a Breakdown Before It Happens

Insulation doesn't fail suddenly

When a flashover occurs in switchgear, it looks like an instant catastrophe from the outside: an arc, a trip, in the worst case a fire and injuries. In reality, insulation degrades over months and years — through corona, surface tracking and partial discharge, which quietly destroy the dielectric long before the fatal breakdown.

If you learn to detect these early discharge phenomena, an emergency shutdown turns into a planned replacement of the defective component at the next maintenance window. The primary tool for this is ultrasound (and, in some cases, ultraviolet inspection).

Three phenomena to catch

• Corona — ionisation of the air around a conductor under high field strength. Occurs at sharp edges, damaged fittings, poor contacts in outdoor switchyards. Accompanied by hissing and the release of ozone and nitrogen oxides (which in turn degrade insulation and metal).
• Surface tracking — conductive paths forming across contaminated or moist insulator surfaces. Each micro-discharge chars the surface a little more, and the track grows until it bridges the gap.
• Partial discharge (PD) — micro-breakdowns in voids and delaminations inside solid insulation (cable joints, terminations, support insulators). The most dangerous of the three because they occur inside the material and are invisible to the eye.

All three phenomena radiate energy in the ultrasonic range — which is exactly what makes them "audible" to a diagnostic instrument.

Why ultrasound, not just thermal imaging

A thermal camera is an excellent tool, but it only sees a defect once it's already heating up: a poor contact, an overloaded busbar. Corona and partial discharge at an early stage generate almost no heat, but they do generate an immediate ultrasonic signal. That's why ultrasound and thermography complement each other:

Method	What it finds	Stage
Ultrasound	Corona, tracking, partial discharge	Early — before heating begins
Thermal imaging	Poor contacts, overheating, overload	Late — once it's already heating up

Combining ultrasound and thermography during a substation walkdown gives the most complete picture of insulation and contact condition.

How it works in practice

The SDT340, fitted with a directional sensor and parabolic concentrator, lets you "listen" to switchgear, outdoor switchyards, cable assemblies and transformers from a safe distance, without opening live cubicles:

• in the headphones, discharge activity is heard as a characteristic "crackling" or "buzzing" at line frequency;
• the instrument measures signal level numerically (RMS, Peak) and records an audio clip for analysis;
• by the signal's waveform and its relationship to the mains phase, a specialist can distinguish corona, tracking and internal PD;
• results are logged into a trend, and the defect's progression is tracked from one round to the next.

For acoustic imaging — when you need to quickly pinpoint the source of a discharge on a large panel or outdoor switchyard — the SDT SonaVu acoustic camera is used: it overlays the ultrasound source directly on a video image as a "heat" spot, letting the operator instantly see which insulator or contact is "noisy."

Safety first

The main advantage of ultrasonic diagnostics for electrical equipment is non-contact, stand-off inspection. The survey is carried out with equipment energised, without de-energising and without opening cubicles, which:

• eliminates the risk of electric shock to personnel;
• requires no equipment outage;
• allows an entire substation to be surveyed in a single shift.

When working on electrical installations, always follow electrical safety requirements and maintain the mandated clearance distances from live parts.

When to inspect

• On a planned basis — for critical switchgear, substations and cable lines (quarterly/semi-annually).
• When an ozone smell, unusual "crackling," or soot traces on insulators appear.
• After lightning or switching overvoltages.
• As part of a combined walkdown together with thermal imaging.

Related uses of ultrasound

The same instrument that finds partial discharge handles other reliability tasks during the same shift — which makes ultrasound one of the most versatile tools in a maintenance department's toolkit:

• partial discharge in generator winding insulation;
• finding compressed air and gas leaks;
• bearing condition monitoring and metered lubrication;
• steam trap testing;
• early diagnosis of bearing defects and pump cavitation.

Conclusion

Partial discharge, corona and tracking are a "countdown" to insulation breakdown that runs for months. Ultrasound hears that countdown at the earliest possible stage, safely and without taking equipment out of service, and paired with thermal imaging it covers the full spectrum of electrical equipment defects. That turns the risk of an emergency shutdown and fire into a manageable, scheduled task.

KEG TRK supplies SDT ultrasonic instruments and acoustic cameras and trains personnel in electrical equipment inspection methodology. Get in touch — we'll help you build an ultrasonic diagnostics programme at your site.