Partial discharge (PD) and tan-delta testing on MV cables — finding a developing defect before it causes a failure

The guide on cable fault location with the Murray bridge and the guide on cable fault location with TDR cover how the location of an already-occurred cable fault is determined. This article covers a fundamentally different kind of diagnostics: techniques that detect a developing insulation defect, before it leads to an actual fault.

What is a partial discharge?

A partial discharge (PD) is a small electrical discharge that occurs within part of the insulation — for example in a gas-filled void, around a contamination site, or at a sharp field concentration — without the discharge bridging the full insulation between conductor and earth (hence "partial"). Such voids and field concentrations occur more often in manually made cable joints and terminations (see the related guide) than in factory-extruded cable material, which is why joints and terminations are often the most likely location of a PD source. Over time, sustained PD activity gradually degrades the insulation (electrical treeing), which can eventually lead to a complete breakdown.

PD measurement per IEC 60270

IEC 60270 describes the standardised measurement method for partial discharges: via a capacitive coupling to the high-voltage circuit, with a blocking impedance, the apparent charge of each discharge is measured, expressed in picocoulombs (pC). This measurement can be carried out either offline (the cable is de-energised and fed separately, typically from a VLF source) or online (the cable stays in service, using non-invasive sensors such as a high-frequency current transformer, HFCT); with the right measurement setup, the location of the PD source along the cable can also be determined via time-difference analysis.

VLF tan-delta: a complementary, bulk-oriented indicator

Alongside PD measurement, tan-delta (dissipation factor) is often also measured, typically using a very-low-frequency (VLF, usually around 0.1 Hz) voltage source. Tan-delta assesses the overall ageing of the insulation as a whole, rather than individual defect locations: the measurement is typically carried out at several voltage levels (for example 0.5×U0, 1.0×U0 and 1.5×U0, where U0 is the rated phase-to-earth voltage), and a clear increase in tan-delta with increasing voltage (the "tip-up") indicates the onset of PD activity in the insulation. PD and tan-delta measurement are therefore often combined: tan-delta gives an overall condition picture of the cable, while PD measurement can localise a specific defect.

Why this is predictive rather than reactive

The essential difference from the Murray bridge and TDR methods is sequence: those methods are used after a cable has already failed, to quickly find the fault location so that digging or repair can begin. PD and tan-delta measurements are instead carried out before a fault occurs, as periodic diagnostics on a cable that is still fully functional, to detect a developing defect and address it in a targeted way (for example at a specific joint) or plan for replacement, before that defect leads to an unexpected outage.

Note: interpreting a measured PD level (in pC) as "concerning" or "acceptable" strongly depends on the cable type, voltage class and measurement setup, and requires experience and reference data; there is no absolute pC threshold that applies equally to every cable and every measurement method.

Why a trend outweighs a single measurement

A single, isolated PD measurement showing an elevated value does not automatically mean an acute risk of failure — some defects develop over years, others faster. A rising trend across successive, periodic measurements of the same cable joint or the same cable route is a significantly stronger indicator of a genuinely progressing defect than a single snapshot, and forms the basis for a predictive maintenance programme on medium-voltage cables.

Practical relevance

For periodic condition assessment of a medium-voltage cable network, it is worthwhile applying PD and/or tan-delta measurements as a supplement to the regular voltage-withstand testing, particularly on older cable routes or at locations with known joints and terminations, and to record the results systematically so that a trend across successive measurements becomes visible instead of assessing each measurement in isolation.

Common mistakes

Confusing PD measurement with cable fault location (Murray bridge/TDR) — PD measurement detects a developing defect that has not yet caused a fault; Murray bridge and TDR locate an already-occurred fault.
Carrying out PD or tan-delta measurements without recording a baseline for later trend comparison — without reference data, a later measurement is difficult to interpret.
Treating a single elevated PD reading on its own as proof of an acute risk of failure — a rising trend across multiple measurements is a significantly stronger indicator than a single reading.
Not distinguishing between online and offline measurement capability when planning a measurement — a cable that cannot be de-energised requires an online measurement technique (for example HFCT), not a VLF offline setup.