1. What Is Partial Discharge?

Partial discharges (PD) are small, localised breakdowns in the insulation in HV equipment. PD does not span the whole insulation, hence ‘partial’. PD activity is an indicator of issues inside and outside the insulation. Testing for PD activity provides an early warning of a potential equipment failure.

2 . What Causes Partial Discharge?

PD can be caused by one or a combination of factors.

During manufacture – Solid insulators are designed to give an even distri bution of electrical stress between the conducting electrodes. However, in practice, defects can arise during manufacture that give rise to small cavities or voids in the insulation bulk.

Equipment installation – When electrical equipment is factory assemb led or installed on site, mistakes can be made that either damage and therefore weaken the insulation or cause increased electrical stress across the insulation.

Ageing and deterioration – Most insulating materials naturally deteriorate with age as interna l chemical bonds break down. This process makes the insulation weaker and less durable when withstanding the electrical stresses arising under normal working conditions.

Over stressed in – service – A short circuit fault or lightening impulse is likely to im pose stress on the insulation due to a fault current or an overvoltage. Although such events usually have a short duration, the increased electrical stress or heating from the curre nt overload can cause permanent damage to the insulation.

In – service damage – Electrical equipment can be physically damaged whilst in service due to external factors. Underground cables are particularly susceptible to third party damage, by, for instance road works near buried cables or the cumulative effect of he avy vehicles pa ssing over them.

3 . Why Survey For Partial Discharge?

The effects of PD can be highly detrimental to the insulation medium. PD changes the nature of the insulation which gets increasingly worse over time. Left unchecked, the insulation can become unable to withstand the stress o f high voltage and a complete failure occurs. This final breakdown is catastrophic causing large scale damage to equipment and a danger to personnel. It is accepted that over 80% of sub – station failures are caused by PD. These failures are entirely preventable by regular surveying for PD.

4. Do I Need To Interrupt My Supply?

Different equipment requires different methods. For the purpose of an initial PD survey, we use a handheld switchgear PD detector that requires the equipment to be energised, so no outage is required.

If PD is detected a more in – depth study would be required.