Partial discharge (PD) is an early indication of the degradation of an insulation system for medium and high voltage networks. Insulation breakdown in an electrical power system can lead to high voltage asset damage, unplanned outages and production delays; with the potential for significant financial impact to your business. Luckily, PD can be identified quickly and non-intrusively – whilst equipment is live and in normal operation – allowing targeted HV maintenance and avoiding unnecessary equipment damage and downtime.
What is partial discharge and how can you mitigate the substantial economic and safety risks involved in the degradation of your power system?
What is Partial Discharge?
As defined by IEC 60270, partial discharge is “a localised electrical discharge that only partially bridges the insulation between conductors and which can or can not occur adjacent to a conductor”. Areas of insulation that are exposed to higher concentrations of electric field stress will generally result in the prevalence of PD, however PD can occur anywhere in the insulation. The PD “electron avalanche” will be initiated once the electric field strength exceeds the opposing breakdown strength provided by the insulation.
PD can occur in all insulation mediums including gas, liquid and solid. Insulation voids, cracks and bubbles will likely be the source of the electrical discharge - however, they can also be the result. Periodic and irreparable damage of the insulation system can be caused by partial discharge and therefore PD will become more prevalent and dangerous as time progresses. Once the insulation is fully degraded, flashover will occur.
Identifying Partial Discharge
When partial discharge takes place, small emissions of energy occur. This energy may be in the form of acoustic wave propagation and electromagnetic wave propagation. PD induced acoustic wave propagation will typically be within the range of audible and ultrasonic frequencies. Electromagnetic wave propagation caused by partial discharge will typically be in the low frequency spectrum, including heat and visible light. There are multiple methods of identifying the prevalence of PD through this energy emission, including transient earth voltage (TEV) sensing and ultrasound (US) detection
TEV PD testing can be implemented on switchgear, HV cabling and other grounded metallic systems. If an electric discharge occurs within the insulation between the earth and the phase conductor, a small capacitive charge is transferred from the electrical conductor to the earthed structure. When this process occurs, the induced electromagnetic wave can transmit through the air gap, causing a transient earth voltage (TEV) on the tested surface. Specialised TEV testing equipment can be used to measure these fluctuations.
Ultrasonic testing is used to identify surface discharge, where electrical arcing and tracking occurs on the degrading insulation system. The electrical current incorporated in arcing and tracking ionises the surrounding air, greatly increasing the air molecules’ ambient temperature. The rapidly expanding air molecules will cause sonic vibrations that can be heard audibly through ultrasonic PD equipment as a crackling or popping, with the energy intensity measured quantitatively.
Why should you conduct partial discharge testing?
PD testing can be easily integrated into standard maintenance routines as online partial discharge testing is non-intrusive and quickly applied, therefore not requiring a shutdown. PD testing can indicate the prevalence of partial discharge in an insulation system before the insulation has been deteriorated to a critical level. Allowing HV and MV insulation to degrade unmonitored can have sudden and catastrophic effects.
How often should you conduct partial discharge testing?
As partial discharge is the result of insulation breakdown, while also being a cause of insulation breakdown, the intensity and prevalence will increase over time. Generally, partial discharge testing should be conducted every 12 months. This frequency may increase depending on the equipment age and criticality to the site. Only once partial discharge is detected, are additional offline intrusive inspections recommended to determine the extent of the PD damage.
Online PD testing can be undertaken without shutdown and is an extremely important and cost-effective maintenance routine that can prevent unnecessary safety issues and financial losses.
PCE currently operate specialised TEV and US PD monitoring equipment for routine PD testing. If you would like more information on partial discharge or to discuss your site requirements around PD testing, contact Power Control Engineers.