Introduction to partial discharge
- 1. © Partial Discharge UK 2015 www.partialdischarge.co.uk What is partial discharge? The insulation in high voltage (HV) systems is constantly ageing from thermal and mechanical stresses. It is subject to detrimental external influences such as heat, humidity, chemical and other contaminating agents. Additionally, insulation can contain defects imparted during its manufacture that will not become apparent until it is in service. Incorrectly installed cables, terminations, joints and switchgear can further compound these issues. Predicting the life of HV insulation is very difficult as many of the factors above are impossible to predict and their effects vary from one installation to another. Destructive techniques that require sampling and analysis require plant to be shutdown and can only be done infrequently. Therefore, non-destructive methods of determining the insulation quality are highly desirable, especially in today’s industrial environment where plant shutdowns are minimised or don’t happen at all! As HV insulation fails, localised electrical discharges occur that only partially bridge the insulation in gas filled voids in the bulk of the insulating material, in air on the surface, or at protrusions whose raised profiles intensify the electrostatic field. These localised discharges are known as Partial Discharges (PD). PD can persist for many years, or in some cases rapidly escalate, until catastrophic failure occurs. Fortunately, detection of PD can be accomplished using a variety of non-destructive methods without the need to shutdown plant. PD detection and monitoring is therefore a diagnostic tool that gives a prognosis of insulation condition. Detecting PD levels over time will indicate and pre-warn of the failure of the insulation in HV cables, switchgear, terminations, joints, transformers, motors and generators. Leading research indicates that over 85% of all failures in HV equipment are due to partial discharge. Early detection and monitoring therefore prevents major events and their implications for personnel safety. Because of the high field strengths required to break down air, PD detection is only suitable for HV equipment operating at voltages greater than 3.3kV. Effects of PD PD pulses have a distinct characteristic. Generally, they have a steep rise time (1-2nS) and short duration in the order of 5nS, combined with typical amplitude of 1V. In AC systems, they occur on the positive and negative rising quarter-cycles. Due their short and spiky nature, PD pulses contain high frequency components of several hundred MHz, and are usually accompanied by other energy releases, as summarised below. • Electromagnetic radiation in 300-2000MHz range • Ultrasonic radiation • Audible, heard as a fizzy sound • Localised heat at the source of the PD • Ultra-violet and visible corona 33kV cable damage due to PDEvidence of PD on 11kV Spouts (deposits & tracking) 11kV cable housing
- 2. © Partial Discharge UK 2015 www.partialdischarge.co.uk PD causes chemical changes in the insulating medium. This fact enables PD, as well as other failure modes, to be detected in the insulating oil of transformers and switchgear. Detecting PD PD detection can be accomplished by sensing the various energy releases as outlined above. These methods are detailed below. Energy type Sensing technique Electromagnetic Transient Earth Voltage & UHF reception Acoustic 20kHz ultrasonic & audio microphones Thermal Thermal imaging, hot-spot measurements Light Optical & Ultra-violet Direct electrical methods using high frequency coupling capacitors and current transformers can also be used. These are employed in motors and generators as outlined in IEC60270. Dissolved gas analysis (DGA) of insulating oils can identify a multitude of potential issues in transformers and switchgear. In practice more than one sensing technique will be employed. This is mainly to distinguish between true PD activity and other electrical noise in the vicinity. A brief description of each sensing method follows. Electromagnetic PD activity is accompanied by electromagnetic (radio) waves that propagate from the PD site. These waves induce minute currents in the metal enclosure. Due the high frequency of the pulses, the impedance of the earthed metal is substantial enough to create small voltages. These were discovered in the 1980’s are known as Transient Earth Voltages (TEV). Detection is by the contact of a capacitive probe to the surface of the metal in the form of a handheld detector. TEV is an established method for detecting PD within the insulating medium. PD derived electromagnetic energy can be in excess of 1GHz, although the majority is around 200MHz. This energy is detectable using UHF equipment. Multiple antennas enable time-of- flight techniques to locate the source of PD. Acoustic Both in the audible and ultrasonic ranges. PD activity on the surface of an insulator causes rapid expansion of the nearby air and a consequent pressure wave is generated. Audible as a fizzing sound (rather like bacon frying in fat), and in the 20kHz ultrasonic range. Detection is via a sensitive microphone tuned to 20kHz and converted to audio heard in a set of headphones. This is a useful technique when gaps in the enclosure allow the sound energy to escape. It should be noted that due the complex propagation path, the sound level does not easily relate to the magnitude of the PD. The energy will rebound and attenuate inside the enclosure. Ultrasonic microphones on long leads with flexible goose necks enable detection in difficult locations. Thermal PD causes hot spots due to the high current density at the PD site. Irregular surfaces due to poor installation, contamination or manufacturing defects can lead to lumps and bumps on the insulation. Detection using thermal imaging is possible. However, as most HV gear is metal clad access to the hot spot is difficult and in any case the thermal effect is small in comparison to the overall heat generated. For these reasons, thermal imaging is not practical in most cases.
- 3. © Partial Discharge UK 2015 www.partialdischarge.co.uk Optical methods PD excites photons due to the ionisation and recombination processes during the discharge event. The light intensity depends mainly on the magnitude of the discharge and the insulation medium. The spectrum can be from visible light through ultra-violet to infra-red. PD in air is dominated by the abundance of nitrogen. Consequently, 90% of optical energy generated is in the ultra-violet region with some in the visible range in total darkness. Of course one needs line of sight with the PD site in order for detection. Surveying for partial discharges in HV equipment A regular testing schedule of HV assets provides predictive data on the condition of the insulation. PDUK uses a variety of techniques to detect PD in switchgear, cables, transformers and machines. From initial detection we locate the source and, if necessary, install monitoring equipment to establish the trend of the PD levels. A comprehensive report is provided detailing the PD levels. We use a traffic light system to show any assets requiring attention. Contact us to discuss your PD survey requirements info@partialdischarge.co.uk www.partialdischarge.co.uk