Partial discharge is the one of most failure maker in electric distribution system. IEC 60270 defines the term partial discharge as: “a localized electrical discharge that only partially bridges the insulation between conductors and which can or can not occur adjacent to a conductor”. In general PD occurs in areas of the insulation or on the surface of the insulation that is subjected to higher local electric field stress concentrations, such as in a cavity in a solid dielectric, a sharp protrusion, or in low density regions of a liquid dielectric. PD initiates once the electric field is high enough, or the mean-free path is short enough, to cause impact ionization to create a “streamer”, otherwise known as an electron avalanche. Furthermore, a free electron must be present in order to start the discharge process, which can be generated by cosmic rays or naturally occurring radiation. Due to this, PD initiation is subject to a statistical time lag. The figures below show some typical sources of PD in air (e.g., insulators, outer cable terminations), liquid (e.g., transformers), and solid insulation (e.g., cable systems).
Defects in medium and high voltage systems not only require costly repairs, but can lead to failures of entire network sections with corresponding consequences. Therefore, it is in the interest of all network operators to recognize signs of impending defects as early as possible and to be able to take appropriate countermeasures on time.
With the Partial Discharge Scan & Analyzer (PD Scan), offers a lightweight, robust and portable PD scanner that can quickly inspect various type of equipment for partial discharge signals.
The primary task of the PD scan is to survey MV switchgear and check for potential problems such as defects in terminations, current and voltage transformers, bushings or bus bars. As each type of switchgear differs it needs different type of sensors to couple out the partial discharge signals. For enclosed switchgear it is recommend to use a combination of TEV and acoustic contact sensors, whereas for air insulated switchgear a flexible acoustic sensor is recommended.
Bus bars are critical components within the substation, if these fail it can cause massive damage, high costs and long outage times. Using the parabolic receiver these bus bars can be surveyed for partial discharge activity. If these bus bars are encapsulated then other sensors such as TEV and acoustic contact probe are recommended.
Bushings within MV substations can be within the switchgear or in larger plants also be in walls or ceilings. Especially the latter is common for older type of plants. These bushings are often used with bus bars, so same criticality applies. A breakdown of one of the bushing can cause therefore massive damage and long outage times. Wall and ceiling bushings can be analyzed using the parabolic acoustic receiver, bushings within enclosed switchgear via TEV and acoustic contact probe sensor.
Using the HFCT sensor partial discharges in medium-voltage cables can be detected, making the PD scan the ideal pre-screening tool for offline PD measurements. Resources can be allocated on those cables which have the highest priority for offline PD measurements only. For commissioning tests on new cable circuits still an offline PD diagnosis is recommended as higher test voltages than operating voltage is needed to check the quality of workmanship.
Also distribution transformers can be analysed with the PD scan. Applying the flexible acoustic receiver or parabolic acoustic receiver, terminations and bushings can be scanned for PD activity and countermeasures can be taken if PD activity exists.
Outdoor HV Components
The parabolic receiver is also ideal for outdoor applications to scan HV components such as terminations, surge arresters, transformer bushings and isolators for corona and surface discharges. Corona discharges are common and not that dangerous, surface discharges however can cause premature failures. Therefore it is of utmost importance to properly scan these components using the directionality of the receiver and its integrated laser pointer.