Partial discharge may occur along cables in electric power transmission and distribution systems when cable insulation breaks down. For example, a cavity in cable insulation can cause partial discharges under normal operating conditions and test conditions when the cable is energized, that is, when high voltage signals are introduced into the cable. When a partial discharge (PD) occurs, high frequency current and voltage pulses emanate from the site of the discharge, which is hereinafter referred to as a fault site. These current and voltage pulses can be useful as an indication of the presence of a fault (e.g., an insulation defect) for partial discharge site location and assessment. The type of cable fault that causes PD is non-reversible and damage to the cable progressively degrades the insulation until a catastrophic failure occurs. A cable that is exhibiting PD and is unattended deteriorates due to a combination of factors such as moisture ingression, mechanical fatigue and thermal cycling, among others. It is most cost effective to monitor the cable at the time of manufacture and once installed to detect partial discharge activity and determine its location as soon as such conditions arise to allow for preventive maintenance to be performed before catastrophic failure occurs.
Partial discharge site location (PDSL) is a technique for determining the position of a fault within a high voltage (HV) cable by using detection means at a near end of the cable to detect pulses which are generated by PD within the fault. The reflection of those pulses from the far end of the cable is also detected at the near end. The time difference between the PD pulse and its reflected pulse is proportional with respect to the distance of the fault site from the far end of the cable.
An existing partial discharge measurement (PDM) system employs a relatively simple discharge site location system which allows a user to perform a single time-related capture of pulse activity on a sample cable being tested. A user can employ the graphic user interface and processing capability of the PDM system to zoom in on pulses and their reflections and to determine the position of a fault within the cable. This single time-related capture technique has some deficiencies when compared with other equipment such as a digitizing oscilloscope. Since the above-described technique is a single shot process, no averaging of the data occurs. Thus, the PDM system sensitivity is limited by the non-correlated noise on the system. A digitizing oscilloscope is advantageous in that it can average pulse activity by digitizing the pulse activity in short bursts based on its time-base setting in response to a triggering event created by the presence of a pulse. These short bursts of digitized pulse activity represent multiple passes of the cable being tested in contrast a single time-related capture. The result of these digitizing operations of multiple passes are averaged together to produce a composite image of the pulse activity. An additional benefit of a digitizing oscilloscope is that it allows the possibility of resolving the presence of multiple sites within the cable.
To provide the performance of a digital oscilloscope in a PDM system by mimicking the operation of a digital oscilloscope using pulse triggering is not a viable option since the PDM system would need to be redesigned to include features that may not be currently included in the PDM system hardware such as a settable threshold detection operation on the incoming data stream with an additional data path to process the threshold signals. Such a redesign is not desirable since it may compromise the partial discharge detection and measurement facilities that are already included in the PDM system. The PDM system is designed to operate on a pulse-by-pulse basis to identify individual pulse events rather than relying on a repetitive signal, as is typically used for PDSL measurement. The redesign would involve considerable effort and expense to include the triggering system of an oscilloscope, as well as a philosophical change to the operation of the PDM system. The PDM system would change from an essentially free-running system to a triggered system so that, rather than looking for events in terms of peak heights (i.e., the factor of interest), the system would detect events crossing a defined threshold. This presents problems for PDM because of a possibility that events will be overlooked for failure to meet the trigger criteria. Accordingly, a need exists for a PDM system which can perform pulse averaging for PDSL without triggering and other operations associated with oscilloscopes.