Partial Discharge (PD) Measurement Instruments for High Voltage Cables

Partial Discharge - Identify, Evaluate & Locate in HV Cable Systems

SEBA KMT oscillating wave test system (OWTS) is used to identify, evaluate and locate partial discharge (PD) faults in HV high voltage cable insulation, cable joints and cable terminations.

The HV Partial Dischagre unit generates the attenuated oscillating frequency up to the peak voltage of the system. An embedded controller for the partial discharge (PD) operation and the digital signal processing and storage from two 100 MHz AD converter systems is integrated in the cylindrical  housing. The communication to the user interface of the notebook is done by W-LAN. Optional optical data link is possible. The OWTS partial discharge instrument can also be coupled with a VLF System.  

Introduction to Partial Discharge (PD)
Even in times of deregulation, the underground cable networks of the power utility and DNO's are one of the most important assets. This fact is often not considered the way it should be in the planning of inverstment budgets i.e. incident-oriented HV cable network mainteanance leads step-by-step to exhaustion of the available safety reserves. Since the 1980's, a large number of XLPE first generation high voltage cables have shown "water-treeing" and have been exchanged or renovated for the most part. It is possible to determine the condition of the power cables with the know dielectric diagnosis methods. Much older paper oil filled cables are inconspicuous regarding their operating performance.

The recording, location and evalulation of partial discharge (PD) inside the cable insulation (including within high voltage cable joints and terminations) offers the possibility of an early diagnosis of cable network failure. In order to be able to carry out an evalutation of the risk factor of partial discharge defects, the applied voltage for a partial discharge (PD) diagnosis should be within the range of the operating frequency, because the typical PD parameters, such as inception and extinction voltage, partial discharge level and partial discharge pattern then correspond to the relevant values under operating conditions.

Partial Discharge Faults in High Voltage Cable Systems (XLPE Polymeric & PILC Paper Cables)
Partial discharge is understood to be the partial electrical breakdown of an insulation system, i.e. only a limited section of the total cable insulation section is bypassed. Inside cables, partial discharge defects are generally ionisable, gas-filled voids, which either developed already during the production of the cable insulation, were caused by mechanical damage or are present inside the high voltage cable joints or terminations. In addition, thermal degradation processes inside cable joints with improperly performed cable jointing workmanship can also lead to partial discharge (PD) inception.

However, partial discharges can also occur when the electrical strength of the respective insulation material is locally surpassed by the stressing field strength - e.g. by large water trees or uneven semi-conducting layers on the inner conductor of a PE or XLPE cable.

Inside paper insulated cables (PILC), joints and terminations a completely different partial discharge behaviour is observed. In the laminated and impregnated paper insulation, partial discharge can occur in dry areas, but may disappear again due to the mass migration in the case of thermal load changes. In the same way, carbonisations have the effect of conductive bridges due to the reacting partial discharge so that the potential differences are "short-circuited" and the PD discontinued. Due to the barrier effect of the laminated paper cable insulation, the propogation of partial discharge channels is largely impeded so that PILC cables with PD in the insulation may be safe for operation even for many years - similar partial discharge behaviour can be found on oil-filled cable joints. The operation of high voltage drained cable terminations, however is directly threatened, because self-healing by a continuous flow of mass or cable oil cannot occur.

From a multitude of partial discharge (PD) measurements on high voltage PE, XLPE and PILC cables, there is knowledge and experience for the evaluation of the threat to the respective insulation systems by partial discharge.

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Insulation Condition Assessment of High Voltage Distribution Power Cables - Paper

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Partial Discharge (PD) Test & Measurement for High Voltage Cables - SEBA OWTS

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Partial Discharge (PD) Measurement Instruments for High Voltage Cables