Qualification tests for partial discharges instruments to be used for insulation condition monitoring of HVDC cable system using HFCT sensors

ABSTRACT

Partial discharges (PD) monitoring in HVDC cable systems used for transmission and distribution networks involves very different strategic approaches compared to PD monitoring in HVAC cable systems. PD activity in HVAC cable systems appears when its main insulation has an internal defect (several hundreds of pulses per second can be sustained for hours or days), on the contrary in HVDC cable systems very few PD pulses related to internal defects occur (for example, some units per minute). However, simultaneously there may be many thousands of pulses per minute associated to external phenomena such as corona. PD instruments must be able to discriminate different pulse-type signal sources to reject those that are not related to an internal defect, such as background noise, corona effect, etc. After a different PD cluster is identified next step is to recognize it is due to a defect. As the classic phase resolved PD patterns are not available in HVDC measurements, more complex artificial intelligence tools are required recognize a PD pattern. Numerical sequential series of PD pulses, which amplitudes and time intervals between consecutive pulses, are the only input data to identify what PD defect is involved in this PD source. Powerful noise rejection tools are also required; otherwise, incorrect numerical sequential series will be recorded for the analysis by the artificial intelligence tool and the defect identification may be incorrect.

In this paper, a set of tests is proposed for the qualification of PD instruments to be used for monitoring the insulation condition of HVDC cable systems. The classical conceptual bases of PD measurements are reviewed. The concept of apparent charge, QIEC, as the largest PD magnitude that occurs repeatedly according to IEC 60270 is not applicable when the PD rate is below 1 pulse per second. Besides, apparent charge cannot be measured when monitoring applications are used on site. Therefore, the classical measuring principles of PD measurement technologies should not be applied in HVDC cable systems monitoring. The following PD instruments features are now interesting to be qualified: a) PD pulse sensitivity in the specific frequency range considered for HVDC cables, b) noise rejection capability related to AC/DC converter stations, and c) ability to identify series of PD pulses generated in internal defects. The qualification tests developed will be described in detail in the paper.

The research here described has been developed in the framework of an EU-funded project called 19ENG02 FutureEnergy “Metrology for future energy transmission” EURAMET Project [1]. In this project, a special PD calibrator is being developed that uses a complete characterization procedure to cover all the functionalities required for the evaluation of the PD measurements instruments

 

KEYWORDS

Partial discharges in HVDC, HFCT sensor, PD instrument qualification, transfer impedance, Electrical noise, PD sensitivity, PD clustering, Insulation defects, PD pulse trains, PD patterns in HVDC.