Using a field probe to study the mechanism of partial discharges in very small air gaps under direct voltage

Abstract

Due to growing use of high-voltage direct current (HVDC) for power transmission, it is of interest whether diagnostic tools introduced in past for insulation condition assessment of high-voltage alternating current (HVAC) apparatus are also applicable for quality assurance tests of HVDC equipment. This refers in particular to measurement of partial (PD) in compliance with international standard IEC 60270 [1], [2]. As PD events in gaseous cavities embedded in bulk dielectric of HV equipment are most harmful ones, mechanism of such internal should be well known to judge PD severity as well as to specify appropriate procedures and quantities for PD tests under direct voltage. According to Devins cavity can be classified as Townsendlike discharges and streamer-like discharges [3]. Even if this classification seems to be reasonable for cavity under both alternating and direct voltage [4]-[6], this has been rejected by Bartnikas. So, he argued in [7] that the term 'streamer' by itself, when applied to PD in relatively short gaps or small cavity diameters, introduces unnecessarily a misleading term in PD lexicon. PD, when occurring in short gaps, may assume different forms: rapid and slow rise time spark-type pulses. This is also consistent with worldwide established PD model according to Kreuger [8], which is based on hypothesis that virtual cavity capacitance is discharged via a conductive spark channel. Despite simplicity, this concept was rejected by Pedersen and his coworkers [9], [10]. As an alternative, they promoted a more sophisticated field-theoretical approach based on a dipole model. As their concept is not easily understandable, this was presented in a more simplified manner in this Magazine [11], [12] as well as in [13].