Analysis of PD phase-resolved images at low frequency of testing voltage

Reliable operation of high voltage insulation systems is still one of the main issues of electric power systems, allowing for uninterrupted power delivery. High voltage tests on large capacitance objects like cables are carried out mainly using DC, oscillating or AC voltage at lower frequency, due to required power of supply source. That fact leads to combine simultaneously low frequency test with partial discharge (PD) measurement. However, the verification of diagnostic usefulness of this approach should be confirmed by laboratory investigations. This paper presents influence of testing voltage frequency on registered phase-resolved PD images occurring in high voltage insulation. Both method and specialized measurement system for PD patterns registration in wide frequency range of testing voltage are described. Results of measurements at frequencies from 0.1 Hz up to 50 Hz performed on different objects are presented. Changes of selected discharge parameters and their analysis based on equivalent circuit models are discussed.