Diagnostics of power cables – DC-measurements Condition Monitoring of Electrical Equipment L School of Electrical Engineering Pyry Karhu Slide 1/ Diagnostics of power cables – DC-measurements Condition Monitoring of Electrical Equipment L

DC-Measurements ON-SITE –Light, compact, easy to use & assemble OFFLINE Measurements Insulation resistance –Quick and simple –Several kV voltage –No expensive equipment needed Polarization Index Dielectric response Diagnostics of power cables – DC-measurements 2/

Insulation Resistance Detects moisture and serious insulation degradation Can’t detect partial discharge Measured resistivity after certain period of time Low value of IR indicates that the insulation is frail Unreliable alone because IR is strongly dependent on temperature Diagnostics of power cables – DC-measurements 3/

Polarization Index - PI Variation of the insulation resistance test –Less sensitive to temperature Ratio between measuret insulation resitance at 10 minutes and 1 minute after applying voltage Low value might indictate moisture Diagnostics of power cables – DC-measurements 4/

Dielectric response - DR General overview of cable condition Detection of water content –Water treeing (XLPE) –Moisture (Oil-paper) DR Measurements –Time domain (PDC) –Frequency domain (FDS) Diagnostics of power cables – DC-measurements 5/

Polarization and Depolarization currents - PDC The DC conductivity estimations Polarization current ─Charging current of insulation material Depolarization current ─Measured with power supply removed Return voltage ─Voltage that insulator returns after short circuit is removed Diagnostics of power cables – DC-measurements 6/ chargingshort circuit polarization current i pol (t) depolarization current i depol (t) return voltage U R (t) t 1 t 2 time U 0 open circuit u,i

Polarization and Depolarization currents - PDC Diagnostics is based on polarization phenomenom variations and nonlinearity Polarization and depolarization currents should exhibit similar trends Polarization index > 4 means good condition Can’t locate the problem Diagnostics of power cables – DC-measurements 7/ Good cable Bad cable

Frequency Domain Spectroscopy - FDS Allows characterization of electrical properties of a insulation material at different frequencies Loss angle δ –How non-ideal the insulator is –Dissipation factor tan δ –Power factor cos δ Smaller tan δ is, the less moisture content Diagnostics of power cables – DC-measurements 8/

Summary Many different DC-measurements for Power Cables –Insulation Resistance –Polarization Index –Dielectric response Time domain Frequency domain Diagnostics of power cables – DC-measurements 9/