Diagnostics of power cables DCmeasurements Condition Monitoring of

Diagnostics of power cables – DC-measurements Condition Monitoring of Electrical Equipment L School of Electrical Engineering Pyry Karhu Condition Monitoring of Electrical Equipment L Diagnostics of power cables – DC-measurements Slide 1/9 25. 4. 2016

DC-Measurements • ON-SITE – Light, compact, easy to use & assemble • OFFLINE Measurements • Insulation resistance – Quick and simple – Several k. V voltage – No expensive equipment needed • Polarization Index • Dielectric response 25. 4. 2016 Diagnostics of power cables – DC-measurements 2/9

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 25. 4. 2016 Diagnostics of power cables – DC-measurements 3/9

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 25. 4. 2016 Diagnostics of power cables – DC-measurements 4/9

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) 25. 4. 2016 Diagnostics of power cables – DC-measurements 5/9

Polarization and Depolarization currents - PDC • The DC conductivity estimations u, i • Polarization current charging U 0 short circuit ─ Charging current of insulation material open circuit return voltage U R (t) • Depolarization current ─ Measured with power supply removed t 1 t 2 time • Return voltage ─ Voltage that insulator returns after short circuit is removed 25. 4. 2016 polarization current i pol (t) Diagnostics of power cables – DC-measurements depolarization current i depol (t) 6/9

Polarization and Depolarization currents - PDC • Diagnostics is based on polarization phenom variations and nonlinearity • Polarization and depolarization currents should exhibit similar trends • Polarization index > 4 means good condition Good cable Bad cable • Can’t locate the problem 25. 4. 2016 Diagnostics of power cables – DC-measurements 7/9

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 25. 4. 2016 Diagnostics of power cables – DC-measurements 8/9

Summary • Many different DC-measurements for Power Cables – Insulation Resistance – Polarization Index – Dielectric response • Time domain • Frequency domain 25. 4. 2016 Diagnostics of power cables – DC-measurements 9/9