16,17/5/2010

PD Mapping:
PD (partial discharges) are small electrical sparks or discharges that occurs in defects in the insulator.
The discharges do not bridge the insulation between conductors and the defects may be:
-entirely within the insulation
-along interfaces between insulating materials
-or along surfaces
Partial discharges characteristics depend on:
-type
-size
-location of defects
-insulating material
-applied voltage
-cable temperature
The damage caused by PD depends on several factors and can be range from negligible to causing failure within days to years.
Important parameters of PD-diagnosis:
-Ground noise level:
Inferences of external noise sources like radio stations ,corona from OH lines ,bad grounding conditions .PD signals below ground noise level can be not determined .Ground Noise Level on site normally between 20 to 100 pC.
-Partial Discharge Inception Voltage –PDIV
Voltage level where first measureable PD pulses are to be seen ,determined by stepwise increase of the test voltage .PDIV should be higher than Uo(normal service voltage).That means the cable is under normal service condition PD free .New installed cables should be PD free up to 2 Uo.
Partial Discharge Extinction Voltage-PDEV
PDEV is often below PDIV,PD starts above Uo in case of an overvoltage can be extinguish below nominal voltage .PDEV should be above Uo.
Condition assess using PD techniques:
-Analysis of HV switchgear fault causes
-Consequences of failure
-EA technology experience and credentials
Reason For Partial Discharge Test
-Insulation failure can be devastating
-Switchgear insulation is difficult to test during routine maintenance
-Many failure of HV insultion sre associated with Partial Discharges
Partial Discharges Measurement Techniques
-Internal Discharge Activity
-Transcient Earth Voltage (TEV)Detection
-Higher frequency(~3 to 80 MHZ)
-Surface Discharge Activity
-Ultrasonic Emission~40KHZ
-TEV detection-when high amplitude surface discharge