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Friday, 20 July 2018

Online Partial Discharge Testing


Partial discharge
Online Partial Discharge Testing:
Online Partial Discharge Testing is performed online without disrupting plant or facility operations. This nondestructive, noninvasive predictive maintenance tool assesses the condition of switchgear, feeder cables, and bus duct to provide critical information regarding levels of partial discharge and its potential impact on the health of the electrical asset. The cost to perform online testing is relatively inexpensive compared with offline testing that requires interruption of service and production. For critical facilities that operate 24x7, online testing is considered the single best option for detecting insulation condition.

 
partial discharge
Technology:
·         ACOUSTIC METHOD
·         ULTRASONIC DETECTION
·         HFCT
 Acoustic Method:Acoustic partial discharge measurement can be easily applied on AIS
without the need of interrupting the operation. Acoustic Partial discharge measurements rely
on close acoustic contact of the area producing discharge. This method is also very effective in
localization of PD.
 Ultrasonic Detection:
All electrical equipment produces a broad range of sound. The basic electrical problems that
produce distinct ultrasound waves that can be detected by Ultrasonic Testing include partial
discharge, corona and tracking. Ultrasonic measurement is most powerful on comparative basis
and will significantly increase the reliability of correct detection of partial discharge.
 HFCT
HFCT is used to detect high frequency current pulse discharging through earth from system
ground. It is also helpful in getting PD Pattern.


PD Tolerance Levels for Switchgear:
mV Level
Condition and Action
< 6
Discharge within acceptable limits*
612
Monitoring recommended
 (Recommended to conduct PD after 1 year.)

1232
Regular monitoring recommended
(Recommended to conduct the PD after 56 months. The panel having increasing trends in PD value should be monitored with help of IR thermography immediately.
> 32
Major concern, located PD with alternate technology

Benefits of On-Line Partial Discharge Field Measurements
·         It is truly a predictive test, indicating insulation degradation in advance of the failure.
·         It is a nonintrusive test, requiring no interruption of service and is performed under
normal operating voltage, load and environmental conditions.
·         It is a nondestructive test; it does not test to failure or adversely affect the equipment under test in any way.
·         It does not use any overvoltage, thereby not exposing the tested equipment to higher voltage stresses than those encountered under normal operating conditions.
·         Trending can be accomplished by storing baseline measurement results to allow comparison with future tests.
·         In many instances the site of the partial discharge occurrence can be located within
the cable/plant under test, so the localised problem can be repaired.
·         The cost to perform a PD-
survey is relatively inexpensive compared with off-line testing, allowing annual surveys to be performed economically at most facilities.

Examples of HV & MV Plant that can be tested
·         Cables and Cable Accessories (terminations and joints)
·         Switchgear (AIS, SIS and GIS)
·         Instrument transformers (voltage and current)
·         Power transformers and bushings
·         Motors and generators
·         Surge arrestors
·         Capacitors


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