Partial Discharge Detection in MV Switchgear
Background
The On-Line Partial Discharge testing of Medium Voltage Switchgear originated in the UK in the early 1980’s through the use of Transient Earth Voltage (TEV)/External Capacitive Coupler sensors which are attached to the outer casing of the switchgear at strategic positions. These sensors pick-up the high frequency (radio-frequency) pulses (in the frequency range of 4MHz to 100MHz) which emanate from discharges occurring within the insulation of the metal clad switchgear.
Motivation for Switchgear PD Testing
Save costs of failure
- Full replacement costs
- Possible serious supply interruptions
Safety of staff and contractors
- Legal liabilities
- Very expensive to compensate for any injuries
- Possible public liabilities
Need engineering data for any replacement program
- Replacement often a multi-reason decision
- Large lead times often needed for switchgear replacement programs
What can switchgear be ‘condition monitored’ for?
- High voltage insulation using PD activity and other insulation tests
- Oil condition using sampling, replacement etc
- Circuit breaker mechanism operation times
- Circuit breaker duty cycles (often in digital protection)
HVPD recommends that for the maximum sensitivity and to detect the widest range of PD types that all three of the non-intrusive PD sensors are used to carry out On-Line PD Testing of MV Switchgear. It is through the combination of the TEV, HFCT and Airborne Acoustic probes that this maximum PD test sensitivity can be achieved in the field.
Improved sensitivities for PD measurements on switchgear can be made using a combination of the TEV, HFCT and Airborne Acoustic Probes. Results of in-field testing show, in general, an improvement of about 5 or 6 times in the sensitivity of PD measurements (for internal discharge) can be achieved using an HFCT connected around the cable conductor compared with a TEV type aerial probe. A similar improvement in sensitivity is seen with the HVPD Airborne Acoustic Sensor compared to the TEV when detecting surface and air discharge.
These results have significant implications for on-line testing for switchgear, where much better sensitivity can be used to monitor switchgear using HFCT’s and Acoustic Probes. These results do not discount the use of TEV sensors for a number of reasons, as follows:
- Locations of PD to a particular Switchgear Panel within the substation can be made using Time-Of-Flight (TOF) analysis of TEV signals as discussed herein.
- There is a large, historical database in the UK for TEV testing of MV switchgear (level of activity given in dB). Results from switchgear testing with TEV probes can therefore be compared back to this reference data.
- The combination of the three sensors allows for ‘type of discharge’ diagnosis to be made, providing the opportunity to assess the seriousness of the PD activity in more detail.
To summarise, for testing switchgear with an identified PD problem, measurements using a combination of sensors (TEV’s, HFCT’s and Acoustic Probes) give a much better monitoring outlook than only using conventional TEV probes alone.
For further detailed information, visit our Applications Notes page.