Online Partial Discharge detection in MV GIS switchgear

Partial Discharge (PD) can occur in metal clad switchgear operating above 2500V and can have catastrophic results.  It occurs in the presence of high voltage field stress and is often undetected until it is too late. Partial discharge has a number of effects that allow it to be detected without direct galvanic connection to the conductors. These effects include ultrasonic energy, radio wave emission, UV and visible light emission, high frequency current pulses, and ozone smell.

Ultrasonic, Transient Earth Voltage, and UHF detection techniques are commonly used for online testing of Air Insulated Switchgear (AIS). Typical AIS switchgear is quite porous to sound waves, skin effect current pulses, and UHF radio waves making these techniques very effective.

Typical MV GIS Construction

Medium voltage SF6 Gas Insulated Switchgear (GIS) is constructed quite differently from IAS.  Typically, a vacuum bottle contains the switching contacts much like AIS. However, the vacuum bottle and associated conductors are surrounded by SF6 gas, not air.  Often the VT and CT as well as the bus bars are also surrounded by SF6.  The SF6 compartment is tightly sealed to keep the SF6 from escaping.  This tightly sealed enclosure affects all the signals we describe above for online PD measurement.

- The SF6 compartment largely contains the ultrasonic sound waves and makes airborne detection virtually impossible.  A contact sensor on the SF6 compartment might work but its sensitivity would be highly reduced.

- High frequency skin effect currents may generate TEV on the SF6 enclosure but that is often buried within the gear and then surrounded by a structural/cosmetic cabinet that will have a highly reduced TEV signature, if any.

- UHF signals are effectively contained within two faraday cages, the SF6 enclosure and the outer cabinet, thereby reducing the effectiveness of this method.

- While not primary PD detection methods, visual inspection and infrared inspection are also much less likely to be effective due to a lack of line of sight to insulators.

EA Technology set out to find the best method for PD testing this type of gear. We ran a series of tests using real PD sources embedded into various locations in a popular brand of 24KV switchgear with GIS insulation and a vacuum circuit breaker.  The switchgear had internal cone cable connections.

We used a lab quality directly connected PD measurement system to confirm our generated PD test levels. 

PD Source mounted inside GIS compartment

As expected, Ultrasonic detection was ineffective at most locations.  Clearly if conventional air insulated cable terminations were employed, PD there would have been detectible via ultrasonics.  As this gear used cone type cables, ultrasonic was not a viable method.

A custom- built device that generates internal PD was attached to the bus bars inside the gear and TEV / HFCT readings were taken.  Both amplitude and phase resolved plots were recorded.

With a source generating a low level (246 pC) of internal discharge at the bus bar, The HFCT on the cable shield produced a strong signal and clear phase resolved data.  TEV signals were clearly visible on both the rear panel and on the cable sheath.

Phase Resolved Partial Discharge plot recorded with HFCT

Corona generated in the bus bar compartment was also easily discerned by the HFCT on the cable shield. TEV detection of corona was less reliable.

This leads us to a number of conclusions about PD and MV GIS:

- PD within GIS switchgear generates a significant signature which can be detected.

- Both TEV and HFCT sensors work well for detecting PD inside the gas compartment.

- Ultrasonic only testing may not find PD in medium voltage GIS switchgear.

- It is possible to test for critical PD activity without taking outages  – no need to de-energize.

- The sensors included with the UltraTEV Plus 2 can be used for periodic PD surveys on all switchgear

- The sensor technology used by the Astute monitor is similar and will work on GIS gear as well.

- Permanently mounted HFCT allow for fast easy surveying

In summary, you can and should do periodic surveys of MV GIS gear just like AIS gear.  The sensor of choice is an HFCT and permanently mounting them makes the survey process quick and easy.