Mansoor – 27.03.2012

COMPARISON BETWEEN AIS COMPACT AND HYBRID SWITCHGEAR

Mixed technology switchgear as defined by IEC 62271-205 includes both AIS compact and Hybrid (AIS

busbars and switchgear in GIS). Though AIS compact is more cost effective it fails to address the issue

of safety, flexibility and lower maintenance requirements expected of compact and future

switchgear.

One example is the Disconnecting breaker which is a live tank circuit breaker whose breaking

contacts act as a disconnector i.e the distance between the CB contacts is large enough to meet the

disconnector electrical requirements and avoid an additional disconnector in the bay.

1. As per the study done by CIGRE Metal enclosed circuit breakers are more reliable and have

less maintenance than non-metal enclosed circuit breakers

2. The Current transformers in AIS are generally oil filled which requires a daily check and more

frequent analysis of CT accuracy and condition whereas a resin clad CT does not change it’s

intrinsic properties once installed and maintenance cycle is much longer.

3. Oil filler instrument transformer is a source of fire and explosion which damage surrounding

equipment and porcelain clad circuit breakers.

4. Conventional current transformer is the weakest link in a AIS SS and using the same for

compact SS reduces the overall safety and reliability of the substation.

5. The present regulations and safety requirements mandate a separate disconnector for

isolating transmission lines and busbars for safety of personnel and equipment which is not

possible in DCB.

6. The number of switching elements are more for a double busbar system which has two DCB’s

and four earthing switches to isolate and earth all live parts which necessitates more number

of equipment to maintain and a higher chance of failure.

7. Protection and relaying is complicated in a double bus system as two sets of current

transformers are used and two DCB’s are operated which requires both the breakers to

operate simultaneously and trip the faulty feeder.

8. In a hybrid switchgear the complete switchgear and instrument transformers are in SF6 gas

(CT are resin clad) and further possibility of lighting arrestor also in sf6 metal enclosed.

9. Direct High voltage cable connection is not possible in AIS compact a separate Cable to Air

bushing is to be used whereas the cable can be directly connected to the hybrid switchgear.

10. The number of high voltage connectors are more in AIS compact which is a source of

constant power loss and thermal breakdown

11. Number of foundations are more ( for CT’s and VT’s) .

12. Life cycle cost of hybrid switchgear is the lowest as calculated by CIGRE.