Disconnector and earth switch in 220kV GIS 2

[ravindranathan]

Why do we need disconnector + earth switch after every component in 220kV GIS ?
This is not the case in 33kV GIS.
For e.g In a typical Bay we have "disconnector+earth switch" after busbar assembly,after Circuit breaker,and again after current transformer.
Pl advise.

 

[crshears]

In air insulated switchgear, maintenance crews can apply temporary working grounds wherever they need to.

In GIS they can't, hence the provision of grounds for every isolatable component.

Theoretically, one could reduce the number of earthing switches and provide grounding continuity via intervening closed disconnect switches...but this can rapidly become an out-of-control monster with significant impact on system reliability, as far too much equipment may need to be removed from service simply for earthing purposes.

Why the same philosophy isn't followed in the 33 kV switchgear you mention I can't say...less system impact due to outages, perhaps?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ravindranathan]

Need more inputs on this topics.Please share your inputs.

 

[Mbrooke]

Can you post a single line diagram of the 33kv GIS?

 

[cuky2000]

It is virtually impossibly compare the 220 kV GIS design and application With the 33 kV design.

For 33 kV applications, there is available medium-voltage(MV) gas-insulated switchgear (GIS), SF6 & vacuum circuit breaker designs with a maximum design voltage up to 38 kV (42 kV on request), up to 200 kV BIL and interrupting ratings up to 40 kA SC without the need of many or none disconnector + earth switch after every component.

The 220 kV GIS design every phase is in a single compartment subjected to very fast transient wave (VFT) quite different than the 33 kV design an even different than the 3 poles GIS design rated for 69 kV to 145 kV.

 

[ravindranathan]

Cuky2000--Do you mean that it is because of Very Fast transient wave(VFT)and resultant voltage that may develop, earth switch is provided for every component?

 

[cuky2000]

The issues related to very fast transients overvoltage (VFTO) is of greater concern at the highest rated voltages and particularly for GIS design with tubes with segregated phases such as the GIS design 220 kV and above.
Typical 33 kV design is build in metal-clad cubicles with SF6 buses in a relative simpler configuration. In contrast, the 220 kV GIS designis build with phase-segregated-bus with isolation and grounding switch required for operation and maintenance.

The very fast transient overvoltages (VFTO) arise within a GIS any time there is an instantaneous change in voltage. Most often this change occurs mainly as a result of the opening or closing of a disconnector switch (DS) or ground switch.

As note in the links above, the 33 kV and the 220 kV GIS design are very differents and hard to compare since the number and requirement of disconnect switches are not comparable.