Intertrip and Interlock Operation

[bee79]

Hi All,

I've seen the intertrip and interlock function is provided for transformer feeder and incomers. Is this protection required for transformers only or can be applicable for plain feeders as well?

Tripping the breaker upstream of a transformer will intertrip the downstream breaker. This is required to avoid current back feeding. And if the downstream breaker trips due to fault condition it will intertrip the upstream breaker to avoid feeding to fault location. This is my understanding about transformer breaker intertripping.

It shall not be possible to close the downstream breaker unless the upstream breaker is closed (interlock). This is given in a switchgear functional description. But the same FD also stated that upstream breaker cannot be closed if the downstream breaker is closed. I wonder how is it possible to close first the downstream breaker in this situation?

Appreciate your answers.

 

[bacon4life]

Interlocking/intertripping is not universally applied across the industry, but your switchgear specifications seem to require it in your specific case. For example, in the case of undervoltage/underfrequency load shedding we trip only the incoming transformer breaker and avoid tripping the lockout(86 relay)/feeder breakers.

If everything works correctly, you are correct. However, adding the additional logic to the upstream breaker covers abnormal situations such as manually operated breakers or breaker failures to open. Also, the intertripping scheme may not be in service during testing or maintenance activities.

 

[ScottyUK]

A few possbiles:
[ul]
[li]Breaker fail condition[/li]
[li]Breaker not opened after test[/li]
[li]Breaker closed by direct operation of the mechanism (i.e. bypassing of interlocks)[/li]
[/ul]

 

[bee79]

I was thinking about bypassing of interlock for direct closing of breaker.It seems like there could be other possibilities as well.

bacon4life, as you have pointed out the Interlocking/intertripping is not universally applied across the industry, how about its requirement on which part of electrical system to be used? Is it for transformer feeders only or can be applicable for a plain feeder lets say 11kV to 11kV?

 

[krisys]

Different interlocks for different types of feeders depending on the degree of operational safety to be achieved. Also the interlocking requirement depends on the criticality of the installation, wattededmanned or unmanned etc.

In an industrial facility normally the interlocks will be provided for the plain feeder, typically for the higher voltage feeders. These interlocks are more significant, if the down stream system is double ended (i.e; two incomers feeding the same switchgear having bus tie).

In essence, you have to take a call after evaluating these parameters (named only a few). But in my opinion, if the interlocks are already provided, why to remove, unless it is really causing some operational problem or safety issue. As such there is no harm in having a redundant interlock. So retain it.