Transformer Differential for Arc Flash Mitigation on Unit Substations

[rockman7892]

I'm looking into AF mitigation techniques on the secondary's of 4.16kV-480V unit substation transformers for several new installations. The secondary equipment for these unit substations consist of either 480V Switchgear or Switchboards with transformers ranging from aprox 2-3MVA.

In trying to mitigation AF on secondary of unit subs the initial thought is to use transformer differential protection with the secondary CT located on the load side of main breaker in either swgr or swbd. The thought here is that we can gain the benefits of both transformer differential protection and AF mitigation on line side of secondary main. I know transformer diff is typically reserved for transformers 5MVA and larger but with the advent of electronic relays I think the economics become more practical for an application such as this.

The other approach here would be to locate the CT's directly on the secondary of the transformer (upstream of swbd) and use these CT's with a maintenance mode setting for mitigating AF at downstream swbd (both line and load side)

I was curious to hear opinions from others on both these approaches and which one is the preferred option?

Is there usually a large difference in clearing time between the two options? With differential I have typically used 100ms including relay time, 1 cycle for Lockout relay and 3-cycle for breaker operation?

 

[veritas]

What do you mean by maintenance mode?

 

[RonShap]

In both cases, the clearing time will be detection plus the clearing time of the primary protection device opening. The diff protection detection time should be the same as it seems the position of the secondary CT's are just either on the line side or load side of the 480V main. The maintenance mode function makes it so that you can set it really tight with little concern for false activation, so it will be even more limiting.

 

[wroggent]

We've been putting differential protection on lots of 4.16kV-480V 1.5MVA transformers for arc flash mitigation.

 

[JG2828]

Each solution provides different zones of protection. The transformer diff zone of protection will only include the transformer and secondary main breaker. The maintenance mode will include the secondary main breaker, feeder bus, and feeder breakers. One disadvantage of the maintenance mode switch is the operator forgetting to switch it into maintenance mode before starting work and forgetting to switch it out when the work is completed.

Transformer Diff zone

Maintenance mode zone

 

[veritas]

What puzzles me is in normal mode the diff relay CT is on the load side of the LV breaker and in maint. mode it is on the line side. Two different CT's? If so does maint. mode mean switching between them? Does this make it really expensive? After all it is only 1.5MVA.

 

[JG2828]

Veritas,
I was explaining two completely different protection schemes. One using a transformer differential relay and the second using an overcurrent relay with a maintenance mode switch. You could technically do both using the same transformer differential relay. As long as it supports the logic. You could connect the CTs as shown in the top sketch, since the breaker is already protected by the transformer zone it doesn't need to be in the maintenance mode zone.

 

[veritas]

Agreed, thanks.