Switchgear control voltage and undervoltage relay

[Live2learn]

Yesterday we turned commercial power off to an older piece of switchgear and it is a Main-Tie-Main and when the left side main went away so did the control power – I am assuming they don’t have a rollover control power scheme otherwise this wouldn’t have happened. To solve this problem I was going to provide it 120V source from batteries via an inverter, so I can disconnect a commercial power feed as needed. Now the undervoltage relay that locks that breaker will never engage, and will have the ability to open and close the breaker without a voltage presenet – what are the possible problems that could arise from this as well, and why wouldn’t you want to control the circuit breaker if there isn’t power to the board?

 

[pwrengrds]

If you don't have control power for the other side of the switchgear and it's energized that protection is also disabled. This would be a problem.

I suspect that it may have 2 sources of power and the transfer scheme didn't work and may be able to be shifted manually.

Typically you would want to have control power on loss of power. That's why a lot of switchgear have DC power to operate the breakers.

I am not quite following the information about the under voltage relay, would think that it would work on a loss of voltage, they typically fail to a safe position (indicating a loss of voltage)

 

[Live2learn]

Thanks for the response. Just looked through the shops and saw only 1 CPT connected to only 1 side with no rollover shceme or anything like that.

Now my concern is

"I am not quite following the information about the under voltage relay, would think that it would work on a loss of voltage, they typically fail to a safe position (indicating a loss of voltage)"

I would not be able to fail to a safe position upon loss of voltage because the UV relay will be supplied by battery backed up source indicating there is no loss of voltage when there is a loss in voltage-here lies my concern.

 

[Zogzog]

Typically a UV device requires no external power, the coil is held in by the line voltage, drops out on a loss of voltage (40% ish) which trips the breaker.

Or am I misunderstanding your question?, you are confusing me with conflicting info in different forums.

 

[rbulsara]

You have two different issues.

1. Control power: You need to fix this first either by roll over scheme or a reliable DC/inverter power source.

2. Understanding of UV relaying and system operation.

As for the UV, if I understand correctly you have disconnected the power to the switchgear by opening a switch upstream of the switchgear, is that correct?

If so now you have no power on line side of the main CB or on the switchgear left bus. Are you trying to close the left main in this condition? This is not recommend/permitted to prevent back feed to the source/utility for obvious safety reasons. Only way you can open and close that breaker would be in test mode, racked out.

Judging by your post, you would do well to involve someone experienced and extra set of eyes in whatever you are doing.

Rafiq Bulsara

http://www.srengineersct.com