Automatic Transfer Systems

[daveR]

I'm looking at an automatic transfer sysmtem to swap between two 400V power sources ("normal" mains supply and a standby generator) using 2 interlocked ACB's.

My question is what side of the main ACB device should we put the voltage sensing relays?

I've been told we should not put the relays on the incoming side since a transient or fault within the site could trip the ACB and the generator would not receive a signal to start. I think that if we put the relays on the output (or switchboard) side of the ACB, then if we have a fault within the site, the main suppply ACB would trip and the ATS system would operate and close the generator onto a fault. Also, when the mains is healthy again, the relays would not see the voltage is OK and operate the ATS to swap back from the generator source......

 

[JBD]

Most automatic transfer switch installations I have seen, have their sensing circuits on the line side of the switch. This position allows them to monitor the power supplies.

That said, all of these sensing circuits have been "downstream" from the service OCPD. This location would cause a transfer if the OCPD tripped due to a fault condition. To prevent closing on a fault you could add a "tripped" position contact to your OCPD for use as a lockout.

 

[vishalmehta]

I agree with JBD regarding location of voltage sensing PTs to be on line side, which is generally a normal practice.

To add on a few more points:

1) ATS should only happen between two incomers incase of "undervoltage". And also a delay of 1 sec is generally given along with undervoltage relay to open the breaker.
I think 1 sec is sufficient time for a transient condition to subside.

2) Also ATS should not take place if the normal supply breaker has tripped on protections like overcurrent and earthfault. Operation of these protections would mean that incomer has sensed a "Bus fault" and hence starting of generator should be inhibited in this condition. As JBD rightly said lockout relay contact may to taken to inhibit start of generator and auto transfer.

3) Undervoltage relays being used must be of "self reset" type which would ensure that manual intervention to reset the relay is not required while normal supply resumes.

Vishal