CB Closing Time

You got to change the Breaker Close Time and Close Fail Delay (if you are using them in your relay)

Do you have automatic synchronizer in your relay?

With the Ta = 120ms setting, the synchronizer sends the breaker closing signal 120ms seconds before its indicator goes to 12 Hour position (complete vertical position). This is because there is a time delay between the time of sending signal and the actual closing of the breaker due to the breaker closing time and other system delays. In your case, you are using 25ms (90ms – 65ms) faster breaker so you can set Ta = 120ms – 25ms = 95ms.

Hope it helps.

- MS

Infact, we have 8 DG sets with CB of closing time 90ms. We are going to retrofit only one of the CB with SF6 CB of closing time 65ms. That rules out the possibility of changing the settings of the CSR. What will happen to the generator if the closing time of the SF6 CB is not changed to 90ms.

Usually problems occur at opening. For inductive loads there are dangers when breakers interrupt the current before passing through zero- that is with fast breakers, like SF6 breakers but mostly vacuum breakers. This creates switching overvoltages. When closing a breaker the best way to do it is to close each pole when its voltage passes through zero- like I have for a 400 kV reactor. At closing, the faster it closes the better, especially when connecting generators in parallel because you have to connect them in the exact syncronizing moment.

Electonaut,

What is happening to the CB tripping time - is it also lower with new CB. I guess so.

That being the case, you may need to look at the CB specifications as the CB would be asked to break greater DC component when the CB opening tim is lower.

Existing CB closing time - 90ms, tripping time - 45ms.

New CB closing time - 65ms, tripping time - 55ms.