Undervoltage protection (27) delay time

[yk052052]

Hi, folks.

In our plant, we have 6kV switchgear A/B and there is NO bus tie. And the bus tie is supposed to be closed when there's undervoltage in upstream of A or B (called ATS : Auto Transfer System).
We got an undervoltage trip accident recently on 6kV swichgear A(due to cable problem on incoming A line) and as it is supposed to be, bus-tie was closed automatically. And 6kV Switchgear B kept supplying the power to all load in downstream.
But the thing is, all the pump loads(including 6kV motors) were stopped by this accident and we needed to restart all the pumps.
We've been complained from operation team that any loads shouldn't be dead from undervoltage (only one of A or B, if both dead there is no way).

I looked into undervoltage protection configuration and I assume that the reason why the pumps were all dead was due to undervoltage protection delay time.
It's currently set to 2 sec at 80%.

Here are my questions.

1. Is undervoltage protection delay time this long (2sec) general configuration causing the pump loads dead?

2. They request us to set this delay time to minimum value as short as pump loads can keep running in case of undervoltage accident.
Like 0.5 sec or even 0.
I know that the delay time is to prevent undervoltage trip in case of instantaneous voltage sag. And I'm trying to see what problem will be caused if we change the delay time to 0.
And I see some.
1) Due to unstable incoming power, there is fluctuation and even if there's very short voltage dip undervoltage trip will occur.
2) In case short circuit accident occurs in downstream(400V), there could be short voltage dip even in upstream(6kv) and undervoltage trip will be activated in upstream.
3) I found even over current protection has delay time before it's activated. So if I set undervoltage time delay less then over current time delay, there is possibility that undervoltage protection will be activated before over current protection activated. (I said possibility since I didn't see in real if there's enough voltage drop by short circuit)

Am I right with those thought and are there any more reason not to set the undervoltage protection delay that short?
If

3. Then isn't there any way to make pump loads alive even after undervoltage accident and bus-tie close?

If I my concerns above are not correct and find reasonable reason to change the undervoltage delay time then we should change it. Otherwise I need to explain and convince operation team.

 

[ScottyUK]

Do you have a standard ATS, or a fast bus transfer scheme? The latter would 'probably' keep the loads supplied, but it has the potential to cause widespread power loss if a breaker fails to operate on demand because the open bus section breaker is signalled to close before the faulted incomer breaker has fully opened, with the risk of closing the good source onto whatever fault tripped the failed one.

 

[waross]

You can't successfully use the same undervoltage relay signal for both undervoltage ATS closing and under voltage trip.
Your time sequence should be:
1> Fault clearing.
2> Close ATS on undervoltage.
3> Trip on undervoltage.
One solution:
1> Fault clearing, instantaneous.
2> ATS transfer on undervoltage, short time.
3> Undervoltage trip on undervoltage, longer time.
Another solution:
1> Fault clearing.
2> Close ATS at undervoltage = 85%
3> Trip on undervoltage at undervoltage = 75%
Another solution:
1> Fault clearing.
2> Close ATS at undervoltage = 80%
3> If ATS is closed, trip on undervoltage after a short delay.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burntcoil]

I believe you had an under voltage dip on side A. Standard MCCs use control power from the line side connected CPTs, so the under voltage caused the contactors to dropout. the transfer was an open transition transfer and that time was enough to drop the contactors. You need to have voltage dip ride through capability for all your critical motor contactors.

 

[krisys]

If sustaining the motor running is so important, either ride thru or the auto re-acceleration scheme should be implemented.

But the cost of scheme implementation shall be justified by the facility operators.