ETAP Power Station and TCC's

[kingtutley]

I am in the process of updating/replacing some relays for a hydro plant and am running into problems using ETAP. I have looked in the no-help files and found nothing. There is no one in the office here that understands ETAP even as well as I do (sadly). Here is a brief overview of the problem I am having, and I will attach a graphical representation of the system I am working with.

I have a 480V breaker as main feed to the main aux board. This breaker has an integral trip device that id being used as primary protection (according to the plant). There is also a Schweitzer 551 relay being used as backup protection. There are 2 main aux boards with identical protection schemes tie together via bus tie breaker and relay. The system coordinates as well as can be expected with so much over-protection. The problem is with the fault current through these breakers/relays.

With one configuration, one of the mains is open and the tie is closed, thereby feeding both boards from one main breaker. In this configuration ETAP's TCC shows that the OPEN breaker sees a fault current of 12.492kA and the relay sees a fault current of 13.325kA. I have verified that the load flow study converges, though it's usefullness is questionable because of the way this facility uses ETAP (There are no static loads of any size. Only motors are modeled.).

My hope is that someone here can point guide me to an answer as to why this breaker see ANYthing from a fault current when it is open, and why they should be different.

Thanks.

 

[7anoter4]

I suppose is a 4 phases distribution: 3 lives and 1 grounded neutral. The no.2 breaker disconnects only the phases and the neutral remains uninterrupted. So, if a phase to neutral short-circuit occurs, the current splits in two ways [see the attachment] and the OCR no.2 sees a half [approximate] of short circuit current.
Regards

 

[jnunner]

My guess is that you ran the short circuit module with the breaker closed and then reran it with the breaker open. Have you rerun the fault current calculation? Also, it could be that ETAP won't update the fault current with a value of zero.

 

[kingtutley]

Sorry it has taken me so long to get back to this article. Very busy here.

The transformers are both open wye primary delta secondary.

The plant in question has directed me to use only the cases they provided with thier model. Case 1 (Normal) has the generators online, the mains closed, the tie open. Case 2 (Alternate) has the generators offline with their respective breakers open, one of the mains to the aux boards open and the tie closed. In this configuration the plant is black and is drawing power from the grid.

With each attempt with this system I reran the shorct circuit study and arc flash study (just for kicks) then updated the TCC's and made certai that the curves had not been pinned. The results were always the same (thankfully); however, I still do not have an explanation as to why the OPEN tie breaker sees any fault current at all. I have tried to generate the Thevinine equivalent for the model to determine if this is the higher of the possible fauls at the terminals of the breaker (primary vs. secondary), but I have had no success getting these numbers.

 

[davidbeach]

Maybe it is for fault current at that tie breaker. Breakers do have faults every now and then.