Advantages of Parallel TX Operation

[KenAlmon]

In considering operating a double ended substation (2 x 100% rated TX) with the tie breaker closed, one of the obvious considerations are fault levels. However, another consideration is reliability relating to "survivability" for the process in this mode. For common faults, such as a fault on the tie breaker, you could conceivably lose both transformers. However, for high side TX faults, you may be able to isolate the transformers automatically without a process upset (transients notwithstanding). On the other hand, if you operate with the tie breaker open, you minimize exposure to common faults, but risk losing the process where the TX is taken out of service. Is anyone aware of a step by step evaluation of why you may want to operate in one mode vs the other?

Thanks in advance.

Ken Almon

 

[RajT]

The selection of mode of operation (ie tie cb closed or open)in such a case is mearly a question of choice.
You have assessed the pros and cons so no need to go into that.
From a survey aspect however, it may be noted that in a majority of the double ended system arrangement the tie breaker is normally open with a fast auto close control in the event of a fault free loss of any one of the Tx incomer.

 

[peebee]

Agreed. Generally you'd want the tie normally open. However, if you'd take a large loss on interruption of power, but can't quite justify a UPS system, then a normally closed tie might make sense.

 

[RRaghunath]

KenAlmon,

For 'tie breaker closed'mode operation I feel Partial differential protection shall be provided for each of the bus sections for prompt isolation of faulty section without affecting the power supplies to the healthy bus section.

The partial differential protection is provided in each of the transformer incomers like OC / EF protection. The difference lies in that instead of incomer CT alone connected to the protection, it is parallel combination of incomer and bus tie CTs that is connected to the partial differential protection. The settings and coordination is done similar to normal incomer OC /EF protection.

Another advantage with this scheme is that there is no separate OC/EF protection provided for the bus tie and thus one step is avoided in protection coordination.

 

[busbar]

IEEE Std 493-1997 may provide a starting point to build your case for particular operating-mode tradeoffs.

Tease at

http://standards.ieee.org/colorbooks/sampler/Goldbook.pdf

 

[KenAlmon]

Thanks for your comments.

rraghunath - Yes, partial diff is in the cards for the scheme, if we do decide to go with the ties NC, we will use partial diff.

Busbar - I have the 1990 version of the standard and while discussing reliability with regards to a tie breaker on a secondary selective scheme, they define a "failure" as having an outage greater than 5 seconds. I am afraid application of that definition exceeds (greatly) our definition. As a result, the formulae relating to reliability cannot be used with any accuracy. Thanks anyway for the link.

I am not sure there is anymore to be discussed on this subject; almost knew the answer before writing...

Ken

 

[busbar]

Good point about the IEEE text, Ken. Five seconds is spitting in the face of sub-cycle power continuity, I guess.