35kV M-T-M vs Double Bus - Double Breaker Switchgear

[rockman7892]

I'm looking at an application where a site has (2) 34.5kV OH utility services coming into a 35kV M-T-M switchgear lineup which has a feeder on each side to feed a 35kv-5kV 20MVA transformer. The existing switchgear with the (2) feeds has become obsolete so customer is looking to have new lineup to feed transformers and keep existing lineup as a backup.

One of the options being kicked around is extending the (2) 35kV services to a new M-T-M switchgear and using the new switchgear as the primary means to feed existing transformers. For redundancy sake the customer wishes to keep existing switchgear as a redundant backup to still be able to power transformers with new switchgear out of service. The proposed means to do this is to install a new MTS at the primary of each transformer to reconnect existing transformer feed from existing switchgear as well as feed from new switchgear. (Another option here was to remove MTS and just connect feeds from each switchgear lineup to transformer primary with breaker interlocks. I don't see this as common having two different feeders to transformer primary)

A new MTS presents some challenges with physical space and trying to re-sue existing feeders as well as the fact that this concept still will utilize an aging and almost obsolete lineup as a redundant backup.

In my opinion the better approach here would be to build the new switchgear to have the same reliability and redundancy as the approach above would provide. One thought I had based off a past project was to make the new switchgear lineup a double-bus / double-breaker lineup to give a high degree of reliability. My main question is how much reliability is gained with the double-bus / double-breaker vs a standard M-T-M when it comes to cost? Also when looking at the application here with having (2) M-T-M lineups with one being almost obsolete does a new double-bus / double breaker make sense?

Appreciate any perspective on this.

 

[FacEngrPE]

I would consider the potential reliability of old obsolete gear (whatever it is) suspect. If old equipment is paralleled with new equipment the resulting reliability is limited by whatever has the lower reliability.

 

[RRaghunath]

Customer is King and, we are supposed to give what he desires, without compromising on safety and reliability.
Considering the constraints you mentioned for installing MTS, how about removing the cable terminations at the transformer and leaving them in the pit (with proper preservation measures).
The new switchgear and new cables will connect to the transformers and, if any time in future the customer needs to supply the transformer from old switchgear, it could be a matter of lowering the connected cables and terminating the old cables lying in the trench.
If client is agreeable to new switchgear that is extremely reliable and discard the old one completely (which I suppose is the neat option), I would recommend Breaker-and-half configuration for the new switchgear.
This is more economical than double bus - double breaker scheme with same level of reliability, allowing for bus or breaker outage any time without affecting the transformer supplies.
You may evaluate with the vendors on space and cost requirements with both options and decide.

 

[rockman7892]

Thanks for the responses!

@RRaghunath - I'm not sure how much room there will be in transformer primary pit (Air terminal chamber in this case)to lay a spare set of cables for termination. Do you see an issue with leaving both sets of cables terminated from two different feeders with the respective feeder breakers interlocked so that only one can be closed at a given time? The main issue i see here is that you will always be back-feeding to the line side of the feeder breaker that is not closed to feed the transformer which could present maintenance or other challenges.

Regarding a potential breaker-and-half scheme I sketched out attached options of M-T-M and Breaker-and-half for comparison in this application. A Breaker-And-Half scheme will only add one breaker so I don't know the cost different will be that significant but I'm not really seeing a huge benefit with reliability in this application with the Breaker-And-Half. The main reliability I see is if the feeder breaker in the M-T-M arrangement failed you cannot feed transformer where in the Breaker-And-Half scheme you have two breakers which can feed transformer.

Perhaps I'm missing some other benefits to Breaker-And-Half in this application beyond just the potential benefit for feeder breaker failure in M-T-M?

 

[rockman7892]

For some reason attachment didn't attach in previous e-mail so re-attaching here.