Emergency Power System - mode of operation

[mayanees]

We are developing a design for the upgrade of a 40 year-old, 13.8 kV Emergency Power System.
The existing system consists of two (2) 3.25 MW, 13.8 kV diesel gensets. These sets energize fourteen (14) 13.8:0.48 kV substations throughout a campus-wide system. Genset operation is initiated by low-voltage (LV) sensing such that upon detection of a 480V loss at any of 250 ATSs, the system is on-line within 10 seconds and all 14 substations are energized.
Only the ATS that lost power is sourced from the emergency system during the genset operation.

I'm writing to get opinions from folks experienced with this type of system, as to the advantages or disadvantages of this mode of operation, that being an emergency system that rests de-energized, waiting for power loss.

I think it would make more sense from a reliability standpoint to bring a Utility source to the generator distribution board that would keep all 14 substation transformers energized, rather than have the system sit idle in a de-energized state. I think the transformers and cables would have less issues with condensation and such if they sat energized.

But the system has operated like this for 40 years, with only about 1100 hours on the gensets, so I don't know if I can present an argument to change the mode of operation. Transformer losses will cause increased cost of operation, but for an emergency system, I think the increase in reliability would be valued.

Thoughts, opinions and comments are appreciated.

John M

 

[davidbeach]

If it were mine, I'd want those cables and transformers energized at all times. You know nothing about the status or condition of a deenergized equipment. Thermal cycling the cables will accelerate the aging process and water ingress, while energization is always prime time for things to fail. That said, keeping them energized may not be a trivial matter; your gen power distribution gear will become more complex, both in configuration and in protection & control.

 

[mayanees]

Thanks for the reply David. That's my thinking as well, but having 40 years of satisfactory performance, I think they'll be reluctant to change. .. unless I can make a strong case for it.

 

[waross]

A suggestion that may allow you to energize but keep operating costs the same or less than with the old system
I imagine that if only a few of the 250 ATSs need power the fuel consumption of a 13.25 MW set may be a little high.
How about an N+1 system.
Some sets may be sized to provide energization power plus two or three ATSs. You will have spinning reserve in the event of a local outage.
A controller will be able to start and run only the capacity needed for any outages.
A first reality check will be a review of the present annual fuel cost. Compare with the number of times the sets ran for local failures and you may have the information you need to justify keeping the equipment energized and running an N+1 configuration.
If you will continue to use the existing 40 year old cables and transformers, keeping them energized may extend their life expectancy considerably.
Your points:
Increased reliability.
Spinning reserve for local outages.
Possible reduction in fuel costs or little increase in costs.
No wet stacking.
As the sets will be operated at substantial load levels, there will be little concern with wet stacking and the associated hazards.
I had one new set that in the first few hours of operation, pumped out so much oil that it shut down on low oil pressure. A 3.25 MW set with only a few loads is a prime candidate for low load issues.
Size of new sets?
Two or three sized so that energization loads are enough to alleviate low load concerns. Add enough extra capacity to handle a small number of local failures if feasible.
The balance of the sets may be sized to each handle several substations.
Alternatively, all the same size sets but set the compression rings in each engine with a load bank run for about 5 days. Easy to say but takes a lot of fuel and water.

Bill
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"Why not the best?"
Jimmy Carter