Distribution Voltage 1

[nightfox1925]

I have encountered one project in which they have used 480VAC, 3phase, 60Hz as their main distribution voltage. Unfortunately, their proposed plant is catering several big motors up to 150~200HP motors and a lot of motors around 20~75HP. They also wish to connect to the utility and build a future co-generation. I suggested that they have to use 4.16kV or 13.8kV distribution level with couple of step down transformers at service points inorder to provide lesser cable sizes at the main distribution. Further, it will also help utilize lower short circuit rated equipment at the main distribution and better voltage profile considering the future modifications they are planning soon. Are there any other concerns of using low voltage as a main plant distribution voltage other than voltage drops and available short circuit?

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[davidbeach]

Capacity; unless they are going to have multiple 480V systems there is a limit on capacity that will be about 1/10 what 4160 could provide and about 1/30 what could be done at 12 or 13 kV.

 

[waross]

For awhile on this forum the answer "It depends" was used quite often.
This is another instance.
I have seen a lot of plants such as you describe running happily on 480 volts.
Utilization will probably be 480 volts.
distribution throughout the plant will depend on a lot of local factors.
I was at a plant years ago where they bought their power at the primary voltage, (Probably 13 kv but I don't remember.)
They carried the primary current overhead on poles, built to the same standards as the utility, to 3 or 4 areas where they installed transformer banks and stepped down to 480 volts.
It worked well.
Some installations are best served with 480 V distribution, some are best served with primary distribution, some have to be evaluated and could go either way.
respectfully

 

[nightfox1925]

We are presently modelling the proposed power system in ETAP with the plant distrbution to be 480VAC. I got a electrical load list showing 2.7MW at 480VAC as the maximum plant peak load. They have sized their in-plant generation to be 2 x 1.5MW STGs plus a future 1-1.5MW STG (for future power export). If I will assume a 12% P.U. STG reactance, STG P.F. =0.80, for 2 STGs, it will yield 2 x 18.8kA=37.6kA and for 3 STGs = 56.4kA Sym. I will end up using a 80kA or a 100kA Switchgear and MCC or worse if utility and motor contribution will be included. This is for a 480VAC distribution.

In addition, is it advisable to load 2.7MW to a 2 x 1.5MW prime rated generator? I am thinking that I could not possibly load more than 80% of the generators' total rating. Furthermore, if one generator is tripped, then the other gets overloaded at around 200% and would lead to a total plant shutdown. Even with 3 x 1.5MW it would lead to same scenario. I am looking on at least 3 x 3MW STGs so 2 STG gets 50% load sharing while the other future 3MW is for power export one the utility gets interconnected. Due to short circuit related reasons, I may end up using a 4160V distribution.

Is my initial recommendations correct? Your comments and suggestions is appreciated

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[waross]

The physical distances between utilization areas will have a bearing on the voltage choice.
I would investigate the possibility of using the utility supply voltage for distribution rather than introducing a third voltage into the system.
respectfully

 

[davidbeach]

If the utility primary supply is less than 25kV I would only consider 480V and the utility primary voltage. 4160V would give you most of the disadvantages of both other voltage levels without out a significant offsetting advantage. That would change if a significant portion of the load could utilize 4160V motors.

With 480V distribution and generation (assuming solidly grounded) and the generators operating in parallel with the utility you need to watch your SLG fault currents, they will be significantly higher than your 3-phase fault currents.

If the plant load is truly 2.7MW and won't/can't exceed 3MW you should be fine with 3MW worth of generation as long as you can keep both machines on line. Three 1.5MW would be vastly better if you can't count on the utility to instantaneously make up the loss of a single (or both) generator. Motor starting (with out utility support) might push your requirements up somewhat; that depends on how large the motors are and what voltage sag you can endure.

I'd look at bringing in the utility primary voltage into a switchgear line-up. I'd try for breakers all the way along, but certainly you need breakers for the main and the generators; if you want cheap (you get what you pay for) you could use fused switches for the distribution to the loads with a breaker between the main bus and the load bus. For the generators I'd pair each with a 2MVA transformer, delta on the generator side and grounded wye on the high side; a 480V breaker on the generator would not be necessary; bringing up the generator with the transformer connected eliminates transformer inrush.

Power can be significantly less expensive at primary compared to secondary (4160V would be secondary and billed at the same rate as 480V and might incur additional charges for special transformers). Fault current issues will be reduced, both in the main gear and in the 480V unit substations. The generators could be impedance grounded with impacting the ability to have phase-to-neutral loads. You have options as to how you ground the high side of the transformers and can limit the SLG fault current while on generator and/or limit the increase in SLG fault current while paralleled with the utility. It is unlikely that you will be able to get an impedance grounded connection from the utility.

 

[nightfox1925]

Thanks david, waross. David, the utility voltage turns out to be 13.8kV. Did you mean looking into 3 x 1.5MW STGs rated at 480VAC, each connected to 3 x 2MVA, 0.48/13.8kV delta-wye (wye impedance grounded if SLG fault is high) step up transformers connected to a paralleling switchgear with a Utility interconnection? The loads will be connected to a separate 13.8kV bus and interconnected to the generation/utility bus through a bus tie breaker. Option of using MV fuses for the outgoing loads. Is this the arrangement you are suggesting for me to look into as an option on your last two paragraphs?

If the running loads can be safely taken up by the 2 or 3 x 1.5MW STGs at 480VAC without utility generation. Then I can say that I may interconnect later with the utility using a 13.8/0.48kV step down transformer. Of course, we will look into the available SLG fault values + the short circuit values. If the short circuit goes way up high for the three generators + motor contribution (without utility) that it would take a 100kA switchgear alone, is it going to be a good reason for me to go after the 13.8kV distribution which would yield me lower short circuit and better voltage profile on the downstream end?

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[davidbeach]

Sounds like a much better direction to head. Lots of project specific details that I'll never know so I don't know exactly how it should work out, but I think you're headed in a better direction now.

 

[nightfox1925]

THANK YOU SIR. I will take over from here. Glad to have some technical partners were I can get second opinion. More power...

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