Internal Generation vs. Grid Power 2

[steve58]

I have been reviewing a proposal from a company to install on-site power generation (natural gas fired engine generators) to handle a part of their building load. The remaining load would still be supplied from the local utility power grid. Has anyone dealt with this situation? I am particularly interested in how you segregate (or if you can) the grid supplied power from the on-site generator to avoid an inadvertant out-of-phase sync of the two systems. Is this something someone has done in a commercial office building?

 

[peterb]

The usual way to handle this would be to synchronize the on-site generation to the utility supply, so that the utility is available as a no-break backup for the loads being supplied by the generator. Of course, it would be better if the generator capacity were able to also act as a backup for the utility supply.
In order for the generators to continue to carry their portion of the load, you would need switchgear to separate those loads when the utility goes offline, also to re-synchronize when the utility comes back (the synch process can be automated). This approach can become really challenging when you are working with an existing power system. The utility should have standard requirements for this type of installation.
If you are not already doing so, you should consider a cogeneration plant instead of a base load simple cycle generator, as this can greatly improve the building energy efficiency.

 

[Newton1Law]

Steve58,

The process of operating on site generation for your building is considerable and can be very expensive. Part of your decision to own and operate a plant should be based on the economics of doing so. If your utlilty supplies you with power using a rate charge that is based on energy use (kWH) and demand (kW) you can determine how much you can save by generating your own power by looking at what reduction you expect to have due to your generation displacing the power your purchase from the utility. Example: Assume you have a 10 megawatt demand with a usage of 3600 MWH. Also assume you pay $100/MW of demand and pay an energy rate of $70/MWH. Your bill for the month would be (10MW)($100/MW)+(3600 MWH)($70/MWH) = $235,000/month. Now lets assume you put a 5 MW generator in place to help reduce the amount of energy and lower the peak demand you have supplied by the utility. Assume you run your generator 24 hours a day. This means you would be supplying 3600 MWH of energy per month, some used by your plant and some sold to the utility during those time periods when your plant has less than a 5 MW demand. When your plant requires the 10 MW mentioned above, the utility will supply the additioanl 5 MW at a cost to you of $500/Month. The total bill per month would be very close to the 5 MW demand charge only from the utility. The $234,500 you save would have to go for paying off your capital investiment in the plant, cost of maintenance, fuel, interconnection equipment costs, etc. This is the first thing to consider before you place a generator on line. There may be additional expenses you will have to incurr if the utility has special provider costs to supply you if your generation fails or if you have a rate agreement with them for interruptable loads during peak demand days, etc.

The rest of the equipment and engineering required to place the unit on line is part of the installation costs you will have to invest in.

Hope this helps.

 

[rhatcher]

I will focus on the engineering aspects versus the economics, but do note that economics and regulatory (environmental, etc.) considerations are of major importance for considering a project like this. Presumably this is addressed in your proposal.

To the "engineering" aspects, you may install a unit that will parallel (synchronize) with your incoming feed and reduce your power draw fom the utility, but there are some major considerations beyond the obvious benefit of saving on power bills. The most obvious, as mentioned by electricpete, is what happens when the utility goes off line. Clearly, the auxilliary generator must be disconnected from the grid to prevent backfeeding. This can be accomplished pretty easily with relays.

However, if you want the aux generator to supply power to any loads during the utility outage, a dedicated switchgear section is usually required. There are numerous ways that this may be accomplished, from the extreme case (you have unlimited money like the government, you think like NASA, and you desire triple redundancy) to the simple approach (you have done so much with so little for so long that you can now do anything with nothing!). If you are interested in more on this I may be able to start the discussion in the direction of suggested approaches.

 

[TBP]

Every project is different, of course, but what usually determines the economics of jobs like this is having a suitable "home" for the recovered heat from the generator drive. The base heating & cooling loads typically determines the size of the generator. If there's a chilled water load in this building, you could consider using absorbers for this requirement, instead of screws or centrifugals. This gives you a home for the exhaust heat in the summer, and can usually shed several hundred horsepower (often more) in drives.

The ideal situation is a year round process heating or cooling application that is independent of seasonal temperature fluctuations. The spring and fall "dead zones" in commercial building heating & cooling often kill these projects, or at least greatly reduces the size of the generator. Sometimes, relatively small generator projects can pay back nicely on just shaving the peak charges.

If you have an induction generator, you won't need an elaborate relay system. With a synchronous generator, you can have a stand-alone generating capability & correct power factor, but since you can also run independent of the grid, the utility will require much more in the way of relays.

 

[steve58]

Very good insight. We have considered operating in parallel with the local utility as an option. However, we are interested if we can operate our in-house generation (it is a cogen) to feed particular loads while the utility feed supplies other loads and the in-house generation is not operating in parallel with the utility. What special precautions/designs might be necessary to ensure there is no inadvertant (out of phase) cross feed? Can it be done simply by seggregating the circuits or is there something else that should be done?

 

[peterb]

Steve58 -
Yes,you can certainly segregate the loads on a dedicated switchgear bus. What would be needed in this case is a transfer switch (or equivalent circuit breaker arrangement, with interlocking) to transfer from the generator to the utility source when the generator is down. This transfer can be auto or manual and there would be no question of inadvertent parallel operation if the interlocking is properly arranged. You haven't mentioned the proposed generator system rating, which would be a major factor in selecting the transfer switchgear arrangement.

(note to rhatcher - I am actually not the same person as electricpete, although our views seem to coincide on a lot of these posts)

 

[BJC]

Good answers.
I think it would be possible to use a natural gas fired reciprocating engine generator as a "ups or standby source". This would not be the leaglly required standby source but the load that is usually hooked up to a ups. Chip plants have large UPS loads that are connected to UPS systems. the trend seems to be moving away from solid state UPS units to rotary units.
I think the large low rpm gas generators would be more reliable than the solid state UPS units. Theres fewer parts in the system, the technology is more mature and once the engine-gen set is in the bottom of the bathtub curve they don't fail.
Summary: If I had a large UPS or critical load I would evaluate that as well as the waste heat into the gas generator equation.

 

[rhatcher]

peterb is correct on all of his points. The only precaution that I may add is that if provisions for an 'emergency' tie is made to the utility as suggested by peterb, then you should make sure that the tie connection has the same phase sequence as the utility source.

peterb, excellent post. Sorry about the name thing, it was simply a typo on my part. I know who you are from following your posts and hope to see more from you.

 

[jbartos]

Suggestions:
1. Please, would you post some figures since there may be a better way to approach your project over specific numbers for kWs or MWs? E.g. Generator size in kWs, Utility supplied kWs without generator, number of utility services to the building, etc. There is:
IEEE Std 241-1990 (R1997) IEEE Recommended Practice for Electric Power Systems in Commercial Buildings (IEEE Gray Book) [Description]
Print: 768 pages [1-55937-088-2] [SH13912-NYF] $93.00 * IEEE Mbr: $74.00
PDF: [0-7381-1144-9] [SS13912-NYF] $140.00 * IEEE Mbr: $112.00
available on

http://www.ieee.org

2. The power distribution system and nature of loads will also be useful to know or to have it somewhat described, although you have already mentioned the "commercial office building." There, may be load priorities involved and power quality to be considered to some loads.
3. It appears that some rerouting of power distribution installations may be needed.

 

[steve58]

Building is a 15-story office building. Total estimated peak electrical load is 7500 kW. Cogen is sized to match building thermal load and will provide a peak output of 3,800 kW with remainder of load served from utility connection. Expect to split the building sevice such that the cogen supplies two circuits (a total peak of 3,400 KW). Each cogen-fed circuit would have an ATS to transfer load to utility if the cogen trips. this would mean that 1/2 of the building would normally be supplied by the cogen and 1/2 by the utility. Concerning loads, approx. 1200 kW will supply the building LAN. The remaining loads are typical building loads.

Thanks for the info on IEEE

 

[skylerman]

As you can tell from the number of responses this issue of on-site power generation, induction or synchronous, is an up and coming scenario in the wake of higher electrical and natural gas costs. Depending upon your type of facitlity cogeneration might be a very viable answer. In order to suggest the best option a description of your facility and/or load analysis would have to be conducted, rates for electricity and gas would have to be known, and utility regs and expectation of generator use would also have to be known. If you would like - please contact me as to your wishes and location and I can discuss this further. It is very doable and viable and I would welcome the opportunity of providing the information from which you can make an informed decision.

 

[jbartos]

Suggestions to steve58 (Mechanical) Jul 20, 2001 marked by ///\\Building is a 15-story office building. Total estimated peak electrical load is 7500 kW. Cogen is sized to match building thermal load and will provide a peak output of 3,800 kW with remainder of load served from utility connection. Expect to split the building sevice such that the cogen supplies two circuits (a total peak of 3,400 KW).
///Considering the size of the peak (3800kW), it may be a good idea to increase that peak. The 400kW margin appears to be on the lower side, even though the trend in electronics indicates some reductions in the electrical power consumption. A load growth analysis is often useful to include.\\Each cogen-fed circuit would have an ATS to transfer load to utility if the cogen trips. this would mean that 1/2 of the building would normally be supplied by the cogen and 1/2 by the utility.
///The increase in the cogen size, mentioned above, could power some necessary loads to be on, in case of the Utility blackout.\\Concerning loads, approx. 1200 kW will supply the building LAN.
///There may be need for some power quality analysis because of harmonics. Usually, harmonic filters are considered.\\The remaining loads are typical building loads.