Mixing Cogeneration and Grid Power?? 1

[EASIJon]

Hello,

I have asked another question here recently and received several quick and accurate replies. I am not an engineer, but I do work with industrial companies to implement full-system electrical cost reduction projects. An existing client has a question that I cannot answer. I hope that someone on this board can help.

This customer is a large tile manufacturing facility, and presently they are on interruptible electric rate schedule. In other words, in exchange for low rates the power company can curtail a significant amount of power during periods of peak demand.

They are guaranteed to receive at least enough power for 800 kw but this will still result in quite a few people standing around during a period of curtailment. The facility has the capacity to generate at least 1000 kw of it's own power using natural gas. Presently these generators are only configured to provide emergency power. The plant engineer is interested to utilize this generating capacity to supplement grid power during upcoming periods of curtailment. She has been advised that this will be difficult because the phases, voltage or frequency may not be in sync and would offer potential for damage.

What type of transfer devices would be necessary to combine grid power with power that is generated on site? Is this a costly solution? What are the potential dangers of mixing these two sources of power without some way to balance the phases and match voltage etc.?

I hope that this provides enough information so that we will recieve helpful responses.

Thank you.

 

[electricpete]

Just some thoughts.

For one thing, you'll need to check with the utility. They will have some requirements for relaying to protect themselves from you. Among other things may include a reverse power trip to prevent you from carrying the feeder (including your neighbors) when the utility goes out... otherwise they'd be exposed to liability for whatever damages your generators might cause other customers' equipment due to strange voltage/frequency that might occur. I'm willing to bet they have a set of fairly prescriptive guidelines (requirements) for this type of connection, and an engineer who reviews the requests.

There is a procedure for paralleling sources of power. It involves checking voltage phase and magnitude accross the syncronizing breaker... also sometimes interlock may be required to help make sure you can't screw it up.

You'll need some kind of real and reactive power control including frequency and voltage feedback to make sure your generator picks up the proper amount of load.

To avoid some of these complexities.... is it possible to split the plant load onto two separate buses... one powered from the grid and one powered from the emergency generator with no interconnection. Would still require some thought as to the transition from full grid power to half grid power and whether you will deenergize half the plant or momentarily parallel.

 

[rhatcher]

One thing that I might add to electricpete's excellent answer (star) is based on some of the current events in California (US) where utility power is in shortage. Some industrial facilities there sought (are seeking) to use emergency power units as permanent power supplies since the utility power is subject to blackout and the cost is very very high. I know of one facility that installed 54 750KVA, 13.8KVAC diesel driven emergency power units and intended to disconnect from the grid. Others seek to use gas turbines as well as any other type of prime mover that you can think of.

The problem is that once you run a generator for permanent service (versus emergency power) you are subject to regulation as a power plant. As well, even emergency only use or peak shaving use may be subject to regulation if the system capacity is large enough. In the case of diesel driven generators, they definitely do not meet the environmental standards for emissions (in the US) and cannot be put into service for that use without an emission control system (ie. electrostatic precipitator, scrubber, etc.).
Natural gas prime movers are, of course, much cleaner than diesel. However, I would still check local regulations before you invest in the equipment to make sure that you can meet all of the requirements for operating that equipment for cogeneration. After all, wouldn't you hate to be the guy that spent the money to buy and install the 54 diesel generators but cannot turn them on!

 

[jbartos]

Suggestions: There are additional areas that may be feasible and have not been addressed yet.
1. Identify loads that could be supplied by the Utility allotment, e.g. 800kW less some buffer (perhaps 50kW to 80kW)
This load would be dedicated to the Utility power supply.
2. The remainder of your loads would be powered by your facility power sources and connected as economically as possible. Some cost trade-off analysis may be appropriate.
This way, there would not be need for the interaction with Utility, if the Utility would not appreciate/allow purchase of your surplus generated electrical energy.
3. If 1. and 2. above are not feasible, e.g. some unreasonable installation modifications would have to be performed, then the Utility has to be contacted regarding terms of paralleling your power generation facility with the Utility. There is a way the genset control can be set that the energy from the Utility is set/controlled up to allowed 800kW and your genset would supply the rest without supplying power to the Utility. Also, the Utility would appreciate, if you produce compensation for your load kVARs that the Utility usually does not get paid for.
4. Some modeling or simulation of your power distribution and loads is recommended since this usually pays for itself handsomely. Also, the energy distribution/consumption margins would be known beforehand and fast.

 

[peterb]

The major bases seem to have been covered above, except for the heat recovery side of the equation.
If you are going to implement an actual cogeneration system solution, this implies that the waste heat from the generation of electric energy will be recovered and used in the plant process. Heat recovery methods are many & varied, depending on the type of prime mover, but the bottom line is that you can typically recover up to say 50% of the generator load from the waste heat. The trick is of course in being able to use the recovered waste heat energy - the plant process needs must be carefully evaluated to determine feasibility.
Cogen projects are usually very attractive in offering a short payback period, dependent largely on the electric energy and fuel costs involved.