Gas generator vs Grid load sharing 1

[MikiBg]

Hi all!
Waste water treatment plant produces gas during cleaning process, and it should be use for gas generator supply, giving electric power and heat at the output.
0.4kV PCC is feed from grid via 20/0.4 distribution transformer, and from two generators.
How would generator and grid share the load when they're coupled?
If rated electrical output of gen is available, and load is less than that, will gen limit it's output to load demand, or inject rated output in PCC, part of it to load, and part back into grid via transformer? Is this scenario always the same, or it can be determined by generator's control cubicle?
I'll appreciate your answers and links.

 

[waross]

The generator is connected to a common point and can not tell what the local load is. If the generator is supplying more power than the local demand the excess will backfeed the grid.
It may be possible to use an external load control panel to limit the output of the generator to the local load, but there would be no advantage and there may be some interesting issues with rapid load changes and with loss of the utility supply.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

I have put a number of these systems in using Woodward based load controls, mostly DSLC and MSLC based systems and recently one using their EasyGen controls.

Essentially the generators are in a baseload control mode, and the setpoint can be be varied using site load demand, gas supply availablity, allowable utility sell back, etc.

We have had systems where the design allowed for island mode capability, with the intent being that if the grid went away the generators would switch to island mode and carry the site load. In reality gas engines running on digester fuel are pretty unstable and depending on load rates, fuel quality, and engine condition the ability to pickup the plant on the fly is usually pretty limited. Our best success was supplying a diesel genset sized to provide site critical loads, including the digester fuel treatment and supply systems, and get the plant back up on the diesel, then bring on a gas engine to parallel to it. A properly designed system can then parallel back to grid and return to baseload mode when it returns.

In addition to the generator load and VAR controls, a real power sensor is usually added at the PCC, and depending on your utilites requirments, a master controller like a PLC or DCS can manage to load rates of the gas engines.

Woodward has some very good information on the subject, and in Europe SEG and Woodward both did a lot of control systems for CAT, Cummins and Jenbacher for digester fueled CHP projects.

Hope that helps.

 

[rbulsara]

A properly designed generator paralleling control system will have the control to set how much load the gen would provide and how much utility will.



Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Thanks catserveng. LPS
Am I correct that you can set a base load with the basic governor on a gen-set, but you need some type of load control panel to track a varying base load?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

Usually when baseloading to a grid (or large stand alone system) the engine governor by itself isn't enough. Originally smaller units operated in droop, but required a lot of operator intervention, especially if the system was a bit "soft" (varying voltage and frequency). Older systems used a Generator Loading Control feeding a signal to the load share lines on a Woodward 2301. The DSLC came out in the early 1980's and made doing baseload and remotely controlled loading systems pretty easy once you figured out all the settings. We started seeing both computer and PLC based systems driving an analog voltage signal out being used for load controllers in the mid 80's. Now there are a fairly large assortment of solutions, each with their pros and cons.

Bill, you're correct, the engine governor, either a Woodward or someone else's, and now days the built in governing in the ECM, is still used. The governor controls the engine to rated speed until the breaker closes, then some form of bias on the speed raise/lower reference input is used by whatever is controlling the load to set the load output. More fuel=more load, less fuel=less load, just got to make sure it doesn't over or underload the unit, and depending on how critical the system is drives how much protection you add to make sure the load controls don't harm the generator or the system or both.

I guess the toughest part is assuring the correct load control dynamics for the installed system, since i haven't found a "one size fits all" solution. Gas engines on bio-fuels can be really tough, especially when you add the current exhaust emissions regulations on top of trying to make a an engine running on half fuel and half inert try to keep running, not to mention all the junk that can come in with the fuel. Digester fuel can be a challenge due to daily changes in fuel energy content that affect power output and response.

Woodward has a very good general publication on load control, reference number 26260, Governing Fundamentals and Load Management. Use link below and seach for title or reference number.

http://www.woodward.com/pubs/pubpage.cfm

Hope that helps.

 

[waross]

Thanks catserveng
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MikiBg]

Very useful link, catserveng. Thanks. Others too.