Running a DG to compensate reactive power demand

[Power0020]

I wonder if anyone came across this application where a facility with heavy poor power factor penalty choose to run a diesel generator in reactive compensation mode (overexcited).

How to estimate the fuel consumption in the case of running with no active power?

would there be any special precautions with the controller?

 

[FreddyNurk]

Since its possible to do the same thing with a synchronous motor without having to worry about the generator, the only reason I can think of that people would bother is that it might be easier to get hold of a generator set than set up a motor.

Its also possible for inverters to do the same thing these days, but its likely to be even less cost effective, as the equivalent size inverter is a lot more expensive than the other options.

There are operational issues in terms of running a diesel set under little load, and whilst the common rail electronic injection units are far better at handling such conditions than the older sets used to be, I'd still expect trouble.

 

[waross]

I have seen it done for an area that included the third largest city in the country.
The diesel sets were run over excited to compensate for reactive transmission line losses.
Fuel consumption. Look at the manufacturer's specs for 25% load fuel consumption.
To estimate low load fuel consumption, assume that the set uses 13 Gallons per Kilo Watt Hour to service the load.
From the consumption at 25% load, subtract the consumption of 25% at 13 Gallons per Kilo Watt Hours to get the no load consumption.
To that, add the consumption for the intended load.
Rough calcs but it will get you in the ball park.
Issues at light loading.
Slobbering or oil pumping. The design of the piston rings uses the pressure of combustion to force the rings against the cylinder walls.
If the rings are not well seated to the cylinder walls, they may pump oil out the exhaust.
This can be an issue with new sets and with high hour sets.
I have had two sets that pumped oil badly on the first run. The load on a standby set is what it is. After all the advice and direction as to avoiding light loading, many standby sets must accept load profiles that include too many hours at too little load.
It is what it is.
The remedy for a new set is to bring in a load bank and run the set loaded for a few days. Expect to use a lot of fuel and to boil off a lot of water.
If you are using a standby set, any amount of oil pumped out is dangerous.
After extended use at no load or light load you may have oil in the exhaust system. In the event of a power failure when the set is called upon to deliver full power, you may have balls of burning oil blowing out the exhaust stack.
Run the Automatic Voltage Regulator in PF mode.
In the event of a power failure you should make provision for the AVR to switch over to voltage control mode.
Be sure to incorporate a reverse power relay. Use it to shunt trip the generator breaker.
Many hours at light loading may eventually lead to slobbering on an originally well performing engine.
Look for a surplus synchronous motor. That is a better solution.


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

 

[ScottyUK]

Any reason why a stepped capacitor bank can't be used to keep the PF within a certain tolerance of setpoint? Cap banks are cheap compared to ongoing fuel burn and the risks to the engine which Bill has already covered, which is why they are the common solution.

 

[itsmoked]

Bill, did you mean 1.3gal/kWhr?

13 seems WAY too high.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Power0020]

Cap banks are cheaper for sure, but the facility have no funding at the moment to upgrade them while the have an emergency DG rarely used, this is why I thought about using generator to compensate reactive demand.

One point here, I believe the generator PF can never run too low leading, this will cause a lot of heating, so we may have to supply some real power as well to keep the generator well within its capability.

 

[waross]

Hi 'smoked. That is the figure that I remember, but I thought at the time of posting that it sounded high.
Give me a few minutes to recheck and post back.
.................
May I reciprocate to your post by suggesting that the correct figure is 13 KWHr per gallon, not 13 gallons per KWHr.
I apologize for my "Seniors moment".

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

 

[cuky2000]

The reactive power for a DG could be estimated from the reactive capability curve of the unit. (See typical curve below)
The engine fuel consumption is a relatively small portion of the regular consumption when generate kW. The fuel required is to turn the engine/gen set plus the losses.
I do not have figure for DG fuel consumption. However for large synchronous condenser the require power range from 3% to 5% of the MVAR rating
Hope this help

 

[waross]

Another issue with lighly loaded diesel engines is engine temperature. Some applications have problems keeping the engine hot enough with light loads.
Running a diesel engine too cool will increase fuel consumption noticeably, and lead to eventual problems with the engine.

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

 

[ScottyUK]

One point here, I believe the generator PF can never run too low leading, this will cause a lot of heating, so we may have to supply some real power as well to keep the generator well within its capability.

I'd be surprised if your generator runs at a leading PF, especially if your plant is attracting a reactive penalty: most industrial plants operate at a lagging power factor due to the induction motor loads. A leading generator absorbs Vars just like a lagging load does, so it would worsen your problem, not help improve it.

Your generator is designed to produce active power and export Vars. With little active load it will operate over to the far right of the capability curve where machine capability may become limited by rotor heating from the field winding rather than the stator current limit. By contrast a generator operating in the leading area of its curve with a weak field may have stability problems when operated in parallel with a larger system and most AVR's will have an under-excitation limiter to prevent operation too far into this region.

 

[Power0020]

I got confused between generator PF and load PF.

I understand that the generator is said to be running with a lagging power factor if it is generating reactive power to compensate for lagging power factor loads.

Other than synchrochecks and anti-motoring protections, any "special" precautions to adjust the generator running in parallel with utility line to generate reactive power?

 

[Jebb01]

If you're in the U.S. I'd double check that you're "permitted" (air quality) to operate your DG for this purpose. Especially since you mentioned it's "emergency DG" that's "rarely used". I'm pretty sure cap banks are way cheaper than the fines associated with an air quality violation.

 

[ScottyUK]

Check your utility will even allow you to parallel with their system for a prolonged period. Some reluctantly accept short term paralleling, but impose fairly onerous conditions on long term paralleling because of difficulties it can cause by altering power flows, affecting the behaviour of protection and control schemes and posing a safety risk from a source outside of their control.