Fuel consumption for VAR flow

[Schwatzernov]

Hi,

I got the query from customer that;
they operate diesel generators(4x5.9MW)to supply power to their customer cement plant. Their customer also has 1x4MW steam turbine generator. The problem is that their customer's loads vary between 2-4 MW and customer's steam turbine runs at 4MW base load and excitation operated in power factor control mode set at 1.0 PF.
So the diesel plant always operated in zero PF condition without supplying active power to customer while MVar always flow to customer plant to support Var requirement.
when customer's STG is operating at 4MW, there is no MW export to cement plant. The diesel power plant cannot get the revenue from selling power to customer but have to export Var to customer.
when customer's STG is operating at 2-3MW, there is reversed power together with Var export to cement plant.
My customer said that they suffer from diesel fuel cost due to Var exporting to client without getting paid for MW.

I check the fuel consumption of diesel generator and it is based on kWh only. There is no relationship between fuel consumption and Var generation from any manufacturers.

So my question is that whether my customer's understanding is correct that whenever diesel generator has to generate Var, there is fuel consumption ?

Thank you for any advice.

 

[davidbeach]

How else are you going to spin the generator if not by consuming fuel?

 

[Schwatzernov]

At starting up of diesel generator, yes it needs active power to spin it off from standstill. This certainly requires fuel.

But the point is that when it is running and when active power becomes zero and only Var is flowing out, the fuel consumption is just to cover losses in primover and generator only. So the fuel consumption is insignificant. However my client understand that if exported MW is zero and if exported Var increases, the consumed fuel increases proportionally.

 

[FreddyNurk]

I wouldn't have thought that fuel consumption at idle for a diesel that size would have been insignificant, at least not compared to supplying vars by some other means.

The other consideration would be the loading on the generator, I'd be sure its not going to be good for it to be operated in such a manner.

I wouldn't, however, consider that diesel loading would increase along with Var only load.

 

[GTstartup]

"However my client understand that if exported MW is zero and if exported Var increases, the consumed fuel increases proportionally"

your client is wrong.

Why don't you just measure the fuel consumed while not exporting VARs and then measure the fuel consumed while exporting VARs?

The difference will be very very small.

 

[rmw]

If it can even be measured at all.

rmw

 

[Schwatzernov]

One thing I thought is that it might be because the Var exporting increases (active power is zero or negligible, from V-curve of generator, at zero power factor the field current increases and armature current increases. This might be one reason since the losses will be higher even though the active power is zero.

Hence as long as losses in rotor and stator increase, the fuel consumption is definitely increase.

Is this my assumption sensible ?

 

[abeltio]

To absorb Vars (negative Vars) what is used in these cases is a generator operated as synchronous condenser.

A clutch should be installed between the diesel and the generator.

the generator is synchronized to the grid with the excitation on, and the diesel is shut off.

the generator stays spinning at rated speed using the excitation for Var absorption.

it will need as well a modification of the lube oil and control system amd speed pick ups (for the generator).

this is "foul play" from the customer... keeping the diesel running does not help the generator to stay synchronized: if you stop and brake the diesel (with the clutch in between) the generator does not know that there is no prime mover.



saludos.
a.

 

[dpc]

Your assumption is correct from a qualitative viewpoint, but your quantitative assumptions seem to be a little off.

As the current increases, there is an incremental increase in the copper losses in the machine. No question about that.

However, these losses are much less than the friction and windage losses in the system, so the total losses are definitely NOT proportional to the vars.

Vars produced by synchronous machines are about as "free" as it gets.

Whether or not their method of operation makes sense depends on the fuel costs and the fuel consumption rate needed to keep the machine in 0 kW spinning reserve.