Generator and Capacitor Bank

We are using two 30,000 kVA, 0.8PF, 2 pole, 11KV, 50Hz alternator for captive power generation for a large fertilizer complex (captive power) in a gas based power plant. Both GTs' run in parallel. One is in Iso and one is in droop.

Till recently we were using a capacitor bank to maintain power factor (PF) of generation. Now,a senior officer has objected to it and have removed the capacitor bank. earlier our power factor was maintained around 0.86-0.88 now it has come down to 0.82-0.83. the reason which he gave, is that capacitor bank is maintaining the line load rather than generated load and this may result in generator trip owing to some line fault!

As far as i understand, alternator only reacts to the external load demand and as most of the load in complex is inductive henceforth obviously there has to be a lag in voltage and current.

Now my question is, is it really justifiable to remove capacitor bank entirely?

We were or are wrong?

Regards
fluidflow
 
Fluidflow... I apologize for having missed your post!

What did you mean by "LINE LOAD" in your senior officer's position that "CAPACITOR BANK is maintaining the LINE LOAD rather than GENERATED LOAD"?

Regards,
Phil Corso
 
Hello Phil,

The problem or dilemma is still persisting.

Senior Manager is telling that Capacitor bank has led to some trips in the past owing to "Third Harmonic Oscillations." Now being a Mechanical Engineer, I am not in a position to counter him.

I have studied some standard texts and have discussed the matter with some seniors, but still I do not understand why Capacitor bank is kept outside the circuit. if we can think of it just as a capacitive load.

In the managers' opinion keeping the capacitor bank in line is not a good solution and since Generator is designed to operate till 0.80 PF, then there is no need for it.

I hope to receive your kind concern.

Regards,
fluidflow
 
Dear Phil,

I have send you the single line diagram via email.

Subject: Fluidflow sending single line diagram.
Hope to hear from you very soon.

Regards,
fluidflow
 
FF, problem is clearer. Following is a summary, based on information provided to date:

A) Gen1's output is 19.8 MW & 12 MVAr, or about 23.2 MVA (77.4% of rating).

B) Gen2's output is 15.5 MW & 12 MVAr, or about 19.6 MVA (65.3% of rating).

C) Total load is 35.4 MW & 24 MVAr (without Cap-Banks) or 42.8 MVA at 0.83.

D) Without Cap-Banks all of Load-MVAr is produced by the generators. This appreciably increases both stator and rotor currents.

E) Unfortunately, increasing current increases losses, and the increase is proportional to the square of current.

F) The gas-turbines will require more fuel, thus increasing heat-rate, and lowering plant efficiency.

G) My conclusion is that re-commissioning the Cap-Banks will significantly reduce losses and lower heat-rate.

H) An approximate magnitude of cost-savings can be determined if the Generators’ rated efficiency is known.

I) I need much more info to review your Senior Officer's charge that the Cap-bank is the cause of unwarranted plant trips.

Regards,
Phil
 
FF... Please play particular attention to Candl_Pete's excellent post.

He was able to illustrate my thesis with an example that proves MVAr influences losses.

Regards
Phil
 
Dear Phil,

I will try to send you a scanned copy of Generator Capability Curves, which in my opinion shall be of some aid to you.

nevertheless, the amount of attention which you have paid fills me with respect and gratitude for you.

Regards,
fluidflow
 
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