Power Factor Improvement For Generators 2

[hamad522452123]

Respected Members
I read and visited many sites but I m not clear about Power Factor Improvement Panel should install where only Generators supplied the electricity
I am working at compression station. We have 634 KW Generators installed. The current Power factor is 0.8, My boss said it should be improve to 0.98.
Remember we r not given any electricity to any user.
But I read that if we improve power factor the it may be damaged generators by Over excitation. I am unable to understand its theory and still confuse i am going to install or not?

Please help me with theory or share any article about this.

Regards

 

[waross]

Your boss must be an MBA.
Show him a number and he will demand that it be higher.
One way to do this, (and we did this once as an experiment) is to connect a second generator in parallel.
Adjust the throttles so that one generator is supplying 100% of the real power and the other generator is supplying 0.0% of the real power.
Now adjust the excitation so that the set supplying the real power is supplying 0.0% of the reactive power and the set supplying 0.0% real power is supplying 100% reactive power.

The adjustments interact and also interact with the voltage.
Trim the throttles for real power.
Trim the excitation for the 100%/0.0% division of reactive power.
Trim excitation again to correct the voltage.
Go back and trim the throttles.
Trim the excitation for reactive power sharing.
Trim the excitation for voltage.
Rinse and repeat, about 4 or 5 times.

It is fun but it is pointless.

Back to the real world:
If your compressors are running 0.8 PF, than they need that much reactive power to operate.
On a grid based system, you may add capacitors to improve the PF as seen by the grid, but that does not change the PF of the motor.
The combination of motor and capacitor may have 100% PF but the motor by itself is still running at 0.8 PF.
It needs those VARS that cause the poor power factor.
Normally, an induction motor draws the needed VARs from the grid, but the VARs may be supplied locally by capacitors.

So, what will happen if the PF is corrected with capacitors?
1. The current between the generator and the motor will drop slightly, and so the voltage drop between the generator and the motor will drop slightly.
That's good, right?
How good?
Let's assume that the voltage drop is 3%. We will save a small part of 3% of the losses, which are already a small percentage of our cost.
Adding capacitors here will be spending a lot of money for "Feel Good" but with actual savings.
The time to repay the investment will be decades.
If interest is on the cost of capacitors is considered, you will never save enough to pay the interest.

The downside.
Adding capacitors will drive the voltage up.
How much? Hard to say.
What can happen?
The generator may be driven into saturation. That will probably be self limiting and the voltage will go so high and no higher.
The motor may be driven into saturation. That may damage the motor, it may damage the generator, or it may damage both.

And, by the way, an almost universal design and rating for generators of that size is for 0.8 PF.
Check the nameplate. (The set nameplate, not just the generator end nameplate).
Take a picture of the 0.8 PF rating.
Share this post and the picture with your boss.
Good luck.
ps; You may edit out the MBA comments. grin


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

It is rare to install power factor correction equipment on standalone generators.

The 0.8 power factor rating Waross mentioned is when the generator is running at nameplate kW output. When the real power output is lower than nameplate, running at lower than 0.8 power factor may be acceptable depending on the exact thermal limits for various components in the generator.

 

[edison123]

KW is fixed by the prime mover and KVA by the generator. Improving the PF reduces the cable currents and hence lower cable I[sup]2[/sup]R losses and cable temperatures. PF correction up to 0.95 lag should not affect the generator voltage much.

In my shop, all the motors have PF correction caps connected at the motor terminal box for 0.95, and I face no issues when supply is switched from utility to standby generator.

Muthu

http://www.edison.co.in

 

[RRaghunath]

Generator field is strong, giving better stability to he generators when the generators run at lagging power factor. That's why it is always recommended to run generators on lagging power factor. Since your generators are not connected to grid and dedicated to your plant, it is best to leave it as it is, allow them to run at 0.8 pf which is presumably the rated power factor for the generators.
Adding power factor correction capacitors may add hardly any value to your system but may drive the generators towards leading power factor and risk over voltages / instability in the functioning of generators / trip of generators.

R Raghunath

 

[che12345]

"...We have 634 KW Generators installed. The current Power factor is 0.8, My boss said it should be improve to 0.98. Remember we r not given any electricity to any user. But I read that if we improve power factor the it may be damaged generators by Over excitation. I am unable to understand its theory..."
I have the following opinion for your consideration.
1. If you are taking the supply from the public utility, usually the authority imposes very heavy penalty on consumer with low pf. There is incentive for the consumer to improve it. This is not your case.
2. with your own Gens.
a) run one Gen up to full kVA , instead of two or more combine.
b) Improve the system pf from 0.8lag to 0.98lag does lower the kAV. But the deduction of current that result to lower I2 R losses is usually NOT economical, consider the investment of the capacitors.
2.1 If your total system kVA is close to the limit at 0.8pf, it could be beneficial to improve to 0.98pf ; to delay procurement of a larger Gen.
3. Improving from 0.8pf to 0.98pf does NO damage the Gens.
4. There may be a slight voltage increase. This can be easily trimmed down by the voltage (NOT frequency/speed) regulator adjustment.
Che Kuan Yau (Singapore)

 

[waross]

2.1 If your total system kVA is close to the limit at 0.8pf, it could be beneficial to improve to 0.98pf ; to delay procurement of a larger Gen.

Have you ever actually tried that?
I have.
The set was rated at 600 kW/750 KVA.
The motivation was to delay adding a second gen-set as the load increased.
With several sets in our small plant, we were able to pick up the VARs on a second set and run one set at the rated 600 kW at unity power factor.
As we attempted to load the set past 600 kW, the frequency started to drop.
Surprise, surprise.
The diesel engine on the 600 kW set was capable of 600 kW and no more. (The set was standby rated. Had the set been prime rated, we could have gone to 660 kW and 825 KVA safely for one hour out of 12.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cranky108]

Look at the name plate on the generator. It should give you a kVA rating at a power factor. That is not the maximum it can provide. It is a point on which two heating curves come together. The generator can provide more power out, provided the prime mover can provide that, and you keep the Vars lower on the machine. The power factor (PF) can be worse than the nameplate value, but the generator will over heat before you get to full kVA output.

That said, the only reason you would want to add capacitors on a simple system as you described, is if you need more power out of the generator (and you don't want to run a second generator). It is a pure economic issue.
If you do add capacitors, the voltage regulators should reduce the field, to a point. If you add too much capacitance the generator field may become too weak, so don't do that. Besides adding capacitance beyond the unity PF doesn't make since.

From a power company perspective, we want to run as few generators as possible, so we want the most power out of the most efficient units as possible, and lightly load the less efficient units. To this end, we add capacitors to the system to keep the Vars off the generators, and run them as close to unity PF as possible.

 

[waross]

Generators and KVA.
Contrary to popular belief, the limit on generator heating capacity is not KVA but Amps.
Example:
I ordered a three phase generator rated at 50 KVA and 208 Volts. The maximum safe current is 139 Amps.
I got a three phase generator rated at 50 KVA and 240 Volts. The maximum safe current is 120 Amps.
The 50 KVA set was now a 43 KVA set.
The set was bought in the US and shipped by sea to Central America. Exchanging the set was not an option.
The load was fixed at over 43 KVA.
Part of the load had to be curtailed when the set was operating.
The owner was not happy.
When you see "KVA" that should be read as "KVA at Rated Voltage". The limit is current regardless of the voltage.
The set could now be safely worked at 93% PF, but that didn't help us. We needed the KVA or current capacity.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cranky108]

Correct the limit is amps, either on the stator, or on the rotor, or both. The kVA at PF number is the point where the two curves cross.

 

[waross]

Nope!
For diesel sets rated KVA equals stator Rated Amps times Rated Volts.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Let us get it right
1. In SI , KVA is INCORRECT !
2. It should be kVA NOT KVA.
3. This SI in kVA (NOT KVA) is recognized by IEC and IEEE Standards, with NO acception.
Che Kuan Yau (Singapore)

 

[waross]

Call it what you will.
Is the rating different in kVA versus KVA?

 

[che12345]

Let us get it right
1. The International system of unit (SI), the statutory units of basic measurement are ... e.g. Length (unit symbol m ) for unit name metre. The thermodynamic temperature (unit symbol K ) for unit name kelvin.
2. For multiples and sub-multiples of units, decimal power 10[sup]3[/sup] ; prefix Kilo with symbol k.
3. Attention: K is NOT equal to k.
4. In SI, KVA i.e. kelvin VA, does NOT make sense. It should be kVA; with NO exception in IEC or IEEE world.
Che Kuan Yau (Singapore)

 

[edison123]

No way KVA is going to be interpreted as Kelvin VA. Ridiculous statement.

I have been using KV, KA, KVA, KW, KVAR, MVA, MW, MVAR, mV, mA, etc. for both IEC and IEEE world clients for over 30 years and none had any problem understanding them.

Muthu

http://www.edison.co.in

 

[cranky108]

Maybe on diesel generators you are correct, but since most of my experience has been with Turban driven or Hydro driven, those units are rated in kVA and PF.

 

[crshears]

Hello che12345,

3. Attention: K is NOT equal to k.
4. In SI, KVA i.e. kelvin VA, does NOT make sense. It should be kVA; with NO exception in IEC or IEEE world.

Yes, you are technically correct.

That being said: in the relevant context, IMNSHO and generally speaking, not many will upbraid others or kick up a fuss about those who type KVA in place of kVA.

I say this as one who has tainted working relationships over less; who wins in a contest between the picayune and the childish?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[cranky108]

OK someone used the wrong case of a letter, let it go and move on.

 

[crshears]

Hi cranky, my point exactly.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[wcaseyharman]

To reiterate the point, if your generators have sufficient VAR capability and your voltage drop is not excessive it seems like a waste of money to correct your power factor.

Casey