Diesel Generator Test Result 2

[sgtan1]

Can some one advise me on the following?

I recently witness a testing for a 300kVA Genset
I realize that typically the suppliers will be refering to their machine as 240kW at 0.8 power factor.

The test was done on a pure resisitive load
The manufacturers tried to pass the testing at 240kW loading and claiming it to be 100% loaded. Ie 240kW at P.F of 1

I insisted on a 300kW P.F 1 load test but was shot down by the others.

I need advice on the matter as I am unfamiliar with genset. Does a genset some how give constant kW independent of the P.F?

 

[davidbeach]

The engine can produce 240kW, not 300kW. With a 300kW engine, your generator (the electrical machine, independent of the prime mover) could be rated 300kW at unity power factor, but then the output would have to go down as the power factor went down to avoid exceeding the 300kVA rating.

 

[7anoter4]

Hi sgtan1
I totally agree with davidbeach :you cannot increase the active power more than 240 kW. The diesel machine power is limited to ,let say, 250 kW. If the generator efficiency is 96% you'll get 240 kW. If you change the cosinus Fi to 1 the generator will produce only active power [Q=0] then S=P=240 KVA.
BTW cosinus Fi=0.8 that means the generator produces reactive power not requires [as a capacitor does and not as an inductance].
Best Regards

 

[waross]

I agree with davidbeach and 7anoter4.
The kW rating (240 kW) basically describes the capability or the power of the prime mover.
The KVA rating (300 KVA) describes the capability of the generator end.
The test at 240 kW with a resistive load proves the capability of the engine and is usually adequate for a 240 kW/300 KVA machine.
To test the generator end's ability to deliver 300 KVA without overheating you need a load with a power factor of 0.8 Such a load is not readily available and a test at full KVA rating is usually omitted. Observe the temperature at 240 kW loading and extrapolate to estimate the temperature at 300 KVA loading.
The easiest way to test at 300 KVA is to parallel the machine with a larger machine. You then use the governor control to load the machine to 240 kW real power and then use the voltage control to over-excite the generator so as to produce enough KVARs to get the KVA up to 300 KVA.
You want someone with some generator experience to vet the setup so that you don't inadvertently push the larger machine into instability.
Another possibility is to drop the voltage 20% so that you can push full rated current through the windings at 240 kW.
The full voltage resistive test will prove the capability of the prime mover.
A test at 80% voltage and rated current will demonstrate the ability of the generator to withstand full rated current without overheating.


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

 

[golftennis]

Hi all,
Regarding the load test of 300KVA gen set if you are testing wiht resistive load PF is unity so KVA=KW then I would raise the load on the generator to 300 kw.
Besides this for a new generator the load test should be done loading to 110% for 1 hr And also there are other parameters to be checked voltage droop , governer responce
when you drop full load from the generator you got to see the governer responce wheather it is keeping the speed in limit or not, some times when the generator trips on full
load the overspped shutdown takes place.
when you trip the generator on full load check the condition of the main contacts of the circuit breaker.
if you are running in parellel then you have to look at load shearing
check if both the generators shearing same KW load and the Amps.
Alex

 

[davidbeach]

golftennis, how do you propose to get 300kW out of a prime mover that can not produce 300kW? In a new system the engine controls will shut you down well before 300kW. Reread the other responses; you won't get 300kW, and certainly won't get 330kW. Your post is highly irresponsible.

 

[sgtan1]

Thanks everyone for your advice.

I think I understand a bit more.

Davidbeach, what golftennis had posted seems to be in line with the accepted testing norm in my country (well from what I learn from the the test anyway.)

I think the confusion came from defining what is meant by a 300kVA Generator. While I believe it to be a machine that can handle 300kVA load at unity; the industry standard might take it to mean 300kVA only at 0.8 P.F thus giving us a 240kW engine...

I am curious on what is the accepted defination. Or how would you specify your requirements.

 

[edison123]

sgtan1

All generators come with a KVA and a pf rating. If yours came with 300 KVA and unity pf, then your engine should be rated for 300 KW and should be rightfully tested at 300 KW.

But if yours came with 300 KVA, 0.8 pf rating but you choose to run it at upf , then you can't expect the engine supplier to fulfil your wish.

Of course, you can always test the machine at 300 KVA, 0.8 pf if you can find such a load.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[davidbeach]

If you have a gen set package with a 240kW 0.8pf rating all of my comments apply. Yes, you have a 300kVA generator but only a 240kW prime mover. On the other hand you can get 240kW out of the set for any power factor better than 0.8.

If you have a gen set package with a 300kVA 1.0 pf rating you can and should test it at 300kW at unity power factor. But unity power factor is the only way you will be able to get 300kW out of the set. At 0.8 pf you will only be able to get 240kW out (300kVA), leaving 60kW of prime move capacity unusable.

I'm much more familiar with the former but am aware of the latter type of rating. Personally I'd rather have a prime mover-generator combination that allows full output of the prime mover over the range of all realistic power factors. For central station generators, a 0.9 or even 0.95 pf rating is fine. For a emergency/standby gen-set application I would expect a 0.8 pf rating allowing full engine output over a broad range of loads. In those sizes a kW of engine is more expensive than a kVA of generator. You're money ahead to have too much generator and the right sized prime mover instead of a right sized generator and an over sized prime mover.

 

[waross]

I have spent more than a few hours pouring over specifications of diesel gen-sets in this size range.
The standard rating is 0.8 power factor.
BUT, some sets rated for prime power will allow a 10% overload, 1 hour out of 12. The maximum fuel setting is often set a little on the high side so that the engine will still be able to produce full power when it is getting tired and ready for an overhaul.
So, you may often be able to push a prime power rated set up to the KVA rating on the initial test when the engine is new.
A standby set is not quite as forgiving.
The same engine, alternator combination may be rated at 100 kW as a prime power set, (with a 10% overload allowed 1 hour in 12) but will be rated at 110 kW as a standby set with no overload allowed.
Some prime power sets are spec'ed with the engine 25% oversized. This will allow operation for extended periods of time between overhauls. These sets will easily produce full KVA at unity power factor when new.
Bottom line;
A prime power set will often be capable of producing full KVA at unity power factor when new, but don't count on it and don't demand it unless the engine has been spec'ed 25% oversized.
A standby set will not be as forgiving as a prime power set. It will be the same basic engine and alternator starting with a 10% higher rating.
A prime power set may have accessories that may not be fitted to a standby set, such as a larger oil sump, an oil; cooler, and/or a larger radiator.
A resistive test at rated kW will prove the capability of the prime mover and demonstrate the stability of the voltage regulator.
The main issue with full KVA ratings is heating. This is dependent on the current. The heating of a set at full current may be evaluated at reduced voltage. If you wish to test a set at full rated current, set the Automatic Voltage Regulator down 20%. At the reduced voltage the prime mover will be able to drive full rated current through the generator with a resistive load.
If anyone wishes to verify my information, go to a web site such as Cat-Power or F G Wilson. Look at standby sets and prime power sets and compare motor numbers and alternator numbers. You will find the same engine and alternator rated 10% higher for standby duty than for prime power duty.
davidbeach; I agree with you for station sets with good protective relaying and competent operators.
In the field where sets of this size are miserably neglected and abused, the 0.8 PF rating gives you a built in safety cushion. Even if the breaker has been bypassed, there is still a good level of self protection when the prime mover can not drive the alternator to 100% output. I am sure that that feature has saved an untold number of sets from meltdown in the third world.
Single phase sets are not so lucky. They are typically set up as three phase sets and then reconnected for single phase. That leaves them over powered even at unity PF.
There's a thought. You can temporarily reconnect a three phase set in double delta or zig-zag and easily run full rated current through a resistive load bank. That will give an accurate indication of the heating to be expected at full KVA loading.

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

 

[rbulsara]

sgtan1:

A simple way to remember if a machine has a kW rating and a kvA rating, you cannot exceed either of them.

Now in a genset, the altenator does not care if you load it up to unity pf, the engine does. Engine can only provide real power or kW. So if you wanted 300kW of capacity you should have specified so.

A 300kVA rating is meaningless unless specified with the power factor. Norm around the world is to assume 0.8pf for the gensets unless specified otherwise. I am not sure what part of the world you are referring to.

 

[rovineye]

I have never worked on a gen set of any size that matched engine and generator ratings at unity. Uneconomical to buy or build that unneeded capacity into the prime mover. Piece of cake for me as an island, no way for a load of any significance to have pf to get above .8.

When mated, the load test is usually only a full load test of the engine,as it is usually done at unity. Some times we have the capability to test at .8 pf, but not usually at the factory, only on site after installation.

We sometimes have factory tests of the generator alone, and in your case can test at 240 kVA at .8 or 300 at unity, and usually at reduced voltage as stated by waross. That is usually only done on first article testing on a new series as well.

 

[Hoxton]

All good advice, guys. Back to basics, since sgtan1 is abviously concerned that the test did not prove the performance of his genset.

All small genset manufacturers offer a resistive kw test only. Full kVA tests at 0.8 pf can sometimes be arranged as a special, but in fact prove little (in my humble view).

The genset factory kW test shows the prime mover at rated load (COP / PRP / LTP or whatever it was sold on). This demonstrates the mechanical integrity of the combination of engine / coupling / generator / baseframe / radiator / enclosure / silencers etc

Putting the generator on full electrical load (kVA) puts only a slight increase in mechanical load (the generator losses will be slightly higher at 300kVA than 240kVA).

The ac generator will have been tested at the manufacturers to prove its capability to IEC or ANSI.

Volume genset manufacturers often "type test" the first one at full kVA, then production units are kW tested only.

Second part : why 0.8pf? Most small gensets supply a disctret load - say a building. Typical site power fctors are better than 0.8, so if you select the genset to match the building load in kVA, then the mechanical rating of the prime mover will not be exceeded.

 

[waross]

Hi Hoxton;

All small genset manufacturers offer a resistive kw test only. Full kVA tests at 0.8 pf can sometimes be arranged as a special, but in fact prove little (in my humble view).

I agree as to full KVA tests, but a test at full current may be desired to observe the heating effects. This may be done either at full load at 0.8 PF if possible, or at reduced voltage and full kW.

Second part : why 0.8pf? Most small gensets supply a disctret load - say a building. Typical site power fctors are better than 0.8, so if you select the genset to match the building load in kVA, then the mechanical rating of the prime mover will not be exceeded.

If you determine that a building has a demand of 500 KVA and select a set rated at 500 KVA/400 kW at 0.8 PF, you will be in trouble if you discover that the building load has a PF of 0.9 and demands 450 kW.
Prudent planning would be to consider both kW demands and KVA demands so as to ensure that the set is capable of supplying both.

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

 

[Hoxton]

Waross

1) do not disagree
2) Again do not disagree, but 0.8 pf is right 99.99% of the time. Look at the hire guys.

 

[rbulsara]

Hoxton:

0.8 pf is right 99% of the time only for the genset alternator rating. Not for building load power factors. So as waross said, you still have to figure out both kW and kVA requirement for the loads and make sure that the genset can meet both criteria.

So if the building load is 500kVA at 0.9 pf (not uncommon), and you need a generator to support that, a typcial genset would be a 450kW/562kVA at 0.8pf or 500kW/625kVA.

You cannot just specify a 500kVA generator and expect it to meet the 450kW/500kVA load requirement.