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
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"Why not the best?"
Jimmy Carter