Power Generation: Site Design, Installation, and Connection

[SyncCheck]

Hello all,

I have a few questions about the CAT C9 250KWe generator set...

This generator is rated for 480VAC 3PH operation at 0.8PF

- Will I have to derate the generator if I run it at 460VAC?      

- If so what factor do you generally use?

- If you run a generator below 0.8PF do you have to derate?

Thanks

[mikel]

Simply lowering the voltage from 480 to 460 shouldn't require any derate, as long as the kVA rating of the generator is not exceeded.

For your question about operating at a lower power factor, you need to have a copy of the generators reactive capability curve, and assure your operating in the safe areas of that curve.  The curve is available from the TMI data, your dealer should be able to provide a copy if it wasn't provided in your submittal information.

Mike L.

[wtc2008]

Answer:

1. You can decrease the voltage, but can not increase the rated current, because the winding can not be changed;

2. According to CAT, maybe the current can be increase 5% when you decrease the voltage, because there are tolerence for generator rating;

3. IF the PF is differenct, the SKVA maybe different, because the active power is from engine, which is fixed; see table below.

[Pierre123]

As far as I know you have a built in +/-5% adjustment allowance for voltage around the rated voltage, it's all over Cat documentation. This will allow adjustment to the voltage you speak of.

Have it checked out, provide your dealer with the Serial number or generator group number and have them run it through TMI cross referencing the ratng available (should take less thn a minute)

Pierre

[arkadiy]

In general, if you go below 0.8 PF you will need less power from engine. So, you shouldn’t care about engine derate. In other word, the engine will have some spare power.

Good luck to everybody

[JvanSchaick]

Power factor (PF) and reactive power can be confusing ideas for everyday use. While it's true that you should limit your loads such that you remain within the reactive power curve of your generator who the heck knows what that is or how do you even find out?

What we do usually know is the nominal voltage setting and the amperage rating of the generator. In fact many of my customers ignore the kW / kVA rating of their machines and simply consider the amperage rating knowing that exceeding the amperage rating means they are, likely, exceeding the thermal limits of the generator. Bear in mind I'm not talking about momentary over-amperage situations like when motors are started. The momentary overcurrent flows are usually not sustained long enough to overheat the insulation. I'm referring to steady-state conditions when too much amperage is creating too much heat.

So, taking amperage as your generator's limit you simply need to remember that changing the voltage means you will either bump into a shortage of engine power limit before you hit the generator's rated amperage (when raising the voltage) or you hit the generator's rated amperage before you run out of engine power (when lowering the voltage).

Generally speaking most Caterpillar voltage regulators have a limit on the range of adjustment to avoid under / over excitation problems. Normally I wouldn't be overly concerned with adjusting output voltage to those range range limits to suit your needs providing there are no extreme circumstances such as excessive harmonics (usually electronic noise) or lots of underloaded motors in the load.

Heat destroys insulation. Amperage is heat. Knowing the amperage rating of your machine and then monitoring the amperage output of the machine is an easy way to keep yourself from exceeding the machine's design limits. Your Cat Dealer is a great resource to assist you in knowing what the machine can do and what to look for in order to stay within those limits.

[josh_lukejan]

Reducing a working voltage from 480v to 460v has nothing to do with derating a generator.

Derating can only be applied if your generator operates at  prime or continous operation.

Since you have a 250kw generator, say operates at prime or continous rating, by derating

the solution is 250 (.70)=175kw. The derate factor is 30% of rated kw. Meaning, at prime or continous operation

you will only required to operate at maximum rating of 175kw and not 250kw. So your operating current will also based (computed) on 175kw.

If your generator operates at emergency rating only, no need of derating the unit.

Hope this answers your questions.

[Srivatsa]

Gents,

It is important to understand how to rate the generator on the basis of voltages. I have taken a case of LC5014L frame C9 250 KW standby rated generator. Generators are to be understood on the basis of the KVA rating and not by KW.. As long as you do not exceed the thermal KVA limit you are OK.. The full load current of the said 250 KW or 313 KVA generator is what is important.. @ 313 KVA and 480 V the full load current is 376.5 amps. At 460 V the full load current for the 313 KVA is 392.9 amps. This translated to the 480 V rating will be 326.6 KVA.. If we see the generator data sheet from the TMI there is a section on the Thermal limits of the generator based on the temperature rise..

If you see the thermal limits table in the attached data sheet you will notice that ratings are given for the generator at various temperature rise. For this application the limit is 105 Deg C

From the above the BR is the base rating and the PR is the peak rating. For a rise of 105 Deg C the base rating of the generator is 362 KVA which is well above the 326 KVA needed and hence there is no derate needed..

The suitability at power factors below 0.8 can also be obtained from the operating chart that will be available in the TMI with the subject genset.. @ 0.7 pf it usually derates by 5 - 7 %...

Hope this will be helpful..