Rating Underated

[vennivivi]

Good day to all,

We are looking at the opportunity to purchase a second hand generator 6.0 MW 6,6 k V rated . However, our network is rated @ 5,5 k V. Basically,running the generator at 5,5 k V and obviously at the rated speed, the capacity of the generator will be limited by the ratio 6,6/5,5 that is 5,0 MW.The generator will be protected as before by a generator protection relay.Is there any issue to look at?

Grundig

 

[00123456]

Kamin Dave
So, many places i have seen, there is no problem to run under rated condition, but you need to take some precuation with excitation system like some times AVR fails & automatic switch over on manual regulator then it is very difficult to control genrator over voltage & it can exceed the voltage up to 1.1 times of rated (6.6kV*1.1).

For that puspose you need to set over excitation & over voltage protection properly.

Another thing, as you said: it have been protected by generator protection relays, but settig should be based on 5.5kV voltage & 5MW capacity.

 

[edison123]

Generators are designed to run at the knee point of magnetic saturation curve (Voltage vs excitation current) at the rated voltage. If you reduce the rated voltage, then it will run at straight portion of the curve, which will lead to stability issues.

I would recommend a 6.6 KV / 5.5 KV trafo (with associated gear) to avoid the above stability issue and also the derating of the machine.

*Why make the same mistake twice when there are so many new ones available ?*

 

[waross]

My experience is with sub-megawatt generators. I have worked on a few rated above 1 MW but voltage rating was not an issue.
In the sub megawatt range, typically alternator will be used for voltages over a range of 120%.
That is, the same alternator will be used at voltages from 200 volts to 240 volts.
The KVA rating is the product of allowable current and utilization voltage. As the utilization voltage drops, the KVA rating drops proportionately.
I had one issue where the generator was rated at 240 volts instead of the 208 volts specified and the loading had to be curtailled.
The KW is limited be the power of the prime mover. This is a level of backup protection. If the main breaker fails to open on an overload, the power limit limit of the prime mover will limit the amount of energy into the generator. This will not prevent burnout on overloads but it will delay it.
Some manufacturers set the maximum fuel position according to the KVA/power factor at the rated voltage.
Some manufacturers set the maximum fuel for the maximum rated KVA/power factor and include a table of ratings at all common voltages.
Although the power factor rating is nominally 80%, the maximum allowable power factor will increase if you drop the utilization voltage without adjusting the governor.
Power factor is the ration of rated input power divided by allowable KVA. As the KVA drops and the power of your prime mover stays the same, the ratio increases.
As I said, voltage issues are ignored by some manufacturers and if you have adequate electrical protection on the set, I would not worry about the governor max. fuel adjustment. If left as is, it will give you faster recovery from block loading.
respectfully

 

[ScottyUK]

If you reduce the rated voltage, then it will run at straight portion of the curve, which will lead to stability issues.

Interesting - any reference for that? A decent AVR, properly tuned and with a suitable supply voltage should not be unstable at a reduced voltage provided the AVR is working within its design limits. A self-excited AVR drawing power from the generator terminals will at some point get into trouble at reduced voltage, especially if operating at a heavy lagging (over-excited) power factor.


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Sometimes I only open my mouth to swap feet...

 

[matrixabc]

Is the AVR designed to run at the lower voltage?

 

[00123456]

Yes, Why not? It is possible.

If you can see in AVR for KVAR adjustment & SPEED controller for KW adjustment, there are settings to set the input voltage & out put gain, trim etc. But, authorized person can able to set AVR & SPEED CONTROLLER not others. It's not so easy.

 

[waross]

If the voltage regulator will not set down to the desired voltage, you can use a transformer or autotransformer to "fool" the voltage regulator.
If you boost the sense voltage 20%, (6.6/5.5), the regulator will happily control at 5.5 thinking it is controlling at 6.6.
You may wish to use the same or a similar setup to boost the power to the regulator.
This may not be needed.
respectfully