voltage vs. frequency operating limits for sync. generator

[ZapSib]

I went acrros several generator data sheets.
A standard capability curve is "voltage vs. frequency operating limits".

I do understand there is a min. and a max. values for both frequency and voltage.

I also understand there is 2 limits zones : one for permanent operation whithin the second one ( operation bounded in time).

let's gives some figures :
first zone :
Freq. +- 2%
U +- 5%

second zone ( allowable for less than 1 hour) :
Freq. + 3% - 5%
U +- 8%

> first issue is than generator is "more sensible" to overfrequency rather than underfrequency. ( Gen. is able to whistand a 5% underfreq. but not a 5% overfrequency)

> Main issue is that gen. may handle at the same time max (resp. min) U allowable value and max ( resp. min) f allowable value.
BUT the same gen. shall not operate with at the time max( resp. min) U allowable value and min( resp. max) f allowable value.

Any clues to explain such limits ?

 

[RRaghunath]

It is a matter of design. The limits in IEC have come about from the operating practices and utility requirements.

Overfrequency means excess generation in the system which can easily be controlled by simply reducing the input the turbines. Whereas, the underfrequency operation for extended periods is a necessity in many of the countries where the peak load outstrips the generation available and load shedding is not always easy to practice.

 

[ZapSib]

You mean that generator is not allowed to operate at Umax, fmin (resp. Umin, f max)on purpose by designer.
But as per curve, it shall be able to operate at Umax, f max ( resp; Umin, f min)
Is there any physical explanation (such as overheating, ...) to explain such "strange curve" ( it is my feeling)?.
I do agree that it is easier to manage overfrequency rather than underfrequency.

 

[RRaghunath]

ZapSib,

Everything boils down to money. If we look for wider tolerances in voltage & frequency, the magnetic circuit of the generator is going to be expensive and will result in higher cost.
So, if we can manage easily the overfrequency, why do we spend money by overspecifying the generator.

 

[ZapSib]

rraghunath,

OK not to go for wider tolerances.
let's go back to the OP :
One gen set is given with the following tolerences :
voltage within Umin and Umax.
freq. within fmin and fmax.

data sheet give the following curve : U vs. f

as per datasheet, it is allowed to operate at
U max and f max
U min and f min

But, limits remaining the same, it is not allowed ( according to this datasheet) to operate at
U max and f min
U min and f max

Why ?

 

[GTstartup]

U max and F min is oversaturation (V/Hz) probably.

Can't help you with U Min and F max, no idea why you can't do that, sorry

 

[olicg]

Generator emf/voltage is given by the equation:

Emax = 2 * pi * B A N f

Now 2, pi, A, B, N are all constants for a given machine. So we can conclude E directly proportional f or E/f is constant. This is mathematical expression.

For a given machine with a certain magnetic flux density limitations due to core size and heating limitations, the machine can handle certain flux. Flux depends upon f and v which are directly prop to each other. So basically VPFL is overexcitation protection.

Overexcitation can happen due to either overvoltage or underfrequency.

Underexcitation conditions which occur otherwise are for stability and loss of excitation protections for the machine.

I hope i have understood the question.

Olic

 

[ZapSib]

Olic,

I did review my OP.
What I forgot to state is that the zone is made of linear line ( and not a full square).

I had continued my research and come to the following :
The curves are approximations : instead of hyperbola ( f(x) = 1/x), data sheets gives linear line.

So, the limits are underexcitation/overexitation.
also, others limits come from others phenomenons such as stator current, heating, etc ...