Thermal Image

[jbond]

Hi,
Perhaps you can help me.

How does one determine what settings to install into a generator protection relay for the case of the thermal image protection function?

I presume it starts with the gen-set manufacturer, but what information do i request from them? Thermal overload curves?

The Sepam protection relay manual seems to indicate different heating and cooling time constants to be set, a continuous time current parameter determined by that heating constant, and it seems to take into account harmonics (contribution to heating) etc

So my point is, it sounds complicated to set the function up. And I'm not sure what info i need from the genset manufacturer, as I'd like to get all the information at once rather than keep going back to them for additional data that I didn't know I'd need the first time around.

Thanks,
John.

 

[Alex68]

This is my first time I listen somebody using SEPAM to protect a generator.
And it is also the first time that somebody wants to protect a generator with a thermal image function.
This function can be "simply" used for protecting transformers and motors whose operating conditions are stable. Anyway my opinion is to avoid a thermal image whose reliability should be demonstrated. I prefer thermal sensors.

A generator can operate in very different conditions. What algorythm could really calculate its thermal condition?
A generator never overheats if it operates within the limit of its capability curve:
1) The stator limit is respected because the generator has been correctly sized considering the maximum power production of the prime mover.
2) the rotor limit is respected by the exciting system if it has been correctly set
3) the overflux is controlled by a dedicated protection(59/81)
4) the negative sequence is controlled by a dedicated protection (46)

In conclusion, I suggest to avoid using the thermal function to protect a generator but to set correctly all the other protections.

 

[mohanlal]

Sepam type of relays could be employed for the generator protection provided all protective functions required for the generator is available like
Differential prot
Over current prot
Earth fault prot
REF
Neg sequence
Over voltage
Under voltage
Voltage controlled over current etc..
The normal prot for the thermal heating is winding tempr prot as I have come across so far.
Subramanain

 

[davidbeach]

3) the overflux is controlled by a dedicated protection(59/81)

Actually, the protection for overflux would be 24, the Volts/Hz element. 59 is overvoltage and 81 is frequency; it is entirely possible to be just below the pickup of the 58 and just above the pickup of the first underfrequency setting, so no tripping, but still be violating a V/Hz requirement. If you truly want overflux (over excitation) protection, you'd be much better off using an element designed for that purpose.

 

[Alex68]

You are rigth Davidbeach: the correct number is 24 but yesterday I wasn't able to remember it.
Sometime the overflux function is also indicated as 59/81.

Please note that overflux is not the same as over-excitation, because you can reach that in particular voltage and frequency conditions.

Anyway the question of Jbond was how to set the thermal image function to protect a generator. My opinion is not to use that. and yours?

 

[davidbeach]

I don't know enough about the finer distinctions between motor operation and generator operation to know whether or not thermal image would work for generator protection as it does for motor protection. But in the motor case, thermal image alway works much better when paired with RTD inputs.

For transformers and induction machines, over-excitation and overflux would be essentially the same thing. For synchronous machines, there can be some distinction. The device number, though, is commonly referred to as over-excitation protection. The IEEE definition more accurately calls it "volts per hertz" (though I would think it "Volts per Hertz" with the proper name based units capitalized).