## Generator Damage Curve Understanding

## Generator Damage Curve Understanding

(OP)

Within SKM to model the damage/decrement curve of a generator, I have noticed that the two variables which have the greatest impact on where the damage curve lies (on the TCC curve) are the If and the Ifg.

The defaults for a generator within SKM are If=3 and Ifg=1.00. I usually just leave these as-is...

I will give 'my' understanding of the situation, which sometimes matches reality and sometimes not. I am thinking back to some of my core engineering courses and differential equations for modeling the transient response of systems that are first order, second order, etc. I would think that the system of a generator driving a load is a second order DE, and the Xd'' and Xd' and etc that come off the generator cutsheets are 'constants' in the simplified solution to the differential equation. This all ties into the transient modeling, or TMS module of the SKM software (which I really haven't fiddled with much).

I want to confirm this and get any additional information anyone has on this..

and

I understand the If and Ifg to only impact the TCC damage curve of the generator, which I understand to basically represent what amount of current the generator is capable of sustaining for a given amount of time without damaging itself.

So the follow-up and what I am concerned with is: where do If and Ifg come from, what do they mean, and where can I get information on the theoretical framework to understand these two variables?

Thanks much

The defaults for a generator within SKM are If=3 and Ifg=1.00. I usually just leave these as-is...

I will give 'my' understanding of the situation, which sometimes matches reality and sometimes not. I am thinking back to some of my core engineering courses and differential equations for modeling the transient response of systems that are first order, second order, etc. I would think that the system of a generator driving a load is a second order DE, and the Xd'' and Xd' and etc that come off the generator cutsheets are 'constants' in the simplified solution to the differential equation. This all ties into the transient modeling, or TMS module of the SKM software (which I really haven't fiddled with much).

I want to confirm this and get any additional information anyone has on this..

and

I understand the If and Ifg to only impact the TCC damage curve of the generator, which I understand to basically represent what amount of current the generator is capable of sustaining for a given amount of time without damaging itself.

So the follow-up and what I am concerned with is: where do If and Ifg come from, what do they mean, and where can I get information on the theoretical framework to understand these two variables?

Thanks much

## RE: Generator Damage Curve Understanding

The generator impedance and time constants are based on Parks transformation that provides an approximation of the generator's dynamic fault response based on a series of impedance models.

## RE: Generator Damage Curve Understanding

The decrement curve is based on a three phase fault at the generator terminals, but I think you can calculate terminal SLG and ph-ph faults using the same logic.

You compare the decrement curve to the generator damage curve. It is really a guestimation because the generator damage curve is for steady state currents, but the current magnitude is changing due to excitation system response.

## RE: Generator Damage Curve Understanding

Also dpc please give more information on If and Ifg and their impact on the generator's response and decrement curve.

## RE: Generator Damage Curve Understanding

Parks transformation is similar to symmetrical components and it converts all of the variables to a new coordinate system.

It eliminates the time-varying inductances that occur due to the rotational energy of the machine.

But you don't need to understand all the theory to use the constants.

## RE: Generator Damage Curve Understanding

## RE: Generator Damage Curve Understanding

And:

When it comes to the decrement curve, it sounds as though this is a 'rough' area in terms of modeling the available fault current that a generator can provide, as you say this varies from between 1pu to 3pu. Is it really that unimportant and unexact? After all, literally the only thing that affects the shape of the decrement curve is the If and Ifg variables. I don't see the point to including a decrement curve if everysingle decrement curve will look the same after adjusted on a per unit basis.

Also, please advise on the relationship between the decrement curve and the protective relay that is protecting the curve. What I had originally thought was a damage curve caused me to believe that it was not in the right area, because it was to the left of the protective relay curve for most (thus causing the gen to get damaged before the relay picked up). But even still, if the decrement curve is meant to show the available fault current of the generator, it still wouldnt make sense for the decrement to be on the left of the protective relay curve, because a generator would then feed a fault indefinitely.

Please advise.

## RE: Generator Damage Curve Understanding

For generator protection, you have to consider the full range of protection provided, not just overcurrent relaying. You have not given us any information on the generators and the protective relaying will depend on the generator size.

But normally, overcurrent relaying is considered backup protection. To account for the decrement curve, a voltage-controlled (51C) or voltage-restrained (51V) overcurrent relay is normally used. This allows the relay to respond to current less than the generator full load current.

You need to get a good text or paper on generator protection and review that if you are trying to do relay coordination for generators.

## RE: Generator Damage Curve Understanding

It is the internal voltage that varies from 1pu to 3 pu, not the current. Use this varying voltage to calculate I over time.

The excitation response time varies from generator to generator, and the Td" and Td' time constantants vary from generator to generator, so each generator decrement curve is a tad different.

You are darn right it is rough. VERY rough. You cannot really plot a decrement curve vs a relay curve. The relay curve is based on a fixed current level, that you do not have. You need to integrate the relay response cycle by cycle to somehow figure out when the relay is really going to trip. And that is if you believe your decrement curve. To make it simple sometimes make wild assumptions of current staying at some constant level, such as V/Xd' and see when all the relays in the string respond given that constant current.