Guidance for OCR Trip curve settings when when a generators decrement Curve 2

[Wfg42438]

Hello Everyone,

In the past, I have heard many engineers place high importance on modeling a generator's decrement Curve.

My basic understanding is that this curve represents the fault current that the generator can produce in the case of a 3-phase fault at the generator terminals.

What I need some clarification on is how this curve affects the protective device trip curve settings.

I know for example for a transformer we want to make sure our trip curve is to the left and below the XFMR damage curve.

However for a generator's decrement curve what exactly is the goal at hand or rule of thumb when adjusting the trip curve of an Overcurrent relay for the generator?

Shown below are generic TCC samples I found online, any help or guidance on this would be much appreciated!

 

[dpc]

The decrement curve is not a damage curve, so it doesn't compare to the transformer through-fault damage curves. It's mainly useful for checking coordination and also to have an idea of the sustained generator current output after the initial subtransient and transient periods so the backup overcurrent protection is set low enough to eventually trip. Without field forcing, the sustained fault current output is generally less than full load current so generally a voltage-restrained or voltage-controlled overcurrent relay is used as a backup. A good text on protective relaying should help you on this.