# Talk:Generator protection

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## Short Circuit Equation

2

I assume the formula to get X'd from x'd is the same as X"d?

In the short circuit current equation for the variable Isc3, what does the 3 signify?

Yes, the formula to get ${\displaystyle X_{d}^{'}}$ from ${\displaystyle x_{d}^{'}}$ is the same as for ${\displaystyle X_{d}^{''}}$ .

To be transparent, I need to check with our experts:

• to confirm what I think = that Icc3 is the short-circuit current for a three line short circuit
• to confirm if this paragraph is still up-to-date (it was not reviewed for some time)

## X'' d equation

3

How can the equation for x"d have x"d in on both sides of the equation?

NVM. I see the difference between X"d and x"d.

Hi @DMullins in fact on the right of the equation you have ${\displaystyle x_{d}^{''}}$ , expressed in %, which is a characteristic of the generator during subtransient phase (= subtransient reactance, expressed in %, only the reactance is used because the resistance is negligible). This characteristic is provided by the manufacturer (see Fig.N5 for an example of manufacturer impedance table in %). It is used in the equation to calculate ${\displaystyle X_{d}^{''}}$ , which is the generator actual short-circuit impedance (in ohms) during subtransient phase. Does that clarify it?

I also made small modifications to this paragraph (fine-tuned the look&feel of the equation, used an inline equation for ${\displaystyle x_{d}^{''}}$ for coherence, I hope it helps making things more clear

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