Need help on how to come up for settings for GE750 2

[budhiman]

Hello Everyone,
I am working on a project which has several GE relays (GE 750 multilins). I am reading through its instruction manual on how to provide the settings for : 32R- Reverse Power, Over Voltage and undervoltage elements.

The confusion I am having for these elements is:

1. Reverse Power - The pick-up here is determined by Settings x Rated power. The manual define rated power as Rated Power = sqrt(3) x Vsec (line - line) x VT ratio x Isec (rated) x CT ratio. Now the question how is the rated power calculated I am assuming the Vsec and Isec is the PT sec and CT sec respectively. Am I correct?

2. Voltage and Undervoltage - It is determined by Settings x VT. Is the VT here the secondary voltage of VT's.

It will be really helpful if someone can help me with an example.

Also is there a good way to learn more on how the settings and how to program these common relays used in the industry ? Like a document with example similar to Basler instruction manuals.

 

[collies99]

Power is calculated as per 3phase formula.
VT x CT x PTR x CTR x root 3 x cos theta.

remember that CT and VT are variables set in the System Sensing page.

Undervoltage is per unit of VT, so just becomes a number. Just have to check if the 27 element operates in a phase-to-phase or phase-to-ground basis.

 

[oldfieldguy]

On the SR750 relay you have two separate settings related to PT secondary voltage. One is the ratio, ie., 120:1. The second is the nominal voltage, which is the secondary voltage you'd see when nominal system voltage is applied to the primary. If you're looking at a 13,800 volt system with 120:1 PT's then the nominal voltage will be 115 volts. Your per unit values for elements are then set on THIS voltage (115 volts in this example). If you miss this little fact, you'll find your PT-related settings will be off.

I learned this the hard way.


old field guy