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Inverse Time Undervoltage Formula
3

Inverse Time Undervoltage Formula

Inverse Time Undervoltage Formula

(OP)
Guys, this may be a dumb question to make but I would appreciate if you will provide me a better understanding.

A G.E. Manual states that its undervoltage delay settings defines the family of curves shown below:

T = D / [1-(V/Vpickup)]

where T = operating time
      D = Undervoltage Delay Setting
          (D = 0 operates instantaneously)
      V = secondary voltage applied to the relay
Vpickup = Pickup Level

Looking at the sample graph, for D = 2 and %voltage pickup of 2%, the operating time is approximately 2 seconds. I'm a little confused on what values I would use for V and Vpickup to coincide with the graph below:

[IMG]http://i532.photobucket.com/albums/ee327/primo_beltran/Inverse_time_UV_curve.jpg[/IMG]

My application is for a 120VAC secondary and with a target operating time of 3 seconds...what will be the Delay setting (D)?

  

RE: Inverse Time Undervoltage Formula

3s at what %Vpickup? D=5 is too long, but D=1 and 2 intersect 3s at different %Vpickups. Both are faster than your target at 2%.

Since Vpickup is expressed as a percentage in the graph, V=100% (of 120V) in the formula above.

RE: Inverse Time Undervoltage Formula

(OP)
Hi stevenal,

Its 85% nominal volts (120VAC) for 3 seconds.

RE: Inverse Time Undervoltage Formula

Depending on the relay, (you may be able to set the time dial to 1/2) I don't think that's going to happen.  You would need to set the relay for a time dial of 1/2 to get a 3 second response at 85%.  (85% of 120 = 102 volts.  The time dial of 1 hits 3 seconds at 65% or 78 volts.  

RE: Inverse Time Undervoltage Formula

None of the curves shown hit that point. Shortest is D=1 at about 7s.

RE: Inverse Time Undervoltage Formula

(OP)
In a secondary selective switchgear, the client specs calls for 0.85P.U. U/V pickup for 2 seconds and initiate the transfer.

Is it acceptable to set a 0.85P.U. voltage pickup using a DEFINITE time of 2 seconds instead of using an inverse voltage curve?

RE: Inverse Time Undervoltage Formula

IMHO - A definite time delay might be better in this application.  If a momentary voltage dip occurs to say 10%V for a few cycles, do you want to transfer immediately or give the system 2 seconds to come back on its own? The inverse time curves will intitate transfer faster than the two seconds specified for low voltages.

RE: Inverse Time Undervoltage Formula

yes, I agree 2, Happy Christ's mass

RE: Inverse Time Undervoltage Formula

Nightfox.
Pay attention on the RCWilson recommendation.
I agree 3.
Best Regards.
Slava

RE: Inverse Time Undervoltage Formula

(OP)
Thank you very much for all the technical assistance. We will get the settings on now.

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