Inverse Time Undervoltage Formula 3

[nightfox1925]

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:

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

 

[stevenal]

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.

 

[nightfox1925]

Hi stevenal,

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

 

[pwrengrds]

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.

 

[stevenal]

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

 

[nightfox1925]

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?

 

[rcw retired EE]

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.

 

[pwrengrds]

yes, I agree 2, Happy Christ's mass

 

[slavag]

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

 

[nightfox1925]

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