IDMT Relays

Please see the following formulae Standard inverse charectristic is t = TMS x 0.14 Divided by [ ( M to the power 0.02 )-1 ] Where TMS - Time Multiplier Setting (0.01- 1), M is current mutiple [ arrived by Fault Current for which time to be arrived divided Actual(Effective ) CT ratio] Effective CT ratio can be arrived by Adopted Ct ratio x PSM (Plug Setting Mutiplier adopted in the relay) For example if you have feeder relay connected to a CT Ratio 400/1A with PSM adopted as 50%, the Effective CT Ratio will become 200/1 Suppose the Fault current is 2000 A, for which the time of operation is to be calculated for a TMS of 1,the value of M will arrive as 10 . on subtituting this you will get a time of 2.97 which is approximatly equal to 3. Or other wise above formulae is intended for 10/ 3 Sec graph of IDMT relay For other graphs also similar formulaes are available From the above formulae you can calculate time graph for intermediate current mutiple other than those specifically mentioned in the Relay name plate itself

Dear Mr. Panner Selvam, many many thanks to provide me such a perfect formula to know the intermediet values of entire range. Sir, just I wanted to know that where u have got this formula? I am really very much thankful to u because from the last 6 moths I am asking this question to no. of expert engineers but unfortunetly they could not provide me any satisfectory aswer.

Sir, Thank U Very Much

Please refer any protection manual for numerical relays by any reputed relay manufacturers like Areva, ABB, SEL, GE,Etc. All the distance relays or differential relays, or generator protection Relays, are provided with O/L relays as backup protection Schemes. These relays are configured to operate only by using different algorithms for different characteristic Standard Inverse, Very Inverse and Extremely inverse, thermal characteristics etc You can Down load manuals from their Websites. All the best.

This algorithm is derived from the IEC Standards (IEC 60255). It gives the algorithm for various curves such as Normally Inverse 3 sec., Normally Inverse 1.3 sec., Very Inverse and Extremely Inverse.

The formula is:

t(I) = {(A) / [(I/Is)a -1]} x TMS

Where,

t (I) = trip time at the actual current 'I'

A = Constant (varies for different curves)

I = Actual current through the relay

Is = Set current

a = another constant (varies for different curves)

TMS = Time Multiplier Setting

For example for NI 3 Curve, 'A' is 0.1414 & 'a' is 0.02.

Thank u Mr. electricalexpert65. Can u please tell me where can I get these curves ? How the value of 'A' & 'a' is obtained? pl. explain me in detail with example.

"A" and "a" are constants that set the extend of "inversity" of the curve, i.e. how fast the operating time drops as current increase (in actality be it is "how fast the slope of dt/dt drops with an increase in current") . The values of these constants are:

For a normal(i.e. moderate) inverse curve: (A,a) = (0.14,0.02)

For a very inverse curve: (A,a) = (13.5,1)

For an extremely inverse: (A,a) = (80,2)

For a long-time inverse:(A,a) =(120,1)

I think normally they use the Greek letter "beta" instead of A used here, and the Greek letter "alpha" instead of the "a". So I think it would be better to call them (beta,alpha) or (B,a) instead of (A,a).

See http://myweb.tiscali.co.uk/robert.booth/uni/docs/Power%20Supply%20Assignment%203.pdf

There is a good explanation on how to calculate the tripping time (both for IEC and IEE) in the link below. The site also has an online calculator which will allow you to work out the tripping time for various curves and settings.

myElectrical - Post on IDMT curves