Protection Riddle No.98 - Grading time of numerical relays protection
Explain and determination of Grading Time  in High set and low set  stages  of Numerical relays Protection ( Delta t DT  And Delta t IDMT )

#1
Thu, February 21st, 2013 - 13:22
The time interval that must be allowed between the operation of two adjacent relays in order to achieve correct discrimination between them is called the grading margin. If a grading margin is not provided, or is insufficient, more than one relay will operate for a fault, leading to difficulties in determining the location of the fault and unnecessary loss of supply to some consumers. The grading margin depends on a number of factors:
1- the fault current interrupting time of the circuit breaker
2- relay timing errors
3- the overshoot time of the relay
4- CT errors
5- final margin on completion of operation

Factors (2) and (3) above depend to a certain extent on the relay technology used – an electromechanical relay, for instance, will have a larger overshoot time than a numerical relay.
Grading is initially carried out for the maximum fault level at the relaying point under consideration, but a check is also made that the required grading margin exists for all current levels between relay pick-up current and maximum fault level.

Reference:
ALSTOM-PRAG

#2
Fri, February 22nd, 2013 - 09:52
Definite time relays, grading time - delta t DT = 2. Ie +Ir + Icb + Im

Inverse Time Relays , Delta t IDMT  = t1 ((1+E1/100)/(1-E2/100) - 1 ) =Ir + Icb + Im
Dear Hamid, Please explain these  Equations.

#3
Sat, February 23rd, 2013 - 08:21
I don’t know the meaning of your concerned formula, but as mentioned above for the best time grading of two definite time relay, the following factors should be considered.

1- the fault current interrupting time of the circuit breaker
2- relay timing errors
3- the overshoot time of the relay
4- CT errors
5- final margin on completion of operation

Also you can refer to following diagram in Siemens technical guides.

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