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# Protection Riddle No.98 – Grading time of numerical relays protection

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• #1086
soman

Explain and determination of Grading Time in High set and low set stages of Numerical relays Protection ( Delta t DT And Delta t IDMT )
Explain and determination of Grading Time in High set and low set stages of Numerical relays Protection ( Delta t DT And Delta t IDMT )

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• #3346
Hamid

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

#3347
soman

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.

#3349
Hamid

I don

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