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 (ii) and (iii) 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.
Circuit Breaker Interrupting Time

  • The circuit breaker interrupting the fault must have completely interrupted the current before the discriminating relay ceases to be energised.
  • The time taken is dependent on the type of circuit breaker used and the fault current to be interrupted.
  • Manufacturers normally provide the fault interrupting time at rated interrupting capacity and this value is invariably used in the calculation of grading margin.

Relay Timing Error

  • All relays have errors in their timing compared to the ideal characteristic as defined in IEC 60255.
  • For a relay specified to IEC 60255, a relay error index is quoted that determines the maximum timing error of the relay.
  • The timing error must be taken into account when determining the grading margin.

Overshoot

  • When the relay is de-energized, operation may continue for a little longer until any stored energy has been dissipated.
  • For example, an induction disc relay will have stored kinetic energy in the motion of the disc; static relay circuits may have energy stored in capacitors.
  • Relay design is directed to minimizing and absorbing these energies, but some allowance is usually necessary.

The overshoot time is defined as the difference between the operating time of a relay at a specified value of input current and the maximum duration of input current, which when suddenly reduced below the relay operating level, is insufficient to cause relay operation.
CT Errors

  • Current transformers have phase and ratio errors due to the exciting current required to magnetize their cores.
  • The result is that the CT secondary current is not an identical scaled replica of the primary current.
  • This leads to errors in the operation of relays, especially in the time of operation. CT errors are not relevant when independent definite-time delay overcurrent relays are being considered.

Final Margin

  • After the above allowances have been made, the discriminating relay must just fail to complete its operation.
  • Some extra allowance, or safety margin, is required to ensure that relay operation does not occur.

Overall Accuracy

  • The overall limits of accuracy according to IEC 60255-4 for an IDMT relay with standard inverse characteristic are shown in Figure 1.

Figure 1.Typical limits of accuracy from IEC 60255-4

AUTHORS
1.Bunty B. Bommera
2.Dakshata U. Kamble

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