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:
- the fault current interrupting time of the circuit breaker
- relay timing errors
- the overshoot time of the relay
- CT errors
- 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.
- 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.
- 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.
- 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.
- 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
1.Bunty B. Bommera
2.Dakshata U. Kamble