Recommended Protection Relay Grading Interval

Typical Protection Relay Timing Errors

Protection relay grading intervals are critical in ensuring selective coordination in power systems. The recommended protection relay grading intervals typically depend on the type of relay and the specific requirements of the power system.

Typical grading intervals

  1. Relay to relay
    The total interval required depends on the operating speed of the circuit breakers and the relay performance. At one time 0.5 s was a normal grading margin. With faster modern circuit breakers and a lower relay overshoot time, 0.4 s is reasonable, while under the best conditions even lower intervals may be practical.
  2. Fuse to fuse
    The operating time of a fuse is a function of both the pre-arcing and arcing time of the fusing element, which follows an I2t law. To achieve proper coordination between two fuses in series, it is necessary to ensure that the total I2t taken by the smaller fuse is not greater than the pre-arcing I2t value of the larger fuse. It has been established by tests that satisfactory grading between the two fuses will generally be achieved if the current rating ratio between them is greater than two.
  3. Fuse to relay
    The basic approach is to ensure that the relay backs up the fuse and not vice versa. If the fuse is upstream of the relay, it is very difficult to maintain correct discrimination at high values of fault current because of the fast operation of the fuse. The extremely inverse (EI) relay characteristic is best suited for co-ordination with fuses. The extremely inverse (EI) characteristic follows a similar I2t characteristic. To ensure satisfactory coordination between relay and fuse, the primary current setting of the relay should be approximately three times the current rating of the fuse. The grading margin for proper coordination, when expressed as a fixed quantity, should not be less than 0.4 s.
Typical relay timing errors - standard IDMT relays
Timing Error Description RELAY TECHNOLOGY
Electro-mechanical Static Digital Numerical
Typical basic timing error (%) 7.5 5 5 5
Overshoot time (s) 0.05 0.03 0.02 0.02
Safety margin (s) 0.1 0.05 0.03 0.03
Typical overall grading margin - relay to relay (s) 0.4 0.35 0.3 0.3
Source: Schneider Network Protection and Automation Guide

Minimum Grading Interval Calculation

A practical solution for determining the optimum grading margin is to assume that the relay nearer to the fault has a maximum possible timing error of +2E, where E is the basic timing error. For the total effective error for the relay, a 10% should be added for the overall current transformer error.

t{prime}={{2E_R + E_CT}/100}*t+t_{CB}+t_o+t_s~~right~~(1)

ER = relay timing error (IEC 60255-4)
ECT = allowance for CT ratio error (%)
t = operating time of relay nearer fault (s)
tCB = CB interrupting time (s)
to = relay overshoot time (s)
ts = safety margin (s)

When the overcurrent relays have independent definite time delay characteristics, it is not necessary to include the allowance for CT error. Hence, equation (1) will be simplified to:


Typical Grading Intervals for Various Types of Relays

Time Overcurrent Relays
Inverse time overcurrent relays grading interval is usually between 0.3 to 0.4 seconds. This interval allows the upstream relay to have sufficient time to detect and clear the fault if the downstream relay fails to operate.

For definite time overcurrent relays, a grading interval of 0.2 to 0.3 seconds is often used.

Differential Relays
Differential protection grading interval can be shorter, often in the range of 0.1 to 0.2 seconds, because differential relays are very fast-acting and precise.
Distance Relays
Distance relays grading interval depends on the zone settings and the protection scheme used. Generally, a grading margin of 0.25 to 0.4 seconds is recommended between the operating times of different zones to ensure proper coordination.
Motor Protection Relays
Motor protection relays grading interval may vary based on the motor's characteristics and the protection scheme. A common grading interval is around 0.3 seconds.
Feeder Protection Relays
Feeder protection relays, especially those protecting multiple downstream feeders, a grading interval of 0.3 to 0.5 seconds is typically recommended.

These intervals ensure that the correct relay operates first, thereby isolating the faulted section while keeping the healthy parts of the system operational. The exact intervals can vary based on several factors, including system voltage, relay types, and specific coordination studies performed during the design of the protection scheme.

It's important to note that these intervals are general recommendations. Detailed coordination studies, often using power system analysis software, should be performed to determine the optimal grading intervals for a specific system, ensuring proper selective coordination and system reliability.