Q2Protection of Power System

The disc-type induction overcurrent relay is a classical electromechanical relay design that produces its inverse-time operating characteristic through the induction motor principle, using a lightweight aluminum disc (analogous to the rotor of an induction motor) mounted to rotate between the poles of an electromagnet whose winding carries the current being monitored, with the electromagnet's core shaped and slotted (or fitted with a shading ring or shading coil on a portion of each pole face) so as to produce two spatially and temporally displaced (out-of-phase) alternating magnetic fluxes threading through the aluminum disc.

Principle of Operation and Torque Equation

Each of the two displaced fluxes (commonly denoted phi1 and phi2, differing in phase due to the shading ring's effect on a portion of the pole flux) independently induces eddy currents in the aluminum disc, and each flux additionally interacts with the eddy currents induced by the other flux, producing a net driving torque on the disc through the same fundamental shaded-pole induction motor principle used in simple shaded-pole fan motors. The resultant driving torque is proportional to the product of the two flux magnitudes and the sine of the phase angle between them: T is proportional to phi1phi2sin(theta), and since both phi1 and phi2 are themselves proportional to the same relay coil current I (differing only by a fixed phase shift determined by the shading ring geometry, not by current magnitude), the driving torque is proportional to the square of the current, T = K*I^2, for some constant K determined by the relay's specific magnetic design.

This driving torque acts against a restraining (control) spring providing a restoring torque proportional to the angular displacement of the disc from its reset position, and, once the disc begins to rotate under sufficient current, against a permanent magnet braking system (a separate permanent magnet positioned so that the moving disc passes through its field, inducing further eddy currents in the disc that oppose its own motion in accordance with Lenz's law, providing a braking torque proportional to the disc's rotational speed) that governs the disc's terminal rotational speed once net accelerating torque falls to zero. The relay is designed so that the disc must travel through a certain fixed angular distance (adjustable via the time setting, as discussed elsewhere in this examination, which sets the initial resting position of the disc relative to the final, contact-closing position) before its contacts close, and since the disc's rotational speed (once accelerating torque and braking torque balance) increases with current magnitude (through the I-squared driving torque relationship above), a higher fault current causes the disc to accelerate to its operating speed more quickly and hence reach the fixed travel distance in a correspondingly shorter time, directly producing the characteristic inverse relationship between current magnitude and relay operating time that defines the IDMT characteristic.

Operating Time Calculation

The plug setting multiplier (PSM) is calculated as the ratio of the actual fault current, referred to the relay's secondary side through the given CT ratio, to the relay's rated current at the given plug setting. With a fault current of 3000A and a CT ratio of 300/3 (equivalently 100:1), the secondary fault current is 3000/100 = 30A.

Since the relay's current rating (plug setting current, given PSM of the plug setting is 1.0, meaning the plug is set at 100% of the relay's 3A rated current, i.e., exactly 3A) is 3A, the plug setting multiplier is PSM = 30/3 = 10.

From the given table of operating times at TMS = 1 for various PSM values (PSM of 2, 4, 6, 7, 8, and 10 giving operating times of 8, 6, 5, 3, 2.8, and 2.4 seconds respectively), the computed PSM of 10 corresponds directly and exactly to a tabulated entry, giving an operating time at TMS = 1 of 2.4 seconds.

Since the time multiplier setting uniformly scales this TMS = 1 operating time in direct proportion to the TMS value itself, the actual operating time at the given TMS = 0.3 is obtained by multiplying the TMS = 1 operating time by 0.3: operating time = 2.4 * 0.3 = 0.72 seconds.

This calculation illustrates the standard two-step procedure used to determine an IDMT relay's actual operating time for a specific fault condition: first, compute the plug setting multiplier from the actual fault current, the CT ratio, and the relay's rated current, then look up (or interpolate, if the computed PSM does not fall exactly on a tabulated value) the corresponding operating time from the relay's published TMS = 1 characteristic curve or table, and finally scale this value by the actual time multiplier setting in use to obtain the true operating time for the specific fault scenario.

Protection Scheme for Feeders

Radial distribution and transmission feeders are commonly protected using a combination of instantaneous and time-delayed (IDMT) overcurrent relays arranged in a graded (time-coordinated) sequence from the load end back toward the source, with relays located further from the source (closer to the load) given progressively shorter operating times (achieved through progressively smaller TMS settings) so that, for a fault occurring at any point along the feeder, the relay immediately upstream of the fault operates first, isolating the smallest possible faulted section, while relays further upstream (closer to the source) remain restrained by their longer time-delayed settings, only operating as backup protection if the more local relay fails to clear the fault within its own designated time. Directional overcurrent relays, as discussed above, are additionally required at locations where fault current could potentially flow in either direction (such as in ring-main or interconnected feeder arrangements, or on the source-side breaker of a feeder that could receive backfeed current from a parallel path during certain fault conditions), ensuring the protection scheme correctly discriminates between faults requiring that specific relay's operation and faults elsewhere in the network that happen to produce current flow through the same relay location but in the reverse (non-tripping) direction. For radial feeders supplying multiple downstream distribution transformers or laterals, a combination of feeder circuit breakers (with IDMT relays) at the main substation, combined with fuses or reclosers at the individual lateral tap-off points, is commonly used to achieve a cost-effective, appropriately graded protection scheme across the full extent of the distribution network, with the fuse or recloser ratings and characteristics at each successive downstream point selected to coordinate correctly with both the loads and faults expected at that specific point in the network.