• Home
  • /
  • Blog
  • /
  • Relay Settings from 1924 Silent Sentinels

Relay Settings from 1924 Silent Sentinels

Silent Sentinels 1924 Excerpt #10

This excerpt from the 1924 version of Silent Sentinels discusses protective relay settings in the electrical system. This is the 10th in the series. Follow these links to learn more about this series and the 1924 version of Silent Sentinels.

Relay Settings

The selection of the proper current tap and time index lever setting for induction-type overcurrent relays demands a knowledge of the load conditions in the circuit, or portion of the circuit to be protected, and the amount of short-circuit current available under minimum generating capacity conditions. It is also desirable to know the amount of short-circuit current available under maximum generating conditions, but in case the data is not available, the interrupting ability of the oil circuit-breaker will serve as a first approximation. While there are minor modifications in the actual settings of the relays dependent on the particular type of protection employed, the following remarks will outline the general considerations with reference to the simpler cases.

Current Tap Settings on Radial and Loop Feeders

The proper current tap is determined largely by the current transformer ratio and the full-load current in the feeder. In general, the current tap is so chosen as to be at least 200 per cent. of the secondary current from the current transformers when carrying full load in the line. In cases where the load is subject to violent fluctuations it may be necessary to make normal operating current for the relay 300 per cent. of the secondary current corresponding to normal full load. The current tap should not, however, be chosen so high that the relay winding receives less than 300 per cent. of its current setting under short circuit with minimum generating capacity on the system, as otherwise the time of operation will be so long as to completely disrupt service.

Time Settings on Radial Feeders

The proper time of operation of the relay is determined by its location with respect to the source of power and other relays on the same circuit. For a radial feeder system, the outermost relay is set for the shortest time, and the relays toward the generating station for successively longer times. The relays at the source of power must be set sufficiently long to allow any outlying faulty section to be cleared. With induction-type relays and direct-current operated circuit-breakers, it is common practice to set successive relays for time intervals 0.5 second apart with the outermost relay set for 0.1 second. Adopting a maximum of 2.1 seconds as the longest time a fault should remain on the system we therefore find that five sections is about the most that should be protected by selective timing alone. Although some systems are operating satisfactorily with relays set 0.3 or 0.4 second apart, such systems achieve success only by using extreme care in the maintenance of the oil circuit-breaker mechanism and careful adjustment of relays and trip-circuit mechanisms. Time intervals closer together than 0.5 second are not to be recommended for average systems. The time of operation should be that required when the relay is operating on the “flat” or minimum time interval of the time-current curve, i.e., when the secondary current is 500 per cent. or more of the relay current tap setting.

Time Settings on Loop Feeders

For the simple loop system, involving only one source of power, the relay time settings are as described in the paragraph on “Time Settings on Radial Feeders.” The loop is opened at one of the generating station breakers and the relays on the outgoing sides of the substations set for successively higher times of operation from the outermost substation (considering the feeder as straight radial) to the generating station. The loop is then closed up and the other generating station breaker opened, the time settings on the remaining relays being determined as already outlined. Starting on the loop at any point and approaching the generating station, it will then be observed that the relays set to trip for power flowing from the generating station will be set for increasingly longer times of operation, whereas the relays set to trip for power flowing towards the generating station will be set for increasingly shorter times of operation.
The loop system which receives power at two or more points in the ring is especially difficult to relay. If the various substation relays are timed for one source of power, as outlined in the preceding paragraph, the timing is wrong when considering the other source or sources of power. Parallel lines between any two points can, of course, use some form of balanced protection for parallel operation, but this is not applicable when the lines are operating singly. In applying a series of time settings to such a system, it is generally necessary to consider both the amount and source of short-circuit current available. Selection usually can be obtained by working on the inverse portion of the time-current curve of the relay.

Parallel Feeders Protected by Type CD Relays

For lines protected by type CD relays it is usual to set the relay to operate on a differential current of 3 amperes. For through short circuits the current in the two equal lines will be balanced so that there will be a very slight amount of differential current tending to trip the relays. For the somewhat infrequent case where type CD relays protect two parallel lines of unequal impedance, but equipped with current transformers of the same ratio, type A current-balance auto-transformers should be used to make equal the currents flowing through the relay windings. The time setting is generally on the order of 0.1 or 0.2 second.

Settings for Ground Relays

Ground relays are set the same as line relays except that ground current flow differs in magnitude from the current flow for a line-to-line short circuit. 



 Did you like this post?

You can share it with these links:

About the Author

Chris is an Electrical Engineering Technologist, a Journeyman Power System Electrician, and a Professional Engineer. He is also the Author of The Relay Testing Handbook series and founder of Valence Electrical Training Services. You can find out more about Chris here.

Read More Articles:

Typical Relay Application from Silent Sentinels 1924

Leave a Reply

Your email address will not be published.

{"email":"Email address invalid","url":"Website address invalid","required":"Required field missing"}