misoperation. Design practices with regard to the use of switches in trip

circuits vary among utilities. The advantage of such switches is that they

can make testing more safe and efficient. The disadvantage is that if such

switches are left in the improper position, protection may not be available

when needed.

Diodes are used in trip circuits to provide the ability for DC current to

flow in one direction through a portion of the circuit. Diodes can facilitate

providing logic into the circuits that would otherwise require the use of

auxiliary relays. The use of diodes reduces cost and complexity of the

circuitry. The disadvantage of using diodes is that they are subject to failure,

which can remain undetected until a misoperation occurs.

14.20.4 TRIP COILS

The function of trip coils is to initiate action to open the main contacts of

breakers. The actual energy to open the contacts is provided by some form of

stored energy such as springs, compressed air, and high-pressure gas. Trip

coils therefore do not directly open the contacts, but act to release a latch or

open a valve that allows the release of some form of stored energy used to

move the contacts. Breaker designs can vary considerably among manufac-

turers and it is important for protection engineers to understand the charac-

teristics of the trip coils and other control devices used in the specific breaker

that is used, so that proper trip circuit designs can be developed.

14.21 TRIP CIRCUIT DESIGN

Trip circuits may be required to trip a single breaker or multiple breakers.

Tripping of a single breaker is usually associated with distribution feeders,

subtransmission lines, and industrial circuits. Multiple breakers need to be

tripped with bus arrangements such as breaker-and-a-half, ring bus, or double

bus–double breaker designs. Such designs are usually associated with higher-

voltage systems.

Various arrangements are possible for trip circuit designs, which are

highly influenced by individual company practices and experience. A special

publication by the IEEE Power System Relaying Committee ‘‘Relay

Trip Circuit Design’’ should be used as reference for more detail on this

subject.

Regardless of the design details, some general requirements that need to

be addressed when developing designs for trip circuits are as follows:

1. Adequate voltage must be delivered to the trip coil(s) to assure reliable

breaker operation. All possible voltage drops that may exist in the

circuit must be considered when determining voltage available at

the trip coil(s).

� 2006 by Taylor & Francis Group, LLC.

2. Sufficient DC current must be avai lable to operate tar gets on all relays

that have operate d. Proper tap settings and coil rating s need to be

selected for target and seal-in units.

3. Care must be taken to avoi d connec tions between DC circuits . Such

connections can result in the loss of both prima ry and backup relays for

a sing le failure on the DC system .

4. Currents drawn by auxi liary relays must not cause inco rrect operation

of seal-in units or the failur e of such units to drop out prope rly .

5. Protection against surge s caused by the operation of the vario us coils

that exist in the cont rol circuitry should be appl ied to protect other

equipment connec ted in the circui t. This is especia lly import ant to

protect d elicate elect ronic equi pment that is used in many o f the

more mode rn subst ations.

6. Relay coils shoul d be connecte d to the negat ive DC bus.

This is required to minimi ze the potential failure of such coils be-

cause of a build-u p of corrosion . Coil s that are direct ly connecte d to

the positive DC bus are subject to corrosi on wi th the pres ence of

moisture .

14.22 TRIP CIRCUIT MONITORING AND ALARMS

Su pervision of cont rol cir cuitry that affects the ability of trippin g functio ns to

perf orm prope rly is an import ant enhanc ement for protectio n relia bility.

Im portant conditions for which monit oring should be consider ed include the

fol lowing:

1. Inadequate DC tripping volt age includes low DC voltage leve ls

(battery problems) , open brea ker trip coil, blown fuse in a DC circui t,

or an opening in the DC circuit wiring.

2. Loss of AC pote ntial to prima ry and backup relaying.

3. Loss of communication channels that are used to initiate tripping.

4. Relay problems when self-checking features are available.

A common means for monitoring availability of DC tripping voltage is by

connecting lamps in the DC tripping circuit. A simple arrangement for

connec ting such lamps in a trip circui t is illus trated in Figur e 14.10.

When the breaker is closed, the red lamp provides indication of the availabil-

ity of DC voltage to the trip coil. When the breaker is open, the green lamp

provides similar indication. The red and green lamps also provide indication

for the breaker position—the red lamp will be lit when the breaker is closed

and the green lamp will be lit when the breaker is open. The lamps are usually

mounted on the control panel within the substation at a location where the

associated breaker is clearly identified. The lamps that are used must meet

requirements of the DC circuit into which they are installed and should

� 2006 by Taylor & Francis Group, LLC.