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DEPARTMENT OF THE ARMY
ETL 1110-8-5(FR)
US Army Corps of Engineers
CECW-EE
Washington, DC 20314-1000
Engineer Technical Letter
No. 1110-8-5(FR)
26 July 1991
Engineering and Design
HYDROELECTRIC PLANT CONTROL
1. Purpose
4.
Breaker Failure Schemes
In applying breaker failure schemes, some thought
The purpose of this engineer technical letter (ETL) is
should be given to the overall reduction in system
to encourage Divisions and Districts to reevaluate
reliability and consequences that could result from
present generator breaker control schemes for pos-
sible modifications to correct deficiencies in unit
incorrect relay operation.
protection.
a. Most breaker failure relaying schemes
2. Applicability
operate on high phase or ground currents. When a
trip signal is applied to the breaker, the breaker
This letter applies to all USACE Commands involved
should open, and the current should cease to flow
in the design and operation of hydroelectric projects.
within the breaker interrupting time. The breaker
failure relay is usually applied to operate lockout
3. Background
relays to trip backup breakers after a time delay
based on the assumption the breaker must have failed
In several cases, failure of breakers to open due to
if current continues to flow after the breaker trip
faulty breaker trip mechanisms has resulted in severe
circuit has been energized.
damage to the associated generator caused by induc-
tion motoring of the generator for an extended period.
b. Time-delayed reverse power relaying
The turbine wicket gates were closed, and the unit
schemes are sometimes used for generators and could
was in a shutdown condition; however, the generator
be applied to protect against generator breaker failure.
breaker remained closed, and the unit remained ener-
These schemes will not operate for a breaker failure
gized from the system. In other cases, breaker clo-
under normal fault conditions and would have to be
sure has occurred without excitation being applied to
coordinated with synchronous condenser operation.
the unit. This ETL presents for consideration breaker
failure schemes, breaker trip coil supervision and
c. One scheme without the above drawbacks
alarm schemes, and fail-safe breaker tripping schemes
is to use a breaker auxiliary contact to detect breaker
that will initiate automatic control to protect against
failure. A breaker auxiliary contact connected as
such failures. The schemes presented in the examples
shown in Figure 1 will drop out Timing Relay 2 if
of this ETL may not be applicable to the specific
the breaker opens after energization of the trip circuit.
situation for all facilities. In those instances, the
Protective relay contact closing, or operation of the
schemes should be modified to make them applicable
breaker control switch to the trip position, energizes
and to avoid improper application.
Timing Relay 2. If the auxiliary contact does not
open after a time equivalent to the breaker interrupt-
ing time, plus a margin for relaying time, the timing
relay will close its contacts to alarm breaker failure
This engineer technical letter supersedes
and to initiate the required backup trip functions.
ETL 1110-2-226, 19 December 1977.
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