Protective relays must operate correctly when abnormal conditions require and must
not operate at any other time.
Electrical protective relays are calibrated with settings derived from system fault and
load studies. Initial settings are provided when relays are installed or replaced.
However, electrical power systems change as new generation and transmission lines
are added or modified. This may mean that relay settings are no longer appropriate.
Outdated relay settings can be hazardous to personnel, to the integrity of the
powerplant and power system, and to the equipment itself. Therefore, it is necessary
to periodically conduct a fault and load study and review protective relay settings to
ensure safe and reliable operation.
Fault and load studies and relay settings are provided by the Electrical Design Group
(D-8440) at 303-445-2813. Field-initiated changes to relay settings should be verified
by this group.
Protective relays currently in use in Reclamation include electro-mechanical, solid-
state, and microprocessor-based packages. Calibration and maintenance
recommendations differ from type to type because of their different design and
Calibration: This process usually includes removal of the relay from service to a test
environment. Injecting current and/or voltage into the relay and observing the
response according to the manufacture's test procedure verifies the recommended
settings. Calibration of electro-mechanical relays is recommended fairly frequently
since operating mechanisms can wear and get out of adjustment. Calibration of solid-
state and micro-processor-based relays is recommended less frequently since there are
fewer ways for them to get out of calibration.
Relay Functional Test: This process verifies that the protective outputs of the relay
(e.g., contact closures) actually operate as intended. This can be accomplished as part
of the calibration procedure in most cases, but relay functional testing should be
verified according to the maintenance schedule.
Protective relays operate into protection circuits to accomplish the desired protective
action. Similar to control circuits, protection circuit integrity may be compromised by
construction, modifications, deterioration, or inadvertent damage. A compromised
protection circuit may not provide the system and plant protection desired. Periodic
functional testing is recommended to ensure the integrity of protection circuits.
Protection Circuit Functional Testing: This process verifies that the entire
protective "trip path" from protective relay through circuit breakers (or other
protective equipment) is intact and functional. This requires actually operating the
entire circuit to verify correct operation of all components.