Protective Relaying Flashcards
Per SOO PC 600.12, no unit will be started after any relay operation resulting in targets showing:
Ground fault (64) Differential (87) Sudden pressure (63)
What are the three major faults that affect power systems?
Three phase
Line to line (phase to phase)
Single line to ground (phase to ground)
Primary relaying is:
The first line of defense or protection against electrical or mechanical trouble.
Back-up relaying functions only:
When primary relaying fails
Auxiliary relays operate:
In response to the opening or closing of its operating circuit
and
to assist another relay or other device in performance of a function.
Monitoring or verification relay is used to:
Verify power system conditions with respect to prescribed limits
and
to initiate or permit automatic functions other than opening a circuit breaker during fault conditions
The two groups of protective relays:
Primary and backup
It is preferable to have adjacent zones of protection:
Overlap
Relays operate on two fundamentally different operating principles:
Electromagnetic attraction
Electromagnetic induction
Relays that employ the electromagnetic induction principle provide:
A time-delayed operation, which can be adjusted on some, by changing the distance the moving contact must travel
An inverse-time curve is one in which:
The operating time becomes less as the magnitude of the actuating quantity is increased
Seal-in function:
Prevents arcing
Shunted in parallel
Keeps contacts closed until reset
Protective relays have been described as:
The watchdogs of the system
Four classes of relays:
- Auxiliary
- Protective
- Regulating
- Verification
Most faults provide several characteristics by which they can be located:
- Increased current
- Reduced voltage
- Fault power always flows towards the fault
- Temperature rise
A contact that is open when the relay is de-energized is termed:
An open “A” contact and a contact that is closed when the relay is de-energized is termed a closed or “B” contact
High speed relays operate in:
3 cycles or less
The 59N (over-voltage neutral) is used as a separate relay to provide sensitive protection to the grounded neutral because:
It can be set without regard to load current
Current in the neutral transformer produces a voltage in the secondary which operates the relay if the voltage exceeds a predetermined level
The 51V ( over-current relay with voltage restraint) is desirable for pumping unit where inrush amps are high because:
It can properly protect the unit against overloads caused by a fault while preventing a trip on a starting amps
The induction disc relays are used with:
Resistance Temperature Detectors (RTD) located in the critical temperature ares of the apparatus being used to provide either an alarm or a direct trip action.
The most common type of fault is:
Phase to ground
