Protective Relays and Instrument Transformers
Protective Relays and Instrument Transformers
This domain tests secondary injection of relays, CT and VT ratio/polarity/burden, pickup and dropout, timing, and trip-circuit verification.
Core rules
- CT polarity: P1 enters, S1 leaves; verify with a DC kick or polarity test. Wrong polarity reverses directional and differential decisions.
- Measure CT ratio and excitation; compare to nameplate. Check burden so the CT does not saturate.
- Never open-circuit an energized CT secondary; dangerous voltage develops. Short unused windings.
- Relay tests: pickup, dropout, time delay, and target; use three-phase injection for phase and ground elements.
- Verify the trip circuit from relay output to breaker trip coil, including DC supply and auxiliary contacts.
Worked example
A 600/5 CT should have a 120:1 ratio. Inject 120 A primary; expected secondary is 1.00 A. The meter reads 1.05 A. Ratio error = (1.05 - 1.00)/1.00 x 100 = +5%, which is excessive for a metering CT and should be investigated. The reasoning: the secondary current is higher than expected, so the CT may be misconnected, the burden may be wrong, or the test lead may be bypassing part of the circuit.
Traps
- Open-circuiting a CT secondary.
- Testing a relay only single-phase when the element is three-phase.
- Ignoring CT burden and saturation.
- Not verifying trip-coil continuity.