Protective Relays and Instrument Transformers
Protective Relays and Instrument Transformers tests ratio, polarity, burden, pickup, and coordination. The exam expects conversion between primary and secondary quantities and recognition of unsafe instrument-transformer conditions.
Core rules
- CT ratio: primary current divided by secondary current. A 600:5 CT has a ratio of 120:1.
- Never open-circuit an energized CT secondary; high voltage can result. Short the secondary before removing burdens.
- A VT secondary must not be shorted; it is a voltage source.
- Polarity marks (H1/X1, P1/S1) determine directional and differential relay operation.
- Burden is the total VA or impedance connected to the secondary; excessive burden causes CT saturation and ratio error.
- Relay pickup is tested with primary or secondary injection and then converted by ratio.
Worked example
A 600:5 CT supplies a relay set to pick up at 4.0 A secondary. Primary pickup = 4.0 × (600/5) = 480 A. If primary injection actually picks up at 540 A, the secondary equivalent is 540/120 = 4.5 A, 12.5% high. Check the relay setting, CT ratio switch, and test-lead polarity. If the burden is 0.5 Ω at 5 A, the CT must supply 2.5 V; at higher burden, saturation risk rises.
Traps
Opening a CT secondary under load; shorting a VT secondary; ignoring polarity; using a CT ratio that does not match the nameplate; testing pickup without confirming the relay setting; neglecting DC offset in transient tests.