How do protective relays work and what are common relay types?

Protective relays monitor system conditions through CTs and VTs, and trip circuit breakers when abnormal conditions are detected. Common types include: overcurrent relays (operate on current magnitude), differential relays (compare currents entering/leaving zone), distance relays (measure impedance to fault), directional relays (determine fault direction), and undervoltage relays. Modern microprocessor relays combine multiple functions, provide event recording, and communicate with SCADA systems.