What type of relay operates based on the ratio of potential transformer to current transformer measurements?

• A. Differential relay
• B. Distance relay
• C. Lockout relay
• D. Time overcurrent relay

Answer: (B)

The distance relay is specifically designed to operate based on the ratio of the measurements from potential transformers (PTs) and current transformers (CTs). This type of relay analyzes the impedance in a power line, which is determined by the voltage (from PTs) and the current (from CTs). The relationship between these two values allows the distance relay to identify issues such as faults and determine their distance from the relay location along the transmission line, which is crucial for effective fault protection and isolation. Through this impedance measurement, the relay can ascertain whether a fault condition exists and enable appropriate corrective actions, such as tripping circuit breakers.

The other relay types mentioned do not utilize the same principle of operation based on the direct ratio of PT and CT measurements for protection purposes.