What type of relay operates based on the ratio of PT measurements to CT measurements?

A distance relay operates based on the ratio of measurements from potential transformers (PTs) and current transformers (CTs). This relay is designed to measure the impedance of the transmission line, which correlates with both voltage (from PTs) and current (from CTs). By calculating this impedance, the distance relay can determine the fault location along the transmission line and operate when it detects a change in that impedance that exceeds pre-set thresholds.

This functionality is critical for protecting transmission lines, as it allows the relay to detect and isolate faults based on distance rather than just the magnitude of current or voltage. The distance relays take into consideration both the voltage and current levels to accurately assess line conditions and can differentiate between various types of faults, leading to reliable and effective protection of the electrical grid.

Other options presented do not operate based on the specific ratio of PT to CT measurements. Overcurrent relays respond to current levels alone, voltage relays operate based on voltage conditions, and current transformer relays, while related to CTs, do not involve the ratio of PT and CT measurements in determining their operation or faults.