Which protective relay operates based on a predetermined value of polyphase voltage in the desired phase sequence?

The protective relay that operates based on a predetermined value of polyphase voltage in the desired phase sequence is indeed the relay type that is designed specifically for monitoring voltage conditions. Relay Type 47 is commonly known as a phase sequence relay. It functions by ensuring that the sequence of voltage phases is correct, and it can recognize if the phase sequence is out of order. This is crucial in preventing damage to three-phase motors and equipment that could occur if they operate with an incorrect phase sequence.

By detecting variations in the polyphase voltage and ensuring they are within set thresholds, the relay can provide safeguards against potentially harmful conditions. In contrast, other relay types mentioned do not specifically focus on the phase sequence and instead have different operational characteristics such as timer functions, overcurrent protection, or other types of voltage monitoring. Understanding the specific application and functionality of each relay type helps in selecting the appropriate protection strategy for electrical systems.