Which transformer is specially designed for the measurement of electrical quantities and control purposes?

The transformer specifically designed for the measurement of electrical quantities and control purposes is known as an instrument transformer. Instrument transformers serve a crucial function in electrical power systems by providing accurate readings of current and voltage at various points within an electrical circuit. This is essential for monitoring, protection, and control of power systems, allowing operators to maintain efficiency and safety.

Instrument transformers include current transformers (CTs) and potential transformers (PTs), which are utilized to step down high current or voltage levels to manageable values for measuring instruments. By transforming the electrical quantities down while maintaining the necessary proportionality, these transformers ensure that sensors, meters, and electrical relays can operate safely and effectively without exposing them to the high voltages or currents present in the system.

Step-down transformers, while important in reducing voltage levels for general use, are not specifically designated for measurement purposes. Potential transformers serve a similar purpose in measuring voltage but are a subset of instrument transformers rather than a standalone type defined strictly for measurement. Lastly, service transformers are primarily used for supplying power to end-users and don't focus on measurement or control functions. This categorical distinction highlights why the instrument transformer is the most appropriate answer for the question posed.