What is the primary function of a current transformer (CT)?

The primary function of a current transformer (CT) is to reduce high line current to a proportionally lower current. This is crucial in electrical systems where direct measurement of high currents is impractical and potentially hazardous. By transforming a high current to a manageable level, the CT allows for accurate measurement and monitoring, enabling the use of standard instrumentation and protection devices without the danger associated with high voltage levels.

This reduced current can be safely monitored and utilized for various applications, such as in metering, protection relays, and control systems. The proportionality factor of the transformer ensures that any readings taken or protections activated reflect the actual high current conditions in the system, thus maintaining accuracy and safety in electrical systems.

The other options, while related to electrical concepts, do not accurately describe the fundamental role of current transformers. For example, increasing line voltage for transmission relates to transformers designed for voltage elevation, while serving as a protective relay or measuring current directly in high voltage lines is not the primary function of a CT, which focuses specifically on current reduction and management.