What is the difference between current transformers and voltage transformers?

Within the vast framework of the power system, there is an abundance of powerful energy in the form of high voltages and large currents. Humans cannot directly perceive or measure this immense energy, which is why current transformers and voltage transformers are needed to act as the "eyes" and "ears" of the system. Among them, current transformers and voltage transformers are the two most core types. Although they have similar names, their functions are quite different, jointly forming the foundation for measurement, metering and protection in the power system.

Ⅰ. Core missions differ: Measuring current vs. measuring voltage

This is the most fundamental difference between the two, which determines all their design and usage characteristics.

⑴ Current Transformer (CT): Its primary function is to measure and proportionally transform the current. It precisely converts the large current on the primary side (the main circuit), such as 1000A, into a standard small current on the secondary side (such as 5A or 1A), which is then used by instruments and protection devices.

⑵ Voltage Transformer (PT/VT): Its main function is to measure and proportionally transform voltage. It precisely converts the high voltage on the primary side (such as 10kV) into a standard low voltage on the secondary side (typically 100V or 100/√3 V), which is used by instruments and protection devices.

II. Different working states and structural principles: Series connection and parallel connection

⑴ The root cause of structural differences:

When CT is connected in the circuit, its own impedance must be extremely low to avoid affecting the measured current and causing unnecessary voltage drop.

When PT is connected in parallel in the circuit, its own impedance must be extremely high to prevent excessive current (excitation current) from being drawn from the main circuit, thereby avoiding any impact on the system voltage.

Although current transformers (CTs) and voltage transformers (PTs/VTs) have fundamental differences in their working principles, structures and usage rules, they are indispensable "golden partners" in modern power systems. Together, they convert the original high-voltage and high-current signals into safe and standardized low-voltage and low-current signals, providing the sole data source for system monitoring, measurement, protection and control.