Current transformers and potential transformers are two core types of instrument transformers used in power distribution and transmission systems. Both devices operate on the principle of electromagnetic induction and are mainly used to convert high power system parameters into standard low‑level signals for measurement, monitoring and system protection. They also provide electrical isolation between high voltage primary circuits and low voltage secondary control equipment to ensure operational safety.
A current transformer is installed to measure and transform circuit current. It is connected in series with the power line so that the entire load current flows through its primary winding. It steps down high primary current to a standardized secondary current of one ampere or five amperes for ordinary meters and protective relays. During normal operation, the secondary circuit of a current transformer maintains a closed loop state. An open secondary circuit will induce extremely high voltage, which may damage equipment and cause safety hazards. For this reason, current transformers are strictly prohibited from open‑circuit operation.
A potential transformer, also known as a voltage transformer, focuses on voltage conversion and monitoring. It is connected in parallel across the power line to bear the full system voltage. The transformer reduces high primary voltage to a standard low secondary voltage of one hundred volts, which is suitable for voltage meters, power meters and protection devices. Unlike current transformers, potential transformers require an open secondary state under normal working conditions. Short circuit faults on the secondary side will generate excessive current and burn out internal windings, leading to device failure and system risks.
In practical power applications, current transformers are mainly used for current measurement, load monitoring and overcurrent protection, while potential transformers are responsible for voltage measurement, power factor calculation and overvoltage protection. The two types of transformers are usually used cooperatively in power cabinets and substations. Their structural and operational differences make them professional and targeted in respective working scenarios, ensuring the stability, accuracy and safety of the entire power system.
