Current transformers are important equipment for energy metering and protection control in power systems. They are a key component of energy metering, relay protection, system diagnosis, and detection analysis. Their measurement accuracy and operational reliability are prerequisites for achieving safe and economical operation of power systems.

1. Measurement of AC current

To determine whether equipment operates at the rated current value, installing current measurement devices is a necessary technical measure. Relevant regulations stipulate that equipment above 40KW must be equipped with ammeters for monitoring.

AC current measurement can be done through direct measurement or expanded measurement via current transformers. Direct measurement involves connecting an appropriate ammeter in series in the current circuit. Circuits are typically connected through current transformers for measurement.  

1) Use one current transformer to measure current in a three-phase balanced line. The current transformer is selected based on the ammeter range, which is determined by the actual load current, and should occupy between 2/3 and full scale of the ammeter's total range.  

2) Use two current transformers connected in an incomplete star configuration to measure three-phase current in balanced or unbalanced lines.

3) Use three current transformers connected in a complete star configuration to measure three-phase current in balanced or unbalanced lines.

2. Measurement of AC energy

To expand the range of AC energy meters, the most common method in factories is to use current transformers to increase the range.

1) Using a single-phase energy meter with a current transformer can be used for measuring three-phase balanced or single-phase energy metering.

2) Using a two-element three-phase three-wire active energy meter with two current transformers can be used for energy metering in balanced or unbalanced three-phase lines.

3) Using a three-phase four-wire active energy meter with three current transformers can measure energy metering in balanced or unbalanced three-phase lines.

3. Protection in electric drive circuits

Overload protection in high-power motors often cannot use corresponding thermal relays due to high current. In such cases, the common solution is to install current transformers. The essence is to convert large currents into small currents, so that a thermal relay within 5A can meet the overload protection requirements.

1) Used for overload protection in motors with high-power auto-transformer step-down starting.

2) Used for overload protection in motors with high-power frequency-sensitive rheostat starting.

3) Used for overload protection in motors with high-power star-delta step-down starting.