A current transformer (CT) operates on the principle of electromagnetic induction, similar to a standard transformer. Its primary purpose is to step down the high current from the power system to a lower, manageable value for metering, protection, and control equipment. Here’s how a current transformer works:
1. Basic Construction:
A CT typically consists of:
Primary Winding: A conductor carrying high current, which can be a single-turn winding or a system conductor passing through the CT’s core.
Core: Made of ferromagnetic material, it concentrates the magnetic field generated by the current in the primary winding.
Secondary Winding: Wound around the core, with more turns than the primary. This winding is connected to a meter, relay, or other measuring/protective device.
2. Operating Principle:
Electromagnetic Induction:
The operating principle of a CT is based on Faraday’s law of electromagnetic induction, which states that a changing magnetic field within a conductor induces a voltage in another nearby conductor.
Primary Current:
When a high current flows through the primary winding (or conductor), it generates a magnetic field around the primary conductor.
This magnetic field is concentrated by the CT’s core and connected to the secondary winding.
Secondary Induced Current:
The changing magnetic field generated by the primary current induces an electromotive force (EMF) in the secondary winding, causing current to flow through it.
The secondary current is proportional to the primary current but is reduced by the turns ratio of the primary and secondary windings.
Turns Ratio:
The ratio of the number of turns in the secondary winding to the number of turns in the primary winding determines the reduction in current.
For example, if the primary consists of one turn (wire passing through the core) and the secondary has 1000 turns, a primary current of 1000 A will result in a secondary current of 1 A (a 1000:1 ratio).
3. Closed Secondary Circuit Operation:
When primary current is flowing, the secondary winding must always be closed (either connected to a load or short-circuited). This allows the magnetic field generated by the primary current to induce a proportional current in the secondary.
The secondary winding is typically connected to a low-impedance measuring device such as an ammeter, relay, or protection system.
4. Purpose and Application:
Current Measurement: CTs reduce high currents in primary circuits to a smaller, standardized value (e.g., 1 A or 5 A) that can be safely measured by instruments.
Protection: CTs provide current inputs to protective relays, triggering circuit breakers in overcurrent or fault conditions.
Isolation: CTs electrically isolate high-voltage primary circuits from low-voltage measuring or protective devices for safety.
5. Accuracy Class:
CTs are designed for high-precision operation, so the secondary current is proportional to the primary current within a specified accuracy level (e.g., 1% or 0.5% error). Accuracy is critical in metering and protection applications.
Work Summary:
Primary current generates a magnetic field in the CT core.
This changing magnetic field induces a current in the secondary winding.
The secondary current is a scaled-down version of the primary current, based on the turns ratio.
The secondary current can then be measured or used for control/protection purposes.
This allows for safe and accurate measurement of high currents in power systems, protecting personnel and equipment.
Post time: Oct-15-2025
