A current transformer (CT) is an instrument that reduces high primary currents into smaller, manageable currents on the secondary side, enabling safe and accurate measurement. This process operates on the principle of electromagnetic induction.
Components:
The CT comprises a closed magnetic core and windings. The primary winding has fewer turns and is connected directly in the line carrying the current to be measured, while the secondary winding has more turns and is connected to measuring instruments and protection circuits.
Primary to Secondary Transformation:
The large current on the primary side is converted into a smaller, proportional current on the secondary side.
Closed Circuit Requirement:
During operation, the secondary circuit of the CT must remain closed. Since the measuring and protection devices connected in series have very low impedance, the CT operates close to a short-circuit condition. Opening the secondary circuit while the primary is energized can lead to hazardous situations.
Measurement:
CTs measure currents in high-voltage lines by reducing them to safe, manageable levels for instruments.
Protection:
CTs provide inputs to protection relays that monitor current and initiate protective actions during abnormal conditions, such as faults.
Safe Monitoring:
By reducing large currents, CTs allow safe and precise monitoring of AC transmission lines in applications like national power grids.
If the CT's secondary circuit is open while the primary remains energized, several risks arise:
High Voltage Transients:
Dangerous high-voltage transients can damage the CT’s winding insulation and render it inoperable.
Core Saturation and Errors:
The core may experience harmful magnetization, leading to measurement inaccuracies.
Operational Safety Risks:
These transients can pose safety risks to personnel and equipment.
To mitigate the risks of an open secondary circuit, IEEE C57.13 recommends equipping CTs with voltage-limiting devices:
Voltage Limiting Devices:
These devices prevent high voltage buildup by withstanding open-circuit conditions for a specific duration without damaging the CT.
Metrosil Current Transformer Protection Units (CTPUs):
Unlike traditional devices, Metrosil CTPUs do not require immediate replacement after abnormal conditions and can remain operational without user intervention.
Varistors:
Limit voltage across CT terminals during open circuits by dissipating energy stored in the CT core.
Thermostatic Switches:
Manage internal thermal conditions and provide remote monitoring options when mounted on radiators.
Durability:
Designed to withstand high transients without impairing CT functionality, ensuring consistent performance and safety.
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