Current Transformers

CTs are factory calibrated using certified reference instruments, but periodic verification is recommended every 3-5 years in standard applications and more frequently in critical environments. Calibration can be performed by comparing the secondary current to a known reference or using automatic calibration systems. Regular maintenance ensures accuracy class and compliance with regulations, reducing the risk of systematic errors and extending the transformer’s useful life.

The secondary load (expressed in VA) represents the power required by the instruments connected to the CT’s secondary winding. If the load exceeds the nominal value, the measurement error increases and may exceed the limits set by the accuracy class. To avoid this problem, it is necessary to sum the impedances of all connected devices and connection cables and ensure they are lower than the CT’s nominal power. Correctly sizing the secondary load ensures accurate measurements and consistent performance over time.

Core saturation occurs when the magnetic flux exceeds the capacity of the ferromagnetic material, compromising the linearity of the transformation and introducing significant errors in the measurement. This phenomenon can occur with currents exceeding the nominal value or excessively high secondary loads. To avoid this, it is important to select a CT with a ratio and nominal power appropriate for the application and verify the magnetic behavior declared by the manufacturer. Saturation negatively impacts the accuracy and ability of the CT to properly power downstream devices.

Current transformers must comply with international technical standards such as IEC 61869-2, which defines construction, accuracy, and safety requirements for current transformers intended for measurement and protection. Other relevant standards include IEC 60044-1 for general characteristics, IEC 61010-1 for electrical safety, and IEC 61326 for electromagnetic compatibility. Compliance with these standards ensures reliable performance, operational safety, and interoperability with certified instruments and measurement systems.

The choice of a CT depends primarily on the value of the primary current to be measured, the required secondary current (typically 1 A or 5 A), the accuracy class, the type of installation (through busbar, wound or split primary), and the secondary load. It is important to select a model with an accuracy class appropriate to the application (0.2 or 0.5 for measurement, 1 or 3 for protection) and ensure that the total connected load does not exceed the CT’s rated power. The type of insulation, voltage level, and mains frequency must also be compatible with the system.

Through-bar CTs are the most common and are passed through a rigid conductor or bar, offering high precision and robustness. Wound-primary CTs are used for low primary currents and integrate the primary winding directly into the transformer, making them compact and suitable for precision measurements. Split-core CTs are ideal for retrofits and installations without interrupting the circuit: while offering slightly lower precision, they allow for quick maintenance and are widely used in temporary monitoring applications or modular systems.

The accuracy class indicates the maximum permissible error between the actual primary current and the proportional secondary current. CTs with class 0.2 or 0.5 are suitable for precision measurements and metering, while classes 1 or 3 are sufficient for protection applications. An inadequate class can compromise the quality of the measurement or the response of downstream devices, such as meters and relays. In critical applications, accuracy directly affects the reliability of the monitoring system and compliance with electricity metering regulations.

Split-core CTs are particularly useful in retrofit or maintenance operations, when it’s necessary to install or replace a transformer without interrupting the electrical circuit. While they offer slightly less accuracy than traditional CTs, they’re the ideal solution for rapid installations, temporary monitoring, or systems where power cannot be interrupted. Furthermore, they simplify periodic checks and can be reused at different points in the system, making them extremely versatile in industrial settings.

The secondary winding of a CT should never be left open when the primary is carrying current, as this can generate high, dangerous voltages that damage the winding or pose a safety risk to the operator. Furthermore, an open secondary winding interrupts the current transformation, causing incorrect readings and possible overvoltages. For this reason, it is good practice to short-circuit the secondary winding during maintenance or replacement of connected instruments.