High Accuracy Current Transformer

High Accuracy Current Transformer

Accurate electric current measurement is essential for the safe operation of power networks.high accuracy current transformer The current transformer (CT) serves as a critical interface between high-voltage power networks and measuring or protection devices. However, CTs are susceptible to many errors and uncertainties that can degrade the reliability of their measurements. This article examines the types and causes of these errors, with special focus on ratio error and phase displacement. It also discusses how international standards, such as IEC 61869-2, determine the accuracy class of non-toroidal CTs and their permitted error limits.

The accuracy of a CT depends on its design, operating conditions, and primary cable position.high accuracy current transformer The main factors affecting its performance are the ratio error and phase displacement, which are defined in IEC 61869-2 as a quality criterion and an accuracy limit. The tolerance for these errors is defined as a specified accuracy class, which specifies the maximum allowable percentage of deviation from the root mean square of the primary current under steady-state conditions.

A CT operates based on electromagnetic induction: an alternating current flowing through the primary conductor generates a magnetic flux that connects to the secondary winding and induces a proportional secondary current.high accuracy current transformer This allows standard low-range instruments to measure large system currents indirectly and safely, without exposing them to high voltage. To achieve high accuracy, the core must be made of a quality material and have a precise winding ratio.

CTs are used in a wide range of applications to track current flow and support energy monitoring and billing, power quality analysis, and industrial automation for electric motors and drives. They are also vital for protecting circuits against overloads and short circuits by reducing large currents to manageable levels. However, these applications often involve dynamic conditions, with currents ranging up to several times the rated value. This means that the CT must accurately reproduce the shape and amplitude of the current waveform to ensure that protection relays respond appropriately.

In this study, the performance of a non-toroidal CT was evaluated through FEA simulations and experimental tests. The simulated results showed that the positions of the primary cables in the airgap of the CT have a significant effect on its accuracy, reaching errors in the current measurement up to 25% when the cables are located at extreme left and right positions. In contrast, the centered position produced an accuracy close to the assigned limit. These results were validated through experimental tests where the primary cables were moved in a mesh of positions in x and y extremes inside the CT airgap. The resulting FEA simulations and the test data allowed the definition of an accuracy zone where the CT can reliably perform its function. This limiting zone can be determined from the relationship between Ec, the composite error, Rt, the transformer current ratio, I1, the instantaneous primary current, and i2, the instantaneous secondary current.

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