The transformation ratio of a current transformer (CT) refers to the ratio between its rated primary current and its rated secondary current; it serves to reflect the CT's capability to proportionally convert large currents into smaller ones. Common standard secondary currents are typically 5A or 1A, whereas the primary current is designed based on the actual load conditions of the power system-for instance, 100A, 200A, or even higher. Consequently, the transformation ratio is typically expressed in forms such as 100/5A or 300/5A, making it a critical parameter in the selection and application of current transformers.
The transformation ratio not only determines the measurement range but also directly impacts metering accuracy and protection performance. If the selected ratio is excessively high, it may lead to inaccurate measurements during light-load operations; conversely, if the ratio is too low, it could result in core saturation under high-current conditions, thereby compromising output linearity. Therefore, in engineering applications, it is essential to establish a rational match between the CT's specifications and the system's maximum load current and normal operating current, thereby ensuring that the transformer operates within its optimal working range.
Furthermore, the transformation ratio of a current transformer is closely linked to its accuracy class. Within the rated load range, a smaller ratio error indicates more stable measurement performance from the transformer. For current transformers used for metering purposes, a higher accuracy class is generally required to ensure the precision of energy measurement; conversely, current transformers used for protection purposes place greater emphasis on reliable response capabilities under fault current conditions. Thus, in the actual design and selection process, the transformation ratio is not merely a simple proportional relationship, but rather a crucial basis for comprehensively balancing system safety with measurement accuracy.
