The principle of a current transformer is based on electromagnetic induction. A current transformer consists of a closed iron core and windings. Its primary winding has very few turns and is connected in series with the circuit whose current needs to be measured; therefore, it often carries the entire current of the circuit. The secondary winding has more turns and is connected in series with the measuring instrument and protection circuit. When the current transformer is working, its secondary circuit is always closed. Therefore, the impedance of the series coils of the measuring instrument and protection circuit is very small, and the operating state of the current transformer is close to a short circuit. The current transformer converts a large primary current into a small secondary current for use; the secondary side must not be open-circuited.
First of all, it is feasible to use the secondary side of a current transformer to control a current relay. However, this can only be considered when the load current is very large and there is no suitable high-capacity current relay to match it. The reason is that in automation control, as long as the function meets the requirements, the fewer components used, the better. This will reduce the failure rate and ensure that the system operates stably and reliably.
Since the secondary side of a current transformer cannot be open-circuited, the coil of the current relay must be reliably connected to the secondary side of the transformer during use. Furthermore, short-circuit and overload protection devices must not be installed. A resistor-capacitor circuit should also be connected in parallel across the coil (to prevent the current relay coil from burning out); otherwise, high voltage will be generated on the secondary side of the current transformer.
When selecting a model, you can consider the load current and the maximum allowable current value of the existing current relay.
If the load current is 100A and the maximum allowable current of the current relay is 3A, then a 100/5 current transformer cannot be selected (when the primary current is 100A, the secondary current is 5A). Instead, a 200/5 current transformer should be selected. The reason is that when the load current is 100A, the secondary current of the transformer is 2.5A, which is lower than the maximum allowable value of the current relay. Furthermore, the operating current of the current relay can generally be adjusted.
If the current required to operate is greater than the rated current, select an overcurrent relay. If the current required to operate is less than the rated current, select an undercurrent relay. Then select the specific model based on the rated current.
