Studyofoverload Current Influence On Residual Current Protection Characteristics

As the important application of residual current protection technology, residual current devices are widely used in low voltage electric distribution network to prevent electric accidents occurring. Whether residual current devices can work reliably has been paid much attention. According to related international standards, under no condition can residual current devices operate falsely. In the condition of overload, great load current often leads to false operation. Among various factors, false residual current is the one of main factors those lead to false operation.This paper defines the false residual current definitely. On the basis of analyzing characteristics of residual current transformer output signal, small signal amplifying circuit was designed. The circuit can avoid EMI and noise disturbance effectively.The relationship between false residual current and the angle formed by current transformer and load conductors is studied. The causes of false residual current is analyzed theoretically and the coiling structure of transformer causes the unbalance of magnet, which leads to the occurrence of false residual current. The air gap between strips boosts the unbalance and this is called shielding effect. The coiling structure of current transformer is modeling and the relationship between false residual current and the angle is analyzed through the method of finite elements with the assistance of computer. Then the value of false residual current is measured under different angles in the lab. The simulation and test is in accordance with theory. The coiling structure causes the false residual current and the angle betweencurrent transformer and load conductors has direct relationship with the value of false residual current.The relationship between false residual current and the second winding’s uniformity of current transformer is studied theoretically and practically. The test results are in accordance with theory. The uniformity of the second winding has great influence on the value of false residual current.In the end, it is proposed to decrease the unexpected effect of false residual current through using the magnet shield and percentage restraint protection. The effective of magnet shield is verified by tests.