The main function of RCD (residual current action protection device) in charging piles is to prevent electric shock accidents, electrical equipment leakage damage, and electrical fires, as well as to protect the safety of electric vehicle charging process.
RCD, as a type of leakage protector, is widely used in low-voltage distribution systems. Its basic function is to cut off the power supply by detecting the residual current in the circuit, thereby avoiding the danger caused by leakage. In the field of electric vehicle charging, RCD is widely used as a basic electrical protection device, especially in electric vehicle charging piles, where its role is particularly important.
The core component of RCD is the zero sequence current transformer, which is actually a ring-shaped iron core made of Permalloy. Both the phase and N wires pass through the iron core, and a measuring coil is wound around the iron core. When the current of the phase line and the N-line is equal in magnitude and opposite in direction, the magnetic field lines generated in the iron core cancel each other out, and there is no magnetic flux inside the iron core. Of course, there is no induced current in the coil. The coil will only induce current when the phase line current is not equal in magnitude to the N-line current. If the induced current exceeds the threshold value of the accumulator or electromagnetic coil, the leakage switch will produce corresponding action.
The zero sequence current transformer inside the RCD needs to pass through three phase lines and one N-line. The relationship between the phasor of the three phase line currents and the N-line current is:
That is to say, the phasor of the three phase lines is equal in magnitude and opposite in direction to the current of the N-line.
So when the line is normal, the output value of the zero sequence current transformer, which is also the residual current:
When a leakage occurs, due to one of the three phase lines experiencing leakage, the phasor sum of the three phase lines is not equal to the current of the N-line, resulting in residual current:
RCD senses and determines that leakage has occurred, and will trip the circuit breaker.
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