Electromagnetic leakage protection working principle

   The electromagnetic leakage protector is generally composed of five parts: residual current transformer, leakage detection circuit (signal conditioning circuit and trip drive circuit), magnetic release device, test button and circuit breaker action mechanism, as shown in Figure 1.

Fig.1 Schematic diagram of electromagnetic leakage circuit breaker

   When the power line runs normally, the sum of current phasors of live line L and neutral line N is zero, and the residual current transformer has no inductive electrical signal output on the secondary side, and the leakage protector does not operate. When there is a leakage fault in the power line, the phasor sum of the L-phase current and N-phase current is no longer zero, the unbalanced current I1 (leakage current) exists on the primary side of the residual current transformer, and the output voltage on the secondary side increases with the increase of leakage current. The output signal is output to the trip drive circuit through the signal conditioning circuit. When the value exceeds the trip drive circuit action threshold, the trip drive circuit will send out a trip signal to drive the magnetic trip device to trip, and the leakage protector will disconnect the line to achieve leakage protection.

The principle of electromagnetic leakage detection circuit is shown in Figure 2, and the signal conditioning circuit includes a compensation circuit and an energy storage circuit. The compensation circuit is composed of the compensation resistor R1 and the compensation capacitor C1, the energy storage circuit is composed of the rectifier bridge and the energy storage capacitor C2, and the threshold voltage of the voltage detection chip in the trip drive circuit is Uact.

Fig.2 Schematic diagram of leakage detection circuit

   In order to facilitate analysis and calculation, the working state of the leakage detection circuit can be divided into three types according to the voltage at both ends of the compensation capacitor C1 and the energy storage capacitor C2, as shown in Figure 3. In FIG. 3, width=9 and height=15 represent the equivalent current converted from the primary current to the secondary current. i2 is the secondary current; width=10,height=15 are the equivalent current converted from the excitation current to the secondary side; L0 is the equivalent inductance converted from the excitation inductance to the secondary side; Lr is the inductance of the magnetic trip coil.

Fig.3 Simplified circuit of leakage detection circuit

   When the compensating capacitor voltage UC1 is less than the storage capacitor voltage UC2, the rectifier bridge is not switched on, the output load of the residual current transformer is R1 and C1, and the circuit works in state Ⅰ. When UC1=UC2, the rectifier bridge is on, the output load of the residual current transformer is R1, C1 and C2, and the circuit works in state II. When UC2=Uact, the output voltage of the voltage detection chip makes the transistor Q2 on, and the circuit works in state III.

If the leakage current I1 is less than the leakage action value IDd, the rectifier bridge is not on during steady state operation, the maximum charging voltage of the energy storage capacitor UC2max is the maximum voltage on the compensating capacitor C1, and UC2max < Uact, the trip drive circuit is not on, and the magnetic trip device is not on. If the leakage current I1= IDd, UC2max can exactly reach Uact. Therefore, the influencing factors of leakage action value IDd can be analyzed according to Figure 3a.

If the leakage current I1 > IDd, the theoretical steady state value of UC2 will be greater than Uact, but in practice, once the value reaches Uact, the voltage detection chip will send a signal to drive the transistor Q2 on, the energy storage capacitor C2 will discharge along the path shown in ir in Figure 3c, and the magnetic trip breaker will drive the leakage circuit breaker to disconnect the line to achieve leakage protection. Therefore, the influencing factors of leakage action time can be analyzed according to Figure 3b.

 

Although the leakage detection circuit is simple, its characteristic factors are complicated. Due to the limitation of leakage action value IDd, the stability of electromagnetic leakage protection cannot be improved simply by improving the performance of residual current transformer. Therefore, this paper tries to reveal the reasons for the low pass rate of electromagnetic leakage protector, through the matching design of detection circuit parameters, improve the stability of leakage protection characteristics, solve the problem in mass production, and improve the pass rate.

 

The action of the magnetic trip device in the electromagnetic leakage protector requires a certain drive energy, so the energy storage capacitor C2 and the Uact of the voltage detection chip in the leakage detection circuit must be greater than a certain value. Based on this, this paper assumes that the energy storage capacitor C2 and the voltage detection chip Uact are to determine the value and analyze the factors affecting the leakage protection characteristics. The following problems need to be solved:

(1) The matching relationship between the compensation resistor R1 and the compensation capacitor C1 and their influence on the leakage protection characteristics.

(2) Influence of residual current transformer performance on leakage protection characteristics.

(3) electromagnetic leakage protection characteristic consistency and robust design.

 

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