1. Leakage protection circuit breaker is a kind of low-voltage electrical equipment that must be installed in the power distribution system. According to different performance parameters and application occasions, it is generally divided into three types: universal circuit breaker, molded case circuit breaker and terminal circuit breaker. With the recent development of new energy vehicles, especially electric vehicles, as a strategic need for national development and social construction, major research institutes, universities and production companies are carrying out related research and development. Due to the harsh working environment of the charging pile, the process requirements and performance parameters of the corresponding leakage protection circuit breaker are naturally higher than the current leakage protection circuit breakers in the market. For the nature of the leakage current, the corresponding circuit breaker can be required. Detect smooth DC leakage signals and perform tripping actions. However, the traditional leakage protector is based on the zero-sequence current transformer. Based on the change of the alternating current magnetic field, the magnetic field change caused by the leakage current induces a voltage on the secondary of the transformer. The leakage detection chip (such as the commonly used 54123) Leakage detection chip) The non-inverting and inverting input terminals of the operational amplifier. If the leakage current signal reaches the preset operation threshold, the leakage detection chip issues an alarm command or cuts off the power distribution circuit to protect personal safety and normal operation of the device. When the DC power system or the leakage current signal is smooth DC, the traditional typical circuit is not applicable. If you want to detect the leakage current and then operate the protection, you need a DC leakage protector that is different from the traditional leakage protection principle. With the rise of new energy electric vehicles and the development of charging pile supporting technology, the development of circuit breakers capable of detecting DC leakage signals has been increasingly valued by related companies.
2. Basic design ideas for DC leakage protection circuit breaker
The conventional AC type leakage protection circuit breaker can detect the leakage signal because the leakage signal is an alternating current signal, so the secondary winding voltage can be induced by the magnetic field. For the DC leakage protection circuit breaker, the basic idea is to artificially superimpose an AC signal on the sensor, and then learn the leakage current detection method of the AC type leakage protection circuit breaker, and finally detect the DC leakage signal, and design the hardware circuit based on this. Since the leakage current sampling method of the DC leakage protection circuit breaker is different from the AC type, and there is no corresponding professional leakage detection chip on the market, the DC leakage protection circuit breaker designed in this paper cancels the design of the test button in appearance, and increases the indication. The lamp is used as a visual indication of normal operation of the product (green light is always on) or fault state (red light flashing), and the original test button position is changed to the dial switch, and the corresponding positions correspond to only the alarm does not trip, alarm and trip. The implementation method is convenient for the user to dynamically select according to the actual use situation, instead of changing the model once the model is selected, the basic alarm mode is red light flashing. Therefore, the DC leakage protection circuit breaker designed in this paper uses the single chip as the control core. A typical functional framework is shown. The single-chip microcomputer processes according to a specific logic algorithm to realize the real-time running state display and the system self-diagnosis function of the DC leakage protection circuit breaker, wherein the DC leakage current sensor is composed of a power source, a transformer, a square wave excitation source, a low-pass filter, and an absolute value circuit. And the amplification circuit is composed.
3. DC leakage detection method
The DC leakage current sampling circuit is shown in the figure. The design idea is to induce the voltage on the sampling resistor Rs through the magnetic ring with the excitation square wave voltage as the carrier. Obviously, the voltage value is inextricably linked with the excitation square wave voltage and Rs resistance. . When there is a DC leakage current in the magnetic ring, the DC current generates a constant magnetic field, which is superimposed with the inherent magnetic property of the magnetic ring. The voltage reflected on the sampling resistor Rs is a fixed offset with respect to the Y axis, and the offset is The direction depends on the direction of the DC leakage current. Based on the schematic diagram of Fig. 2, the mathematical derivation and the Fourier expansion of the square wave excitation voltage contain only odd harmonic components. When there is DC leakage current I0 in the magnetic ring, the winding current I_leak obtained on the sampling resistor only contains DC. The component and the odd harmonic component, and the DC component is proportional to the DC leakage current I0. The derivation of the mathematical formula also confirms the feasibility of the design from the side. To obtain the DC leakage current signal, the high-frequency component needs to be filtered out by a suitable low-pass filter. Since the DC leakage current has positive and negative directions, the focus of the leakage protection circuit breaker is the magnitude of the leakage current rather than the direction. Therefore, it is necessary to pass the appropriate signal conditioning circuit to the AD conversion pin of the single chip microcomputer. The signal conditioning circuit includes a low pass filter circuit, an absolute value circuit, and an amplification circuit.
4. Single-chip trigger thyristor
This design is based on the specific hardware circuit design of the single-chip microcomputer. Different from the traditional AC-type leakage detection chip 54123, the internal differential output terminal 4 pin and the latch 5 pin are connected. When the latch is high level for a certain period of time Pull the 7-pin level high to trigger the thyristor, and the latch level is released after the trigger, so that the distribution line can be closed again after troubleshooting; if the thyristor is directly triggered by the IO port of the microcontroller, The thyristor trigger current and the trigger voltage are considered to meet the requirements, but the thyristor passes a strong current at the turn-on instant. To avoid the impact of strong current on the IO port of the microcontroller, it needs to be controlled in the IO port of the microcontroller. Isolation circuits such as common optocouplers are added between silicon. From the perspective of cost, this design uses a triode, and its circuit schematic is shown in the figure. Select a single-chip microcomputer whose IO port is high by default when power-on. At this time, the S9013 transistor is turned on, the trigger level of the thyristor SCR is almost 0, and the DC leakage protection circuit breaker is in normal closing. State; when the DC leakage current signal is detected, the SCR_ON of the MCU IO port is set to 0 level by the program. At this time, the S9013 transistor is turned off, and the trigger level of the SCR SCR is triggered by almost 12V. At this time, the coil Coil passes. The large current causes the DC leakage circuit breaker to trip and protect it.
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