Design of Intelligent Leakage Circuit Breaker

　　Arc-type short circuit to ground in electrical grounding fault is an important cause of electrical fire. Arc-type short circuit to ground has a large impedance and voltage drop, which limits the fault current, making the overcurrent protector unable to operate or unable to operate in time to cut off the power supply, and the local high temperature generated by the leakage arc of several hundred milliamperes can reach more than 2000℃, which is enough to ignite the surrounding combustibles and cause fire. Moreover, electrical equipment is distributed in every corner of the building, and the scope of harm is wide. If the leakage of the system is not monitored and controlled, it will pose a threat to personal and property safety and there is a great fire hazard. Intelligent leakage circuit breakers can accurately monitor the faults and abnormal conditions of electrical lines, and can effectively prevent common electrical fire accidents in buildings caused by grounding arcs caused by leakage. In order to ensure the safety of people's lives and property, it is necessary to install intelligent leakage circuit breakers at the power supply line and trunk line of the building.

　　1 Causes of residual current and protection principles

　　Let the three-phase four-wire conductor pass through a zero-sequence current transformer CT together, or install a zero-sequence current transformer CT on the neutral line N, and use these CTs to detect the current vector sum of the three phases, that is, the residual current, as shown in Figure 1. According to the circuit principle, when there is no equipment leakage or grounding fault in the circuit and the three-phase load is completely balanced, the vector sum of the instantaneous current in the primary side is zero, that is, Ia+Ib+Ic+IN=0, and the vector sum of the magnetic flux generated in the current transformer is equal to zero. At this time, the induced current IL in the secondary coil is 0. When an insulation fault occurs in the protected circuit, there is a load leakage current to the ground on the load side, and the vector sum of the zero-sequence current transformer is not zero, that is, Ia+Ib+Ic+IN≠0, and the vector sum of the magnetic flux generated in the current transformer is also not equal to zero. At this time, an induced current is generated in the secondary winding of the zero-sequence current transformer, that is, the residual current IL≠0.

　　The leakage circuit breaker is mainly composed of a zero-sequence current transformer CT, a leakage detection circuit, and a release. When there is leakage in the protected circuit or electric shock to the human body, as long as the leakage or electric shock current reaches the leakage action current value, the secondary winding of the zero-sequence current transformer will output a signal, which will be amplified by the integrated circuit amplifier and sent to the CPU. The CPU outputs a driving signal to drive the leakage release to drive the circuit breaker to trip, thereby cutting off the power supply to protect against leakage and electric shock.

　　2 Design of circuit breaker controller

　　2.1 Basic functions of the system

　　Intelligent leakage circuit breaker integrates the monitoring, analysis, alarm and control of electrical faults such as residual current, short circuit, overload, overvoltage and undervoltage (phase loss), and has the following functions:

　　1) It has residual current detection and protection functions. When leakage is detected, that is, the residual current IL≠0, the signal is sampled and calculated by the single-chip microcomputer for rapid judgment. When the residual current reaches the set action value, the thyristor is driven to turn on the power supply of the electromagnetic release, the electromagnetic release is attracted, and the circuit breaker trips, thereby achieving the function of leakage protection.

　　2) The protection action current and breaking time are adjustable: when used as the total protection of the substation, the residual current action value can be set to 300-1000 mA, and the breaking time can be set to 0.6 s. When used as the secondary protection, the action current can be set to 200 mA, and the breaking time can be set to 0.3 s. Such settings can avoid the occurrence of large-scale power outages caused by over-tripping.

　　3) It can intelligently identify sudden residual current and slow-changing residual current, thereby distinguishing equipment leakage and live electric shock. Separate identification of slow-changing and sudden-changing leakage is suitable for the characteristics of my country's rural low-voltage power grid and has been widely used. It is an effective technical measure for safe electricity use in rural areas.

　　4) It has three protection functions: long overcurrent delay, short overcurrent delay and short circuit instantaneous protection, forming the required intelligent leakage circuit breaker protection characteristics. Intelligently set the setting value and warning value of leakage current, long overcurrent delay, short overcurrent delay and instantaneous overcurrent.

　　5) Display and store the line address, fault type, fault time, leakage current, and three-phase current value of the fault point. Up to 200 historical faults can be recorded and stored for a long time until deleted by command.

　　6) RS485 bus communication technology is adopted, and the bus and the host can be used to form a master-slave monitoring system to realize user networking. 1 to 250 intelligent circuit breakers can be monitored online remotely on a computer, and the safe use of electricity by each user can be checked at any time. The power supply lines of each user can be connected or disconnected at any time, and various parameters of the circuit breakers can be remotely set.

　　2.2 Overall hardware design

　　The intelligent leakage circuit breaker is mainly composed of a power supply circuit, a single-chip microcomputer PIC24FJ64, a three-phase AC voltage and current detection circuit, a residual current detection circuit, a serial communication interface circuit, a human-machine interface circuit and an alarm, as shown in Figure 2.

　　Its main working principle: after conditioning the three-phase current, leakage and voltage signals obtained from the current transformer and linear optical isolator, they are input into the A/D conversion of the single-chip microcomputer, which samples and analyzes them, and outputs corresponding display and alarm signals, etc. The analysis results can also be transmitted to the host computer through the RS485 bus. [page]

　　2.2.1 Single-chip microcomputer circuit

　　The single-chip microcomputer uses PIC24FJ64, which is a high-performance CPU with improved Harvard architecture designed by Microchip. It is the core of the intelligent circuit breaker. It completes various control functions of the intelligent circuit breaker, including sampling of three-phase voltage, three-phase current and leakage current, data processing, alarm output, communication with the host computer, LCD display and buttons, etc. Microchip develops, manufactures and produces single-chip microcomputers with the following advantages: 1) Harvard bus structure; 2) Reduced Instruction Set (RISC) technology; 3) Simple addressing mode; 4) High code compression rate; 5) High running speed; 6) Extremely low power consumption; 7) PIC16F877 chip has A/D, MSSP, USART serial bus ports, etc., and has the characteristics of simple external circuit, convenient development, C language programming, strong program confidentiality, etc.

　　2.2.2 Residual current detection circuit

　　The residual current detection circuit is a zero-sequence current transformer. The protected phase line and neutral line pass through the annular core to form the primary coil of the transformer, and the winding wound on the annular core forms the secondary coil of the transformer. If there is no leakage, the vector sum of the current flowing through the phase line and the neutral line is equal to zero, so the corresponding induced electromotive force cannot be generated on the secondary coil. If leakage occurs, the vector sum of the current of the phase line and the neutral line is not equal to zero, which will generate an induced electromotive force on the secondary coil, and this signal will be sent to the intermediate link for further processing, as shown in Figure 3.

　　After the AC signal is processed by the absolute value amplifier circuit, it is full-wave rectified and the processed signal is sent to the single-chip microcomputer. The single-chip microcomputer samples 36 points in each cycle, and the effective value of the residual current can be calculated according to formula (1), where X is the sampled value.

　　2.2.3 Three-phase voltage, current and phase sequence detection

　　The current detection is composed of two-phase and three-phase AC transformers, operational amplifiers and rectifier-filter circuits. The three-phase AC transformer converts the current into a voltage signal, which is conditioned by the circuit composed of the operational amplifier and then rectified and filtered before being input into the A/D converter of the microcontroller for conversion.

　　The traditional voltage detection method is to use a voltage transformer or a linear optical isolator. The disadvantage of using a voltage transformer for voltage detection is that the transformer is relatively large in size. In many cases, the designed product requires the controller to be small in size so that it is easy to install and use. The disadvantage of using a linear optical isolator is that the voltage detection accuracy is not high. This system uses a current transformer and a resistor in series to detect voltage, which greatly reduces the size of the controller and ensures high accuracy of voltage detection. Its schematic diagram is shown in Figure 4.

　　The current transformer uses the 1:1 current transformer TV16 produced by Yaohua Electronics. Since the primary and secondary sides of the current transformer have equal transformation ratios, the secondary voltage is equal to the primary voltage. By selecting a suitable resistor R1, the peak value of the secondary output voltage of the current transformer does not exceed the maximum allowable sampling voltage. The secondary voltage of the transformer becomes a single-phase full wave after passing through the rectifier bridge, and the A/D converter of the single-chip microcomputer can sample and analyze the full wave.

　　The power phase sequence detection adopts the peak detection method. The phase difference of the three-phase voltages A, B, and C is 120°. The detection method is to start timing when the maximum value of phase A is detected, and stop timing when the maximum value of phase B is detected. The time interval between the peak values ​​of phases A and B can be obtained, set as △t, and the phase difference φ between phases A and B can be calculated based on △t. The calculation formula is:

　　If the calculated phase difference is 110°≤φ≤130°, the phase sequence can be considered normal. If it exceeds this range, it is determined to be a phase sequence error.

　　2.2.4 RS485 bus hardware circuit

　　The intelligent leakage circuit breaker communicates with the host computer using RS485 bus. One host can control up to 250 circuit breakers. The RS485 communication system adopts a master-slave structure. The slave does not actively send commands or data. Everything is controlled by the host. Therefore, in a communication system, only one host computer is used as the host. Other slaves cannot communicate with each other. Even if there is information exchange, it must be forwarded through the host. The hardware circuit for communication with the host computer is shown in Figure 5.

　　The communication between the intelligent circuit breaker and the host computer adopts the Modbus communication protocol, which is one of the mainstream communication protocols currently used by international intelligent instruments. The master-slave communication method is adopted between the two. When the host computer sends a communication command to the circuit breaker, the slave that meets the corresponding address code receives the communication command and reads the information according to the function code and related requirements. If the CRC check is correct, the corresponding task is executed, and then the execution result is returned to the host.

　　3. Software Design of Intelligent Circuit Breaker

　　The software completes the functions of the entire circuit breaker and adopts a modular structured C language programming scheme. The main programs include:

　　1) System main program. It mainly completes the initialization of the system's ports, timers, A/D converters and other modules, and also completes the LCD interface display.

　　2) The timer interrupt service subroutine and A/D conversion subroutine mainly complete the A/D conversion task and key processing function. The microcontroller needs to sample 36 times in one cycle (20 ms) and save the sampled data.

　　3) Data processing subroutine, mainly completes the calculation of leakage current, judgment of leakage current, processing of tripping or not, etc.

　　4) The key processing subroutine mainly provides a human-computer dialogue channel. The user can set the setting value of the leakage protection, delayed tripping time, etc. through keys. The modification of its parameters is password protected.

　　The main software system block diagram is shown in Figure 6.

　　4 Conclusion

　　Intelligent leakage circuit breaker IRCCB is a low-voltage electrical appliance widely used in distribution networks. It is mainly used to prevent electric shock and equipment leakage failures. The correctness of its operation directly affects the safety and reliability of power supply. The intelligent design of intelligent leakage circuit breaker using PIC microcontroller has reliable quality and strong anti-interference ability. It can also realize the systematization and networking of circuit breaker control through the application of bus communication technology.