This paper describes the cause and harm of residual current, designs a set of Intelligent Residual current circuit breaker, and gives the hardware structure and software design. The test results show that the intelligent circuit breaker has reliable quality, stable performance, and fully meets the requirements of national standards.
Arcing short circuit to ground is an important cause of electrical fire in electrical grounding fault. Arc short circuit to ground has great impedance and voltage drop, which limits the fault current, so that the over-current protector can not act or can not act in time to cut off the power supply, and the local high temperature generated by hundreds of Ma leakage arc can reach more than 2000 ℃, which is enough to ignite the surrounding combustible and cause fire. Moreover, electrical equipment is distributed in every corner of the building, with a wide range of hazards. If the leakage of the system is not monitored and prevented, it will pose a threat to personal and property safety, and there are great fire hazards. The intelligent residual current circuit breaker can accurately monitor the fault and abnormal state of electrical circuit, and can effectively prevent the common building electrical fire accidents caused by grounding arc caused by leakage. In order to ensure the safety of people’s life and property, it is necessary to install intelligent residual current circuit breaker on the power line and main line of buildings.
1. Causes of residual current and protection principle
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 CT to detect the sum of three-phase current vectors, i.e. 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 fully balanced, the vector sum of the instantaneous current in the primary side is zero, that is, Ia + IB + IC + in = 0, 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 the insulation fault occurs in the protected circuit, the load side has the load discharge current to the ground, the vector sum of the zero sequence current transformer is not zero, that is, Ia + IB + IC + in ≠ 0, the vector sum of the magnetic flux generated in the current transformer is not equal to zero, at this time, the induced current is generated in the secondary winding of the zero sequence current transformer, that is, the residual current IL ≠ 0.
The residual current circuit breaker is mainly composed of zero sequence current transformer CT, leakage detection circuit and release. When there is leakage or electric shock in the protected circuit, as long as the leakage or electric shock current reaches the leakage action current value, the secondary winding of the zero sequence current transformer outputs a signal, which is amplified by the integrated circuit amplifier and sent to the cup. The CPU outputs the driving signal to make the leakage release act and drive the circuit breaker to release, so as to cut off the power supply and play the role of leakage and electric shock protection.
Design of 2 circuit breaker controller
2.1 basic functions of the system
The intelligent residual current circuit breaker integrates the monitoring, analysis, alarm and control of residual current, short circuit, overload, overvoltage, undervoltage and other electrical faults
1) It has the function of residual current detection and protection. When the residual current is detected, that is, the residual current IL ≠ 0, the signal is quickly judged after sampling and calculation by the single chip microcomputer. When the residual current reaches the set action value, the thyristor is driven to connect the power supply of the electromagnetic release, and the electromagnetic release is pulled in to make the circuit breaker trip, so as to achieve the function of leakage protection.
2) The protection action current and breaking time can be adjusted: when it is used as the total protection of the station area, the action value of residual current can be set to 300 ~ 1000mA, and the breaking time can be set to 0.6s. When it is used as the secondary protection, the action current can be set to 200mA, and the breaking time can be set to 0.3s. This setting can avoid the occurrence of large area power failure caused by override trip.
3) It can intelligently identify abrupt residual current and gradual residual current, so as to identify equipment leakage and live electric shock. It is suitable for the characteristics of rural low-voltage power grid in China and widely used. It is an effective technical measure for the safe use of electricity in rural areas.
4) It has three protection functions: long time delay of over-current, short time delay of over-current and instantaneous protection of short circuit, which constitute the required protection characteristics of Intelligent Residual current circuit breaker. Intelligent setting of leakage current, over-current long delay, over-current short delay and over-current instantaneous setting value and warning value.
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 they are deleted by command.
6) Using RS485 bus communication technology, the master-slave monitoring system can be formed by using bus and host to realize user networking. 1-250 intelligent circuit breakers can be monitored online and remotely on one computer. The safe power consumption of 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 circuit breakers can be set remotely.
2.2 overall hardware design
The intelligent residual current circuit breaker is mainly composed of power supply circuit, MCU pic24fj64, three-phase AC voltage and current detection circuit, residual current detection circuit, serial communication interface circuit, man-machine interface circuit and 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 to the A / D conversion of the single-chip microcomputer, and the single-chip microcomputer analyzes them after sampling, and outputs the corresponding display and alarm signals. The analysis results can also be transmitted to the host computer through RS485 bus.
2.2.1 single chip microcomputer circuit
Pic24fj64 is a high performance CPU with improved Harvard architecture designed by microchip company. It is the core of intelligent circuit breaker. It completes various control functions of intelligent circuit breaker, including sampling of three-phase voltage, three-phase current and leakage current, data processing, alarm output, communication with host computer, LCD display and key functions. Microchip company develops, develops and produces microcontrollers 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, and has the characteristics of simple external circuit, convenient development, C language programming and strong program confidentiality.
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 ring iron core to form the primary coil of the transformer. The winding wound on the ring iron core forms the secondary coil of the transformer. If there is no leakage, the sum of the current vector passing through the phase line and neutral line is equal to zero, so the corresponding induced electromotive force can not be generated on the secondary coil. If leakage occurs, the sum of the current vectors of the phase line and the neutral line is not equal to zero, the induced electromotive force will be generated on the secondary coil, and the 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, the full wave rectifier is obtained, and the processed signal is sent to the MCU. According to equation (1), the effective value of residual current can be calculated, where x is the sampling value.
2.2.3 three phase voltage, current and phase sequence detection
The current detection is composed of two-phase and three-phase AC transformer, operational amplifier and rectifier filter circuit. Among them, the three-phase AC transformer converts the current into voltage signal, which is adjusted by the circuit composed of operational amplifier and then input to the A / D converter of MCU for conversion.
The traditional voltage detection method is to use voltage transformer or linear optical isolator. The disadvantage of using voltage transformer for voltage detection is that the volume of the transformer is too large. Many times, the designed products require the volume of the controller to be small, so it is easy to install and use. The disadvantage of using linear optical isolator is that the voltage detection accuracy is not high. This system uses the method of current transformer and resistance in series to detect the voltage, which not only greatly reduces the volume of the controller, but also ensures the high precision of voltage detection. The schematic diagram is shown in Figure 4.
The 1:1 current transformer TV16 produced by Yaohua electronics is used as the current transformer. Because the primary side and secondary side of the current transformer have the same transformation ratio, the secondary side voltage is equal to the primary side voltage. By selecting the appropriate resistance R1, the peak value of the secondary output voltage of the current transformer does not exceed the maximum allowable sampling voltage. The secondary output voltage of the current transformer becomes a single-phase full wave after passing through the rectifier bridge. The A / D converter of the single-chip microcomputer can sample and analyze the full wave.
The peak detection method is used to detect the phase sequence of power supply, and the phase difference of a, B and C three-phase voltage 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 phase A and phase B can be obtained, which is set as △ T. according to △ T, the phase difference of phase a and phase B can be calculated
If the calculated phase difference is 110 °≤ φ≤ 130 °, the phase sequence can be considered as normal. If it is beyond this range, it is judged as phase sequence error.
2.2.4 RS485 bus hardware circuit
The RS485 bus is used to communicate between the intelligent residual current circuit breaker and the host computer. A host computer can control up to 250 circuit breakers. The RS485 communication system adopts the master-slave structure. The slave computer does not actively send commands or data, and everything is controlled by the host computer. Therefore, in a communication system, only one upper computer is used as the host, and other slave computers cannot communicate with each other. Even if there is information exchange, it must be transmitted through the host. The hardware circuit of communication with upper computer is shown in Figure 5.
Modbus communication protocol is used between the intelligent circuit breaker and the host computer. Modbus communication protocol is one of the mainstream communication protocols widely used in the international intelligent instruments. The master-slave communication mode is adopted between the two. When the upper computer sends the communication command to the circuit breaker, the slave computer meeting the corresponding address code receives the communication command and reads the information according to the function code and relevant requirements. If the CRC check is correct, the corresponding task is executed, and then the execution result is sent back to the host.
3 software design of intelligent circuit breaker
The software completes the function of the whole circuit breaker, and adopts modularized and structured C language program design scheme. It mainly includes the following procedures:
1) System main program. It mainly completes the initialization of the port, timer, a / D converter and other modules of the system, and completes the LCD interface display at the same time.
2) The timing interrupt service subroutine and a / D conversion subroutine mainly complete the A / D conversion task and key processing function. The MCU needs to sample 36 times in a cycle (20ms) and save the sample.
3) Data processing subroutine, mainly to complete the calculation of leakage current, leakage current judgment, trip or not processing.
4) The key processing subroutine mainly provides a man-machine dialogue channel. The user can set the setting value of leakage protection, delay trip time, etc. through the key, and the modification of its parameters is password protected.
The main software system block diagram is shown in Figure 6.
Editor’s comments:Intelligent residual current circuit breaker (irccb) is a kind of low-voltage electrical apparatus widely used in distribution network. It is mainly used to prevent personal electric shock and equipment leakage fault. The correctness of its work directly affects the safety and reliability of power supply PIC single chip microcomputer is used for intelligent design of Intelligent Residual current circuit breaker, which is reliable in quality and strong in anti-interference, and can realize the systematization and networking of circuit breaker control through the application of bus communication technology.
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