Introduction to intelligent circuit breakers and analysis of their working principles

1 Introduction

Low voltage circuit breakers As the main control electrical appliances widely used in power supply and distribution systems, in addition to being able to normally disconnect and connect the rated current of the relevant system, they must also be able to quickly and selectively and reliably disconnect the short-circuit fault current of the relevant system when the relevant system fails, and there must be no over-tripping or refusal to operate.

In particular, with the development and application of the digitalization process of power system control and the widespread application of power system integrated automation, the requirements for system visualization, automation, networking, real-time and precision are getting higher and higher. Correspondingly, higher requirements are put forward for low-voltage circuit breakers with wide application area, complex network structure, frequent operation and high failure rate. Traditional circuit breakers cannot meet the needs of modern power system integrated automation at all. The application of intelligent technology has become an important development and application direction of low-voltage circuit breakers.

2 Introduction to Intelligent Circuit Breakers

Intelligent circuit breakers integrate modern high-voltage zero arc technology, electronic technology, electrical automation technology, network communication technology, computer and software technology, etc., adopt modular structure, completely break through many shortcomings of traditional circuit breakers, and integrate protection, measurement and monitoring. In addition to conventional control, protection, alarm and setting functions such as overload, overcurrent, quick break, leakage and grounding, it also has functions such as human-computer dialogue display, storage, memory, logic analysis, judgment and selection, and network communication. It can display various characteristic parameters such as temperature, current, voltage, power factor, active power, reactive power, etc. in real time, and store fault parameters and types. It has self-diagnosis ability, so as to provide actual operation data on site for operation and maintenance personnel to conduct corresponding information query and fault judgment and processing, laying the foundation for the optimization of system operation mode. Through its network communication technology, multiple intelligent circuit breakers can realize two-way communication with the central control computer, constitute an intelligent power supply and distribution system, realize the "four remote" function, and create necessary equipment and technical guarantees for unmanned stations and regional interlocking, remote monitoring, and operator adjustment.

3 Composition and working principle of intelligent circuit breaker technology

The core of intelligent technology in intelligent circuit breakers is a multifunctional trip unit that integrates protection, measurement and monitoring. It is mainly composed of a microprocessor unit, a signal detection and acquisition unit, a switch input unit, a display and keyboard unit, an execution output unit, a communication interface, a power supply, etc. As shown in Figure 1.

The working principle of each part is as follows:

3.1 Microprocessor Unit

The single-chip microcomputer has become the first choice for intelligent circuit breaker intelligent control system with its high cost performance and reliability. The microprocessor unit is composed of a single-chip microcomputer and its peripheral electronic circuits with high-performance built-in A/D conversion, watchdog monitor, I2C serial bus and high-speed input and output channels, communication interface and standard JTAG program burning port. With optimized software, the single-chip microcomputer control system requires fewer peripheral components, making the design simple and wiring convenient, and greatly improving the stability and anti-interference ability.

Each AC quantity is sent to the multi-way switch controlled by the CPU through the signal input circuit and low-pass filter. After analog-to-digital conversion, it is sent to the data storage (RAM) through the DB data bus. The CPU calculates the collected data by calling the program in the program storage (EPROM), and compares the calculation result with the set value stored in the electrically erasable memory (E2PROM) to make corresponding fault judgment and processing. Then the processed signal is sent to the corresponding peripheral (signal and output) through the input and output port (I/O) to issue an alarm signal or execute tripping. In addition to completing the measurement, protection, logic and other functions of the entire system, the microprocessor unit also has the ability of self-fault diagnosis and monitoring. When the circuit breaker itself fails or the ambient temperature exceeds the allowable range, it can issue a corresponding signal display or alarm, and restart at the same time. The self-diagnosis items mainly include EPROM error, A/D conversion error, environmental overtemperature, CT disconnection, trip coil disconnection, circuit breaker refusal to move and contact maintenance. The self-diagnosis function of the microprocessor unit not only greatly improves the operating reliability of the circuit breaker, but also provides great convenience for subsequent maintenance and fault diagnosis.

3.2 Signal Detection and Acquisition Unit

As a very important component of intelligent circuit breakers, the signal detection and acquisition unit requires high conversion accuracy, sensitivity, reliability, frequency response, measurement range and anti-interference ability, so that the microprocessor unit can make accurate judgments and processing. Therefore, the signal detection and acquisition unit takes the protection signal and the measurement signal from different types of current transformers to meet the requirements of protection and measurement. When measuring large currents (short-circuit currents), hollow current transformers with good linearity and high accuracy are basically used to detect protection signals; iron core transformers are used to detect small currents and electrical parameters; and voltage signals for measurement and protection are obtained by voltage transformers. After the above signals pass through the signal processing circuit, the voltage and current signals in the main circuit can be linearly converted into level signals that can be processed by digital circuits and single-chip microcomputers, and after analysis and judgment by the single-chip microcomputer, signals are sent or the action of the circuit breaker is controlled.

According to the sampling values ​​of current and voltage, different algorithms can be used to calculate the corresponding voltage, current effective value, active power, reactive power, frequency, power factor, electric energy and other parameters to realize various meter functions, which is of great benefit to reducing costs, simplifying wiring and improving the reliability of low-voltage power distribution.

3.3 Switching input unit

In order to meet the needs of control logic, the high-speed input channel (HSI) and high-speed output channel (HSO) inside the single-chip microcomputer can be easily used for the input and output of switch quantities, which is convenient for the monitoring and operation control of the host computer. The input of the switch quantity is mainly the auxiliary contact status of the circuit breaker, which can be easily input into the CPU through photoelectric isolation to determine the status of the circuit breaker.

3.4 Display and keyboard unit

In intelligent equipment, it is necessary not only to realize automatic control, but also to be able to transmit relevant information to the operating personnel, and to be able to accept external input and respond. A good user interface is indispensable for human-computer dialogue. Flexible keyboard management and intuitive information display provide great convenience for users. Through the LCD screen or light-emitting tube, the parameter values ​​of various states and loads, fault current, fault type and protection action, and test setting conditions can be displayed in a timely manner. In combination with buttons, protection setting, warning value setting, switch testing and various function detection can also be performed. 3.5 Execution output unit

The actuator of the intelligent circuit breaker is a flux converter with a permanent magnet, which is characterized by small size, low power consumption and large tripping force. During normal operation, the permanent magnet keeps the moving and static iron cores attracted. The overload, short circuit, grounding and other fault signals from the transformer are processed by the microprocessor unit according to the pre-designed protection characteristics, and a trip pulse (negative square wave pulse) of a certain width is sent to the coil of the flux converter to generate a reverse magnetic field to offset the permanent magnetic flux. The mechanical energy released by the moving iron core pushes the circuit breaker's trip device to disconnect the circuit breaker. Its tripping execution circuit is shown in Figure 2.

3.6 Communication Interface

The serial communication interface of the intelligent circuit breaker can connect the intelligent circuit breaker to the field bus, forming a local area network of the power distribution system, with a computer as the master station and several intelligent circuit breakers with communication interfaces as slave stations. The circuit breaker number, opening and closing status, various set values, operating current, voltage, fault current, action time and fault status and other parameters are transmitted over the network to achieve information exchange with the system upper management computer or the control and dispatch center computer, receive data upload requirements from the upper computer or the remote dispatch control center, and complete the remote monitoring and telemetry, telecommunication, teleadjustment and remote control functions of the field equipment.

3.7 Power Supply

The intelligent circuit breaker adopts dual power supply mode. As long as any one of the power supplies works normally, it can reliably supply power to the multifunctional release. One power supply is self-generated power supply, which uses fast saturated iron core current transformer to obtain power from the main circuit. The other power supply is auxiliary power supply, which is provided by the outside in an "OR" way. It not only provides power when the main power supply cannot work, but also can perform parameter setting, function inspection, test, status display, communication on the controller before the main circuit is powered off or the circuit breaker is put into operation, and can maintain various status indications and fault inspections after the circuit breaker is disconnected due to fault, so as to maintain its normal operation and various displays.

4. Software Design of Intelligent Release

The software design is mainly divided into two parts, the main program and the interrupt program. The main program includes fault processing, keyboard processing, display processing, communication processing and other subroutines; the interrupt program includes timer interrupt, keyboard interrupt, communication interrupt, etc.

The single-chip microcomputer samples the power frequency current signal and uses an improved algorithm based on wavelet analysis and FFT to calculate the effective value of the current, which can improve the sampling accuracy and meet the system's high-precision requirements for delayed protection. After the wavelet algorithm detects a suspicious signal point during the sampling process, the FFT algorithm performs an effective value judgment. If it does not exceed the threshold value, the suspicious signal point is invalid, and the wavelet algorithm is returned to continue looking for the sampling suspicious point; if the effective value exceeds the threshold value, the suspicious point is considered valid, and the corresponding signal is output according to each protection condition.

The real-time control of the multifunctional release adopts the timer interrupt mode, judging the instantaneous fault as the highest priority interrupt; judging the short delay, long delay and ground fault as the second priority interrupt, and the key operation as the low-level interrupt. Each phase current is sampled in turn, and compared with the data saved in the previous time, and the larger data is saved. Then the maximum phase current is calculated, compared with the instantaneous set current value, and it is judged whether it is an instantaneous fault. When the key is closed, an interrupt request is generated, I/0 responds to the interrupt, and the interrupt service program is executed to scan the key, identify the key number of the closed key on the keyboard and make corresponding processing. At this time, it is also necessary to eliminate the misoperation caused by key jitter, and avoid the error of multiple processing of one closure of the same key. The double-key blocking keyboard working mode is used to eliminate the above misoperation. If two keys are pressed during the de-jitter cycle, they are considered to be pressed at the same time, and only the last released key is closed. The key number calculated by the root executes the corresponding program to realize various operation protection functions of the release.

5 Anti-interference measures

Since the circuit breaker works in a relatively harsh electromagnetic environment, strong interference can cause the system monitoring program to lose control, deviate from the normal execution sequence, and even send out wrong control signals, causing the circuit breaker to malfunction. Therefore, the anti-interference performance of the release largely determines whether the circuit breaker can work normally and the accuracy and reliability of its action. In order to ensure the reliability of the system, intelligent circuit breakers often use anti-interference technology that combines software and hardware. In the hardware design, power supply filtering technology, shielding technology, isolation technology, grounding technology, reasonable wiring, SMD packaging, moisture-proof and insulation, hardware watchdog and other measures are adopted; in the software design, digital filtering technology, software traps, empty instructions, symmetric detection method, limit detection method, wavelet analysis and FFT improved algorithm, software watchdog and other technologies are adopted, thereby ensuring the reliable and accurate opening and closing of the intelligent circuit breaker, avoiding the occurrence of failure to open or malfunction caused by system failure.

6 Protection Settings

In order to meet the actual needs of the site, the intelligent circuit breaker is equipped with four protection functions: long delay for overload, short delay for short circuit, instantaneous short circuit and grounding. Among them, instantaneous protection is a special protection mode. In order to improve reliability, it does not need to compare the effective value, but adopts the method of sampling and comparing immediately. Once it is found that several sampling points exceed the specified threshold continuously, the system will immediately send a trip signal to trip the circuit breaker to prevent excessive short-circuit current from damaging the system equipment. The protection setting value of the intelligent circuit breaker is very convenient to set. It can be set on site according to the user's requirements. After the setting value is set, the setting curve can also be displayed on the display device, so that a single action characteristic can achieve multiple action characteristics for one protection function, and the protection function is more complete and reliable. The action current and action time values ​​of each protection function can be pre-set by the keyboard and encoder, and these values ​​are transmitted to the internal memory of the microprocessor unit for storage. When various faults occur in the main circuit, such as when the fault current or voltage reaches or exceeds the set value, the CPU will send an action signal to the drive circuit after calculation, judgment and comparison, causing the circuit breaker to trip and cut off the fault circuit, thereby protecting the normal operation of the system.

7 Conclusion

With the continuous development and application of high-performance, low-cost chips, large-scale programmable logic devices and software technology, intelligent circuit breakers will have great improvements in protection diversity, judgment accuracy and anti-interference, self-diagnosis ability, real-time communication and display. In particular, the application of large-scale programmable logic devices and high-speed digital signal processors and the successful development of new voltage and current converters or special chips for intelligent circuit breakers will make a qualitative leap in the application of intelligent circuit breakers, and take the comprehensive automation of low-voltage electrical systems with a wide application area and complex network structure to a higher level.