Abstract: This paper analyzes the current status of molded case circuit breaker (MCCB) technology in China, focusing on the hardware modules (including current input, power supply, signal conditioning, microcontroller system, and operating mode) and software design (including algorithm analysis, data sampling, frequency measurement, and program structure) of an intelligent trip controller for MCCBs based on the TI MSP430F167 microprocessor. Finally, it provides a technological and market outlook for China's low-voltage electrical appliance industry, particularly for intelligent MCCBs. Keywords: Low-voltage electrical appliances, molded case circuit breaker, intelligent controller 0 Introduction With the introduction of computer and communication technologies into low-voltage electrical appliances, the concept of intelligent electrical appliances is increasingly being discussed. Intelligent electrical appliances are those with functions such as automatic fault detection, automatic measurement, automatic control, automatic adjustment, and communication with a remote control center. Current domestic research on the intelligentization of low-voltage electrical appliances mainly focuses on the following aspects: online monitoring of equipment, research on new methods of signal acquisition and processing, research on the electrical appliance itself, research on the reliability of intelligent electrical appliances and the anti-interference capability of the control part, and communication implementation methods. The molded case circuit breaker is a very important low-voltage electrical appliance. Currently, molded case circuit breakers (MCCBs) produced in my country are broadly classified into three generations. The first generation currently holds approximately 35% of the market share; the second generation, comprised of replacement products and those manufactured using imported technology, currently holds about 40% of the market, but its share has declined significantly with the entry of large foreign companies; the third generation, developed independently to keep pace with new foreign technologies, currently holds less than 25% of the market. Western countries introduced intelligent, communicative fourth-generation products in the late 1990s, which possess many advantages of previous generations, demonstrating the development trend of MCCBs. In general, the technological content of Chinese MCCB products is relatively low, and their intelligentization has received increasing attention from the industry. This paper, based on the current status of MCCBs both domestically and internationally, proposes a specific method for implementing intelligentization in the MB30 series MCCB. 1. Molded Case Circuit Breakers Molded case circuit breakers (MCCBs) are widely used in low-voltage power distribution systems. They are generally used for control and protection of distribution feeders, main switches for the low-voltage side of small distribution transformers, control and protection of power distribution terminals, and control and protection of residential distribution terminals. They can also be used as power switches for various production machinery. Small-capacity MCCBs below 50A use non-energy-storage closing mechanisms and are manually operated; large-capacity circuit breakers use energy-storage closing mechanisms and can be operated manually or by a motor, which allows for remote control operation. The rated current of MCCBs is generally 6–630A, and they are available in single-pole, two-pole, three-pole, and four-pole configurations. Currently, large MCCBs with rated currents of 800–3000A are available. The protection functions of MCCBs are achieved by various trip units such as undervoltage release, overcurrent release, and shunt trip. Undervoltage releases monitor fluctuations in the operating voltage. When the mains voltage drops to 70%–35% of the rated voltage or a grid fault occurs, the circuit breaker can immediately trip. When the power supply voltage is below 35% of the rated voltage, it prevents the circuit breaker from closing. Undervoltage releases with time-delay action prevent inappropriate tripping caused by voltage fluctuations due to sudden load increases. Shunt releases are used for remote control or thermal relay tripping of circuit breakers. Overcurrent releases can be further divided into overload releases and short-circuit releases, used to prevent overload and short circuits on the load side. General circuit breakers also have a short-circuit locking function to prevent reclosing after tripping due to a short circuit fault but before the fault is cleared. Circuit breakers have auxiliary contacts, generally normally open and normally closed, which are used by signaling devices and intelligent control devices. Traditional molded case circuit breakers use a thermal tripping control method, utilizing the thermal effect of the load current to cause the bimetallic strip to bend and deform, controlling the tripping. Because the shape, structure, and thermal deformation precision of bimetallic strips are difficult to guarantee, the tripping delay time of circuit breakers is difficult to control precisely, resulting in low interruption accuracy. Furthermore, it suffers from functional deficiencies, such as the inability to diagnose phase loss faults, the inability of users to identify the cause of circuit breaker failures, and inconvenient maintenance. Using an intelligent trip controller to control circuit breaker tripping completely eliminates the shortcomings of traditional trip devices and lays the foundation for technological innovation in molded case circuit breakers. This makes molded case circuit breakers easier to use in low-voltage distribution and power information network systems, developing them into intelligent, networked, and information-based electrical appliances supported by fieldbus. 2 Intelligent Controller for Molded Case Circuit Breakers 2.1 Hardware Module Design The hardware of the intelligent controller for molded case circuit breakers includes current input, power supply module, preprocessing, CPU system, potentiometer adjustment, communication interface, human-machine interface plug-in, output driver, indicator lights, button operation, maintenance interface, etc., as shown in Figure 1. Figure 1 System Hardware Block Diagram 2.1.1 Current Input: Three current transformers convert the three-phase current into an AC current signal matching the intelligent controller. This signal is then converted into an AC voltage signal by a voltage sampling circuit. 2.1.2 Power Module: A molded case circuit breaker introduces the three-phase AC voltage. The intelligent controller obtains energy from the input AC voltage, processes it to obtain two independent +5V power supplies, which are used on both sides of the optocoupler to achieve electrical isolation. Simultaneously, a +12V power supply is also obtained to drive the output relay. 2.1.3 Signal Conditioning: After voltage sampling, the current signal passes through an emitter follower, a first-order passive anti-aliasing low-pass filter, and an amplification and conditioning circuit to become an AC voltage signal compatible with the microcontroller system. An emitter follower isolates the voltage sampling circuit from the post-processing circuit, reducing the impact of the post-processing circuit impedance on the voltage sampling circuit. The cutoff frequency of the first-order passive anti-aliasing low-pass filter satisfies Shannon's sampling theorem, filtering out signals with frequencies above 2fmax. Current transformers exhibit good linearity over a wide range; the primary current varies from a few amperes to tens, hundreds, or even thousands of amperes; small signals are easily affected by interference, so the acquired signal is input to the microcontroller system in two paths, with different amplification factors for the two signals. When the signal is small, the microcontroller system uses the path with the larger amplification factor; when the signal is large, the microcontroller system uses the path with the smaller amplification factor, as shown in Figure 2. Figure 2 Single-channel signal conditioning circuit 2.1.4 The microcontroller system uses the TI MSP430F167 microcontroller as the main control chip. This chip has a built-in 32K FLASH memory, 1024B RAM, two USART interfaces, an 8-channel 12-bit A/D converter, 48 I/O ports, a 16-bit watchdog timer, one 16-bit Timer_A (3 capture/compare registers), and one 16-bit Timer_B (7 capture/compare registers); it adopts a RISC instruction set architecture. The chip meets the intelligent requirements of various types of molded case circuit breakers without the need for any external auxiliary circuits, greatly enhancing the anti-interference performance. In order to improve the reliability of the system, the microcontroller system has been extended with power monitoring, reset control and hardware watchdog circuits. 2.1.5 Adjustment of operating mode The operating mode of the molded case circuit breaker includes the rated current, protection on/off, current setting, time setting, time limit characteristics, etc. The operating mode of the molded case circuit breaker can be adjusted in two ways. (1) Potentiometer adjustment method. For molded case circuit breakers with rated current ≤225A, considering the limitations of cost space and size space, a metal film potentiometer with an insulated plastic knob is used to adjust the operating mode of the circuit breaker. Since the temperature characteristics of the potentiometer are poor, the error caused in the range of 40℃ may exceed 2%, so this method will cause errors, requiring the potentiometer resistance value to have a large redundancy. (2) Maintenance port and human-machine interface adjustment method. For molded case circuit breakers with a rated current > 225A, due to the greater cost and size flexibility, maintenance software is used to adjust the circuit breaker's operating mode via a PC. The maintenance port uses an RS-232 interface and employs the MODBUS message format. A human-machine interface is also configured for the molded case circuit breaker, displaying various operating parameters via a digital tube and a keypad. When not in use, the digital tube automatically turns off after 1 minute to reduce power consumption. All settings require password verification to prevent accidental operation. 2.1.6 Communication Interface The communication interface uses RS-485 communication. The communication protocol adopts the MODBUS communication protocol required for low-voltage electrical equipment, connecting to a communication adapter, the upper-level communication equipment, or directly to the substation computer management system. Figure 3 is a schematic diagram of the communication structure using a communication adapter. Figure 3 Intelligent Molded Case Circuit Breaker Communication Network 2.1.7 Indicator Lights The provided indicator lights include operating indicator lights, overload indicator lights, communication indicator lights, fault indicator lights, and verification indicator lights. 2.1.8 The output drive controller has power outage and low load lockout functions, and dual-value inverted logic output function to ensure the reliability of the output operation and avoid malfunctions caused by power-on, power-off, and interference. See Figure 4. Figure 4 Output drive circuit 2.2 Software design 2.2.1 Algorithm analysis The AC sampling technology and full-wave Fourier algorithm are adopted to extract the real and imaginary parts of the equivalent complex vector based on the sampled data. (1) Where n is the fundamental or harmonic order to be extracted; N is the number of sampling points in one cycle; x (k) is the real-time sampling value. The equivalent complex vector can be obtained from the real-time sampling data according to equation (1) (2) According to equation (2), the complex value and phase angle of the fundamental or a certain harmonic can be obtained from equation (3) and equation (4) (3) (4) The full-wave Fourier algorithm can completely extract the fundamental or the required harmonic components, filter out the DC components and the unwanted integer harmonic components, but has poor suppression ability for non-periodic components. When a fault occurs, the power system is in a transient process, containing aperiodic components such as attenuated DC components, which will cause errors in the Fourier algorithm. Therefore, a differential filter is used to suppress aperiodic components and make it work in conjunction with the Fourier algorithm. y(n)=x)(n)-x(n=k) (5) Where k is the order of the differential filter. In the plastic shell intelligent controller, k is 2, that is, a second-order differential filter is used. When the grid frequency deviates, the extraction of fundamental or harmonic components using the Fourier algorithm will produce a mixing phenomenon, which will cause errors in the measurement of electrical power. In order to eliminate the errors caused by the grid frequency deviation, the intelligent controller can use frequency tracking technology to improve it. The sampling interval is adjusted in real time according to the measured frequency so as to always satisfy equation (6). fs/f1=24 (6) fs is the sampling frequency, f1 is the system frequency, and 24 points are sampled per cycle. In this way, even if the frequency fluctuates, it will not cause a large measurement error. 2.2.2 Data Sampling: Two sampling data buffers are established: one for differential filtering of the sampled data, and the other for storing the real-time data after differential filtering. Due to the use of a second-order differential filter, a 3-word filtering buffer is required for each current path. The buffer adopts a first-in-first-out (FIFO) queue structure, sequentially storing the real-time sampled data at points x(n-2), x(n-1), and x(n), as shown in Figure 5. Figure 5: FIFO-structured filtering buffer. The full-wave Fourier algorithm requires the data to be arranged in a continuous period of sampled data. Since a 24-point full-wave Fourier algorithm is used, using a FIFO queue structure would consume a significant amount of time. Therefore, a 48-word circular sampling buffer is allocated for each current. 2.2.3 Frequency Measurement: A linear fitting zero-crossing algorithm based on AC sampled values ​​is used to accurately calculate the frequency value of the power system, as shown in Figure 6. Points B and A are two sampling points before and after the zero crossing, and B′ and A′ are two sampling points before and after the next zero crossing. Points B′ and A differ by K sampling points, so the measured power frequency period is (7) where Ts is the sampling period and the frequency is the reciprocal of the power frequency period. Please log in to: Power Transmission and Distribution Equipment Network to browse more information Figure 6 Software measurement frequency method 2.2.4 Program structure The program structure adopts a combination of main program and interrupt program, as shown in Figure 7. The main program realizes the functions of circuit breaker status monitoring, hardware self-test, display processing, keyboard processing, indicator light processing and maintenance port communication; the sampling interrupt mainly realizes data acquisition, data processing and fault processing, and the communication interrupt mainly realizes the communication function. The priority of the sampling interrupt is higher than the priority of the communication interrupt. Figure 7 Program flowchart 3 Intelligent controller function analysis and technical characteristics (1) The intelligent controller greatly enhances the protection function of the circuit breaker. The short delay in its three-stage protection characteristics can be set to I2t characteristics so as to better match with the next stage of protection and can realize ground fault protection. (2) The protection characteristics of the intelligent trip unit can be easily adjusted, and warning characteristics can also be set. The intelligent circuit breaker can reflect the effective value of the load current, eliminate high-order harmonics in the input signal, and avoid malfunctions caused by high-order harmonics. (3) The intelligent controller can improve the self-diagnosis and monitoring functions of the circuit breaker, and can monitor and detect voltage, current and protection characteristics. When the internal temperature rise of the circuit breaker exceeds the allowable value, or the wear of the contacts exceeds the limit value, an alarm can be issued. (4) The intelligent circuit breaker has high operating accuracy, a wide setting adjustment range, and can realize overload, phase loss, three-phase imbalance, grounding, undervoltage and other protection and alarm functions. (5) After the intelligent circuit breaker is networked with the control computer, it can also automatically record the circuit breaker operation status and realize telemetry, telesignal and remote control. (6) The intelligent controller can be set with a maintenance port, which makes it more convenient to adjust and maintain the circuit breaker. A display module can be set to realize the functions of ammeter, voltmeter, wattmeter and frequency measurement. (7) Intelligent controllers can use software to verify system errors, including gain error and zero-point offset, truly achieving debugging-free operation. 4 Low-voltage electrical appliance technology and market outlook In the future, the focus of the low-voltage electrical appliance industry will be on the following aspects: targeting high technology, developing environmentally friendly, intelligent, networked, communicable, design-drawing-free, and manufacturing-efficient low-voltage electrical products with independent intellectual property rights; improving and perfecting traditional low-voltage electrical products, developing economical and practical products, and consolidating the market advantages of traditional products; strengthening reliability research and improving the quality stability and reliability of domestic products; reorganizing the industrial structure and product structure with intellectual property rights and product brands as the guide; researching and developing China's fourth-generation intelligent and communicable low-voltage electrical products, developing China's low-voltage electrical fieldbus, and gradually narrowing the gap with advanced foreign levels. The focus is on prioritizing the development of intelligent communicable products and fieldbus products. The development direction of the low-voltage electrical appliance industry determines that the future development direction of molded case circuit breakers is: miniaturization, high breaking capacity, multi-functionality, modular accessories, intelligence, communicability, and support for fieldbus. The market potential of intelligent molded case circuit breakers is enormous in the future. 5. Summary Introducing an intelligent controller into molded case circuit breakers (MCCBs) can significantly improve their performance. It can not only more accurately and flexibly achieve instantaneous tripping, short-delay tripping, and long-delay tripping, but also grounding protection tripping. The intelligent controller provides a more user-friendly interface, enabling various fault alarms, online circuit breaker monitoring, and integrating the functions of ammeters, voltmeters, and wattmeters. The intelligent controller provides a communication interface, supports fieldbus, and is more suitable for information exchange in low-voltage power distribution and utilization networks, enabling telemetry, remote signaling, and remote control functions. 6 References [1] Chen Degui. New technologies for low-voltage electrical appliances for the 21st century. Low Voltage Electrical Appliances, 2001(1):3-8. [2] Chen Chun, Qiao Wei, Ye Fansheng. Intelligent low-voltage circuit breaker based on single-chip microcomputer. Electric Power Automation Equipment, 2003, 23(2):49-51. [3] Zeng Qingjun, Jin Shengfu, Huang Qiaoliang, et al. On intelligent controller of universal circuit breaker. Electric Power Automation Equipment, 2004, 24(2):79-83.