Abstract: This paper introduces a microcomputer protection controller for motors based on the high-performance embedded microcontroller MB90F543G. The controller integrates CPU, RAM, ROM, timer, UART, and CAN network port, and features high reliability, low power consumption, fast real-time calculation speed, networking, redundancy backup, and resource sharing. It also has a compact structure and good anti-electromagnetic interference and vibration resistance. Keywords: motor, microcomputer, protection controller 1 Introduction Various high and low voltage motors are one of the main loads in the power system, so the protection of motors is also the most important. With the development of computer technology, motor protectors are developing towards microcomputerization, intelligence, integration, and high reliability [1]. The most important and basic requirement of protection devices is reliability. The reliability of the microcomputer protection device for motors depends to some extent on the reliability of the CPU itself and its anti-electromagnetic interference capability. The most reliable solution is the CPU with a bus that does not leave the chip. Since the CPU's computing speed and bit width do not have a direct impact on reliability [2], considering both reliability and computing speed, this protector uses Fujitsu's MB90F543G as the protection CPU. The MB90F543G, as a new generation of enhanced microcontroller, features a DSP-like structure, a 4MHz external clock frequency, embedded 32-bit computing instructions, high-speed 32-bit multiplication, division, addition, and subtraction, a minimum instruction cycle of 62.5ns, 128K FLASH memory, and abundant peripherals (dual CAN, dual UART, SPI, and abundant I/O ports), making it easy to implement a multi-tasking real-time operating system. The protector is described below. 2. Protector Hardware System The protector consists of an AC sampling board (AC), a main control board (CPU), a power logic board (LOGIC), a trip board (TRIP), and a human-machine interface board (MMI). 2.1 AC Board The AC board includes two parts: voltage input and current input, as shown in Figure 1. The voltage input element consists of a voltage transformer, and the current input element consists of a current transformer and a parallel resistor. [align=center]Figure 1 AC Board Schematic[/align] 2.2 CPU Board The CPU board consists of seven parts: microcontroller, E2PROM, digital input circuit, digital output circuit, communication line opto-isolation circuit, real-time clock (RTC), and AD converter. The core of the microcontroller is the highly integrated chip MB90F543G. Therefore, there is no external bus on the CPU board, which greatly improves the device's anti-electromagnetic interference capability. The relationship between the parts on the CPU board is shown in Figure 2. [align=center]Figure 2 CPU Board Schematic[/align] 2.3 LOGIC Board The LOGIC board uses miniature relays to form trip, close, and alarm signal logic, as shown in Figure 3. It has the functions of issuing protection trip, remote trip, and remote close commands when the motor fails; cutting off the 24V power supply to the CPU board when the device fails; and issuing alarm signals when the device alarms. It also has the functions of signal reset and converting the 220V AC power supply to 24V DC. [align=center]Figure 3 Schematic diagram of LOGIC board[/align] 2.4 TRIP board The TRIP board has functions such as protection closing hold, protection tripping hold, manual closing, manual tripping, and anti-pumping. It can also monitor the integrity of the control circuit. 2.5 MMI board The MMI board consists of a CPU chip (another MB90F543G), a CAN network driver, a keyboard and LCD, signal indicator lights, a clock, and a reset button. It can perform functions such as keyboard response, menu operation, LCD display, communication with the CPU, signal indication, reset, and confirmation operations. The relationship between the parts is shown in Figure 4. The MMI board uses a 128*64 dot matrix LCD screen, and its interface is connected to the CPU board via a CAN network, offering high speed and fast response. [align=center] Figure 4 MMI Board Schematic[/align] 3 Software Design of the Protector 3.1 Main Program of the Protector The main program flowchart of the motor microcomputer protector is shown in Figure 5. Initialization (I) is the initialization of the single-chip microcomputer and its expansion chips, which initializes the switch output of the protection output and assigns normal values ​​to ensure that the output relays do not operate. Initialization (I) is an initialization program that is needed for both the running and monitoring programs. Initialization (II) includes the initialization of the sampling timer, the control of the sampling interval time, and the clearing of all software counters and various flag bits used in the RAM area during operation. After initialization, before entering the running phase, analog-to-digital conversion should begin, and a series of sampling calculations should be performed. A sampling interrupt request signal should be issued every 0.625ms (32 sampling points). After each interrupt service routine ends, it returns to the self-test loop and continues to wait for the interrupt request signal. The main program repeatedly performs self-tests and interrupts in a continuous loop, which is an important part of the protection operation program. [align=center] Figure 5 Main Program Flowchart[/align] 3.2 Sampling Interrupt Service Routine The sampling interrupt service routine mainly includes three parts: sampling calculation, PT disconnection detection, and protection starting element. Upon entering the sampling interrupt service routine, sampling is performed first. After sampling, the real-time effective value is calculated using the FFT algorithm, and then each effective value is stored in the corresponding address unit of the random access memory (RAM). Before the protection judgment starts, it is first checked whether the PT secondary is disconnected. In order to improve the reliability of the protection action, the output of the protection device is locked by the starting element. The lockout of the protection device output is only released after the protection starting element is started. In the microcomputer protection device, the starting element is implemented by software. The protection adopts the phase current change amount starting method. The sampling interrupt service routine flowchart is shown in Figure 6. [align=center] Figure 6 Sampling Interrupt Service Routine Flowchart[/align] 3.3 Fault Handling Program The fault handling program includes protection soft pressure plate switching check, protection setting comparison, protection logic judgment, trip handling program, and post-acceleration part. The protection logic functions include overcurrent protection glbh(), overload protection gffbh(), motor start-up time too long protection qdbh(), stall protection dubh(), zero-sequence overcurrent protection lxbh(), negative-sequence overcurrent protection fxbh(), undervoltage protection dybh(), PT disconnection detection chkPT(), underfrequency load shedding dzjz(), and undervoltage protection sybh(). In each protection logic judgment, if the numerical setting of phase A does not exceed the set value or the logic judgment program does not determine the protection action, the logic judgment and fault handling program for phases B and C will proceed. The fault handling program flowchart is shown in Figure 7. [align=center] Figure 7 Fault Handling Program Flowchart[/align] 4 Type Test of Motor Microcomputer Protection Controller Based on MB90F543G According to the above introduction, the structure of the motor microcomputer protection controller is shown in Figure 8. As a new generation of embedded microcomputer protector, the protection device adopts a high-performance, low-power enhanced microcontroller with a DSP-like structure and 32-bit calculation instructions. The high-reliability CPU, RAM, ROM, timers, UART, and CAN network port are integrated into a single unit, achieving low power consumption, high speed, and networking capabilities. A high-precision real-time clock with GPS automatic time synchronization enables synchronous sampling within the station system, and its E2PROM stores equipment parameters and protection settings. A large-capacity ferroelectric memory can store fault waveform data and 32 event reports; all data is retained even after power loss. The protection device uses a standard 4U height chassis with a front MMI board and rear plug-in structure. The AC board, CPU board, LOGIC board, and TRIP board are connected via a backplane. The MMI board at the front of the device communicates with the CPU board via the A82C250 CAN driver. Internal plug-ins are removable, facilitating debugging and maintenance, and forming an organic whole with the MMI board, resulting in a compact structure and excellent anti-interference and vibration resistance. Currently, the microprocessor-based motor protection controller based on MB90F543G has undergone type testing. The test results show that its main performance is superior to the national standard requirements (see Appendix 1 and Appendix 2 for details). It can be widely used in high-voltage asynchronous motors below 2000kW without differential protection, or as backup protection for asynchronous or synchronous motors above 2000kW. [align=center] Figure 8 Schematic diagram of protection device[/align] Appendix 1 Appendix 2 The author's innovation: This independently developed device uses a high-performance embedded microcontroller with a DSP-like structure as the protection CPU and a 14-bit high-precision AD chip as the protection analog-to-digital converter, which improves its measurement accuracy and protection reliability compared to similar products. In addition, the internal structure adopts a front MMI board and rear plug-in structure, making the device compact and providing good anti-interference and anti-vibration performance. References: [1] Wang Daoqian. Application of embedded system in substation relay protection system [J] Industrial Control Technology, 2006, 6 [2] Shi Huili, Zhang Xuejuan. Design of intelligent motor protector based on DSP [J] Microcomputer Information, 2006, 14: 179-181 [3] Liu Qiang. Discussion on the selection of central processing unit of microcomputer protection device [J] Journal of Jiangxi Electric Power Vocational and Technical College, 2006 [4] Chen Deshu. Microcomputer relay protection [M]. Beijing: China Electric Power Press, 2000