Abstract: This paper introduces the hardware and software design of an intelligent circuit breaker measurement and control unit based on a TMS320LF2407 digital signal processor (DSP). The system integrates measurement, protection, control, communication, and display functions, realizing the microcomputer-based automatic monitoring function of the circuit breaker and featuring three-stage current protection and grounding/leakage protection. Both hardware and software adopt a modular design. The digital signal processing system is the core of the measurement and control unit, and its signal conditioning, programmable logic, clock, and communication interface modules are described. The software design employs new digital filtering and protection algorithms, and utilizes a real-time multi-task scheduling operating system. The design incorporates a loop querying of the interrupt flag in the main program, dividing the entire program into a main program and interrupt subroutines. Flowcharts of the main program and some interrupt subroutines are provided. Currently, the prototype of this measurement and control unit has been put into operation, and the operating results have met the expected design requirements. Keywords: Intelligentization; Digital Signal Processor; Digital Filtering; Protection Algorithm 1 Introduction With the rapid development and application of computers, microprocessors, fiber optic transmission technology, sensor technology, and digital processing technology, intelligent control electrical products with microprocessors have emerged. Intelligent operation of intelligent circuit breakers is a completely new concept in the development of intelligent circuit breakers. The purpose of intelligent circuit breaker operation is to control the circuit breaker to quickly disconnect the faulty part of the system when a fault occurs in the system, preventing the fault from escalating, ensuring the safety of equipment and personnel, and ensuring the normal operation of other parts of the system. The control unit is the core component for realizing intelligent operation. Its basic task is to provide corresponding control information by collecting and processing power grid parameters to obtain the required breaking time delay of the circuit breaker. This paper aims to provide conditions for the realization of intelligent operation of circuit breakers through the development of the control unit. The central component of an intelligent circuit breaker is the intelligent measurement and control unit, which undertakes various protection, alarm, display, and control functions of the circuit breaker. Due to the adoption of computer technology, digital processing technology, control theory, programmable logic technology, and serial communication technology, the functions of intelligent measurement and control units are becoming increasingly sophisticated. Besides implementing various selective protection functions, they also possess various auxiliary functions such as display, fault recording, self-diagnosis, testing, and control. Furthermore, they can achieve "four remote" functions (telemetry, remote adjustment, remote control, and remote signaling) through network cards or interface converters, making them suitable for network systems and allowing centralized monitoring and control via a host computer. Therefore, circuit breakers with intelligent measurement and control units are widely used in low-voltage power distribution systems. Based on the ST intelligent controller developed by Jiangsu Huanghai Electric Control Equipment Factory, the author independently developed an intelligent measurement and control unit and its host computer monitoring system, analyzing and discussing the relevant issues involved in the design of the intelligent measurement and control unit. 2. Overall Structure and Working Principle The basic task of this control unit is to provide corresponding control information by collecting and processing power grid parameters to obtain the required breaking time delay of the circuit breaker. To determine the state of the power grid, the control unit must first have the function of accurately measuring power grid parameters. The overall structure of the intelligent measurement and control unit is shown in Figure 1. It consists of four parts: a switching power supply, a signal detection unit, a microprocessor system, and an actuator. The switching power supply provides the operating power for the measurement and control unit; the signal detection unit consists of current transformers (CTs) and voltage transformers (PTs), which, through pre-conditioning sampling circuits and combined with microelectronics technology, convert the strong current and high voltage in the power supply circuit into current and voltage signals suitable for electronic circuits and microcontroller processing, providing accurate and reliable sampling signals for the microprocessor system; the microprocessor system consists of Texas Instruments' TMS320LF2407 digital signal processor and Altera's MAX7000 series CPLD and their external interface circuits, which perform real-time acquisition, processing, and storage of current and voltage signals to realize various protection and auxiliary functions; the actuator is a flux-transfer separation trip unit, which can be driven to trip with relatively small energy, and the trip signal comes from the microprocessor system. Among them, the microprocessor system is the core of the entire measurement and control unit and is the hallmark of the circuit breaker's digital intelligence. The current and voltage signals of the normal power supply main line are converted into 0-5A current and 0-100V voltage signals by high-power CT and PT for secondary system acquisition. Considering that the signal input standard of LF2407DSP is low-power 0-3.3V power supply, small CT and PT must be used to condition the signal again, and then rectified and filtered to supply the microprocessor. The microprocessor acquires electrical signals in real time and performs further software filtering and real-time processing on the sampled signals. If a fault is found, the corresponding alarm, display and tripping operations are performed according to the fault type. 3 Hardware Design The microprocessor system is the core component of the intelligent measurement and control unit and is also the mark of the intelligentization of the measurement and control unit. The design of the microprocessor functional modules in this section is described as follows: (1) The minimum DSP system includes CPU (TMS320LF2407), EEPROM (X25045), RAM (CY7C1021), address latch 74LS373, etc. Its main function is to process the acquired data, complete the metering and protection functions, and communicate with the host computer through the communication interface. The LF2407DSP is a low-power digital signal processing chip powered by 3.3V and is widely used in modern signal processing systems. In addition, this system uses the on-chip AD module of the DSP, which saves manufacturing costs and system size. (2) The CPLD module uses Altera's MAX7000 series. This device has high integration, fast working speed and convenient programming. In this system, it completes fast chip select signal, tri-state drive, decoding circuit and latching function, which greatly simplifies the hardware complexity of the system, improves the reliability of the system and reduces power consumption. (3) The signal conditioning module consists of level conversion adjustment circuit, sample and hold circuit, multiplexer, etc. Its main function is to condition large electrical signals into signals that can be received by the A/D module input of the DSP. And implement hardware low-pass filtering function. (4) The monitoring module uses MAX706 dedicated monitoring chip produced by MAXIM. It has watchdog, voltage detection and power-on reset functions, which can improve the reliability and accuracy of the system. (5) The clock module uses DALLAS's DS1302 serial real-time clock chip. It provides the system with a time standard. The chip has the main features of small size, low power consumption, easy interface, and less CPU I/O bus occupation, and is widely used in intelligent instruments and meters. (6) The user operation panel module includes a keyboard operation panel and an LCD display panel, which provides a human-machine interaction interface. The functions of setting, testing, and detection can be realized through the buttons. The LCD displays a large amount of information, such as data display, parameter setting, and fault information. (7) The communication interface module adopts the MAX483 interface chip produced by MAXIM and the ADAM4520 conversion module produced by Advantech. The communication between the lower computer and the upper computer is realized through the RS-485 interface, which is suitable for network systems. 4 Signal acquisition and filtering algorithm of the measurement and control unit At present, the discrete Fourier algorithm is a widely used method in the quantitative analysis of fault signals. The discrete Fourier algorithm not only has a strong filtering function, but also obtains the real part and imaginary part of the signal through the algorithm, which greatly facilitates the determination of the magnitude, nature (capacitive or inductive) of short-circuit current and voltage and the calculation of power, and is therefore widely used. In this device, harmonic analysis is first performed on the current and voltage, and then the effective value and phase angle of each harmonic are calculated after the compensation calculation of the attenuated DC component. The following analysis is used for the processing of the attenuated DC component. Let the input signal be Ks and Kc in equations (2) and (3), which are the compensation coefficients of the sine and cosine components, respectively. The calculation formula is N in equations (4) and (5), where N is the number of sampling points. After the AD sampling of a certain channel signal for one cycle is completed, the r value is calculated in real time, and then the sine and cosine components are compensated after the Fourier analysis is completed. In this system, only the fundamental wave is compensated. The program uses the Fast Fourier Transform (FFT) algorithm to realize the filtering function of the power grid signal. Since the DSP instruction speed is fast and has suitable hardware conditions and instructions for the implementation of this algorithm, the running time of this program can meet the real-time requirements of the intelligent operation of the circuit breaker. 5. Protection Characteristics of the Measurement and Control Unit 5.1 Overview The intelligent measurement and control unit has grounding leakage protection, three-stage current protection, phase loss and imbalance protection, undervoltage protection, and single-phase grounding protection. This section discusses the long-delay, short-delay, and instantaneous three-stage current protection. The overcurrent protection characteristics are represented by a time-current curve in a rectangular coordinate system. Both the vertical and horizontal axes are logarithmically transformed, with the vertical axis representing the operating time and the horizontal axis representing the overcurrent multiple. Figure 2 shows a typical three-stage overcurrent protection characteristic curve. It includes long-delay, short-delay, and instantaneous current protection. The current setting range of the three current protection stages can be represented by a number axis, as shown in Figure 3. By setting different current setting values, the measurement and control unit can have three protection characteristics simultaneously or separately. The short-delay current coverage range overlaps with the long-delay current and instantaneous current coverage ranges, respectively. Depending on the current setting value, the circuit breaker can have three protection characteristics simultaneously or separately, as shown in Table 1. 5.2 Protection Characteristics The long-delay protection characteristic is inverse time, and the mathematical expression is: The short-delay protection characteristic consists of two parts: inverse time and definite time. 8Ir is the dividing point, and the definite time has multiple specified values ​​for its operating time; the mathematical expression of the inverse time characteristic is: In equation (6) and equation (7), Ir is the current setting value, I is the overcurrent value, T is the operating time, tL is the long-delay operating time setting value, and tS is the short-delay operating time setting value. When the short-circuit short-delay operating current is greater than 8Ir, the short-delay characteristic of the intelligent trip unit is automatically converted to definite time, and the operating time is independent of its current setting value. The short-circuit instantaneous operating characteristic is definite time, and the operating time is generally 10 to 20 ms. Intelligent circuit breakers generally have an OFF instantaneous locking function, which can be used to turn off the instantaneous protection function in power distribution systems where instantaneous action is not required. 5.3 Protection Principle The protection principle of definite time protection is relatively simple. When the fault current value is greater than the current setting value, a timer is started. When the timer expires, the protection operates. If the fault current value is less than the setting value within the timer period, the protection deactivates. Inverse-time protection is essentially thermal protection; its operating time is inversely proportional to the square of the current. To improve the real-time performance of inverse-time current protection, this measurement and control unit uses a lookup table method to determine the protection's operating characteristics. Based on the measured overcurrent multiple, the operating time is calculated by looking up the table; when the timer expires, the protection operates. 6. Software Design Real-time multi-task scheduling has three states: running, ready, and waiting. Each task switches between these three states under the scheduling of the real-time operating system, as shown in Figure 4. Switching between tasks should be based on priority. The software design of the low-voltage circuit breaker digital measurement and control unit adopts a hierarchical and modular design approach. The software structure diagram is shown in Figure 5. The program uses task status words to correspond to different priorities. As shown in Figure 5, the software system is divided into 3 interrupts, 1 scheduler, and multiple tasks. The real-time multi-task scheduling system is the core of the entire software system, enabling the rational and orderly execution of multiple tasks. As the core of the real-time multi-task system, task scheduling must be executed once within a very short time and cannot be interrupted by other programs to ensure that high-priority tasks are executed promptly. In this measurement and control unit, task scheduling is executed within the data acquisition interrupt handler. This ensures that the highest-priority task waits at most one sampling cycle, approximately 1.67ms, before execution, meeting real-time requirements. The lower-level software design of this intelligent measurement and control unit utilizes a mixed programming and debugging environment using assembly and C languages ​​on the LF2407. The program structure features modularity and subroutines, and anti-interference processing is incorporated into the program design. The software program is divided into two main parts: the main program and the interrupt routines. The main program includes fault handling subroutines, communication handling subroutines, keyboard handling subroutines, and display handling subroutines. The interrupt routines include external interrupt subroutines, timer interrupt subroutines, communication interrupt subroutines, and keyboard interrupt subroutines. The interrupt priorities are, in order: external interrupt, timer interrupt, communication interrupt, and keyboard interrupt. (1) The main program is a loop program, and its flow is shown in Figure 6. After initialization, it enters the main loop, queries the subroutine flag register, and if the flag is found to be set, it enters the corresponding subroutine for processing. (2) External interrupts: The external interrupt 1 (XINT1) subroutine is used to detect system power failures, and the external interrupt (XINT2) subroutine is used to detect grounding failures. Once a fault is detected, an alarm signal is immediately issued, and the corresponding fault processing is initiated. The priority of external interrupt 1 (XINT1) is higher than that of external interrupt (XINT2). (3) Timer interrupt: Provides the signal sampling period. The timing period is 1ms, that is, 20 sampling points are collected in one cycle. The sampling frequency is 1kHz, which is 20 times the signal frequency (50Hz), and non-distortion sampling can be achieved. For the TMS320LF2407 processor with an external 15MHz crystal oscillator, the time required to complete each A/D is much less than 1ms, and this waiting time can be used to process the sampled values. The program flow is shown in Figure 7.