Abstract: In power systems, the functions of primary switchgear operating status, temperature and humidity control, and high-voltage energization indication in high-voltage switchgear are generally achieved by indicator lights and independent electrical components. This inevitably leads to low integration, complex wiring, and poor reliability. This paper introduces an intelligent measurement and control device for switchgear, suitable for 3-35kV indoor high-voltage switchgear. It is used for primary switchgear status simulation display, high-voltage energization indication, anti-condensation temperature and humidity control, and electrical parameter measurement, greatly improving the integration and intelligence of switchgear operation and measurement.
Keywords: MC9S08AW32, switchgear, primary system diagram, intelligent monitoring and control device
Design and application of the Intelligent Monitoring and Control Device for Switchboard Based on the Chip of MC9S08AW32
Abstract: According to the survey, working state of the switching device, control of temperature and humidity and high-voltage live instruction are usually achieved by some signal lamps and several independent electronic devices in a high-voltage switchboard of power system, which will inevitably bring about the shortcomings of low integration, complex wiring, and lower reliability. An intelligent monitoring and control device for switchboard named ASD is introduced in this paper, which is used in 3 ~ 35kV indoor high voltage switchboard. The device is used for the of switching device status, high-voltage live instructions, anti-condensing temperature and humidity control, electrical parameter measurement and so on, which is highly increased the integration and intelligence of manipulation and measurement of the switchboard.
Key words: MC9S08AW32; switchboard; primary system diagram; intelligent monitoring and control device
0 Introduction
Switchgear typically includes primary switching equipment such as circuit breakers (load switches), isolating switches, and grounding switches. Monitoring the status of these primary switching devices is crucial during operation and commissioning. Traditionally, indicator lights are used to indicate these statuses in switchgear, but this method is not intuitive and wiring is inconvenient. The intelligent monitoring and control device for switchgear combines the primary equipment status display with the switchgear's primary circuit diagram. LED displays are placed at the locations of the equipment symbols on the diagram, making the circuit status clear, vivid, and intuitive, as shown in Figure 1.
Meanwhile, the integrated high-voltage live display, automatic temperature and humidity control, and electrical parameter measurement functions make the switch cabinet panel simple and elegant, reducing the workload of secondary wiring.
1 Hardware Design Methodology
1.1 Design Platform
The central processing unit (CPU) utilizes Freescale's first device based on the highly energy-efficient S08 core, the MC9S08AW32 high-performance microcontroller. This microcontroller boasts abundant on-chip resources, supports BDM on-chip debugging, integrates a watchdog circuit, exhibits outstanding anti-interference capabilities, and possesses industry-leading EMC performance. The CPU bus frequency can reach up to 20MHz, and the maximum operating speed can reach 40MHz. Abundant on-chip resources include: 32KB in-system programmable FLASH memory, an internal clock generator, two programmable timers, and a rich array of I/O ports: dual SCI ports, SPI, I2C, and other interfaces, greatly facilitating hardware expansion.
The energy metering chip uses the high-precision three-phase energy measurement chip ADE7758 from Analog Devices (ADI). This chip boasts high measurement accuracy and powerful functionality. The IC embeds a high-precision analog-to-digital converter and a fixed-mode digital signal processor (DSP), providing digital integration, digital filtering, practical energy monitoring, and metering functions. The chip features an SPI serial port, active energy pulse output, and reactive energy pulse output, enabling it to measure active power, reactive power, energy, and RMS voltage and current in various three-phase systems, as well as providing the necessary signal processing circuitry for digitally correcting system errors.
The ADE7758 provides system calibration functions for each phase, including RMS offset calibration, phase calibration, and power calibration.
1.2 Design Flowchart
The hardware circuit design block diagram of the device is shown below. The entire system is based on the MC9S08AW32 and can be divided into a central processing unit, a power supply module, a voltage and current sampling and calculation module, a switch quantity control module, a temperature and humidity acquisition module, a human-machine interaction module, and a communication module, etc.
1.3 Part of the circuit
1.3.1 Central Processing Unit
The central processing unit circuit diagram is shown in Figure 3. The CPU processes and calculates the sampled signals, comparing the measured current, voltage, temperature, and humidity values with various preset protection values to determine whether the voltage and current, temperature, and humidity of the switchgear are normal. If abnormal, it outputs corresponding alarm information. An external ferroelectric memory is added to store important parameters, ensuring that previous important data is not lost even if the program is upgraded later.
1.3.2 Switch control module
The digital input control module includes digital inputs and alarm outputs, and its circuit diagram is shown in Figure 4. The digital inputs are connected to the CPU via optocouplers; the alarm outputs are connected to the relay output via GPIO ports and optocouplers. There are 8 digital inputs, corresponding sequentially to the circuit breaker closed, circuit breaker open, trolley working position, trolley test position, grounding switch position, and spring energy storage indicator in the primary circuit diagram; the remaining inputs are reserved. The digital inputs are programmable according to the primary circuit diagram. There are 6 digital outputs, sequentially outputting the status of heater 1, heater 2, fan, alarm, lighting, and interlock.
1.3.3 Human-computer interaction unit
The high-end version of this device uses an LCD display module for its human-machine interface. The LCD uses a 128*128 dot matrix display, initially displaying electrical parameters. Users can then access the menu settings interface via button input. All menu options are displayed in Chinese, making operation intuitive and easy to understand. Parameters such as wiring method, voltage ratio, current ratio, alarm settings, and communication address/baud rate can be set through the menu options. The low-end version uses a dual-row four-digit LED display to show temperature, humidity, and various programmable information. Users can set various alarm settings, communication address, and baud rate according to their actual needs.
1.4 Commentary
The power supply module used in this device is a switching power supply module. This power supply module has an input voltage of AC90～285V or DC100～300V, an input frequency of 45～60Hz, stable output voltage, low failure rate, output ripple <1%, and conversion efficiency ≥75%. It features overvoltage and overcurrent protection. This module has been tested in actual field applications and demonstrates high stability, reliability, and anti-interference capability.
The temperature and humidity sensor uses the SHT10 series, a highly integrated temperature and humidity sensor chip with advantages such as ultra-fast response and strong anti-interference capability, providing fully calibrated digital output. The CPU and SHT10 use a serial interface, and optimizations have been made in terms of sensor signal reading and power consumption.
The high-voltage live display module receives an electrical signal from a high-voltage live sensor to determine whether the high-voltage cabinet is energized. Due to the high bus voltage, the high-voltage live display circuit employs various overvoltage protection and isolation protection devices to ensure the normal operation of the internal circuitry.
In addition, this device also integrates control functions, human body sensing functions, and voice error prevention prompts.
2 Software Design Process
The system software design consists of two parts: the main program and the communication module.
The main program completes functions such as power-on or reset initialization, power chip initialization, other peripheral initialization, temperature and humidity measurement, reading electrical parameters, power calculation, status display and alarm handling, LCD display refresh and key handling. The program design flow is shown in Figure 5.
CPU initialization mainly refers to configuring the CPU's Special Status Registers (SFRs), setting the input/output states and initial states of I/O ports, and reading ferroelectric register data, etc.; power chip initialization mainly refers to configuring the ADE7758 function registers; the rest of the main program is to complete various functions. Only by reasonably arranging the program flow to complete these functions can the device work reliably.
The communication module is implemented using interrupts and mainly performs functions such as receiving data and processing protocols. The communication protocol adopts the standard MODBUS-RTU protocol, which facilitates communication with the host computer and allows for networking with other network instruments to achieve real-time monitoring of the switchgear status.
3. Achieved technical specifications and performance
The technical specifications of the ASD series switchgear intelligent monitoring and control device are shown in Table 1. The product design employs superior electromagnetic interference PCB design technology, and the entire unit undergoes energized aging and factory inspection tests during production to ensure the long-term stability and reliability of the product.
Table 1 Technical Specifications of ASD Devices
Technical parameters | index | |
enter | network | Three-phase three-wire, three-wire four-wire |
frequency | 45–60 Hz | |
Voltage /current | Rated value: AC 100V/5A | |
Overload: 1.2 times the rated value (continuous); 2 times the rated value for 1 second | ||
Power consumption: voltage loop less than 0.5VA; current loop less than 1VA | ||
Output | Electricity | Output method: Open collector optocoupler pulse |
Pulse constant: 32000 imp/kWh | ||
communication | RS485 interface, MODBUS-RTU protocol | |
accuracy | Electrical parameters | Frequency 0.05Hz, reactive power level 1, all others level 0.5. |
Temperature and humidity | Temperature: ±1℃; Humidity: ±4% | |
digital input | 8-channel dry contact input, built-in +12V power supply | |
Switch output | Output method: 6-channel relay output | |
Contact capacity: AC250V/5A | ||
power supply | AC90-285V, DC100-300V, power consumption ≤8VA | |
Security | Power frequency withstand voltage: AC2kV/1min; Insulation resistance: >100MΩ | |
Electromagnetic compatibility compatible | Electrostatic discharge test: Level 3; Burst test: Level 4; Surge test: Level 4 | |
environment | Operating temperature: -10～55℃; Storage temperature: -20～70℃; Relative humidity: 5%～95% (non-condensing); Altitude: less than 2500 meters | |
4 Application Cases
In the design of high-voltage switchgear at a cement plant, the ASD series intelligent monitoring and control devices were used throughout. The application scheme is shown in Figure 6. In the figure, the signal indication circuit uses switch input to acquire primary circuit diagram indications; the energized display circuit uses high-voltage sensor input to indicate whether the busbar is energized; and the temperature and humidity control circuit uses temperature and humidity sensor input to control temperature and humidity. It also features Modbus communication output for remote monitoring and control by the user. This eliminates the need for multiple products from different manufacturers for each switchgear, reducing the workload, saving costs, and improving system reliability. The intelligent monitoring and control devices deployed in the trial have ensured the protection of primary equipment within the switchgear with their excellent stability and reliability, guaranteeing accurate measurement of parameters such as voltage, current, and power, as well as accurate metering of electrical energy, while facilitating real-time monitoring of the switchgear by the user.
5 Conclusion
The intelligent monitoring and control device for switchgear is designed to meet the control requirements of power switchgear and is suitable for various switchgear types, including medium-voltage switchgear, handcart switchgear, fixed switchgear, and ring main unit. This product has been serialized, integrating operation and display to meet the needs of diverse customers. It features comprehensive functions, reliable operation, and excellent anti-interference performance, and has a very broad application prospect.
References
[1] Ren Zhicheng, Zhou Zhong. Principles and Application Guide of Digital Instruments for Power Measurement [M]. Beijing: China Electric Power Press, 2007.
[2] Wang Wei. Embedded Microcontroller S08AW Principles and Practice [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 2009.
[3] ADE7758 Datasheet [EB/OL]. Analog Devices, 2006.
[4] Sun Xingli, Liu Shuguang. Design and implementation of an intelligent control device for switchgear [J]. High Voltage Apparatus, 2009.
