Design and application of intelligent measurement and control device for switch cabinet based on single chip microcomputer

At the same time, 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 processor uses Freescale's first high-performance MC9S08AW32 microcontroller based on the highly energy-saving S08 core. The microcontroller has rich on-chip resources, supports BDM on-chip debugging, has an integrated watchdog circuit, has outstanding anti-interference ability, and has the best EMC performance in the industry. The CPU bus frequency can reach up to 20MHz, and the maximum operating speed can reach 40MHz. Rich on-chip resources: 32KB online programmable FLASH memory, internal clock generator, with 2 programmable timers, rich I/O ports: dual SCI ports, SPI, I2C and other interfaces, which greatly facilitates hardware expansion.

Switchgear intelligent measurement and control principle block diagram

The energy metering chip uses the high-precision three-phase energy measurement chip ADE7758 from ADI of the United States. The chip has high measurement accuracy and powerful functions. The IC has a built-in high-precision analog-to-digital converter and a fixed-mode digital processing signal processor (DSP), which has digital integration, digital filtering, practical energy monitoring, and metering functions. The chip has an SPI serial port, active energy pulse output, and reactive energy pulse output, and can be used for the measurement of active power, reactive power, electric energy, voltage and current effective values ​​in various three-phase systems, as well as the signal processing circuits necessary for digitally correcting system errors.

Switch cabinet intelligent measurement and control circuit diagram

ADE7758 provides system calibration functions for each phase, including RMS offset calibration, phase calibration, and power calibration.

The block diagram of the device hardware circuit design is as follows. The entire system is based on MC9S08AW32 and can be divided into central processing unit,

1.3 Partial Circuit

1.3.1 Central Processing Unit

The circuit diagram of the central processing unit is shown in Figure 3. The CPU processes and calculates the sampled signal, and compares the measured current, voltage, temperature and humidity values ​​with the various pre-set protection values ​​to determine whether the voltage and current of the switch cabinet are normal, and whether the temperature and humidity conditions are normal. If not, the corresponding alarm information is output. The external ferroelectric memory is expanded to store some important parameters, so that the previous important data will not be lost even if the program is upgraded in the future.

Switchgear intelligent measurement and control central processing unit[page]

1.3.2 Switching Control Module

The switch quantity control module includes switch quantity input and alarm output, and its circuit diagram is shown in Figure 4. The switch quantity input is connected to the CPU through photoelectric coupling; the alarm is connected to the relay output through the GPIO port through the photoelectric coupler. There are 8 switch quantity inputs, which correspond to the circuit breaker closing, circuit breaker opening, trolley working position, trolley test position, grounding knife position and spring energy storage indication in the primary diagram, and the rest are reserved. The switch quantity input corresponds to the primary diagram and can be programmed. There are 6 switch quantity outputs, which output the status of heater 1, heater 2, fan, alarm, lighting, and lockout in turn.

Switch cabinet intelligent measurement and control switch quantity control module

1.3.3 Human-computer interaction unit

The human-computer interaction interface of the high-end product of this device adopts LCD liquid crystal display module. LCD adopts 128*128 dot matrix display, and the initial interface is the electrical parameter display interface. The menu setting interface is entered by key input. The menu options are all displayed in Chinese, making the operation intuitive and easy to understand. Through the menu options, parameters such as wiring mode, voltage ratio, current ratio, alarm setting value, communication address baud rate, etc. can be set. Low-end products use double-row four-digit LED digital tubes to display temperature and humidity information and various programmable information. Users can set various alarm setting parameters, communication address baud rate, etc. according to actual needs.

1.4 Comments

The power module used in this device is a switching power module. The input voltage of this power module is AC90~285V or DC100~300V, the input frequency is 45~60Hz, the output voltage is stable, the failure rate is small, the output ripple is less than 1%, and the conversion efficiency is ≥75%. It has overvoltage and overcurrent protection. The module has been used in actual field and has high stability, reliability and anti-interference ability.

The temperature and humidity sensor uses SHT10, which is a highly integrated temperature and humidity sensor chip with the advantages of ultra-fast response and strong anti-interference ability, and provides fully calibrated digital output. The CPU and SHT10 use a serial interface, and optimization has been done in terms of sensor signal reading and power consumption.

The high-voltage live display module receives an electrical signal from the high-voltage live sensor to determine whether the high-voltage cabinet is live. Due to the high bus voltage, the high-voltage live display circuit uses various overvoltage protection and isolation protection devices to ensure the normal operation of the internal circuit of the device.

In addition, the device also integrates control function, human body sensing function, voice error prevention prompt function, etc.

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2 Software Design Process

The system software design includes two parts: the main program and the communication module.

The main program completes power-on or reset initialization, energy chip initialization, other peripheral initialization, temperature and humidity measurement, reading electrical parameters, power calculation, status display and alarm processing, LCD display refresh and key processing and other functions. The program design process is shown in Figure 5.

CPU initialization mainly refers to the configuration of the CPU's special status register SFR, setting the input and output status and initial status of the I/O port, reading the ferroelectric register data, etc.; the power chip initialization mainly refers to the configuration of the ADE7758 function register; the rest of the main program is the completion of various functions. Only by arranging the program flow reasonably to complete these functions can the device work reliably.

The communication module is implemented in an interrupt mode, mainly completing functions such as receiving data and protocol processing. The communication protocol adopts the standard MODBUS-RTU protocol, which is convenient for communication with the host computer and networking with other network instruments to achieve real-time monitoring of the switch cabinet status.

3 Achieved technical indicators and performance

The technical indicators of the ASD series switch cabinet intelligent measurement and control device are shown in Table 1. The product is designed with excellent electromagnetic interference PCB design technology, and undergoes live aging and factory inspection tests during production to ensure the long-term stability and reliability of the product.

Table 1 Technical indicators of ASD device

4 Application Cases

In the design of a high-voltage switchgear in a cement plant, all ASD series switchgear intelligent measurement and control devices are used. The application scheme is shown in Figure 5. In the figure, the signal indication circuit realizes the primary schematic indication through the switch quantity input acquisition; the live display circuit indicates whether the bus is energized through the high-voltage sensor input; the temperature and humidity control circuit realizes temperature and humidity control through the temperature and humidity sensor input. And it has Modbus communication output for users to remotely monitor and control. It has changed the previous situation that each switchgear needs to be equipped with multiple products from different manufacturers, which not only reduces the engineering workload and saves costs, but also improves the reliability of the system. The switchgear intelligent measurement and control device put into trial use guarantees the protection of the primary equipment in the switchgear with its good stability and reliability, and guarantees the accurate measurement of parameters such as voltage, current, power and accurate measurement of electric energy, while facilitating users to monitor the switchgear in real time.