Application of CAN bus in online detection system of high voltage switch cabinet in substation

introduction

High-voltage switchgear plays a dual role of control and protection in the power system. With the increasing expansion of the power grid and the increasing reliability of power supply, as well as the popularization and promotion of the unmanned management mode of substations , the safe operation of high-voltage switchgear is becoming more and more important. Therefore, it is urgent to have high reliability of high-voltage switchgear and to monitor the operating status of the switchgear in real time so as to take preventive measures to prevent accidents. Due to the progress of electronic information technology, fieldbus technology has been widely used in the intelligent system of high-voltage switchgear functions, making the online detection system of high-voltage switchgear form a distributed monitoring system, and making the whole system more powerful and more functional than the traditional centralized monitoring system.

1. CAN bus technology and its characteristics

In traditional substation monitoring systems , BITBUS bus and RS-485 bus are generally used, but they have the following defects in practical applications:

① There can only be one master node on the BITBUS bus and RS-485 bus, and a multi-master system cannot be formed. Once the master node fails, the system cannot operate normally, resulting in poor reliability of the entire system; ② The BITBUS bus and RS-485 bus have poor anti-interference capabilities, are not suitable for operation in a high electromagnetic environment, and have poor error handling capabilities;

③The data communication method is command-response. The lower-end node can respond only after receiving the command from the main node, resulting in the data not being uploaded in time, causing the entire system to have poor real-time performance. It is not suitable for systems that require high real-time performance, such as online detection of switch cabinets.

In order to solve a series of problems in the application of BITBUS bus and RS-485 bus, this article adopts the controller area network CAN (Controller Area Network) which is currently commonly used in industrial measurement and control systems. The CAN bus uses differential drive, is suitable for environments with high noise interference and has strong error correction capabilities. It has now formed the international standard ISO11898 specification and has been recognized as one of the most promising field buses for its low price, high reliability and flexible structure.
CAN uses two of the seven layers of the OSI/ISO model, namely the physical layer and the data link layer. Users can use these two as the basis to develop corresponding application layer communication protocols according to actual needs. The main features of CAN are:
① It can work in multi-master mode. Any node on the network can actively send information to other nodes on the bus at any time, thus forming a multi-host system;

② When a CAN node has a serious error, it has the function of automatically shutting down the output, cutting off the connection between the node and the bus, so that other nodes and communications on the bus are not affected, so it has strong anti-interference ability.

③ Using non-destructive bus arbitration technology, when two nodes send information to the bus at the same time, the node with low priority will actively stop sending data, while the node with high priority can continue to transmit data without being affected, saving bus conflict arbitration time and avoiding bus conflicts;

④Node information on the bus can be divided into different priorities to meet different real-time requirements;

⑤ Data can be sent and received in point-to-point, point-to-multipoint and global broadcast modes;

⑥ The maximum communication distance of CAN can reach 10km/5kbps, the maximum communication rate can reach 1Mbps/40m, and the actual number of nodes on CAN can reach 110;

⑦ CAN uses a short frame structure, each frame contains 8 bytes, and has error detection measures such as CRC check, short transmission time, ensuring low data error rate;

⑧ The communication medium uses twisted pair, coaxial cable or optical fiber, which provides flexible selection.

In short , the CAN bus has strong real-time performance, high reliability and simple user interface, and is very suitable for forming an online fault detection system for high-voltage switchgear in substations.

2. Overall system design

The structure of the high-voltage switchgear online detection system is shown in Figure 1.

The whole system connects each acquisition unit module and monitoring computer unit into a distributed structure through the CAN bus.

The computer and each detection unit module in the network have their own ID mark, and ensure that their respective IDs cannot be repeated. The data acquisition unit module collects various signal parameters of the switch cabinet and sends them to the system monitoring unit through the CAN bus to establish a database of the high-voltage switch cabinet of the entire substation. The computer monitors and manages the entire CAN bus system, and has functions such as system parameter (such as transmission rate, node address, etc.) setting, data sending, data receiving, local status query, node status query, interrupt status query, etc. The system monitoring unit is composed of an industrial control computer with a CAN bus communication interface card. It receives the data sent by the data acquisition unit module, and the computer performs corresponding calculations to obtain the values ​​of various parameters of the high-voltage switch cabinet. According to the relevant properties such as various parameters and their waveforms, the working status of the high-voltage switch cabinet is obtained through specific analysis. The CAN bus communication interface card enables the industrial control computer to be easily connected to the CAN bus. It consists of a CAN interface circuit and its connection circuit with the computer serial port to ensure that data can accurately flow between the CAN bus and the computer.

3. Hardware circuit design

Since the computer serial port is a standard RS-232 interface, the data signal sent is transmitted in bytes, while the CAN bus signal is transmitted in frames. Therefore, if we want to transmit data between the computer serial port and the CAN bus, we must have a CAN bus communication interface card . The function of this card is to convert the format of the signal sent by the computer serial port and the signal sent by the CAN bus, so that each party can recognize the signal sent by the other party and realize data flow.

The hardware circuit design of the CAN bus communication interface card in this article is shown in Figure 2, in which the oscillation circuit and reset circuit are omitted.

3.1 CAN interface circuit

3.1.1 Selection of CAN controller

This article selects the SJA1000 chip, which supports the CAN2.0B communication protocol. It is fully compatible with the CAN controller PCA82C200 that only supports CAN2.0A in hardware and software, and adds new functions on its basis: standard frame data structure and extended frame data structure, and both frame formats have single /dual receive filters; 64-byte receive FIFO; error counters and error limit alarms with read and write access, as well as listen-only mode, etc.

3.1.2 CAN transceiver

PCA82C250 is the interface between PHILIPS's CAN controller and the physical bus, which can provide differential sending and receiving capabilities for the bus. It has three different working modes: high speed, standby and slope control. Slope control is generally used. PCA82C250 is used in hardware circuits to increase communication distance, improve the system's instantaneous anti-interference ability, and protect the bus.

Figure 2 CAN bus communication interface card Hardware Circuit diagram[page]

3.1.3 Photoelectric Isolation

In order to further improve the anti-interference ability of the system, an isolation circuit consisting of a high-speed isolation device 6N137 is added between the controller SJA1000 and the transceiver PCA82C250.

3.1.4 Selection of MCU

This device uses AT89C51 as the processor. AT89C51 is an 8-bit single-chip microcomputer of ATMEL Company, which is fully compatible with MCS51. It has an internal integrated 4Kbps flash E2PROM ^and the operating frequency can reach up to 20MHZ.

3.1.5 Selection of Level Conversion Chip

In the CAN interface circuit, the AT89C51 chip uses TTL level, while the output from the computer serial port is RS-232 level. Therefore, in order to connect the computer serial port to the microcontroller, level conversion must be performed. The MAX202 chip produced by MAXIM is a RS-232/TTL bidirectional level conversion chip in the absence of ±12V power supply. Its power supply is single +5V, and the maximum level conversion speed is not less than 120kbps. In order to prevent the power supply of the computer and the power supply of the CAN interface circuit from interfering with each other, we also use a photocoupler between MAX202 and AT89C51 to achieve the purpose of power isolation.

3.1.6 Data Acquisition Unit Circuit

The acquisition unit circuit is generally composed of a microprocessor plus a display keyboard circuit, an analog input circuit, a switch input and output circuit, a CAN bus communication interface circuit, a real-time clock circuit, etc. The specific circuit is not introduced in detail here.

4. Software Design

This system uses C language to develop software. The design of software is the key and also the difficulty for each node of CAN bus to complete data communication with AT89C51 effectively and in real time. It mainly includes computer serial port communication program, AT89C51 communication program, CAN node initialization program, CAN data sending and receiving program and CAN bus error handling program, etc. The internal registers of CAN controller SJA1000 are the off-chip registers of AT89C51 chip. The exchange of status, control and data between AT89C51 and SJA1000 is completed by reading and writing these registers by SJA1000 in reset mode or working mode. When initializing the internal registers of CAN, pay attention to make the bit rate of each node consistent, and the receiving and sending sides must be synchronized. There are two main ways to receive data: interrupt and query reception. In order to improve the real-time performance of communication, the interrupt reception method is adopted in this paper, and this can also ensure that there will be no data overflow in the receiving buffer. The following is the initialization flow chart of SJA1OOO working in Basic CAN mode and the source program of data reception.

/*CAN receive interrupt subroutine*/

Void canreceive(void) interrupt 0 {

Unsighed char xdata *data p;

Unsighed char xdata i, temp[15];

if ((can_sr&0x01)==0x01) { /*Judge whether the SJA1000 receive buffer is valid*/

p=&canrxbuff; /*canrxbuff is the first address of the receive buffer*/

for(i=0;i<0;i++) {

temp[i] =*(p+j); /*Assign the data of the receive buffer to the temp array*/

}

can_cmr=0x04; /*Release the receive buffer*/

i=can_ir; /*Reset CAN interrupt*/

}

}

The substation high-voltage switchgear online detection system composed of CAN bus technology in this paper has been applied in small and medium-sized substations. After actual operation, it has good reliability and anti-interference performance, can better grasp the operating status of the high-voltage switchgear , and stably display the waveform of each parameter, so that the equipment can be repaired in time to avoid accidents. It has been unanimously recognized by substation maintenance personnel.

5. Conclusion

CAN bus is gaining more and more attention due to its high reliability and excellent performance, and is recognized as one of the most promising buses. This paper applies this bus technology to the online detection system of high-voltage switch cabinets in substations, designs the main hardware circuits and software, and puts it into practical use. The design methods of its hardware and software circuits are also suitable for the node design of other distributed control systems based on CAN bus.