“In the power supply and distribution system, the distribution transformer monitoring terminal (TTU) is used to collect and control the information of the distribution transformer. It monitors the operating conditions of the distribution transformer in real time, and can transmit the collected information to the main station or Other smart devices provide data required for the operation control and management of the power distribution system. TTU is generally required to be able to monitor the operating conditions of the line, pole-mounted power distribution substation or box-type substation in real time, detect and deal with accidents and emergencies in time, and have the functions of on-site and remote reactive power compensation and on-load voltage regulation. It can be seen that in addition to data acquisition and control functions, another important function of TTU is communication.
In the power supply and distribution system, the distribution transformer monitoring terminal (TTU) is used to collect and control the information of the distribution transformer. It monitors the operating conditions of the distribution transformer in real time, and can transmit the collected information to the main station or Other smart devices provide data required for the operation control and management of the power distribution system. TTU is generally required to be able to monitor the operating conditions of the line, pole-mounted power distribution substation or box-type substation in real time, detect and deal with accidents and emergencies in time, and have the functions of on-site and remote reactive power compensation and on-load voltage regulation. It can be seen that in addition to data acquisition and control functions, another important function of TTU is communication.
Power line carrier communication technology appeared in the early 1920s. It uses power lines as transmission channels. It has unique advantages such as high reliability, low investment, quick results, and synchronization with power grid construction. Power line carrier is divided into high voltage power line carrier (usually refers to 35 kV and above voltage level in power line carrier), medium voltage power line carrier (10 kV voltage level) and low voltage power line carrier (380/220 V voltage level).
1. Hardware design of the communication module of the distribution transformer monitoring terminal
1.1 Requirements for TTU communication in power distribution automation
According to the requirements of the distribution automation system, the distribution transformer monitoring terminal TTU pair should be able to communicate with the distribution electronics station or the master station, report the real-time information collected by the terminal, and receive various control commands from the substation/master station. The following requirements can communicate with the nearby distribution transformer monitoring terminal (TTU) or other smart devices. Therefore, the requirements for the communication function of the distribution transformer monitoring terminal are relatively strict, and the requirements of the distribution network automation system must be met regardless of the communication mode, communication protocol, and communication interface, including:
(1) Reliability of communication: The communication of the distribution transformer monitoring terminal should be able to withstand harsh weather conditions, such as rain, snow, hail and thundershowers, as well as long-term ultraviolet radiation and strong electromagnetic interference.
(2) Higher cost performance: Consider the cost of the communication system, choose the best combination of cost, function and technological advancement, and pursue the best cost performance.
(3) The real-time nature of distribution communication: TTU can quickly and timely transmit a large amount of fault data when the power grid fails, and the communication system of the distribution transformer monitoring terminal must have the ability of two-way communication, and have the ability of half-duplex or full-duplex.
(4) Standardization and versatility of communication methods: The communication system of the distribution transformer monitoring terminal includes a transmitter and a receiver. It is often necessary to communicate with other power distribution equipment during use. Therefore, communication methods and equipment that are versatile and highly standardized should be selected as far as possible to facilitate use and maintenance.
1.2 The composition of TTU communication module
1.2.1 The overall block diagram of the communication module
The overall block diagram of TTU’s communication module is shown in Figure 1.
Based on the power line carrier chip ST7538, the design interface of the TTU communication module is realized. The command information from the master station is received through the power line. After filtering and amplifying, the command is demodulated and sent to the controller, and then the controller sends the master station command to the data acquisition and data acquisition through the serial port. Processing module. The data acquisition and processing module collects the data of the distribution transformer according to the received master station command. After analysis and processing, the data information is sent to the controller of the communication module through the serial port, and then modulated, and finally sent to the power line via the interface , Waiting for the master station to receive.
1.2.2 Selection of Power Line Carrier Chip
In the power line carrier communication, the power line carrier chip plays a vital role, it directly affects the accurate transmission of information, so the choice of the power line carrier chip is very important.
XR2210/XR2206 chip or LM1893 is an earlier power line carrier chip. XR2210/XR2206 is a set of FSK mode modulation and demodulation chips, not specifically designed for power line carrier communication. LM1893 is a modem chip produced by National semiconductor Corporation. It adopts FSK modulation and demodulation method. It is only a slight improvement on the general FSK modulation and demodulation chip. At present, these two modem chips are basically not used in China. SSC P300 is a power line carrier modem chip designed by Intellon with the latest modern communication technology. It adopts spread spectrum (Chirp mode) modulation and demodulation technology, modern DSP technology, CSMA technology and standard CEBus protocol. It can be called an intelligent modem chip, which reflects the development trend of modem chips. But it is designed by Intellon in accordance with North American frequency standards and power grid characteristics, especially for home automation. The frequency range is 100 kHz “400 kHz, and the grid voltage is 480 Y/277 Vac, 208 Y/120 Vac, 60 Hz, which is not suitable for my country’s 50 Hz grid frequency. The ST75xx chip is a modem chip designed for power line carrier communication by SGS-THOMSON .Because it is a dedicated modem chip, in addition to the signal modulation and demodulation function of the general modem chip, many special signal processing methods have been added for power line applications. At present, it is widely used in the field of power line carrier meter reading in China.
This article selects SGS-THOMSON’s power line carrier chip ST7538, which is a half-duplex, synchronous/asynchronous FSK modem chip designed for home and industrial power line network communications based on ST7536 and ST7537. ST7538 integrates all the functions of sending and receiving data. Through serial communication, it can be easily connected to the microprocessor. It has automatic voltage control and automatic current control. It can be connected to the power grid through a small amount of external devices such as coupling transformers. In, reliable communication can be achieved in a channel environment with a wide noise frequency band. ST7538 also provides watchdog, zero-crossing detection, operational amplifier, clock output, timeout output, +5 V power supply and +5 V power supply status output, etc., which greatly reduces the number of peripheral components of the ST7538 application circuit. It is a function Powerful, highly integrated power carrier chip, designed for home and industrial environment applications, adopts a variety of anti-jamming technologies.
1.2.3 Design of hardware circuit
The communication module includes a microprocessor part, a carrier part, a signal filtering part and a power line signal coupling protection part. The connection of each part of the entire communication module is shown in Figure 2.
The microprocessor selects the ARM chip, which communicates with the data acquisition and processing module of the TTU through the serial port, and sends the command of the master station in time and transmits the distribution data collected by the TTU.
The power line carrier chip ST7538 communicates with the microprocessor through the SPI port, and the data exchange between the microcontroller and ST7538 can be realized through the serial port RxD, TxD and CLR/T of the microprocessor and ST7538. The working mode of ST7538 is determined by the status of REG_DATA and RxTx. The communication between the microprocessor and ST7538 adopts the synchronous way, CLR/T is regarded as the reference clock. When ST7538 is in the state of receiving data, RxTx is low, and the data to be sent enters ST7538 from the TxD pin. It is sampled at the rising edge of the clock and sent to the FSK modulator for modulation. The modulated signal is converted by D/A, filtered, and the automatic level control circuit (ALC), and then output to the power line through a differential amplifier. When ST7538 is in the state of receiving data, RxTx is high, and the signal enters ST7538 from the analog input terminal RAI pin, passes through a bandpass filter with a bandwidth of ±10 kHz, and sends it to an amplifier with automatic gain AGC. This signal is then demodulated, filtered and phase-locked into a serial digital signal, which is output to the microprocessor ARM.
The signal filtering part includes two parts: input narrowband filter and output narrowband filter. The input filter circuit adopts a parallel current resonance circuit to filter out unnecessary signals and noises other than the specified frequency. The output filter circuit adopts a series voltage resonance circuit to prevent unwanted signals from coupling to the power line.
The power line signal coupling protection circuit is composed of three basic parts: a power amplifier, an output protection matching circuit, and an input gain balance matching circuit. The coupling method adopts capacitive coupling.
2. Software design of the communication module of the distribution transformer monitoring terminal
2.1 Formulation of communication protocol
The communication protocol of the communication module is formulated according to the DNP3.0 statute, and the data of the data link layer adopts a variable frame length format: FT3. An FT3 frame is defined as a fixed-length header, followed by optional data blocks, and each data block is accompanied by a 16-bit CRC check code. The fixed header contains a two-byte start word, a byte length (LENGH), a byte link layer control word (CONTROL), a 16-bit destination address, a 16-bit source address, and a The 16-bit CRC check code, its frame format is shown in Table 1.
Start word: 2 bytes, 0x0564.
Length: 1 byte, which is the sum of control word, destination address, source address and user data, 255≥length≥5.
Destination address: 2 bytes, low byte first.
Source address: 2 bytes, low byte first.
User data: the data block following the header, one block for every 16 bytes, the last block contains the remaining bytes, which can be 1”16 bytes. Each data block has a CRC cyclic redundancy code. behind.
CRC cyclic redundancy code: 2 bytes. In a frame, hang after each data block.
Control word and function code: The communication control word contains the transmission direction of the frame, the type of the frame and the control information of the data flow. The specific settings of the function code are:
For the original sender’s frame:
0: Reset the remote link
1: Reset the remote process
3: Sending user data requires the other party to confirm
4: Send user data without the other party’s confirmation
9: Ask the link status
For the frame sent from the slave:
0: Affirmative confirmation
1: Negative confirmation
11: Answer the link status
2.2 Software design scheme of communication module
The communication module usually works in the carrier receiving state. After receiving a frame of data, it demodulates it to the TTU data acquisition terminal. The TTU acquisition terminal receives and returns the data. After the data is modulated, it is transmitted to the master station through the power line and demodulated to the acquisition terminal. When the RS485 receives the data within the specified time, it will send the carrier wave, and return to the receiving state after the data is sent. If RS485 does not receive data within the specified time, it will automatically return to the carrier receiving state. The software flow of the communication module is shown as in Fig. 3.
When the communication module judges that a frame command is received, it starts demodulation, that is, carrier receiving, and the limited time is 5 s. The frame command is judged at the same time during the demodulation process, and if there is, the demodulation is continued again. The serial port is only allowed to receive when a frame command is received, and the time limit is 1.5 s. After receiving the frame command to demodulate, it sends the command to the serial port. After receiving the command, the TTU data collection terminal collects and processes data according to the instructions of the command, analyzes the data status, and sends the collection and analysis results to the serial port. When the communication module receives the return data from the acquisition terminal within 1.5 s after the frame header is found, it modulates the data and sends it to the power line, that is, carrier wave transmission. The time limit is 5 s. If the frame header is found, the return data from the acquisition terminal is not received within 1.5 s Data, it is forbidden to receive data from the TTU collection terminal. After the carrier is sent, the communication module returns to the receiving state again, waiting for the next command from the master station.
This paper realizes the design of the communication module of the distribution transformer monitoring system. The module is based on the power line carrier communication technology and communicates with the power distribution master station through the power line. It does not need another line. It has the advantages of good cost performance, high integration and reliable operation. The communication module is combined with the data acquisition and analysis processing module of the distribution transformer to form the monitoring terminal of the distribution transformer, which enables the distribution transformer monitoring terminal to integrate collection, processing, and communication, and improves the function of the distribution transformer monitoring terminal. Optimized its design, improved the performance of the entire monitoring terminal, and has good development prospects.