Research on distribution automation communication system based on OFDM modulation technology

　　The communication methods that can be used in the distribution network automation system include optical fiber, distribution line carrier (DLC), wireless, wired, etc. The 10kV distribution line can extend from the substation to any measurement and control point on the line, so DLC is one of the most economical and reliable communication methods and the preferred communication method for distribution automation.

　　Distribution line carrier communication (DLC) is different from the original high-voltage system transmission line carrier communication (PLC: Power Line Carrier) method of the power system. PLC generally sends and downloads high-frequency signals between two points through wave traps and combined filters. The transmission target is clear and the structure is simple. DLC is a one-to-many communication method without wave traps. The communication signal is transmitted in the 10kV and 380kV distribution network. Any communication node installed on it can be used as a signal source and receiver, and the transformer (distribution transformer and substation transformer) is a natural barrier to the signal. The distribution line channel has the characteristics of high noise, large impedance variation range, and large loss. Therefore, it is much more difficult to use the power line of the distribution network as a communication medium than the high-voltage transmission line. The use of distribution line carrier communication requires solving a series of related problems.

　　Orthogonal Frequency Division Multiplexing (OFDM) is a highly efficient modulation technology used in high-speed digital communication on power lines. It has the following advantages:

　　⑴High frequency band utilization;

　　⑵Strong anti-ISI interference capability;

　　⑶ Anti-channel fading;

　　⑷ Anti-noise interference.

　　OFDM Basic Principle

　　The basic principle of OFDM is to convert the encoded data from serial transmission to parallel transmission, and modulate them separately using N subcarriers with equal frequency intervals. The modulated N subcarrier signals are added and sent simultaneously. Since the symbol rate is greatly slowed down, the transmission time is greatly extended, thus improving the ability to resist multipath fading and delay spread, reducing the influence of inter-symbol interference, and by selecting the carrier interval (△f=1overT, where T is the symbol period), these subcarriers maintain the orthogonality of the spectrum over the entire symbol period. At the receiving end, the orthogonality between carriers is used to ensure the distortion-free recovery of the transmitted signal.

　　System composition and working principle

　　This communication system uses 10kV distribution lines as the transmission medium, and the network topology is mainly "bus + tree". Different bus segments can be interconnected to form an open-loop ring network. The system consists of two parts: the communication substation system and the master station system. The master station system is set up in the 110kV substation to collect telemetry information from all substations on each 10kV line, and realizes safety monitoring, automatic control and protection of the entire distribution network according to the determined algorithm and control strategy. The communication substation is used in conjunction with FTU, TTU, etc. and is distributed throughout the distribution network. Here we first introduce the substation system part.

　　Communication substation system

　　The substation system is mainly composed of Intellon's INT51X1 chip, Winbond's W90N740 microprocessor, level converter MAX3232, analog front end (AFE) and coupler.

　　INT51X1 chip

　　The INT51X1 chip[1][2] is an integrated MAC/PHY power line transceiver. It enables high-speed data communication over power lines. The INT51X1 fully complies with the HomePlug Power Alliance industry standard V1.0.1 and uses Intellon's patented PowerPacket OFDM technology. The specific technical parameters of PowerPacket are as follows:

　　Speed ​​14MB/s; frequency band 4.3~20.9MHz; OFDM signal modulation, 84 carrier channels, automatic channel adaptation, forward error correction; carrier modulation mode supports DQPSK, DBPSK, ROBO; read and write mode supports CSMA/CA; meets FCC-15 radiation standards; communication encryption; 0.25 m technology processing; operating frequency 100MHz; operating voltage core voltage 2.5V, 3.3V/5V input and output voltage; working environment 0~70℃.

　　INT51X1 can achieve a data transmission speed of 14Mbps in a harsh power line communication environment. It can select available frequencies according to the signal-to-noise ratio (SNR) on the channel, resist deep attenuation grooves, noise and multipath fading, and achieve synchronization without using pilots in low SNR channels. In INT51X1, MAC adopts a carrier sense multiple access (CSMA/CA) scheme with conflict avoidance, and has priority setting and automatic repeat request (ARQ). Through packet encapsulation, it supports the reliable transmission of Ethernet packets. Under the premise of ensuring the quality of service (QoS), it provides the necessary bandwidth for multimedia payloads (including voice, data, audio and video).

　　INT51X1 provides three interfaces: MII/GPSI, USB1.1 and Ethernet. The three modes are USB mode, PHY mode and Host/DTE mode. This system selects PHY mode to interface with W90N740 and AFE respectively to complete the mutual conversion between Ethernet packets and power packets.

　　W90N740 Microcontroller

　　W90N740[3][4] uses a reduced instruction set and is a 32-bit microcontroller based on ARM7TDMI. It has two built-in Ethernet MAC controllers 0 and 1, 8K instruction cache and 2K data cache. It can be interconnected with external SDRAM, ROM/SRAM, flash memory or I/O through the external bus interface (EBI). Through the on-chip Ethernet MAC Controller 0 and INT51X1 interface, RS232 standard data and IEEE802.3 standard data can be converted to each other. By connecting a MAX3232 level conversion chip to the on-chip serial port UART, standard RS232 communication functions can be realized and connected to automation equipment such as FTU and TTU. [page]

　　Hardware connection between INT51X1 and W90N740

　　Since this system uses PHY mode, it is connected to the microcontroller through the MII interface. MII (Media Independent Interface) is an industrial standard interface that provides interconnection between PowerPacket MAC and IEEE802.3 Ethernet MAC controller.

　　The frame structure of the MII interface is as follows:

　　The analog front end (AFE) completes the amplification and filtering functions, and an 8v AFE is selected.

　　The coupler is coupled to the 10KV power line.

　　Software Design

　　The initialization of INT51X1 is completed by configuring EEPROM (AT93C46), and its own MAC address should be written (this MAC address is unique).

　　Because ARM7TDMI is the core of W90N740, it is programmed in ARM7 assembly language. Functions: Receive FTU/TTU data from RS232 port, convert it into 802.3 protocol IP packet, and send it to INT51X1 via MAC; receive INT51X1 data via MAC, unpack it, and send it to FTU/TTU via RS232 port.

　　W90N740 is initialized after power-on. After the initialization is completed, it sends a special call message to the master station (64 bytes all 5), and enters communication operation after the handshake process is completed.

　　The handshake process is as follows:

　　First, the substation broadcasts to the main station (sends a call message with 64 bytes of all 5s). After receiving its own response message (64 bytes of all 3s), it fills in the destination address of the substation communication (i.e. the address of the main station that sent the reply) and enters normal communication. If no response message is received, the call message is sent repeatedly every 1 minute.

　　Any substation will ignore the broadcast call information (identified by address) received from other substations. This strategy can ensure the communication between each substation and the master station on the bus network under normal power line operation. When power is restored after a power outage, each substation communicates with the master station of the network or other temporary master stations to facilitate engineering installation.

　　If a substation receives a call from a master station, it should write the address of the calling master station into the destination address of this substation, and then send a response message, and then enter normal communication with the master station. This strategy can enable a substation temporarily added to the management scope of another master station to return to the network, or restore contact with the original master station after power failure.

　　Master Station System

　　The master station system is composed of several communication terminals, 8/16* switches (HUB) and PCs. The communication terminals include RJ-45, RTL8201, W90N740, INT51X1, AFE and Coupler, as shown in Figure 3. INT51X1 also uses PHY mode to form an ETH-PLC router. The MII interface on the host side is connected to W90N740 Ethenet MAC Controller 0, and W90N740 is connected to ETH PHY (RTL8201) through Ethenet MAC Controller 1. The RJ-45 interface is derived from RTL8201 and connected to the network switch; the power line side is similar to the substation system.

　　Each communication terminal has a unique MAC address on the power grid and a unique IP address on the local area network. Each communication terminal acts as a communication host of this network (bus + tree type), communicates with up to 63 substations in a 1:N manner, and manages the substations of this network. In this network, the addresses of all substations are recorded and stored as the destination address for sending data. When a broadcast call (all 64 bytes are 5) is received from a substation, the substation address is recorded and a response (all 64 bytes are 3) is sent back. After that, normal communication is carried out with the substation. (In this case, the substation may not belong to this network.) If a substation under the jurisdiction of this station is lost, that is, its message cannot be received for a long time (such as 5 minutes), it is necessary to call these substations every 1 minute until the response information of the corresponding substation is received and normal communication is entered. If there is still no response after 24 hours, the call is abandoned.

　　The IP machine receives IP packets from each communication terminal and locates the source of the IP packets. It records all substation addresses and clearly identifies the network where each substation is located. This ensures that when sending data, the corresponding communication host can be accurately found and then forwarded to the destination substation under it.

　　in conclusion

　　This article introduces a distribution automation communication system based on OFDM technology, describing its communication substation, main station part and corresponding hardware circuit and software solution. By introducing the INT51X chip, it can be found that OFDM technology has strong advantages in power line communication.

References:

[1]. PLC datasheet http://www.dzsc.com/datasheet/PLC_1248813.html.
[2]. INT51X1 datasheet http://www.dzsc.com/datasheet/INT51X1_1136773.html.
[3]. W90N740 datasheet http://www.dzsc.com/datasheet/W90N740_1095266.tml.
[4]. MAX3232 datasheet http://www.dzsc.com/datasheet/MAX3232_1107769.html.
[5]. ARM7TDMI datasheet http://www. dzsc.com/datasheet/ARM7TDMI_139812.html.
[6]. RS232 datasheet http://www.dzsc.com/datasheet/RS232_585128.html.
[7]. AT93C46 datasheet http://www.dzsc.com/datasheet/AT93C46_143850.html.
[8]. RTL8201 datasheet http://www.dzsc.com/datasheet/RTL8201_586322.html.