Scheme of distribution automation communication system using OFDM modulation technology
The communication systems that can be used in distribution network automation systems include optical fiber, distribution carrier (DLC: DistribuTIon Line Carrier), wireless, and wired. The 10kV distribution line can be extended 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 transmits and downloads high-frequency signals between the two points through a wave blocker and a combined filter, with clear transmission goals and simple structure. DLC is a one-to-many communication method without a wave blocker. Communication signals are transmitted in 10kV and 380kV distribution networks. Any communication node installed on it can be used as a signal source and receiver, and a transformer (with Transformers and substation transformers) are natural barriers to signals. The distribution line channel has the characteristics of high noise, large impedance change range, and large loss. Therefore, it is much more difficult to use the distribution line power line as the communication medium than the high-voltage transmission line. The use of distribution line carrier communication needs to solve a series of related problems.
Orthogonal frequency division multiplexing (OFDM: Orthogonal Frequency Division MulTIplexing) is an efficient modulation technique, applied to high-speed digital communications on power lines, has the following advantages:
â‘´High frequency band utilization;
⑵Strong ability to resist ISI interference;
⑶Anti-channel fading;
â‘·Anti-noise interference.
Basic principles of OFDM
The basic principle of OFDM is to convert the encoded data from serial transmission to parallel transmission, using N sub-carriers equally spaced in frequency for modulation, and the modulated N sub-carrier signals are added and sent simultaneously. Since the symbol rate is greatly slowed and the transmission time is greatly extended, the ability to resist multipath fading and delay spread is improved, and the effect of inter-symbol interference is reduced. By selecting the carrier interval (â–³ f = 1overT, where, T Is the symbol period) so that these subcarriers maintain the orthogonality of the spectrum throughout the symbol period. At the receiving end, the orthogonal performance between the carriers is used to ensure that the transmitted signal is recovered without distortion.
System composition and working principle
This communication system uses 10kV distribution line as the transmission medium, the network topology is mainly "bus + tree", and different bus segments can be interconnected as an open-loop ring network. The system consists of two parts: the communication sub-station system and the main station system. The main station system is installed in the 110kV substation, collects the telemetry information of all the substations on each 10kV line, and implements safety monitoring, automatic control and protection of the entire distribution network according to the determined algorithm and control strategy. Communication substations are used in conjunction with FTU, TTU, etc. and are distributed throughout the distribution network. Here first introduces the substation system part.
Communication Substation System
The substation system is mainly composed of Intellon's INT51X1 chip, Winbond's W90N740 microprocessor, level shifting MAX3232, analog front end (AFE) and coupler (Coupler).
INT51X1 chip
INT51X1 chip [1] [2] is an integrated MAC / PHY power line transceiver. Using it, high-speed data communication can be carried out through the power line. 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; conforms to FCC- 15 radiation standard; communication encryption; 0.25 m technical processing; working frequency 100MHz; working 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 the available frequency according to the signal-to-noise ratio (SNR) on the channel to resist deep attenuation slots, noise, and multipath fading. Using pilots can achieve synchronization. In INT51X1, MAC adopts the carrier sense multiple access (CSMA / CA) scheme with collision avoidance, and has priority setting and automatic repeat request (ARQ). Through packet encapsulation, it supports the reliable transmission of Ethernet packets. Under the prerequisite of ensuring quality of service (QoS), provide the necessary bandwidth for multimedia payloads (including voice, data, audio and video).
INT51X1 provides three interfaces MII / GPSI, USB1.1 and Ethernet port. The three modes are USB mode, PHY mode and Host / DTE mode. This system selects the PHY mode and interfaces with W90N740 and AFE respectively to complete the conversion between the Ethernet packet and the power packet.
W90N740 microcontroller
W90N740 [3] [4] uses a reduced instruction system and is a 32-bit microcontroller based on ARM7TDMI. There are two Ethernet MAC controllers 0 and 1, 8K instruction cache and 2K data cache. It can be connected with external SDRAM, ROM / SRAM, flash memory or I / O through the external bus interface (EBI). Through the on-chip Ethenet MAC Controller 0 and INT51X1 interface, complete the conversion between RS232 standard data and IEEE802.3 standard data. By connecting an MAX3232 level conversion chip to the on-chip serial UART, the standard RS232 communication function can be realized, and it can be connected to FTU, TTU and other automation equipment.
Hardware connection between INT51X1 and W90N740
Because this system chooses PHY mode, it is connected to the microcontroller through the MII interface. MII (Media Independent Interface) is an industry standard interface that provides the 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 selects 8v AFE.
The coupler (Coupler) is coupled to the 10KV power line.
software design
The initialization of INT51X1 is done by configuring EEPROM (AT93C46), and it should write its own MAC address (this MAC address is unique).
Because ARM7TDMI is the core of W90N740, it is programmed with ARM7 assembly language. Realization function: Receive FTU / TTU data from RS232 port, convert to 802.3 protocol IP packet, send to INT51X1 via MAC; receive INT51X1 data via MAC, unpack, send FTU / TTU via RS232 port.
W90N740 initializes after power on, and sends special call information to the master station after the initialization is completed (64 bytes all 5). After the handshake process is completed, it enters the communication operation.
The handshake process is as follows:
First, the sub station broadcasts to the master station (sends call information 64 bytes all 5), after receiving the response message (64 bytes all 3), fills in the destination address of the sub station communication (that is, the address of the master station sent back ) To enter normal communication. If no response information is received, the call information is repeatedly sent every 1 minute.
Any sub-station receives the broadcast call information (identified by the address) of other sub-stations, and ignores it. This strategy can ensure the communication between each sub-station and the main station on the bus network under the normal operation of the power line. When the power is restored after the fault is cut off, each sub-station communicates with the main station of this network or other main stations temporarily contacted, which is convenient for engineering installation.
If the sub-station receives a call from a master station, write the address of the calling master station to the destination address of the sub-station, and then send a response message, and then enter normal communication with the master station. This strategy can make the sub-station temporarily added to the management scope of other main stations return to this network, or restore contact with the original main station after power failure.
Master station system
The main station system is composed of several communication terminals, 8 / 16-port exchange (HUB) and PC. The communication terminals include RJ-45, RTL8201, W90N740, INT51X1, AFE and Coupler, as shown in Figure 3. INT51X1 also selects PHY mode to form an ETH-PLC router. The MII interface on the host side is connected to the W90N740 Ethenet MAC Controller 0, and the W90N740 is connected to the ETH PHY (RTL8201) via the Ethenet MAC Controller 1, and the RJ-45 interface is led out from the RTL8201 to connect 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 network and a unique IP address on the local area network. Each communication terminal serves as the communication host of this network (bus + tree), communicates with up to 63 substations 1: N, and manages substations of this network. In this network, record and store the addresses of all sub-stations as the destination address for sending data. When receiving a broadcast call from a substation (64 bytes all 5), record the substation address and send back a response (64 bytes all 3). After that, normal communication with the sub-station. (In this case, the substation may not belong to the home network.) If you lose a substation under the jurisdiction of this site, that is, you cannot receive its message for a long time (such as 5 minutes), you need to call these substations every 1 minute Once, until the response information of the corresponding sub-station is received and normal communication is entered. If there is no response after 24 hours, the call is abandoned.
The IP machine receives the IP packets from each communication terminal and locates the source of the IP packets. Record the addresses of all the sub-stations, and clarify which sub-station is in the network where the communication host is located. This can ensure that when sending data, the corresponding communication host can be accurately found and then forwarded to the destination sub-station under it.
in conclusion
This paper introduces a distribution automation communication system based on OFDM technology, and describes its communication sub-station, the main station part and the corresponding hardware circuit and software solution. By introducing the INT51X chip, it can be found that the application of OFDM technology to power line communication has a strong advantage.
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