At present, the construction of smart grids is gradually being carried out on a global scale. When constructing a new generation of AMI (Advanced Metering System) systems, power supply companies will consider adopting a hybrid network of multiple communication technologies to overcome some of the inherent shortcomings of the existing technologies. In order to meet the requirements of system performance and return on investment. These AMI system requirements include: high-reliability bi-directional channel for direct communication with each meter, extremely high single-reading success rate, real-time fast system response, adequate data bandwidth remote break/delivery, cardless remote prepaid Fees, remote adjustment of future rates, etc. Of course, the most important thing is to get a high return on investment by reducing line loss and improving efficiency.
Although there are many manufacturers involved in the field of automatic meter reading in China, and the products are complex and diverse, the basic structure of the automatic meter reading system is shown by comprehensive comparison.
The upper computer is a computer system placed in the management department or the monitoring center; the concentrator is a device for collecting data in the area; the collector is a device for collecting data of the electric meter in a partition. In this system networking scheme, the upper channel can use wired local calls, wireless communication (including GPRS/CDMA), optical fiber communication, etc., and the reliability of these channels is unquestionable. However, the communication problem of the automatic meter reading system is now more in the lower channel, that is, from the concentrator to the meter. This channel has a large amount of use and the most heated debate.
Some of the so-called "last mile" technologies, such as low-voltage power line carrier (PLC) and RS485 wired communication, have various drawbacks. (See Table 1)
With the continuous development of wireless communication technology, in recent years, there has been a technology for wireless networking requirements for low-cost devices, called ZIGBEE, which is a close-range, low-complexity, low-power, low data rate, low-cost Two-way wireless communication technology, mainly suitable for automatic control, remote control field and home equipment networking.
Due to the superior characteristics of ZIGBEE, wireless networking based on ZIGBEE technology is a suitable means for implementing downlink channels. It is suitable for the networking of some short-distance wireless networks, such as office, office buildings, dormitory buildings, factories and other wireless meter reading networks. It is suitable for internal energy consumption monitoring and management systems, especially for some wiring difficulties. In the energy management system. And if it is combined with mature industrial Ethernet and GPRS/CDMA uplink channels, and the background management master station constitutes a complete collection and monitoring system, it can provide an effective solution for remote management.
Table 1. Comparison of ZIGBEE with other “last mile” technologies
1.2 ZIGBEE technical characteristics
The ZIGBEE protocol is based on the IEEE 802.15.4 standard. From the release of ZIGBEE V1.0 in 2004 to the latest version of ZIGBEE 2006 with the addition of the ZIGBEE-PRO extended instruction set, ZIGBEE functionality continues to grow. ZIGBEE has powerful device networking capabilities (see Figure 2) that support three main types of self-organizing wireless networks, namely Star, Mesh, and Cluster Tree, especially Mesh structure, with strong network robustness and system reliability. Compared with the commonly used short-range wireless communication technologies such as wi-Fi and Bluetooth, the main features of ZIGBEE are:
(1) The working cycle is short, the power consumption of the transmitting and receiving information is low, and the RFD (Reduced Function Device) adopts the sleep mode, and can enter the sleep mode when not working.
(2) Low cost. By greatly simplifying the protocol (less than 1/10 of Bluetooth), the requirements for the communication controller are reduced. With the 8051 8-bit microcontroller, the full-featured master node requires 32KB of code, and the sub-function node has as few as 4 KB of code. .
(3) Low rate, short delay. ZIGBEE's maximum communication rate of 250 kb / s (working at 2.4 GHz), to meet the needs of low-speed data transmission applications. ZIGBEE's response speed is relatively fast, generally only 15ms from sleep to working state, node connection into the network only 30ms, further saving energy. In comparison, Bluetooth requires 3 to 10 S and Wi-Fi requires 3 S.
(4) Close range, high capacity. The transmission range is generally between 10 and 100 m. After increasing the RF transmission power, it can also be increased to 1 to 3 km. This refers to the distance between adjacent nodes. If the relay is used for routing and communication between nodes, it can reach several hundred meters or even several kilometers after expansion. ZIGBEE can be used in star, patch and mesh networks. A master node manages several child nodes, and at most one master node can manage 254 child nodes.
(6) Exemption from the license band. Direct sequence spread spectrum in the Industrial Scientific Medical (Industrial Scientific Medical, ISM) band, respectively, 2.4 GHz (worldwide), 915 MHz (USA) and 868 MHz (Europe).
1.3 Architecture of ZIGBEE wireless communication meter reading system
Figure 3 is a network topology diagram of a wireless communication meter reading system. The whole network is mainly composed of four parts: a meter, a local wireless communication network, a remote communication network, and a data exchange device. The architecture of ZIGBEE wireless communication meter reading system also inherits the wireless communication meter reading system. Its structure is almost the same as that of the wireless meter reading system. The whole network is also composed of measuring instruments and ZIGBEE collectors (responsible for communication with metering instruments). ZIGBEE network terminal (responsible for interfacing with the upper communication network, such as industrial Ethernet, etc.), upper communication network and data exchange storage device. The networking mode generally adopted by the ZIGBEE wireless communication meter reading system is the mesh network of MESH. The MESH network can better guarantee the communication quality and ensure that the communication state of other nodes is not affected when a single node fails.
1.4 ZIGBEE wireless meter reading solution
Whether wired or wireless, meter reading systems are always affected by factors such as the environment, distance and occasions, each with its own solution. Based on the ZIGBEE meter reading system, it will not break away from this constraint. It will also vary depending on factors such as environment, distance and occasion, and there are different solutions. Since the positioning of ZIGBEE is short-distance communication, it is relatively less considered when applied to wireless meter reading networks such as office buildings, office buildings, dormitory buildings, factories, etc.
Figure 4 is a schematic diagram of the composition of a single subnet of the ZIGBEE wireless collection system. The system architecture described in the previous system is the same, mainly composed of the uplink network industrial Ethernet and the downlink network ZIGBEE wireless local area network. The entire subnet consists mainly of an electric meter, a ZIGBEE collector, and a ZIGBEE network terminal. The electric meter can use ACREL's 1352 series card-type electric energy meter and ACR network power meter. They use RS485 communication with ZIGBEE collector and adopt MODBUS communication protocol. Up to 32 tables can be connected under ZIGBEE collector; due to limited MODBUS address, Up to 255 tables can be connected to the entire ZIGBEE subnet; in order to ensure the reliability of the communication connection, sometimes the network node (the ZIGBEE collector is configured as a routing function) with several routing functions depends on the environment and the distance. In order to ensure the communication of some remote nodes is normal; in addition, considering the congestion degree and real-time transmission of the wireless network, it is recommended that the number of wireless nodes (ie, ZIGBEE collectors) in the entire subnet should not exceed 60, thus ensuring the network. Communication quality in the middle. Each ZIGBEE subnet has its own ID identification and band division, which can help expand more table counts.
1.5 Performance parameters