Solar energy (also called photovoltaic power station) mostly consists of hundreds and thousands of photovoltaic panels.Stability of Photovoltaic power station is related to the output power of the photovoltaic array and even the working statue of every panel. How to monitor and diagnose the failure is the main problem for managing the photovoltaic station. At present, the main problem of the photovoltaic is “hot spot” .The so called hot spot phenomenon is part of the photovoltaic cells in photovoltaic panels monomer the obscured for a long time.According to Kirchhoff Voltage Law, these obscured photovoltaic cells monomer will get negative voltage, becomes the load circuit, and other work in the form of heat consumption of monomer of photovoltaic battery power, the heat accumulation for a long time can damage photovoltaic panels of packaging materials, even destroy the physical structure of the photovoltaic panels, and will cause permanent damage.
At present, there are two ways for monitoring Photovoltaic array direct method and indirect method. Direct method is to directly measure the voltage and current of each panel, and then transfer the data to the pc by mod bus. But there exist a lot of problems in planning layout, default interface, line detection, line capacity etc..Indirect is to judge the working statue of battery by measuring the temperature difference of the battery. However, this method has some defects, such as can not distinguish between the state of the temperature difference is not obvious, real-time performance, accuracy and efficiency of fault detection depends on the testing equipment (infrared thermal imager) level, is not easy to implement online fault analysis and alarm, and so on.
In comparison of these two methods, using wireless sensor to monitor the photovoltaic is of incomparable superiority. Wireless sensor transferring the data to three dimensional, space,conductor medium is no needed thus saving the lavor cost and maintain cost.
This Solution is based on ZigBee Technology, Four-Faith ZigBee device is need for networking.
1 Solution
1.1 System
The whole system is divided into three parts: terminal acquisition layer, data transmission layer and application managing layer. Terminal acquisition acts as performer, including monitoring node and center node. Monitoring node will transfer the date to the center node in form of multi-hop. Center node will transfer the data from Zigee monitoring area to the data transmission layer. Data transmission layer will act as agent will transfer the data from collection layer to application layer. Application layer is the monitoring center,as as the decider, responsible for data analysis and system managing maintain to realize remote monitoring in time and alarm.
1.1.1 Collection Terminal Device
It needs to monitor multiply parameter, collection terminal device is used for realizing the parameter collection for each monitoring node. Such as photovoltaic modules voltage, Current sampling module is responsible for sampling photovoltaic components of voltage and current values to judge if there is failure. Temperature, humidity and light sensor is responsible for collection the environment data, and provide some data support. Acquisition terminal will transfer it via rs232 to ZigBee transfer unit.
1.12 ZigBee Data transmission Unit
ZigBee data transmission layer use Four-Faith F8914 and F8114 to establish the data transmission turn channel. When the ZigBee device receive data from acquisition layer or through ZigBee, the data will be transf fed to the managing layer. Meanwhile, application layer will also send the control command to each node via ZigBee network to realize double way communication realize remote monitoring.
In this case, we use two model of ZigBee Products: One is ZigBee terminal F8914, another is ZigBee GPRS modem F8114. F8914 fistly send the data to the center node via ZigBee network, then F8114 works as center node, then transfer thedata to the monitoring center via gprs network.
1.1.3 Application managing layer
Application layer is mainly center monitoring system, which consist of pc server and platform software. When the center system receive the data from the transmission layer, it will analysis the data and operate according to the analysis like send our alarm information or send out control command.
1.2 Network Structure
According to the distance between the managing center and the photovoltaic array, we use two kinds of structure.
When the center is near the photovoltaic array, we will use only F8914, only Zigbee network is needed.
As below shows:
When the center managing center is far away from the photovoltaic,F8914 will work as monitoring node and F8114 as center node, F8914 will transfer the data to F8114 via ZigBee network, and then F8114
RS422 ZigBee will transfer the ZigBee data to TCP/IP data and then via gprs network to send to managing center.
When F8114 transfer the data to the monitoring , there are several access to the network
1. The center use APN special line, all the terminal use fixed ip. The client center access the mobile company via gprs wireless network, an GRE channel is established between GGSN and router. An special APN is assigned for the client, others can not apply for the APN.The user can establish RADIUS server internal to ensure the internal security. User can build DHCP server, and assign the internal address for the authentication user.
This solution is greatly improved in real time , security and stability compared with the previous solution.
The control center adapt ADSL connection, and use IP+DNS. User need to open the dynamic domain name .IP MODEM need to connect to the DNS server by domain addressing way first, and then find center pubic IP by DNS server ,establishing a connection.This way can greatly save the cost of the public static IP, but stability is subject to the stability of the DNS server, so looking for reliable DNS service is necessary .This solution is suitable for small-scale applications.
3.Control center use ADSL INTELNET public network connections, such as using public fixed IP service.This solution need o apply to operators for ADSL broadband business, center has a public static IP.IP MODEM directly connect to the center. It is reliable connection, this solution is recommended.
Other network
Four-Faith Zigbee device not only can be used for transparent data transmission, also support IO acquisition. In this solution, we can also use Zigbee device for terminal IO acquisition and data transmission. Four-Faith ZigBee device supply 5 IO.3 analog input ,2 digital input, puzzle input and output.Analog voltage input is 0-5v, current input is 0-20ma, digital input is 0-0.33v, acquisition accuracy is 12bit.
When Four-Faith device used as IO acquisition, the working mode should set as AT or API. The difference between this working mode and transparent mode is that for transparent mode, the center send out the data in the form of broadcast, the terminal can not be set unless the address is changed every time. For AT mode,it is used in the data acquisition terminal and receiver terminal, IO acquisition report the data in the form of AT command. Meanwhile, under the mode of AT,the center mode can use AT+TXA= Address, the value is for which
ZigBee terminal to send to. The function of API mode is same as AT mode, hexadecimal is used by API mode, and there is check code, xor and inspection.
When ZigBee Device is used as IO data acquisition, the working mode should be set as AT. When data is collected, there are two mode for data reporting. When the reporting time is set as non zero value, it is automatically reported,the IO will report the data every several time according to the setting time. When the reporting time is set as zero, it needs the upper software to send the command AT+Nvn=to consult the data, analog acquisition need to be transferred get the real value as below:
Voltage=(collected value)*3.3*20.16/(2047*12.1)(V)
3.3: Voltage
20.16: Resistance Value// Two resistance 12.1(Resistance 1)+8.06(Resistance 2)
2047: ADC Range //211
12.1:(Resistance 1)
Current=（Collected Value）*3.3*1000/(2047*150) （mA）
3.3: Voltage
1000: mA unit transfer
2047: ADC Range //211
150：Resistance Value
Meanwhile, when ZigBee device used as IO acquisition, the upper software should be developed according to Four-Faith protocol, that is should according to the AT command in the user manual.
2. Features:
1.Wireless, reduce the complexity of the photovoltaic array monitoring system
2.Low cost: Simple and free of patent change
3.Intelligent, each node will set up the connection automatically
4.Support the star, tree, the net network with strong network connection capability, can support 65000 nodes
5.Industrial grade design with metal shell, DC12V/500mA power supply.Operate temperature:
-25~+65ºC. Storage temperature -40~+85ºC. Relative humidity 95%(unfreezing); Multi WDT design, suitable for oil well application.
6.The communication range for ZigBee terminal is 2k meter.
7.No cost is needed for ZigBee communication, large cost is saved for the whole project.
8.Easy configuration, easy installation
With the increase of the request and technology, ZigBee will have a broad application in remote monitoring.
Ⅲ. Conclusion
ZigBee wireless data transmission is stable, reliable and convenient. Low installation cost, easy maintaining, no need to upgrade, no need to rent the public internet, also do not need huge operation cost. This solution is the best one for modern data acquisition and remote transmission system.
