Recovering the Communication of Power System Protection Quickly Via ZigBee Wireless Network for the Intelligent Protection Center Research

Transcription

1 2016 International Conference on Material Science and Civil Engineering (MSCE 2016) ISBN: Recovering the Communication of Power System Protection Quickly Via ZigBee Wireless Network for the Intelligent Protection Center Research Zi-kun ZHANG 1, Zhi-qian BO 2,, Xin-zhou DONG 3, Shen-xing SHI 3, Xin CHEN 3, Rui-tong LIU 4 and Wei FAN 4 1 Tsinghua University, Beijing, China, 2 Xuji Group Corporation, Xuchang, China 3 State Key Laboratory of Control and Simulation of Power System and Generation Equipments, Tsinghua University, Beijing, China 4 Electric Power Research Institute, State Grid Liaoning Electric Power Supply Co., Ltd, Liaoning, China @163.com The Intelligent Protection Center can make it to be possible to recover the power system protection quickly after the power grid was destroyed by some disaster. Then there will be one question: how to recover the communication channel rapidly between information island based on the Intelligent Protection Center. Only if this question is solved, can power grid be reconstructed quickly. In this paper, ZigBee technology is chosen to rebuild the wireless channel for information island. According the theory of ZigBee technology and power system protection, the maximum distance of this wireless channel can be gotten by calculation and simulation. Keywords: Intelligent Protection Center; ZigBee technology; "information island"; Directional protection. 1. Introduction In recent years, with the development of national economy, the social electricity consumption is increasing quickly. At the same time, the power transmission and distribution technology is also following up. When the power grid was destroyed after a disaster occurred, how to recover the power grid quickly is a question people need to pay attention to. The recovery of power grid includes the recovery of primary equipment and power system protection. What restricts the speed of the recovery of power grid is the speed of the recovery of power system protection. If the Intelligent Protection Center is used in power grid, the information island can be found rapidly after disaster by fast topology analysis. After that, the wireless channel for those information islands can be rebuilt Corresponding author 594

2 quickly, so the time being used to recover the power system protection is decreased. In this paper, ZigBee technology is chosen to rebuild the wireless channel. This choice is made after we referred to the performances of many short-distance wireless technologies like Wi-Fi, Bluetooth, etc. At present, the mainstream ways of longitudinal joint protection include directional protection and differential protection. According to the theory of those two protection, it is decided to rebuild the directional protection by using ZigBee wireless network. After this decision, a lot of calculation and simulation has been done to get the requirement for the ZigBee wireless network. At the same time, it is possible to calculate the maximum distance of this wireless channel. 2. The Power System Protection At present, people usually use the directional protection or differential protection to protect the power grid. Directional protection and differential protection have different principles. If the differential protection is used in power grid, then it is necessary to know the difference of values of voltage or current of both ends of the line. In order to get the difference, the speed of communication should be very fast so that the protection device can get the electric parameters of the other end of this line almost at the same time it monitors the electric parameters itself. Because of it, the differential protection has a high requirement for time delay. However, if the power grid uses directional protection as its protection, the requirement for time delay is not as high as the differential protection s. What s more, the directional protection needs less electric parameters, only the polarity information, so the requirements for band width is lower. Those two protections have different requirements for the performance of communication. We compared those differences in Table 1. Requirements of Protection Technology Directional Protection Differential Protection Table 1. Requirements for communications comparison of different protection technology. Time Delay Band Width Error Rate Not Very High Low High Very High Medium High According to TABLE I, it is obvious that the requirements of directional protection is lower. In order to improve the possibility to recover the power 595

3 system protection, the directional protection is chosen to protect power grid. In the calculation and simulation, traveling wave directional protection is used as an example. 2.1 Principle of Traveling Wave Directional Protection When there is a fault on transmission line somewhere, it is possible to analyze this system by using superposition theorem. According to the superposition theorem, the electrical quantities can be divided into fault components and normal components. m im F i n n U m U n x Power System with Fault F U F Fault component system Figure 1. Process of analyzing the traveling wave. In the theory of traveling wave directional protection, the only requirement is to analyze the fault components. In order to get those components, it is required to put a voltage source which is equal and opposite compared to the voltage before the fault to the place where the fault occurs. After that, the forward traveling wave and back traveling wave can be gotten. This analysis process is shown in Fig.1. According to this process, the expressions of forward traveling wave and back traveling wave are gotten as follows. 596

4 2 1 C u u Z i 2 2 C u u Z i (1) (2) It can be concluded that if the fault is forward, the monitoring points will get back traveling wave, so the voltage traveling wave and current traveling wave have different polarity. On the other hand, those two kinds of waves have the same polarity for the monitoring points. According to these principles, we can judge the direction of fault. 2.2 Refraction and Reflection of Traveling Wave incident wave refracted wave 1 2 A ZC1 Z C 2 reflected wave Figure 2. Reflection and refraction of the traveling wave. The traveling wave will refract and reflect on the transmission lines, what is shown in Fig.2. But what is required is to test the initial traveling wave to judge the direction of fault. So there will be a requirement for communication. In order to get the fault interval, there must be at least two monitoring points at both ends of the line. When the monitoring points communicate with each other, they must transform the information of initial traveling wave, so the delay jitter should less than the time during which the reflected wave or refracted wave transfer to any of those monitoring points. After the initial wave produce at the fault point, the time it needs to get to the monitoring point is: t t 0 (3) The time the reflection wave produced at the fault point needs to get to the monitoring point is: 597

5 t 3 t (4) The time difference between initial wave and reflection wave is decided by the distance of fault point and monitoring point. It is obvious that the delay jitter must not be longer than this time difference. According to our calculation, the delay jitter shouldn t be longer than 4ms if the transmission lines is longer than 600km. 2.3 Scheme of ZigBee Wireless Network Since the directional protection is chosen, it is necessary to design a scheme of ZigBee wireless network aimed at the requirements of this protection. The scheme can be seen in Fig.3. 0 Protection device Protection device Disaster area ZigBee wireless network C R R E Figure 3. Reflection and refraction of the traveling wave. As is shown in Fig.3, when a disaster occurs and a part of the communication channel is cut up because of it. However, the directional protection needs the communication between both ends of a line, so it is necessary to rebuild the communication channel to recover the protection. In this scheme, a ZigBee wireless network replaces the original fiber channel, which can be used by the protection devices to communicate with each other. Then the power system protection will be recovered quickly via ZigBee wireless network. 2.4 Simulation of ZigBee Wireless Network Requirements Numerical Value Table 2. Requirements for communications comparison of traveling wave directional protection. Time Delay[ms] Band Width[Mbit] Daily Protection of the Total Locking Time[ms] Delay Jitter[ms] 598

6 After the calculation and simulation, the requirements for communication of traveling wave directional protection are shown in Table 2. Because of those requirements, there will be a maximum distance of ZigBee wireless network. At present, the band width of ZigBee wireless network and electric communication network is more than 10 Mbit. And the daily protection of the total locking time is less than 500ms when using SESR and BBER to calculate. So what restrict the distance are requirements of time delay and delay jitter. In order to get the maximum distance of the wireless network, we design a simulation experiment. According to the scheme of ZigBee wireless network, we use OPNET 14.5 to create two scene models which are shown in Fig.4. Scene model 1 Scene model 2 Figure 4. OPNET 14.5 simulation scene models. In model 1, there are seven routers in this ZigBee wireless network, and the information sent from end device needs to be forwarded seven times by router. In model 2, however, there are only six routers which can forward information. The 599

7 different time delay and delay jitter is shown in Fig. 5. According to the curve of time delay, the delay jitter can also be gotten from Fig. 5. It is concluded that the number of router should be less than seven, otherwise the time delay will exceed 20ms, which cannot meet the requirements of traveling wave directional protection. When there are six routers in the ZigBee wireless network, both the time delay and delay jitter are less than the numerical value required. According to our simulation, the maximum distance one router can forward is about 100m when it is working in low power state. So the maximum distance of ZigBee wireless network is about 600m. Figure 5. Curve of time delay. 3. Conclusion In the process of recovering the power system, it is necessary rebuild the communication channel by using ZigBee wireless network for Information Island. Then it is possible for those relay protection devices in Information Island to communicate with each other. When a disaster occurs and Information Island appears in the disaster area, relay protection devices cannot communicate with wired way, then it is possible use wireless network to recover the communication. As the development of power grid, Information Island will appear more and more frequently. So it is very important to do research on how to realize wireless communication in power grid. Acknowledgments Work partially supported by grant of the Swiss National Science Foundation. 600

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