Research On Wireless Sensor Networks And Applications In Smart Grids

Sensor Networks is the fourth information technology revolution after PC, the Internet and wireless communication technology. It has important scientific significance and application value. As the typical representative of sensor network technology, Wireless Sensor Networks(WSN) have great potential for many applications in scenarios such as military, environment monitoring, medicine, natural disaster relief, smart energy, intelligent transportation system. It consists of a number of sensor nodes (few tens to thousands) working together to monitor a region to obtain data about the environment, merge and forward the data. Unlike traditional wireless networks, WSN has its own design and resource constraints. Resource constraints include a limited amount of energy, short communication range, low bandwidth, and limited processing and storage in each node. Protocols designed for traditional wireless network are not suitable for WSN, it is necessary to designed resource efficient protocols for WSN in different applications. Some supporting technologies of WSN and its application in power system are systemically and deeply investigated in this dissertation. The research work and the main contributions are as follows:1. Current clustering algorithms has three problem:the traffic load of cluster heads is highly uneven, the number of elected cluster heads deviate the desired number of cluster heads, and it is unreasonable to define the network lifetime in terms of the round when the first node dies. To address these problems, an unequal cluster-based routing protocol based on variable cluster radius is proposed, which can balance energy consumption of cluster head and maintain the stability of the cluster effectively, and thus prolong the network lifetime. Simulation results show the effectiveness of the algorithm.2. As the sync error is proportional to hops from root node in hierarchical time synchronization, which has a high synchronization overhead. An energy-aware time synchronization algorithm is presented. In this algorithm, the additional parameters including the remaining energy level of child-nodes and their degrees are synthetically considered to optimize the process of reference-node selection, thus to reduce and balance energy-consumption among the nodes, prolong the network lifetime efficiently. Experimental results show the validity of the algorithm.3. In WSN, the clock readings in a sensor are derived from oscillators with only limited stability (due to phase noise, thermal noise, aging, etc). Consequently, clocks on sensors are easily affected by temperature variations, vibration, and interference, and can significantly deviate from the reference sources. These make the clock offset and skew nonlinear and time-varying, so it is critical to track and compensate the clock offset and skew. Based on AR clock model, Sequential Weighted Least Squares (SWLS) is used to track clock offset and clock skew according to the most current timestamp in this paper. Due to lower computation complexity and storage requirement, it reduces energy-consumption and prolongs the network lifetime.4. Based on wireless CT, a wireless current differential protection is proposed, and analysis demonstrates synchronization precision and delay of the wireless differential protection can meet the requirements of the current differential protection. Setting the pickup current and slope of the differential relays of power transformer protection is well known a compromise. However, it is difficult to achieve a high degree of security and sensitivity at the same time by conventional percentage restraint differential criteria based on a fixed predetermined characteristic curve. Therefore, a novel adaptive protection criterion is proposed in this paper, which can self-regulate the parameters of percentage differential characteristic (e.g., pickup current, restraint coefficient, and restraint current at the knee point of the slope characteristic) on the basis of operating conditions of power transformers. Test results show that the proposed scheme provides a high operating sensitivity on internal faults, whilst remains higher security on all external faults.5. Intelligent substation is important foundation and support for strong smart grid, and also important field of application of sensor network. In this paper, Based on the communication architecture of substation automations system (SAS) and WSN, a wireless sensor network for intelligent substation is proposed. According to Layered distributed systems of substation, the merits and feasibility of WSN applied in substation communication are analyzed, a practical and feasible applicable scheme as well as WSN nodes with electronic transformers is planned. Lastly a scheme of energy supply and an approach of access of WSNs and Ethernet are presented.