| Interconnected large power system has become an inevitable trend for today’s power grid evolution. Also, the larger the power grid is, the more rigorous standard is needed for relay protection. Wide area protection has been proposed by researchers after the research of several serious power outage cases all over the world. It has been noticed that the traditional relay protection only aims at single elements, which cannot work efficiently under current interconnected large power system. There are two major parts in Wide area protection, system stability control and relaying protection. In this thesis, we will mainly focus on relaying protection and it will work as back-up protection. It will work much more efficiently on power system safety operation when combining both Wide area protection and the traditional relaying protection method.Wide area current differential protection (WACDP) is mainly discussed in this thesis. Based on the graph theoretic approach and combining the node branch incident matrix in power system, the algorithm has been proposed using the wide area measurement signal in order to find the wide area back up protection area in real time. The algorithm can automatically find the back-up protection area under the situation of main protection failure, breaker failure and local backup protection failure, in order to act on back up protection to solve the system fault. Since the Current differential protection method is highly up to the synchronization of the current in calculation, the current prediction method is used in this work to solve the possible synchronization accuracy problem in measurement or the delay in data transporting and package lost problem when using wide area measurement system. Therefore, the calculation precision has been improved when calculating differential current.In order to measure the current at different circuit location and realize WACDP, wide area measurement device and wide area protection device with DSP core have been made. The measurement device and the protection device are connected by the SPI module of the DSP to communicate with each other. The tripping command for protection and the alert signal are from the GPIO port on DSP.In the design, the data acquisition and transportation between the measurement and protection devices are realized by optical fiber communication network. The utilization of commercial opto-electronic module, connecting the devices with opto-electrical convenor via Ethernet module, is easy to handle, reliable and time saving.The simulation of the test device, protection device and the fiber Ethernet communication system is realized by RT-LAB real time simulation system. Under the power system Simulink simulation system built up in RT-LAB, the measurement device with a DSP as the core, the protection device and the optical Ethernet communication network are connected to RT-LAB. The simulation test runs with the real hardware devices, which verifies that the design is practicable and efficient.The WACDP method discussed in this thesis provides the online real-time determination for the backup protection area. If the breaker fails or backup protection fails, it can further expand the backup protection area and remove the fault. In this work, applying to power transmission line, using current differential principle, running on the measurement and protection devices with a DSP board, the power grid wide area backup protection is realized combining the optical Ethernet communication system based on the proposed graphical theory for backup protection program. Also, the efficiency and the reliability of the design is verified by RT-LAB simulation experiment.
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