Fault Directional Element Studies For Smart Distribution Grid With DG

The requirements of renewable energy generation, such as wind power and solar energy power, in the power system is gradually increasing, because of the approaching depletion of traditional fossil fuel and the strengthening of the consciousness of environmental protection and energy saving. For network security and economy, those renewable energy sources are integrated into distribution networks separately and constitute the distributed generation (DG). And the DG not only improves the energy structure of power grid, but also achieves fewer line loss and higher power quality. At the present time, the important research projects in DG are how to make distribution network to accept more DGs and improve the reliability of distribution network with DGs.The integration of DG can improve the reliability and power quality of the distribution networks, but the traditional relay protection strategies are inapplicability in distribution network with DGs. With the incorporating DGs in distribution network, there will be fault current and voltage at both side of fault point, when fualt occurs, which is quite different with the traditional distribution network. It is very hard to avoid the refuse or wrong operation if the traditional rules of protection strategy and action value was continued to be used. One of the most effective and convenient method solving this problem is installing the directional element to the original protection.The DGs that connected to the intelligent distribution network have different types, capacities and fault attributes. On the other hand, the distribution network itself has different fault attributes to transmission network. So the exiting directional elements cannot be used in distribution network with DGs directly. In order to find out the proper directional elements that can be used in intelligent distribution networks, it is necessary to analyze the characteristic of distribution network and DG, the effect of DG integration to distribution network and the advantage and disadvantage of existing direction elements. Last but not least, it is necessary to analyze the fault current and voltage characteristic of distribution network and DG.This paper has done some research on the principle, operation characteristic and operation scope of90°connection power directional element, zero sequence power directional element, distance protection with directional characteristic and power frequency variation directional element. Then study on the influence of DG connected to the distribution system was conducted carefully, mainly including the analysis on conection positions, DG types, control strategies, compensation devices, fault current limiting Devices and so on. Finally, the necessity of the directional element based on new principle and their indispensable characteristic was concluded.Based on the analysis above, this paper proposed two methods identifying the fault direction, one called voltage variation amplitude comparison directional element, and another called positive sequence polarizing voltage directional element. In order to ensure the speed of fault direction judgment and overcome the influence of fault resistance, the former proposed new setting rule to setting impedance, which can both satisfy the frequent change of impedance characteristics, and automatically adapt to changes in the impedance magnitude, thus ensuring the high reliability of direction element. In order to improve the reliability of the fault direction determination, the latter setting rule makes the setting impedance through comprehensive considering of the load impedance magnitude, line impedance angle and minimum limit. This rule inherits the advantages of traditional direction, such as no voltage dead zone, less affected by the load impedance characteristics and so on. At the same time, it makes up the disadvantage that can not be applied in the whole process of fault.Simulation and analysis of directional element have been conducted by DIgSILENT software, and the model of simulation system is shown in Fig.6. There are all kinds of DGs in the10kV distribution network, including solar energy model PV connecting grid through inverter, doubly-fed induction Machine model DFIG, small capacity synchronous machine model SM and static var device model SVS. The simulation of different fault styles on the DIgSILENT proved the justifiability and reliability of this fault direction identification criterion.