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-Recently, introduction of renewable energy such as photovoltaics and wind energy has gained precedence for greenhouse gas reduction and oil substitution. However, due to the power fluctuation of renewable energy sources, voltage and frequency deviation are problematic in power systems. From the background, smart grids are attracting attention. Already, a smart grid based on AC grid is proposed. However, no study on research is presented or published on a smart grid based on DC grid. DC smart grid has been presented by the authors. The DC smart grid is robust to ac-side fault conditions. However, dc faults in the DC smart grid are not considered in previous papers. This paper analyzes dc faults, their transients, and the resulting protection issues in the DC smart grid. Over-current fault are analyzed in detail and provide an insight into protection system design. The effectiveness of the proposed method is performed by using PSIM environment.

This paper presents the operation under fault and protection of multi DC smart grids connected to the DC transmission line. The smart grid consists of wind turbine and photovoltaic generators, controllable loads, generator side and grid side inverters. The voltage stability of the DC bus voltage is attained by applying the power consumption control of controllable loads, the power control of grid connected converter and power control of renewable plants. On the other hand, the fault protection for short-circuit and low resistance ground fault at the DC transmission line was sighted. Simulation results based on PSIM are provided to demonstrate the effectiveness of the system.

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