Global inclination toward renewable energy sources and far distance between consumption and generation zones has lead to globalized point -to- point HVDC utilization. On the other hand, the line reliability is much lower than the grids, so in recent years extensive has been conducted into DC grid development. In order to implement a DC grid, Voltage Source Converters must be utilized, but they are so vulnerable to the DC fault. Thus the main research theme in this field is recognized as fast and reliable protection against DC faults. As the DC grids operational frequency is zero, the transmission line impedance in steady state is only composed of small resistive quantity. So, the DC fault level is far greater than counterpart AC system. Additionally, the instantaneous value of voltage in a DC grid is a large constant value which leads to very high rate of DC fault current increase. Alternatively, the oscillating pattern of AC voltage leads to much less current increase rate in AC system. Given this challenge, the fast and discriminative DC fault detection and isolation is the first priority in DC grid protection technology. The conventional AC grid protection methods are capable of detection and clearance of the fault in hundreds of milliseconds which means such methods cannot be used in DC grids. In this thesis already proposed methods for DC fault detection are first introduced and their advantages and disadvantages are discussed. Then, two new fault detection strategies are introduced. In order to assess the proposed methods, the proposed detection techniques are implemented for a four-terminal DC grid protection modeled in PSCAD platform. The proposed methods achieve fast detection response, support fault zone identification, and are also capable of high-resistance fault detection. Moreover, the so-called method of bus – bar capacitor (BCC) current benefits from transient DC fault removal detection. Keywords: 1- DC gird. 2- DC fault. 3- DC grid protection 4- Modular multilevel voltage source converter 5- DC Circuit breaker 6- Bus – bar capacitor current fault detection method 7- Asymmetrical pole inductance fault detection method
