Once the pole-to-pole dc fault occurs in a multi-terminal HVDC system, it is desirable that the stations and dc solidstate transformers on healthy cables continue to operate, rather than blocking. To reduce the fault current of a modular multilevel converter based dc solid-state transformer, active fault current control is proposed, where the dc and ac components of fault arm currents are controlled independently. By dynamically regulating the dc offset of the arm voltage rather than being set at half the rated dc voltage, the dc component in the fault current is reduced significantly. Additionally, reduced ac voltage operation of the dc solid-state transformer during the fault is proposed, where the ac voltage of transformer is actively restricted in the controllable range of both converters in the transformer to effectively suppress the ac component of the fault current. The fault arm current peak and the energy absorbed by the surge arrester in the dc circuit breakers are reduced by 31.8% and 4.9% respectively, thereby lowering the capacities of switching devices and circuit breakers. The novel active fault current control mechanism and the essential control strategy are presented and simulation results confirm its feasibility. I. INTRODUCTIONThe deregulation of international energy markets and the trend to decentralized power generation are increasing the demand for advanced power electronic systems. For this application field multilevel converters with a high number of voltage levels seem to be the most suitable types, MMC was first proposed for HVDC applications at 2003from a Marquardt and it is first employed commercially in Trans Bay Cable project in a San Francisco. In the present scenario the Modular Multi-Level Converter (MMC) is becoming the most common type of a voltage source converter for HVDC applications. DC fault protection is an issue to be resolved for the development of modular multilevel converter (MMC) based HVDC transmission systems. [1][2][3][4]. The dc circuit breakers (DCCBs) have the potential to isolate a dc fault and protect stations from damage and limiting reactors are series connected with the fast acting DCCBs (e.g. solid-state DCCBs, hybrid DCCBs) to limit the fault current di/dt and decrease the fault current peak. Recently, the concept of the dc solid-state transformer (DCT) has been proposed which uses active controlled power electronic components to optimize converter performance. [6][7][8][9]. Similar to the ac transformer, the dc solid-state transformer can adapt the dc voltage to any higher or lower voltage level. By blocking all the converters of the dc solid-state transformer, dc faults can be isolated without significantly affecting the healthy system parts. Thus, the dc solid-state transformer appears the only approach to connect and interconnect existing HVDC links with different dc voltages. These transformers are available in different configurations which include the thyristor based solid-state transformer[10],the dual-active bridge (DAB) transformer [1...