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A control topology input-parallel–output-series converter system is investigated, which is suitable for high-voltage and high-output power applications. In this control method input current sharing (ICS) and output current sharing (OVS) taken into account. As observed, ICS is automatically attained as OVS sharing is controlled. In this design, two modules are connected in series, Module 1 provided variable voltage but in Module 2 by using three-level neutral-point-clamped converter and designing a comprehensive control feedback system, which regulates the DC voltages at the constant magnitude and constant frequency, helps to provide constant output voltages in the event of uncertain disturbance and load variation, A feedback double closed-loop control topology (inner current loop control and outer voltage loop control) with the combination of sliding mode control and a proportional–integral control method. A simulation model is built, by using MATLAB/Simulink, and the simulation results show excellent performance and feasibility of this system. This will be beneficial for research and improvement for the grid-connected converter.

The high power dc-dc converters emerged as the key circuit element for the future multi-terminal dc grid due to rapid increase in the number of HVDC transmission lines. Beside power conversion, it can provide many functionalities such as bidirectional power flow, power flow control and dc fault isolation. Recently, many high power dc-dc converter topologies have been proposed in the literature for applications in the dc grid. The problem with these, however, is that they are not classified in to various categories based on different parameters such as internal isolation, power density or conversion ratio. This review proposes a taxonomy of most efficient state-of-the-art dc-dc converter topologies based on the structural affinities. Most prominent converters are classified in to two major groups based on the internal isolation. Then, the basic structure, operating principle and performance capabilities accompanied by key advantages and major limitations of each converter is discussed. Furthermore, this study highlights the most suitable converter topologies for the application of HVDC grid interconnection.

Recently, there is rapid increase in the number of voltage source converter based HVDC transmission lines in order to couple the large offshore windfarms to the main ac power system. In comparison to the well-established point to point HVDC links, the multi-terminal HVDC grid is considered as the most efficient solution for high renewable energy penetration. It can provide many advantages such as secure power flow control, increased redundancy and higher efficiency. However, it still requires key enabling technologies such as dc-dc converter and dc circuit breaker for practical realization. Although, the dc-dc converter can provide many functionalities such as bi-directional power flow and dc voltage matching. However, the dc fault isolation in multi-terminal HVDC system has great importance. This paper firstly proposes a taxonomy of the most suitable converter topologies based on the structural affinities. Most prominent converters are classified into two major groups based on the internal isolation. Then, the basic structure, operating principle and performance capabilities of each converter is discussed in detail. Furthermore, the most promising dc-dc converter topologies are reviewed from the aspect of dc fault blocking capability and are compared in terms of cost effectiveness. Finally, we highlight prominent open research issues and provide few recommendations for the future research work.

With the expending scale of the power grid and the substantial development of renewable energy generation system, the demand for power quality is becoming more and more significant. The doubly fed pumped storage can be an effective method to control the power of the power grid and restrain the power grid fluctuation caused by renewable energy generation. First, this study introduces the structure of doubly fed pumped storage unit, and analyses the mathematical model of reversible hydraulic pump turbine, doubly fed induction generator (DFIG) and control strategy of the converter. On the basis of the model of reversible hydraulic pump turbine and DFIG, control strategies of the dq decoupling and synchronous rotating coordinate system are studied to output the active and reactive power. Second, the models of generating mode and pumping mode for the doubly fed pumped storage unit are established. Third, for the grid power regulation, active power and speed control strategy in generating mode, pumping mode are put forward. Finally, the model of doubly fed pumped storage unit of 300 MW is established in MATLAB/Simulink. The proposed control strategy is verified by the simulation results.