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This paper proposes a unified power flow algorithm based on the Newton–Raphson (NR) method to obtain a feasible solution for hybrid AC/DC grids incorporating voltage source converters (VSCs). The power flow model of hybrid AC/DC grids is composed of an AC grid model, a VSC station model, and a DC grid model. Detailed converter losses are involved in VSC station modeling. In the DC grid model, a unified dc control expression is applied to adapt to different control methods. To obtain sparse Jacobian matrix, the mismatch equations and unknown variables are classified into three categories corresponding to AC, VSC, and DC models. Then the proposed algorithm achieved quadratic convergence with the NR method. Once the solution is infeasible (variable out-of-limit) or diverging, the constrained load flow (CLF) method is applied automatically to achieve a feasible solution by revising the references of ac/dc grids. Simulations were carried out for the hybrid AC/DC test system composed of two AC grids and two DC grids. The results under different DC voltage control modes show that the proposed algorithm is able to obtain a feasible solution with high accuracy and convergence speed.

In this paper, the electromechanical model of VSC HVDC-integrated AC/DC hybrid system is established. The VSC station model is comprised of the physical model and the control model. In the mathematical control model, the inner current-loop regulator, the outer loop regulator, the modulation and the phase-lock loop are taken into consideration. The proposed model is validated through the simulation of a 5-port VSC-HVDC integrated IEEE 39 system with DIgSILENT. The transient-stable closed-loop control algorithm is utilized and the stability study is performed, demonstrating the accuracy of the proposed model and the effectiveness of the control scheme in hybrid systems.

Hybrid HVDC system combines the advantages of line commuted converter HVDC (LCC-HVDC) and voltage sourced converter HVDC (VSC-HVDC) and has become a popular trend for future grids. In this paper, the hybrid HVDC system, integrated in either a synchronous grid or connecting asynchronous grids, is introduced and studied. The LCC station model, VSC station model as well as the dc network model are established. The tap ratio of converter transformer is considered and its effect on HVDC is analyzed through case study. The control scheme of the hybrid HVDC system is proposed and the corresponding equation is developed. Based on this, a sequential iterative power flow calculation algorithm of AC/DC networks incorporating hybrid HVDC is proposed, and its validity and effectiveness are verified by case study.

Hybrid HVDC is a novel topology which combines advantages of both line commuted converter (LCC) and the voltage source converter (VSC). In this paper, the power flow model of hybrid HVDC, which contains mathematical models of LCC and VSC stations, is built first, concerning the control modes of hybrid HVDC and converter transformers ratios. Based on that, the unified iterative power flow calculation algorithm of hybrid AC/DC networks incorporating hybrid HVDC is proposed. The algorithm is suitable for hybrid HVDC embedded in an AC grid or connecting different asynchronous AC grids. Finally, the accuracy of the mathematical model and the validity of the proposed algorithm are verified by a case study.

The application of modular multilevel converter (MMC) to High Voltage Direct Current projects has to consider voltage balancing of the multiple floating capacitors. The common voltage balancing strategy, which based on the nearest level modulation (NLM) method, benefits from satisfied voltage balancing, but draws back the performance by its high switching frequency and high switching losses. This paper proposes a novel low switching frequency (NLSF) strategy to reduce the switching frequency by using the switching state of submodules (SMs) on the basis of good voltage balancing. A detailed 21-level back to back MMC-HVDC model has been built in PSCAD/EMTDC for demonstration. The simulation results show acceptable voltage balancing with dramatic reduction of the switching frequency by 61% in contrast to the common voltage balancing strategy.

This paper introduces the structure and operational mechanism of Modular Multilevel Converter-High Voltage Direct Current (MMC-HVDC) and the mathematical model for the MMC-HVDC system in d-q synchronous reference frame is developed. When supplying power to passive networks, the MMC-HVDC controller at the passive network side, which contains a novel inner fast current loop and an outer voltage loop, is designed. The MMC-HVDC connected to passive network simulation model is set up in PSCAD/EMTDC. Simulation results, in the case of various operation conditions, verify that the designed control strategy is effective.