Electromagnetic transient (EMT) and transient stability hybrid simulations are predominantly used to analyze the interactions between HVDC systems and the AC grids accurately and efficiently. However, the dynamics of the converters will be affected by the waveforms of adjacent AC systems greatly. Waveform distortion as well as time delay caused by interfacing would significantly increase interface errors, resulting in the decrease of the overall accuracy of the simulations. To resolve such problems, a dynamic phasor based interface model (DPIM) is proposed in this paper to improve the accuracy of interfaces, especially when the fault occurs near the converters. In doing so, the whole system is partitioned into three parts: the transient stability (TS) subsystem, the EMT subsystem, and the DPIM. During each iteration, the interfaces between the TS subsystem and DPIM are represented by their Norton equivalents at fundamental frequency and Thevenin equivalents in the dynamic phasor form. Similarly, the interfaces between DPIM and EMT subsystems are represented by their three phase Norton equivalents and Thevenin equivalents in dynamic phasors, respectively. The effectiveness of the proposed method in terms of accuracy and efficiency is validated by simulating a practical HVDC Project.

A power electronic transformer (PET) is expected to be an important device within active distribution networks due to its high flexibility and controllability of AC/DC conversion. However, the multiple power conversion stages of a PET increase its volume and decrease its efficiency. Therefore, we present novel time-sharing multiplexing control using the basic H-bridge unit of PETs. This control scheme integrates the line-frequency and high-frequency power transmission in each medium-frequency modulation cycle and eliminates one power conversion stage compared with the typical four-stage PET. The underlying principles of H-bridge time-sharing multiplexing are discussed, emphasizing key concepts of mode-switching and the operational constraints of high-frequency transformers emphasized. Then, the topology, control strategies, and parameter design of the constructed PETs are introduced in detail. Finally, experiments and auxiliary simulations verify the effectiveness of the proposed principles and circuits, which provides a new paradigm for the operational application of PETs.

Rapid development of wind power is necessary in China to supply the energy source gap. Wind power development in China has the characteristics of large-scaled and concentrated exploitation, long-distance and large-capacity transportation. Integration of large scale wind power into the grid is a bottleneck for wind power development in China. In order to accommodate wind power and ensure the security and stability of the power grid, some important issues about integration are presented and analyzed in this paper. Followed issues are concerned: firstly, how to improve the wind power digestion capability of the power grid, secondly, advance transmission system plan and design of large-scale wind bases should be carried out, thirdly, a strong and reasonable grid should be configured, fourthly, the economic advantages of wind power integration must be improved.

With the increasing integration of renewable energy into power grids, voltage source converter-based high-voltage direct current (VSC-HVDC) stations often adopt hybrid grid-following (GFL) and grid-forming (GFM) control strategies to improve adaptability to varying grid strengths. In many existing schemes, the hybrid coefficient changes abruptly, which may produce large transient current overshoots and compromise the safe and stable operation of converters. An adaptive hybrid GFL-GFM control framework equipped with a hybrid coefficient transition regulation is proposed. Small-signal state–space models are established and eigenvalue analysis confirms stability over the considered short-circuit ratio (SCR) range. The regulating method is activated only during coefficient transitions and is inactive in steady-state, thereby preserving the operating-point eigenvalue properties. Dynamic equations of the converter current change rate are derived to reveal the key role of the hybrid-coefficient change rate in driving transient current overshoots, based on which a real-time hybrid coefficient regulating method is developed to shape coefficient transitions. Simulations on a 500 kV/2100 MW VSC-HVDC project demonstrate reduced transient current overshoot and power oscillations during SCR variations, with robustness under moderate parameter deviations as well as representative SCR assessment error and update delay.