The existing power system experimental platforms have the main problems such as deviation from engineering application, low effective utilization and poor flexibility. Based on modular and flexible networking method, a new integrated teaching experiment and research platform of power system is designed. The equipment in this platform have plug and play function, and the central-distributed architecture is adopted in the control system. The experimental platform can simulate the scenarios of different voltage levels and different power supply distances such as low-voltage AC/DC microgrid, medium-voltage flexible distribution network and high-voltage AC/DC network. With the help of experimental platform, teaching experiments and scientific research exploration experiments are designed respectively, which helps to enhance students’ cognition and understanding of the new power system and cultivate high-level talents under the construction of new engineering.

For dynamic stability analysis and instability mechanism understanding of multi-converter medium voltage DC power systems with droop-based double-loop control, an advanced system-level model reduction method is proposed. With this method, mathematical relationships of control parameters (e.g., current and voltage control parameters) between the system and its equivalent reduced-order model are established. First, open-loop and closed-loop equivalent reduced-order models of current control loop considering dynamic interaction among converters are established. An instability mechanism (e.g., unreasonable current control parameters) of the system can be revealed intuitively. Theoretical guidance for adjustment of current control parameters can also be given. Then, considering dynamic interaction of current control among converters, open-loop and closed-loop equivalent reduced-order models of voltage control loop are established. Oscillation frequency and damping factor of DC bus voltage in a wide oscillation frequency range (e.g., 10-50 Hz) can be evaluated accurately. More importantly, accuracy of advanced system-level model reduction method is not compromised, even for MVDC power systems with inconsistent control parameters and different number of converters. Finally, experiments in RT-BOX hardware-in-the-loop experimental platform are conducted to validate the advanced system-level model reduction method.

The coordinated control of parallel three-phase four-wire converters in autonomous AC microgrids is investigated in this paper. First, based on droop control, virtual impedance is inserted in positive-, negative- and zero-sequences to enhance system damping and imbalance power sharing. Then, to facilitate virtual impedance design, small signal models of the three-sequence equivalent circuits are established respectively. Corresponding indexes are proposed to comprehensively evaluate the impact of sequence virtual impedance on current sharing accuracy, voltage quality at the point of common coupling (PCC) and system stability. In addition, constraint of DC-link voltage is also considered to avoid over modulation when subjected to unbalanced loads. Furthermore, to address the PCC voltage degradation resulting from virtual impedance, a voltage imbalance compensation method, based on low-bandwidth communication, is proposed. Finally, simulation and experimental results are provided to verify the correctness of the theory model, indicating that the proposed method can achieve PCC voltage restoration while guaranteeing the current sharing accuracy with desirable dynamics.

The concept of a flexible power electronics substation (FPES) was first applied in the Zhangbei DC distribution network demonstration project. As a multi-port power electronics transformer (PET) with different AC and DC voltage levels, the FPES has adopted a novel topology integrating modular multilevel converter (MMC) and four-winding medium frequency transformer (FWMFT) based multiport DC-DC converter, which can significantly reduce capacitance in each sub-module (SM) of a MMC and also save space and cost. In this paper, in order to accelerate speed of electromagnetic transient (EMT) simulations of FPES based hybrid AC/DC distribution systems, an averaged-value model (AVM) is proposed for efficient and accurate representation of FPES. Assume that all SM capacitor voltages are perfectly balanced in the MMC, then the MMC behavior can be modeled using controlled voltage sources based on modulation voltages from control systems. In terms of the averaged current transfer characteristics among the windings of the FWMFT, we consider that all multiport DC-DC converters are controlled with the same dynamics, a lumped averaged model using controlled current and voltage sources has been developed for these four-port DC-DC converters connected to the upper or lower arms of the MMC. The presented FPES AVM model has been tested and validated by comparison with a detailed IGBT-based EMT model. Results show that the AVM is significantly more efficient while maintaining its accuracy in an EMT simulation.