The existing research on voltage-source converter HVDC (VSC-HVDC) system stability mostly focuses on small-signal stability analysis methods. However, when large-signal disturbances occur, the intrinsic nonlinearity of the VSC-HVDC system becomes inevitable so that the small-signal studies are no longer valid. In this paper, a large-signal stability analysis method of the VSC-HVDC system is proposed based on the estimation of domain of attraction (DOA). This method can not only analyze the asymptotic stability of the operating point, but also answer the question of how large deviations from the operating point can be tolerated by the system. By using Takagi-Sugeno (T-S) fuzzy model and Lyapunov stability theorem, the DOA of the VSC-HVDC System at the operating point is estimated, and the large-signal stability of VSC-HVDC System is analyzed. On this foundation, the influence of system parameters on the large-signal stability is studied. The results can be used to guide the adjustment of system parameters to solve the problem of system instability. The time-domain simulations conducted in Matlab/Simulink are used to test the stability of the system under large-signal disturbance in the range of DOA. The simulation results verify the feasibility and effectiveness of the proposed method.

Stability studies of voltage-source converter HVDC (VSC-HVDC) systems are mainly based on small-signal linearization techniques. However, when VSC-HVDC systems become unstable due to large-signal disturbances, the small-signal analysis method would be no longer available. In this paper, the large signal stability of VSC-HVDC system is studied based on mixed potential theory. A simplified state-space averaging model of VSC-HVDC system is established under the synchronous rotating reference frame. Based on this model, the stability prediction criteria for VSC-HVDC system under large-signal disturbances is obtained through Brayton-Moser's nonlinear potential method. On this foundation, the influence of system parameters on the large-signal stability is studied and the boundaries of stability are located as well. Time-domain simulations conducted in Matlab/Simulink are used to test the stability of the working points next to the stable boundary, which aim to prove the correctness of the obtained stability criterion. Aiming at the unstable situation, the time-domain simulation results prove the feasibility of the method of adjusting the system parameters according to the stability criterion to make the system stable, which further verifies the correctness of the stability criterion.

Flexible interconnection transformation of a distribution network using all-control power electronics technology will help to overcome the technical bottleneck of distributed generation (DG) penetration and high-quality power supply in the traditional distribution network. However, the flexible interconnection form that uses back-to-back power electronic converters to replace tie switches has disadvantages such as high cost and large footprint, so its mass promotion and application are bound to be limited. Therefore, a new topology of flexible interconnected distribution network with embedded DC system (FDN+EDC) with a better economy is proposed. Firstly, the topology of FDN+EDC is introduced to improve the controllability level of the distribution network under high percentage DG penetration, and is compared with the conventional flexible interconnection method. Secondly, in order to realize the coordinated optimal economic operation of source, network and load in the FDN+EDC, a dynamic reconfiguration model of switches status and continuous adjustment of flexible interconnection device (FID) are proposed. Then, to achieve a fast and globally optimal solution of the model, a hierarchical coordinated optimization strategy is designed based on an improved particle swarm algorithm for the optimization of discrete and continuous variables. Finally, the validity of the algorithm is verified, and an 88-node FDN+EDC is constructed to verify the advantages of the proposed embedded DC flexible interconnection topology and realize the joint optimal dispatch of network-source-load.

DC microgrid offers high efficiency, simple control, and reduced conversion stages advantages, but suffers from induced instability caused by its low inertia. Due to the strong nonlinearity of the DC microgrid, the analysis of its large-signal stability (LSS) is a very challenging task. This paper uses the T-S fuzzy model (TSFM) method to investigate the LSS of the DC microgrid with various loads, which is connected to the AC grid. The TSFM of the DC microgrid with grid-connected voltage source converter (VSC) as the main power supply is established. The analysis of the LSS of the DC microgrid is carried out using an estimate of the domain of attraction (DOA) of the working point. On the basis of the DOA, the effects of different types of loads and system parameters on the LSS are researched. The research results can serve as a guideline for adjusting the physical and control parameters to solve the DC microgrid instability problem. Finally, the large-signal stability analysis (LSSA) of the DC microgrid is performed using the MATLAB/Simulink simulation platform. The results of the simulation confirm the rightness of the proposed LSSA approach. They indicate that constant power load (CPL) has the greatest effect on the LSS of the DC microgrid compared to the constant impedance load (CIL) and the constant current load (CCL). In addition, the results of the time-domain simulation also validate the effectiveness of this method, which is a way to enhance the LSS of the DC microgrid through the adjustment of system parameters.

In this paper, a series hybrid circuit breaker based on a flying capacitor multilevel injector (FCM-SHCB) is proposed for MVDC and LVDC, which utilizes the multilevel characteristic of FCM to realize the fault-current-limitation modulation and extend operation windows for mechanical switch (MS). Meanwhile, by fully exploring the redundant states of the FCM structure, the proposed FCM-SHCB can avoid the high-frequency operation of one single device in conventional S-HCBs, realizing balanced switching operation of the power semiconductor devices and reducing switching frequency without sacrificing the injection performance. The operation principle and parameters design consideration are detailed. A proof-of-concept FCM-SHCB prototype experimentally demonstrates the proposed solution can maintain the high-speed fault-current breaking ability (150V/15A/6μs) while obtaining balanced switching operation (half-reduced switching frequency) and current limiting capability.

The use of the conventional dual closed-loop control strategy by photovoltaic (PV) plants with grid-connected inverters may weaken the damping of a power system, which may aggravate low-frequency oscillations (LFOs). This influence will become more severe as the penetration of PV plants increases. Therefore, it is necessary to incorporate damping controls into PV plants to suppress LFOs. This paper proposed an additional compound damping control (ACDC) system that combines additional damping control (ADC) for the inverter with ADC-based dynamic power compensation control (DPCC), allowing hybrid energy storage systems (HESSs) to suppress LFOs. First, the feasibility of suppressing low-frequency oscillations in PV plants is demonstrated by the torque method and a small signal model. Then, an additional damping controller is added to the active power control link of the PV inverter to enhance the damping abilities of the system. However, given that the damping performance of PV plants with only ADC is limited by the compensated power, PV plants require devices that can rapidly compensate for the damping power. Therefore, we added the HESS to the DC bus and proposed DPCC. Finally, a three-machine nine-node system for a PV plant was modeled and simulated in the PSCAD platform. The simulation results showed that the proposed control strategy could provide effective damping for interarea oscillation.