In this study, a multi-stage planning model of the distribution network with high photovoltaic (PV) penetration considering operational reliability is presented. The operation reliability cost consisting of three modified reliability indices is imposed in the objective function in the proposed distribution systems planning model. Additionally, the distribution network reconfiguration including microgrid is considered at the opera-tion level to improve system operation reliability. Nonlinear ex-pressions of modified reliability indices aggravate the complexity of the proposed planning model. To address the computational burden, the planning model is simplified as a mixed integer linear programming (MILP) model, which have an efficient computational behavior with exiting MILP solvers. The benefits of imposing operation reliability cost in the objective function and network reconfiguration including microgrid as operation strategies, are discussed. The effectiveness of the proposed model is tested by numerical results.

This paper presents a novel power injection priority optimization-based control strategy for a PMSG-based wind farm. The proposed strategy seeks to provide voltage support while mitigating the frequency excursion resulting from voltage dip during the fault. To achieve this objective, the conventional fixed reactive power injection priority plan is replaced by an adaptive power injection priority plan in the proposed method. The control problem is formulated as a multi-objective function optimization problem, where the power injection priority plan is calculated by minimizing the shortfall of WF active power and maximizing the reactive power injection of WF. During the fault period, the optimized result is directly applied to the outer control of machine-side converter and grid-side converter to regulate the active/reactive power generation. The simulation system model and proposed optimal model are built in EMTDC/PSCAD and MATLAB, respectively, to verify the effectiveness of the proposed strategy. Test results demonstrate that the proposed control strategy is applicable to mitigate frequency excursion while complying with the voltage support requirement during the fault.

Low inertia is becoming an issue that cannot be avoided as renewable energy sources, represented by wind energy, are increasingly integrated into the power systems. Wind farms have taken on the role of supplying frequency support during system disturbances. In this case, the signal source of wind farm frequency support becomes a critical factor for the control performance. The Center of Inertia (COI) frequency can better reflect the dynamic change of the system frequency after disturbance compared with the Point of Common Coupling (PCC) frequency. However, due to the time delay in signal communication and calculation, the direct utilization of the COI frequency may bring a detrimental influence on the performance of frequency support control, while the PCC frequency is measured locally with a rare time delay. Therefore, how to combine both advantages of the PCC frequency and COI frequency into the auxiliary frequency control signal of wind farms requires specific attention. This paper presents a comparative analysis of the control effect of using two frequency signals. Simultaneously, the effect of different delays of the control signals on the control effect is firstly analyzed. To minimize the detrimental influence of time lag in COI frequency, Long Short-Term Memory (LSTM) neural network is introduced to correct the time lag of the COI frequency. Furthermore, based on the delay-corrected COI frequency signal, an auxiliary frequency control strategy based on the PCC-COI frequency is proposed. This novel strategy aims to combine the advantages of both signals, effectively enhancing the efficiency of auxiliary frequency control. Finally, the efficiency of the suggested control strategy is validated in the modified IEEE 39 bus system.

Grounding fault analysis is of vital importance for low voltage direct current (LVDC) supply and utilization systems. However, the existing DC grounding fault model is inappropriate for LVDC supply and utilization system. In order to provide an appropriate assessment model for the DC grounding fault impact on the LVDC supply and utilization system, an LVDC supply and utilization system grounding fault model is proposed in this paper. Firstly, the model is derived by utilizing capacitor current and voltage as the system state variable, which considers the impact of the converter switch state on the topology of the fault circuit. The variation of system state parameters under various fault conditions can be easily obtained by inputting system state data in normal conditions as the initial value. Then, a model solution algorithm for the proposed model is utilized to calculated the maximum fault current, the system maximum fault current with different grounding resistance is simple to acquired based on the solution algorithm. The calculation results demonstrate that grounding resistance and structure of LVDC supply and utilization system have remarkable impacts on the transient current. The effectiveness of the proposed model is verified in PSCAD/EMTDC. The simulation results indicate that the proposed method is appropriate for the system fault analysis under various fault conditions with different grounding resistance and the proposed model can offer theoretical guidance for system fault protection.

The worldwide promotion of carbon-neutral policies is leading to a continuously growing percentage of electricity being derived from renewable energy, which makes it feasible to design power systems composed of 100% renewable energy in the future. The question of how to realize stable transmission for 100% renewable energy-integrating grids under different operating conditions needs to receive more attention. Voltage source converter-based high-voltage direct current (VSC-HVDC) technology is one of the prospective solutions for large-scale renewable energy integration due to its unique dominance in areas such as independent reactivity and active control. In this study, we design a novel, 100% renewable energy system through grid integration via a VSC-HVDC system structure and a control strategy. Unlike in other research, a mixed control strategy based on grid-forming control (PSL) and grid-following control (GFL) is developed to realize smooth switching in order to ensure secure transmission and consistent operation when the operating conditions of the 100% renewable energy-integrating grid changes. The simulation results indicate that the proposed system structure and control could stabilize renewable energy transmission under normal operation conditions and provide necessary grid support under different system disturbances.