The configuration of a direct-current (DC) distribution system has prominent advantages in accommodating distributed photovoltaics, energy storage, and other DC power sources, making it an effective approach for achieving emission peaks and carbon neutrality. Random power fluctuations in photovoltaics can cause bidirectional changes in the power flow, frequent voltage beyond the limit, and increased network losses. A sensitivity-based adaptive voltage/network loss-coordinated optimal control strategy is proposed for a DC distribution system with integrated photovoltaics and energy storage (photovoltaic-storage DC distribution system). First, the voltage control characteristics of the photovoltaic-storage DC distribution system were analyzed, and analytical expressions for the voltage and network loss sensitivity suitable for photovoltaic and storage DC distribution networks were derived. Subsequently, a coordinated optimization control strategy based on sensitivity was designed, which included adaptive switching between two modes: voltage correction control and network loss optimization. This strategy performs coordinated control of the voltage source converter (VSC), DC transformer, and controllable energy-storage resources to achieve voltage safety control and network loss optimization. Finally, through simulations and programming combining DIgSILENT and Python, models of 33-node and 69-node photovoltaic-storage DC distribution systems were constructed, and the effectiveness of the proposed control strategy was verified.

In order to solve the problem of voltage imbalance in three-phase three-leg off-grid inverter, a fractional order proportional integral (PI) quasi proportional resonant voltage control strategy with virtual impedance is proposed. In the d-q rotating coordinates, the fractional order PI tracking control is designed for the positive sequence component of the fundamental wave, and the quasi-proportional resonant tracking control is designed for the negative sequence component of the fundamental wave. Because of the difference of bandwidth between fractional order PI and quasi proportional resonant controller, the compound voltage controller does not need the separation of positive and negative sequence of voltage, which avoids the complexity of program implementation and the problem of delay and precision degradation caused by separation. Considering the influence of unbalanced voltage drop on the line impedance on the common bus voltage, the negative-sequence virtual impedance is designed to reduce the total negative sequence impedance, thus realizing the three-phase voltage balance at the point of common coupling. Finally, the effectiveness of the proposed control strategy is verified by simulation and experiment.

DC distribution network, with its advantage of efficient absorption of direct current energy, will become an important way of connecting the future distributed generators. However, the unbalanced faults of the AC distribution networks will cause the voltage fluctuation of the DC distribution networks, which will harm the normal operation of the DC distribution system. To tackle the problem aforementioned, this paper firstly analyzes the characteristics of active power and DC voltage fluctuations in the case of single-phase-to-ground fault in AC distribution network, especially deduces the expression of instantaneous active power of filter inductance, and points out that the instantaneous active power of filter inductance is the important cause of DC voltage fluctuation. Secondly, an improved phase lock loop (PLL) is proposed to track the phase of positive-sequence voltage. The proposed method only redesigns the compensator of the traditional PLL, which is clear in principle and easy to implement. Finally, a simulation model of typical two-terminal AC-DC distribution system is built in PSCAD/EMTDC to verify the correctness of the theoretical analysis and the effectiveness of the proposed control method.

Distributed energy storage (DES) has flexible operating characteristics and can be properly configured to effectively serve the voltage management in the active distribution network. The existing studies on voltage management-oriented DES optimization configuration are usually focused on the balanced network model to analyze the impact of energy storage operation characteristics on the system voltage, while the real distribution networks are actually unbalanced. To this end, a method for sequence optimization of DES in unbalanced distribution networks applying voltage sensitivity analysis is proposed, and the optimal configuration of DES in unbalanced distribution networks is studied from the perspective of improving the network voltage. Firstly, the DES access location is determined according to the comprehensive voltage sensitivity analysis; secondly, taking into account the economy of system operation, the access capacity of DES with the goal of minimizing the investment and maintenance cost of DES is optimized. Finally, by improving the IEEE 33-node three-phase distribution network to carry out a calculation example analysis, it is verified that the proposed DES sequence optimization configuration method can effectively improve the voltage quality of the unbalanced distribution network.

When photovoltaic power is connected to the distribution network, it will cause the voltage at the end of the line to rise and threshold crossing in severe cases. In order to solve this problem, to improve the ability and sensitivity of reactive power voltage regulation, it is proposed to add an adjustable reactor in series with distribution line as well as to use the reactive power of solar inverter for voltage regulation. The cause of the voltage rise and the effect of the photovoltaic inverter and the series reactor on voltage regulation are analyzed in this paper, and the relationship between voltage drop and series reactance and the reactive power of the photovoltaic inverter is obtained. Taking full advantage of the reactive power of the photovoltaic inverter, taking minimum active line loss as the objective function, the optimal allocation approach of series reactor is found. Finally, with the example of the line in Xinjiang Autonomous Region, the limitation of the reactive power of the photovoltaic inverter is analyzed, and the effectiveness of the method is verified.