Fast and accurate acquisition of current components is a key factor for an active power filter to realize transient control under unbalanced conditions. In this letter, a robust real-time algorithm, which rapidly separates the positive-sequence component (PSC) from multiple decaying dc (DDC) components, dc bias component, negative-sequence component, and harmonics, is proposed. To this end, first, the multiple DDC components are detected in the multiple disturbance and unbalanced grid context, by making use of the periodicity of remaining components. The dc bias can be obtained accordingly, followed by the detection of a compound signal encompassing the positive-sequence component, negative-sequence component, and harmonics. Specifically, the compound signal can be extracted precisely, based on the detected DDC and dc bias components, with one grid cycle response time, or approximately, within half grid cycle. A switching logic of the two approaches is designed to shorten the overall convergence time and improve steady-state accuracy. The PSC is then effectively extracted by constructing the virtual orthogonal signal of the compound signal and using dq -frame filtering. Compared with the existing transient control techniques, the proposed scheme guarantees one grid cycle response time and simultaneously suppresses multiple disturbances. Finally, experimental results verify the effectiveness of the proposed method.

For physically understanding the dynamic response of the photovoltaic (PV) integrated power system for electromechanical oscillations damping, firstly, this paper develops the linear mathematical model of a single machine infinite bus system integrated by the PV grid-tied inverter under the electromechanical time scale. Then, based on the electric torque analysis method, and the functional route of the PV grid-tied inverter variable active power control (P-control) and reactive power control (Q-control) with the power angle or grid frequency feedback as the input control signal, the key factors and influence laws of the integrated system inertia, damping level and synchronization capability are investigated. The developed model shows that enlarging the proportional-integral-derivative (PID) controller parameters of P and/or Q-control loop can effectively change electromagnetic power distribution to suppress the imbalance power, and equivalently improve the system damping level, synchronization capability and inertial effect, respectively, contributing to actively adjust the grid frequency oscillation amplitude, period and recovery speed to maintain the system control stability according to the grid code demand. Besides, under the same inverter capacity constraint, P-control mode is proved more advantageous than Q-control mode to damp electromechanical oscillations. Finally, the correctness of the physical mechanism analysis is verified by MATLAB simulations and RT-LAB experiments.

With high penetration of renewable energy, DC distributed power systems (DDPSs) need to improve the inertia response and damping capacity of the power grid. The effects of main circuit parameters and control factors on the inertia, damping and synchronization of the DDPS were studied in this paper. Firstly, the dynamic model of DDPSs based on frequency droop control is established in the DC voltage control (DVC) timescale. Then, a static synchronous generator (SSG) model is used to analyze the parameters that affect the inertial level, damping effect and synchronization capability of the DDPS. The analysis results show that an optimal design of the frequency droop coefficient and proportional integral (PI) parameters of the DC bus voltage control loop can equivalently change the characteristics of inertia and damping when the frequency droop control strategy is applied to the DC/DC converter and the DC bus voltage control strategy is used in the grid-tied inverter. Simulation results verify the correctness of the conclusions. This paper helps to design an effective control strategy for DDPSs to enhance the inertial level and damping effect of the power grid and to improve the stable operation capability of renewable energy systems.

With the rapid development of ultra high-voltage DC (UHVDC) transmission technology, the total electric field problem in high-altitude dc transmission lines is more important. Therefore, this article proposes a calculation model of the ion flow field at high altitude based on the natural wind model and the gas motion theory. First, the initial charge density is assumed, and then, Poisson’s equation is solved by the finite-element method (FEM). The measured wind speed and altitude are introduced into the natural wind model and the ion mobility model, and the natural wind speed and the ion mobility of each node are obtained. Subsequently, the current continuity equation is calculated by this information and the upstream FEM. When the calculation results do not meet the steady-state convergence conditions, the charge density on the conductor surface is iteratively corrected. The calculation model is verified by the measurement results of the ±800-kV UHVDC transmission line at the National Engineering Laboratory for Ultrahigh Voltage Technology (Kunming) of the China Southern Power Grid. The research shows that with the increase of altitude, the ion mobility gradually becomes larger, and the total electric field at ground level also increases. Due to the influence of wind, the peak value of the total electric field at ground level is obviously shifted, and the maximum peak values are on the downwind side. This model in this article can accurately calculate the total electric field of UHVDC transmission lines at high altitude considering the natural wind and provide a reference for the design of transmission lines.

In order to study the influence of drizzle weather on the total electric field of ultra-high voltage direct current (UHVDC) transmission lines, an ion flow field model considering drizzle weather is established. This model calculates not only the positive and negative ions, but also the charged tiny water droplets, dust particles and falling raindrops, enabling accurate simulation of the total electric field in drizzle weather. Meanwhile, the ion flow field model is solved by the improved upstream finite element method. Firstly, the influence factors and mechanism of drizzle weather on the ion flow field of UHVDC transmission lines are studied in this paper. Then, the improved upstream finite element method is used to solve the ion flow field under different degrees of drizzle weather. Finally, the influence rule of drizzle weather on the total electric field at ground level is obtained. In this paper, the accuracy of the ion flow field model is proved by the measurement data of the ground-level ion flow field of the ±800 kV UHVDC transmission line from Chuxiong to Suidong. The research results show that the ion flow field of drizzle weather at ground level are larger than that of sunny weather. With the degree increase of drizzle weather, the peak value and distribution curve of the ground-level total electric field gradually increases. This paper has a guiding role for the construction and the safe operation of UHVDC transmission lines considering drizzle weather.