Fast and accurate acquisition of voltage components is the basic requirement for grid-tied inverters to realize various control under unbalanced grids. In view of this situation, a delay sampling period filter (DSPF), which rapidly separates each voltage component by delaying two sampling periods in dq frame, is proposed. In view of the noise interference, a delay operation period filter (DOPF) is proposed to scale the operation period and limit the noise to an acceptable range. The complete positive sequence components (PSC) extraction method based on the developed DOPF+MAF algorithm is presented, combing the advantages of DOPF and MAF for eliminating the double-frequency AC components and high-frequency noise, respectively. Finally, the feasibility and priority of the DOPF+MAF algorithm and PSC extraction method have been verified by the physical experimental results.

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.

High proportion of wind power generation weakens the power system inertia, causing concerns over frequency stability. The direct-drive permanent magnet synchronous generators-based wind turbine system (WTS) usually uses virtual inertia control (VIC) to support frequency. However, the related controller parameters are difficult to design, and the deep participation in frequency regulation (FR) will reduce the economic benefits of WTS. To this end, this paper breaks out of the VIC framework, and proposes a transient FR strategy based on frequency trajectory planning (FTP). The basic idea is to actively plan a frequency trajectory with a safety margin according to the grid frequency threshold when the system is disturbed, so that the FR power provided by the WTS can be controlled according to the planned frequency trajectory and achieve rapid support for the system transient frequency, ensuring the safe and stable operation of the grid while achieving the economy of wind power generation. Simulation and hardware-in-loop experimental results demonstrate that the proposed FTP-based transient FR strategy for WTS is effective and economical.

Due to the rapid development of renewable energy technology, renewable energy systems with power electronic equipment have gradually become the main components of the power system. However, the traditional detection methods do not always satisfy the need for accurate and high-speed sequence component acquisition of the power grid at point of common coupling (PCC) under distortion grids, which leads to the decline of the reliability of renewable energy system. To this end, a detection scheme based on the cascaded enhanced delayed signal cancellation (CEDSC), triangular orthogonal signal construction (TOSC) and adaptive stabilization algorithms is derived mathematically. The CEDSC can effectively suppress multiple harmonics and noise. Afterwards, the sequence components can be calculated by the TOSC and proposed mathematical calculation algorithms. The adaptive stabilization algorithm of the proposed scheme can avoid calculation errors and thus improves the robustness. The results show that in distorted grids, the dynamic response time of this scheme to detect positive and negative sequence components is about 8 ms. Compared with the traditional methods, this scheme has obvious comprehensive advantages in the detection accuracy and response time. Moreover, the proposed scheme has the characteristics of strong robustness, which improves the grid-connected stability of renewable energy systems.

Given the increasing penetration of wind power generation, the power system faces great challenges in frequency regulation due to reduction in system inertia. This paper proposes an adaptive power compensation based frequency regulation (FR) strategy of the wind turbine system (WTS). In a frequency transient event, the proposed strategy exploits power excitation method to estimate the disturbance level, which enables the WTS to timely and accurately participate in FR. After the system frequency and the rate of change of frequency meet their requirements, the droop control is adopted to smooth the WTS out of the FR mode and further improve the frequency quality. By analyzing the operation mode and determining the switching logic, detailed implementation of the proposed strategy is developed. Its excellent frequency response performance is verified via results of hardware-in-the-loop experiment and MATLAB/Simulink-based simulation.