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The grid voltage modulated direct power control (GVM-DPC)-based inverter is an attractive solution to regulate the instantaneous real and reactive powers injected into power grids. However, the power coupling in GVM-DPC, i.e., interactions between the real and reactive power loops, may deteriorate its transient performance. In this paper, a power decoupling strategy for GVM-DPC is proposed based on a dynamic feedforward power compensation (DFPC) algorithm. It is proved that the proposed strategy has better decoupling performance than that of the traditional virtual impedance method (VIM). Firstly, power coupling characteristics in GVM-DPC are analyzed, indicating that the ignorance of voltage angle variation at the point of common coupling (PCC) will result in severe power coupling. Then, power coupling magnitudes are derived according to relationships among the real power, reactive power and PCC voltage. Next, the coupling magnitudes are compensated into the power control loop of the GVM-DPC for the better decoupling performance. The transient performance of inverter using DFPC is studied based on the impedance analysis. Also, the relationship between coupling magnitude and grid side impedance is studied, which indicates the limited power decoupling capability of VIM. Finally, the proposed method is validated by the simulations and a hardware-in-loop system.

Accurate estimation of local bus frequency is important for effectively controlling both synchronous and nonsynchronous generators. As the power grid evolves to accommodate essential reliability services such as virtual inertia from nonsynchronous generators, conventional techniques to estimate frequency face challenges. This paper proposes a new frequency estimation method that can effectively include the inertia contributions from double-fed wind generators (DFIGs). This is achieved through the proposed extended frequency divider formula (FDF) to include the contributions of DFIGs via Thevenin equivalents. The proposed extended FDF does not suffer from numerical issues as compared to existing phasor angle derivative-based approaches. Moreover, the knowledge of the rotor speeds of the synchronous machine and DFIGs as well as of the network admittance matrix allows estimating the frequencies of all buses in the grid, thereby significantly improving system situational awareness with a limited number of measurements. Numerical results on the IEEE 39-bus power system with DFIGs show that the proposed method achieves more accurate bus frequency estimations than the original FDF formula and other approaches based on the numerical derivation of the bus voltage phase angles.

This paper proposes a novel Multivariate Quotient-Difference (MQD) method to obtain the approximate analytical solution for AC power flow equations. Hence, in the online environment, the power flow solutions covering different operating conditions can be directly obtained by plugging in values into multiple symbolic variables, such that the power injections and consumptions of selected buses or areas can be independently adjusted. This method first derives a power flow solution by Multivariate Power Series (MPS). Next, the MQD method is applied to transform the obtained MPS to Multivariate Pade Approximants (MPA) to expand the Radius of Convergence (ROC), so that the accuracy of the derived analytical solution can be significantly increased. Besides, the hypersurface of voltage stability boundary can be identified by an analytical formula obtained from the coefficients of MPA. This direct method for power flow solution and voltage stability boundary is fast for many online applications, since such analytical solutions can be derived offline and evaluated online by only plugging values into the symbolic variables according to the actual operating conditions. The proposed method is validated in detail on the New England 39-bus and IEEE 118-bus systems with independent load variations in multi-regions.

This paper proposes a control system of active filter, which is embedded in the grid-side converters (GSCs) of wind turbines (WTs), to mitigate the harmonics on remote busbars within offshore wind power plants (OWPPs). Superior to the existing filters with extra devices, the proposed active filter is embedded in the control system of GSC without additional device. Firstly, the embedded active filter (EAF) detects the harmonics and calculates the compensation current based on instantaneous measurements in the time-domain. In addition, both general compensation module for total harmonic distortion (THD) and specific compensation module for selected-order harmonics are combined in EAF. Furthermore, the proposed EAF improves the power quality for remote Point of Common Couplings (PCCs). Finally, an OWPP connected with island distribution grid and external grid is implemented in DIgSILENT/PowerFactory with field measurements to validate the performance of proposed EAF.

This paper proposes a filter control system embedded in the control system of the wind turbine, which adopts the d-q transformation to detect harmonics, and then controls the output of the wind turbine to generate harmonic compensation signal to achieve the purpose of harmonic management of the wind power plant. Moreover, it is also proposed and proved that the detection of harmonics by d-q transformation will cause the problem of harmonic confusion and multiple embedded filter control systems will influence each other. Finally, this paper uses DIG-SILENT/PowerFactory to establish a wind power plant model with multiple wind turbines, simulating and verifying the above content.

This paper proposes a novel analytical method to calculate the Static Voltage Stability Boundary (SVSB) for power systems as a hypersurface in a high-dimensional space on power flow solutions. First, the proposed method derives the analytical voltage solution of AC power flows in the form of Multivariate Power Series (MPS) using the Multidimensional Holomorphic Embedding Method (MDHEM). Second, based on the Cauchy-Hadamard theorem, the proposed method is able to derive an analytical expression of the SVSB using the obtained voltage MPS. Besides, a theoretical proof is given in this paper to generalize the proposed method from unidimensional to multidimensional scenarios. In general, the SVSB is found to be analyzable and its analytical expression can be derived offline. Therefore, in the online stage, the proposed method can be used to judge the feasibility of operating conditions very quickly. IEEE 14-bus system, New England 39-bus system, and IEEE 118-bus system are adopted to validate the effectiveness of the proposed method.

This paper proposes a novel method of identifying buses vulnerable to power system voltage instability. Unlike traditional sensitivity and modal analysis methods, the proposed method can preserve the nonlinearity of power flow equations, and visually pinpoint the vulnerable buses in advance for any possible load increasing scenarios. Based on the power flow equations on decomposed two-bus equivalent channels from the power network, a vulnerability index is introduced and visualized for each channel in the solvability region of its decoupled power flow equations bounded by a parabola. It is found that the arrival at the parabolic boundary indicates a potentially vulnerable bus but not necessarily results in voltage collapse. Thus, a criterion on the gradient of vulnerability index trajectory is proposed to discover and differentiate two types of potentially vulnerable buses: one type has trajectories that are tangent to the boundary but stay inside the solvability region; the other type has trajectories crossing the boundary to result in unsolvable power flow equations, which are identified as the vulnerable buses. The proposed method is validated on the IEEE 14-bus, IEEE 118-bus, and New England power systems with comparison to the traditional methods, and the test results show that the proposed method performs better than the traditional methods with satisfactory robustness under voltage fluctuation. In particular, the proposed method is 33.3% and 66.7% more accurate than the voltage sensitivity method in prospective operating conditions on the IEEE 14-bus and 118-bus systems, respectively.