Compared to the conventional power systems, large wind power plants (WPPs) present a more challenging system, where the interactions between the passive elements and the wideband control systems of power converters may result in harmonic instability and new resonance frequencies. Most of researches about harmonic stability focus on small-scale systems, and it has not paid much attention yet to identify the mentioned resonance frequencies. This paper models and analyzes the harmonic instability and resonance frequencies in large permanent magnet synchronous generator (PMSG) based WPPs with full-scale converters, where linearized models of inner control loops of the power converters are considered. A large PMSG-based WPP introduces as a multi-input mulit-output (MIMO) control system, therefore, the stability of the whole power system is analyzed based on the real parts of the poles of the introduced MIMO system and the resonance frequencies are identified based on the element amplitudes of the MIMO matrix. An active damping controller is used to set the poles of the WPP in a desired location in order to mitigate the harmonic instability problems. Multiple case studies are provided to depict that wind turbine connections or disconnections in a WPP, as well as grid impedance variations can affect the harmonic stability and resonance frequencies. The effectiveness of the presented theoretical analysis is validated by time-domain simulations of a 400-MW WPP in PSCAD/EMTDC software environment.

With the increased prevalence of power converters in power systems, especially three-phase voltage source converters (VSCs), the stability analysis of power electronics-based power systems has received much attention recently. To this end, different impedance models, such as the dq-domain, sequence-domain, and phasor-domain impedance models among others, have been developed for three-phase VSCs in recent years. A common trend in all these impedance models, which have no noticeable practical advantage compared to each other, is considering a standard synchronous reference frame phase-locked loop (SRF-PLL) for the synchronization of the VSC with the power grid. The standard SRF-PLL, however, has a limited filtering ability and, therefore, may not be very practical in most applications. To deal with this shortcoming of the SRF-PLL, a great number of advanced three-phase PLLs have been proposed in the literature. These advanced PLLs may have different feedback/feedforward loops and filters in their structures, which make including their dynamics in the available impedance models complicated. Bridging this gap in research is the objective of this article. To this end, it is demonstrated that advanced three-phase PLLs have an alternative representation, which can be easily obtained and included in the available dq-frame impedance model. Several case studies are presented to verify this idea.

Compared with the conventional power systems, large-scale power electronics based power systems present a more complex situation, where harmonic instability may be induced by the mutual interactions between the inner control loops of the converters. This paper presents an approach to locate which power converters and buses are more sensitive and have significant contribution to the harmonic instability. In the approach, a power electronics based system is introduced as a Multi-Input Multi-Output (MIMO) dynamic system by means of a dynamic admittance matrix. Bus Participation Factors (PFs) are calculated by the oscillatory mode sensitivity analysis versus the elements of the MIMO transfer function matrix. The PF analysis detects which power electronic converters or buses have a higher participation in harmonic instability excitation than others or at which buses such instability problems have a higher impact. In order to confirm the effectiveness of the presented approach, time-domain simulation results are provided for a 400-MW wind farm in PSCAD software environment.

The proportional-resonant (PR) controller is highly popular for controlling grid-connected voltage source converters. The resonant part of this controller provides an infinite gain at the nominal frequency and, in this way, ensures a zero steady-state tracking error when the grid frequency is at its nominal value. In the presence of frequency drifts, nevertheless, a zero tracking error may not be guaranteed. To deal with this problem, the resonance frequency of the PR controller may be updated using the frequency estimated by the synchronization unit, which is often a phase-locked loop. In recent years, however, there is a growing attention toward eliminating the need for a dedicated synchronization unit and designing integrated synchronization and control structures as they benefit from a simpler and more compact structure. In this letter, based on a structural resemblance between a resonant current regulator and second-order generalized integrator-based frequency-locked loop, a frequency-adaptive resonant controller is presented. The tuning procedure of the proposed controller is discussed, and some experimental results are presented to verify its effectiveness.

During the past two decades, the doubly fed induction generator (DFIG) based wind farm has been under rapid growth, and the increasing wind power penetration has been seen. Practically, these wind farms are connected to the three-phase ac grid through long transmission cable which can be modeled as several Π units. The impedance of this cable cannot be neglected and requires careful investigation due to its long distance. As a result, the impedance interaction between the DFIG-based wind farm and the long cable is inevitable, and may produce high-frequency resonance (HFR) in the wind farm. This paper discusses the HFR of the large-scale DFIG-based wind farm connected to the long cable. Several influencing factors, including 1) the length of the cable, 2) the output active power, and 3) the rotor speed, are investigated. The transformer leakage inductances in the transmission system are taken into consideration when investigating the HFR. Simulation validations using MATLAB/Simulink have been conducted to verify the theoretical analysis.

The single-phase nonlinear loads are going to increase in the future and power quality concerns in the residential distribution grid. Since most of nonlinear loads have a dispersed nature, compensating these phenomena may be complicated. On the other hand, the increased capacity of single-phase grid-connected roof-top PV inverters in residential distribution grid can be an opportunity to engage these systems in the power quality issues as custom power devices. By implementing a proper control for roof-top PV inverters, these systems may in addition to inject the fundamental current, additionally act like a virtual harmonic resistance, and dedicate their additional current capacity to compensate the harmonics of residential distribution grid. In this paper, each roof-top PV system is a grid-harmonic supervisor, where it continually measures the PCC voltage harmonics by the sliding discrete Fourier transform algorithm and then individually compensates the measured harmonics by the proposed adaptive-harmonic compensator. In the proposed approach, a current reference is separately determined for compensating each harmonic component. Then, a specific amount of additional current capacity will be dedicated for each component to compensate by using adaptive gain. Finally, the current reference is controlled by a model predictive current controller. It has been tested in a single-phase transformerless inverter (highly efficient and reliable inverter concept) with an LCL filter at the output. Also, the effectiveness of the control scheme has been verified by MATLAB Simulink and experimental tests.