Recently, sustained power oscillation at subsynchronous frequency was captured in direct-drive permanent magnetic synchronous generator (PMSG) based wind farms in Xinjiang Uygur Autonomous Region, China. This new type of subsynchronous interaction (SSI) detected in practical systems has never been reported and analyzed before. Therefore, its mechanism and characteristics are not yet clearly clarified. In this paper, a simplified but representative system model with multiple PMSGs interfaced with AC networks is established first based on the actual system and the PMSG model provided by the manufacturer. Then, small-signal eigenanalysis, time-domain simulation, and impedance model analysis are carried out to investigate the interactive dynamics between them. The results show that such interaction between direct-drive PMSG wind farms and weak ac grids characterized by low short-circuit ratio would cause negative-resistance effect for the SSI mode, leading to unstable oscillation. In such cases, the controller of PMSG would saturate soon, resulting in sustained power oscillation in the system. If unfortunately the oscillation frequency matches the torsional mode of any nearby turbogenerator, severe torsional vibration would be excited on the shaft of the latter. The analysis results are finally validated with field measurements of an actual SSI event. To address the problem, a supplementary subsynchronous damping control loop is attached to the controllers of PMSGs to reshape the impedance and thus to stabilize the SSI.

Subsynchronous Interaction Between Direct-Drive PMSG Based Wind Farms and Weak AC Networks

Traditionally, inertia in power systems has been determined by considering all the rotating masses directly connected to the grid. During the last decade, the integration of renewable energy sources, mainly photovoltaic installations and wind power plants, has led to a significant dynamic characteristic change in power systems. This change is mainly due to the fact that most renewables have power electronics at the grid interface. The overall impact on stability and reliability analysis of power systems is very significant. The power systems become more dynamic and require a new set of strategies modifying traditional generation control algorithms. Indeed, renewable generation units are decoupled from the grid by electronic converters, decreasing the overall inertia of the grid. ‘Hidden inertia’, ‘synthetic inertia’ or ‘virtual inertia’ are terms currently used to represent artificial inertia created by converter control of the renewable sources. Alternative spinning reserves are then needed in the new power system with high penetration renewables, where the lack of rotating masses directly connected to the grid must be emulated to maintain an acceptable power system reliability. This paper reviews the inertia concept in terms of values and their evolution in the last decades, as well as the damping factor values. A comparison of the rotational grid inertia for traditional and current averaged generation mix scenarios is also carried out. In addition, an extensive discussion on wind and photovoltaic power plants and their contributions to inertia in terms of frequency control strategies is included in the paper.

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Power systems with high renewable energy sources: A review of inertia and frequency control strategies over time

The strengthening of electric energy security and the reduction of greenhouse gas emissions have gained enormous momentum in previous decades. The integration of large-scale intermittent renewable energy resources (RER) like wind energy into the existing electricity grids has increased significantly in the last decade. However, this integration poses many operational and control challenges that hamper the reliable and stable operation of the grids. This article aims to review the reported challenges caused by the integration of wind energy and the proposed solutions methodologies. Among the various challenges, the generation uncertainty, power quality issues, angular and voltage stability, reactive power support, and fault ride-through capability are reviewed and discussed. Besides, socioeconomic, environmental, and electricity market challenges due to the grid integration of wind power are also investigated. Many of the solutions used and proposed to mitigate the impact of these challenges, such as energy storage systems, wind energy policy, and grid codes, are also reviewed and discussed. This paper will assist the enthusiastic readers in seeing the full picture of wind energy integration challenges. It also puts in the hands of policymakers all aspects of the challenges so that they can adopt sustainable policies that support and overcome the difficulties facing the integration of wind energy into electricity grids.

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Grid Integration Challenges of Wind Energy: A Review

The utilization of power electronic inverters in power grids has increased tremendously, along with advancements in renewable energy sources. The usage of power electronic inverters results in the decoupling of sources from loads, leading to a decrease in the inertia of power systems. This decrease results in a high rate of change of frequency and frequency deviations under power imbalance that substantially affect the frequency stability of the system. This study focuses on the requirements of inertia and the corresponding issues that challenge the various country grid operators during the large-scale integration of renewable energy sources. This study reviews the various control techniques and technologies that offset a decrease in inertia and discusses the inertia emulation control techniques available for inverters, wind turbines, photovoltaic systems, and microgrid. This study attempts to explore future research directions and may assist researchers in choosing an appropriate topology, depending on requirements.

Future low-inertia power systems: Requirements, issues, and solutions - A review

This paper presents a methodology for the analysis of frequency dynamics in large-scale power systems with high level of wind energy penetration by means of a simplified model for DFIG-based wind turbines. In addition, a virtual inertia controller version of the optimized power point tracking (OPPT) method is implemented for this kind of wind turbines, where the maximum power point tracking curve is shifted to drive variations in the active power injection as a function of both the grid frequency deviation and its time derivative. The proposed methodology integrates the model in a primary frequency control scheme to analyze the interaction with the rest of the plants in the power system. It is also proven that, under real wind conditions, the proposed version of the OPPT method allows us to smooth the wind power injected into the grid, thereby reducing frequency fluctuations.

Fast-Frequency Response Provided by DFIG-Wind Turbines and its Impact on the Grid

This paper investigates an improved active power control method for variable speed wind turbine to enhance the inertial response and damping capability during transient events. The optimized power point tracking (OPPT) controller, which shifts the turbine operating point from the maximum power point tracking (MPPT) curve to the virtual inertia control (VIC) curves according to the frequency deviation, is proposed to release the “hidden” kinetic energy and provide dynamic frequency support to the grid. The effects of the VIC on power oscillation damping capability are theoretically evaluated. Compared to the conventional supplementary derivative regulator-based inertia control, the proposed control scheme can not only provide fast inertial response, but also increase the system damping capability during transient events. Thus, inertial response and power oscillation damping function can be obtained in a single controller by the proposed OPPT control. A prototype three-machine system containing two synchronous generators and a PMSG-based wind turbine with 31% of wind penetration is tested to validate the proposed control strategy on providing rapid inertial response and enhanced system damping.

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Control of PMSG-Based Wind Turbines for System Inertial Response and Power Oscillation Damping

In a dc grid, the inherent inertial support from the dc capacitors is too small to resist step changes or random fluctuations from the intermittent power resources, which results in lower dc voltage quality. In this paper, an adaptive droop control (ADC) strategy is proposed to achieve an increased inertia from the droop controlled converter. The adaptable droop coefficient according to the dc voltage variation enables fast swing of the droop curve, so that the converter can provide inertial power for the dc grid like synchronous generators in ac grids. The design of the ADC including the calculation and limitation of the adaptable droop coefficient is analyzed in detail. The small-signal analysis of the dc grid with ADC is provided to identify its stability issue. Experimental tests on a controller hardware-in-the-loop platform of a low-voltage (LV) dc grid are carried out to validate the proposed method. In this LV dc grid, the proposed ADC is implemented on the energy storage system which provides inertial support to improve the dc voltage quality under different power fluctuations, and smooths the power transmitted to ac grid.

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Adjustable Inertial Response From the Converter With Adaptive Droop Control in DC Grids

This paper investigates virtual inertia control for dc microgrids using voltage droop control as the power-sharing strategy. An adaptive virtual inertia control (AVIC) strategy with the variable droop coefficient is proposed to increase the inertia of the dc microgrid. By swinging the droop curve during transient events, the dc voltage change rate is reduced. The variable droop coefficient is modified by the arc-tangent function, which makes the AVIC possess good adaptability to different operation states and good controllability of the supplied virtual inertia. Matlab/Simulink simulations are carried out to validate the proposed strategy. The results reveal that with AVIC, the system inertia is increased and the quality of the dc bus voltage is improved.

Adaptive virtual inertia control for DC microgrid with variable droop coefficient

Distributed generators using power electronic converter interface have no inherent inertial response similar to that from the rotating mass of conventional synchronous generators. Virtual synchrono...

Flexible Virtual Synchronous Generator Control for Distributed Generator with Adaptive Inertia

Dual active bridge (DAB) DC-DC converter has been widely used in the DC grid. A virtual DC machine control strategy (VDCM) for DAB DC-DC converter in DC distribution network is proposed in this paper, aiming at the DC voltage dip caused by sudden power change of micro-source, load switching and power disturbance of AC grid. Based on the original phase-shift control, the strategy makes the converter mimic the inertia characteristics of the DC machine. The DC microgrid model based on the proposed control strategy is established in Matlab/Simulink, with which the theoretical analysis and the effectiveness of the control strategy are verified.

Jianhui Meng

Papers

Control of PMSG-Based Wind Turbines for System Inertial Response and Power Oscillation Damping

Adjustable Inertial Response From the Converter With Adaptive Droop Control in DC Grids

Adaptive virtual inertia control for DC microgrid with variable droop coefficient

Flexible Virtual Synchronous Generator Control for Distributed Generator with Adaptive Inertia

A Virtual DC Machine Control Strategy for Dual Active Bridge DC-DC Converter