DFIG-based Wind Turbine Generators Research Articles

Grid-Forming Control of DFIG-based Wind Turbine Generator by Using Internal Voltage Vectors for Asymmetrical Fault Ride-Through

Grid-Forming Control of DFIG-based Wind Turbine Generator by Using Internal Voltage Vectors for Asymmetrical Fault Ride-Through

A novel virtual synchronous generator control scheme of DFIG-based wind turbine generators based on the rotor current-induced electromotive force

This paper focuses on the research of virtual synchronous generator (VSG) control technology for the doubly-fed induction generator (DFIG)-based wind turbines. The key for DFIG VSG operation is to construct an internal electromotive force (EMF) with the virtual rotor motion characteristic in the stator. The existing control schemes use the rotor or mutual flux-induced EMF as the internal EMF. Since the flux corresponds to a composite magnetic field generated together by the stator and rotor current. Any variations in the stator current caused by external disturbances will reflect on the rotor current to keep the flux constant. The rotor overcurrent is inevitable under a large external disturbance, threatening the safety of the rotor-side converter (RSC). Moreover, the flux is immeasurable. The accurate flux control depends on flux observers, increasing the complexity; while the control without observers cannot ensure the effectiveness, leading to control failure. To address these problems, a novel DFIG VSG control scheme with the rotor current-induced EMF as the internal EMF is proposed in this paper. The internal EMF is corresponded to the single magnetic field generated only by the rotor current. This can lead to the following benefits: the rotor current will not vary with the stator current and can be limited under large external disturbances, ensuring the safety of the RSC; the rotor current can be directly measured without the need for extra observers, reducing the difficulty of control implementations; the underlying controlled vector is unified with the traditional grid-following DFIG, favoring the retrofit of the installed DFIGs. Compared to the existing schemes, this scheme has a larger equivalent internal impedance. An additional rotor current-terminal voltage magnitude droop control is added to effectively restrain the problems brought by this larger impedance. Simulation and analysis results demonstrate that the proposed DFIG VSG control scheme effectively addresses problems in the existing schemes. Moreover, the results also exhibit that the scheme has a robust adaptability across a wide range of short-circuit ratios and possesses the grid-forming ability independent of SGs.

Hierarchical bi-level load frequency control for multi-area interconnected power systems

To overcome the shortcomings of proportional–integral (PI) control, a linear quadratic regulator (LQR) is proposed in this study, to determine the set points of the PI controllers in a two-level hierarchical manner. The application of the proposed scheme is studied on load frequency control (LFC) of a power system with multiple control areas (CAs). At the lower control level, a discrete PI controllers are used to regulate the frequency and tie-line powers of each of the CAs. To achieve an improved closed loop performance, the local PI controllers are optimally tuned using moth flame optimization (MFO) algorithm by minimizing the Integral Square Error (ISE) of the state errors with respect to local information and interactions with neighboring CAs. While at the upper control layer, an optimized finite-horizon LQR is applied as a guide to establish the optimal set-points of the lower level PI controllers. Since only few of the state variables can be accessed, Kalman filter (KF) is applied as an observer for the estimation of the remaining states. The efficacy of the proposed scheme is verified by implementing it on a three-CA system perturbed with a step and random net disturbances formed by combining the fluctuations in the output power of DFIG-based wind turbine generator and random load demand. From the simulation results, it is established that the proposed LQR-PI supervisory scheme has outperformed the conventional PI control scheme in optimality and stability.

Comparison of internal voltage vectors of DFIG-based wind turbine generator and synchronous generator during asymmetrical fault

Fault signatures of doubly fed induction generator (DFIG) based wind turbine generators (WTGs) are different and more complex than that of synchronous generators (SGs). This paper proposes a new approach to elaborate the formation of the fault signatures of DFIG-based WTGs under asymmetrical faults and various fault ride through control. The proposed internal voltage vectors and equivalent circuit enable reorganizing the control schemes, electric and magnetic circuits of DFIG in terms of transient, positive- and negative-sequence components. This way, the characteristics of DFIG-based WTGs and SGs can be compared in a similar frame through their internal voltage vectors. The differences and similarities are summarized in support of protection and control designs.

Short-Circuit Modeling of DFIG-Based WTG in Sequence Domain Considering Various Fault- Ride-Through Requirements and Solutions

The doubly fed induction generator (DFIG) based wind turbine generator (WTG) can activate different fault ride through (FRT) control schemes and circuits to comply with various grid code or utility interconnection requirements. These FRT solutions play a key role in the resulting fault current of the DFIG-based WTG as well as the voltages and currents distributed in faulted networks. This paper accounts for their impact by proposing a new generic sequence domain model of the DFIG-based WTG for static short circuit calculations. In this paper, the FRT configurations are classified into three types. The proposed model first calculates the positive- and negative-sequence current phasors under the specified FRT control. Then based on the control, the desired currents and voltages are calculated by explicit equations to determine whether the crowbar could operate during FRT operation. By comparing with detailed electromagnetic transient (EMT) simulations using an EPRI benchmark system, the proposed model, incorporated into an iterative steady state solver, is shown to be accurate to calculate voltage and current phasors in an efficient manner. The proposed model provides a rapid tool for short circuit computations and protective relaying studies considering various grid code requirements and FRT implementations.

Wind energy penetration impact on active power flow in developing grids

Renewable energy (RE) generators are typically connected to the grid via the distribution network or transmission network load buses. With today’s large-capacity RE generators, such as wind turbines, and the environmental concerns, these RE generators could become the grid’s major power source. Thus, renewable energies such as wind are predicted to gradually replace traditional generators. This raises questions about which generator to be replaced first, and what the acceptable limit of RE penetration will be? In this paper, the impact of wind energy penetration on the active power flow in the Nigerian grid was investigated. The integration via load buses was considered, as well as the integration by gradually replacing conventional generators with DFIG-based wind turbine generators (WTG). In each case, the maximum wind energy penetration was determined. The results show that the penetration of wind energy is more when connected to load buses than when it replaces existing generators. Furthermore, when replacing a Nigerian gas-fired generation with offshore WTG, Bus 2 generator (Delta plant) is the first contender, which minimizes the active power loss. However, if the WTG will link via a PQ bus, Bus 11 (Aja) is the best connection point.

Analytical characterization of DFIG response to asymmetrical voltage dips for efficient design

The fault current characteristics of Doubly Fed Induction Generator (DFIG) based wind turbine generator (WTGs) differ from those of traditional generators. Although asymmetrical faults are far more frequent, the studies on transient current characteristics of DFIG-based WTGs are mainly focused on symmetrical faults and consider the variations in voltage magnitude only. This paper presents highly accurate and concise analytical expressions for the stator and rotor currents of DFIG-based WTGs under crowbar protection considering the variations in magnitude and phase angle of both the positive- and negative-sequence voltages during asymmetrical faults. The proposed expressions are validated using a detailed simulation model of a 1.5 MW DFIG-based WTG. The expressions can be directly integrated into protection tools for the evaluation of dynamic performance of relays and design of fault ride through (FRT) solutions.

Dynamic Frequency Support from a DFIG-Based Wind Turbine Generator via Virtual Inertia Control

As the penetrated level of wind in power grids increases, the online system inertia becomes weak. Doubly-fed induction generator (DFIG)-based wind turbine generators (WTGs) are required to provide virtual inertia response to support system frequency. The present inertia control strategy with fixed control gain is not suitable and may cause stall of the DFIG-based WTG, as the virtual inertia response potential from the DFIG-based WTG is different with various wind speed conditions. This paper addresses a virtual inertia control method for the DFIG-based WTGs to improve the system frequency stability without causing stalling of the wind turbine for various wind speed conditions. The effectiveness of the proposed virtual inertia control method is investigated in a small system embedded with the DFIG-based WTG. Results demonstrate that the proposed virtual inertia strategy improves the frequency stability without causing the rotor speed security issue. Thus, the proposed control strategy can secure the dynamic system frequency security of power systems under the scenarios of full and partial loads, and, consequently, the proposed method provides a promising solution of ancillary services to power systems.

Short-Term Frequency Response of a DFIG-Based Wind Turbine Generator for Rapid Frequency Stabilization

This paper proposes a short-term frequency-response scheme of a doubly-fed induction generator (DFIG)-based wind turbine generator (WTG) for improving rotor speed recovery and frequency nadir. In the energy-releasing period, to improve the frequency nadir and rotor speed convergence by releasing a large amount of kinetic energy stored in the rotating masses in a DFIG-based WTG, the power reference is increased up to the torque limit referred to the power and reduces along with it for a predefined period which is determined based on the occurrence time of the frequency nadir in a power grid. Then, the reference decreases so that the rotor speed is forced to be converged to the preset value in the stable operating region of the rotor speed. In the energy-absorbing period, to quickly recover the rotor speed, the reference smoothly decreases with the rotor speed and time during a predefined period until it intersects with the maximum power point tracking curve. The simulation results demonstrate that the proposed scheme successfully achieves rapid frequency stabilization with the improved frequency nadir under various wind conditions based on the IEEE 14-bus system.

Modelling and analysis of transient state during improved coupling procedure with the grid for DFIG based wind turbine generator

The aim of this study is to enhance DFIG based Wind Energy Conversion Systems (WECS) dynamics during grid coupling. In this paper, a system modelling and a starting/coupling procedure for this generator to the grid are proposed. The proposed non-linear system is a variable structure system (VSS) and has two different states, before and after coupling. So, two different state models are given to the system to analyse transient stability during the coupling. The given model represents well the transient state of the machine, through which, a behaviour assessment of the generator before, during and after connection is given based on simulation results. For this, a 300 kW DFIG based wind generation system model was simulated on the Matlab/SIMULINK environment. We judge the proposed procedure to be practical, smooth and stability improved.

Synchronization and Frequency Regulation of DFIG-Based Wind Turbine Generators With Synchronized Control

Synchronized control (SYNC) is widely adopted for doubly fed induction generator (DFIG)-based wind turbine generators (WTGs) in microgrids and weak grids, which applies P-f droop control to achieve grid synchronization instead of phase-locked loop. The DFIG-based WTG with SYNC will reach a new equilibrium of rotor speed under frequency deviation, resulting in the WTG's acceleration or deceleration. The acceleration/deceleration process can utilize the kinetic energy stored in the rotating mass of WTG to provide active power support for the power grid, but the WTG may lose synchronous stability simultaneously. This stability problem occurs when the equilibrium of rotor speed is lost and the rotor speed exceeds the admissible range during the frequency deviations, which will be particularly analyzed in this paper. It is demonstrated that the synchronous stability can be improved by increasing the P-f droop coefficient. However, increasing the P-f droop coefficient will deteriorate the system's small signal stability. To address this contradiction, a modified synchronized control strategy is proposed. Simulation results verify the effectiveness of the analysis and the proposed control strategy.

Impacts of PLL on the DFIG-based WTG's electromechanical response under transient conditions: analysis and modeling

Increased penetration of wind energy in the electric grid has necessitated studying the impact of wind integration on the transient stability of the power system, with urgency to develop appropriate electromechanical models of the wind turbine generator (WTG). The representation and control of the WTG's electric signals are typically in a rotational dq coordinate system whose reference angle is provided by a phase-locked-loop (PLL). The PLL is commonly considered as a measurement device and is often absent in existing WTG electromechanical models. This paper studies the impact of PLL on the DFIG-based WTG electromechanical response by theoretical and simulation analyses. The dynamics of the PLL are found to greatly influence the WTG electromechanical response, suggesting that PLL should be regarded as an indispensable control loop rather than a measurement device, and its impact should be modeled when establishing the WTG electromechanical model.

Dynamics of a Flywheel Energy Storage System Supporting a Wind Turbine Generator in a Microgrid

Abstract Integration of an induction machine based flywheel energy storage system with a wind energy conversion system is implemented in this paper. The nonlinear and linearized models of the flywheel are studied, compared and a reduced order model of the same simulated to analyze the influence of the flywheel inertia and control in system response during a wind power change. A quantification of the relation between the inertia of the flywheel and the controller gain is obtained which allows the system to be considered as a reduced order model that is more controllable in nature. A microgrid setup comprising of the flywheel energy storage system, a two mass model of a DFIG based wind turbine generator and a reduced order model of a diesel generator is utilized to analyse the microgrid dynamics accurately in the event of frequency variations arising due to wind power change. The response of the microgrid with and without the flywheel is studied.

Impact of Increased Penetration of DFIG-Based Wind Turbine Generators on Transient and Small Signal Stability of Power Systems

The targeted and current development of wind energy in various countries around the world reveals that wind power is the fastest growing power generation technology. Among the several wind generation technologies, variable speed wind turbines utilizing doubly fed induction generators (DFIGs) are gaining momentum in the power industry. With the increase in penetration of these wind turbines, the power system dominated by synchronous machines will experience a change in dynamics and operational characteristics. Given this assertion, the present paper develops an approach to analyze the impact of increased penetration of DFIG-based wind turbines on transient and small signal stability of a large power system. The primary basis of the method is to convert the DFIG machines into equivalent conventional round rotor synchronous machines and then evaluate the sensitivity of the eigenvalues with respect to inertia. In this regard, modes that are both detrimentally and beneficially affected by the change in inertia are identified. These modes are then excited by appropriate disturbances and the impact of reduced inertia on transient stability performance is also examined. The proposed technique is tested on a large test system representing the Midwestern portion of the U.S. interconnection. The results obtained indicate that the proposed method effectively identifies both detrimental and beneficial impacts of increased DFIG penetration both for transient stability and small signal stability related performance.