Stator Side Converter Research Articles

Intelligent Control for Islanded Operation of Doubly Fed Induction Generator based Wind Energy Conversion System coupled to DC Micro-Grid

A single parameter based fuzzy model for controlling the Doubly Fed Induction Generator (DFIG) based Wind Energy Conversion System (WECS) to operate in islanded mode is discussed in this paper. The system employs Rotor Side Converter (RSC) and Stator Side Converter (SSC) connected back-to-back to a dc micro grid. The control scheme for both RSC and SSC employ cascaded control loops: Outer speed and inner current loop for RSC and Outer voltage and inner current loop for SSC respectively so as to provide better response during dynamic conditions. In both control schemes, the outer loop is processed and the error along with its derivative is found to produce a single variable for feeding the fuzzy model. As only one variable is involved, less memory occupancy, less complex control and computation is faster. The RSC control employs the speed reference obtained through power calculator based maximum power tracking method which is novel in its kind. The SSC employs the terminal voltage at PCC for the outer voltage loop. The simulation of the system under constant and variable wind speeds is carried out using Simpower Systems toolbox of MATLAB. The results indicate that the efficacious working of the system with faster response to dynamic variations.

Open-circuit faults diagnosis and Fault-Tolerant Control scheme based on Sliding-Mode Observer for DFIG back-to-back converters: Wind turbine applications

One of the most common types of wind turbines is the Doubly-Fed Induction Generator with a back-to-back converter. The Insulated Gate Bipolar Transistor switch’s open-circuit fault rate compared to the fault total rate in the wind turbine is noteworthy. The converter Insulated Gate Bipolar Transistor open-circuit causes a drawback in output current and as a result, the production performance of the turbine is reduced. In this article, a new method is presented for multiple Insulated Gate Bipolar Transistor open-circuit faults for open-circuit faults diagnosis. After the fault occurs, to maintain the performance of the turbine, fault compensation is performed based on the new Fault-Tolerant Control scheme. The proposed method is inexpensive, and for fault detection and location determining no need for additional sensors and hardware. This method uses residual output based on the Sliding Mode Observer with SAT function to detect faulty phase and overcome chattering Doubly-Fed Induction Generator converter. To residual evaluate and reduce the false alarm rate against changes in operating points and transient currents, the adaptive threshold is used. For efficient control of active or reactive power and dc-link voltage, a six-leg converter control based on Simplified Space Pulse Width Modulation is replaced by a five-leg converter based on Space Vector Modulation. Several experimental results using hardware in the loop Doubly-Fed Induction Generator test setup with a back-to-back converter are presented. It is shown the fault diagnostic algorithm is effective and robust against false alarms for both rotor and stator-side converters. The experimental result presents that the Fault Tolerant Control scheme can make power switch fault easily managed by the hardware reconfiguration and new control strategy adjusting.

Asynchronous Machine with Wind Turbine IRFO Control

The paper deals with a squirrel cage induction generator connected to the grid through a back-to-back converter driven by vector control. The stator-side converter controls the generator torque by means of an indirect vector control scheme. In order to reduce the system dependence from the mechanical system behavior, a torque loop is used in the current reference calculations. The battery energy storage system (BESS) plays a fundamental role in controlling and improving the efficiency of renewable energy sources. Stochasticity of wind speed and reliability of the main system components are considered. The grid-side converter controls the DC bus voltage and the reactive power in order to accomplish the grid codes. Speed control using flow directional control, indirect conventionally uses proportional integral (PI) type current regulators, which achieve satisfactory objectives on torque and flow dynamics. The objective of this article is to present an indirect vector control strategy with oriented rotor flux using current regulators of the proportional integral (PI) type, applied to an asynchronous machine supplied by a voltage inverter, capable of supplying during restrictive stresses, more satisfactory torque and flux responses. The obtained simulation results upon simulation tests of the global system are developed under the MATLAB / Simulink environment and are satisfactory.

Generator and grid side converter control for wind energy conversion system

<p>This paper focuses on the modeling and control of a wind energy conversion chain using a permanent magnet synchronous machine. This system behaves a turbine, a generator, DC/DC and DC/AC power converters. These are connected on both sides to the DC bus, where the inverter is followed by a filter which is connected to the grid. In this paper, we have been used two types of controllers. For the stator side converter, we consider the Takagi-Sugeno approach where the parameters of controller have been computed by the theory of linear matrix inequalities. The stability synthesis has been checked using the Lyapunov theory. According to the grid side converter, the proportional integral controller is exploited to keep a constant voltage on the DC bus and control both types of powers. The simulation results demonstrate the robustness of the approach used.</p>

Three-Phase to Single-Phase Generation System Based on Doubly-Fed Induction Generator

This paper presents a three-phase to singlephase six-leg converter for a wind energy conversion system based on Doubly-Fed Induction Generator (DFIG). The converter is composed of six legs and is divided in three parts: the grid-side converter (GSC), which is responsible for power factor and DC-link voltage controls, the stator-side converter (SSC), which guarantees the control of the stator voltage, and the rotor-side converter (RSC), which is responsible for vector control of the generator. One of the converter’s legs is shared by gridside and stator-side converters. The main advantages of the proposed configuration compared to other topologies presented in the literature are providing balanced threephase voltages to the DFIG, simple control system and not having any limitation with respect to synchronization between grid and stator voltages and DC-link voltage. The system model, pulse-width modulation (PWM) and control strategies are discussed and simulation and experimental results are presented in order to prove the feasibility of the system.

Development and Application of Type-III Turbine Impedance Models Including DC Bus Dynamics

Several small-signal impedance models have been reported for type-III wind turbines for use in impedance-based system stability studies. Common to these models is the assumption that the dc bus between the rotor- and stator-side converters is an ideal voltage source. Under this assumption, there is no dynamic coupling between the two converters and the turbine can be represented by the impedance of the stator-side converter in parallel with the impedance of the induction generator with the rotor-side converter behind it. The assumption is well justified above the second harmonic frequency but introduces considerable error at lower frequencies, especially within 20–30 Hz of the fundamental frequency in which dc bus voltage control is active and dc bus capacitor impedance is high. This paper presents the development and application of full-order sequence impedance models for type-III turbines that include dc bus dynamics. The new models also consider the coupled current responses created by each of the converters as well as their coupling across the dc bus. Inclusion of these additional dynamics leads to more accurate prediction of turbine impedance responses and turbine-grid system stability. A comparison with existing models is presented to show the utility and limitation of each model. A practical resonance problem is also presented to demonstrate the application.

Determination of the most effective switching signal and position of braking resistor in DFIG wind turbine under transient conditions

A quick post fault recovery of wind generator system could be achieved with the help of a series dynamic braking resistor (SDBR). This paper investigates the dynamics, size, optimal location of a series dynamic braking resistor in the doubly fed induction generator (DFIG) structure based on different switching strategy of the braking resistor. Three signals were investigated in switching the SDBR during transient conditions of the DFIG wind generator; the DC-link voltage, rotor current and grid voltage, respectively. Also, three locations were considered in connecting the SDBR in the DFIG system; the stator, rotor and grid side converter, respectively. Furthermore, the optimum size and best insertion time of the SDBR were investigated based on the most effective performance of the SDBR scheme considering its switching signal and position in the DFIG system. Simulations were run in power system computer-aided design and electromagnetic transient including DC (PSCAD/EMTDC). The results show that when the SDBR is connected to the DFIG, improved performance was achieved; however, the stator based SDBR topology in conjunction with a grid voltage actuating signal, fast insertion time and short duration of operation quickly recovered the DFIG wind generator variables during transient conditions.

Effect of Doubly Fed Induction Generator on Transient Stability Analysis under Fault Conditions

Wind is one of the most widely used non-conventional sources of energy. The majority of wind farms use a variable speed Wind Turbines Generator (WTGs) equipped with Doubly-Fed Induction Generator (DFIG) due to their advantages over other WTGs. The analysis of Wind Energy (WE) dynamics with the DFIG – Wind Turbines (WTs) has become an interesting research issue during transient faults. This paper investigates the effects of the different faults on transient stability. The analysis is performed on a 6-bus test system during transient faults and loss of excitation in synchronous generators. The stator-flux oriented vector control approach used for both stator and Rotor Side Converter (RSC) is proposed to mitigate DFIGs impacts on the system stability. The dynamic performance of DFIG variable speed WT under faults conditions are simulated using Neplan® program, considering different disturbance natures.

Study on the Subsynchronous Resonance Control Method Based on the DFIG Grid-Side Converter

DFIG-based series-compensated can increase the capacity of wind power transmission, but it can also cause the sub-synchronous resonance (SSR) problem. Based on the mechanism of subsynchronous resonance in DFIG, the electromagnetic torque variation under the variation of rotor speed can be divided into two parts, i.e. rotor torque variation and stator torque variation. Supplementary controller is added to the stator side converter, which is generated to provide positive damping. MATLAB simulation is carried out to compare the result of supplementary damping. Results show that supplementary damping controller makes DFIG keep stable under various rotor speeds and RSC inner-loop gain.

An Optimum Design for a DC-Based DFIG System by Regulating Gearbox Ratio

This paper proposes an optimum design for the dc-based DFIG generation system in a dc grid to make its cost optimal. As the stator is connected to a dc bus by the stator side converter, its voltage and frequency are not enslaved to an ac grid, which makes it possible to decrease the synchronous speed, rotor speed, and gearbox ratio. Three aspects are improved. First, control targets are optimized to minimize the increased generator current with decreasing gearbox ratio. Second, optimum design of a generation system is made by regulating gearbox ratio at the cost of increase in the generator current. Significant decreases in cost and losses are analytically obtained from the elaborated cost and losses models, whose parameters are taken directly from the practical and commercial wind generation system. Third, the coordinated model predictive control is adopted in the stator flux-oriented frame with a stator flux and rotor current predictive method. Finally, as the synchronous speed and rotor speed decrease to low levels, simulation and experiment studies are carried out to verify the operational feasibility of DFIG and analyze its steady and transient performance.

An overall Control of BDFIG using Direct Power Control for WECS under Unbalanced Grid Voltage Conditions

The paper presents the complete modelling and control strategy of variable speed wind turbine system (WTS) driven brushless doubly fed induction generators (BDFIG). A back-to-back converter is employed for the power conversion exchanged between BDFIG and grid. The wind turbine is operated on the maximum power point tracking (MPPT) mode its maximum efficiency. The control strategy of CW stator side converter (MSC) combines the technique of MPPT and conventional direct power control. In the control system of the grid side converter (GSC) the direct power control based an improved switching table (IST-DPC has been used to maintain a constant DC-Link voltage and the reactive power is set to zero. In addition, the behaviours of BDFIG under unbalanced grid voltage conditions are studied and a power compensation scheme is developed to improve the power quality generation. Simulations results using MATLAB/SIMULINK are presented and discussed on a 3.5 KW BDFIG wind generation system based an cascaded doubly fed induction generator  demonstrate the effectiveness of the proposed control. several advantages, particularly providing sinusoidal line current when the supply voltage is not ideal.

Control scheme for DFIG converter system based on DC‐transmission

This study proposes a novel control scheme for doubly-fed induction generator (DFIG) in DC-grid where its stator and rotor are connected to DC-bus via insulated-gate bipolar transistor (IGBT)-based converters, namely rotor side converter and stator side converter (SSC). Maximum slip frequency strategy is proposed to determine stator frequency and improve stator voltage at low-rotor speed, which could lower the design difficulty of SSC. Stator voltage decreases with stator frequency at low-rotor speed, and keeps its rated value at normal rotor speed. The control strategy of constant stator voltage and constant rotor exciting current is adopted to make DFIG operate at normal and low-rotor speed, respectively, and low-speed wind energy is utilised accordingly. Furthermore hysteresis control strategy is adopted to avoid frequent switching in the strategy switching process. Experiment results validate the feasibility and effectiveness of the proposed control scheme.

Optimized and coordinated model predictive control scheme for DFIGs with DC-based converter system

This paper proposes an optimized and coordinated model predictive control (MPC) scheme for doubly-fed induction generators (DFIGs) with DC-based converter system to improve the efficiency and dynamic performance in DC grids. In this configuration, the stator and rotor of the DFIG are connected to the DC bus via voltage source converters, namely, a rotor side converter (RSC) and a stator side converter (SSC). Optimized trajectories for rotor flux and stator current are proposed to minimize Joule losses of the DFIG, which is particularly advantageous at low and moderate torque. The coordinated MPC scheme is applied to overcome the weaknesses of the field-oriented control technique in the rotor flux-oriented frame, which makes the rotor flux stable and the stator current track its reference closely and quickly. Lastly, simulations and experiments are carried out to validate the feasibility of the control scheme and to analyze the steady-state and dynamic performance of the DFIG.

Micro grid control strategy of DFIG unit based on improved DC grid connected topology

Aiming to the application of the DFIG connected to DC-Microgrids, an improved topology for the DFIG connected to DC-Microgrids is taken into account in this thesis. The stator side loses the support of voltage and frequency of AC point of common coupling bus. A novel control method suitable to the stator side converter (SSC) and the rotor side converter (RSC) of the topology is proposed. The independent control of stator voltage and frequency, the decoupled control of power and variable speed constant frequency of DFIG are achieved in the doubly-fed induction generator connected to DC-Microgrids. which can enhance the capacity of active power transmission of DFIG during the voltage variation.

Microgrid Restraining Strategy Based on Improved DC Grid Connected DFIG Torque Ripple

Aiming to the voltage of the stator side is generated by the modulation of the SSC in the improved topology, especially under the circumstance with the asymmTeric fault of stator side, DFIG’s electromagnTeic torque, amplifies ripple of grid-connected power for the grid side. The novel control mTehod suitable to stator side converter and rotor side converter based on reduced-order resonant controller (RORC) is proposed in this thesis, DFIG’s torque and output power performance are improved. Under the RORC control conditions the transfer functions of stator current and torque control system are established, the amplitude characteristic and the system stability of RORC control are analysed. The simulation results in Matlab/Simulink verify the correctness and validity of the proposed mTehod.

A Control Strategy for DFIM to Minimize DC Bus Voltage Fluctuations and Torque Oscillations

The Direct Power Contro; has many advantages like it avoids the usage of integration of PWM voltages which leads to stable operation even at zero rotor frequency, it is position sensor less and hence will not depend on machine parameters like stator or rotor resistance. In case of network unbalance, if the system is operated with constant active and reactive powers, it leads to oscillations in the electromagnetic torque and currents exchange with the grid will become non- sinusoidal, which is not good for the system as it increases the mechanical stress. In this paper, both the rotor connected converter and grid connected converter are fed with DPC strategy along with that a Torque Oscillations Cancellation scheme is applied to RSC and Proportional Integral control based power references generation strategy without calculating the sequence components and with elimination of DC bus voltage oscillations is applied to stator-side converter in order to achieve non-oscillating torque accompanied by quality improved current exchange with the grid. The simulation results of Doubly Fed Induction Generator with and without fault clearly shows that the performance of the proposed scheme is validated.

Backstepping flux observer for nonlinear control of the direct-drive permanent magnet synchronous generator wind turbines

This article described a backstepping flux observer and a backstepping control strategy applied to the wind energy conversion system based on synchronous generator. The proposed control technique is developed for both converters: the stator-side converter and the grid-side converter. The stator flux is recovered by the backstepping flux observer, and simultaneously, we introduce stator resistance variations to test the robustness of nonlinear backstepping control strategy. Then, MATLAB simulation works show high performances in terms of tracking, stability, and robustness against parameter variations.

A new control strategy of WFSG-based wind turbine to enhance the LVRT capability

This paper proposes a competent and effective scheme to enhance the low voltage ride through (LVRT) capability of wound field synchronous generator (WFSG) based wind turbines (WTs) under unbalanced voltage dip conditions. A technique for grid synchronization against voltage excursions, i.e., a PLL using positive grid voltages with a high selectivity filters (HSFs) is utilized to extract a robust grid voltage synchronization signal irrespective of the mains condition to enhance the overall system performance. Besides, a new controller (adaptive fuzzy RST) for both the stator side converter (SSC) and grid side converter (GSC) current regulation are employed to further improve dynamic performance. Also, a reactive power support scheme to manage the WFSG reactive power during contingencies and fulfill the grid codes obligations is presented. Moreover, an additional device such as braking chopper (BC) circuit is used in the DC-link circuit for stable operation of the wind energy conversion system (WECS) under-line fault. Effectiveness of the proposed control strategy is verified by the numerical simulations.

Improved control scheme of stand-alone DFIG-based wind energy conversion system with battery energy storage system

An improved control scheme for stand-alone DFIG-based wind energy conversion system is proposed. The intermediate DC-link of back-to-back PWM converter is supported with a battery energy storage system to smooth the fluctuation of power caused by the variation of wind speed and loads. A three-phase four-leg converter is employed as the Stator-Side Converter (SSC) to provide a path for the zero-sequence current. Based on the modified topology, the stator voltage of DFIG is directly controlled by the SSC. And the rotor speed of DFIG is regulated by the rotor-side converter for maximum power point tracking. Modified repetitive controllers incorporated with the state-feedback controllers are adopted for the SSC to ensure the desirable stator voltage even under the unbalanced loads or non-linear loads conditions. The proposal avoids the complicated operation of the harmonic extraction, and the positive- and negative-sequence current decomposition, which is verified by the simulation results.

An Adaptive Backstepping Flux Observer for two Nonlinear Control Strategies Applied to WGS based on PMSG

A nonlinear backstepping flux observer (BFO) for two control strategies applied to directly driven wind synchronous generator is proposed. A backstepping control (BC) and a sliding mode control (SMC) strategies are developed for both converters: the stator side converter (SSC) and the grid side converter (GSC). Using Lyapunov stability technique, one may guarantee the global stability of the wind energy conversion. The efficiency of the two non linear control strategies has been proved in terms of tracking and precision. Then, a comparative study between four possible combinations of control is illustrated and simulation results have shown high performances of the wind system controlled by BC strategy. Finally, stator resistance variation is introduced and it proves the robustness of the BC strategy applied to both converters with the BFO.