Wound Rotor Synchronous Generator Research Articles

Alternative Surface-Mounted Permanent Magnet Topology for Reducing Voltage and Torque Harmonics in Shaft Generators

Traditional diesel generators on a merchant ship, composed of a wound rotor synchronous generator and a four-stroke diesel engine, supply electrical power for various loads. Recently, shaft generators for merchant ships have been increasingly replacing diesel generators to reduce CO2 emissions through fuel efficiency improvement. In particular, permanent magnet synchronous generators have replaced induction generators due to their high-efficiency characteristics at light loads. The surface-mounted permanent magnet rotor can be a suitable topology owing to the relatively short constant power range. This generator can also operate as a motor according to the propulsion mode, so minimizing the harmonics of the induced voltage with the torque pulsation being essential. This paper proposes an alternative surface permanent magnet topology. Three magnets comprise one pole, with one bread-loaf magnet and two rectangular magnets. It helps to simplify the magnetization and assembly of the rotor because of the flat bottom shape of the magnet. Due to the low remanence of two rectangular magnets at the pole edge, this rotor structure effectively makes the air-gap magnetic flux density sinusoidal with production costs reduced. The step-skew suppresses higher-order harmonics. The total harmonic distortion comparison of the two-dimensional finite element analysis and the no-load test result shows under 6% difference from the interior permanent magnet prototype machine. A comparison of harmonic characteristics with other rotors shows that the proposed modular pole has sufficient competitiveness compared to the tapered bread-loaf type. It can be applied as a substitute for the tapered bread-loaf magnet in direct-drive ship propulsion systems and is expected to shorten the manufacturing process and time.

Simple and Robust MPPT Current Control of a Wound Rotor Synchronous Wind Generator

In the search for efficient non-permanent magnet variable-speed wind generator solutions, this paper proposes a maximum power point tracking (MPPT) current-control method for a wound rotor synchronous wind generator. The focus is on direct-drive, medium-speed wind generators. In the proposed method, the currents of the wound rotor synchronous generator (WRSG) are optimally adjusted according to the generator speed to ensure maximum power generation from the wind turbine without needing information on wind speed. The design, modeling, and simulation of the MPPT current controllers are done in Matlab/Simulink with the WRSG in the synchronous reference frame. The controller is put to the test using different wind speed profiles between cut-in and rated speeds. The simulation results indicate that the proposed current control method is simple, effective, and robust, suggesting its practical implementation. To validate the simulation results, experimental work on a 4.2 kW WRSG prototype system is presented to demonstrate the stability and robustness of the MPPT current control method in operating the turbine at or near the maximum power point.

A grid forming control for wind energy conversion systems

This paper presents an effective and practical structure to improve voltage and frequency regulation in grid-forming control of self-excited induction generator-based wind energy conversion systems. In the proposed control structure, in addition to a fixed parallel excitation capacitor, a small wound rotor synchronous generator is also used as a reactive power compensator. The excitation voltage of the small synchronous generator is controlled in such a way that by injecting the required amount of reactive power, this generator maintains the balance between the inductive and the capacitive reactive powers in the isolated system, and thus, improves the system voltage regulation. In addition, an active power control is proposed in which the speed of the prime mover of the self-excited squirrel cage induction generator is adjusted in such a way that the active power of the small synchronous generator is maintained at zero value. The proposed power controller, by creating the balance between the generated and the consumed active powers, improves the frequency regulation of the wind energy conversion system. The validity and effectiveness of the proposed structure are demonstrated through both simulation studies and experimental implementation. The obtained results confirm that the poor regulation of voltage and frequency, which is one of the major drawbacks of self-excited induction generator-based wind energy conversion systems, has been significantly mitigated by using the proposed method.

Performance Comparison of Large-Scale Design-Optimized Non-Overlap and Overlap Winding Wound Rotor Synchronous Generators

The use of non-overlap windings rather than overlap windings is a topic for debate. Non-overlap windings are generally cheaper and easier to manufacture but result in a high amount of magnetomotive force sub- and higher-order harmonics. A strategy to reduce these harmonics has been previously published and this study aims to provide a performance comparison of a large-scale wound rotor synchronous generator with a modified non-overlap winding and a classical overlap winding. Hence, design optimisations of a 16/18 and 16/72 pole/slot grid-connected 3 MW wound rotor synchronous generator for medium-speed wind generation are conducted. The optimisations are performed using the modified method of feasible direction gradient-based optimisation because of its simplicity and fast results. Optimal solutions for the 16/18 and 16/72 3 MW models are obtained and the performance of the solutions compared. The results show that the 3 MW 16/18 model performs favourably against the 3 MW 16/72 model, especially regarding the lower active mass.

Enhancing Diesel Generator Fuel Efficiency by Operating in Optimum Variable-Speed Combined with Vienna Converter

Diesel engine generators are commonly used in refinery systems to provide electrical power. The engines are generally Constant Speed Diesel Wound Rotor Synchronous Generator (CSD-WRSG) type and are operated in parallel with ones. In this study, a Variable Speed Diesel Wound Rotor Synchronous Generator (VSD-WRSG) system is proposed, combined with additional AC-to-DC and DC-to-AC converters, in order to meet the low load condition and save the fuel. The Diesel Generator's current rotation, which is set at a constant speed, is then changed to the optimum speed. It varies depending on the load demand. The electrical frequency is controlled through the converters. This research focuses on a single diesel generator with a capacity of 880 kW for supplying 343 kW average load demand. The AC-to-DC converter employs a Vienna Rectifier with hysteresis control. In order to enable smooth power transmission during load changes, a DC-to-AC converter is used with a Virtual Synchronous Generator control method. All the simulations are completed using MatLab/Simulink. The simulation results indicate that the proposed model is effective. When daily load data is employed, the VSD-WRSG system can save up to 5.3% or 121 liters of fuel per day if compared to the CSD-WRSG system.

Prediction of Power Generation Performance of Wound Rotor Synchronous Generator Using Nonlinear Magnetic Equivalent Circuit Method

This paper presents a nonlinear magnetic equivalent circuit method and an electromagnetic characteristic analysis and verification of a wound rotor synchronous generator (WRSG). The reluctance generated by the stator, rotor, and air gap is subdivided to form a reluctance construction. A nonlinear magnetic equivalent circuit (MEC) for the WRSG is constructed and solved by an iteration method. Moreover, to calculate the inductance of the generator, the reluctance circuit of the d−qaxis is constructed, and the inductance of the generator is obtained using the initial relative permeability of the material. Using the electromagnetic parameters obtained via the MEC method, the power generation characteristics of the generator are predicted. The results of this MEC method are also verified by comparing them with the finite element analysis (FEA) results and experimental results.

Experimental implementation of a novel scheduling algorithm for adaptive and modified P&O MPPT controller using fuzzy logic for WECS

SummaryThis article proposes an adaptive and modified Perturb and Observe (P&O) maximum power point tracking (MPPT) algorithm, using fuzzy logic step size controller, for a wind energy conversion system (WECS) based on wound rotor synchronous generator. The modified P&O controller uses additional current information of the dc‐link, to decrease power oscillation around MPP, and choose the correct direction of the perturbation, while wind speed increase condition, depending on the sign of dc‐link current change. So, in this contribution, a modified drift‐free P&O algorithm was chosen because it gives better results, but the duty cycle step size in the proposed MPPT controller has been varied adaptively, using a fuzzy logic inference system based on the variation of the rectifier output power and the previous change of the duty cycle to get fast convergence until the MPP is reached in increasing and decreasing wind speed conditions. Performances and robustness of the proposed speed sensorless MPPT controller are evaluated in an experiment implementation, using a WECS emulator and dSpace DS1104 controller board, where experimental results show that the proposed method guarantees fast convergence with better energy quality and high robustness against speed or load change conditions.

Indirect Vector-Controlled Brushless Doubly-Fed Twin-Stator Induction Generator for Wind Energy Conversion Application

Wind energy conversion systems (WECSs) seem certain to play a major part in the world’s energy future due to their known high power capacity. The maximum power tracking is unavoidable due to the wind velocity variation and the non-linear relation with the turbine mechanical power. Commercial wind turbines are commonly coupled to either doubly-fed induction generators (DFIGs), wound rotor synchronous generators (WRSG) or permanent magnet synchronous generators (PMSGs). The DFIG-based WECS has several advantages over others. One of which is the power converter in such systems only deals with rotor power, hence the converter rating can run at reduced power rating. However, DFIG has the famous disadvantage of the presence of slip rings which leads to increased maintenance costs and outage times. Hence, brushless doublyfed induction machines (BDFIMs) can be considered as a viable alternative at the penalty of complicated controller requirement and limited decoupling control capability due to the machine’s non-linearity. In this paper, an enhanced performance indirect vector controller is proposed for WECS based on brushless doubly-fed twin-stator induction generator (BDFTSIG). The presented controller offers (i) simplified implementation, (ii) decoupled active-reactive power control, and (iii) a wide range of operation. The proposed controller performance is investigated under various loading conditions showing enhanced transient and minimal steady-state oscillations in addition to complete active/reactive power decoupling. The rigorous simulation and experimental results verify the claimed controller effectiveness under all possible operating conditions for sub- and super-synchronous speed range.

Wind Turbines Offshore Foundations and Connections to Grid

Most offshore wind farms built thus far are based on waters below 30 m deep, either using big diameter steel monopiles or a gravity base. Now, offshore windfarms are starting to be installed in deeper waters and the use of these structures—used for oil and gas like jackets and tripods—is becoming more competitive. Setting aside these calls for direct or fixed foundations, and thinking of water depths beyond 50 m, there is a completely new line of investigation focused on the usage of floating structures; TLP (tension leg platform), Spar (large deep craft cylindrical floating caisson), and semisubmersible are the most studied. We analyze these in detail at the end of this document. Nevertheless, it is foreseen that we must still wait sometime before these solutions, based on floating structures, can become truth from a commercial point of view, due to the higher cost, rather than direct or fixed foundations. In addition, it is more likely that some technical modifications in the wind turbines will have to be implemented to improve their function. Regarding wind farm connections to grid, it can be found from traditional designs such as radial, star or ring. On the other hand, for wind generator modeling, classifications can be established, modeling the wind turbine and modeling the wind farm. Finally, for the wind generator control, the main strategies are: passive stall, active stall, and pitch control; and when it is based on wind generation zone: fixed speed and variable speed. Lastly, the trend is to use strategies based on synchronous machines, as the permanent magnet synchronous generator (PMSG) and the wound rotor synchronous generator (WRSG).

Experimental verification and electromagnetic characteristics analysis of wound-rotor synchronous generator using magnetic equivalent circuit method

This paper presents the magnetic field characteristics of a synchronous generator with a salient pole structure using a simple magnetic equivalent circuit (MEC) method. This method aims to reduce the analysis time considering nonlinearity. By using the MEC, a solution for the magnetic flux density is obtained. The linkage flux is derived using the magnetic flux density of the air gap and the armature voltage is obtained. Additionally, results obtained using the finite-element method and MEC are validated against experimental data.

Excitation procedure for brushless wound‐rotor synchronous starter generator with seamless transitions

An analytical model of excitation system of brushless wound-rotor synchronous starter generator for aircraft applications is developed and presented in this study. During engine start-up process, the starter generator operates as a motor to start the aircraft turbine and to assist it up to the self-sustaining speed. On the basis of rotation speed of the starter generator in the engine start-up process, there are three excitation modes for the exciter and two transient modes for the field current of the main generator (MG). In this study, for each excitation mode, the relationship between the excitation current (field current of the exciter) and the field current of MG is investigated by taking into account the influence of power electronic parts. This study also proposes a control method for the generator field current in all excitation modes which can detect optimum operating points for transition between excitation modes. Furthermore, a seamless transient control strategy is proposed for the transient modes of the generator field current. Simulation and experimental results are provided to demonstrate the feasibility of the proposed solution.

유한요소해석을 이용한 권선형 동기발전기의 특성 해석

유한요소해석을 이용한 권선형 동기발전기의 특성 해석

슬롯 수에 따른 100MW급 권선형 동기발전기 발전특성 및 운전특성 비교

슬롯 수에 따른 100MW급 권선형 동기발전기 발전특성 및 운전특성 비교

Sliding Mode Observer-Based Load Angle Estimation for Salient-Pole Wound Rotor Synchronous Generators

Synchronous generator load angle is a fundamental quantity for power system stability assessment, with possible real-time applications in protection and excitation control systems. Commonly used methods of load angle determination require additional measuring equipment, while existing research on load angle estimation for wound rotor synchronous generator has been limited to the estimator based on the generator’s phasor diagram and estimators based on artificial neural networks. In this paper, a load angle estimator for salient-pole wound rotor synchronous generator, based on a simple sliding mode observer (SMO) which utilizes field current, stator voltages, and stator currents measurements, is proposed. The conventional SMO structure is improved with use of hyperbolic tangent sigmoid functions, implementation of the second order low-pass filters accompanied with corresponding phase delay compensation, and introduction of an adaptive observer gain proportional to the measured field current value. Several case studies conducted on a generator connected to a power system suggest that the proposed estimator provides an adequate accuracy during active and reactive power disturbances during stable generator operation, outperforming the classical phasor diagram-based estimator by reducing mean squared error by up to 14.10%, mean absolute error by up to 41.55%, and maximum absolute error by up to 8.81%.

Design, Analysis, and Comparison of Automatic Flux Regulator With Automatic Voltage Regulator-Based Generation System for DC Marine Vessels

The generation system in the dc marine vessel is expected to be comprised of wound rotor synchronous generator (WRSG) which is interfaced with an active front-end (AFE) rectifier. For such system, WRSG flux and dc voltage at the AFE rectifier output are controlled independently. WRSG flux is maintained by the field excitation circuit of WRSG for which automatic voltage regulator (AVR) has been traditionally employed. AVR operates by regulating the WRSG terminal voltage, thus demands additional tuned filter for the application in the dc marine vessel. This filter increases the size of the generation system along with additional cooling requirements at higher load. With this regard, this paper introduces an automatic flux regulator (AFR)-based field excitation system where the WRSG flux is maintained directly and is independent of the measurement of the terminal voltage. Small-signal analysis is done to design the control loop of the AFR-based field excitation circuit. To verify its efficacy, comparison with the AVR-based dc generation system is conducted. The comparison has been done on the basis of control response and marine operation.

Marine turbine system directly connected to electrical grid: Experimental implementations using a nonlinear and robust control

This paper deals with real-time implementations of a nonlinear and robust control applied on a marine turbine system. For the experimental validation which uses real data from Raz de Sein site (Brittany, France), the DC motor drives a wound rotor synchronous generator. A computer pilots this DC motor in order to stand in for the marine turbine behavior. Experimental results show the control successful of both outputs (terminal voltage and speed) making possible direct connection to electrical grid. In addition, we verify the robustness properties of the complete system under hard mechanical and electrical perturbations. Finally, a comparative study highlight the proposed control performances compared with conventional AVR-PSS which is one of the most widely adopted controller in industry for electrical grid stabilization.

Field Current Estimation for Wound-Rotor Synchronous Starter–Generator With Asynchronous Brushless Exciters

Accurate information about the field current of the wound-rotor synchronous starter generator (WRSSG) is prerequisite for optimal start control research for WRSSG in the starting mode. However, as the field winding of WRSSG is in the rotating parts and there is no slip ring and brush mounted in the shaft, the measurement of the field current is not possible. A field current estimation method for the WRSSG from the brushless exciter side was proposed in this paper. In this estimation method, the rotor currents and voltages of the asynchronous brushless exciter were first estimated and then used to calculate the field current of WRSSG through the parametric average-value model of the rotating rectifier. Experimental results verified that the proposed estimation method showed sufficiently accurate estimation performance both in steady and transient states.

Extended SMC for a stand-alone wound rotor synchronous generator

In this paper, a sliding mode controller for the stator voltage amplitude of a stand-alone wound rotor synchronous generator is presented. The standard dq-model of the electrical machine connected to an isolated inductive load is obtained. Then, the control law is designed using sliding mode techniques. The controller introduces a dynamic extension so that the stator voltage amplitude has relative degree one. As a result, a fast controller, robust to both machine and load parameters variations, is obtained. The control algorithm, that does not requires the knowledge of the parameters, is implemented and validated experimentally.

Hybrid Model for Wound-Rotor Synchronous Generator to Detect and Diagnose Turn-to-Turn Short-Circuit Fault in Stator Windings

In this paper, a hybrid modeling approach to model synchronous generator by combining the dq0 modeling with the winding function approach has been proposed. The proposed hybrid approach improves accuracy and reduces complexity of the synchronous generator model for winding fault such as turn-to-turn short-circuit (TTSC) . This paper also proposes a simple method to model localized saturation effect due to TTSC and presents an air-gap function for synchronous machine with winding fault to improve model accuracy under high degrees of TTSC fault. Experimental validation results show that the developed synchronous generator model with TTSC fault is able to provide improved accuracy and consistent performance under wider degrees of TTSC faults in the stator windings.

DC-DC Chopper Excitation Control of WRSG for MPPT in Offshore Wind Farm

Abstract This paper gives an idea of different state of the art of offshore wind farm configurations. One of the configurations uses Wound Rotor Synchronous Generator (WRSG), which is connected to HVDC grid through diode bridge rectifier. Field excitation of the generator is controlled for achieving Maximum Power Point Tracking (MPPT) to have full utilization of wind power. The most important feature of the suggested prototype is its low cost and high efficiency. A working model of this configuration is implemented in hardware using DSPIC and the results are verified.