Search Results (20)

This work analyzes high-power wind turbines (WTs) from the Oravita region, Romania. These WTs are based on slip ring induction generator with wound rotor and we propose a modified architecture with two power converters on both the stator and on the rotor, functioning at variable wind speeds spanning a large interval. Investigations developed around a realistic WT model with doubly fed induction generator show how WT control enables variable wind speed operations at optimal mechanical angular speed (MAS), guaranteeing maximal power point (MPP), but only up to a critical wind speed value, after which the electrical power must saturate for reliable operation. In this reliable operating region, blade pitch angle control must be enforced. Variable wind speed acts as a time-varying parameter disturbance but also imposes the MPP operation setpoint in one of the two analyzed regions. To achieve null tracking errors, a double integrator must appear within the MAS controller when the wind speed disturbance is realistically modeled as a ramp-like input; however, inspecting the linearized model reveals several difficulties as described in the paper, together with the proposed solution tradeoff. The study developed around the Fuhrlander-FL-MD-70 1.5[MW] WT model shows that several competitive controllers are designed and tested in the identified operating regions of interest, as they validate the reliable and performant functioning specifications. Full article

The conversion of air currents through wind turbine technology stands as one of the most significant and effective means of generating green electricity. Wind turbines featuring a horizontal axis exhibit the greatest installed capacity. The study establishes a mathematical model for large wind turbines, categorized by megawatt output, utilizing measured data for key parameters, including wind speed, power output from the generator, and rotational speed. The analysis of the system’s behavior on startup—the cut-in wind speed, is conducted by transitioning the electric generator into motor mode. A mathematical model has been established for the dual-powered motor configuration, wherein both the stator and rotor are connected to a common frequency network, facilitating a shift to synchronous motor functionality. The equation that describes the kinetic moment highlights the importance of attaining optimal velocity, while simultaneously accounting for variations in the load angle. These fluctuations are observable in both the power output and the electrical currents. The simulations that have been processed are derived from experimental data, specifically inputs obtained from a 1.5 MW wind turbine located in the Oravita region of southwestern Romania. The paper thus outlines essential elements concerning the functionality of high-power wind turbines that utilize wound rotor induction generators, aiming to guarantee optimal performance from the moment the wind speed reaches the cut-in threshold. Full article

Wound rotor synchronous condensers (WRSCs) are DC-excited rotor machines that utilize rotor winding instead of permanent magnets. Their voltage regulator controls the rotor field to generate or absorb reactive power, thereby regulating grid voltage or improving power factor. A key characteristic of a WRSC is the compounding curve, which shows the required rotor current under specific stator current and voltage conditions. This paper presents an approach for quickly calculating the electromagnetic parameters of a WRSC using a mathematical method. After determining magnetic flux density, induced voltage, and inductance through analytical methods, the Park and Clarke transformations are applied to derive the dq-frame quantities, enabling prediction of active and reactive powers and compounding curve characteristics. The 60 Hz model was evaluated through comparison with finite element method (FEM) simulations. Results of flux density, induced voltage, and the compounding curve under varying rotor and stator current conditions showed that the proposed method achieved comparable performance to FEM simulation while reducing computational time by half. Full article

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. Full article

This paper presents a new online method based on low frequency signal injection to estimate the stator resistance of a Wound Rotor Synchronous Machine (WRSM). The proposed estimator provides a parameter-free method for estimating the stator resistance, in which there is no need to know the values of the parameters of the machine model, such as the stator and rotor inductances or the rotor flux linkage. In this method, a low frequency sinusoidal current is injected in the d axis of the stator current to produce a sinusoidal flux in the stator. In this paper, it is shown that the phase difference between the generated sinusoidal flux and the injected sinusoidal current is related to the stator resistance mismatch. Using this phase difference, the stator resistance is estimated. To validate the proposed model-free estimator, simulations were performed with Matlab Simulink and the results were compared with the extended Kalman filter observer. Finally, experimental tests, under different conditions, were performed to estimate the stator resistance of a WRSM. Full article

This paper presents a topology for the brushless operation of a wound-rotor synchronous machine based on the subharmonic excitation technique by using two sets of multi-pole windings on the armature as well as on the rotor. The armature windings consist of a four-pole three-phase main winding and a two-pole single-phase additional winding, responsible for the generation of fundamental and subharmonic components of magnetomotive force (MMF), respectively. The rotor contains four-pole field winding and two-pole excitation winding. From the generated air gap MMF, the additional winding is responsible for induction in excitation winding, which feeds DC to the field winding through a rotating rectifier without the need of brushes. Then, the interaction of the magnetic field from the main and the field windings produces torque. The proposed topology is analyzed using 2D finite element analysis (FEM). From the analysis, the generation of the subharmonic component of MMF is verified, which helps in achieving the brushless operation of the wound-rotor synchronous machine. Furthermore, the performance of the proposed brushless multi-pole topology is compared with the existing dual three-phase winding multi-pole topology in terms of current due to induction, output torque, torque ripples, and efficiency. Full article

Data-driven methods have prominently featured in the progressive research and development of modern condition monitoring systems for electrical machines. These methods have the advantage of simplicity when it comes to the implementation of effective fault detection and diagnostic systems. Despite their many advantages, the practical implementation of data-driven approaches still faces challenges such as data imbalance. The lack of sufficient and reliable labeled fault data from machines in the field often poses a challenge in developing accurate supervised learning-based condition monitoring systems. This research investigates the use of a Naïve Bayes classifier, support vector machine, and k-nearest neighbors together with synthetic minority oversampling technique, Tomek link, and the combination of these two resampling techniques for fault classification with simulation and experimental imbalanced data. A comparative analysis of these techniques is conducted for different imbalanced data cases to determine the suitability thereof for condition monitoring on a wound-rotor induction generator. The precision, recall, and f1-score matrices are applied for performance evaluation. The results indicate that the technique combining the synthetic minority oversampling technique with the Tomek link provides the best performance across all tested classifiers. The k-nearest neighbors, together with this combination resampling technique yielded the most accurate classification results. This research is of interest to researchers and practitioners working in the area of condition monitoring in electrical machines, and the findings and presented approach of the comparative analysis will assist with the selection of the most suitable technique for handling imbalanced fault data. This is especially important in the practice of condition monitoring on electrical rotating machines, where fault data are very limited. Full article

This paper proposes a new scheme for a brushless wound rotor synchronous machine (WRSM) by generating an additional, two-pole component of magneto-motive force (MMF) with a series-connected additional three-phase winding with the armature three-phase winding. Unlike existing brushless excitation schemes, which use the inverter to inject harmonic currents in the stator windings, the proposed scheme uses series-connected additional winding on the stator with the armature winding in a two-pole configuration. Consequently, as the current flows in the armature winding, it creates a fundamental rotating air gap flux to interact with the field flux. At the same time, additional rotating flux is created from the additional three-phase winding, which cannot synchronize with the field winding. This additional flux can cause the induction of a voltage in a winding with exactly the same number of poles. For this purpose, a harmonic winding is installed in the rotor along with the field winding connected through a diode bridge rectifier, in order to feed the direct current (DC) to the field winding for rotor excitation without an input current from the brush-slip-ring assembly. The 2D finite-element analysis (FEA) was performed to validate the brushless operation of the proposed machine system. Full article

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 $d-q$axis 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. Full article

Accurate online diagnosis of incipient faults and condition assessment on generators is especially challenging to automate through supervised learning techniques, because of data imbalance. Fault-condition training and test data are either not available or are experimentally emulated, and therefore do not precisely account for all the eventualities and nuances of practical operating conditions. Thus, it would be more convenient to harness the ability of unsupervised learning in these applications. An investigation into the use of unsupervised learning as a means of recognizing incipient fault patterns and assessing the condition of a wound-rotor induction generator is presented. High-dimension clustering is performed using stator and rotor current and voltage signatures measured under healthy and varying fault conditions on an experimental wound-rotor induction generator. An analysis and validation of the clustering results are carried out to determine the performance and suitability of the technique. Results indicate that the presented technique can accurately distinguish the different incipient faults investigated in an unsupervised manner. This research will contribute to the ongoing development of unsupervised learning frameworks in data-driven diagnostic systems for WRIGs and similar electrical machines. Full article

Monitoring and early fault prediction of large electrical machines is important to maintain a sustainable and safe power system. With the ever-increasing computational power of modern processors, real-time simulation based monitoring of electrical machines is becoming a topic of interest. This work describes the development of a real-time digital twin (RTDT) of a wound rotor induction machine (WRIM) using a precomputed finite element model fed with online measurements. It computes accurate outputs in real-time of electromagnetic quantities otherwise difficult to measure such as local magnetic flux, current in bars and torque. In addition, it considers space harmonics, magnetic imbalance and fault conditions. The development process of the RTDT is described thoroughly and outputs are compared in real-time to measurements taken from the actual machine in rotation. Results show that they are accurate with harmonic content respected. Full article

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. Full article

To present the characteristics of pole number and pole shape of the core, the five-phase wound-field doubly salient generators (WFDSGs) with symmetric phase inductance are studied and optimised in this paper, considering the split ratio, slot fill factor and core fringing effect. Based on the principle and structure of the five-phase WFDSGs, the winding induced electro-motive force under different number of poles is theoretically analysed. The constraints for parameter optimisation design including slot fill factor, split ratio and magnetic density characteristic are given. The finite element models of 30/24-pole and 20/16-pole WFDSG are established, and the comparative simulation analysis is carried out. It is pointed out that when the inner and outer diameters of the stator and rotor, the axial length and the maximum magnetic density are constant, the induction electromotive forces of the WFDSGs with different pole numbers and same phase coil number are same. Considering the pole fringing effect, the rotor pole equivalent width is the sum of the rotor pole actual width and 4 times of the air gap. The comparison experiments between the 30/24-pole and 20/16-pole WFDSGs were carried out, which verified the correctness of the theoretical analysis and finite element analysis (FEA). Full article

In this paper the dynamic behavior of different wind turbine generator configurations including doubly fed induction generators (DFIG), squirrel cage induction generator (SCIG), wound rotor induction generator (WRIG), and permanent magnet synchronous generator (PMSG) under ferroresonant conditions of energization and de-energization was investigated using Simulink/MATLAB (version 2017B, MathWorks, Natick, MA, USA). The result showed that SCIG had the highest overvoltage of 10.1 PU during energization, followed by WRIG and PMSG, while the least was DFIG. During de-energization, PMSG had the highest overvoltage of 9.58 PU while WRIG had the least. Characterization of the ferroresonance was done using a phase plane diagram to identify the harmfulness of the ferroresonance existing in the system. It was observed that for most of the wind turbine configurations, a chaotic mode of ferroresonance exists for both energization and de-energization scenarios. Although overvoltage during energization for wind turbine generator configurations was higher than in the de-energization with an exception of PMSG, their phase plane diagrams showed that de-energization scenarios were more chaotic than energization scenarios. The study showed that WRIG was the least susceptible to ferroresonance while PMSG was the most susceptible to ferroresonance. Full article

The equivalent inductance of three-phase induction motors is experimentally investigated in this paper, with particular reference to the frequency range from 1 kHz to 20 kHz, typical for the switching frequency in inverter-fed electrical drives. The equivalent inductance is a basic parameter when determining the inverter-motor current distortion introduced by switching modulation, such as rms of current ripple, peak-to-peak current ripple amplitude, total harmonic distortion (THD), and synthesis of the optimal PWM strategy to minimize the THD itself. In case of squirrel-cage rotors, the experimental evidence shows that the equivalent inductance cannot be considered constant in the frequency range up to 20 kHz, and it considerably differs from the value measured at 50 Hz. This frequency-dependent behaviour can be justified mainly by the skin effect in rotor bars affecting the rotor leakage inductance in the considered frequency range. Experimental results are presented for a set of squirrel-cage induction motors with different rated power and one wound-rotor motor in order to emphasize the aforesaid phenomenon. The measurements were carried out by a three-phase sinusoidal generator with the maximum operating frequency of 5 kHz and a voltage source inverter operating in the six-step mode with the frequency up to 20 kHz. Full article