Stator Circuit Research Articles

A Strategy to Eliminate Instantaneous Active Power Oscillations and Reduce High Currents in the DFIG During Asymmetric Voltage Sags

Wind turbines are widespread around the globe, and the number of windfarms connected to the grid is continually increasing. The Doubly-Fed Induction Generator (DFIG) plays an important role, since it is one of the most used configurations for wind power generation. DFIG-based wind plants, however, are very sensitive to grid disturbances, specially to voltage sags, as these machines have their stator circuit directly connected to the grid. Voltage sags can result in oscillations in active power, torque and DC link voltage, as well as damage the machine and the back-to-back converter due to high currents that arise in such type of contingency. This work proposes a control strategy applied to the rotor-side converter (RSC) of the DFIG, in order to protect the machine and the back-to-back converter during voltage sags. The aim of the control strategy is to reduce the machine currents and also to remove the oscillating active power caused by unbalanced voltage sags.

Decrease of the thermal overloads of a variable-frequency electric drive at damages in the electric circuit of an induction motor stator

Decrease of the thermal overloads of a variable-frequency electric drive at damages in the electric circuit of an induction motor stator

Performance evaluation of a hybrid fuzzy logic controller based on genetic algorithm for three phase induction motor drive

<p class="Author"><span>It is known that controlling the speed of a three phase Induction Motor (IM) under different operating conditions is an important task and this can be accomplished through the process of controlling the applied voltage on its stator circuit. Conventional Proportional- Integral- Differeantional (PID) controller takes long time in selecting the error signal gain values. In this paper a hybrid Fuzzy Logic Controller (FLC) with Genetic Algorithm (GA) is proposed to reduce the selected time for the optimized error signal gain values and as a result inhances the controller and system performance. The proposed controller FL with GA is designed, modeled and simulated using MATLAB/ software under different load torque motor operating condition. The simulation result shows that the closed loop system performance efficiency under the controller has a maximum value of 95.92%. In terms of efficiency and at reference speed signal of 146.53 rad/sec, this system performance shows an inhancement of 0.67%,0.49% and 0.05% with respect to the closed loop system efficiency performance of the PID, FL, and PID with GA controllers respectively. Also the simulation result of the well designed and efficient GA in speeding up the process of selecting the gain values, makes the system to have an efficiency improvement of 14.42% with respect to the open loop system performance.</span></p>

Simulated topology of doubly powered asynchronous motor with diode clamped inverter

This paper deals with the problem of modelling, control and analysis of a doubly-powered induction machine (DPIM) using a three-level three-phase voltage source inverter known simply as diode clamped inverter (DCI) with a three-phase diode rectifier and a DC link in between. A mathematical model of DPIM and the associated DCI circuit is developed. The stator circuit of the DPIM is supplied directly from an ac system network while its rotor side is supplied via a variable frequency DCI. A multi-loop nonlinear field controller is developed to meet an accurate motor speed, regulation of the mechanical load torque and the overall system power. The results obtained from the simulation in the environment of Simplorer 7 show that the proposed DCI inverter offers great optimisation, little overshoot, and reduced voltage ripples. Consequently, the dynamic behaviour of the motor and the energy efficiency of the overall system are progressively improved and a complete suppress of the influence of the variation in the motor load on the speed of the motor is achieved.

Simulated topology of doubly powered asynchronous motor with diode clamped inverter

This paper deals with the problem of modelling, control and analysis of a doubly-powered induction machine (DPIM) using a three-level three-phase voltage source inverter known simply as diode clamped inverter (DCI) with a three-phase diode rectifier and a DC link in between. A mathematical model of DPIM and the associated DCI circuit is developed. The stator circuit of the DPIM is supplied directly from an ac system network while its rotor side is supplied via a variable frequency DCI. A multi-loop nonlinear field controller is developed to meet an accurate motor speed, regulation of the mechanical load torque and the overall system power. The results obtained from the simulation in the environment of Simplorer 7 show that the proposed DCI inverter offers great optimisation, little overshoot, and reduced voltage ripples. Consequently, the dynamic behaviour of the motor and the energy efficiency of the overall system are progressively improved and a complete suppress of the influence of the variation in the motor load on the speed of the motor is achieved.

On the issue of reactive power compensation in electrical systems

Purpose. Investigation of the effect of reactive power compensation on the regime parameters and the stability of the electrical system. Determination of the permissible levels of reactive power compensation for the stability of load nodes and synchronous generators. Research of self-excitation area of synchronous generator at compensation of reactive power. Methodology. The mathematical models of the electric system in the form of nodal equations, which were solved by the modified Newton method, were used for the research. Mathematical modelling of stability of load nodes and synchronous generators was carried out in Matlab package. Results. Functional dependences of the degree of reactive power compensation on the parameters of the regime of the electric system according to the "deep input" scheme taking into account the daily unevenness of the load are obtained. The limits of reactive power compensation in electrical systems are set, taking into account the stable operation of load nodes and synchronous generator. It is established, that at a certain combination of parameters of an electric network with capacitive sequential compensation of reactive resistances of a stator circuit at compensation of reactive power of loading by means of batteries of static capacitors it is possible Violation of static stability of the type of self-excitation of synchronous generator. Originality. It is established that in networks with capacitive sequential compensation of reactive resistances of the stator circuit of synchronous generator, compensation of reactive power of loading by means of batteries of static capacitors can lead to violation of static Resistance type of self-excitation. Practical value. The technical and economic efficiency of the degree of compensation of reactive power on the parameters of the regime of the electric system according to the "deep input" scheme taking into account the daily unevenness of the load is established. The levels of compensation of reactive power of loading at which the value of a reserve coefficient of static stability on voltage in normal and repairs modes will be not less than the minimum rationed value are determined. The value of the reserve coefficient of static stability on the active power in normal and repairs modes will be not less than the minimum rationed value, and the power factor of the generators should be close to the nominal.

Design of DFIG Converters to Overcome Grid Faults Using Improved Stator Flux Based Field Oriented Control and STATCOM Controller

The doublyfed induction generator based wind energy conversion system is attracting the energy production market for the last decade. It is due to the facts like large rating single unit, withstand to grid disturbances, independent real and reactive power flow control and mainly low power converter ratings. However, under severe short circuit faults, the doubly fed induction generator (DFIG) is constrained to be in synchronism with the grid as is posed by the modern grid codes. For this, dynamic and transient response of DFIG converter unit control needs to be modified for sustainability and reliability. The article will show that DFIG will follow better grid code requirements using proposed improved stator flux based field oriented control scheme in Rotor Side Converter. Further, a three levels STATCOM controller is externally placed near the grid point to further increase its capability under transients. This can be achieved by minimizing DC offset currents to zero by controlling stator flux decay during transients. The stator d and q axis flux wave is circular during steady state and also deviate its shape and characteristics during transients. This feature is restored using proposed FOC technique and helps in maintaining a minimum voltage and current in rotor and stator circuit. This technique not only improves LVRT but also ensures a longer lifetime of the machine during major disturbances.

Start-up of large-power synchronous motor with the 6 kV voltage source inverter and microprocessor-controlled unit for excitation supply

Abstract The paper presents idea and practical implementation of a medium voltage synchronous drive with a voltage source inverter in the stator circuit and a microprocessor controlled unit for excitation supply. Construction of both devices was presented, and methods of their cooperation were described. Selected start-up methods of large power synchronous motors were presented. Exemplary realization was discussed. Measurement results of the direct full-voltage start-up and frequency start-up of a real 1.25 MW 6 kV fan drive system were compared and discussed.

Simulation of Start-Up Behaviour of Induction Motor with Direct Online Connection

In this paper, a simulation of dynamical start-up behaviour of induction motor with direct online connection to the network has been carried out using COMSOL Multiphysics software. The simulated model of the motor mainly consists of a basic FEM model, an electrical model with stator and rotor circuits and a~mechanical model. The chosen motor parameters both electrical and mechanical have been studied and their results presented in this paper. An experimental measurement of the start-up transient of the no-loaded motor was carried out and some its results have been compared to the results from the simulated motor model and analysed.

Research on Short Circuit Current Characteristics of Doubly-fed Wind Power Generator Considering Converter Regulation

When fault occurs in the grid, the short circuit characteristics of doubly-fed induction generator (DFIG) are more complicated due to the coupling influence of generator electromagnetic transient and converter regulation. In view of this problem, a DFIG stator/rotor short circuit current calculation method considering the transient regulation of both rotor-side and grid-side converters is proposed in this paper. First, DFIG fault equivalent network considering the rotor-side converter control is established, and the analytical expressions of stator/rotor currents in this network are derived. On this basis, considering the DC bus voltage fluctuation, the double-loop transient control process of the gird converter in the short circuit process is described quantitatively. Thus, the mechanism of the second harmonic in the stator short circuit current is revealed, and the expression of the harmonic component is derived. Finally, the analytical expressions of stator and rotor short circuit full currents are obtained. Simulation results verify the correctness and feasibility of the proposed method.

Induction Motor Stator Fault Diagnosis by Rotor Slots Harmonics Tracking Using Prony Improved Approach

The short-circuit fault diagnosis is essential in order to avoid irreversible damages that could affect the stator circuit of the induction motor. For this purpose, this paper addresses a new approach based on Prony's high resolution spectral analysis method to avoid the drawbacks of the well-known method based on the power spectral density estimation by Periodogram technique. This new approach, called Root-Prony, allows a better frequency resolution even on a very short acquisition time as well as a better detection even on very small magnitudes. Moreover, and in order to reduce the computation time of this approach, this paper proposes to analyze only the band around the first rotor slot frequencies. The choice of this band allows avoiding any confusion with the frequency signatures of external phenomena which appear most often in very low frequencies. The experimental results obtained show the efficiency and the reliability of this new approach in tracking the induction motor stator winding degradation.

Nonlinear dynamic modeling for fault ride-through capability of DFIG-based wind farm

Fault ride-through (FRT) is a control model enhanced to protect doubly fed induction generator (DFIG) during voltage dip occurring in grid. In this study, stator and rotor circuit dynamic modeling enhanced in terms of simulation performance and fast system responses during instability in DFIG-based wind farm, besides, a FRT capability strategy were enhanced for nonlinear supercapacitor modeling in DFIG-based wind farm. The transient stability analyses of the DFIG with and without supercapacitor as well as positive–negative-sequence dynamic modeling (PNSDM) were compared for three phases, two phases, two-phase-ground and a-phase-ground faults. Furthermore, variations such as DFIG output voltage, DFIG angular speed, DFIG electrical torque and DFIG d–q axis stator current variations were also evaluated. It was found that the DFIG-based wind farm became stable within a short time using the PNSDM and supercapacitor.

Evaluation of the energy characteristics of an asynchronous inverter-fed induction motor at different approximations of the magnetization curve

Results of research on the effect of magnetization-curve approximation of the electrotechnical steel and excitation frequency of an asynchronous inverter-fed induction motor (IFIM) on its energy performance with extremal control algorithms based on energy-saving criteria are presented in this article. An IFIM is a variant of a double-fed motor with static frequency converters in the stator and rotor circuits. Its application in a variable-speed drive meets to the greatest extent the main requirements for an ac machine in terms of speed regulation and providing high energy performance. It is shown that application of piecewise linear approximation by three sections and the spline function provides improved values of energy performance relative to the simple approximation by two linear sections due to becoming close to a real magnetization curve. It provides, under otherwise equal values of electromagnetic torque, a higher flux linkage with decrease in magnetization current. Control of the excitation frequency of the asynchronous inverter-fed induction motor has a significant influence on the steel losses of the machine. In the region of low loads, they are particularly important relative to the active losses in the motor windings.

Enhancement of demagnetization control for low-voltage ride-through capability in DFIG-based wind farm

Low voltage ride through (LVRT) is one of the most popular methods to protect doubly fed induction generator (DFIG) against balanced and unbalanced voltage dips. In this study, a novel LVRT capability strategy is enhanced using forcing demagnetization controller (FDC) in DFIG-based wind farm. Moreover, not only stator circuit but also rotor circuit were developed by electromotor force (EMF) for LVRT in DFIG-based wind farm. The transient stability performances of the DFIG with and without the FDC and EMF were compared for three- and two-phase faults. In addition to variations such as 34.5 kV bus voltage and terminal voltage of DFIG, speed of DFIG, electrical torque of DFIG and d-q axis rotor-stator current variations of DFIG were also evaluated. It was seen that the system became stable within a short time using the FDC and EMF.

Stator and Rotor Faults Diagnosis of Squirrel Cage Motor Based on Fundamental Component Extraction Method

Nowadays, stator current analysis used for detecting the incipient fault in squirrel cage motor has received much attention. However, in the case of interturn short circuit in stator, the traditional symmetrical component method has lost the precondition due to the harmonics and noise; the negative sequence component (NSC) is hard to be obtained accurately. For broken rotor bars, the new added fault feature blanked by fundamental component is also difficult to be discriminated in the current spectrum. To solve the above problems, a fundamental component extraction (FCE) method is proposed in this paper. On one hand, via the antisynchronous speed coordinate (ASC) transformation, NSC of extracted signals is transformed into the DC value. The amplitude of synthetic vector of NSC is used to evaluate the severity of stator fault. On the other hand, the extracted fundamental component can be filtered out to make the rotor fault feature emerge from the stator current spectrum. Experiment results indicate that this method is feasible and effective in both interturn short circuit and broken rotor bars fault diagnosis. Furthermore, only stator currents and voltage frequency are needed to be recorded, and this method is easy to implement.

Advanced Generator Ground Fault Protections in Pulp and Paper Mill Applications

Ground faults in field/rotor and generator stator circuits are serious events that can lead to damage, costly repair, extended outages, and loss of revenue. This paper explores advances in field/rotor circuit ground fault and stator ground fault protection. These advanced protection strategies employ ac injection, ground differential protection, and the use of hybrid grounding to reduce both internal generator ground fault levels and facility ground levels in utility-paralleled operating modes.

Modeling of rectangular pulse oscillator of electrical tonal rail circuit

Purpose. Selecting rectangular pulse oscillator circuits, mathematical and physical modeling for the analysis of their work. Methodology. For the manufacture of oscillator circuit it is necessary to determine the rectangular-pulse oscillator circuit based on integrated circuits, perform the analysis, mathematical and physical modelling, to define the parameters of each output signal. Findings. The obtained data allow to form a stationary circuit of rectangular signals oscillator of electrical tonal rail circuit excluding the output frequency and power tuning. Originality. During the work performed the simulation of amplitude-manipulated signal oscillator of electrical tonal rail circuit based on logic elements. Practical value. The data obtained allow to optimize the costs of production of tonal rail circuits equipment.

Parametric Method based Inter-Turn Incipient Short Circuit Stator Fault Detection of Induction Motor

Objectives: The parametric method is proposed to identify the inter-turn incipient short circuit fault of induction motor. Methods/Statistical Analysis: The Parametric methods are the high resolution methods, which are used to detect the frequencies with low signal to noise (SNR) ratio using smaller number of data samples. The subspace parametric methods use the Multi-signal Classification (MUSIC) algorithm to identify the inter-turn incipient fault and its severity. The pseudo spectrum from the MUSIC algorithm estimates the frequencies related to the faulty condition of the induction motor. Findings: The frequencies which are related inter-turn short circuit fault are obtained from the pseudo spectrum and are compared with the estimated frequencies and thus it proves the estimated frequencies. Application/Improvements: The proposed algorithm is used to estimate the harmonic components from the pseudo spectrum which helps to differentiate the healthy and faulty conditions of the motors. Also the severity of the fault is identified from the pseudo spectrum.

Motor current signatures and their envelopes as tools for fault diagnosis

AbstractInduction motor inter-turn short circuit stator fault is identified using motor current signals with the help of Discrete Wavelet Transform along with Power Spectral Density. This methodology is adopted for motor current signals and their envelopes in order to identify a better tool for fault diagnosis. First, the analysis is performed using the original stator current signal, which has the fundamental frequency components along with the slip frequency and other harmonic/fault components. The fundamental component in the stator current can be eliminated by using the envelope of the signal. The transient in the wavelet decomposition can be suppressed by modifying the current envelope. This is done by pre-multiplying the original envelope with the Tukey window. The Power Spectral Density is calculated for both the signatures. Comparison of these two methods show that the proposed analysis based on modified envelope using Discrete Wavelet Transform along with Power Spectral Density offers better resu...

Stator Shape Optimization for Electrical Motor Torque Density Improvement

The shape optimization of the stator and the rotor is important for electrical motor design. Among many motor design parameters, the stator tooth and yoke width are a few of the determinants of noload back-EMF and load torque. In this study, we proposed an equivalent magnetic circuit of motor stator for efficient stator tooth and yoke width shape optimization. Using the proposed equivalent magnetic circuit, we found the optimal tooth and yoke width for minimal magnetic resistance. To verify if load torque is truly maximized for the optimal tooth and yoke width indicated by the proposed method, we performed finite element analysis (FEA) to calculate load torque for different tooth and yoke widths. From the study, we confirmed reliability and usability of the proposed equivalent magnetic circuit.