Generator Stator Currents Research Articles

An Improved Current-Source-Converter-Based Series-Connected Wind Energy Conversion System

The passive rectifier-based current source converter (CSC) used in the series-connected wind energy conversion system features low cost, high reliability, and simple control, but suffers highly distorted generator stator currents. A new CSC-based wind energy conversion system composed of a multi-phase generator, a passive rectifier, a modular medium-frequency transformer-based converter, and a CSC, is proposed. In addition, two versions of the proposed converter are developed for low- and medium-voltage turbine systems respectively. The operation principle is presented, controls are developed, and simulations and experiments are conducted. Both converters offer excellent generator stator current harmonics performance, while also retaining the advantages of existing CSC-based converters.

Power system harmonic analysis with real geomagnetically induced current from the 2003 halloween storm

It is well understood that geomagnetically induced currents (GICs) flow through the grounded neutrals of wye-connected transformers, generating harmonic currents that flow along frequency-dependent transmission lines. Harmonic currents affect protection systems and cause harmonic voltage drops that disturb power system voltage stability. In the literature, GICs are modelled as dc-currents for harmonic analysis. However, GICs are multi-frequency currents having both positive and negative polarities. Therefore, a representative harmonic analysis of power systems is only possible when GICs are modelled as multi-frequency AC currents. In this paper, the authors subjected a multi-machine power system model to real GICs from the 2003 halloween storm. Representative time intervals within the storm were selected, and a harmonic analysis of transmission line and generator stator currents was carried out. The results showed that the harmonic spectrum of currents fluctuated with a real GIC instead of reaching a steady state with a DC model for GIC. Moreover, harmonics induced on generator windings increase the risk of dynamic voltage and power system instability. The results confirm the susceptibility of power system generators to GICs and validate the use of synthetic GIC for modelling the effect of power system stability.

Novel strategy for fault e-diagnosis of wind energy conversion systems using wavelet analysis based on Rt-Lab and Arduino

The diagnosis of wind energy conversion systems (WECS) turns out to be necessary because of their relatively high cost of operation and maintenance. Wind turbines are hard-to-access structures, and they are often located in remote areas. Therefore, a remote diagnosis (e-diagnosis) is required. This paper proposes an alternative approach for the e-diagnosis of a WECS based on the discrete wavelet transform (DWT) and frequency analysis of the aero generator stator currents. To validate this approach, real-time hardware in the loop (HIL) is used to simulate in real-time the mathematical model of the induction generator on the OPAL-RT OP5600 platform to generate the stator currents and the rotor speed. The DWT is applied to the current signal, to generate the DWT signal, which has a huge number of points that are not supported for direct transmission by the Arduino Mega RobotDyn because of its limited sample time. The absolute values of the DWT peak points (MDWT) are sent as point’s packages form to the diagnosis station via the ESP8266 integrated Wi-Fi board of the Arduino Mega RobotDyn to monitor the SCIG states and determine the number of broken bars.

Encoderless Control of PMa-SynRG Based-Stator Current Vector for Wind Generation Systems

In this paper, a novel encoderless control of permanent magnet assisted synchronous reluctance generator (PMa-SynRG) based-stator current vector for renewable energy systems is presented. For the encoderless control, the PMa-SynRG rotor position angle along with rotor angular speed are estimated ultimately from the generator's stator currents, as the stator inductance is only requisite nevertheless the stator resistance. Indeed, this encoderless control technique employs one current control loop for regulating the magnitude of the stator current instead of employing two current control loops. For eliminating the current measurement noise and enhancing the phase angle tracking performance, a modified second-order generalized integrator frequency-locked loop (MSOGI-FLL) technique is employed and compared with the standard SOGI-FLL under different voltage/frequency conditions. The proposed technique has a simple computational implementation while accomplishing a rapid along with precise estimation performance of the PMa-SynRG rotor position angle and speed. The performance is validated using simulations and real-time hardware-in-the-loop Typhoon HIL implementation.

A SDTOGI based speed control method for SPMSG in WECS

Abstract The sensorless techniques are incorporated to optimize the cost and performance of the Surface Mounted Permanent Magnet Synchronous Generator (SPMSG) in the Wind Energy Conversion System (WECS) in this manuscript. Strengthen Dual third-Order Generalized Integrator (SDTOGI) with Synchronous Reference Frame-Phase Locked Loop (SRF-PLL) is applied in SPMSG to estimate the speed & position of the measured generator stator current. The proposed technique is applied in low-speed direct-driven 1.5 MW SPMSG. SDTOGI with SRF-PLL technique is compared with conventional techniques to investigate the efficacy of the proposed sensorless technique due to rapid change in the torque of the turbine. The stability of the proposed system is higher, as shown in the Nyquist plot. Further, the obtained simulation result shows that the proposed method estimates position and speed efficiently. The results are verified in the OPAL-RT 4510 real-time simulator environment to ensure the efficacy of the proposed technique.

Combining synergetic control and super twisting algorithm to reduce the active power undulations of doubly fed induction generator for dual-rotor wind turbine system

Aim. This work presents the amelioration of direct power control using synergetic-super twisting algorithms for asynchronous generators integrated into dual-rotor wind turbine systems. Method. The main role of the direct power control is to control the active and reactive powers and reduce the harmonic distortion of stator current of asynchronous generator for variable speed dual-rotor wind turbine systems. The traditional strategy is more attractive due to its high efficiency and simple algorithm. Super twisting algorithms are a non-linear command strategy; characterized by robustness against the parameters change or disturbances, it gives a good power quality under different conditions such as changing generator parameters. Novelty. Synergetic-super twisting algorithms are designed. Synergetic-super twisting algorithms construction is based on synergetic command and super twisting algorithms in order to obtain a robust control strategy and a fast system with acceptable precision. We use in our study a 1.5 MW asynchronous generator integrated to dual-rotor wind turbine system in order to regulate the active and reactive powers. Results. As shown in the results figures using synergetic-super twisting algorithms the ameliorate performances especially minimizes the torque, active and reactive power undulations, and reduces harmonic distortion of stator current (THD = 0.19 %) compared to traditional strategy.

Wave Excitation Force Estimation Using an Electrical-Based Extended Kalman Filter for Point Absorber Wave Energy Converters

Accurate real-time knowledge of the wave excitation force affecting a wave energy converter (WEC) – either through measurement or by estimation – is crucial for implementing effective control strategies that ensure optimum power absorption, system reliability, and durability. The estimation of the excitation force using other readily available measurements is deemed a cost-effective solution given the technical difficulties associated with directly measuring the excitation force on the WEC’s floater hull. In this study, an electrical-based extended Kalman filter (E-EKF) estimator for estimating the wave excitation force, floater’s heave displacement, and velocity is proposed. The estimator is derived using a holistic nonlinear wave-to-wire model of a direct-drive heaving WEC. A continuous and differentiable approximation of the well-known Tustin friction model is utilized to incorporate the friction force model into the estimator. The proposed E-EKF estimator requires only the measurement of the three-phase permanent magnet linear generator stator currents using current transducers. A practical approach is provided to overcome the need for measuring the wave surface elevation and velocity. Simulations are conducted to assess the goodness of the proposed E-EKF under various sea-state conditions, modeling mismatches, and electric loading scenarios. For the sake of comparison, the performance of the E-EKF estimator is measured against mechanical-based extended Kalman filter and linearized mechanical Kalman filter estimators. The E-EKF estimator exhibits superior performance in terms of nearly all performance metrics, with an excitation energy percentage error score not exceeding 9 %, while being immune to measurement noise.

Direct Power Control of Grid Connected Double-Fed Induction Generator in Wind Energy Conversion System

Direct Power Control is a rotor side control of a doubly fed induction generator by which decoupled control of stator active and reactive power can be obtained is presented. It is based on the measurement of active and reactive power on the grid side where voltages and currents are alternating at fixed frequency. The active and reactive powers are made to track references using hysteresis controllers. The measurements are taken on stator side and the control is made on rotor side. This control system eliminates the need for rotor position sensing and gives an excellent dynamic performance with simulation results for a variable speed constant frequency induction generator system. The system can be operated below, at and above synchronous speed. The modeling of the Doubly Fed Induction Generator (DFIG) system is discussed in this paper. The power is controlled by rotor current injection. The controlling scheme is explained and the modeling of the complete system is done in MATLAB-SIMULINK. The performance of this control technique is observed in wind power applications. Optimum power control and pitch angle control for wind turbine is proposed and the response of generator speed, pitch angle, optimum power, turbine torque, stator current, stator and rotor powers for different wind speeds are shown by simulation results. The direction of stator and rotor power flow is explained with simulation results.

Analysis on SSR frequency components by studying two‐dimensional modulation of synchronous generator's flux linkage

As China's AC transmission system with series capacitor and HVDC transmission technology are widely used in long-distance and large-scale power transmission network, sub-synchronous resonance (SSR) problem becomes more prominent. This paper studied induction generator effect (IGE) and torsional interaction (TI) of synchronous generator by focusing on amplitude and frequency modulation of synchronous generator's flux linkage. IGE involves only electric system dynamics, which will only cause amplitude modulation of flux linkage, resulting in one or two frequency component added in stator current of non-salient pole synchronous generator (NPSG) or salient pole synchronous generator (SPSG) respectively. TI involves both electric and mechanical system dynamics, which will cause both amplitude and frequency modulation of flux linkage, resulting in multiple sub and super-synchronous frequency components added in stator current by means of Bessel function. This result will provide a new reference to understand the SSR phenomena containing multiple frequency components in practical power system. The IEEE first benchmark model is used to demonstrate the accuracy of the analysis results.

Planetary Gear Fault Detection Based on Mechanical Torque and Stator Current Signatures of a Wound Rotor Induction Generator

In this paper, an approach for tooth localized fault detection in the sun gear of a planetary gear, using the measured mechanical torque and the stator current of a wound rotor induction generator (WRIG), is proposed. A theoretical background is developed to demonstrate that the faulty sun gear produces periodic fault signatures in the mechanical torque and consequently fault-related frequencies in the stator current of the WRIG. The employed technique needs knowledge of mechanical system torsional natural frequencies, which aid in identifying the most sensitive fault characteristic harmonics in both mechanical torque and stator current spectra. It is illustrated by simulation and experimental results that the amplitude of these harmonics can be used as a fault index for diagnosis of tooth localized defect in planetary gears. A test rig based on a 5.5 kW WRIG, shaft connected to the sun gear of a planetary gear through a high-speed shaft and couplings, was utilized for validation of the proposed technique.

Current-Based Gear Fault Detection for Wind Turbine Gearboxes

Gearbox faults contribute to a significant portion of the faults and downtime of wind turbines. Gearbox fault detection using the electrical signals acquired from generator terminals has advantages over traditional vibration-based methods in terms of cost, hardware complexity, implementation, and reliability. This paper analyzes the principle of using the nonstationary stator current signals of a generator for the fault detection of a multistage gearbox connected to the generator in varying-speed conditions. Based on the analysis, the characteristic frequencies of various gearbox faults in the frequency spectra of the generator stator currents are identified. A method is then proposed for the fault detection of the gearbox using the current signals. The method consists of an adaptive signal resampling algorithm to convert the nonstationary characteristic frequencies of gearbox faults in the current signals to constant values when the gearbox operates in varying-speed conditions, a statistical analysis algorithm to extract the fault features from the frequency spectra of the resampled stator current signals, and two fault detectors based on the extracted fault features. Experimental results on a two-stage gearbox connected to an electric generator are given to show the effectiveness of the proposed analysis and method for detection of a variety of gear faults in the gearbox.

Detection and modelling of incipient failures in internal combustion engine driven generators using Electrical Signature Analysis

Condition-based maintenance of electric generators have been gaining increasing importance due to the electricity demand and the criticality that this equipment represents to electrical power systems. In this context, this paper proposes a methodology and a system for detection and modeling of incipient failures in the components of internal combustion engine-driven generators based on Electrical Signature Analysis (ESA). The proposed methodology enables the detection of incipient faults both in the prime mover and in the coupled synchronous generator, only relying on measurements of the generator stator voltages and currents. The proposed ESA failure patterns are based on defined frequencies and the structural features of the machine, so they can be reproduced in a wide range of engine-generators sets. The main advantages of the proposed system are its low intrusiveness, feasible installation and cost efficiency. A scale model laboratory has been designed to simulate faults in a small diesel generator and apply the ESA methodology to detect these faults and obtain the failure patterns. Experimental results are presented to prove the effectiveness of the proposed methodology. The main results include the findings that exciter generator unbalance induces electrical unbalance components, exciter diode short circuit induces even harmonics, intake valve failure and piston ring failure induce multiples of rotation frequency components, and mechanical misalignment of the engine generator set induces multiples of half order speed frequency components on ESA. Moreover, the proposed prototype is installed at two large in-service internal combustion engine-driven generators and examples of signal analysis are provided.

Stator Current Harmonic Reduction in a Novel Half Quasi-Z-Source Wind Power Generation System

The generator stator current gets distorted with unacceptable levels of total harmonic distortion (THD) because impedance-source wind power generation systems use three-phase diode rectifiers. The stator current harmonics will cause increasing losses and torque ripple, which reduce the efficiency and stability of the system. This paper proposes a novel half quasi-Z-source inverter (H-qZSI) for grid-connected wind power generation systems, which can reduce the generator stator current harmonics a great deal. When H-qZSI operates in the shoot-through zero state, the derivative of the generator stator current is only determined by the instantaneous value of the generator stator voltage, so the nonlinear relationship between generator stator current and stator voltage is improved compared with the traditional impedance-source inverter. Theoretically, it is indicated that the stator current harmonics can be reduced effectively by means of the proposed H-qZSI. Finally, simulation and experimental results are given to verify the theoretical analysis.

Research on Transient Characteristics of Doubly Fed Wind Power Generator under Symmetrical Short-circuit Fault Considering Grid-side Converter Control

When grid fault occurs, the doubly fed induction generator short-circuit currents are very complex due to the interaction between the doubly fed induction generator and the grid. As the capacity of wind power generation increases, it has become vital to analyze and evaluate the doubly fed induction generator stator/rotor short-circuit currents correctly so that the generator's low-voltage ride-through can be realized, as well as the analysis and calculation of fault electrical variables. In this article, a doubly fed induction generator fault transient model fully considering grid-side converter control and the doubly fed induction generator terminal phase jump is established first. On this basis, the transient expressions of doubly fed induction generator rotor and stator currents under a grid three-phase symmetrical fault are derived. The mechanism in which the short-circuit current is affected by the doubly fed induction generator terminal voltage phase jump and grid-side converter control is then revealed. Thus, a more accurate description of the doubly fed induction generator fault transient process under grid fault is achieved. Time-domain simulation and dynamic experiments both verify that the expression of the short-circuit current obtained according to the proposed method is correct.

A Fault Equivalent Network Based Method for Calculating the Doubly Fed Wind Power Generator Short-circuit Currents

—Current research on short-circuit currents of the doubly fed induction generator is all based on the traditional flux analysis theory. In this article, a novel method to calculate the doubly fed induction generator stator and rotor short-circuit currents is proposed based on the fault equivalent network. First, the fault network is decomposed into a no-fault (normal operation) network and a fault supplementary network. On this basis, considering the influence of the terminal voltage phase jump and the rotor-side converter control, from the perspective of function decoupling, the fault supplementary network is further decomposed into the stator-side fault supplementary network and the rotor-side fault supplementary network. Laplace transformation is then used to calculate the current additions of the two fault supplementary networks. Thus, the analytical expressions of all currents under the grid symmetrical and asymmetrical short circuit could be derived. Finally, both simulation and dynamic experiment results verify that the doubly fed induction generator short-circuit current expressions derived with the proposed method are correct, which facilitates the short-circuit current calculation and relay protection setting of a power grid with a doubly fed induction generator connection.

An investigation of the dynamics of a wind power unit equipped with synchronous generators with permanent magnets

Sudden gusts of wind exert significant forces on the blades of a wind power unit (WPU), creating a torque jump on the shaft of the unit. When operating a wind power unit on the grid, such a regime threatens to distort the dynamic stability of the system. A change in wind speed that is close to real fluctograms, structure of the wind velocity model that allows one to set the difference speed value rise and fall of the wind flow, the maximum value of a gust of wind, and the wind speed before and after a gust of wind are suggested. The system of equations constituting a mathematical model of a synchronous generator with permanent magnets, which is an electromechanical transducer in wind turbines were shown. Research that is carried out on these models determine the conditions under which the system is dynamically stable for different values and forms of perturbing actions of internal constant and electromagnetic coupling of stator and rotor generator circuits. The study allowed us to find the influence of the magnetic energy of a permanent magnet synchronous generator of a wind turbine on the dynamic stability. It is revealed that a larger value of the magnetic energy of permanent magnets per unit volume at constant values of generators active power will increase its dynamic stability. It was also revealed that the most effective way to improve the dynamic stability of a synchronous generator with a wind turbine’s permanent magnet is rapid frequency decrease of the generator stator current during a gust of wind to a certain value, with a further frequency increase to its original value after a gust of wind speed.

Space Vector Modulation Technique Applied to Doubly Fed Induction Generator

In this paper, implementation of SVPWM switching technique for a constant frequency, variable speed wind turbine is carried out. The modeling and simulation is done using MATLAB/Simulink for doubly-fed induction generator. The total harmonic distortion for various parameters like stator current, rotor speed and electro-magnetic torque of this wind electric generator has been obtained. The reduction in distortion level is higher compared to other pulse width modulation techniques.

The Simulation Research of Double Fed Wind Generator Fault Diagnosis Based on MCSA

In order to research the possibility of Motor Current Signature Analysis (MCSA) applied to fault diagnosis of double fed wind turbine generator system and overcome the defect of sensor stalled installed inconveniently with regular vibration inspection method constructs a simulation model of double fed wind turbine generator with SIMULINK, which uses PWM module for stator excitation and the signal of rotating speed and torque of external dynamic changes to simulate the fault of wind turbine motor transmission system, and then verifying the accuracy of fault features of stator current when generator system has fault by emulating and analyzing the response process of signal in the stator current of generator with different situations.

Imbalance Fault Detection of Direct-Drive Wind Turbines Using Generator Current Signals

Imbalance faults constitute a significant portion of all faults in wind turbine generators (WTGs). WTG imbalance fault detection using generator current measurements has advantages over traditional vibration-based methods in terms of cost, implementation, and system reliability. However, there are challenges in using current signals for imbalance fault detection due to low signal-to-noise ratio of the useful information in current signals and nonstationary characteristic frequencies of imbalance faults. This paper proposes a method of using generator stator currents for imbalance fault detection of direct-drive WTGs. In the proposed method, the variable shaft rotating frequency of a WTG is estimated from one phase stator current measured from the generator terminal by using a phase-locked loop method. The estimated shaft rotating frequency is then processed by using appropriate upsampling and variable-rate downsampling algorithms. Consequently, the variable characteristic frequencies of imbalance faults in the spectrum of the estimated shaft rotating frequency are converted to constant values. Therefore, the signatures of wind turbine imbalance faults can be clearly identified from power spectral density analysis of the converted shaft rotating frequency signal. Simulation and experimental results show that the proposed method is effective to detect various imbalance faults in direct-drive WTGs.