Control Winding Current Research Articles

A Proportional-Integral-Resonant Current Control Strategy for a Wind-Driven Brushless Doubly Fed Generator during Network Unbalance

This article proposes a proportional-integral-resonant (PIR) current control strategy for a wind-driven brushless doubly fed generator (WDBDFG) during network unbalance. Firstly, four control objectives of WDBDFG, including eliminating unbalanced currents of power winding (PW), pulsations of control winding (CW) currents, torque, and PW power, are discussed and different from current controls in which the references to PW currents were computed; the CW current references are derived here. Then, an improved CW current controller using a PIR controller is proposed to achieve different control objectives. In contrast with current controls, CW currents are not involved with sequence extraction in the proposed control and can be totally regulated only in a positive synchronous reference frame. Hence, the system control structure is greatly simplified, and dynamic characteristics are improved. Furthermore, in order to obtain completely decoupled control of current and average power, feedforward control, considering all the couplings and perturbances, is also applied in CW current loops. Simulation results for a 2 MW grid-connected WDBDFG show that the proposed control is capable of achieving four control objectives, including canceling CW current distortion, PW current unbalance, pulsations of PW active power or pulsations of reactive power, and machine torque. Its dynamic process is much more smoothly and quickly than that of current controls, and therefore the proposed control has better dynamic control characteristics during network unbalance.

Optimal Control of Brushless Doubly Fed Wind Power Generator under Zero-Voltage Ride-Through

In the grid-connected operation dynamics of brushless doubly fed generators (BDFGs), a dip in the grid voltage is equivalent to suddenly adding a reverse voltage source at the parallel node. By deriving the expressions of the transient current of power winding (PW), control winding (CW), and rotor winding (RW) of a BDFG in the complex frequency domain under a natural state, it was concluded that the overshoot and oscillation time are affected by the CW voltage, the drop degree and phase of the grid voltage, and the rotor speed. Therefore, an optimal control strategy is proposed. A state model with the CW current as the state variable was constructed using the Pontryagin minimum principle. The finite-time integral value of the square of the electromagnetic torque was set as the objective function to achieve the minimum value that could suppress the overshoot and oscillation of the electromagnetic torque, and the optimal CW voltage command value was directly solved to accelerate the convergence of the BDFG’s physical quantities, thereby reducing the amplitude. Finally, the feasibility of the optimal control algorithm was verified using tests on an experimental platform.

Control of Cascaded/Brushless Doubly-Fed Induction Motors With Real-Time Torque Optimization

Brushless doubly-fed induction machines offer a potential alternative to regular doubly-fed induction generators for wind turbines, due to higher reliability and lower maintenance costs. As motors and generators, the brushless machines could also be part of new technological solutions for hybrid-electric propulsion, including aircraft propulsion. A main advantage is that most of the power would be transferred from generators to motors without electronic conversion. The paper proposes a new speed control algorithm that maximizes the available torque within the limits of the control winding currents and ensures anti-windup protection. The maximum torque and the control variables are obtained through simple analytical computations. The derivations of the paper are based on a complex-variable representation that simplifies the mathematical model and the control algorithm. Compact formulas specify the controller parameters as functions of the machine parameters and of the desired closed-loop behavior. The results are tested on a laboratory testbed using a cascaded doubly-fed induction motor and demonstrate fast responses reaching torque limits that are computed in real-time by the algorithm. While control is achieved through the converter interfaced with the control windings, the data shows that most of the active and reactive power is exchanged directly through the power windings. Similar results are obtained in simulations of a larger brushless doubly-fed induction motor.

High-Efficient Nonlinear Control for Brushless Doubly-Fed Induction Machines

The brushless doubly-fed induction machine (BDFIM) is suitable for applications that need adjustable speed within limited ranges. It is a promising machine if its control strategies, such as the maximum torque per total copper losses (MTPCL) control, are well designed. A BDFIM has two stator windings, a power winding (not controllable) and a control winding (CW) (controllable through a partially rated back-to-back converter). Unlike singly-fed electrical machines, which are fully controllable and the MTPCL can be easily implemented to them, implementing the MTPCL control in a BDFIM is challenging. In this paper, the model-based MTPCL control strategy in the presence of a nonlinear controller in the BDFIM drive is developed. To realize this strategy, we first determine an expression in terms of the angle of the CW's current as a controllable variable. The optimal angle of the CW's current, which guarantees the realization of the MTPCL strategy, is then mathematically obtained using the numerical minimization approach. Next, the proposed passivity-based nonlinear controller regulates both the MTPCL criterion and the electromagnetic torque directly as the output variables. The proposed strategy is validated by a TMS320F2833 microcontroller synchronized with a personal computer for a 3 kW prototype D132s-BDFIM.

Backstepping-Based Direct Power Control for Dual-Cage Rotor Brushless Doubly Fed Induction Generator

In this article, we propose a backstepping (BS)-based direct power control (DPC) strategy for the dual-cage rotor brushless doubly fed induction generator (DCR-BDFIG). The relationship between the power winding (PW) output power and the control winding (CW) voltage is derived according to the complete mathematical model of DCR-BDFIG. Based on that, a BS-based DPC controller, also known as a nonlinear controller, is designed with the global asymptotic stability analyzed for the grid-connected DCR-BDFIG. Compared with the dual-closed-loop control in the advanced vector control method, only single-closed-loop is required for the PW power in the proposed BS-based control scheme, and thus it has more simple structure and faster responses. The global stability of the proposed control scheme can be easily guaranteed with a Lyapunov function properly selected. In addition, in order to improve the performance of the proposed BS-based DPC strategy under unbalanced network, the resonant controller is introduced to suppress the oscillating components in the PW power, PW current, and CW current, whose harmonic order is twice of the grid frequency referring to the synchronous reference frame. Finally, the detailed experimental results demonstrate the effectiveness of the proposed control strategy.

Dynamic Low Voltage Ride through Detection and Mitigation in Brushless Doubly Fed Induction Generators

Brushless doubly-fed induction generators have higher reliability, making them an attractive choice for not only offshore applications but also for remote locations. These machines are composed of two back-to-back voltage source converters: the grid side converter and the rotor side converter. The rotor side converter is typically used for reactive current control of the power winding using the control winding current. A low voltage ride through (LVRT) fault is detected using a hysterisis comparison of the power winding voltage. This approach leads to two problems, firstly, the use of only voltage to detect faults results in erroneous or slow response, and secondly, sub-optimal control of voltage drop because of static reference values for reactive current compensation. This paper solves these problems by using an analytical model of the voltage drop caused by a short circuit. Moreover, using a fuzzy logic controller, the proposed technique employs the voltage frequency in addition to the power winding voltage magnitude to detect LVRT conditions. The analytical model helps in reducing the power winding voltage drop while the fuzzy logic controller leads to better and faster detection of faults, leading to an overall faster response of the system. Simulations in Matlab/Simulink show that the proposed technique can reduce the voltage drop by up to 0.12 p.u. and result in significantly lower transients in the power winding voltage as compared to existing techniques.

Dual-Mode Control for Brushless Doubly Fed Induction Generation System Based on Control-Winding-Current Orientation

Brushless doubly-fed induction generator (BDFIG) has huge potential due to its high reliability and low maintenance costs. In order to meet the requirements of the modern grid code, the BDFIG system should have dual-mode (i.e., operating in both stand-alone and grid-connected modes) capability. Rather than using two separated systems for two modes, this article proposes a unified control architecture for dual-mode operation. Such a unified architecture is based on the stand-alone control-winding (CW) current orientation, and the grid-connected functions are realized by adding three plug-in functional units: 1) the phase-lock function is realized by adding an increment adjustment controller into the CW-angle control; 2) the inrush current during mode switching is suppressed by adding a high-pass feedforward controller into the inner-loop controller; 3) the active power and reactive power are regulated by adding an amplitude-angle controller of CW current. With the proposed unified architecture, the high-performance dual-mode operation is achieved without breaking the original CW-current orientation system, facilitating the overall system design a lot. The detailed analysis and designs are given in this article, and the experiments based on a BDFIG prototype are given to verify the feasibility.

Dual-Objective Control Using an SMC-Based CW Current Controller for Cascaded Brushless Doubly Fed Induction Generator

This article proposes a dual-objective control method using a sliding-mode control (SMC)-based control winding (CW) current controller for cascaded brushless doubly fed induction generator (CBDFIG), where the smooth grid synchronization and flexible grid-connected power regulation are achieved with the same CW current controller. A so-called virtual grid flux method is introduced to obtain the CW current reference value for the grid synchronization. In addition, according to the relationship between the power winding (PW) power and CW current, a novel direct power control method is proposed for CBDFIG. Furthermore, a unified form of the CW current references is investigated to achieve the dual-objective control for CBDFIG, where no mode switching operation is required and constant switching frequency can be achieved in both the control processes with space vector pulsewidth modulation. The proposed control method shows a strong robustness against the grid voltage disturbances, which is necessarily required in both the grid synchronization and grid-connected power regulation. The simulation and experiments are conducted to validate the effectiveness of the proposed control method.

Sensorless Control of Brushless Doubly Fed Induction Generator with Nonlinear Loads for Stand-Alone Power Generation Systems

This paper presents a sensorless control scheme for the stand-alone brushless doubly fed induction generator (BDFIG) feeding nonlinear loads. The fundamental and harmonic components of the distorted power winding (PW) voltage caused by the nonlinear loads are extracted and controlled separately. A rotor speed observer is employed to estimate the speed based on the PW voltage and control winding (CW) current without the need of any other machine parameters except for the number of pole pairs. Since the d- and q-axis references of the CW current from the PW voltage control loop contain both dc and ac components, which cannot be tracked easily by conventional PI controllers, a CW predictive current controller is designed to regulate the CW current. Finally, the performance of the proposed control scheme is verified by comprehensive experiments on a 35-kVA prototype BDFIG.

An Improved Rotor Speed Observer for Standalone Brushless Doubly-Fed Induction Generator Under Unbalanced and Nonlinear Loads

The conventional control methods for brushless doubly-fed induction generator (BDFIG) normally employ mechanical sensors to acquire the information of rotor speed, which brings many disadvantages in the cost, complexity, reliability, etc. This paper presents an improved rotor speed observer (RSO) for the sensorless operation of standalone BDFIGs, which is based on the power winding (PW) voltage and control winding (CW) current. In order to eliminate the impact of unbalanced and nonlinear loads on the RSO, second-order generalized integrators (SOGIs) and low-pass filters (LPFs) are introduced to pre-filter the PW voltage and CW current, respectively. Through comprehensive parameter design, the response speed of the improved RSO will be not lower than that of the basic RSO with ensuring the filtering effect of these additional filters. In addition, the proposed RSO is independent to machine parameters except the pole pairs. Comprehensive experiments are conducted and results verify the proposed improved RSO applied to the standalone BDFIG. Also, the applicability of the proposed RSO on another dual-electrical-port machine, doubly-fed induction generator, is confirmed by simulation results.

Protection scheme for turn-to-turn faults of magnetically controlled shunt reactor based on waveform similarity comparison

Magnetically controlled shunt reactor (MCSR) is one of the key elements applied in EHV/UHV transmission systems for reactive power compensation and overvoltage suppression. Due to its complex core structure and winding connection mode, the accurate identification of turn-to-turn faults in MCSRs becomes challenging. In addition, the existing protection scheme of MCSR is liable to mal-operate under energization due to the similar features of internal fault. In order to solve this problem, a novel protection scheme is proposed by analyzing the waveform similarity and frequency spectrum of three-phase control winding currents. The waveform similarity is measured by the Euclidean distance (ED) between any two of three-phase control winding currents after phase shift. The EDs approach to zero under normal operation but increase under internal faults or pre-excited energization. Internal faults can be further distinguished from the pre-excited energization according to frequency spectrum differences of control winding currents. Finally, simulations in MATLAB/Simulink verify the proposed algorithm, which helps to promote the application of MCSRs in engineering.

Virtual Synchronous Control Based on Control Winding Orientation for Brushless Doubly Fed Induction Generator (BDFIG) Wind Turbines Under Symmetrical Grid Faults

The Brushless Doubly Fed Induction Generator (BDFIG) has huge potential for wind power systems due to its high reliability and low maintenance cost. To add inertia for system stability enhancement, as well as to maintain the uninterrupted operation during symmetrical grid faults, this study proposes a Virtual Synchronous Control (VSC) with a transient current compensation strategy for BDFIG. The proposed VSC is realized by regulating the virtual inner electrical potential and angular velocity of BDFIG under Control Winding (CW) current oriented vector control, and compensating for the transient CW current to weaken the transient inner electrical potential under symmetrical grid faults. Modeling and analysis of such a VSC strategy are presented in this paper, and a simulation is also made to compare the performances of existing and proposed VSC strategies. It is shown that the merits of the proposed VSC can enhance the fault ride through the ability of the BDFIG system and support the recovery of grid voltage.

Sensorless Control of Brushless Doubly Fed Induction Machine Using a Control Winding Current MRAS Observer

In order to realize sensorless control for brushless doubly fed induction machine (BDFIM), this paper presents a model reference adaptive system (MRAS) observer, designed based on the error of the control winding current. Furthermore, a phase-locked loop is employed to estimate the current winding position and rotor speed. Consequently, a detailed theoretical derivation proves that the MRAS observer is stable and the dynamic performance is good. Thus, it does not cause any estimated speed error in steady state. Moreover, the estimated position error is bounded and trivial, thus its effects on the sensorless control of the BDFIM are neglected. The correctness, feasibility, and robustness of the proposed sensorless control method are verified by means of experimental validation on a 30 kW test rig.

A Robust Control Scheme Based on ISMC for the Brushless Doubly Fed Induction Machine

The brushless doubly fed induction machine (BDFM) has great potential in the wind energy generation system because it offers the advantages of higher reliability and maintenance-free operation. In this paper, a mathematical model in the power windings stator flux orientation frame is discussed by taking the rotor flux as the disturbance. Consequently, the control of the BDFM system is implemented by generating suitable control winding currents. Moreover, in order to eliminate the speed error and compensate the system uncertainties caused by the rotor flux and parameter perturbations, an integral sliding mode variable structure controller for the rotor speed is developed, and the reactive power is adjusted by a proportional-integral (PI) controller in the outer-loop. Furthermore, the control winding currents are regulated by the PI control scheme in the inner-loop. The proposed control frame achieves a good static and satisfactory dynamic control performance. Finally, simulation and experimental results are used to verify the correctness and feasibility of the proposed method.

Analytical expression for LVRT of BDFIG with enhanced current control to CW and reactive power support from GSC

During the low-voltage ride-through (LVRT) of the brushless doubly-fed induction generator (BDFIG), the oscillation of the power winding (PW) flux will induce high current in the control winding (CW), causing possible violation to the current constraint and weakening the power support capability of the machine-side converter (MSC). The existing PI control is designed based on the steady-state operation thus sensitive to the intense oscillation of the PW flux. To isolate the CW current control from the disturbances of the oscillating PW flux, the enhanced CW current control is newly proposed by introducing the change rates of the PW flux and the rotor flux to the inner-loop current control of the MSC as the feed-forward compensation terms. The analytical expressions of the PW current, the CW current, the rotor current, and the CW voltage are newly derived to quantify the LVRT effect of the enhanced control. With the DC-voltage constraint considered, the capacity of the grid-side converter (GSC) is newly utilized to improve the reactive power support capability. Dynamic simulation results show that the enhanced control reduces the oscillation of the CW current, the PW current and the rotor current. The accuracy of the analytical expression is validated by comparing it with the step-by-step simulation results. The selected references of the GSC current provide the additional reactive power support without violation to the constraint of the DC-voltage.

A Current Control Scheme With Back EMF Cancellation and Tracking Error Adapted Commutation Shift for Switched-Reluctance Motor Drive

In this paper, an improved winding current control scheme for switched-reluctance motor drive is developed and evaluated. The current feedback controller (CFBC) is augmented with an observed back electromotive force (EMF) current feedforward controller (CFFC) and a current tracking error adapted dynamic commutation shift controller (DCSC). In the proposed DCSC, the rising winding current tracking errors of all phases are extracted, synthesized, and processed to generate the suitable advanced commutation shift angle. The effects of motor back EMF and nonlinear winding inductance can be reduced to yield improved winding current tracking waveforms under a wide operation range. The gradual improvements in current responses and efficiencies by augmenting the CFBC with the CFFC and then the DCSC are verified experimentally. Then, the robust speed control scheme is properly designed to achieve good acceleration/deceleration, reversible running, and dynamic characteristics.

Comparison of Peak Armature and Field Winding Currents for Different Topologies of 10-MW Superconducting Generators Under Short-Circuit Conditions

This paper studies the peak armature and peak field winding currents for three different topologies of 10 MW partial high temperature superconducting generators (HTSGs) under short-circuit conditions (SCC) by simulation. The investigated partial HTSGs employ copper armature windings and superconducting field windings with different armature and rotor topologies, i.e., iron-cored armature and rotor, air-cored armature and rotor, and iron-cored armature and air-cored rotor. For each HTSG topology, the investigation includes: 1) the field winding current control strategies; 2) the influence of operating field current; and 3) the ratings of circuit breakers for limiting the peak armature and peak field winding currents. The results can provide guidelines for determining the peak armature and peak field currents of HTSGs and also the possibility of limiting them by employing circuit breakers under SCC.

Control Design of the Brushless Doubly-Fed Machines for Stand-Alone VSCF Ship Shaft Generator Systems

This paper presents a stand-alone variable speed constant frequency (VSCF) ship shaft generator system based on a brushless doubly-fed machine (BDFM). In this system, the output voltage amplitude and frequency of the BDFM are kept constant under a variable rotor speed and load by utilizing a well-designed current vector controller to regulate the control winding (CW) current. The control scheme is proposed, and the hardware design for the control system is developed. The proposed generator system is tested on a 325 TEU container vessel, and the test results show the good dynamic performance of the CW current vector controller and the whole control system. A harmonic analysis of the output voltage and a fuel consumption analysis of the generator system are also implemented. Finally, the total efficiency of the generator system is presented under different rotor speeds and load conditions.

Control design and experimental verification of the brushless doubly‐fed machine for stand‐alone power generation applications

In this study, a control scheme for the stand-alone generation system based on brushless doubly-fed machine (BDFM) is presented. The output voltage amplitude and frequency are kept constant under variable rotor speed and load by regulating the amplitude and frequency of the control winding (CW) current of BDFM appropriately. The control scheme utilises a CW current vector controller as the inner loop, and a power winding voltage amplitude controller and a frequency controller as the outer loop. The proposed control scheme has been implemented on a prototype BDFM designed for the variable-speed stand-alone ship shaft generation application. Moreover, the satisfactory dynamic performance and low total harmonic distortion of the output voltage in the proposed stand-alone generation system is verified by experiments with two kinds of typical loads: three-phase induction motor load and three-phase RL inductive load.

Research of Voltage Amplitude Fluctuation and Compensation for Wound Rotor Brushless Doubly-Fed Machine

This paper presents an unexpected fluctuation of voltage for a 30-kW wound rotor brushless doubly-fed machine (BDFM) in a standalone power generation system. To address this issue, an improved method of harmonic injection is introduced. A voltage and current double closed-loop control strategy, based on control winding (CW) current decoupling, is applied through the entire experiment. Consequently, the experimental outcomes are fairly aligned with the dynamic simulations in most conditions. However, the voltage fluctuation in the power winding (PW) can be observed for a certain speed range. Harmonic analysis is then carried out on experimental data of PW voltage and CW current. A pair of interharmonic waves, which are symmetric with respect to the fundamental wave, is demonstrated to exist in PW voltage and CW current. Due to the analysis, these harmonic conjugate waves are caused by the high-order magnetomotive force tooth harmonics and current harmonics within a specific speed range. Accordingly, the control strategy of harmonic injection is employed to effectively eliminate these harmonic conjugate waves. The experimental results further highlight that the voltage amplitude can be maintained with less harmonic contents by our tailored control scheme. In addition, the formation and presence of harmonic conjugate waves may lead to the voltage amplitude fluctuation of BDFM in the standalone power generation system.