Quantitative Comparison of Partitioned-Stator Machines for Hybrid Electric Vehicles

In this paper,three partitioned-stator(PS)machines,namely the PS flux-switching DC-field(PS-FSDC)machine,the PS-FS hybrid-excitation(PS-FSHE)machine,and the flux adjuster FS permanent-magnet(FA-FSPM)machine are proposed.With different flux-regulating mechanisms,all three proposed machines can offer satisfactory flux-weakening capabilities for wide-speed range operations.Unlike the traditional PS machine that installs the armature windings and the excitation sources in the outer-stator and inner-stator,respectively;the proposed machines purposely swap the installation arrangements.Upon the proposed structure,the FA-FSPM machine can fully utilize the stator core for PM material accommodations.As a result,excellent power and torque densities can be achieved.To verify the proposed concepts,these three PS machines are quantitatively compared based on the hybrid electric vehicle(HEV)specifications.

Message from Editors

ELECTRICAL machine systems have found extensive applications,for which various machine topologies have been proposed in the recent years.Performance analysis and optimal design with theoretical methods and multi-field finite element methods are key issues for developing high-end products.Meanwhile,control strategies linked with specific machine topologies and design considerations are rather beneficial for the electrical machine systems,in the aspects of energy efficiency,output smoothness,control precision,dynamic response,and many others.

Optimized Auxiliary Frequency Control of Wind Farm Based on Piecewise Reduced-order Frequency Response Model

With the increasing wind power penetration in the power system,the auxiliary frequency control(AFC)of wind farm(WF)has been widely used.The traditional system frequency response(SFR)model is not suitable for the wind power generation system due to its poor accuracy and applicability.In this paper,a piecewise reduced-order frequency response(PROFR)model is proposed,and an optimized auxiliary frequency control(O-AFC)scheme of WF based on the P-ROFR model is proposed.Firstly,a full-order frequency response model considering the change in operating point of wind turbine is established to improve the applicability.In order to simplify the fullorder model,a P-ROFR model with second-order structure and high accuracy at each frequency response stage is proposed.Based on the proposed P-ROFR model,the relationship between the frequency response indexes and the auxiliary frequency controller coefficients is expressed explicitly.Then,an OAFC scheme with the derived explicit expression as the optimization objective is proposed in order to improve the frequency support capability on the premise of ensuring the full release of the rotor kinetic energy and the full use of the effect of time delay on frequency regulation.Finally,the effectiveness of the proposed P-ROFR model and the performance of the proposed OAFC scheme are verified by simulation studies.

Multiple target implementation for a doubly fed induction generator based on direct power control under unbalanced and distorted grid voltage

This paper presents a multiple target implementation technique for a doubly fed induction generator （DFIG） under unbalanced and distorted grid voltage based on direct power control （DPC）. Based on the mathematical model of DFIG under unbalanced and distorted voltage, the proportional and integral （PI） regulator is adopted to regulate the DFIG average active and reactive powers, while the vector P1 （VPI） resonant regulator is used to achieve three alternative control targets： （1） balanced and sinusoidal stator current; （2） smooth instantaneous stator active and reactive powers; （3） smooth electromagnetic torque and instantaneous stator reactive power. The major advantage of the proposed control strategy over the conventional method is that neither negative and harmonic sequence decomposition of grid voltage nor complicated control reference calculation is required. The insensitivity of the proposed control strategy to DFIG parameter deviation is analyzed. Finally, the DFIG experimental system is developed to validate the availability of the proposed DPC strategy under unbalanced and distorted grid voltage,

Comparison of resonant current regulators for DFIG during grid voltage distortion

We investigate two different kinds of resonant current regulators for a doubly fed induction generator(DFIG) under distorted grid voltage conditions: proportional integral resonant(PIR) regulator with traditional resonant part and vector proportional integral(VPI) regulator with VPI resonant part. Based on the mathematical model of DFIG under distorted grid voltage, the transfer function and frequency response characteristics of the two current regulators are analyzed and compared. The superiority of the VPI current regulator over the PIR regulator is pointed out, and the influence of discretization methods on the performance of the resonant current regulator is studied. All the results are validated by MATLAB simulation and experiments.

Improved Model Predictive Direct Power Control of Grid Side Converter in Weak Grid Using Kalman Filter and DSOGI

When grid side converter(GSC)is connected to weak grid,the small signal instability can happen due to terminal characteristics of the GSC that are incompatible with the grid impedance.This issue is not of wide concern in previous studies of direct power control(DPC)of GSC.By small signal analysis,it is shown that the impedance characteristic of the conventional direct power control GSC is not compatible with the inductive grid impedance in weak grid due to its constant power load behavior.To solve the problem,instead of feeding the DPC controller with direct measured voltage and current,a Kalman filter(KF)is used to obtain filtered output current and a double second-order generalized integrator(DSOGI)is used to obtain filtered voltage at the point of common coupling(PCC).These strategies change the impedance characteristic of the GSC dramatically and make it suitable to operate in weak grid where SCR is 2,while the rapid power response is preserved.The proposed strategy is verified through simulation and controller hardware in loop(CHIL)tests.

Using Inverter-Based Renewable Generators to Improve the Grid Power Quality-A Review

With the increasing penetration of renewable power in the grid,renewable generators are expected to play more important roles rather than merely working as active power sources.Considering the high controllability and considerable idle capacity of the inverter-based renewable generators,various auxiliary functions for them have been studied to benefit the grid.Among these auxiliary functions,the grid power quality improvement is very promising.This paper presents a review of the emerging control strategies for inverter-based renewable generators to improve grid power quality.The related papers are classified with different power quality issues and technical routes,and a comprehensive comparison is presented.The future trends of this research area are also discussed.

High Frequency Resonance in DFIG-Based Wind Farm with Variable Power Capacity

As wind power penetration has been gaining in the power grid for decades,a large number of the doubly fed induction generator(DFIG)based wind farms are being established around the globe.The power capacities of these wind farms may vary around hundreds of MW,and most of the wind farms are connected to long transmission cables whose impedances can not be ignored and require careful attention.Several works have investigated the impedance interaction between the DFIG based wind farm and long transmission cables which may unfortunately cause high frequency resonance(HFR).The main contribution of this paper is to investigate the influence of the variable wind farm capacity on the behavior of the HFR when certain transmission cables are provided.It is found out that the potential HFR may happen in certain wind farms,and the larger wind farm capacity causes more severe HFR due to the relatively weaker grid transmission capability.Simulation results based on Matlab/Simulink are given to validate the analysis of HFR.

Improved three-vector based dead-beat model predictive direct power control strategy for grid-connected inverters

Since only one inverter voltage vector is applied during each duty cycle, traditional model predictive direct power control(MPDPC) for grid-connected inverters(GCIs) results in serious harmonics in current and power. Moreover, a high sampling frequency is needed to ensure satisfactory steady-state performance, which is contradictory to its long execution time due to the iterative prediction calculations. To solve these problems, a novel dead-beat MPDPC strategy is proposed, using two active inverter voltage vectors and one zero inverter voltage vector during each duty cycle. Adoption of three inverter vectors ensures a constant switching frequency. Thus, smooth steady-state performance of both current and power can be obtained. Unlike the traditional three-vector based MPDPC strategy, the proposed three vectors are selected based on the power errors rather than the sector where the grid voltage vector is located, which ensures that the duration times of the selected vectors are positive all the time. Iterative calculations of the cost function in traditional predictive control are also removed, which makes the proposed strategy easy to implement on digital signal processors(DSPs) for industrial applications. Results of experiments based on a 1 kW inverter setup validate the feasibility of the proposed three-vector based dead-beat MPDPC strategy.