Current Control Performance Research Articles

Stability and performance of current harmonic controllers for multiphase PMSMs

Multiphase electric machines can provide significant benefits over conventional three-phase machines. A drawback to multiphase machines is that they are known to have problems with stator current harmonics. The harmonics can be eliminated by various current harmonic control methods. However, there appears to be no clear agreement on the most suitable method for multiphase machines. This paper aims to compare different current harmonic controllers in terms of stability and performance under model uncertainty. A detailed theoretical analysis of the harmonic controllers is given by taking a modern multi-input multi-output approach based on a structured singular value analysis. Further, the performance of the harmonic controllers is studied with experimental results from a dual three-phase permanent magnet synchronous machine. The analysis and results of this paper show how to design robust high-performance current harmonic controllers for multiphase machines.

Deadbeat Predictive Current Control of Permanent-Magnet Synchronous Motors with Stator Current and Disturbance Observer

In order to optimize the current-control performance of the permanent-magnet synchronous motor (PMSM) system with model parameter mismatch and one-step control delay, an improved deadbeat predictive current control (DPCC) algorithm for the PMSM drive systems is proposed in this paper. First, the performance of the conventional predictive current control, when parameter mismatch exist, is analyzed, and then a stator current and disturbance observer (SCDO) based on sliding-mode exponential reaching law, which is able to simultaneously predict future value of stator current and track system disturbance caused by parameter mismatch in real time, is proposed. Based on this SCDO, prediction currents are used for replacing the sampled current in DPCC to compensate one-step delay, and estimated parameter disturbances are considered as the feedforward value to compensate the voltage reference calculated by deadbeat predictive current controller. Thus, a composite control method combining the DPCC part and current prediction and feedforward compensation part based on SCDO, called DPCC + SCDO method, is developed. Moreover, based on conventional exponential reaching law, a novel sliding-mode exponential reaching law is proposed to further improve the performance of the DPCC + SCDO method. Simulation and experimental results both show the validity of the proposed current control approach.

Adaptive finite‐control‐set model predictive current control for IPMSM drives with inductance variation

Finite-control-set model predictive control (FCS-MPC) has many advantages in electric drive control systems but needs the accurate knowledge of the system parameters. The performance of the FCS-MPC will be deteriorated under parameter mismatches. This study proposes an adaptive FCS-MPC current control method for interior permanent magnet synchronous machine (IPMSM) drives subject to the inductance variations. The inductances are identified online by an adaptive observer with a recursive algorithm, which is inherently incorporated into the FCS-MPC control process to reduce the additional computational cost. Compensation methods are also proposed to improve the identification accuracy. The simulation and experimental results validate that, the IPMSM current control performance, speed-extension capability and drive efficiency are all improved by the proposed method.

A Three-Phase Digital Current Controller With Improved Performance Indices

Performance of conventional digital current controllers is constrained by transport delays within the feedback acquisition chain, as well as by delays inherent to the pulse width modulation. In this paper, we introduce a novel current controller which provides a very high closed-loop bandwidth, improves the robustness and disturbance rejection, and eliminates the noise and sampling errors in the feedback path. In order to achieve these goals, we suppress the transport delays by introducing an improved execution schedule of the control interrupt and by inserting a cascaded multiplier of differential character. With the novel gain setting rule, the closed-loop bandwidth reaches 17% of the sampling frequency, disturbance rejection capability is doubled, the step response has a negligible overshoot, and the robustness is characterized by the vector margin of 0.65. Experimental verification is performed using an experimental setup with a three-phase inverter, digital controller, and a permanent magnet synchronous motor.

Static Current Error Elimination Algorithm for Induction Motor Predictive Current Control

Predictive current control of induction motors can effectively avoid performance deterioration of control caused by delays in the current loop and improve the dynamic performance of current control. However, owing to measurement errors and parameter changes, deviations can appear between the predictive controller parameters and the actual motor parameters. This might lead to static current error, which can cause problems, including decrease in system’s efficiency, inability to deliver nominal torque, and to operate in torque control mode, among others. Based on an induction motor model, this paper quantitatively analyzes the influence on current control stability caused by errors in the predictive control model parameters. In addition, we present the mathematical relation between errors in model parameters and static current error, and propose an algorithm to eliminate this type of error. The algorithm corrected the parameters for predictive control using $dq$ axis current feedback and eliminated the static error caused by parameter mismatch. Through experimental results, the stability and effectiveness of the proposed method were shown.

Multilevel Power Conditioner and its Model Predictive Control for Railway Traction System

In the rapid development process of a high-speed electrified railway, power quality problems in traction power grid have become increasingly deteriorative. In order to ensure a three-phase balanced traction power grid, a modular multilevel converter based railway traction power conditioner (RTPC) is presented. The RTPC consists of four H-Bridge clusters and filter inductors, and it can be directly connected to traction feeders in a co-phase traction system without insulation transformers. According to the equivalent circuit analysis of RTPC system, it can be considered as four single-phase inversion systems. Based on the equivalent control model of each single-phase cluster, the relationship between the multilevel output voltage and current slopes in a control period is analyzed, and an improved model predictive control is proposed. Moreover, a linear combination of two different output levels is proposed to improve tracking performance of cluster current control, and enhance the waveform quality of ac current. Finally, both simulation and experiment results are presented to verify the effectiveness of the structure and its control method.

A Model Predictive Control-Based Common-Mode Voltage Suppression Strategy for Voltage-Source Inverter

Model predictive control (MPC) method has been developed as a simple and effective current control technique for voltage-source inverters (VSI). The conventional MPC method applied to VSI adopts all the seven voltage vectors (VVs), including one zero VV and six nonzero VVs, to implement the predictive current control in order to improve the current control performance. However, the common-mode voltage (CMV) is large due to the use of zero VV. Although the MPC strategy only using six nonzero VVs can reduce the CMV, its switching frequency is relatively high due to too many switches between nonadjacent VVs. In this paper, a new MPC-based CMV suppression strategy for VSI is proposed and applied to a permanent-magnet synchronous generator (PMSG)-based control system. Only four nonzero VVs, including three adjacent nonzero VVs and one nonadjacent nonzero VV, are utilized to perform the optimization in every sampling period, resulting in switching frequency and calculation effort reduction with no adverse effect on the current response. Moreover, the influence of the MPC-based CMV reduction algorithm on the speed control of PMSG is tested in this paper. The experimental results confirm the effectiveness of the proposed strategy.

Dynamics estimation and generalized tuning of stationary frame current controller for grid-tied power converters

The integration of ac-dc power converters to manage the connection of generation to the grid has increased exponentially over the last years. PV or wind generation plants are one of the main applications showing this trend. High power converters are increasingly installed for integrating the renewables in a larger scale. The control design for these converters becomes more challenging due to the reduced control bandwidth and increased complexity in the grid connection filter. A generalized and optimized control tuning approach for converters becomes more favored. This paper proposes an algorithm for estimating the dynamic performance of the stationary frame current controllers, and based on it a generalized and optimized tuning approach is developed. The experience-based specifications of the tuning inputs are not necessary through the tuning approach. Simulation and experimental results in different scenarios are shown to evaluate the proposal.

Fuzzy‐PI‐based sensorless frequency and voltage controller for doubly fed induction generator connected to a DC microgrid

This study presents fuzzy-proportional–integral (PI)-based sensorless frequency and voltage control strategy for doubly fed induction generator connected to a dc microgrid system. A significant reduction of the costs can be achieved by this topology because only a single dc/ac converter and a diode rectifier are required instead of the traditional back-to-back converter topology. To improve the performance of the rotor current controller, a fuzzy PI-based control algorithm is used. Furthermore, a simple sensorless control technique based on the detection of the stator frequency is employed to detect the rotor position. The sensorless control technique can operate without the knowledge of the machine parameters. The main aim of this study is to keep the stator frequency as well as the dc bus voltage at the desired value under different load and wind speed conditions without using position sensor. Simulation and experimental studies were performed to verify the dynamic and steady-state performances of the proposed control strategy The results show that the proposed strategy not only has an excellent steady state and dynamic performance, but also it is robust against the variation of system parameters such as wind speed and load.

A Sensorless Implementation of the Parabolic Current Control for Single-Phase Stand-Alone Inverters

Parabolic current control is an attractive current control method with fast transient response and constant switching frequency. Due to the good dynamics of the parabolic current control, it can be employed in voltage source inverters to improve the system performance such as minimizing the distortion of current waveforms or voltage waveforms. To implement the parabolic current control, a current sensor is required, associated with the current conditioning circuit and parabolic carrier generators. Since the parabolic current control is based on the real-time information of the inductor current, any phase delay or propagation delay of the sensor itself and the conditioning circuitry, or limited resolution of parabolic carrier generators, could impact the current control performance. Since the parabolic current control compares analog signals to generate the required control signals, noise from the control board impacts the control precision as well. This paper will explore solutions to these problems. First, the inductor current of the voltage source inverter is analyzed and the parabolic current control strategy is studied, then a sensorless parabolic current control method is proposed. The new sensorless parabolic control method utilizes a current emulator to rebuild the inductor current on a microcontroller. To avoid a dc offset on the ac-side output voltage caused by the current emulator, an additional control loop in the current emulator is added. The effectiveness of the proposed methods is experimentally verified by the use of an H-bridge voltage source inverter.

Simplified torque modulated microstepping for position control of permanent magnet stepper motors

In previous methods designed based on field oriented (and/or weakening) control, high performance of current control is required for position control of permanent magnet (PM) stepper motors. This paper proposes a simplified torque modulated microstepping (STMMS) based on singular perturbation theory for position control of the PM stepper motors. Since a two phase frame PM stepper motor model involves slow and fast dynamics, singular perturbation theory is applied to improve the transient response and to reduce the ripple. The torque modulated microstepping method is applied with a simplified current tracking control law obtained from singular perturbed system analysis. Using applied conditions and stability proofs, we find that the proposed control design procedure is simplified. This eliminates the high-bandwidth control efforts for the current tracking required in the previous methods. Furthermore, the STMMS does not use current feedback for the current tracking. It is shown that high performance in the current tracking does not necessarily entail high performance in the position tracking. The proposed method was validated by simulation and experimental results. It is observed that the STMMS not only improved transient response but also reduced ripple in position control over the previous method. Furthermore, the STMMS used the input voltages less than the previous method does.

Characteristics of nearest level modulation method with circulating current control for modular multilevel converter

The nearest level modulation combined with circulating current control (denoted by NLM-C method) is applied for modular multilevel converter. The characteristic of the NLM-C method in terms of output voltage level and total harmonic distortion (THD) is studied and is compared with conventional NLM and the modified level-increasing NLM method without circulating current control (M-NLM method). The impact of circulating current control on the waveform of the ac output voltage with NLM-C method is studied. Compared with the conventional NLM method, besides suppressing the second order harmonic in the circulating current, the NLM-C method can also increase the output voltage level number and reduce the output voltage and/or current THD, which is similar to the M-NLM method. The impact of sampling frequency on the accuracy of circulating current control with NLM-C is analysed. A step-by-step method is presented in order to select the sampling frequency for achieving optimal circulating current control performance when using the NLM-C method. The analysis in this study is verified by simulation results.

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.

New Permanent Magnet Synchronous Motor Current Sensing Phase Delay Compensation Method

This paper presents a method that can improve the performance of permanent magnet synchronous motor current control by minimizing the measured current phase delay caused by the Low Pass Filter(LPF) used to cut off the noises that flowed in when feedback currents are measured. Although existing methods that change the Cutoff Frequency of the LPF can minimize phase delays during high speed rotations, their noise cutoff effects are much lower and this may lead to the decline of control performance. Therefore, in this study, an algorithm that can compensate current phase delays through relatively simple calculations from the synchronous motor d-q axis coordinate transformation matrix and the inverse transformation matrix is proposed and the validity of the proposed method is verified by comparing the waveform of the calculated current with the waveform of actual currents through simulations and experiments.

Small signal analysis for DC bus voltage disturbance resistance of voltage source converter

The DC bus voltage of the voltage source converter (VSC) is prone to large fluctuation and even to losing its stability for the large mutation of grid voltage or DC-link loads. First the characteristics of two-step transmission to power in the VSC are analyzed, proving that it is inevitable for DC bus voltage fluctuation to occur when grid voltage or DC-link loads mutate. A new two-step feedforward control strategy is then put forward for curbing the fluctuation, which overcomes the shortcoming by which the traditional feedforward control strategy is affected for the performance of current control. The small signal models of nonfeedforward control, traditional feedforward control, and two-step feedforward control are established respectively for comparing their performance. It is certified that the two-step feedforward control maximizes the stability of the bus voltage without affecting current control. Finally, the experimental results verify the correctness of the new strategy and the theoretical analysis.

전원 전압 왜곡과 주파수 변동 시 단상 PWM 컨버터의 전류 제어

This paper presents a current control strategy in the synchronous reference frame for a single-phase PWM converter, which ensures sinusoidal input current control under the distorted source voltage and frequency condition. Given that the distorted source voltage distorts the phase angle for PWM converter control, the input current contains the same harmonics as the source voltage. Aside from the distorted voltage, the variation in source frequency reduces the performance of input current control. To achieve sinusoidal input current control under the distorted source voltage and frequency condition, this paper proposes a compensation strategy of current reference with the distortion component extracted from the phase angle and a detection strategy of frequency variation from the output of a synchronous reference frame phase-lock loop. The experimental results confirm the validity of the proposed method under the distorted source voltage and frequency condition.

Tuning of Synchronous-Frame PI Current Controllers in Grid-Connected Converters Operating at a Low Sampling Rate by MIMO Root Locus

Current controller performance is key in grid-connected power converters for renewable energy applications. In this context, a challenging scenario is arising in multi-megawatt wind turbines, where sampling and switching frequencies tend to be lower and lower as power ratings increase. This strongly affects achievable control time constant. With this perspective, this paper presents a systematic procedure for accurate dynamics assessment and tuning of synchronous-frame proportional–integral current controllers, which is based on linear control for multiple-input–multiple-output (MIMO) systems. The dominant eigenvalues of the system are calculated with explicit consideration of time-delay and cross-coupling terms, two factors which clearly impair the system dynamics when considering a low sampling frequency. The proposed methodology is summarized as follows. First, the plant and controller matrices are modeled in state space. Subsequently, the characteristic polynomial of the closed-loop system is obtained and a computer-aided parametric analysis is performed to calculate the MIMO root locus as a function of the control gain. By its inspection, it is possible to identify the gain, which minimizes the current closed-loop time constant. This tuning is suitable for wind turbine applications, taking into consideration cascaded-control structures and grid-code requirements. The validity and accuracy of the analysis is fully supported by experimental verification.

A Real-Time Computation Method With Dual Sampling Mode to Improve the Current Control Performance of the $LCL$-Type Grid-Connected Inverter

Due to the higher attenuation of switching frequency current harmonics, the LCL filter has been widely used in grid-connected inverters. To deal with the resonance of the LCL filter, the capacitor current is usually fed back to damp the resonance actively. However, the computation and pulsewidth modulation (PWM) delays in the digital control system have a significant influence on the active damping method, resulting in poor system robustness. Meanwhile, these delays also reduce the control bandwidth greatly and thus impose a severe limitation on the low-frequency gains. In this paper, a real-time computation method with dual sampling mode is proposed to remove the computation delay from the inner active damping loop and the outer grid-current control loop simultaneously; thus, the system robustness and the control performance can be greatly improved. Moreover, the time duration between the sampling instant and the switching transition of the inverter bridge is extended by the proposed method, which effectively prevents the switching noise distorting the sampled signals. Therefore, the noise immunity of the inverter is also improved greatly. Experimental results from a 6-kW LCL-type single-phase grid-connected inverter confirm the theoretical expectations and the effectiveness of the proposed method.

High-Efficiency Contactless Power Transfer System for Electric Vehicle Battery Charging Application

In this paper, a contactless charging system for an electric vehicle (EV) battery is proposed. The system consists of three parts: 1) a high-frequency power supply from a full-bridge inverter with frequency modulation; 2) a loosely coupled transformer that utilizes series resonant capacitors for both the primary and secondary windings; and 3) a rectification output circuit that uses a full-bridge diode rectifier. With carefully selected compensation network parameters, zero-voltage switching can be ensured for all the primary switches within the full range of an EV battery charging procedure, which allows the use of low ON-state resistance power MOSFETs to achieve high-frequency operation and system efficiency. The design of loosely coupled transformer is simulated and verified by finite element analysis software. For a 4-kW hardware prototype, the peak dc–dc efficiency reaches 98% and 96.6% under 4- and 8-cm air gap conditions, respectively. The prototype was tested with an electronic load and a home-modified EV to verify the performance of constant current and constant voltage control and their transitions.

Current Control for Synchronous Motor Drives: Direct Discrete-Time Pole-Placement Design

This paper deals with discrete-time models and current control methods for synchronous motors with a magnetically salient rotor structure, such as interior permanent-magnet synchronous motors and synchronous reluctance motors (SyRMs). The dynamic performance of current controllers based on the continuous-time motor model is limited, particularly if the ratio of the sampling frequency to the fundamental frequency is low. An exact closed-form hold-equivalent discrete motor model is derived. The zero-order hold of the stator-voltage input is modeled in stationary coordinates, where it physically is. An analytical discrete-time pole-placement design method for two-degrees-of-freedom proportional-integral current control is proposed. The proposed method is easy to apply: only the desired closed-loop bandwidth and the three motor parameters (Rs, Ld, Lq) are required. The robustness of the proposed current control design against parameter errors is analyzed. The controller is experimentally verified using a 6.7-kW SyRM drive.