Fast Current Control Research Articles

Control of a Dual-Air-Gap Axial Flux Permanent Magnet Machine for a Flywheel Energy Storage System: A Model Predictive Control Approach

This paper presents a fast current control scheme for a dual air-gap axial flux permanent magnet (AFPM) machine based on model predictive control (MPC) approach. This AFPM machine functions as both motor and generator in the flywheel energy storage system (FESS), hence its fast acceleration or deceleration is crucial for rapid storage and release of energy respectively. MPC method, which can pre-calculate the system behaviour and subsequently select optimal control variables, has been employed to control this machine. Performance of MPC for this machine is evaluated and compared with the proportional-integral-derivative controllers. The feasibility and potential of the MPC for this AFPM machine in the FESS have been validated by both simulations and experiments on a machine prototype.

Predictive Fast DSP-Based Current Controller for Thyristor Converters

The predictive current controller presented in this paper provides fast current control for thyristor converters without relying on the accurate load model. Unlike load-model-based predictive methods of current control, this method does not use load parameters in the calculations of the firing angle. Instead, thyristors are fired when the sum of the measured instantaneous load current and the load current rise, which is predicted from the voltage-time area on the dc-side inductor, is equal to the current reference. This way, accurate current control is achieved just by knowing the dc-side inductor parameters and by measuring line voltages, the load current, and the load voltage. The proposed predictive current controller exhibits a practically negligible steady-state error, the fastest possible transient response, and the fastest possible and smooth transitions between continuous- and discontinuous-current conduction modes, for both the rectification and inversion modes of operation of the converter. It shows great stability and robustness. The controller also compensates for sudden changes in both line and load voltages, as well as for voltage drops due to the commutation process. Based on the proposed algorithm, the computer simulation model and the laboratory prototype of the system were developed. Both the simulation and experimental results demonstrated an excellent dynamic performance of the proposed controller.

Control and Performance Analysis for a Capacitor-coordinated Static Synchronous Compensator to Enhance Dynamic Compensation Capability

This article proposes a capacitor-coordinated static synchronous compensator, a combination of a relay-switched shunt capacitor and a voltage-sourced inverter based static synchronous compensator, for both steady-state and dynamic reactive power compensation purposes. The utilization of the shunt capacitor is to restore static synchronous compensator capacity such that the fast voltage control for successive transient disturbances is achievable. To attain tight regulation in AC- and DC-link voltages, a voltage-regulation-oriented current command generator derived from a synchronous reference frame is proposed. To facilitate the static synchronous compensator capacity restoration, the equivalent reactive current of the shunt capacitor is considered to modify the current command. A hysteresis current tracker is employed to take place of the conventional pulse width modulated voltage modulation, providing direct and fast current control. The validity of the proposed direct current controlled capacitor-coordinated static synchronous compensator has been verified by laboratory experiments on a 3-kVA digital signal processor based controlled prototype. The experimental results for the system subjected to successive load changes associated with the switching of the shunt capacitor are presented to demonstrate the effectiveness of the proposed system.

Hybrid Continuous-Wave Demodulating Multipixel Terahertz Imaging Systems

We present an electrooptic (EO) terahertz imaging technique providing a demodulating detector array for phase-sensitive multipixel terahertz detection. The terahertz radiation from a quartz-stabilized microelectronic emitter is mixed with the synchronized laser beat signal of a continuous-wave distributed-feedback diode laser pair. A fast laser current control loop provides stable phase locking between the terahertz emitter and the laser difference frequency, whereby a demodulating near-infrared photonic-mixer-device camera is used for depth-resolving EO terahertz imaging. Alternatively, a femtosecond laser is used for the EO read-out.

Modeling and Control Design for a Very Low-Frequency High-Voltage Test System

With the rapid growth of energy generation by regenerative decentralized sources and the continuous replacement of overhead power lines by subterraneous distribution grids, the amount of power supply cables is increasing. In consequence, a large demand for mobile high-voltage cable test systems is expected within the coming years. This contribution deals with modeling and control design of a novel high-voltage test system based on a zero-voltage switching series-parallel resonant converter and a three-stage Cockcroft-Walton voltage multiplier rectifier, generating a true sinus test voltage of 85 kV (rms) at 0.1 Hz. Due to the inherent high dynamics of the power supply as compared to the slowly varying output test voltage, a cascaded control scheme relying on a fast inner current control loop combined with an outer voltage control is established. In order to derive the relevant waveforms, the complex rectifier was significantly simplified to a one-stage voltage doubler rectifier. By applying a suitable generalized averaging method in combination with an extended describing function, a steady-state solution along with the corresponding small signal model can be established, which is utilized for the current control design. The cascaded control strategy using an observer enables substitution of voluminous and costly high-voltage current sensors. The total high-voltage test system was implemented and validated by experimental tests.

Predictive Stator Current Control for Medium Voltage Drives With LC Filters

The switching frequency of power semiconductor devices in high-power medium voltage drives is limited due to high switching losses. These drives often comprise an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> output filter. The filter introduces a resonant circuit that may be excited, for instance, by fast control transients. One option to avoid such oscillations is to apply damping control methods, which on the other hand usually require switching frequencies well above the filter resonance. This paper presents a predictive stator current controller that avoids the excitation of the filter resonance, realizes active damping and enables fast current control while maintaining low switching frequency. Feasibility and good dynamic performance of the proposed control method is demonstrated by simulation results of a 2.4 kV induction motor drive and by experimental results obtained from a 55 kW prototype ac drive.

Development of Adaptive Hysteresis-band Current Control of PWM Three-Phase AC Chopper with Constant Switching Frequency

Three-phase AC choppers are widely used to obtain a variable alternative voltage from a fixed alternative source for the control of alternative motors at variable speeds. The hysteresis current control is one of the simplest techniques used to control the magnitude and phase angle of three-phase motor currents for high-speed drive systems, primarily because of its implementation simplicity, fast current control response, and inherent peak current limiting capability. However, conventional fixed hysteresis-band current control has a variable switching frequency throughout the fundamental period, and consequently, the load current harmonic ripple is not optimum. In this article, a simple, novel adaptive hysteresis-band current control technique is proposed, where the hysteresis band is controlled as variation of input voltage, output voltage, and slope of the current error to achieve constant switching frequency at any operating condition. The high performances of this novel technique are verified by digital simulation.

Study of the fast ion confinement and current profile control on MAST

One of the main operational aims of the MAST experiment (Darke et al 1994 Proc. 18th Symp. on Fusion Technology (Karlsruhe, Germany, 1994) p 799) and the proposed MAST upgrade is to investigate possible mechanisms to control the q-profile and drive off-axis current. Experiments were carried out to determine the extent to which the q-profile may be modified using two different approaches, transient and steady-state. Transient effects during the plasma current ramp-up phase were investigated with the aim of developing a start-up regime that can later be used as a target plasma for non-inductive current drive or to access advanced modes of operation such as the hybrid or improved H-mode. The most significant effect in this case was observed when early neutral beam injection (NBI) was applied to the fast current ramp-rate start-up plasmas causing reversed magnetic shear and the plasma current to ‘pile-up’ off-axis.In steady-state experiments, in which off-axis NBI was studied, results indicate that broadening the fast ion deposition profile by off-axis neutral beam (NB) injection helps to avoid harmful plasma instabilities and significantly extends the operational window of MAST. Long pulse (>0.65 s) H-mode plasmas were achieved with plasma duration limited only by present machine and NBI engineering limits. In order to match the experimentally observed neutron rate and stored energy a low level of anomalous fast ion diffuse ion (Db ∼ 0.5 m2 s−1) is required. The introduction of the fast ion diffusion broadens the neutral beam current drive profile and degrades the relative contribution of NB driven current from ∼40% to ∼30%. To obtain direct measurements of the current profile, a multi-chord motional stark effect diagnostic has been commissioned on MAST and is currently delivering first results in order to confirm the off-axis location of the NB driven current.

A current ripple reduction of a high-speed miniature brushless direct current motor using instantaneous voltage control

High-speed miniature brushless direct current motor (BLDCM) is used in robots and medical applications because of its high-torque and high-speed characteristics. When compared with the general BLDCM, a high-speed miniature BLDCM has a low electrical time-constant. The current and torque ripple are very high when compared with the conventional pulse-width modulation (PWM) control scheme in the conduction period because of the inherent electrical characteristics. The authors propose a simple instantaneous source voltage and phase current control for torque ripple reduction of a high-speed miniature BLDCM. To reduce the switching current ripple, instantaneously controlled source voltage is supplied to the inverter system according to the motor speed and the load torque. In addition, a fast hysteresis current controller can keep the phase current within a limited band. Computer simulations and experimental results up to 40 000 rpm show the effectiveness and verification of the proposed control scheme.

Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults

The increasing penetration of distributed power generation into the power system leads to a continuous evolution of grid interconnection requirements. In particular, active power control will play an important role both during grid faults (low-voltage ride-through capability and controlled current injection) and in normal conditions (reserve function and frequency regulation). The aim of this paper is to propose a flexible active power control based on a fast current controller and a reconfigurable reference current selector. Several strategies to select the current reference are studied and compared using experimental results that are obtained during an unsymmetrical voltage fault. The results of the analysis allow selection of the best reference current in every condition. The proposed methods facilitate multiple choices for fault ride through by simply changing the reference selection criteria.

스위치드 릴럭턴스 전동기의 히스테리시스 및 PI 전류제어기 응답특성

This paper presents a comparison of different current controller response characteristics of SRM. The most common current controllers of the SRM is hysteresis type. The hysteresis controller is easy to implement and fast current control response, but has the inherent disadvantage of switching frequency variations. The other common type of current controller is PI scheme. The design of a classical PI current controller with fixed parameters for SRM is not an easy task due to the extreme nonlinear characteristics. In this paper, some linearization technique is used for design of PI current controller. Experimental results of 1-hp SRM are presented for the basic reference data which can be used to select the proper current control scheme according to the applications.

A Robust Hybrid Current Control for Permanent-Magnet Synchronous Motor Drive

Recently, the permanent-magnet synchronous motor (PMSM) has found widespread utilization in modern adjustable AC drives. This is achieved by using current-controlled voltage source inverter (VSI) systems. Because of its ease of implementation, fast current control response and inherent peak current-limiting capability, hysteresis current control is considered as the simplest technique used to control the motor currents for an AC machine. On the other hand, the ramp comparator controller has some advantages, such as limiting maximum inverter switching frequency to the carrier triangular waveform frequency and producing well-defined harmonics. In order to take advantage of the position features of both these two controllers, this paper presents the design and software implementation of a hybrid current controller. The proposed intelligent controller is a simultaneous combination and contribution of the hysteresis current controller and the ramp comparator. Comparisons using simulations on a 0.9-kW PMSM confirm that the proposed hybrid current controller gives better performance and has the advantage of conceptual simplicity. In particular, harmonic spectra of the stator current, obtained using a fast Fourier transform (FFT), are used for comparison purposes.

Ferraris Sensor – The Key for advanced dynamic Drives

The positioning features of feed drives in machine tools are nowadays almost always guaranteed by a proportional positioning and a proportional and integral velocity controller with an underlying fast current controller. The achievable dynamics of the positioning control are mainly determined by mechanical oscillations within the structure. In this paper it is shown that the bandwidth of the positioning and velocity controller depends also on the acquisition of the state variables. New methods for measuring the relative acceleration according to the Ferraris principle guarantee a velocity of low quantization together with a conventional cascade control. Based on these improvements it is shown that an underlying acceleration control can increase the dynamic disturbance stiffness further. This requires new control structures. Measuring results of single drive systems and a milling operation with a 5-axis machine tool have shown what potential lies in the improved acquisition of state variables by a relative acceleration sensor to achieve highly dynamic and accurate machining results.

Single-stage three-phase buck-boost type AC-DC converter with high power factor

A new control process for single-stage three-phase buck-boost type AC-DC power converters with high power factor, sinusoidal input currents and adjustable output voltage is proposed. This converter allows variable power factor operation, but this work focus on achieving unity power factor. The proposed control method includes a fast and robust input current controller based on a vectorial sliding mode approach. The active nonlinear control strategy applied to this power converter, allows high quality input currents. Given the comparatively slow dynamics of the DC output voltage, a proportional integral (PI) controller is adopted to regulate the converter output voltage. The voltage controller modulates the amplitudes of the current references, which are sinusoidal and synchronous with the input source voltages. Experimental results from a laboratory prototype show the high power factor and the low harmonic distortion characteristics of the circuit.

Modeling Analysis and Control of a Current Source Inverter-Based STATCOM

This article presents a new approach for the dynamic control of a current source inverter (CSI)-based STATCOM. The dq-frame model and the steady-state characteristic of the CSI STATCOM are proposed as a basis for control design. Use of traditional PI controllers leads to a poorly damped high-frequency oscillation between the inductance and capacitance of the CSI output filter. The new approach includes a fast ac current control inner loop and a slower dc current control outer loop. The inner loop, which is a combination of multivariable full state feedback and integral control, allows for rapid nonoscillatory dynamics of the ac current without overshoot or steady-state error. Experimental tests on a 5 kVA laboratory CSI STATCOM setup validate the proposed control design as well as the simulation results.

Fast current control for low harmonic distortion at low switching frequency

The structure of the current control loop of an induction machine drive determines decisively the dynamic performance of the overall system. Fast current control is a prerequisite for dynamic decoupling between the torque and the flux commands. Standard solutions are well established for drives in the low- and medium-power ranges. The low switching frequency of high-power pulsewidth modulation inverters calls for a tradeoff in controller design between the low harmonic losses and torque ripple in the steady state on one hand, and fast dynamic response during the transients on the other. The problem is developed in detail. A variable-structure approach is proposed as the solution.

Full control of a PWM DC-AC converter for AC voltage regulation

The design and implement action of a DSP-based fully digital-controlled single-phase pulsewidth modulated (PWM) dc-ac converter for ac voltage regulation is described. The proposed multiloop digital controller (MDC) consists of a current controller, a voltage controller, and a feedforward controller. This MDC was realized using a single-chip digital signal processor (DSP). The PWM gating signals are determined at every sampling instant by the proposed multiloop digital control scheme using a set of detected feedback signals. A software current control; scheme has been developed to achieve fast current control of the PWM inverter and decouple the inductor of the output filter. Experimental results have been given to verify the proposed digital control scheme. The constructed DSP-based PWM dc-ac converter system can achieve fast dynamic response and with low total harmonic distortion (THD) for rectifier type of loads.

Identification and compensation of torque ripple in high-precision permanent magnet motor drives

Permanent magnet synchronous machines generate parasitic torque pulsations owing to distortion of the stator flux linkage distribution, variable magnetic reluctance at the stator slots, and secondary phenomena. The consequences are speed oscillations which, although small in magnitude, deteriorate the performance of the drive in demanding applications. The parasitic effects are analyzed and modeled using the complex state-variable approach. A fast current control system is employed to produce high-frequency electromagnetic torque components for compensation. A self-commissioning scheme is described which identifies the machine parameters, particularly the torque ripple functions which depend on the angular position of the rotor. Variations of permanent magnet flux density with temperature are compensated by on-line adaptation. The algorithms for adaptation and control are implemented in a standard microcontroller system without additional hardware. The effectiveness of the adaptive torque ripple compensation is demonstrated by experiments.

SLIDING MODE CONTROLLER FOR PARALLEL RESONANT DUAL CONVERTERS

The classical DC drive control structure (outer speed control loop and fast inner current control loop) is applied for induction motors. This paper focuses on the inner current control (indirect current control) loop. Three Park vector based VSC (Variable Structure Control) methods (one for input rectifier two for the output inverter) are studied. In the three cases, a complex error vector, a complex sliding surface and a complex distance vector (which indicates the distance of the actual state from the sliding surface) are defined. The inverter is switched in such a way that the system trajectory gets as close to the sliding surface as possible. In spite of the fact that the error vectors have different physical meanings, similar switching strategies can be applied to all cases due to the similarity of mathematical equations. By the proposed method both the low-order harmonics and the audio noise can be reduced.

Fast current trajectory tracking control based on synchronous optimal pulsewidth modulation

State-of-the-art pulsewidth modulation techniques fail to give satisfactory results when high-bandwidth torque control is required at the low switching frequency of modern high-power AC machine drives. A novel current control method refers to precalculated optimal synchronous pulse patterns. These are used to generate specific current reference trajectories to be adapted on-line to the actual dynamic torque command. A fast tracking controller, operated in parallel to a conventional PI controller, minimizes the trajectory tracking error. The optimized current trajectories ensure maximum response and minimum harmonic distortion.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>