Voltage Feed-forward Compensation Research Articles

Robust fixed-order [formula omitted] controller synthesis for multi-phase converters with input voltage feed-forward

Generated DC voltages of batteries, fuel cells, or supercapacitors as power sources connected in series with DC-DC converters change according to operating conditions. In such cases, the output voltage stability of the multi-phase DC-DC converters is an issue, particularly being subject to noises in the input voltage and load current disturbances. Input voltage feed-forward (IVFF) compensation approaches can improve the output voltage stability of the DC-DC converters in such scenarios. This paper proposes a robust fixed-order H∞ based IVFF control framework by considering sensor noise, input voltage, and load current disturbances to improve output voltage stability and reference tracking performance. The mismatched disturbance transfer functions depending on parasitic resistances are derived to include in the controller synthesis process. An iterative convex-concave optimization-based robust fixed-order controller design algorithm is proposed to optimize the feedback and feed-forward controllers under the stability and performance constraints. This algorithm allows the designing of the controllers simultaneously. The feed-forward controller, whose structure is independent of the system transfer functions, is optimized without any model inversion or extra design steps. The performance of the proposed controller is compared with an integral linear quadratic regulator controller, a sliding mode PID controller augmented with a nonlinear disturbance observer, and a fixed-order robust controller designed with non-smooth techniques. Real-time implementation of the fixed-order control systems is performed on a 2-leg 200 W interleaved boost converter. The performances are discussed under external disturbance effects to show the validity of the proposed control approach.

Discrete-Time Current Regulator for AC Machine Drives

In a practical alternating current (ac) machine drive system, a zero-order hold equivalent discrete-time machine model motivates the development of a direct discrete-time current regulator structure. However, it is complicated to establish the direct discrete-time current regulator for asymmetric machine<xref ref-type="fn" rid="fn1">¹</xref><fn id="fn1"><label>¹</label> Asymmetric machine denotes a ac salient machine in this article, whereas the symmetric machine denotes a ac non-salient machine. </fn> model due to the involvement of complex matrix exponential related to unequal machine inductance parameters. In order to overcome this barrier, a novel direct discrete-time current regulator based on the flux model is proposed, where a modified ac machine model considering delay compensation is developed and a new feed-forward voltage compensation is designed to eliminate the matrix exponential. In addition, a discrete-time flux observer is proposed and can work in the proposed current regulator to suppress the adverse one-step delay effect on the feed-forward voltage compensation and current regulator. The theoretical analysis has shown that the proposed discrete-time current regulator and flux observer have good parameter robustness and stability. Also, the experimental verification is performed on an ac machine drive test rig.

An Improved Impedance Modeling Method of Grid-Tied Inverters With White-Box Property

Small-signal modeling of grid-tied inverters (GTIs) is an essential work in stability analysis of GTIs-weak grid coupling system. Different from the previous small-signal modeling methods of GTIs with black-box property, a reduced-order modeling method has been proposed in this article to provide an impedance model with white-box property. First, impedance and admittance are seamlessly incorporated into the modeling method, where concepts of partial admittances, partial impedance, active power admittance, and reactive power admittance are especially introduced. As a result, simplified expressions of impedance model of GTIs have been obtained with physical interpretations of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> impedances strengthened. After that, dominated relationships between terms of GTI's impedance matrix and its internal physical parts are explicitly revealed to represent white-box property of the proposed impedance model. Specifically, the influence and effect mechanism of feedforward voltage compensation, current controller, phase lock loop, and power points ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_d</i> , <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_q</i> ) are emphasized based on the proposed impedance model. Finally, perturbations generated independent with PLL are injected into a hardware in-loop experiment platform, and impedance measurement results are provided to verify the conclusions drawn from the improved modeling method.

Improved droop control strategy based on voltage feedforward current compensation

In order to achieve the stable operation of different-capacity parallel distributed generators, this paper proposes an improved droop control strategy according to the control strategy of parallel synchronous generators. By the improved control strategy, the active power and reactive power of parallel distributed generators can be automatically allocated by ratio of distributed generators capacity, and the function of plug and play of the distributed generators in microgrids can be realized. It will improve the flexibility of distributed generator’s interconnection and solve the technological problem of different-capacity distributed generator interconnection. In addition, the proposed control strategy can keep unchanged while switching microgrid operation mode, realizing the smooth transition between grid mode and island mode, and improving the stability and reliability of microgrid. Simulation results verify that the improved droop control strategy can make the different-capacity parallel distributed generators work stably under static and transient operation.

Synchronous Control Strategy with Input Voltage Feedforward for a Four-Switch Buck-Boost Converter Used in a Variable-Speed PMSG Energy Storage System

The four-switch Buck-Boost (FSBB) converter can produce voltage conversion within a wide input voltage range, which is suitable for variable-speed permanent magnet synchronous generator (PMSG) energy storage systems with AC inputs and DC outputs. To reduce the interference of input voltage fluctuation on the performance of the FSBB converter, an input voltage feedforward (IVFF) compensation method is proposed in this paper. The switching synchronization strategy is simple. Using the switching average model, the small signal model of a non-ideal FSBB converter in all working modes is established. The effects of input voltage, load current, damping coefficient and right half plane (RHP) zero on the stability of the control system are analyzed in detail. The transfer function of the IVFF of the FSBB converter is derived, and the relationship between input voltage, load current and duty cycle is analyzed. Finally, the design of the parameters of the converter control system is presented. The simulation and experimental results show that this FSBB converter has high efficiency and a good transient response.

Optimal control strategy of state feedback control for surface-mounted PMSM drives based on auto-tuning of seeker optimization algorithm

This paper focus on a state feedback controller (SFC)-based optimal control scheme for surface-mounted permanent-magnet synchronous motor (SPMSM) with auto-tuning of controller built on seeker optimization algorithm (SOA). First, based on the nonlinear state-space model of SPMSM, voltage feedforward compensation is used to design a linear SFC. Then in order to guarantee the steady performance in speed and current, integral models considering the errors of rotor speed and current response in d-axis are added in the state space model of SPMSM. Furthermore, by statically decoupling the load torque in the state equation, feedforward compensation is implemented on the load torque to improve the dynamic performance of the controller. The load torque is estimated by using disturbance observer with reasonable parameter selection. Then, with the consideration of the search capacity of seeker optimization algorithm (SOA), it is adopted to acquire matrix coefficient of the presented controller. Furthermore, in order to suppress the speed overshoot, a penalty term is introduced to the fitness index. The performance of the proposed method has been validated experimentally and compared with the conventional method under different conditions.

Passivity‐based output admittance shaping of the converter‐side current‐controlled grid‐tied inverter to improve the robustness to the grid impedance

In the LCL-filtered grid-tied inverter, the classical low-cost control strategy which is composed of the single converter-side current control and the unit grid voltage feedforward compensation, is commonly employed to inject the sinusoidal current into the utility. However, the inverter output admittance features the non-passive behaviour, which indicates that the harmonic currents may be amplified under the disturbances of the grid voltage and the grid impedance. Therefore, two dedicated current loops based on only the converter-side current feedback are proposed, which consist of the low-pass filter-based current regulation and the high-pass filter-based active damping (HFAD). The mixed asymmetrical regular sampling method is further used to reduce the control delay of the HFAD. Moreover, the low-pass filter-based feedforward is developed to make a compromise between the disturbance rejection and the stability. Fair comparisons with other typical control strategies are made through analysing the virtual resistor in parallel with the filter capacitor, which reveals that the positive virtual resistance at the frequency lower than half of the switching frequency is maintained. With the elaborated control parameters, the exhaustive experimental results finally verify the effectiveness of the proposed method in passivating the inverter output admittance.

Adaptive Torque Ripple Suppression Methods of Three-Phase PMSM During Single-Phase Open-Circuit Fault-Tolerant Operation

This article proposes adaptive torque ripple suppression methods by utilizing resonant controllers during the single-phase open-circuit fault-tolerant operation of a three-phase star-connected permanent magnet synchronous motor (PMSM). A zero-sequence current is supposed to be injected to maintain the postfault torque the same as the prefault torque. According to the postfault PMSM model, a feedforward voltage compensation can be adopted to inject the zero-sequence currents and, hence, attenuate the postfault speed and torque ripples in the vector control framework. However, the feedforward voltage compensation, which is related to the phase resistance and leakage inductance, may be inaccurate due to the temperature variation, etc., and hence, an unexpected second harmonic torque ripple still exists. In this article, the adaptive suppression methods of the second harmonic torque are proposed. The first one constructs a novel closed current loop of the zero-axis and the second one modifies the controllers in the dq -axis current loop. These methods are based on the resonant controllers, which can adaptively compensate for the voltage needed for the zero-sequence current injection. Experiments are implemented based on a four-leg inverter and the performance comparison between the feedforward compensation and adaptive compensation validates that the proposed methods can effectively suppress the postfault torque and speed ripple.

A Torque Control Method Based on I-MR Controller for IPMSM Drive With Small DC-Link Capacitor

In traditional motor drive system, large volume electrolytic capacitors are utilized to buffer energy and stabilize the dc-link voltage. However, these capacitors affect the reliability of the drives and narrow the conduction angle of diode-rectifier. As a result, the grid side current is distorted and the power factor is deteriorated. In order to settle these issues, small dc-link capacitors can replace the electrolytic capacitors. In this paper, a motor torque control method with dc-link voltage feed-forward compensation for an interior permanent magnet synchronous motor (IPMSM) drive system is proposed. Hence, an integration multi-resonant ( I-MR ) controller is established to regulate the motor torque to obtain high input power factor and low grid current harmonics. In addition, a voltage feed-forward compensation is used to diminish the error of motor torque control. The effectiveness of the proposed method is verified through simulation and experiments. Results indicate that the input power factor can reach about 99.4% and the grid current harmonics are considerably lower than the requirement of EN-61000-3-2 standard.

Current Harmonic Suppression Algorithm for Asymmetric Dual Three-Phase PMSM

Based on the vector space decomposition (VSD) transformation, the currents of an asymmetric dual three-phase permanent magnet synchronous motor (ADT-PMSM) with isolated neutral points are decoupled into the torque, producing α-ß components in the fundamental subspace and the loss-producing x-y components in the harmonic subspace. In order to suppress the current harmonics, a feedforward voltage compensation that is calculated according to the dead time, turn on/off time, and the voltage drop is added to the reference voltage of the control algorithm. Due to the limitations of the feedforward voltage compensation method, a feedback current control loop is set up in the x-y subspace, where an improved resonant controller is adopted to generate the compensation voltage. The improved resonant controller can achieve a high gain at a specific frequency. The experimental results verify the effectiveness of the proposed method.

Research on Control Strategy of Quasi-Z Source Network DC/DC Converter for Fuel Cell Vehicle

DC/DC converter for fuel cell vehicles is required to operate in high voltage gain and wide voltage range conditions, which traditional boost converter is difficult to meet. In this paper, the control strategy of a quasi-zsource boost converter used in fuel cell vehicles is studied. Based on the small signal dynamic modeling method, the double-closed-loop control strategy of voltage and current is adopted. In addition, the input voltage feedforward compensation control strategy is adopted to improve performance in rejecting Input voltage disturbance. A 400W prototype is built for experimental verification. Experimental results show that the proposed input voltage feedforward compensation strategy and double- closed-loop control strategy of voltage and current effectively improve the system dynamic response speed and robustness.

State Feedback Control for a PM Hub Motor Based on Gray Wolf Optimization Algorithm

This paper presents an optimal control strategy for a permanent-magnet synchronous hub motor (PMSHM) drive using the state feedback control method plus the gray wolf optimization (GWO) algorithm. First, the linearized PMSHM mathematical model is obtained by voltage feedforward compensation. Second, to acquire satisfactory dynamics of speed response and zero d -axis current, the discretized state-space model of the PMSHM is augmented with the integral of rotor speed error and integral of d -axis current error. Then, the GWO algorithm is employed to acquire the weighting matrices Q and R in linear quadratic regulator optimization process. Moreover, a penalty term is introduced to the fitness index to suppress overshoots effectively. Finally, comparisons among the GWO-based state feedback controller (SFC) with and without the penalty term, the conventional SFC, and the genetic algorithm enhanced proportional–integral controllers are conducted in both simulations and experiments. The comparison results show the superiority of the proposed SFC with the penalty term in fast response.

ILC-Based Voltage Compensation Method for PMSM Sensorless Control Considering Inverter Nonlinearity and Sampling Current DC Bias

Inverter nonlinearity and sampling current dc bias can cause harmonic distortion of estimated back electromotive forces (EMFs), and therefore results in position estimation error and degrades control performance of model-based sensorless techniques of permanent-magnet synchronous motors, especially in low speed region. In this paper, the effects of inverter nonlinearity and sampling current dc bias on back EMF estimation are analyzed. A voltage feedforward compensation method based on an iterative learning control (ILC) algorithm is proposed and it is used to eliminate the harmonics of estimated back EMFs and position estimation error, so as to improve the performance of sensorless control. Compensated voltages at different rotor positions are calculated online by the ILC algorithm and stored in a lookup table. The method does not depend on current polarity and inverter model. And it is simple and easy to implement. The effectiveness of the proposed compensation method is verified by experiments.

Design of Hybrid Control Algorithm for Fault Ride-Through of Photovoltaic System

This paper proposes a novel control algorithm to enhance the fault ride-through (FRT) capability of a photovoltaic (PV) system. In this method, the overcurrent of the grid-tied inverter is suppressed to a preset value using model current predictive control (MCPC) algorithm, and its DC-link overvoltage is removed using a non-maximum power point tracker (non-MPPT) algorithm. Therefore, the inverter overcurrent and its DC-link overvoltage problems can be placed under decoupling control using a DC/AC converter controller and DC/DC converter controller, respectively. Using eight switching modes for the three-phase inverter, the MCPC of DC/AC converter controller establishes a value function between the current reference value and output current value. Then, by introducing the switching mode (corresponding to the minimum value function) into the DC/AC converter controller, if the current reference is set at the rated value, the inverter’s output current can be inhibited to the rated value under fault conditions. Moreover, a balanced three-phase rated current can always be obtained under either a symmetric or asymmetric fault. However, together with the MCPC, a DC-link overvoltage will appear. To remove this DC-link overvoltage quickly, based on the grid system voltage sag level, the non-MPPT algorithm is used to calculate the adjusted power for PV arrays. Then, based on the calculated power, the amended duty cycle can be obtained and immediately introduced into the DC/DC converter controller to tune the PV arrays’ output power, which significantly decreases the power imbalance between the inverter’s AC and DC sides, thus inhibiting the DC-link overvoltage. In addition, to inhibit DC-link voltage fluctuations further, DC-link voltage feedforward compensation is introduced into the DC/DC converter controller. In particular, under an asymmetric fault condition, with the help of the voltage feedforward compensation, the second harmonic frequency components of the DC-link voltage can also be removed. Finally, based on the theoretical derivation and simulation results, it can be proved that the major problems troubling a PV system’s FRT technologies can be resolved by the proposed method, especially the problem related to the harmonic components of the DC-link voltage.

Dynamic Performance Improvement of Five-Phase Permanent-Magnet Motor With Short-Circuit Fault

Multiphase permanent-magnet (PM) brushless motors are popularly adopted for their high efficiency and high power density. However, short-circuit phase fault results in serious problems, such as increased torque fluctuations and deteriorated dynamic performance. This paper proposes a new vectorial approach to minimize pulsating torque and improve dynamic performance in a five-phase PM motor with short-circuit fault. The novelty of the proposed strategy is voltage feedforward compensation based on the relation of the short-circuit current and its fault-phase back electromotive force. First, the compensatory voltages are used to eliminate the impact of the short-circuit current. Then, its combination with the orthogonal reduced-order transformation matrices derived from fault-tolerant current references can improve the dynamic performance of the faulty PM motor. The effect of the short-circuit phase fault on the PM motor model under rotating synchronous frame is also discussed. This control strategy allows minimal reconfiguration of the control structure from healthy operation to fault-tolerant one and exhibits the improved dynamic performance. The simulated and experimental results are presented as validation for the proposed strategy.

Study of the influence of different feed-forward voltage compensation terms on the fault ride-through capability of doubly fed induction generator wind turbines

As a result of the increasing wind power penetration into power systems, the wind farms, especially with doubly fed induction generators, are required to have the fault ride-through capability. The article presents a study on the influence of three different feed-forward voltage compensation terms in rotor current control on the fault ride-through capability of doubly fed induction generator wind turbines. The study focuses on the enhancement of fault ride-through capability for doubly fed induction generator wind turbines by adopting different feed-forward voltage compensation terms in rotor current control. The three control strategies, which are developed based on different feed-forward voltage compensation terms, are evaluated through simulations of doubly fed induction generator wind turbines under both normal conditions and grid voltage fault conditions. The three control methods present different performances. Under normal conditions, the control system 1 presents better performance. However, the control system 3 presents better performance when the doubly fed induction generator wind turbine is subjected to a voltage fault. In order not to have the influence on the performance under normal operating conditions, the added feed-forward voltage compensation terms in the control system 3, which acted as a fault control mode, are enabled only when a voltage fault occurs in the grid.

An Efficient and Robust Hybrid Damper for &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$LCL$&lt;/tex-math&gt;&lt;/inline-formula&gt;- or &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$LLCL$&lt;/tex-math&gt;&lt;/inline-formula&gt;-Based Grid-Tied Inverter With Strong Grid-Side Harmonic Voltage Effect Rejection

A high-order ( $LCL$ or $LLCL$ ) power filter with a small grid-side inductor is becoming more preferred for a grid-tied inverter due to less total inductance and reduced costs. In a microgrid, the background harmonic voltage (BHV) may distort the injected currents of the grid-tied inverters. In order to resist the effect of the BHV, a feedforward voltage compensator and a proportional resonant regulator with harmonic compensation are often adopted. However, they still have their own limitations, particularly when there are higher order BHVs at the point of common coupling and when the equivalent grid impedance widely varies due to the different numbers of grid-tied inverters in parallel. Thus, an extra damper should be inserted to keep the system stable. In this paper, the control bandwidth limitation of a multiloop control active damping (AD) method is analyzed and illustrated by the capacitor-current-feedback AD. Based on this, a single-loop current control with a hybrid damper is proposed for a single-phase $LCL$ - or $LLCL$ -filter-based grid-tied inverter. A step-by-step design of the controller method is also introduced in detail. Experiments on a 2-kW prototype fully demonstrate the strong robustness of the stability and the high harmonic rejection ability of the inverter using the proposed control method.

Based on Feed-Forward Photovoltaic Grid Converter Control Strategy

In photovoltaic grid-connected system, the method of adding voltage feed-forward to improve the shape of grid-connected current wave is presented, the reason for the introduction of the net voltage feed-forward decreasing the phasic difference between grid-connected current and net voltage and reducing the THD of grid-connected current is analyzed. It is confirmed that the net voltage feed-forward compensation can improve grid-connected current availably by the results of simulations and experiments, keep the current frequency and phase synchronous to the grid.

Regulator Design of Vector Control System

A Decoupling method based on feedforward voltage compensation is adopted to eliminate completely the coupling between exciting current and torque current. Based on conventional design methods of single-variable linear system , the paper analyses and designs detailedly the flux loop, speed loop and current loop. A design method based on double closed-loop PI regulator for speed sensorless vector control system is proposed. Through experimental results, we demonstrate its good performance.

A Digital Power Factor Correction (PFC) Control Strategy Optimized for DSP

A predictive algorithm for digital control power factor correction (PFC) is presented in this paper. Based on this algorithm, all of the duty cycles required to achieve unity power factor in one half line period are calculated in advance by digital signal processors (DSP). A boost converter controlled by these precalculated duty cycles can achieve sinusoidal current waveform. One main advantage is that the digital control PFC implementation based on this control strategy can operate at a high switching frequency which is not directly dependent on the processing speed of DSP. Input voltage feed-forward compensation makes the output voltage insensitive to the input voltage variation and guarantees sinusoidal input current even if the input voltage is distorted. A prototype of boost PFC controlled by a DSP evaluation board was set up to implement the proposed predictive control strategy. Both the simulation and experimental results show that the proposed predictive strategy for PFC achieves near unity power factor.