Voltage Feedforward Research Articles

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.

Design of Fast Transient Response Voltage-Mode Buck Converter With Hybrid Feedforward and Feedback Technique

For a voltage-mode buck converter, to improve the transient response and reduce the size of the proportional-integral-differential (PID) compensation network, a hybrid feedforward and feedback technique (HFFT) is presented in this article. First, the HFFT utilizes the input voltage feedforward, the output voltage feedback, and the duty-ratio feedback to achieve fast line and load transient responses simultaneously. Second, the converter system is comprised of two feedback loops. One outer loop employs a voltage feedback control and provides a phase lead (PD) element. The other inner loop uses a duty-ratio feedback control and produces a phase lag (PI) element. Then adopting the PD and PI elements, a proposed implementation of PID compensation with small on-chip components can also be obtained. A buck converter chip with the HFFT has been fabricated in 0.18- $\mu \text{m}$ 1.8/5-V CMOS technology. Measurement results show that the output voltage of the buck converter is almost invariant under a line voltage step of 1.5 V and settled within $4.5~\mu \text{s}$ for a load current step of 500 mA.

Modeling and Analysis of High-Frequency MMC Impedance Considering Different Control Modes and Voltage Feedforward

The sequence impedance of modular multilevel converters (MMC) is the premise to analyze the stability of MMC-based high voltage direct current (HVDC) system. However, few papers have modeled MMC impedance with comprehensive consideration of different control modes and voltage feedforward, the high-frequency impedance characteristics of the MMC have not been intensively studied, either. In fact, the voltage feedforward has an important influence on the high-frequency impedance characteristics of the MMC, which can not be ignored. This article proposes a sequence impedance for MMC considering complete control loops. Not only current inner-loop, circulating current control loop and phase-locked loop (PLL) are considered, but also different control modes including power control, DC voltage control and AC voltage control are considered. It is worth mentioning that especially the voltage feedforward and time delay are taken into account. Based on this impedance model, the characteristics of the MMC are thoroughly analyzed, and the analysis shows that the voltage feedforward and time delay will greatly affect the high-frequency characteristics, which will deeply affect the stability of the system. Meanwhile, the influences of PLL dynamics, outer-loop control, circulating current control and parameters of controller on the sequence impedance are also analyzed in detail.

A Novel Inverter-Side Current Control Method of $LCL$ -Filtered Inverters Based on High-Pass- Filtered Capacitor Voltage Feedforward

LCL filters have been widely used as the interface between inverters and power grids, but damping methods must be taken to solve the resonance of LCL filters. Considering overcurrent protection and inherent active damping, this paper focuses on the inverter-side current feedback (ICF) control. Some literatures have proposed that the capacitor voltage feedforward (CVF) is beneficial to the stability of the ICF control. However, in practical applications, it is found that the ICF control with CVF mainly faces two problems. Firstly, the CVF can be seen as a positive feedback loop, and it tends to cause low-frequency oscillation, especially in weak grid. Secondly, low-frequency harmonics in the grid voltage can easily distort the grid-side current of the inverter, even though the inverter-side current has been regulated well. In order to solve these two problems simultaneously, this paper proposes a novel inverter-side current control method, which adds a high-pass filter into the CVF path. In this paper, the design process of the high-pass filter parameters is analyzed, and the specific voltage feedforward structure of the proposed method to avoid the inverter startup inrush current is also discussed in detail. Simulations in MATLAB/SIMULINK and experiments based on a 6.6 kW prototype have been utilized to verify the effectiveness of the proposed method.

Assessment of Model Predictive Voltage Control for Autonomous Four-Leg Inverter

The Finite control set model predictive control (FCS-MPC) is recently introduced to control inverters without the modulation stage. The absence of the modulation stage gives an unpredictable performance of the control system. In this paper, the performance of FCS-MPC is assessed by comparing with PID control which is based on Scalar Pulse Width Modulation (PWM). The two control techniques are applied for load voltage regulation of the autonomous four-leg voltage source inverter (FLVSI). Practically, the predictive control requires a large number of calculations, resulting in high computation time and delay. In this paper, a new finite control set model predictive voltage control (MPVC) algorithm is proposed to predict the load voltages for 15 switching states instead of 16 switching states for reducing the computation time required for the control algorithm. Moreover, the algorithm is optimized by removing the repeated computations and the delay is compensated using the two-step prediction horizon principle. An accurate discrete-time state-space model of the autonomous FLVSI with output LC-filter is used for predicting the load voltages considering the neutral inductance and damping resistance of the LC filter. A simple PID control scheme with decoupled feedforward voltage and current loops is used in the DQ0 reference frame, while MPVC operates in the ABC reference frame. The simulation and experimental results are used to show the full assessment of the MPVC. The prominent outcomes show the ability of the proposed MPVC algorithm to provide high power quality under unbalanced and non-linear load conditions with high stability and robustness.

Dynamic Capacitance Variations Due to Barrier Modulation in Pinned Photodiode of CMOS Image Sensor

The pinned photodiode (PPD) capacitance is an important parameter in CMOS image sensor design since it provides useful information on the charge handling capacity of the photodiode. The model of the dynamic capacitance of the PPD includes a fixed depletion capacitance and a variable diffusion capacitance. The variable component of the photodiode capacitance is estimated by measuring the feedforward voltage at the floating diffusion node. However, the estimated feedforward voltage is highly nonlinear at its full-well condition. In this article, the transfer gate (TG) channel potential is shown to be dependent on the number of charges integrated in the PPD. The nonlinear behavior of TG channel potential influences the effective potential barrier. The nonlinearity observed in the feedforward voltage is also explained using the nonlinear behavior of TG channel potential. In the literature, the influence of the effective potential barrier on the photodiode capacitance models is not taken into account. In this article, the influence of the effective potential barrier on the full-well capacity, feedforward effect, and PPD capacitance is studied. The improved photodiode capacitance model using the effective potential barrier is in good agreement with the measurement results.

Design of Double-Line-Frequency Ripple Controller for Quasi-Single-Stage AC–DC Converter With Audio Susceptibility Model

Power factor correction (PFC) converter suffers from serious double-line-frequency output voltage ripple due to the difference between instantaneous ac input power and dc output power. To suppress such double-line-frequency output voltage ripple, a ripple cancellation circuit (RCC) is utilized to generate the same magnitude but 180° phase shifted double-line- frequency voltage ripple. This paper provides an insight into the relationship between control scheme and double-line-frequency ripple suppression from the perspective of audio susceptibility model. For the purpose of reducing the double line frequency ripple, the closed-loop audio susceptibility of series RCC should be designed relatively low at double line frequency. According to such requirement, a ripple controller with improved voltage feed-forward (FF) scheme and quasi-proportional-resonant (QPR) compensator is designed. As QPR compensator can increase the denominator of the closed-loop audio susceptibility, while voltage FF scheme can reduce its numerator, its magnitude at double line frequency is thus greatly reduced and double-line-frequency ripple is also significantly suppressed. Moreover, due to the flexibility of QPR compensator, the proposed ripple controller presents good ripple suppression in a different line frequency application. Finally, experimental results of a flyback PFC with buck-type RCC quasi-single-stage ac–dc converter are provided to verify the validity of the proposed ripple controller.

Virtual-Stator-Flux-Based Direct Torque Control of Five-Phase Fault-Tolerant Permanent-Magnet Motor With Open-Circuit Fault

A five-phase fault-tolerant permanent-magnet (FTPM) motor can offer high torque density, low torque ripple, and good fault-tolerance. To improve operational performance under open-circuit fault condition, this article proposes a novel direct torque control (DTC) based on virtual stator flux strategy for an FTPM motor drive. The novelty of the proposed strategy is the development of virtual stator flux, which is used to achieve circular trajectories of current and rotating magneto-motive force vectors, and to derive feedforward voltage. Additionally, optimal fault-tolerant current references get derived based on maximum torque criterion, and then combined with reduced-order orthogonal decomposition matrices deduced from them, the feedforward voltage is used to eliminate asymmetrical motor behavior under the fault condition. Hence, the DTC becomes applicable to the FTPM motor drive under open-circuit fault condition. This control strategy not only enhances the torque performance with smooth torque and high dynamic response but also allows minimal reconfiguration of the control structure from healthy operation to fault-tolerant operation. The simulation and experimental results are given to verify the feasibility of the proposed strategy.

Harmonics and Stability Analysis of Single-Phase Grid-Connected Inverters in Distributed Power Generation Systems Considering Phase-Locked Loop Impact

In the distributed power generation systems (DPGSs) based on the renewable energies, the stability and harmonics of the grid-connected inverter are seriously affected by the uncertainties of the grid at the point of common coupling (PCC). More importantly, as several inverters are usually tied at the PCC together, the interactions among the inverters and the grid impedance can cause more serious harmonics and instabilities at the low frequencies. Hence, the phase-locked loop (PLL) affecting the low-frequency behaviors must be considered. This study aims to provide a comprehensive study of the stability and harmonics of single-phase inverters in the weak grid. Modelings of single-inverter and multiple-inverter systems considering the PLL are established. The impacts of different PLLs on the low-order current harmonics and stability are then studied. It has been proved that the maximum ability of inverters in suppressing the low-order grid current harmonics closely relates to the PLL, even if a harmonic resonant controller and/or PCC voltage feedforward is used. Thus, using different PLLs yields quite different performances. Simulation and experimental results will be given to demonstrate the analysis, which is helpful for the readers to design a proper PLL for DPGSs.

Grid-Synchronization Stability Analysis and Loop Shaping for PLL-Based Power Converters With Different Reactive Power Control

Power converters may lose synchronization with the remaining network when integrated in a weak power grid. Such an instability phenomenon [known as grid-synchronization instability (GSI)] features the frequency divergence of phase-locked loop (PLL) and oscillations of the converter’s power output. In this paper, we focus on the influences of reactive power control (RPC) methods on GSI. We develop a single-input–single-output model to explicitly reveal how the PLL interacts with the other parts of the converter system in terms of grid synchronization. Then, after deriving the open-loop transfer function and sensitivity function of the entire converter system, we compare the stability margins for different RPC methods. Furthermore, we elaborate on the interactions among RPC, PLL, and voltage feedforward (VFF), and then demonstrate that different design methods of RPC and VFF will lead to different stability margins. The subsequent stability analysis provides insightful guidelines for coordinating the design of multiple control loops, i.e., RPC, VFF, and PLL. In particular, we demonstrate how the loop shaping of PLL takes effect in increasing the stability margin and eventually preventing the converter from GSI. The validity of the stability analysis is verified through simulations in MATLAB/Simulink.

Effects on oscillation mechanism and design of grid‐voltage feedforward in grid‐tied converter under weak grid

Grid-voltage feedforward control is widely used in grid-tied converters. The reported studies show that the grid-voltage feedforward may reduce the system stability margin, and may lead to oscillation problem in renewable power generation system. This study revisits the effect of voltage feedforward control on system stability and especially focuses on low frequency oscillation with impedance-based method. The low frequency oscillation usually happens in industry around 100 Hz and shows frequency coupling characteristics. A converter equivalent impedance which includes the effects of voltage feedforward and also covers the frequency coupling introduced by phase locked loop (PLL) is established. Based on the built impedance, the effect on oscillation mechanism with voltage feedforward and PLL is revealed. The results show that the voltage feedforward could increase the system stability margin and may be employed to improve the system stability. Then how to design an optimal proportional voltage feedforward control is demonstrated. Experimental results verify the theoretical analysis and the design of the proportional grid-voltage feedforward control.

Novel Soft-Start Technique for Grid Integration of LCL-filtered Inverters

Start-up inrush current is the major threat on the reliability of the power electronic converters which may occur due to even a micro-level mismatch in the grid synchronization of inverters. This paper presents a novel technique to limit the inrush current while simultaneously reducing the synchronizing time. A feedforward voltage loop is introduced along with a current control loop with varying weightage (gain factors). In addition, the current control loop gain is adjusted at the start-up so that it catches up faster to further reduce the inrush current. A detailed investigation of the causes of the inrush current and the effect of the control loop dynamics is carried out. The effectiveness of the proposed technique is compared with various other start-up techniques in the literature. The results are validated through simulation and experimentation.

Low-Cost Position Sensorless Speed Control of PMSM Drive Using Four-Switch Inverter

A low-cost position sensorless speed control method for permanent magnet synchronous motors (PMSMs) is proposed using a space vector PWM based four-switch three-phase (FSTP) inverter. The stator feedforward d q -axes voltages are obtained for the position sensorless PMSM drive. The q-axis current controller output with a first order low-pass filter formulates the rotor speed estimation algorithm in a closed-loop fashion similar to PLL (Phase Lock Loop) and the output of the d-axis current controller acts as the derivative representation in the stator feedforward voltage equation. The proposed method is quite insensitive to multiple simultaneous parameter variations such as rotor flux linkage and stator resistance due to the dynamic effects of the PI current regulator outputs that are used in the stator feedforward voltages with a proper value of K gain in the q-axis stator voltage equation. The feasibility and effectiveness of the proposed position sensorless speed control scheme for the PMSM drive using an FSTP inverter are verified by simulation and experimental studies.

Using feed-forward voltage-control to increase the ion removal rate during batch electrodialysis desalination of brackish water

Batch electrodialysis (ED) desalination, which relies on diluate recirculation to produce a desired product, is often conducted at constant voltage. Here we show that constant-voltage operation under-utilizes membrane area because the applied current is much lower than the limiting current early in a batch cycle. Time-variant voltage-control, targeted at raising the average ratio of applied to limiting current during a batch cycle, can therefore increase the rate of ion-transfer achievable using a fixed membrane area. We designed a feed-forward voltage controller, which provided within −15 to +20% of the desired current, and used it to raise the production rate by up to 37% ± 2%, relative to constant-voltage operation, without exceeding the limiting current density. Furthermore, an analytical prediction of the batch completion times was derived and validated under varying feeds (1500, 2000, and 3000 mg/L), products (200, 300, and 500 mg/L), and flow velocities (4.3, 6.4, and 8.5 cm/s). Supported by experiments, the predictive model indicates that time-variant voltage-control can provide the greatest increase in production rate at high feed-to-product concentration ratios and low flow velocities. This work will assist designers and operators seeking to size, evaluate, and maximize the production performance of new and existing batch ED processes.

Improved single‐phase self‐synchronised synchronverter with enhanced dynamics and current limitation capability

Inertia-less power converters are one of the main obstructions to increase the penetration level of distributed energy resources in the utility grid. The stability of the power system mainly depends on the system inertia. Virtual synchronous machine and synchronverter-based grid-tied inverters have been introduced as a solution to this problem. Here, a modified self-synchronised synchronverter is introduced to enhance the dynamic response of the active power loop and to limit the injected active/reactive power to the grid. Furthermore, the injected current to the grid is limited during faults employing proportional-resonant-based inner current loop. Feed-forward voltage is implemented in the current control loop to achieve seamless transfer between standalone and grid-connection modes of operation. Extensive simulation and experimental results are obtained to verify the proposed enhanced controller.

Anti-Islanding Detection Method Using Phase-Shifted Feed-Forward Voltage in Grid-Connected Inverter

For safety and maintenance problems, the grid-connected inverters have been required to detect whether the utility voltage source is connected or disconnected to the grid. If the inverter continues to energize local loads while the utility voltage source is disconnected, it is called islanding operation. This paper proposes a new islanding detection method using phase shifted feed-forward voltage. In the grid- connected inverter system, the PCC voltage is simultaneously measured, and those values are used as the feed-forward values in the current controller. In this paper, the phase of feed-forward voltage is modified to detect the islanding condition. As the measured PCC frequency changes, the proposed method modifies the phase of feed-forward voltage. Because the phase angle of feed-forward voltage is determined according to the variation of measured frequency of PCC voltage, the proposed method makes positive feedback to the frequency. If the frequency variation exceeds the allowable range, it is judged to be islanding operation. Since the frequency response of the proposed method is fast, the islanding operation could be detected in 90 ms despite high quality-factor ( $Q_{f}$ ), 10. Moreover, when the grid is normal condition but grid frequency changes in allowable range, the proposed method does not produce continuous reactive power which reduces the quality of inverter output power. Lastly, proposed method has very small NDZ because there is no limitation on reactive power injection. The performance of the proposed islanding detection method was verified by a computer simulation and experiment with 600 W single/three phase grid-connected inverters.

Robust Grid Current Control With Impedance-Phase Shaping for LCL-Filtered Inverters in Weak and Distorted Grid

For the grid-connected LCL -filtered inverter, the grid current feedback active damping (AD) can assure a desirable bandwidth and a good robustness against certain parameter variations by using a minimal number of sensors. However, the inverter performance was seriously endangered by nonideal conditions at the point of common coupling (PCC), including background voltage distortions and grid impedance variation. The grid current AD could not work well for wide grid impedance variation. Hence, in this study, the robustness with such control is discussed and its relations with system parameters are derived through mathematic derivations. It is proved that, for the typical grid current AD control, a contradiction between high bandwidth and high robustness exists. On the basis of analyzing the reasons for the limitations of the typical control, a systematic approach for enhancing the robustness under complex grid conditions has been proposed. The proposed fundamental PCC voltage feedforward enhances the robustness, while the proposed phase shaping compensator assures a higher robustness with a desired bandwidth. Comparative waveforms with the typical grid current AD control have been provided to verify the correctness of the analysis and the improvement of the proposed approach.

Analysis and Comparison of Notch Filter and Capacitor Voltage Feedforward Active Damping Techniques for LCL Grid-Connected Converters

The use of LCL filters is a well-accepted solution to attenuate the harmonics created by the pulsewidth modulation. However, inherently LCL filters have a resonance region where the unwanted harmonics are amplified, which can compromise stability. Several techniques have been developed in order to tackle this issue. At first the use of passive damping, by intentionally increasing the resistance of the LCL filter components, is a simple, robust, and straightforward solution; however, it decreases the overall efficiency of the system, and hence in general is unwanted. Alternatively, active damping strategies, where the resonance damping is provided by the current controller, are of major interest. This paper analyzes the robustness of the closed-loop dynamics when different active damping techniques are implemented. The analyzed active damping techniques, which have been selected because of their readiness and simplicity, are: 1) filtered capacitor voltage feedforward and 2) second-order filters in cascade with the main current controller. The impedance/admittance stability formulation is used to model the system, which has been proven to be very convenient for the assessment of robustness. Experimental tests are provided in order to show the accuracy of the analysis and verify the findings. This paper proves that filtered capacitor voltage feedforward is a more robust and reliable solution than implementations based on cascade notch filters.

A method of thrust ripple suppression for long stator linear synchronous motor

With the advantages of high speed, low noise and high efficiency, the electromagnetic suspension (EMS) type maglev train has a good prospect in railway transportation. It is based on the long stator linear synchronous motor (LSLSM). However, due to cogging effect, end effect and the harmonics in the stator current and flux density distribution around the air-gap, the thrust generated by the LSLSM fluctuates. The thrust ripple brings noise, drop of control accuracy, even causes the resonance of train. In this paper, the thrust ripple produced by the cogging effect and flux linkage harmonics is analyzed. Then a method of harmonic current injection is proposed to compensate cogging force and reduce the thrust ripple, without influence the decoupling control of traction and suspension system. The injected current harmonics are controlled under multiple rotating reference frames independently. Finally, based on voltage equations of harmonics, the decoupled harmonic current controllers with harmonic voltage feedforward are designed, which improve the performance of current harmonics response and thrust ripple suppression. Simulation results on Simulink verify the effectiveness of proposed thrust ripple suppression method for LSLSM.

Stability Improvement of Current Control by Voltage Feedforward Considering a Large Synchronous Inductance of a Diesel Generator

In this paper, the damping ability of point of common coupling (PCC) voltage feedforward is analyzed in order to improve the stability of current control, in which interference with an LCL filter of a battery energy storage system (BESS) and synchronous inductance of a diesel generator is considered in a stand-alone microgrid. A BESS can be charged by a diesel generator when the state of charge of the battery bank is significantly insufficient and where the BESS operates with constant current constant voltage control and the diesel generator provides constant voltage constant frequency control. In this case, large synchronous when the diesel generator is in a steady state may severely cause the instability of current control. The stability analysis should be required to guarantee the paralleled operation between BESS and diesel generator. An equivalent circuit model of the diesel generator is needed to perform these processes. To bring availability of analysis, this paper considers a simple equivalent circuit model considering synchronous inductance in the steady state, and it has very large inductance corresponding 1 per unit. The resonance issue is attenuated by the damping ability of PCC voltage feedforward where it is possible to improve the stability by appropriately adjusting the gain of the voltage feedforward. The effects of the synchronous inductance and the feedforward loop are analyzed in the s-domain and z-domain. The gain of the voltage feedforward is determined to be 0.2 and 0.4 based on stability analysis. To verify the validity of the damping effect of PCC voltage feedforward, a demonstration site is constructed and the stability is verified through simulation and experiment with a full model, and total harmonic distortion of current is improved to 2.44%.