A Simple Model Predictive Instantaneous Current Control for Single-Phase PWM Converters in Stationary Reference Frame

The three-phase dual-active bridge converter is a promising topology for the high-power high-frequency dc–dc conversion, due to the soft-switching capability and inherent galvanic isolation. With the state-of-the-art instantaneous current control (ICC) based on the single phase-shift (SPS) operation, a bidirectional power flow can be controlled dynamically between two dc ports without inducing overshoots in the transformer currents. However, a transient dc-bias flux may still occur in the transformer, which may cause the transformer core saturation in the dynamic operation such as start-up, power- off , and abrupt load changes. To analyze and address this issue, this paper presents a set of model-based methods to avoid inducing the dc-bias flux in various transient situations. The proposed double-side SPS operation could nullify the dc-bias flux in abrupt load changes and even instant power-flow reversals. Furthermore, the soft-magnetizing and soft-demagnetizing techniques are presented to eliminate the initial and residual dc-bias flux during start-up and power- off instantly. The proposed flux control method can be integrated with the ICC. Thus, the transformer magnetizing flux and the winding currents can be controlled simultaneously and instantaneously in transient situations. Simulations and experiments on a down-scaled hardware prototype validate the effectiveness of the proposed methods.

The research object of this paper is single-phase PWM rectifier, the purpose is to reduce the total harmonic distortion (THD) of the grid-side current. A model predictive current control (MPCC) with fixed switching frequency and dead-time compensation is proposed. First, a combination of an effective vector and two zero vectors is used to fix the switching frequency, and a current prediction equation based on the effective vector’s optimal action time is derived. The optimal action time is resolved from the cost function. Furthermore, in order to perfect the established prediction model and suppress the current waveform distortion as a consequence of the dead-time effect, the dead-time’s influence on the switching vector’s action time is analyzed, and the current prediction equation is revised. According to the experimental results, the conclusion is that, firstly, compared with finite-control-set model predictive control, proportional-integral-based instantaneous current control (PI-ICC) scheme and model predictive direct power control (MP-DPC), the proposed MPCC has the lowest current THD. In addition, the proposed MPCC has a shorter execution time than MP-DPC and has fewer adjusted parameters than PI-ICC. In addition, the dead-time compensation scheme successfully suppresses the zero-current clamping effects, and reduce the current THD.

Model-predictive current control of Voltage Source Converters has demonstrated superior performance over conventional Proportional-Integral control in terms of total harmonic distortion and dynamic response. However, the small-signal stability properties of the Model Predictive Control (MPC) remain insufficiently explored in the literature. This paper investigates the stability characteristics of the MPC using Impedance-based Stability Analysis. An impedance model of the MPC controlled converter is developed and validated through simulation-based measurements. Furthermore, the effect of model mismatch, switching frequency, and sampling time on the impedance of the MPC-based converter is analysed. A simulation study is conducted, demonstrating that MPC achieves a higher stability margin compared to PI control under weak grid conditions.

In order to reduce the harmonic content normally generated at the input of AC motor drives as a result of using three-phase voltage-fed inverters (VFI) for supplying electric drives with AC motors, and thus, to improve the input power factor of such machines, hysteresis current control technique is usually used to control the amplitude of three-phase motor currents and to decrease the phase shift between these currents and their corresponding voltages. Hysteresis current technique is considered to have a very simple implementation and an instantaneous current control, resulting in a fast response. However, the switching frequency is not constant and varies over a wide range during each half-cycle of the AC input source, and consequently the load current harmonic ripple is not optimal [J. Salmon & H. Zhang, Power and Energy Systems, 2004; Buso et at., IEEE Transactions on Industrial Applications, 2000]. In this paper, an improved implementation of hysteresis current control of three-phase voltage-source inverter with constant frequency is presented. In order to do that, before developing any inverter prototype, it is very convenient to use one of the several dynamic system software simulators available such as the phase-locked-loop (PLL) with the dead beat current control, even at a high rate of output voltage change, such as required in fast drive systems, and other highly demanding applications. Stability analysis and performance are confirmed through simulation results using Matlab/Simulink and Simplorer 6.

This paper investigates the operation of a current controlled Voltage Source Converter (VSC) under weak grid conditions caused by large grid impedance. Three different current controller structures are investigated; the conventional decoupled PI-controller in the synchronously rotating reference frame, Proportional Resonant (PR) controllers in the stationary reference frame, and the phase current hysteresis controllers. The different control strategies are studied by simulation, and the results show how large grid impedances can influence the dynamic response of the system. It is further discussed how the interaction between the current controllers, the Phase Locked Loop (PLL) and the grid inductance can trigger instability when the voltage measurements are highly influenced by the operation of the converter. The results indicate that the tuning of the PLL and the way of utilizing the phase information is of large importance for the stability and dynamic response of the control system.

To solve the problems of slow response speed of the permanent magnet synchronous motor (PMSM) system when the external interference occurs and the poor anti-interference ability, this paper proposes a double model predictive control (MPC) strategy for PMSM to control speed and current. To be specific, firstly, the mathematical model of PMSM is established under the two-phase synchronous rotation coordinate system. Secondly, we propose the double MPC strategy where MPC controller is used in the speed outer loop and deadbeat prediction controller is used in the current inner loop. Finally, By comparing the traditional control strategy (speed outer loop PI controller and current inner loop deadbeat prediction controller) with the double MPC strategy, the simulation results show that the double MPC has no overshot, better steady-state performance, faster dynamic response and stronger anti-load disturbance capability, which proves the effectiveness of the proposed strategy.

To improve the dynamic and steady-state control performance of permanent magnet synchronous motors under the three-vector model predictive current control method, this study proposes a switching current predictive control method based on the exponential moving average algorithm, which evaluates the magnitude of the change of the q-axis current slope in real time to discriminate the motor’s operating conditions and selects the optimal control method for different operating conditions. Meanwhile, the traditional three-vector model predictive current control method is improved by introducing a comparison mechanism for the q-axis current slope to select the second effective voltage vector, avoiding the secondary optimization calculation of the value function and reducing the computational complexity of the traditional method. By comparing the proposed method with the traditional three-vector model predictive current control method, the experimental results prove that the proposed method improves the system’s dynamic response and steady-state performance.

This study proposes a model predictive stator current control (MPSCC) strategy of doubly fed induction generator (DFIG) under unbalanced grid voltage conditions. Sinusoidal and balanced stator currents injected into the power grid can be ensured due to the direct control of stator currents rather than rotor currents. Model predictive control instead of traditional vector control is adopted, which can increase the current loop bandwidth and obtain faster dynamic responses. Conventional resonant regulators such as second‐order generalised integrators or second‐order vector integrators that are usually used to eliminate unbalanced components in stator currents can also be avoided. Moreover, both extractions of negative sequence components in rotor or stator currents and calculation of commanded rotor current are avoided, which simplifies the control scheme. The proposed MPSCC strategy can also implement the grid connection by introducing the virtual stator currents without any changes in the control scheme. Finally, experimental results based on a 1 kW lab DFIG system are provided to validate the effectiveness of the proposed control strategy.

Multi-vector model predictive control of permanent magnet synchronous motor can effectively overcome the shortcomings of conventional single-vector model predictive control such as large motor current ripples and limited control accuracy, but its control performance is more susceptible to the influence of motor parameter perturbation. To improve the robustness of the multi-vector predictive control algorithm under the motor parameter perturbation, a novel nonparametric three-vector model predictive current control method for permanent magnet synchronous motors is proposed in this paper. Through constructing an ultra-local predictive current model based on the input and output signals of the permanent magnet synchronous motor, the influence of motor parameter perturbation on current prediction is avoided, and a disturbance observer is designed to estimate the non-modeled and perturbed parts of the ultra-local model. In addition, the direct calculation method of vector duty cycles based on current error is introduced to suppress the influence of motor parameter uncertainty on the duty cycle calculation link, which further improves the system robustness. Finally, simulations and experiments of the proposed method are demonstrated to compare with the conventional parametric three-vector model predictive current control, and the results show that the proposed control strategy can effectively suppress the disturbances caused by the motor parameters and ensure the steady-state performance during motor operation.

Aiming at the low power level of the two-level Z-source inverter, the current and voltage harmonic distortion rate is high, the output power quality is low, The diode Neutral Point Clamp (NPC) three-level Z source inverter has insufficient boost capacity, and the capacitor voltage stress is low, the Z source network of the three-level inverter is improved and applied to photovoltaic grid-connected. In order to speed up the dynamic response speed of the quasi-Z-source photovoltaic grid-connected system, and for the quasi-Z-source photovoltaic gridconnected system with the existing current inner loop control output harmonic distortion rate is high, and the steady-state error is large. the power feedforward bacterial foraging proportional complex integral (BFOA-PCI) control method is put foreward. The system about this method reduces the steady-state error through the PCI controller, and adds the bacterial foraging optimization proportional coefficient KP and integral coefficient Ki , makes KP and Ki any the optimal conditions can be achieved, and introduces the output power feedforward of the photovoltaic array in the voltage outer loop, and its output is used as the reference value of the current inner loop. The method speeds up the system’s response changes to the external environment, reduces the harmonic distortion rate, improves steady-state accuracy and the output power quality.

Conventional model predictive current control (MPCC) applies only one vector during one control period, which produces large torque and flux ripples and high current harmonics in permanent magnet synchronous motor (PMSM) drives. Recently MPCC with duty cycle control has been proposed to improve the steady state performance by applying one non-zero vector and one null vector during one control period. However, the prior method requires lots of calculations and predictions to find the optimal voltage vectors and calculate their respective duration. Different from prior enumeration-based MPCC, this paper proposes an efficient two-vector MPCC by applying two arbitrary voltage vectors during one control period. The reference voltage vector is firstly calculated based on the principle of deadbeat current control. Two optimal vectors and their duration are then obtained in a very efficient way, which does not require the calculation of current slopes in prior MPCC methods. The proposed method is compared to the state-of-the-art predictive control methods, including conventional MPCC, MPCC with duty cycle control, deadbeat control with space vector modulation (SVM) and modulated model predictive control (M2PC). Both simulation and experimental results prove that the proposed method achieves better steady state performance than conventional MPCC with or without duty cycle and the dynamic response is not degraded. Under the condition of insufficient dc bus voltage, the proposed method outperforms deadbeat control and M2PC by presenting even higher speed range and less torque ripples.

Direct current control has been used in many voltage source power converters connected to utility grid, such as Static Var Generators (SVGs) and parallel Active Power Filters (APFs). In their stationary coordinates control systems, there are usually two closed control loops. One is the current loop and the other is the voltage loop. In practical applications, because the voltage source converters are connected to the grid, the output current will be affected and very often a command-tracking error exists in the current control loop. In this paper, a detailed analysis and design considerations are carried out to investigate this command-tracking error caused by the source voltage. Based on the obtained current loop transfer function and block diagram of the system, analytical expressions of the current control error are derived. The variables that affect the current control errors are then identified. The relationship between the circuit/control parameters and the current control errors are extensively explored. The command-tracking error affects the peak value of the modulation wave. When we design a fully digital control system, the peak value of the modulation wave is smaller than the peak value of the carrier wave. Then, according to choosing the values of the variables that affect the command-tracking error, the methods to design a fully digital direct current controlled voltage source converters connected to the grid are presented. The computer simulation investigations and hardware experimental results verify the analysis and design considerations.

In the present scenario, renewable energy source like solar power is the more convenient source of energy. Sustaining a steady DC link voltage and regulation of grid-current are challenging task in grid-tied photovoltaic (PV) system. These issues are overcome by designing a dual-loop control schemes; outer dc-link voltage loop which generates the reference signal for the inner current loop. The grid-tried inverter is used in photovoltaic (PV) system to inject the current with low total harmonic distortion (THD) into the utility grid by designing the efficient current controller. Generally, proportional-integral (PI) controller, proportional-resonant (PR) controller, dead-beat controller and hysteresis current controller are used in PV system to regulate the grid-current. These controllers are implemented in any one of the reference frames like; synchronous (d-q), stationary (α-β) or natural (a-b-c) reference frame. In this paper, the above-mentioned grid-current controllers are employed in all three reference frames and evaluate the steady-state performance of the controllers in terms of THD. The advantages and disadvantages of all grid-current controllers in each reference frames are briefly discussed and compared. This paper provides the best possible grid-current controller in three different reference frames. Finally, the simulation results from a two-stage 5.5 kW, 440 V (L-L), three-phase grid-tied PV system are provided to confirm the theoretical analysis and effectiveness of the control schemes.

This paper proposes two complex vector models to characterize three-phase unbalanced systems with four wires and containing PWM converters. In the first case, the three-phase system is decomposed in orthogonal odq components. A complex vector model is then built from the dq components and o component. In the second case a complex vector model is introduced by decomposing the system in three single-phase systems. In both cases, a second orthogonal simulated model is introduced to handle a single-phase system, such as the homopolar system or each of the three single-phase systems that compose a three-phase four wires system. From the vector models, a vector controller that employs two different reference frames is proposed and emulated on the stationary reference frame. A digital version for such a controller is also presented. As an example, a current control in a PWM-VSI system is presented.

Photovoltaic (PV) systems with integrated energy storage units traditionally have been complex and required multiple DC-DC converters. In this paper, a novel multiport isolated LLC resonant converter and a modified maximum power point tracking (MMPPT) control method are introduced to reduce complexity. The proposed topology is designed to operate under a wide input voltage range and contains an integrated bidirectional pulse width modulation (PWM) converter and a LLC resonant converter. Frequency modulation control is implemented for regulating the output voltage of the whole system while the PWM converter is controlled by the MMPPT which generates the battery current reference and PI controller. MATLAB/Simulink was used to demonstrate the feasibility of the topology.