A Current Decoupling Control Scheme for LCL-Type Single-Phase Grid-Connected Converter

This paper presents an implementation methodology for a current harmonic compensation technique of three-phase grid-connected voltage-source converter (VSC), which was applied under the grid voltage distortion and frequency variation. The proportional-integral plus multi-resonant controllers (PIMR) were used to mitigate the high order harmonic current. The PIMR controller was transformed to the discrete-time domain and implemented on a TMS320F28379D digital signal processor. The proposed control scheme was validated with a hardware in the loop (HiL) and with a 5 kW three-phase LCL-filtered grid-connected VSC. The harmonic compensators were used in the dq-frame at harmonic orders 6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> and 12 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> for suppression of the grid voltage harmonic orders 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> , 7 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> , 11 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> , and 13 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> . Results from MATLAB/Simulink simulation and experiments with HiL and hardware implementations were found to be in close agreement. The experimental grid current exhibited excellent harmonic rejection with a total harmonic distortion close to 1% and complied with the IEEE 1547 standard. The grid current can tracked the reference command with a fast dynamic response.

An improved virtual current method for single-phase converters under frequency variations

To reduce the consumption of non-renewable energy in the building sector, single-phase grid-connected voltage source converter (VSC) has been widely used to integrate distributed energy resources (DERs) into the building electrical network. As the increasing penetration of DERs into the electrical power system, new grid codes require the DERs should become more interactive and flexible while operating with the utility grid. Some functionalities may include grid fault operation and support capabilities, such as fault ride through, Volt/VAr support, etc. However, these new advanced functions might bring functionality conflict with the existing functions, such as anti-islanding protention. In this paper, the behaviors of single-phase grid-connected VSC under various grid conditions, such as voltage sag and unintentional power outage, are investigated. Following the analysis, a coordinated reactive control strategy including grid status identification criteria development and reactive curve designing, is proposed to enhance the performance of the converter on descriminating various grid status with multiple functionality conflict consideration. Simulation studies are presented to demonstrate the validity of the proposed control strategy.

One of the major concerns during the design of grid-connected converters is meeting the electromagnetic interference (EMI) standards. Ground leakage current is another aspect which has a stringent limit due to safety concerns. Presence of high frequency pulses, with high dv/dt due to switching actions, in common-mode (CM) voltage excite parasitic capacitances of the power converters. This results in injection of peaky current to ground and causes high EMI noise level. In this paper a new method is proposed to make the CM voltage, of a single-phase grid-connected converter, sinusoidal and free of high frequency pulses. The proposed method consists of two parallel connected H-bridge power converters, which uses unipolar PWM with carrier interleaving angle of 180°. The proposed method reduces the ground leakage current by more than an order of magnitude. A modification to the LCL filter is adopted which further reduces the leakage current by 48%. It is shown that these modifications make the overall system insensitive to circuit nonidealities. Experimental results based on ground leakage current and EMI measurements on a 7.5kW parallel single-phase PWM-rectifier are presented. These results validate the effectiveness of the proposed method.

Three-phase active damping LCL-type grid-connected converters are usually used in distributed power generation systems. However, serious inrush current will be aroused when the grid-connected converter starts, especially in rectifier mode, if no effective control method is taken. The point of common coupling (PCC) voltage feedforward is usually used to suppress start-up inrush current. Unfortunately, it will introduce a positive feedback loop related to the grid current and grid impedance under weak grid conditions, and therefore, the grid current will be distorted and the system stability margin will be significantly reduced. To solve the above problems, this paper proposes a simple method based on a d-axis fundamental positive-sequence component of filter capacitor voltage feedforward, without extra sensors and software resources. With the proposed method, it is possible to suppress the start-up inrush current and maintain the grid current quality and system stability under weak grid conditions. The mechanism of start-up inrush current and the effectiveness of the method for inrush current suppression are analyzed in detail. Then, the influences of different feedforward methods on system stability are analyzed under weak grid conditions by the impedance model of grid-connected converter. Finally, experimental results verify the validity of the proposed method.

Series-winding topology (SWT) could improve the DC-link voltage utilization, as open-winding topology does. Meanwhile, it can greatly reduce the number of power devices. Firstly, for the half-bridge power modules (HBPMs)-based inverter, an N-phase series-winding motor only requires N + 1 HBPMs for driving. On the other hand, such SWT also brings new challenges to the drive system. A zero-sequence loop is introduced into the motor windings due to SWT. The generated zero-sequence current would degrade the total harmonic distortion of the phase currents and produce the additional torque ripple. Moreover, current sensors are typically integrated with the HBPMs. However, in SWT, their measured results are the leg currents of the inverter, not the phase currents of the motor, which is crucial to the motor control. Thus, this paper mainly focuses on the aforementioned problems in a three-phase series-winding permanent-magnet synchronous motor (TPSW-PMSM) drive with HBPM-based inverter. Firstly, to control the zero-sequence subspace, the voltage vector distribution of TPSW-PMSM is analyzed. In addition, two voltage vectors with zero-sequence components are selected to generate the zero-sequence voltage. Then, the phase currents are reconstructed according to the leg currents from the current sensors on HBPMs. Based on the above, the deadbeat predictive current control (DBPCC) scheme is proposed for a TPSW-PMSM drive with HBPM-based inverter. It provides the TPSW-PMSM drive with fast dynamic response and effective zero-sequence current suppression. Finally, both simulation and experimental results verify the feasibility and effectiveness of the proposed DBPCC scheme.

A novel scheme for the control of three-phase two-level grid-connected power converters is presented in this paper. A cascade-control structure, based on H ∞ control and sliding mode control (SMC), is proposed, which simultaneously achieves output voltage regulation and unity power factor under dc-load variations. The overall control strategy contains two main loops: first, a current tracking loop (internal loop) based on sliding mode control that guarantees the input currents to follow the desired references; second, a dc-link voltage regulation loop (external loop) based on an H ∞ controller integrated with an extended state observer (ESO), which regulates the output voltage and provides current references for the internal loop. Simulation results are provided to assess the efficiency of the proposed method.

Three-phase grid-connected converters are widely used in renewable and electric power system applications. Traditionally, grid-connected converters are controlled with standard decoupled d-q vector control mechanisms. However, recent studies indicate that such mechanisms show limitations. This paper investigates how to mitigate such problems using a neural network to control a grid-connected rectifier/inverter. The neural network implements a dynamic programming (DP) algorithm and is trained using backpropagation through time. The performance of the DP-based neural controller is studied for typical vector control conditions and compared with conventional vector control methods. The paper also investigates how varying grid and power converter system parameters may affect the performance and stability of the neural control system. Future research issues regarding the control of grid-connected converters using DP-based neural networks are analyzed.

High penetration of wind power requires reliable wind energy generation. A successful low-voltage ride-through (LVRT) scheme is a key requirement to achieve reliable and uninterrupted electrical power generation for wind turbines equipped with doubly fed induction generators (DFIGs). This paper proposes a feed-forward transient current control (FFTCC) scheme for the rotor side converter (RSC) of a DFIG to enhance its LVRT capability. This new control scheme introduces additional feed-forward transient compensations to a conventional current regulator. When three phase faults occur, these compensation terms correctly align the RSC ac-side output voltage with the transient-induced voltage, resulting in minimum transient rotor current and minimum occurrence of crowbar interruptions. With little additional computational effort, the proposed control scheme helps relieve the transient current stress on the RSC and helps maintain an uninterrupted active and reactive power supply from the wind turbines to the power grid. Simulation results are shown to demonstrate the effectiveness of the proposed FFTCC scheme in suppressing transient rotor currents.

Robust tracking control for vehicle electronic throttle using adaptive dynamic sliding mode and extended state observer

This paper presents a smart DC network formed by a multi-terminal VSC-HVDC connecting various micro-grids, independent power generation stations, and passive loads. The corresponding deadbeat current control scheme based on coordinated strategy for multi-terminal VSC-HVDC systems is proposed. The current control scheme is easy to design, implement and has good dynamic response. the case of four-terminal VSC-HVDC interconnected with AC grids, wind farms, and passive loads is developed in MATLAB/Simulink to demonstrate the validity of the proposed control scheme and the united system. Simulation results have demonstrated that the proposed scheme enables optimal operation under conditions of voltage dip, power flow and load changes when used for a large DC smart network.

In a conventional finite control set model predictive control (FCS-MPC) formulation, active and reactive power control loops rely on the predictive controller, whereas the dc-bus voltage is usually governed by a PI-based control loop. This originates from fact that the dynamic equations for describing the predictions of these variables are heavily coupled. In this paper, a cascade-free FCS-MPC for single-phase grid-connected power converters is presented. The proposed control algorithm is formulated in terms of established dynamic references design, which was originally proposed to directly govern active and reactive power, and dc-voltage in three-phase power converters. In this paper, the dynamic reference design concept is extended to control single-phase grid-connected power converters. The proposed control algorithm does not use instantaneous ac-power calculations; instead, it directly formulates the optimal control problem on the grid-current in the original stationary reference frame. The experimental results obtained with a single-phase grid-connected neutral point clamped (NPC) converter confirm a successful design, where system constraints, e.g., maximum power and weighted switching frequency, are easily taken into account.

A novel simplified control scheme for a three-phase switch-mode rectifier is proposed in this paper. The proposed control scheme is based upon a load-conductance rectifier controller, in which reference current signals are obtained. The goal to follow the reference current is converted to follow a reference voltage. A simplified control scheme utilizing space-vector modulation is developed to calculate the duty ratio required to synthesize the reference voltage. The proposed scheme has the advantage of space-vector modulation with fast dynamic response and is simple enough to be implemented in a single-chip microprocessor 80196MC.

This paper presents a predictor-based active disturbance rejection control (PADRC) scheme for input delay systems. In the proposed control scheme, the extended state observer (ESO) is modified by delaying the control signal to compensate the time delay from the observation loop. Moreover, a predictor is designed in time domain to obtain estimated states for current time and cancel the effects of input delay on the closed-loop system, where the approximation of disturbances is considered for corrective prediction purposes. Furthermore, a PD controller is designed to compensate disturbances and control the system. Then, the stability of the closed-loop system is analyzed by the Lyapunov function method. Finally, the effectiveness of the PADRC is demonstrated by simulations.

Although the control of modular multi-level converters (MMCs) in high-voltage direct-current (HVDC) networks has become a mature subject these days, the potential for adverse interactions between different converter controls remains an under-researched challenge attracting the attention from both academia and industry. Even for point-to-point HVDC links (i.e., simple HVDC systems), converter control interactions may result in the shifting of system operating voltages, increased power losses, and unintended power imbalances at converter stations. To bridge this research gap, the risk of multiple cross-over of control characteristics of MMCs is assessed in this paper through mathematical analysis, computational simulation, and experimental validation. Specifically, the following point-to-point HVDC link configurations are examined: (1) one MMC station equipped with a current versus voltage droop control and the other station equipped with a constant power control; and (2) one MMC station equipped with a power versus voltage droop control and the other station equipped with a constant current control. Design guidelines for droop coefficients are provided to prevent adverse control interactions. A 60-kW MMC test-rig is used to experimentally verify the impact of multiple crossing of control characteristics of the DC system configurations, with results verified through software simulation in MATLAB/Simulink using an open access toolbox. Results show that in operating conditions of 650 V and 50 A (DC voltage and DC current), drifts of 7.7% in the DC voltage and of 10% in the DC current occur due to adverse control interactions under the current versus voltage droop and power control scheme. Similarly, drifts of 7.7% both in the DC voltage and power occur under the power versus voltage droop and current control scheme.