An active power synchronizing controller for grid-connected power converters with configurable natural droop characteristics

Control of grid-connected power converters is continuously developing to meet the grid codes, according to which the generation units should keep connected to the grid and further provide ancillary services, such as voltage and frequency support, negative sequence current injection, inertia emulation, etc. A virtual admittance controller is proposed in this paper for the objective of voltage support under asymmetrical grid faults. By using independent and selective admittances for positive and negative sequence current injection, the unbalanced voltage can be significantly compensated during asymmetrical faults. The controller is based on the generic control framework of the synchronous power controller (SPC), which is able to control a power converter with emulated and improved synchronous generator characteristics. Simulation and experimental results based on two paralleled 100 kW grid-connected power converters demonstrate the controller to be effective in supporting unbalanced voltage sags.

This paper proposes a self-tuning model predictive direct power control (MPDPC) strategy for power flow control and power quality improvement in grid-connected power converters. At each sampling instant, a fuzzy logic controller is used to determine online the best weighting factor values for a given operating point. These values are then used to solve the multi-objective optimal control problem associated to the MPDPC. The optimal solution that minimizes the multi-objective cost function is chosen as the input (power switch state). The proposed method is examined through a case study and verified numerically via MAT LAB SIMULINK. A comparative study is conducted to demonstrate the effective performance of this approach. As a result of the proposed weighting factor online tuning, an improved performance in terms of total harmonic distortion and average switching frequency is attained when compared with fixed weighting factors.

We consider model predictive control (MPC) of disturbed linear time-invariant (LTI) multiple-input multiple-output (MIMO) systems without constraints. For the considered system class, the MPC problem has an analytical solution which allows for prediction horizons with arbitrary length. In addition, we introduce integral error feedback which assures steady-state accuracy for (piece-wise) constant disturbances and reference signals. The proposed MPC scheme is applied to grid-side current control of grid-connected power converters with LCL-filter. Simulation results illustrate and compare the control performance of the proposed MPC scheme with and without integral error feedback under (symmetric) grid perturbations.

Renewable energy sources (RESs) have been extensively employed to replace fossil fuels for reducing carbon footprints. Since RESs are normally coupled to the mains grid by power converters without providing any inertia, the system inertia generated by synchronous generators continues to decrease, making modern power systems sensitive to frequency disturbances. To address the inertia concern, this paper proposes an inertia enhancement method implemented by grid-connected power converters. Specifically, the proposed method can be achieved by proportionally linking the time derivative of the system frequency and active power references of power converters. Although the differential operation cannot be applied due to high frequency noises, the presented frequency-locked-loop (FLL) estimates the frequency derivative signal in an alternative and practical way. Through the proposed method, all types of grid-connected converters can easily participate in frequency regulation. Simulation and experimental results are presented to illustrate the effectiveness of the proposed method.

Distributed Generation (DG) provides an alternative to the Centralized Generation (CG) by means of generating electricity near to the end user of power with the employment of small-scale technologies to produce electricity, mainly using Renewable Energy Sources (RES). The prospects of renewable energy integration during the next years are still very optimistic. 
 This PhD dissertation is made to provide an alternative control framework for the grid connected power converter by adopting the virtual flux concept in the control layer. This dissertation can be divided into three main topics. The 1st topic presents the voltage sensorless control system for the grid-connected power converter. The control system presented is done without depending on AC-voltage measurement where the grid synchronization is based on the Virtual Flux (VF) estimation. In this regard, the Frequency Locked Loop (FLL) is used in conjunction with the estimation scheme to make the system fully adaptive to the frequency changes. This voltage sensorless application is useful for reducing cost and complexity of the control hardware. It is also can be utilized in case of limited reliability or availability of voltage measurements at the intended point of synchronization to the grid. Considering that most previous studies are based on the VF estimation for the case of power converter connected to the grid through the L-filter or LC-filter, this dissertation is focused on the power converter connected to the grid through the LCL filter. The Proportional Resonant (PR) current controller is adopted in the inner loop control of the power electronics-based converter to test the performance of such system.
 Another control method based on VF synchronization that permits to control the active and reactive power delivery in a remote point of the grid is also presented in this dissertation. This is due to the fact that the VF is implemented that the voltage in a remote point of the line can be estimated. As it will be shown in simulations and experiments, the proposed control scheme provides a good tracking and dynamic performance under step changes in the reference power. The fast synchronization and the smooth reference tracking achieved in transient conditions have demonstrated the effectiveness of the Dual Second Order Generalized Integrator controlled as Quadrature Signal Generator (DSOGI-QSG) and also the current controller used in the proposed system. In addition to the power control itself, this study could also benefit the frequency and the voltage regulation methods in distributed generation applications as for instance in microgrid.
 Considering the fact that the grid connected power converter can be controlled as a virtual synchronous generator where the flux is a variable to be used for controlling its operation, this dissertation also presents a Virtual Synchronous Flux Controller (VSFC) as a new control framework of the grid connected power converter. In this regard, a new control strategy in the inner loop control of the power converter will be proposed. The main components of the outer loop control of VSFC are based on the active and reactive power control. The results presented show that the VSFC works well to control the active and reactive power without considering any synchronization system. The inner loop control is able to work as it is required, and the measurement flux is able to track the reference flux without any significant delays.
 All the work presented in this dissertation are supported by mathematical and simulation analysis. In order to endorse the conclusions achieved, a complete experimental validations have been conducted before wrapping this dissertation with a conclusion and recommendation for future enhancement of the control strategies that have been presented.

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.

This paper analyzes optimal control of a grid-connected converter (GCC) based on the adaptive critic designs (ACDs), especially on heuristic dynamic programming (HDP). Instead of using a trained model neural network to identify the dynamics of the plant, the paper uses exact GCC plant mathematical model to reflect the system dynamics accurately. Thus, the HDP for GCC only contains a critic neural network and an action neural network, which simplifies the control design. The training cycles of the critic and action networks in our HDP design are combined into one training cycle, which reduces the calculation time and makes it more suitable to online training. Results show that the RNN controller based on the designed HDP exhibits good tracking ability. The weights of the critic and action networks are adjusted online to adapt the changing GCC system automatically.

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.

This paper proposes an extended state observer (ESO) based second-order sliding-mode (SOSM) control for three-phase two-level grid-connected power converters. The proposed control technique forces the input currents to track the desired values, which can indirectly regulate the output voltage while achieving a user-defined power factor. The presented approach has two control loops. A current control loop based on an SOSM and a dc-link voltage regulation loop which consists of an ESO plus SOSM. In this work, the load connected to the dc-link capacitor is considered as an external disturbance. An ESO is used to asymptotically reject this external disturbance. Therefore, its design is considered in the control law derivation to achieve a high performance. Theoretical analysis is given to show the closed-loop behavior of the proposed controller and experimental results are presented to validate the control algorithm under a real power converter prototype.

Grid synchronization algorithms are of great importance in the control of grid-connected power converters, as fast and accurate detection of the grid voltage parameters is crucial in order to implement stable control strategies under generic grid conditions. This paper presents a new grid synchronization method for three-phase three-wire networks, namely dual second-order generalized integrator (SOGI) frequency-locked loop. The method is based on two adaptive filters, implemented by using a SOGI on the stationary αβ reference frame, and it is able to perform an excellent estimation of the instantaneous symmetrical components of the grid voltage under unbalanced and distorted grid conditions. This paper analyzes the performance of the proposed synchronization method including different design issues. Moreover, the behavior of the method for synchronizing with highly unbalanced grid is proven by means of simulation and experimental results, demonstrating its excellent performance.

Increased dc injection from grid-connected power converters and an increased dc-bias sensitivity of recent distribution transformers require unprecedented precision in detecting and suppressing the dc bias in 0.4-kV ac grids. In this paper, an optimum design of the dc-bias sensing reactor is devised, along with a robust, simple-to-use sensing algorithm that does not require calibration or tuning. The sample sensors are used as feedback devices within an experimental setup comprising a three-phase grid-connected inverter and a novel dc-bias suppression algorithm. The steady-state dc bias is suppressed below $65\, \upmu{\rm V}$ , the level acceptable even with the most sensitive distribution transformers.

Power electronic Grid-Connected Converters (GCCs) are widely applied as grid interface in renewable energy sources. This paper proposes an extended Direct Power Control with Space Vector Modulation (DPC-SVM) scheme with improved operation performance under grid distortions. The real-time operated DPC-SVM scheme has to execute several important tasks as: space vector pulse width modulation, active and reactive power feedback control, grid current harmonics and voltage dips compensation. Thus, development and implementation of the DPC-SVM algorithm using single chip floating-point microcontroller TMS320F28335 is described. It combines large peripheral equipment, typical for microcontrollers, with high computation capacity characteristic for Digital Signal Processors (DSPs). The novelty of the proposed system lies in extension of the generic DPC-SVM scheme by additional higher harmonic and voltage dips compensation modules and implementation of the whole algorithm in a single chip floating point microcontroller. Overview of the laboratory setup, description of basic algorithm subtasks sequence, software optimization as well as execution time of specific program modules on fixed-point and floating-point processors are discussed. Selected oscillograms illustrating operation and robustness of the developed algorithm used in 5 kVA laboratory model of the GCC are presented.

The benefits of using LCL filter connecting the grid with converter heavily depends on control method. Researchers in field of predictive control are mainly focused on improved the response dynamics, stability and accuracy of reference current tracking. This paper introduces an extended predictive control scheme of power converter with an LCL filter with improved operation under grid distortions. The proposed modification of control algorithm uses additional feedback form grid current to increase robustness for higher harmonics in grid voltage. The simulation results have proven good performance and verified the validity of the proposed solution. The laboratory test setup provided experimental results showing properties of the described method.

This article deals with a new nonlinear transformation providing representation of unbalanced three-phase signals (voltage and currents) as vectors that have constant components in a new rotating d'q' frame. Using the proposed transformations, the control system of a power electronic converter operating with an unbalanced grid can be equipped with proportional-integral (PI) controllers, without the use of resonant terms, for which antiwindup structures are more complicated and nonintuitive. This article presents the derivation of new direct and inverse transformations, features, a power converter model in a new frame, simulation results showing the waveforms of signals transformed to the new non-Cartesian frame, and simulation and experimental results of the voltage-oriented control for a grid-connected power converter.

This paper proposes a computationally efficient and robust direct model predictive control (DMPC) technique with enhanced steady-state performance for power converters tied to the electric utility. The discrete space vector modulation (DSVM) method is considered in the design of the suggested DMPC, where virtual voltage vectors (VVs) besides the real ones are utilized for improving the steady-state response of the proposed controller. Furthermore, for averting the high computational burden and making the proposed control technique simple, a deadbeat (DB) function is employed for calculating the reference VV based on the required reference current. Subsequently, a discrete-time integral term is combined with this DB function to enhance the robustness of the suggested DMPC technique against variations of the model parameters. Finally, the best virtual or real VV is chosen by a certain quality function, which will be applied to the power converter in the next sample. The suggested technique is verified by simulation results and its performance is compared with the classical DMPC and voltage-oriented control (VOC).