Controlled Voltage Source Inverter Research Articles

HARMONIC DOMAIN MODEL OF AN OPEN-LOOP CONTROLLED PWM CONVERTER

An harmonic domain model is developed for open-loop controlled voltage source inverters, which provides the steady-state solution for integer multiples of the fundamental frequency. The convolution of the switching function spectrum with the converter currents and voltages is used to describe the coupling of the AC side and the DC side. The modeling is based on a single phase leg and is extended to a single-phase and a three-phase voltage source inverter, which results in a non-linear algebraic equation system. For open-loop control the switching function is independent of the converter currents and voltages and the model is simplified to a linear equation system. This straightforward and fast model is verified by time domain simulations and experimental results. The modeling can be adapted to systems of coupled converters, which is presented for a back-to-back converter system showing the harmonic interaction of the connected subsystems.

Fuzzy SVPWM‐based inverter control realisation of grid integrated photovoltaic‐wind system with fuzzy particle swarm optimisation maximum power point tracking algorithm for a grid‐connected PV/wind power generation system: hardware implementation

This research study presents the fuzzy space vector pulse width modulation (FSVPWM) method of current control for three-phase voltage source inverter. The hybrid fuzzy particle swarm optimisation-based maximum power point (MPP) tracking algorithm has been employed to obtain high tracking efficiency as well as optimal MPP under adverse operating states. The FSVPWM technique provides less current harmonic content, fixed switching pattern, protection from over current, low switching losses and able to handle the non-linearities and uncertainties of the photovoltaic-wind grid integrated system. Grid synchronisation with sinusoidal current injection is achieved using the inverter controller. Fuzzy logic controller-based SVPWM controller compensates current error and provides DC-link utilisation with high efficiency. The experimental responses have been validated using MATLAB/Simulink interfaced real-time dSPACE DS 1104 controller. Irrespective of solar irradiance and wind velocity, the proposed hybrid system obeys MPP accurately with high performance.

A Simplified Design Strategy for Multi-Resonant Current Control of a Grid-Connected Voltage Source Inverter with an LCL Filter

A distorted grid voltage or nonlinear behavior in the current control loop can cause low frequency current harmonics in a grid-connected voltage source inverter (VSI). Many efforts have been made to mitigate such phenomena, including hardware and/or control structure improvements. A well-known suitable strategy to reduce current harmonics in a selective manner is to apply a Proportional Multi-Resonant (PMR) current controller. Inverter-grid stability is another common issue when dealing with grid-connected VSI. Stability is influenced by the inverter impedance, which depends on the controller parameters. This paper presents a simplified tuning strategy for the PMR controller, taking into consideration the inverter-grid stability issue. The obtained controller was implemented and tested in a 10 kW three-phase inverter with a passively damped LCL filter. A significant reduction of current harmonics emission from the inverter up to 650 Hz was achieved without any hardware modification. The limits of PMR controllers to mitigate current harmonics were studied, and the influence of the grid impedance was verified.

FPGA implementation and analysis of model predictive current control for three-phase voltage source inverter

A detailed investigation of the model predictive current control (MPCC) for a three-phase voltage source inverter (VSI) with an output RL filter is examined in this manuscript. The investigation on VSI and its operating conditions such as dynamic, steady state, transient state condition with balanced and unbalanced loads are explained. The parameter variations such as sampling time, filter inductance and DC-link voltage variations are also explained in terms of load current Total Harmonic Distortion (THD) and inverter switching frequency. During each sampling period, the discrete mathematical model of the system is used to predict the load current for all the inverter switching state. These predictions are estimated using cost function, and so, the switching state which gives minimum cost function is selected and applied to the inverter. The effectiveness of the control technique is verified in MATLAB software and also in real-time hardware setup using Xilinx XCS500E Spartan 3E FPGA board. The detailed simulation and hardware results are included to investigate the implementation.

FPGA implementation and analysis of model predictive current control for three-phase voltage source inverter

A detailed investigation of the model predictive current control (MPCC) for a three-phase voltage source inverter (VSI) with an output RL filter is examined in this manuscript. The investigation on VSI and its operating conditions such as dynamic, steady state, transient state condition with balanced and unbalanced loads are explained. The parameter variations such as sampling time, filter inductance and DC-link voltage variations are also explained in terms of load current Total Harmonic Distortion (THD) and inverter switching frequency. During each sampling period, the discrete mathematical model of the system is used to predict the load current for all the inverter switching state. These predictions are estimated using cost function, and so, the switching state which gives minimum cost function is selected and applied to the inverter. The effectiveness of the control technique is verified in MATLAB software and also in real-time hardware setup using Xilinx XCS500E Spartan 3E FPGA board. The detailed simulation and hardware results are included to investigate the implementation.

On Stability of Voltage Source Inverters in Weak Grids

As the number of inverters increases in the power grid, the stability of grid-tied inverters becomes an important concern for the power industry. In particular, a weak grid can lead to voltage fluctuations at the inverter terminals and consequently cause inverter instability. In this paper, impacts of circuit and control parameters on the stability of voltage source inverters are studied using a small-signal state-space model in the synchronously rotating $dq$ -frame of reference. The full-order state-space model developed in this paper is directly extracted from the pulsewidth modulation switching pattern and enables the stability analysis of concurrent variations in the three-phase circuit and control parameters. This paper demonstrates that the full-order model of a grid-tied active (P) and reactive (Q) power (PQ)-controlled voltage source inverter (VSI) can be significantly reduced to a second-order model, preserving the overall system stability in the case of grid impedance variations. This paper also shows that a decrease in the grid inductance does not necessarily improve the stability of grid-tied VSIs. The system stability is a function of both the grid R/X ratio and grid inductance. Despite the grid-side inductor of the LCL filter is in series with the grid impedance, they have different impacts on the stability of a grid-tied PQ-controlled VSI, i.e., an increase in the filter inductance may improve the system stability in a weak grid. These findings are verified through simulated and experimentally obtained data.

Fuzzy-Super Twisting Sliding Mode MPPT Control for Three-Phase Grid-Connected PV

In this paper, a new fuzzy-super twisting sliding mode control for maximum power point tracking MPPT of a photovoltaic array connected to a three-phase grid is proposed. A voltage source inverter control using d-q components of the grid line currents is employed in order to regulate DC link voltage and also to control the active and reactive injected powers. The effectiveness of the proposed method is demonstrated under standard and real solar insolation and temperature. Additional supplementary power quality control functions as low harmonic distortion rate and zero reactive power injection are ensured. The system performances are simulated under MATLAB/SIMULINK environment. Â

Selected issues of control voltage source inverter with self-excited synchronous generator as DC grid voltage source

Synchronous self-excited generators are the most popular AC voltage sources installed in power plants of seagoing vessels. Because of fuel savings varying revolutions Diesel drives can be found on selected types of ships (platform support vessels, cable layers, tug boats etc.). Very interesting issue is use of such type of alternator working with inverter acting as rectifier in direct current grid system. In direct current type of electrical grid the problems with synchronization and reactive power balance are absent. As the control method most suitable to use is modified version of field oriented control (FOC) known from induction machines. Aforementioned method involves decoupling of currents and control voltages to flux and torque components and keeping them in the most optimal condition. Theoretical background of inverter and synchronous generator adopted FOC control method along with numerical situations and experimental results obtained in laboratory test bench of such a system were included in following article.

Development of a new control strategy based on two revolving field theory for single-phase VCVSI integrated to DC microgrid

This paper proposes a new control strategy of single-phase Voltage Controlled Voltage Source Inverter (VCVSI) based on Two Revolving Field Theory (TRFT) for integrating the single-phase static as well as dynamic ac loads to the DC Microgrid (DCMG). In the developed control scheme, the output ac voltage/current of the single-phase VCVSI has been decomposed into two counter rotating vectors, referred as Forward-Positive (FP) and Backward-Negative (BN) vectors, by using TRFT principle. The α-β components of the FP and BN rotating vectors have been obtained using the orthogonal transformation. The new control scheme, with dual controllers; one for the FP vectors and another for the BN vectors of the output ac voltage/current of the VCVSI, has been implemented into two rotating d-q synchronous reference frames, using a feed-forward and two feed-back control loops. The objective of the proposed controllers is to establish the VCVSI rated output ac voltage with low total harmonic, faster transient response after disturbances, and to ensure needed power flow from the microgrid to the single-phase loads. Simulations have been carried out to show the effectiveness of the developed control scheme along with the proposed DCMG in the islanded mode, under different operating conditions.

Model-Based Predictive Current Control Method with Constant Switching Frequency for Single-Phase Voltage Source Inverters

Voltage source inverters operated by predictive control methods generally lead to a variable switching frequency, because predictive control methods generate switching operation based on an optimal voltage state selected at every sampling period. Varying switching frequencies make it difficult to design output filters of voltage source inverters. This paper proposes a predictive control algorithm with a constant switching frequency for the load current control of single-phase voltage source inverters. This method selects two future optimal voltage states used in the subsequent sampling period, which are a zero-voltage state and a future optimal voltage state, based on the slope of the reference current at each sampling period. After selecting the two future voltages, the proposed method distributes them to produce a constant switching frequency and symmetric switching pattern. The performance of the proposed method is validated with both simulation and experimental results for single-phase voltage source inverters.

Single-Step Current Control for Voltage Source Inverters With Fast Transient Response and High Convergence Speed

Current control loop response in voltage source inverters impacts the quality of output current and output voltage waveforms. Parabolic current control provides a fast transient response with approximate-constant switching frequency, solving the frequency variation problem of hysteresis current control. This makes it a good candidate for the current control loop of voltage source inverters to achieve a good system performance. Yet, parabolic current control is often implemented with digital-to-analog converters, analog comparators, and field-programmable gate array circuits where increasing switching frequency pushes update speed and bandwidth requirements. Concurrently, even if the transient response of parabolic current control is fast, it can still take up several switching cycles converging to steady state. In order to solve both problems, a new current control strategy, motivated by the convergence analysis of parabolic current control but with a convergence process that takes just one switching operation, is proposed: single-step current control. Since single-step current control samples two data points in a switching cycle instead of using continuous parabolic carriers, it can be easily implemented in a digital microcontroller then high switching frequency can be achieved. The small signal model, dead-time compensation, and stability analysis are also studied in this paper. The convergence speed of the algorithm is verified with experimental hardware prototype and the transient performance of the designed voltage source inverter meets expectation.

Intelligent control strategy in the islanded network of a solar PV microgrid

In the islanded mode of MG network, due to the lack of inertia and under performance of PI based inverter control, variation in power and frequency level can be expected more during the transient conditions as compared to the grid connected mode. Thus, in this study, a Particle Swarm Optimization (PSO) algorithm with the proposed cost function was implemented in the Grid-support grid-forming (GsGfm) type Voltage Source Inverter (VSI) of the DG sources (solar PV and battery unit) for the islanded photovoltaic (PV) based MG model (built in Matlab-Simulink software environment) to reduce the variation in power and frequency level. The islanded MG model was also implemented with a reverse droop based on virtual impedance control strategy for the VSI and power management control strategy to ensure better power sharing among DG sources with regulation of frequency level and power balance with co-ordinated control operation of DG sources. Implementation of PSO based intelligent control strategy for the inverters of DG units, showed substantial improvement in the performance of controller in terms of reduced settling time and overshoot in power level along with the significant reduction of the power variation and frequency variation during the transient conditions (change in load and change in solar irradiance of PV units). Also proposed PSO control strategy ensured the frequency level to be within the acceptable operating range (Australian network standard) in the islanded MG network.

Dead‐beat predictive direct power control of voltage source inverters with optimised switching patterns

This study presents a novel dead-beat predictive direct power control (PDPC) strategy working at nearly constant switching frequency for the voltage source inverter. The proposed strategy employs two active voltage vectors and one null voltage vector during every control period. To solve the problem of negative predicted duration time of active voltage vectors in the conventional dead-beat PDPC strategy, an automatic updating mechanism is designed. When the predicted duration time is negative, the applied voltage vectors will be updated automatically without the need of reselecting voltage vectors or recalculation of the duration time. Accurate tracking of both active and reactive power can be achieved with small power ripples using the strategy proposed in this study. The proposed PDPC strategy shows excellent steady performance and fast dynamic responses. High sampling frequency can also be avoided and the calculation process is simple enough to be implemented on normal DSPs. Simulation and Experimental results are presented to validate the feasibility of the proposed dead-beat PDPC strategy.

HIL simulation of the DTC for a three-level inverter fed a PMSM with neutral-point balancing control based on FPGA

This paper presents a hardware description methodology for the direct torque control (DTC) of a three-level voltage source inverter (VSI) fed a permanent magnet synchronous motor drive with neutral-point balancing control algorithm based on field programmable gate array (FPGA). The main drawback of the conventional DTC during its digital implementation is the high torque ripple, and to overcome this problem, the use of a three-level VSI and a simple hardware description methodology for DTC based on FPGA is proposed. However, the circuit limitations of the three-level inverter, such as neutral-point balance, should be taken into account in the selection of the future space vector applied to the machine; for this reason, a simple and effective procedure to maintain the neutral-point balance is presented. The proposed methodology has been validated using hardware in the loop simulation, and results under different operating points are presented.

A Nonlinear Backstepping Controller for Inverters used in Microgrids

This paper proposes a nonlinear controller for Voltage Source Inverter (VSI) used in microgrids. The controlled system includes the VSI, an LC filter, a coupling inductor, and loads. The controller is designedon the basis of the averaged system model, using the backstepping technique and the droop control method. The controller stability is theoretically analyzed and its performances are illustrated by simulation in different grid operation modes(connected and islanded)and load changes.

A Comparison Study of Sinusoidal PWM and Space Vector PWM Techniques for Voltage Source Inverter

In this paper, the methods used to control voltage source inverters, which have been intensively investigated in recent years, are compared. Although the most efficient result is obtained with the least number of switching elements in the inverter topologies, the method used in the switching is at least as effective as the topology. Besides, the selected switching method to control the inverter will play an effective role in suppressing harmonic components while producing the ideal output voltage. There are many derivatives of pulse width modulation techniques that are commonly used to control voltage source inverters. Some of widespread methods are sinusoidal pulse width modulation and space vector pulse width modulation techniques. These modulation techniques used for generating variable frequency and amplitude output voltage in voltage source inverters, have been simulated by using MATLAB/SIMULINK. In addition, the total harmonic distortions of the output voltages are compared. As a result of simulation studies, sinusoidal pulse width modulation has been found to have more total harmonic distortion in output voltages of voltage source inverters in the simulation. Space vector pulse width modulation has been shown to produce a more efficient output voltage with less total harmonic distortion.

Grid synchronization and control of distributed generation unit with flexible load compensation capabilities using multi-output LMS-filter

This paper deals with grid synchronization and control of single-phase voltage source inverter in distributed generation (DG) systems using a multi-output adaptive filter. Besides the active power injection, the proposed control enables the inverter to compensate load harmonic and reactive currents based on the capacity of inverter. The currents to be compensated are obtained using a multi-output adaptive filter structure that operates on the principle of variable step size (VSS) least mean squares (LMS) algorithm. The main objective of VSS-LMS filter is to decompose the load current into harmonic and reactive current components and compute their RMS values. The currents extracted by the VSS-LMS filter are used in developing a flexible compensation based on load compensation factors. The load compensation factors are calculated based on the remaining capacity of the inverter and priority. Harmonic currents compensation is given first priority over the reactive currents. In addition to harmonic currents detection, the same VSS-LMS filter is used in developing a phase locked loop unit for synchronizing the inverter with fundamental point of common coupling (PCC) voltage. Synchronizing the DG inverter with fundamental PCC voltage would make the proposed control algorithm insensitive to grid voltage harmonics and frequency fluctuations. The feasibility and the efficacy of the proposed control algorithm are demonstrated using hardware-in-the-loop (HIL) based experiments.

Some Aspects of Voltage Source Inverter Control

The model of a voltage source inverter depends on its output filter. The parameters of this filter can differ significantly from their nominal values due to power losses in the core of the output coil and changes in its inductance. Only Multi-Input-Single-Output control systems can effectively reduce inverter output voltage distortions for a standard nonlinear rectifier RC load. However, their design requires knowledge of the actual values of the inverter parameters. The novelty of the paper is that it connects the designs of two MISO control systems with a method of the automated measurements of the voltage source inverter parameters. The breadboard verification is also included.DOI: http://dx.doi.org/10.5755/j01.eie.23.2.17995

Harmonic Mitigation Using DC Self Supported Shunt Active Power Filter

The advent of semiconductor technology has resulted in increased application of power electronic devices in industrial as well as domestic loads, a combination of which acts as a non-linear load. These non-linear loads are responsible for increased harmonics by drawing a distorted current from the supply system, leading to deterioration of source currents and voltages. This work proposes a method to improve the power quality with the aid of a three-phase shunt active power filter (SAPF) implementing a controlled voltage source inverter and synchronous d-q reference current generation technique for reference current generation of hysteresis controller. A discrete PI controller has been implemented to maintain a constant DC capacitor link voltage. The proportional and integral gains of the PI controller have been computed with the help of bacterial foraging optimization technique. The proposed SAPF for a three phase three wire system employing a balanced non-linear load has been simulated on MATLAB software to validate its performance. The simulation results demonstrate satisfactory performance of the proposed SAPF by effectively compensating harmonics of the supply system and lowering THD such that they fall within IEEE 519 standards for harmonic levels.

Design and experimental investigation of digital model predictive current controller for single phase grid integrated photovoltaic systems

Depletion of fossil fuel reserves has lead to identification of renewable sources of energy as alternative sources of energy for power generation. The photovoltaic plant is more prominent in present day scenario. The power electronic converters interfacing the photovoltaic generator with the grid gives rise to various power quality problems such as harmonics in current injected by the inverter into the grid and low output power factor. This paper focuses on the design, analysis and control of single phase grid integrated photovoltaic systems. In this paper a dual stage photovoltaic inverter system is simulated using MATLAB/SIMULINK software. A digital predictive current controller based on model predictive control theory with delay compensation technique is introduced for output current control of the single phase voltage source inverter integrating the photovoltaic generator with the grid. The proposed control strategy exhibits excellent steady state performance and attains a high output power factor. It also achieves very fast transient response during change in reactive power demands and grid voltage distortions. The efficacy of the proposed controller is validated by experimental implementation using low cost digital signal processor.