Voltage Source Converter Output Research Articles

Wavelet-based rapid identification of IGBT switch breakdown in voltage source converter

The power electronic converters have drawn a lot of attention from investigators in recent years. Modern power electronics converters rely heavily on insulated gate bipolar transistors (IGBT). Conventional distribution systems may be made more reliable, stable, and secure by using a microgrid (MG). An acceptable, enticing, and useful option for power distribution networks is inverter-based MGs. This article describes a method for identifying the IGBT switch breakdown failure (IBDF) in a 3-phase, 3-level Voltage Source Converter (VSC) linked to the photovoltaic (PV) grid. The VSC's output phase voltage was subjected to an analysis based on the Discrete Wavelet Transform (DWT) methodology. Kurtosis, skewness, and root-mean-square (RMS) values of the wavelet coefficients of the output phase voltage harmonic spectra of the VSC are all factored in during studies. For the detection of IBDF, comparative learning has been used to identify the specific parameter that is most suitable. Moreover, the comparative analysis and unique contribution of this investigation have been demonstrated.

Adaptive Virtual Impedance Controller for Parallel and Radial Microgrids With Varying X/R Ratios

In this paper, an adaptive virtual impedance-based universal voltage source converter (VSC) control scheme is proposed to improve the stability, and power-sharing performance of VSC interfaced distributed energy resources in hybrid AC/DC microgrids with different feeder characteristics. The proposed control scheme modulates the VSC output impedance by tracking the active and reactive power transfer difference between the inverter terminal and the point of connection to the microgrid. Firstly, the stability limits of microgrid with resistive and inductive microgrid feeders are investigated by theoretical analysis. Subsequently, influence of the feeder parameters on the stability of microgrid is evaluated. From the theoretical analysis, the proposed control scheme shows resistive inverter output impedance at low frequency, while it shows more inductive behavior at system frequency for both the low and medium voltage microgrid feeders. The effectiveness of the proposed control strategy is demonstrated through a range of scenarios for two different microgrid types. The results show that the proposed controller can autonomously vary the output impedance of the VSC and provides significantly improved damping and power-sharing performance of the microgrid.

Stability Analysis of Chaotic Grey-Wolf Optimized Grid-Tied PV-Hybrid Storage System during Dynamic Conditions

This paper presents the stability improvement of the three-phase four-wire (3P-4W) grid-tied PV-hybrid energy storage system (HESS) using chaotic grew wolf optimization (CGWO) for DC bus voltage (Vdc) and AC bus voltage (Vpcc) control. The CGWO tuned fractional order proportional–integral (FOPI) controllers reduce the Vdc and Vpcc variations during diverse, dynamic conditions, i.e., sudden irradiation variations, deep voltage sag/swell, etc. The DC bus is responsible for the current injection/extraction control, maximum PV power extraction, bi-directional power flow, dc second-harmonics component elimination, and active power balance. At the point of common coupling (PCC), the AC bus is accountable for bi-directional power flow and active and reactive power control. The two-level voltage source converter (VSC) is controlled by a novel variable step-size incremental least mean square (VSS-ILMS) in zero voltage regulation (ZVR) mode. Due to its varying step size, VSC control is less prone to noise signals offers better stability, improved convergence rate, dc offset rejection, and tracking speed during dynamics, i.e., large oscillations. A battery and ultracapacitor are coupled to the DC link by buck-boost converters in the HESS. To regulate power transit between the DC bus and the grid, the HESS current control technique is designed to shift frequently from charging to discharging stage and vice versa. The novelty of the PV-HESS system lies in CGWO tuned VSS-ILMS control of VSC, which effectively and efficiently filter out the active fundamental constituents of load current and eliminate dc offset from VSC output. The HESS control maintains the DC bus voltage profile by absorbing and delivering energy (during dynamic conditions) rather than curtailing it. The presented system is simulated in a MATLAB/SIMULINK environment. The simulation results in graphical and numerical forms verify the stable and satisfactory operation of the proposed system as per IEEE519 standard.

A Modified Reference Saturated Third Harmonic Injected Equal Loading PWM for VSC-Based Renewable Energy Systems

Mitigation of excessive heat due to high frequency switching of voltage source converter (VSC) is always an industrial concern for superconducting magnetic energy storage (SMES) or high temperature superconducting (HTS) material based power conditioning system with grid-tied renewable energy interface. The pulse width modulation (PWM) technique employed for the switching of the VSC has a significant impact on joules heating, switching and conduction power losses, total harmonic distortion (THD) profile of the VSC output, and conversion efficiency in the SMES/HTS based grid-tied power system. In this paper, a modified reference saturated third harmonic injected equal loading PWM technique is proposed for a VSC-based grid-tied photovoltaic (PV) system, which offers lower THD and reduced power conversion losses. The proposed PWM technique employed in the VSC can be used in a PV-based SMES/HTS integrated power system to improve the performance of the power conditioner. To prove the superiority of the proposed PWM technique, a simulation in MATLAB/Simulink was carried out and the simulation results were validated by laboratory tests.

A Virtual Space Vector-Based Model Predictive Control for Inherent DC-Link Voltage Balancing of Three-Level T-Type Converters

The finite-control-set model predictive control (FCS-MPC) has been widely studied to control voltage source converters (VSCs) due to its fast-dynamic response and robustness. To design an FCS-MPC for a three-level VSC, it is necessary to design a multiobjective cost function, which usually consists of a term dedicated to control the neutral-point voltage (NP-V) of the VSC dc link. Nevertheless, selecting weighting factors for the multiple control objectives is time consuming. To address this issue, this article proposes a new FCS-MPC, which only uses voltage vectors and virtual voltage vectors that do not affect the NP-V, such that an inherent dc-link voltage balancing can be achieved. The proposed approach not only simplifies the controller design by using a simplified cost function, which significantly reduces the execution time of the MPC, but also eliminates the voltage and current sensors for the dc link. Moreover, the proposed MPC can improve the quality of VSC output waveforms. The simulation studies are carried out to verify the superiority of the proposed MPC over the existing ones. The effectiveness of the presented FCS-MPC is also experimentally validated on a T-type converter prototype.

Design and Implementation of Partial Feedback Linearization Controller for Unified Power Flow Controller

Electric power transmission systems can be effectively controlled with FACTS devices. Unified Power Flow Controller (UPFC) is a multi-functional FACTS device that can control different parameters of the power system under dynamic conditions. UPFC consists of two Voltage Source Converters (VSCs), which are connected back to back to a common DC link. In this paper, a non linear state-space model of UPFC is developed by including VSC output voltage functions, the effect of source impedance and energy variation in the passive elements. The non linear model of the UPFC is controlled with a non linear controller. The paper also proposes a non linear controller Partial Feedback Linearization (PFBLC) for UPFC. PFBLC modifies the control input such that UPFC is able to behave linearly under all non linear dynamics occurs in the system. The proposed system is designed, analyzed and tested in an interconnected power system network. The hardware implementation is also carried out in a five bus test bench. It was observed that the UPFC shows satisfactory performance in all operating conditions.

Enhancing the performance of cascaded three-level VSC STATCOM by ANN controller with SVPWM integegration

This article presents a cascaded three-level voltage source converter (VSC) based STATCOM employing an artificial neuron network (ANN) controller with a new simple circuit of space vector pulse width modulation (SVPWM) technique. The main aim of utilizing ANN controller and SVPWM technique is to minimize response time (RT) of STATCOM and improve its performance regard to PF amplitude, and total harmonic distortion (THD) of VSC output current during the period of lagging/leading PF loads (inductive/capacitive loads). The performance of STATCOM is tested using MATLAB/SIMULINK in IEEE 3-bus system. The simulation results clearly proved that the STATCOM with intelligent controller is more efficient compared to a conventional controller (PI controller), where ANN enables the voltage and current to be in the same phase rapidly (during 1.5 cycles) with THD less than 5%.

Stationary-Frame Modeling of VSC Based on Current-Balancing Driven Internal Voltage Motion for Current Control Timescale Dynamic Analysis

High-frequency oscillations caused by voltage source converters (VSCs) are constantly emerging in power systems with the increasing penetration of renewable energies. VSC models in current control timescale play a pivotal role in the analysis of these oscillation issues. Conventional VSC models show few physical mechanisms of VSC dynamics during system oscillations. Hence, this paper proposes a VSC model from the viewpoint of its internal voltage (namely, VSC output voltage), which is driven by current balancing between the current reference and the feedback in a stationary frame. The proposed model can be used to study VSC dynamic characteristics in an intuitionistic physical way. Based on the proposed model, it is found that VSC current reference in a stationary frame varies with internal voltage dynamic due to the essence of active and reactive current control in a current control timescale. Additionally, in a stationary frame, the dynamic relation between internal voltage and error current embodies a generalized integrating characteristic with adaptable center frequency determined by a phase-locked loop, which guarantees the zero steady state error of VSC current control. Comparisons of simulations between the proposed model and a switch-based model validates the effectiveness of the proposed model.

Modeling of Grid-Connected VSCs for Power System Small-Signal Stability Analysis in DC-Link Voltage Control Timescale

With the increasing use of voltage source converters (VSCs) in power electronics dominated power systems, oscillation phenomena in DC-link voltage control (DVC) timescale (around 10 Hz) among multiple VSCs have occurred. Several studies have tried to analyze these oscillation problems, but all associated with the single VSC situation. To consider the dynamic interactions between VSCs in DVC timescale, especially in the weak grid condition, this paper presents a small-signal model to understand VSC external characteristics based on motion equation concept also featured in synchronous generator (SG). Comparisons of time-domain simulation responses and eigenvalues show that the proposed model can hold the main behaviors of concern. The form of the model is very similar to the rotor motion equation in SG, with which power engineers have been very familiar. In addition, by establishing the relationship between the unbalanced powers and state variables of internal voltage (viz., VSC output voltage), the modeling idea introduced in this paper can be applied to other power electronic devices.

Superior decoupled control of active and reactive power for three-phase voltage source converters

This paper presents an active-reactive power control strategy for voltage source converters (VSCs) based on derivation of the direct and quadrature components of the VSC output current. The proposed method utilizes a multivariable proportional-integral controller and provides almost completely decoupled control capability of the active and reactive power with almost full disturbance rejection due to step changes in the power exchanged between the VSC and the grid. It also imposes fast transient response and zero steady-state error as compared to the conventional power control approaches. The applicability of the proposed power control strategy for providing the robust stability of the system against the uncertainties of the load parameters is also investigated. The superiority of the proposed control strategy over conventional approaches in the new condition of supplying the load is demonstrated. The theoretical aspects of the proposed multivariable-based power control strategy and the conventional approaches are reviewed and simulation results are presented in two separate sections. MATLAB/Simulink 2009a is used to simulate different scenarios of the simulation.

A Novel Voltage Source Converter Behind the StatCom

This article analyzes the structure of an 84-pulse voltage source converter, assembled by one 12-pulse voltage source converter in conjunction with an asymmetric single-phase seven-level converter with the addition of an injection transformer. Through the application of this novel structure, the total harmonic distortion in the voltage source converter's output is reduced, allowing its use in special applications or as the basement for FACTS devices. The proposition allows savings on the total amount of switches employed and a few capacitors to ward off unbalance problems. Emphasis is given on the automatic control of the reactive power exchange, providing a detailed and explicit explanation of the used blocks. A synchronizing strategy to match phase and frequency in an automatic way is also included. Simulations and experimental results are given to show the proposal appropriateness.

A Theoretical Analysis of the Harmonic Content of PWM Waveforms for Multiple-Frequency Modulators

This paper theoretically identifies the harmonic components of a carrier-based pulsewidth-modulated (PWM) voltage-source converter (VSC) output voltage when the modulating wave includes fundamental and baseband harmonic components. This occurs, for example, when a VSC is used as an active power filter. The general analytical solution provided can be applied with a minimum additional mathematical effort to any harmonic combination in the modulation signal. The analysis undertaken in this paper determines how the fundamental and low-order harmonics (in the modulation signal) interact with the carrier to produce the low (and high) order frequencies in the PWM VSC output voltage. The analysis is based on a double Fourier series expansion in two variables. This approach to harmonic identification is evaluated by a comparison with a fast Fourier transform analysis of simulated PWM waveforms.

DStatCom regulation by a fuzzy segmented PI controller

This paper presents a segmented PI controller for the 84-pulses DStatCom connected to the electric power system (EPS) and subjected to several common disturbances in voltage and load levels. The 84-pulses signal is obtained by combining one twelve-pulses voltage source converter (VSC) with an asymmetric single-phase seven-level converter, adding an injection transformer. This structure allows a reduced VSC output's total harmonic distortion. The synchronizing strategy ties the DStatCom to the EPS in an automatic way, preventing phase and frequency problems. PI controllers exhibit good performance around nominal conditions but they do not tolerate severe disturbances. Thus, a segmented PI strategy based on the error and error's variation is utilized and selected according to fuzzy rules. Simulations demonstrate the appropriateness of the proposal.

Active power flow control in a distribution system using discontinuous voltage controller

This paper proposes a hybrid discontinuous control methodology for a voltage source converter (VSC), which is used in an uninterrupted power supply (UPS) application. The UPS controls the voltage at the point of common coupling (PCC). An LC filter is connected at the output of the VSC to bypass switching harmonics. With the help of both filter inductor current and filter capacitor voltage control, the voltage across the filter capacitor is controlled. Based on the voltage error, the control is switched between current and voltage control modes. In this scheme, an extra diode state is used that makes the VSC output current discontinuous. This diode state reduces the switching losses. The UPS controls the active power it supplies to a three-phase, four-wire distribution system. This gives a full flexibility to the grid to buy power from the UPS system depending on its cost and load requirement at any given time. The scheme is validated through simulation using PSCAD.

An Advanced Cascade Controller for Series-Connected VSC for Voltage Dip Mitigation

Power electronic converters connected in series with the grid can provide protection of sensitive processes against voltage dips. In this paper, an advanced cascade controller for series-connected voltage-source converter (VSC), constituted by an outer voltage loop and an inner current loop, is presented. Positive and negative sequence components of voltage and current are controlled separately to achieve perfect compensation of unbalanced dips. Limitation of the VSC output voltage and compensation of the computational time delay are included in the current controller for both sequence components. Experimental results for the series-connected VSC with the proposed cascade controller under balanced and unbalanced voltage dips are presented.

Autonomous Load Sharing of Voltage Source Converters

An autonomous load-sharing technique for parallel connected three-phase voltage source converters is presented. An improved power-frequency droop scheme computes and sets the phase angle of the voltage source converter (VSC) directly to yield more rapid real power sharing without sacrificing frequency regulation. Reactive power sharing in the presence of a mismatch between the VSC output interface inductors is achieved by having each VSC regulate the high side voltage with a drooped voltage reference. Dynamics of the reactive power control can be tuned without interfering with steady-state reactive power sharing. Simulation results that validate the proposed technique are also provided.