Performance Of Voltage Source Converter Research Articles

An Observer-Based Multivariable Controller for Improved Performance of Voltage Source Converters With LCL Filter Connected to Unbalanced Grid

An Observer-Based Multivariable Controller for Improved Performance of Voltage Source Converters With LCL Filter Connected to Unbalanced Grid

Analysis of an improved three-phase voltage source converter with high-performance operation under unbalanced conditions

The power system is rapidly transforming into a new large-scale distributed power generation model using renewable energy. Grid-connected converters, particularly voltage control compensators like STATCOM, will be crucial to the functioning of this new situation. Voltage source converter (VSC) is one of the most commonly used topologies in grid connection converters. As a distributed power-generating device, VSCs can be utilized as a means of connecting sustainable energy sources to the power grid. The three-phase voltage source currents or voltage ripples may appear in the DC chain. This study examines the enhanced three-phase VSC, pinpoints the factors that contribute to the great performance of VSCs, and looks at future trends in three-phase VSCs. VSC is a power electronic device capable of converting DC voltages to three-phase AC voltages and is commonly used in areas such as grid connection, reactive power compensation and harmonic suppression. In practical applications, various types of grid or load disturbances pose hidden dangers to the normal operation of the VSC. Due to the presence of unbalanced grids and loads, significant double frequency.

A coordinated control scheme for VSC-MTDC system with multiple renewable energy sites based on voltage margin and adaptive drooping control

This paper designs a novel coordinated control scheme to improve the control performance of voltage source converters (VSC) based multi-terminal high voltage direct current (MTDC) transmission network by merging the benefits of adaptive drooping and voltage margin controllers. In this proposed scheme, the droop coefficient is used to manage the DC voltage and maintain power sharing by shifting the control modes of the rectifiers and inverters in the presence of considerable power disturbances. The droop coefficient is planned based on the stability limit of DC voltage. The performance of the designed control scheme is compared to that of existing coordinated control and voltage margin droop control approaches. A ±400 kV five-terminal VSC-MTDC network is developed in PSCAD to verify the planned control method. The simulation results indicate that the proposed control scheme can achieve good performance for controlling the DC voltage and maintaining active power in the MTDC system.

A Multivariable Phase-Locked Loop-Integrated Controller for Enhanced Performance of Voltage Source Converters Under Weak Grid Conditions

This article proposes a new control strategy in stationary frame to improve the operational robustness of voltage source converters (VSCs) under weak grid conditions. First, dynamics of the phase-locked loop (PLL) are incorporated into the model of a grid-connected VSC in stationary reference frame. It is rigorously shown that the three-phase PLL introduces cross-coupling terms between the two control channels. Second, based on the augmented model, a multivariable controller is proposed which combines the current control feedback gains with PLL feedback gains. Third, the gains of the controller are designed using an optimal control theory approach. It is shown through theoretical analysis, computer simulations, and a laboratory experimental setup that the proposed controller improves the stability margins of the VSC during weak grid conditions. More specifically, under a given identical operating conditions, while the conventional controller’s responses become increasingly oscillatory for short-circuit ratio (SCR) below 1.95 and eventually becomes unstable at SCR=1.26, the proposed controller exhibits smooth and stable responses for SCR as low as 1. The proposed controller also produces cleaner sinusoidal currents during unbalanced grid voltages.

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.

Generalized Normal Distribution Algorithm-Based Control of 3-Phase 4-Wire Grid-Tied PV-Hybrid Energy Storage System

The presented work employs the multiple random feature kernel mean p-power algorithm (MRFKMP) for the voltage source converter (VSC) control of a three-phase four-wire grid-tied dual-stage photovoltaic-hybrid energy storage system (HESS) to achieve multiple objectives during various induced dynamic conditions. The proposed control enables the VSC to accomplish manifold goals, i.e., reactive power compensation, power quality enhancement, load, power balancing at common coupling point and grid voltage balancing during unity power factor mode of operation. The proposed system is scrutinized under steady-state and numerous dynamic states such as irradiation variation, specified power mode, abnormal grid voltage, load, and grid voltage unbalancing. The seamless control facilitates the swift resynchronization of the grid as well as maintaining stability during islanding and re-synchronization operations while satisfying the necessary load requirements. The associated HESS consisting of battery and ultra-capacitor is competent enough in managing the interruptions occurring on the grid, load and photovoltaic side. The DC bus voltage is controlled by the PI controller, which is tuned by the generalized normal distribution algorithm and kept at the desired level during diverse operating conditions. The optimized DC bus generates an accurate loss component of current and further enhances the VSC performance. The proposed system is investigated by simulation and found acceptable as per IEEE 519 standards.

Power-Synchronization Control for Ultra-Weak AC Networks: Comprehensive Stability and Dynamic Performance Assessment

Power-synchronization control (PSC) is a promising control strategy to improve the stability and performance of voltage-source converters (VSCs) in ultra-weak AC grids. However, evaluation of PSC to date has investigated performance only at single controller operating points, rather than holistically varying multiple controller gains. This paper develops a new methodology, based on small-signal eigenvalue analysis, to comprehensively analyze PSC-VSC stability. The maximum active power transfer of PSC is established across a broad range of controller tunings and the two-way and three-way couplings between the power-synchronization control, AC voltage control and high-pass current filter gains are quantified. A new stable tuning region is introduced, which represents the controller parameter space for stable operation. It is shown that PSC can achieve rated power transfer into an AC grid (short circuit ratio(SCR)=1) at multiple controller operating points, but dynamic performance varies significantly within this region. The robustness of this operating region to SCR changes is also investigated. The stability boundary and dynamic performance are validated using control hardware-in-the-loop experiments with a real-time digital simulator. Practical recommendations arising from this work are a set of controller gains that provide stability and good dynamic performance at high power transfer for ultra-weak grid-connected VSCs employing PSC.

Inter Harmonic THD Amplification of Voltage Source Converter: Concept and Case Study

In this letter, with the consideration of both PLL and the inter-harmonic voltage perturbation, a technical concept of “inter-harmonic total harmonic distortion (THD) amplification” has been presented, by which a hidden output feature of voltage source converter (VSC) has been unveiled. The phenomenon has been explained in theory that the THD of current is greater than the THD of voltage under the inter-harmonic perturbation during the particular frequency range. The detailed frequency range and boundary characteristics have been given for reference. Both the theoretical analysis and experimental results based on RT-LAB validate the proposed inter-harmonic THD amplification phenomenon. This letter could deepen the understanding of the fundamental THD theory, and provide a new way to evaluate the grid integration performance of VSCs.

Modelling, synchronisation, and implementation of the virtual synchronous generator: a study of its reactive power handling

This paper presents an application for the control of a voltage source converter (VSC) through the synchronverter approach. The objective is to show that such an approach is pertinent to handle this type of converters, especially important nowadays that such devices are practically permeating small as well as large electrical networks. The paper helps to demonstrate that the proposed control strategy is capable of satisfying the requirement that a modern VSC must contribute to the Volts/Var regulation problem in the smart grid, which represents a significant challenge. The application is carried out in the context of the hardware-in-the-loop using Matlab/Simulink. The results state the VSC performance under different operating conditions and become quite relevant.

A Robust Power Regulation Controller to Enhance Dynamic Performance of Voltage Source Converters

This paper presents a novel power regulation controller for grid-connected voltage source converter (VSC). The proposed controller is usually applied to achieve smooth active and reactive power dynamics against the variation of parameters. The dynamic performance of grid-connected VSC is sensitive to parameter variation, threatening the safe operation especially when the relative stability is low. Moreover, there is no possibility to modify the dynamic response without the loss of power regulation precision or output voltage and frequency accuracy. In this paper, an improvement in the dynamic response is achieved by introducing feedforward branches into the VSC power regulation controller. Owing to the model reduction mechanism, the VSC provides smooth dynamic response and robustness against the variation of parameters. Simulation and experiment results are presented to prove the effectiveness of this approach, which shows excellent performance as opposed to the conventional controller paradigm.

Transient switching performance of VSC and the DC fault partitions

Two-level voltage source converters (VSC) have been widely used in energy conversion and transmission but they are vulnerable to the high discharge current from the DC-linked capacitor when the DC faults occurs. Previous studies have divided the DC fault process into three stages. The capacitor discharge stage (first stage) and the fault steady-state (final stage) are relatively simple and have been analyzed in detail. In contrast, the transition between these stages, i.e., the uncontrolled rectification stage, exhibits an extremely complex transient current feeding process when a DC fault occurs due to the unilateral conductivity of the internal power electronic devices and charge/discharge characteristics of the internal energy storage components. However, the switching characteristics of the converter in this stage have not been investigated to date. This study focuses on the analysis of the transient current feeding process of the uncontrolled rectification stage. Two time-boundaries of the commutation are defined based on the calculation of the limit of the current feeding rate and the effects of different fault distances on the transient switching performance of the converter are investigated. It is demonstrated that it is relatively easy to divide the DC line into three partitions by using the converter’s switching information instead of measuring the line’s electrical quantities; this provides a basis for developing DC protection. In the simulation analysis of the proposed method, a range of fault distances and initial feeding angles are considered.

A Virtual Impedance Control Strategy for Improving the Stability and Dynamic Performance of VSC–HVDC Operation in Bidirectional Power Flow Mode

It is a common practice that one converter controls DC voltage and the other controls power in two-terminal voltage source converter (VSC)–based high voltage DC (HVDC) systems for AC gird interconnection. The maximum transmission power from a DC-voltage-controlled converter to a power-controlled converter is less than that of the opposite transmission direction. In order to increase the transmission power from a DC-voltage-controlled converter to a power-controlled converter, an improved virtual impedance control strategy is proposed in this paper. Based on the proposed control strategy, the DC impedance model of the VSC–HVDC system is built, including the output impedance of two converters and DC cable impedance. The stability of the system with an improved virtual impedance control is analyzed in Nyquist stability criterion. The proposed control strategy can improve the transmission capacity of the system by changing the DC output impedance of the DC voltage-controlled converter. The effectiveness of the proposed control strategy is verified by simulation. The simulation results show that the proposed control strategy has better dynamic performance than traditional control strategies.

Multi-Objective Optimization of VSC Stations in Multi-Terminal VSC-HVdc Grids, Based on PSO

The objective of this paper is to enhance the control performance of voltage source converter (VSC) stations within multi-terminal high voltage direct current (M-HVdc) grids under dynamic conditions. This paper presents the application of particle swarm optimization (PSO) to tune the control parameters of VSC stations. VSCs are non-linear components of the M-HVdc grids. Conventional techniques use approximated linear models to tune the control parameters which do not produce optimal results. Thus, PSO is employed to optimally tune the control parameters of VSC stations. The algorithm of the proposed objective function applies simultaneous optimization of inner and outer control layers. Dynamic simulations of a four-terminal HVdc system are developed in PSCAD/EMTDC to extol the merits of the presented optimization technique. Results show the steady state and dynamic control performances, both for classically and PSO-based tuned parameters, during load demand change from ac grids, wind power change, and eventual permanent VSC disconnection.

Modeling of VSC Connected to Weak Grid for Stability Analysis of DC-Link Voltage Control

In this paper, a small-signal model of voltage source converter (VSC) is developed to investigate the stability of dc-link voltage control. This model contributes to representing the dc-link voltage dynamics characteristics of VSC integrated to weak grid. Effects of grid strength, operating point, and control loops’ interactions on the performance of VSC are taken into consideration. Based on the proposed small-signal model, eigenvalue analysis is employed to study the stability of dc-link voltage control with varying operating conditions. Analysis results show that control loops’ interactions introduce a partial positive feedback to the dc-link voltage control in weak grid. Furthermore, the effect of control loops’ interactions on dc-link voltage control stability reaches largest when the bandwidth of the phase-locked loop is close to that of the dc-link voltage control. Time-domain simulations and experiments were conducted to validate the analysis.

Hybrid Multilevel Converter With Cascaded H-bridge Cells for HVDC Applications: Operating Principle and Scalability

Hybrid multilevel converters are contemplated in an attempt to optimize the performance of voltage source converters in terms of magnitude of semiconductor losses and converter footprint, and to achieve additional features such as dc short circuit proof, which is essential for a high integrity multiterminal HVDC grid. Therefore, this paper considers an emerging hybrid cascaded converter that offers the dc side short circuit proof feature at reduced loss and footprint compared to the existing multilevel and other hybrid converters. Its operating principle, modulation, and capacitor voltage balancing strategies are described in detail. Furthermore, hybrid converter scalability to high voltage applications is investigated. The validity of the modulation and capacitor voltage strategy presented are confirmed using simulation and experimentation. The hybrid cascaded converter is extendable to a large number of cells, making it applicable to high voltage applications, and operation is independent of modulation index and power factor. On these ground, the converter is expected to be applicable for both real and reactive power applications.

Decentralized Cooperative Control Strategy of Microsources for Stabilizing Autonomous VSC-Based Microgrids

In designing procedure of a power sharing controller for a voltage source converter (VSC)-based microgrid with no communication link, three issues should be considered. Firstly, in VSC-based microgrids, which use droop controller method, the desired frequency of VSCs is altering regarding the output active power. Consequently, the conventional load frequency control techniques are inappropriate since their operation is based on a fixed pre-specified desired frequency. Secondly, to prevent circulating current and thermally overstressing, all DGs should participate in active power supply. In addition, since there is no communication link, the steady state value of each micro-source active power is unknown. Therefore, the conventional fixed active power control method for DGs is not appropriate. Thirdly, when the microgrid loads are increased, the output power of VSCs is increased rapidly; however, the output power of each VSC's primary source could not change in the same rate to respond. It causes the DC voltage of VSCs to decrease, which could affect the appropriate performance of VSCs. In this paper, a novel control strategy for VSCs and an energy storage system in a VSC-based microgrid without communication link accompanied with a novel hybrid model of VSC-based DGs, which considers primary source effect, is proposed.

Exponentially convergent estimator to improve performance of voltage source converters

In this study, the authors introduce an exponentially convergent estimator of load power to improve the performance of voltage source converter (VSC) applications. This estimator, based on a non-linear reduced-order observer, is used to estimate the load power and its time derivative. The estimated signals are feedforwarded to build a non-linear controller in order to improve the whole system performance. The main advantage of the proposed estimator is that it presents linear error dynamics which allows to design the observer by using linear techniques. In this way, the observer transient performance (i.e. convergence rate) is clearly established in the whole VSC operation range. The feedback controller is based on input-output feedback linearisation considering the energy as output so that the DC-voltage internal dynamics is avoided and linear control strategies can be also used in the controller design. Several tests - including performance in presence of switching frequency, parameter uncertainties and disturbances in the AC input voltage - validate the proposed technique.

Modeling synchronous voltage source converters in transmission system planning studies

A voltage source converter (VSC) can be beneficial to power utilities in many ways. To evaluate the VSC performance in potential applications, the device has to be represented appropriately in planning studies. This paper addresses VSC modeling for EMTP, power flow and transient stability studies. First, the VSC operating principles are overviewed and the device model for EMTP studies is presented. The ratings of VSC components are discussed, and the device operating characteristics are derived based on these ratings. A power flow model is presented and various control modes are proposed. A detailed stability model is developed, and its step-by-step initialization procedure is described. A simplified stability model is also derived under stated assumptions. Finally, validation studies are performed to demonstrate performance of developed stability models and to compare it with EMTP simulations.