Control Of Voltage Source Converter Research Articles

A VSC-based Model for Power Flow Assessment of Multi-terminal VSC-HVDC Transmission Systems

This paper puts forward a new practical voltage source converter (VSC) based AC-DC converter model suitable for conducting power flow assessment of multi-terminal VSC-based high-voltage direct current (VSC-MTDC) systems. The model uses an advanced method to handle the operational limits and control modes of VSCs into the power flow formulation. The new model is incorporated into a unified framework encompassing AC and DC power grids and is solved by using the Newton-Raphson method to enable quadratically convergent iterative solutions. The use of complementarity constraints, together with the Fischer-Burmeister function, is proposed to enable the seamless incorporation of operational control modes of VSC and automatic enforcement of any converter's operational limits that become violated during the iterative solution process. Thus, a dedicated process for checking limits is no longer required. Furthermore, all existing relationships between the VSC control laws and their operational limitsare considered directly during the solution of the power flow problem. The applicability of the new model is demonstrated with numerical examples using various multi-terminal AC-DC transmission networks, one of which is a utility-sized power system.

Control Strategy Optimization of Voltage Source Converter Connected to Various Types of AC Systems

Connecting the voltage source converters (VSCs) to various types of AC systems results in different operation characteristics and core problems associated with traditional control strategies. Therefore, it is necessary to optimize the control strategies of the VSCs according to the types of AC systems. For the VSCs connected to islanded renewable power plants, a voltage/frequency (V/f) droop control strategy is proposed to damp fluctuations of AC voltage and frequency in the island, which is vital for bipolar VSC control. In addition, a multi-branch impedance equivalent method for renewable power plants is proposed, with which large-scale renewable power plants can be modeled accurately in the frequency domain to prevent wide-band oscillation. For the VSCs connected to strong AC systems, smart AC voltage and coordinated frequency transient control strategies are proposed, which can improve AC system transient stability. For the VSCs connected to weak AC systems, the relationship between the system stability and strength is analyzed, and then the control strategy of inner-loop control parameter optimization and outer-loop power limiting (if necessary) is proposed to improve the stability of the allied system. The proposed strategies are verified by both software simulation and field commissioning.

Contingency Control Capability of an Optimized HVDC-Based VSC Transmission System

Power network operations are increasingly becoming complex due to upsurge in power transactions, following the ongoing power system deregulation and restructuring. The alteration in dynamics of power operations based on this upsurge, usually results into contingency because power facilities are made to operate in region of their limits. In this regards, high voltage direct current (HVDC) based voltage source converter (VSC) and other flexible alternating current transmission systems (FACTS) imbedded with VSC have established competence for noticeable diverse power network characteristics improvements. Therefore, this study applies optimized VSC-HVDC transmission system for mitigation of bus voltage and line thermal limit violation as a result of n-1-line outage contingency during bilateral, simultaneous and multilateral transactions. Line contingency evaluation through real power performance index (RPPI) is adopted for severe contingency identification purposes. Optimization of VSC-HVDC system is achieved through statically developed sensitivity-based analysis of VSC control parameters, while its performance for both steady state and contingency conditions are verified with IEEE 6 and 30 bus networks. An improvement in power flow, voltage deviation minimization and automatic alleviation of violated thermal and voltage limits during contingency present optimized VSC-HVDC system as a solution for network performance optimization especially during various transactions occasioned by unbundling power processes. Besides, the superiority of the proposed optimization techniques is established through performance comparison with other sensitivity methods involving reactive power (RPSM), active power (APSM) and performance index (PISM), where the proposed sensitivity method (PSM) outperformed these other methods.

IZAQ‐LMS‐based control of PV‐battery interfaced microgrid in grid‐connected and autonomous modes

This study presents a voltage source converter (VSC) control of solar photovoltaic (PV)-battery-based three-phase grid interfaced microgrid. This microgrid is controlled to operate in an autonomous mode and the grid-connected mode. In grid integrated mode, the control aims to improve the power quality of the grid current and to ensure the unity power factor operation at the grid. Therefore, in the grid-connected mode, the improved zero attracting quaternion-valued least mean square (IZAQLMS) control approach with high DC offset rejection capability is used. Owing to this, the reference grid currents become immune to harmonics and DC-offset. The control strategy takes power management into account and additionally assists several vital functions such as compensation of reactive power, power factor correction, and harmonics mitigation. This control scheme also ensures the stable system operation in autonomous mode following an unexpected incident deprived of collapse owing to the grid failure and discrepancy in the generation and load demand. In off-grid mode, the microgrid control generates a good quality of voltages and feeds the power to the load using the PV array and the battery. Simulated and test results on a laboratory prototype validates the effectiveness of the system.

Performance Enhancement of PV–DG–BS Distributed Generation System in Islanded Mode

The photovoltaic (PV)-based distributed generation systems are being promoted for electrification of remote and hilly areas. In the absence of grid, the continuity, quality, and reliability of the power supply need to be regulated through deployment of auxiliary generators, energy storage systems, etc., which is the focus area of this work. The aim of this paper is to address multiple challenges of power management, power quality, maximum energy harvesting from renewable source, battery current control, and load balancing in a diesel generator (DG)–photovoltaic (PV)–battery energy storage (BES)-based standalone system, with reduced number of converters. A voltage source converter (VSC) is used for implementing a modified system identification technique of normalized robust mixed norm (NRMN), which is found better for improving the power quality index as compared to the conventional methods. Further, the given VSC control scheme also achieves balancing of DG currents in case of unbalanced load and coordinates the power obtained from PV array, and DG, to feed the loads at all times. The PV array is connected in single-stage manner, i.e., without interlinking DC–DC converter, for making the system economic and efficient. The function of MPPT operation of PV array, thus, is achieved here through the buck-boost converter (BBC) which is a bidirectional converter connected between BES and the DC side of VSC. The BBC control scheme in this paper is unified with the battery current control to regulate the battery charging/discharging current within permissible limits. The robustness of control is successfully tested in the laboratory for various scenarios on an experimental prototype.

Control of Single-Phase Solar Photovoltaic Supply System

This article presents the modeling, design, and control of a photovoltaic supply (PVS) for single-phase grid system. In the two stage conversion process, a step-up converter (SUC) is employed in between the photovoltaic panel and dc bus of voltage source converter (VSC). For the maximum power tracking (MPT) at PVS terminals, a fuzzy logic control (FLC) based approach is developed for the switching of an SUC. A band-pass filter (BPF) is developed using modified multilayer fifth-order generalized integrator (mMLFOGI) to estimate synchronizing signals of the grid supply for the control of VSC. The mMLFOGI based BPF ensures the proper synchronization of VSC under grid disturbances such as dc offsets, frequency variations, voltage sag, swell, and harmonics. These proposed control approaches are simulated in MATLAB 2018a and prototype of PVS for single-phase grid system is developed in the laboratory. A Texas instrument micro controller board TMS320F28379D is used for real time validations of proposed control algorithms. The performance is obtained using simulation model and validated with test results under field emulated conditions.

Centralized Fuzzy Logic Based Optimization of PI Controllers for VSC Control in MTDC Network

Advancements in the field of power electronics led to global changes in the electrical energy generation, transmission, and distribution. The voltage source converter (VSC) based HVDC system is the future of the power system due to the advantages it offers in terms of renewable power generation, transmission, and integration. Currently, the two-terminal VSC–HVDC systems have been successfully commissioned. Multi-terminal VSC–HVDC system is the developing trend for higher power reliability, large scale integration, and smart operation. The performance of a VSC based multi-terminal direct current (MTDC) system greatly depends upon the tuning of the controller. Standard practice is to tune the PI controller using hit and trial method or based on the operator’s experience. However, the tuning process becomes complex when multiple grids are involved. Thus, for MTDC systems, the manual tuning of the controller does not yield the desired results. This paper presents a centralized fuzzy logic-based optimization technique for VSC control of the MTDC system to obtain the optimized parameters for the PI controllers. The optimized parameters ensure a better system performance in terms of fast settling time, better slew rate, minimum undershoot, and minimum overshoot response. The proposed technique is tested on a three-terminal MTDC network in SIMULINK / MATLAB.

Equal Loading Rate Based Master–Slave Voltage Control for VSC Based DC Distribution Systems

With the increasing penetration of renewable resources and the DC nature loads, voltage source converter (VSC) based DC distribution systems take an important position in the future smart grid architecture due to many distinguishing features. However, DC distribution systems face frequent power disturbances and multi-mode swithcing problem, how to achieve better DC voltage control and reasonable power sharing performance under various disturbances is still a challenge. After analyzing the control structure and master–slave (M–S) method, an equal loading rate based M–S method is proposed based on the power sharing capability of the VSC station. Moreover, error analysis of the proposed M–S method is conducted and a correction method for the proposed M–S method is developed. The proposed M–S method not only preserves the advantage of M–S method, but also changes the power instructions accordingly based on the operation state of the whole DC distribution system. Moreover, the proposed M–S method is simple and easy to implement. Lastly, simulations of a three-terminal VSC based DC distribution system are conducted in PSCAD/EMTDC to validate the effectiveness of the proposed M–S method.

Initialisation of a hybrid AC/DC power system for harmonic stability analysis using a power flow formulation

This study presents the initialisation of a hybrid alternating current/direct current (AC/DC) power system to determine the operation point around which the system is linearised as a starting point for an electromagnetic or harmonic stability analysis. To this end, each AC or DC power system component is modelled in the Fourier transform domain and further adjusted to a simplified representation compatible with a power flow formulation. The study also presents how voltage source converter (VSC) high-voltage DC-based systems can be analysed when the different VSC controls are applied. The result of the power flow solution is then used for the power converters’ initialisation.

A Microgrid Based on Wind-Driven DFIG, DG, and Solar PV Array for Optimal Fuel Consumption

This article presents a green energy solution to a microgrid for a location dependent on a diesel generator (DG) to meet its electricity requirement. This microgrid is powered by two renewable energy sources, namely wind energy using doubly fed induction generator (DFIG) and solar photovoltaic (PV) array. The solar PV array is directly connected to common dc bus of back-to-back voltage source converters (VSCs), which are connected in the rotor side of DFIG. Moreover, a battery energy storage is connected at the same dc bus through a bidirectional buck/boost dc–dc converter to provide a path for excess stator power of DFIG. The extraction of maximum power from both wind and solar is achieved through rotor side VSC control and bidirectional buck/boost dc–dc converter control, respectively. A modified perturb and observe algorithm is presented to extract maximum power from a solar PV array. Moreover, the control of load side VSC is designed to optimize the fuel consumption of DG. A novel generalized concept is used to compute the reference DG power output for optimal fuel consumption. The microgrid is modeled and simulated using SimPowerSystems toolbox of MATLAB for various scenarios such as varying wind speeds, varying insolation, effect of load variation on a bidirectional converter, and unbalanced nonlinear load connected at point of common coupling. The DFIG stator currents and DG currents are found balanced and sinusoidal. Finally, a prototype is developed in the laboratory to validate the design and control of it.

Harmonic-Domain SISO Equivalent Impedance Modeling and Stability Analysis of a Single-Phase Grid-Connected VSC

This article presents a harmonic-domain single-input single-output (SISO) equivalent modeling technique for the impedance modeling and stability analysis of a single-phase grid-connected voltage-source converter (VSC). The basis is a conversion technique that transforms a harmonic transfer function (HTF)-based model into a SISO equivalent model while preserving all the information of frequency couplings. The proposed SISO modeling concept is useful for understanding the meaning and consequence of SISO impedance measurement of an interconnected system with frequency couplings, which further enables a simpler impedance measurement and impedance-based analysis. Applications of this method for the VSC model reduction and stability characteristic analyses are presented. From these results, useful conclusions regarding the accuracy of three types of reduced-order VSC impedance models and the stability effects of the VSC control with and without compensation for dc voltage variation are obtained. The presented examples of applications demonstrate how the proposed SISO modeling technique facilitates a simpler and efficient impedance-based analysis. Finally, experimental results verify the validity of the proposed VSC-SISO admittance and corresponding analyses.

Multifunctional Control for PV-Integrated Battery Energy Storage System With Improved Power Quality

In this work, a multifunctional control is implemented for a solar photovoltaic (PV) integrated battery energy storage (BES) system (PVBES), which operates both in the grid-connected mode (GCM) and a standalone mode (SAM). This system addresses the major issues of integrating power quality enhancement along with the solar PV generation. Thus, a multifunctional control of voltage source converter (VSC) is implemented to provide the load compensation, harmonics mitigation, unity power factor, and an automated transition between GCM and SAM in PVBES. A single VSC is used to control the active power and to maintain the power quality in this PVBES. Furthermore, due to an automated transition in the system, the local loads have continuous power irrespective of the grid accessibility. The main challenges addressed are the operation of an automated transition between GCM and SAM with the least disturbance to the nonlinear loads. The integration of a BES enables the SAM operation and improves the reliability of the PVBES. The charging and discharging of BES are controlled by using a dc–dc bidirectional converter. The performance of the implemented controls is tested using a real-time processor (dSPACE-1202) and validated through test results for steady state and dynamic conditions. In all operating conditions, the harmonics content of the grid current is maintained less than 5%, which follows the IEEE standard 519-2014.

The Impact of VSFPWM on DQ Current Control and a Compensation Method

The variable switching frequency pulsewidth modulation (VSFPWM) is varying the carrier frequency of pulsewidth modulation (PWM) based on the prediction model to reduce the switching losses and lower the electromagnetic interference (EMI). However, it may cause some deterioration in digital dq current control of voltage source converters. This article takes the L -type grid-connected converter as the research object, and first presents the generation mechanism of the varying time delay caused by VSFPWM. In the case of proportional-integral current regulator implemented with VSFPWM, the varying time delay is accompanied by the frame rotation, which causes a varying phase lag in output voltages, degrading the system performance. Subsequently, an analytical method based on complex coefficient transfer functions is developed to provide an accurate explanation of the relation between the VSFPWM and the corresponding impacts. A compensation method is then proposed to eliminate the varying phase lag of output voltages when the VSFPWM is employed, improving the steady state and dynamic performance of dq current control. Finally, the simulation and experimental results are presented to verify the analysis and the effectiveness of the proposed compensation method.

Instantaneous Complex Power Control of a Grid-Tied VSC Supplying a Constant Power Load

In this article, a nonlinear controller for a grid-tied voltage-source converter (VSC) supplying a constant power load is introduced. The control law combines a feedback loop and a feedforward compensator. The feedback loop strategy uses a feedback linearizing method for compensating nonlinearities appearing in the VSC model. The performance in the presence of changes between two different values of constant power is improved by including a fast feedforward compensation. This compensator uses estimates of power load and its time derivative obtained from a nonlinear observer. The resultant strategy is validated via simulation and experimental tests. These experiments show that the proposed scheme performs in a very good way.

Effect of voltage source converters with electrochemical storage systems on dynamics of reduced-inertia bulk power grids

A major concern associated to the massive connection of distributed energy resources is the increasing share of power electronic interfaces resulting in the global inertia reduction of power systems. The recent literature advocated the use of voltage source converter (VSC) interfaced battery energy storage system (BESS) as a potential way to counterbalance this lack of inertia. However, the impact of VSCs on the dynamics of reduced-inertia grids is not well understood especially with respect to large transmission grids interfacing a mix of rotating machines and resources interfaced with power electronics. In this regard, we propose an extension of the IEEE 39-bus test network to quantify the impact of VSCs on reduced-inertia grids. In this respect, a reduced-inertia 39-bus system is obtained by replacing 4 synchronous generators in the original 10-synchronous machine system, with 4 wind power plants modeled as aggregated type-3 wind turbines. Then, a large-scale BESS is integrated into the reduced-inertia network via a three-level neutral-point clamped (NPC) converter, thereby to be used for studying the impact of VSC on the dynamics of the inertia-reduced power system, as well as for comparing different VSC controls. The proposed models are implemented on a real-time simulator to conduct post-contingency analysis, respectively, for the original power system and the reduced-inertia one, with and without the BESS-VSC.

Transient Stability Enhancement Control Strategy Based on Improved PLL for Grid Connected VSC during Severe Grid Fault

In this article, the transient synchronization process of the grid-connected voltage source converters (VSC) is studied detailly. Firstly, the phase-locked loop (PLL)-synchronized VSC is modeled according to the rotor motion equation of synchronous generator (SG). Furthermore, the VSC's damping ratio is derived, and the effects of the VSC's control parameters and the operation status on the equivalent power angle (EPA) is investigated. Moreover, the influence of the PLL's parameters and the voltage dips degree on the VSC's transient frequency behavior are analyzed. Analysis result reveals that the VSC's damping ratio decreases once the grid voltage drops, so that the EPA and frequency of the VSC may appear large overshoot and oscillation, which may trigger the frequency protection and deteriorate the VSC's transient stability. In order to ensure that the VSC can smoothly operate to new equilibrium points, an improved PLL is proposed, which can adaptively adjust the VSC's damping ratio for different voltage sags. The proposed method can not only reduce the overshoot of the EPA but also restrain the VSC's frequency dips degree. Finally, the simulations and experimental tests validate the effectiveness of the theoretical analysis.

Advanced Impedance-Based Control Design for Decoupling Multi-Vendor Converter HVDC Grids

A progressive interconnection of existing HVDC links to form grids and the development of completely new HVDC grids from different vendors are expected shortly. One of the current challenges of such endeavour is unintended interactions due to independently designed controllers. This article proposes a design methodology for decentralized controllers to mitigate such interactions in multi-vendor voltage source converter (VSC)-HVDC grids. The approach presented relies on the unique stand-alone input-output impedance transfer function of each VSC, and the global impedance transfer function as seen from each terminal after interconnection with other VSCs. Subsequently, network-level controllers are designed by attempting to match the global responses at selected locations based on a novel interaction analysis, to the unique transfer function model of the vendors at the corresponding location. This approach reduces the entire problem to an impedance matching problem. We demonstrate the efficacy and flexibility of both the methodology and the designed controllers in mitigating interactions due to the independent design of VSC controllers through nonlinear simulations on a four-terminal droop controlled HVDC grid.

The Thyristor-Augmented Modular Bridge Converter: A Highly Efficient VSC-HVDC Converter With Reduced Energy Storage Requirement

Recently, the idea of combining both thyristors and IGBTs in a voltage-source converter (VSC) structure has gained significant research interests for HVDC applications. The main objectives of deriving these topologies are to achieve forced-commutation of thyristors, increased efficiency, reduced energy-storage requirement and dc-fault tolerance besides retaining the VSC control flexibility. A novel topology named `thyristor-augmented modular bridge converter' (TAMBC) is proposed in this paper. Each TAMBC phase contains four chain-links comprising of series-connected stacks. Each stack contains a parallel connection of an IGBT-based submodule-branch and a thyrsitor-branch. A control technique is developed that allows the main current to flow primarily through the thyrsitor-branch to achieve higher efficiency. In each phase of TAMBC, two chain-links generate the required ac voltage in the first half-cycle and are bypassed through thyrsitor-branch during the next half-cycle. Consequently, it enables current distribution among the chain-links resulting in reduced capacitors size and enhanced efficiency. The ability of TAMBC-HVDC systems to function over a wide operating range, to ride-through ac faults and to provide independent active and reactive power control are verified using PSCAD. Capacitors design, loss analysis and comparative evaluations are carried out to highlight the key features of the TAMBC over the existing ones.

The Post-Fault Current Model of Voltage Source Converter and Its Application in Fault Diagnosis

The open-switch fault of voltage source converters (VSC) may change the switching state and distort the current, which is the basis for model-based fault diagnosis methods. Based on the analysis of the switching state, this letter presents a post-fault current model to estimate the current of each phase after one open-switch fault occurs. Combined with the current model-based method, the proposed post-fault model can be used not only to check the accuracy of the diagnosis result but also to detect multiple open-switch faults. The current, voltage, and driving signals used in this model are all from the VSC control system, therefore, this model can be easily implemented without additional sensors or hardware. Finally, the effectiveness of the proposed model is verified by the experimental results.

Fully decoupled fixed-time cascade control of three-phase voltage source converters with LC output filter

This paper presents a fully decoupled fixed-time cascade control system of three-phase voltage source converters (VSCs) with LC output filter. The proposed controller has a cascade structure and it is designed in the synchronous reference frame d-q. The outer loop is for voltage control and the inner loop is for current control. A fixed-time controller is developed for voltage regulation and reference tracking before the desired pre-fixed time despite the unknown disturbances. The current controller uses PI controllers for current regulation and feedforward terms to decouple the direct and quadrature of voltage and current control loops. Unlike the conventional PI-based synchronous reference frame VSC control, the proposed approach provides better control performance with smaller and smoother control signal and enables the d-q components of voltage and current to be fully decoupled. A comparative simulation study with the conventional control system is provided to highlight the effectiveness of the proposed approach.