Four-leg Voltage Source Converter Research Articles

Sliding Mode Control of a Four-Leg Dynamic Voltage Restorer in a Natural Reference Frame

This paper focuses on designing a sliding mode control (SMC) for a dynamic voltage restorer (DVR) using a four-leg voltage source converter and its output filters in the natural reference frame. The coupling of filter-capacitor voltages through the control inputs of the DVR system leads to cross-coupling in the dynamics of conventional sliding variables with respect to four control input variables, making the design of SMC using Lyapunov stability criteria difficult. To overcome this, a new sliding variable for each converter leg is proposed, which is a function of the respective filter-capacitor voltage and a fictitious voltage that depends on the converter neutral-point voltage (NPV). This allows control of both the compensator voltages and the converter NPV using the proposed scheme. This paper also presents the calculation of the optimum sliding coefficient to maximize the sliding existence region for a four-leg DVR. The performance of the proposed control scheme is experimentally validated under various operating conditions using a laboratory prototype of a four-leg DVR.

Experimental Validation of an Advanced Load-Shift System Operating as Shunt Active Power Filter

This paper presents a load-shift system with advanced functionalities for the power grid. When compared with the conventional approach, an advanced load-shift system allows the compensation of power quality problems on the power grid side, namely problems related to current harmonics, current imbalances, and low power factor. The advanced load-shift system is composed by a bidirectional ac-dc converter to interface the power grid and by a bidirectional dc-dc converter to interface an energy storage system. Since the main innovation is related with the power grid interface, the focus of this work is on the analysis of the ac-dc converter, which is based on a three-phase four-leg voltage source converter. A theoretical study and the details concerning the control algorithm are presented and discussed along the paper. A laboratory prototype of the advanced load-shift system was developed, and the main details of implementation are described in the paper. Experimental results obtained with the developed prototype prove that the advanced load-shift system contributes for the technology progress in this area, validating a new operation concept concerning the power quality on the power grid side. For all the operation modes, the system operates with high power factor and the currents on the power grid side are balanced with a low total harmonic distortion and in phase with the respective power grid voltages.

AC Current Ripple Harmonic Pollution in Three-Phase Four-Leg Active Front-End AC/DC Converter for On-Board EV Chargers

Three-phase four-leg voltage-source converters have been considered for some recent projects in smart grids and in the automotive industry, projects such as on-board electric vehicles (EVs) chargers, thanks to their built-in ability to handle unbalanced AC currents through the 4th wire (neutral). Although conventional carrier-based modulations (CBMs) and space vector modulations (SVMs) have been commonly applied and extensively studied for three-phase four-leg voltage-source converters, very little has been reported concerning their pollution impact on AC grid in terms of switching ripple currents. This paper introduces a thorough analytical derivation of peak-to-peak and RMS values of the AC current ripple under balanced and unbalanced working conditions, in the case of three-phase four-leg converters with uncoupled AC-link inductors. The proposed mathematical approach covers both phase and neutral currents. All analytical findings have been applied to two industry recognized CBM methods, namely sinusoidal pulse-width modulation (PWM) and centered PWM (equivalent to SVM). The derived equations are effective, simple, and ready-to-use for accurate AC current ripple calculations. At the same time, the proposed equations and diagrams can be successfully adopted to design the conversion system basing on the grid codes in terms of current ripple (or total harmonic distortion (THD)/total demand distortion (TDD)) restrictions, enabling the sizing of AC-link inductors and the determination of the proper switching frequency for the given operating conditions. The analytical developments have been thoroughly verified by numerical simulations in MATLAB/Simulink and by extensive experimental tests.

Power Quality Improvement for Grid-connected PV System Based on Distribution Static Compensator with Fuzzy Logic Controller and UVT/ADALINE-based Least Mean Square Controller

This paper presents a novel method of power quality enrichment in a grid-connected photovoltaic (PV) system using a distribution static compensator (DSTATCOM). The paper consists of two-step control processes. In the primary step, a fuzzy logic controller (FLC) is employed in the DC-DC converter to extract the peak power point from the PV panel, where the FLC produces a switching signal for the DC-DC converter. In the secondary step, a unit vector template (UVT)/adaptive linear neuron (ADALINE)-based least mean square (LMS) controller is adopted in the DC-AC converter, i.e., voltage source converter (VSC). The input to this VSC is the boosted DC voltage, which originates from the PV panel as a result of DC-DC conversion. The VSC shunted with the power grid is known as a DSTATCOM, which can maintain the power quality in the distribution system. The UVT controller generates reference source currents from the grid voltages and DC-link voltages. The ADALINE-based LMS controller calculates the online weight according to the previous weights by the sensed load current. The UVT/ADALINE-based LMS controller of a DSTAT-COM performs several tasks such as maintaining the sinusoidal source current, achieving a unity power factor, and performing reactive power compensation. The reference current extracted from the UVT/ADALINE-based LMS controller is fed to the hysteresis current controller to obtain the desired switching signal for the VSC. A 100 kW solar PV system integrated into a three-phase four-wire distribution system through a four-leg VSC is designed in MATLAB/Simulink. The performances of the FLC and UVT/ADALINE-based LMS controllers are demonstrated under various irradiances as well as constant temperature and nonlinear loading conditions.

VSSMLMS‐based control of multifunctional PV‐DSTATCOM system in the distribution network

The variable step size modified least mean square (VSSMLMS) learning algorithm based multifunctional double-stage grid interactive photovoltaic (PV) system is implemented in this paper which feeds power to the grid and the load, ensures unity power factor operation and compensates reactive power. Along with this, it mitigates high grid neutral current during unbalanced loading condition. This algorithm uses variable step sizes to have a smooth dynamic and steady state performances. The boost converter (booster) is used in the first stage to extract peak power from a PV array using perturb and observe peak power point tracking technique. Next to this, a four leg voltage source converter (VSC) is placed to interlink the solar PV system to the grid and the loads. VSSMLMS-based control technique is used to filter the fundamental component of load current and hence gate pulses are generated for the four-leg VSC. This system is simulated in SIMULINK/MATLAB software under several working conditions and the obtained simulation results are validated on a developed laboratory prototype. The performance of the VSSMLMS-based control technique is compared with that of the traditional LMS-based technique. All results validate the IEEE-519 standard.

Voltage Control of Four-Leg VSC for Power System Applications With Nonlinear and Unbalanced Loads

Voltage source converters are presented as the key devices for the future massive integration of distributed renewable energy resources in the network. This article presents a novel approach to control a three-phase four-leg voltage source converter for grid-forming operation. The objective of the controller is to generate a balanced three-phase voltage with a given amplitude and frequency at the point of common coupling. The proposed control algorithm works over the stationary axes. It is based on full state feedback law in combination with a resonant control loop tuned at the fundamental frequency in order to guaranty zero steady-state error on the voltage. The main advantage of the controller with respect to the classical cascade controllers is that this strategy is not modified depending on the type of load connected at the point of common coupling. Moreover, a systematic methodology to compute the controller gains is presented by solving an linear quadratic regulator problem that considers an extended model. This method guarantees small signal stability and provides active damping to the system. A laboratory testbed with different type of loads is used to validate and compare the proposed algorithm with the classical one. The experimental results demonstrate the effectiveness of the proposal by achieving low levels of harmonic distortion and imbalances in steady-state as well as a fast transient response.

Multi-Objective Single-Stage SPV System Integrated to 3P4W Distribution Network Using DMSI-Based Control Technique

This paper proposes a multiobjective solar photovoltaic (SPV) system integrated to a three-phase four wire (3P4W) distribution network by using a digital multistage interpolation (DMSI)-based technique. In addition to the power exchange from SPV array to the 3P4W grid, a four-leg voltage source converter (VSC) is used as a power conditioning unit; therefore, it is named a multiobjective SPV system. For good utilization of SPV array, it is necessary to harvest peak power from SPV string. To fulfill this objective, a perturb and observe-based maximum power point tracking control is used. However, to control the grid integrated VSC, a DMSI-based control technique is presented. The technique is utilized for mitigation of grid neutral current, load harmonics currents, power factor improvement, and grid currents balancing. The incorporation of SPV feed forward loop in control loop minimizes the dependency on proportional integrator controller, and also good dynamic behavior is recorded because of load and climate changes. The operation and control of system topology are validated experimentally under various scenarios.

A Decoupled Adaptive Noise Detection Based Control Approach for a Grid Supportive SPV System

A grid supportive two-stage three-phase four-wire solar photovoltaic (SPV) system is presented in this paper, wherein a boost converter is used as a first stage to serve the function of maximum power point tracking (MPPT) and a four-leg voltage source converter (VSC) is used to feed the extracted SPV energy, along with supporting distribution system for improvement in the power quality. Unlike conventional SPV inverters, the proposed solar energy conversion system provides extra functionalities such as balancing of grid currents, reactive power compensation, mitigation of harmonics, and neutral current elimination in grid side neutral conductor. An extra feed-forward term is added in the control loop to provide fast dynamic responses. The PV array voltage is continuously adjusted using a boost converter to achieve MPPT. A control approach employing decoupled adaptive noise detection (DAND) algorithm is used for controlling the four-leg grid-tied VSC. The DAND algorithm is a simple approach using two multipliers, one integrator, and one summer per phase for detection of useful component of load current. The proposed control algorithm gives features such as simple structure, fast convergence, frequency adaptive detection, and good steady-state performance. The grid currents are found adhering to an IEEE-519 standard, even under nonlinear and unbalanced loads at common point of interconnection.

Grid-forming VSC control in four-wire systems with unbalanced nonlinear loads

A grid-forming voltage source converter (VSC) is responsible to hold voltage and frequency in autonomous operation of isolated systems. In the presence of unbalanced loads, a fourth leg is added to provide current path for neutral currents. In this paper, a novel control scheme for a four-leg VSC feeding unbalanced linear and nonlinear loads is proposed. The control is based on two control blocks. A main control commands the switching sequence to the three-phase VSC ensuring balanced three-phase voltage at the output; and an independent control to the fourth leg drives neutral currents that might appear. The proposed control is noninvasive in the sense that both control blocks are independently implemented, avoiding the use of complex modulation techniques such as 3D-SVPWM. Moreover, the main control is deployed in dqo reference frame, which guarantees zero steady-state error, fast transient response during system disturbances and mitigation of harmonics when nonlinear loads are present. Simulations and experimental results are presented to verify the performance of the proposed control strategy.

Better Control for a Solar Energy System: Using Improved Enhanced Phase-Locked Loop-Based Control Under Variable Solar Intensity

An improved enhanced phase-locked loop (EPL)-based control algorithm for a double-stage solar photovoltaic (PV) grid-interfaced power generating system is presented in this article. It also mitigates power quality (PQ) problems in a three-phase, four-wire (3P4W) distribution system under variable solar intensity. The proposed solar PV grid-interfaced system consists of a solar PV array, a boost converter, a four-leg voltage source converter (VSC), and connected linear/nonlinear loads. The suggested improved EPL based synchronization method is implemented to meet grid requirements such as PQ improvement, distortion-free signals under variable solar intensity, and load unbalancing.

Solar Energy Used for Grid Connection: A Detailed Assessment Including Frequency Response and Algorithm Comparisons for an Energy Conversion System

To help reduce losses in a distributed generation system, this article proposes a suitable method involving a three-phase multifunctional grid-connected solar energy conversion system (SECS). A two-stage circuit topology is used in this work; the first stage is a boost converter, which serves the purpose of maximum power point tracking (MPPT), and the second stage is a four-leg voltage source converter (VSC), which serves the purpose of feeding extracted energy along with improving power quality in the distribution system. The SECS not only feeds solar photovoltaic (SPV) energy into the grid but also acts to balance grid currents, compensate for reactive power, eliminate harmonics, and mitigate neutral current. A feedforward term for the SPV contribution is used to improve the dynamic response for climatic changes. The PV array voltage is continuously adjusted with the help of a boost converter to achieve MPPT, whereas the dc-link voltage of the VSC is kept constant using a proportional-integral (PI) controller. A second-order generalized integrator quadrature (SOGI-Q)-based algorithm is proposed for the control of a four-leg VSC.

Predictive Current Control With Instantaneous Reactive Power Minimization for a Four-Leg Indirect Matrix Converter

This paper presents the experimental validation of a predictive current control strategy with minimization of the instantaneous reactive input power for a Four-Leg Indirect Matrix Converter (4Leg-IMC). The topology includes an input matrix converter stage, which provides the dc voltage for a four-leg voltage source converter (VSC) output stage. The VSC's fourth leg provides a path for the zero sequence load current. The control technique is based on a finite control set model predictive control (FCS-MPC) strategy, whereby the switching states for the input and output converters are selected by evaluating a predictive cost function. This results in a simpler approach than that seen in other well-known modulation methods, such as three-dimensional space vector modulation (3D-SVM). Positive dc voltage, (a requirement for the safe operation of the IMC) and minimization of the instantaneous input reactive power are obtained, while maintaining good tracking of the load reference currents. Furthermore, soft switching is achieved by synchronizing the state changes in the input stage with the application of zero voltage space vectors in the inverter stage. The control strategy is experimentally verified using a laboratory prototype.

Control and Implementation of a Standalone Solar Photovoltaic Hybrid System

A control algorithm for a standalone solar photovoltaic (PV)-diesel-battery hybrid system is implemented in this paper. The proposed system deals with the intermittent nature of the energy generated by the PV array and it also provides power quality improvement. The PV array is integrated through a dc–dc boost converter and is controlled using a maximum power point tracking algorithm to obtain the maximum power under varying operating conditions. The battery energy storage system (BESS) is integrated into the diesel engine generator set for the coordinated load management and power flow within the system. The admittance-based control algorithm is used for load balancing, harmonics elimination, and reactive power compensation under three-phase four-wire linear and nonlinear loads. A four-leg voltage-source converter with BESS also provides neutral current compensation. The performance of the proposed standalone hybrid system is studied under different loading conditions experimentally on a developed prototype of the system.

Performance Evaluation of Fixed-speed and Variable-speed Stand-alone Wind Energy Conversion Systems

This article deals with the performance evaluation of fixed-speed and variable-speed stand-alone wind energy conversion systems using an isolated asynchronous generator. A voltage and frequency controller for an isolated asynchronous generator based variable-speed stand-alone wind energy conversion system is investigated using two back-to-back connected voltage source converters and a battery energy storage system. A three-leg insulated-gate bipolar transistor based voltage source converter is used at the machine side for variable-voltage–variable-frequency energy conversion, and a four-leg insulated-gate bipolar transistor based voltage source converter is used between the load terminals and intermediate DC link for feeding regulated voltage and frequency supply. A voltage and frequency controller for an isolated asynchronous generator based fixed-speed stand-alone wind energy conversion system is also designed and modeled using a four-leg voltage source converter and battery energy storage system. A detailed performance evaluation is made between fixed-speed and variable-speed stand-alone wind energy conversion systems using wind data obtained for Jakhau (Gujarat) situated in India.

Unity power factor operation and neutral current compensation of diesel generator set feeding three‐phase four‐wire loads

This study deals with power quality improvement in a diesel generator (DG) set-based standalone supply system feeding three-phase four-wire(3P4W) loads. The proposed system consists of a diesel engine, three-phase wound field synchronous generator, a four-leg voltage source converter (VSC) and3P4W loads. The four-leg VSC with a capacitor connected on its DC link is used for neutral current compensation and unity power factor operation of the DG set. In addition, the VSC provides harmonics elimination and the load balancing. The voltage of the DG system is controlled by excitation control of the synchronous generator and the frequency is controlled by speed regulating system of the diesel engine. The speed of the engine is regulated by controlling the fuel injection of the diesel engine. An adaptive theory-based notch filter algorithm is used to extract fundamental component of the load currents for estimation of reference source currents.

Neural Network-Based Control Algorithm for DSTATCOM Under Nonideal Source Voltage and Varying Load Conditions

Distribution static compensator (DSTATCOM) is the optimal choice of power quality (PQ) compensator in a three-phase four-wire distribution system for the mitigation of PQ problems. The performance of the PQ compensator under varying load and nonideal source conditions relies on the control strategy. A neural network-based p-q control algorithm is proposed in this paper for the DSTATCOM, which comprises of a four-leg voltage-source converter with a dc capacitor. The proposed control strategy implements five artificial neural network controllers for, the conversion of nonideal voltage source into ideal sinusoidal voltage, the extraction of dc component p of load real power supplied to the load, maintenance of the voltage across the capacitor, and mitigation of neutral current. The performance of the proposed neural network-based p-q control strategy for DSTATCOM is evaluated under various possible source and load conditions by simulating in MATLAB/Simulink environment, and the results obtained through the simulation are validated experimentally by a prototype developed in the laboratory. Both the experimental and simulation results prove that the performance of the proposed neural network-based control strategy is superior to the conventional method.

Neural network controlled grid interfaced solar photovoltaic power generation

This study deals with a NN (neural-network)-based control algorithm of a grid interfaced SPV (solar photovoltaic) generating system. The proposed grid interfaced SPV generating system utilises a NN control algorithm-based on the LMS (least mean-square), known as Adaline (adaptive linear element) to estimate reference grid currents. A DC-DC boost converter is used for achieving the maximum power point tracking between SPV and DC bus of four-leg VSC (voltage source converter) interfaced to a three-phase, four-wire distribution system. The four-leg VSC of SPV generating system is also used for the compensation of the reactive power for zero voltage regulation or for power factor correction along with load balancing, elimination of load harmonics currents and mitigation of neutral current at PCC (point of common coupling) in three-phase four-wire distribution system. The DC bus of VSC is supported by a capacitor which is fed by SPV energy through a DC-DC boost converter. A laboratory prototype of proposed grid interfaced SPV generating system is developed to validate its developed model and the NN-based control algorithm.

Design and Implementation of Four-Leg Voltage-Source-Converter-Based VFC for Autonomous Wind Energy Conversion System

This paper deals with the design and implementation of a voltage and frequency controller (VFC) for a three-phase four-wire autonomous wind energy conversion system (WECS). The VFC is realized using a four-leg voltage source converter and a battery energy storage system. The enhanced phase-locked loop-based control algorithm is used in the VFC. A prototype of a 3.7-kW isolated asynchronous generator-based WECS is developed to demonstrate the performance of the VFC. The obtained test results have shown that the proposed VFC functions as a load leveler, a load balancer, a neutral current compensator, and a harmonic eliminator.

Control Method of the Three-Phase Four-Leg Shunt Active Power Filter

This paper presents a novel all-digital approach applied to three-phase four-wire active power filter (APF), which is made up of three-phase four-leg voltage source converters. The controller just detects the main current and DC side capacitor voltage. Calculation method of the switch duty cycle is deducted on the base of mathematics model formed. This controller has fewer measurement, higher robustness and simpler calculation as comparing to traditional controllers. Simulation verify that the APF with proposed controller can compensate the harmonics effectively.

New control approach for capacitor supported DSTATCOM in three-phase four wire distribution system under non-ideal supply voltage conditions based on synchronous reference frame theory

A large number of single-phase linear and non-linear loads may be supplied from three phase ac mains with neutral conductor. They cause excessive neutral current, harmonics and reactive power burden and unbalance. A four wire DSTATCOM (distribution static compensator) is used for neutral current compensation along with reactive power compensation, harmonics elimination and load balancing. A novel control approach is proposed for the control of four wire DSTATCOM under non-ideal supply voltage conditions. A four-leg voltage–source converter (VSC) with a dc capacitor is used as a four wire DSTATCOM. The proposed control approach is based on synchronous reference frame (SRF) theory and an indirect current control technique. The switching signals for the voltage–source converter (VSC) of the DSTATCOM are derived from the estimated reference supply currents. The load balancing, harmonics elimination and the neutral current compensation are demonstrated along with unity power factor (UPF) and zero voltage regulation (ZVR) modes of operation. Simulation results based on MATLAB software with its Simulink and power system blockset (PSB) toolboxes are presented to validate the control strategy. The DSTATCOM is able to maintain the self-supported dc bus under various disturbances.