Improved direct power control of a grid-connected voltage source converter during network unbalance

This paper deals with an improved direct power control(DPC) strategy for the pulse width modulation(PWM) voltage source converter(VSC) under unbalanced grid voltage conditions.In order to provide enhanced control performance for the VSC,the resonant controllers tuned at the double grid frequency are applied in the DPC design to eliminate the power pulsations and dc link voltage ripples produced by the transient unbalanced grid faults.In this way,the output power and dc link voltage of the VSC can be directly regulated without positive and negative sequential decomposition.As a result,and as has been verified by experiment,the proposed method can provide fast dynamic response with easy implementation.

Proportional integral plus multi-frequency resonant current controller for grid-connected voltage source converter under imbalanced and distorted supply voltage conditions

This paper proposes a current control scheme for a grid-connected pulse width modulator(PWM) voltage source converter(GC-VSC) under imbalanced and distorted supply voltage conditions.The control scheme is implemented in the positive synchronously rotating reference frame and composed of a single proportional integral(PI) regulator and multi-frequency resonant controllers tuned at the frequencies of 2ω and 6ω,respectively.The experimental results,with the target of eliminating the active power oscillations and current harmonics on a prototype GC-VSC system,validate the feasibility of the proposed current control scheme during supply voltage imbalance and distortion.

Novel control scheme for 3-phase PWM current-source converters under unbalanced source voltage conditions

Under unbalanced source voltage supply, considerable output second harmonics and input low-order harmonics in 3-phase PWM current-source converters (PWM-CSC) are generated. This paper proposes a new deadbeat controller based on compensation for unbalanced source voltage and current. With the proposed scheme, the second harmonics of the output current are eliminated and low-order harmonics of the source current are reduced effectively. Simulation and experimental results con- firmed the feasibility of the proposed method.

Multilevel Current Source Converters for High Power Medium Voltage Applications

In medium voltage high power applications,multi-level current source converters(CSCs)are good candidate to increase system power region,reliability,and the quality of output waveforms.Compared with widely researched voltage source multi-level converters(MLCs),the current source MLCs have the advantages of inherent short-circuit protection,high power capability and high quality of output current waveforms.The main features of MLCs include reduced harmonics,lower switching frequency and reduced current stress on each device which is a particularly important for high power application with low voltage and high current requirements.This paper conducts a general review of the current research about MLCs in higher power medium voltage application.The different types of parallel structure based MLCs and the modulation methodologies will be introduced and compared.Specifically,the circuit analysis of the common-mode(CM)loop for parallel structures will be conducted,the common-mode voltage(CMV)and circulating current suppression methods developed on the base of multilevel modulations will be addressed.

Improved Interleaved Single-Ended Primary Inductor-Converter forSingle-Phase Grid-Connected System

The generation of electricity based on renewable energy sources,parti-cularly Photovoltaic(PV)system has been greatly increased and it is simply insti-gated for both domestic and commercial uses.The power generated from the PV system is erratic and hence there is a need for an efficient converter to perform the extraction of maximum power.An improved interleaved Single-ended Primary Inductor-Converter(SEPIC)converter is employed in proposed work to extricate most of power from renewable source.This proposed converter minimizes ripples,reduces electromagnetic interference due tofilter elements and the contin-uous input current improves the power output of PV panel.A Crow Search Algo-rithm(CSA)based Proportional Integral(PI)controller is utilized for controlling the converter switches effectively by optimizing the parameters of PI controller.The optimized PI controller reduces ripples present in Direct Current(DC)vol-tage,maintains constant voltage at proposed converter output and reduces over-shoots with minimum settling and rise time.This voltage is given to single phase grid via 1�Voltage Source Inverter(VSI).The command pulses of 1�VSI are produced by simple PI controller.The response of the proposed converter is thus improved with less input current.After implementing CSA based PI the efficiency of proposed converter obtained is 96%and the Total Harmonic Distor-tion(THD)is found to be 2:4%.The dynamics and closed loop operation is designed and modeled using MATLAB Simulink tool and its behavior is performed.

A Study of Power Loss for 3 Phase Voltage Source Converter of Elevator

In this paper,to study the power loss of converter for elevator,The analysis is to establish relationship between parameters(current,voltages and losses in the inverter and converter) relevant for sizing of major components of the drive was done.) For high PWM(pulse width modulation) frequency like in elevator applications of fpwm =10kHz,switching losses are dominant and are about 2/3 of the total losses on IGBT switch.Transition from continuous 3 phase to discontinuous 2 phase PWM results in 50% reduction of switching loses on IGBT devices providing that PWM is not done over 60deg angle in a particular phase when current has maximum value.Total losses on IGBT(conduction + switching) are reduced approximately by ～1/3 what is still a significant reduction.Two phase PWM with reduced losses can be used for applications when acoustic noise due to increased current ripple is not significant and fall back solution to regular 3 phase PWM when drive operates under rare extreme conditions resulting in increased heat sink temperature.The analysis will be examined by further laboratory testing simulating 60% duty cycle on a dynamometer.

Detection of Nonlinear Behavior Induced by Hard Limiting in Voltage Source Converters in Wind Farms Based on Higher-order Spectral Analysis

In recent years,sub-synchronous oscillation accidents caused by wind power integration have received extensive attention.The recorded constant-amplitude waveforms can be induced by either linear or nonlinear oscillation mechanisms.Hence,the nonlinear behavior needs to be distinguished prior to choosing the analysis method.Since the 1960s,the higher-order statistics(HOS)theory has become a powerful tool for the detection of nonlinear behavior(DNB)in production quality control wherein it has mainly been applied to mechanical condition monitoring and fault diagnosis.This study focuses on the hard limiters of the voltage source converter(VSC)control systems in the wind farms and attempts to detect the nonlinear behavior caused by bi-or uni-Iateral saturation hard limiting using the HOS analysis.First,the conventional describing function is extended to obtain the detailed frequency domain information on the bi-and uni-Iateral saturation hard limiting.Furthermore,the bi-and tri-spectra are introduced as the HOS,which are extended into bi-and tri-coherence spectra to eliminate the effects of the linear parts on the harmonic characteristics of hard limiting in the VSC control system,respectively.The effectiveness of the HOS in the DNB and the classification of the hard-limiting types is proven,and its detailed derivation and estimation procedure is presented.Finally,the quadratic and cubic phase coupling in the signals is illustrated,and the performance of the proposed method is evaluated and discussed.

Recent Developments of Modulation and Control for High-Power Current-Source-Converters Fed Electric Machine Systems

The pulse-width-modulated(PWM)current-source converters(CSCs)fed electric machine systems can be considered as a type of high reliability energy conversion systems,since they work with the long-life DC-link inductor and offer high fault-tolerant capability for short-circuit faults.Besides,they provide motor friendly waveforms and four-quadrant operation ability.Therefore,they are suitable for high-power applications of fans,pumps,compressors and wind power generation.The purpose of this paper is to comprehensively review recent developments of key technologies on modulation and control of high-power(HP)PWM-CSC fed electric machines systems,including reduction of low-order current harmonics,suppression of inductor–capacitor(LC)resonance,mitigation of common-mode voltage(CMV)and control of modular PWM-CSC fed systems.In particular,recent work on the overlapping effects during commutation,LC resonance suppression under fault-tolerant operation and collaboration of modular PMW-CSCs are described.Both theoretical analysis and some results in simulations and experiments are presented.Finally,a brief discussion regarding the future trend of the HP CSC fed electric machines systems is presented.

A Novel Polymorphic Topology with Hybrid Control Strategy Based LLC Resonant Converter for Ultra-Wide Input Voltage Range Applications

To realize effective utilization of renewable energy sources,a novel polymorphic topology with hybrid control strategy based LLC resonant converter was analyzed and designed in this paper.By combining the merits of a full bridge LLC resonant converter,three-level half bridge LLC resonant converter,and variable frequency control mode,the converter realizes an intelligent estimation of input voltage by automatically changing its internal cir-cuit topology.Under this control strategy,different input voltages determine different operation modes.This is achieved in full bridge LLC mode when the input voltage is low.If the input voltage rises to a certain level,it operates in three-level half bridge LLC mode.These switches are digital and entirely carried out by the DSP(Digi-tal Signal Processor),which means that an auxiliary circuit is unnecessary,where a simple strategy of software modification can be utilized.Experimental results of a 500W prototype with 100V~600V input voltage and full load efficiency of up to 92%are developed to verify feasibility and practicability.This type of converter is suitable for applications with an ultra-wide input voltage range,such as wind turbines,photovoltaic generators,bioenergy,and other renewable energy sources.

Current Source Converter Based Wind Energy Conversion Systems

The increase in the installed capacity of wind energy conversion systems(WECS) has triggered the devel-opment of more demanding grid codes and additional requirements on performance.In order to meet these require-ments the industry trend has shifted to full-scale power converter interfaces in modern multi-megawatt WECS.As con-sequence,a wide variety of new power converter topologies and WECS configurations have been introduced in recent years.Among them,current source converter(CSC) based configurations have attracted attention due to a series of advantages like:simple structure,grid friendly waveforms,controllable power factor,and reliable grid short-circuit pro-tection.This paper presents the latest developments in CSC interfaces for WECS and related technologies such as modulation methods,control schemes and grid code compatibility.

Three Phase Dual Input Direct Matrix Converter for Integration of Two AC Sources from Wind Turbines

This project proposes a novel dual-input matrix converter (DIMC) which is used to integrate the output of the wind energy to a power grid. The proposed matrix converter is developed based on the traditional indirect matrix converter under reverse power flow operation mode, but with its six-switch voltage source converter replaced by a nine-switch configuration followed by the current source inverter (CSI). Matrix electric power conversion topologies and their switch functions are flexible and are used for specific applications. With the additional three switches, the proposed DIMC can provide six input terminals, which make it possible to integrate two independent AC sources from two independent wind turbines into a single grid tied power electronics interface. Commanded currents can be extracted from the two input sources to the grid. The proposed PI control and modulation schemes guaranteed sinusoidal input and output waveforms as well as reduced THD. The simulation results are provided to validate the effectiveness of the proposed control and modulation schemes for the proposed converter.

Genetic Algorithm Based Smart Grid System for Distributed Renewable Energy Sources

This work presents the smart grid system for distributed Renewable Energy Sources(RES)with control methods.The hybrid MicroGrids(MG)are trending in small-scale power systems that involve distributed generations,power storage,and various loads.RES of solar are implemented with boost converter using Maximum Power Point Tracking(MPPT)with perturb and observe technique to track the maximum power.Also,the wind system is designed by permanent magnet synchronous generator that includes boost converter with MPPT technique.The battery is also employed with a Direct Current(DC)-DC bidirectional converter,and has a state of charge.The MATLAB/Simulink Simscape software is used to design the proposed model.The switching element of all converters is metal oxide semiconductor field effect transistor.The hybrid system is controlled by a voltage source controller using Proportional Integral Derivative(PID)-Genetic Algorithm(GA),and inductors-capacitors filter is used to reduce the system’s harmonics.Also,smart meters are used for monitoring purposes in residential loads.This paper analyses the performances of the MG against various scenarios.

Inherently Sinusoidal Single-phase Voltage Source Inverter Based on Modified Cuk Cell

Renewable energy has become important for electricity generation because of the high air pollution associated with conventional fossil-based energy systems.Conventional fossil-based power plants are gradually transitioning by incorporating renewable energy sources,such as photovoltaic(PV)cells.In a PV system,an inverter converts DC power from solar panels to AC power required to serve common electrical loads.A conventional H-bridge inverter topology has several disadvantages,such as the voltage being not sinusoidal,switching the DC voltage and high common-mode voltage.The common-mode voltage can cause a large leaked capacitive current,which can result in undesirable operation in solar power applications.A common solution to this problem is the addition of a large filter to the input or output of an inverter.An inherent sinusoidal voltage source inverter based on a modified Cuk converter as its basic cell,which simultaneously generates a sinusoidal output voltage and a lower common-mode voltage,is proposed.The proposed topology does not require additional input or output filters.Analytical expressions are derived to confirm the operation of the proposed topology.Simulation results confirm the mathematical analysis.A laboratory-scale experiment is performed to verify the proposed inverter.

R&D and application of voltage sourced converter based high voltage direct current engineering technology in China

As a new generation of direct current(DC)transmission technology,voltage sourced converter(VSC)based high voltage direct current(HVDC)has been widely developed and applied all over the world.China has also carried out a deep technical research and engineering application in this area,and at present,it has been stepped into a fast growing period.This paper gives a general review over China’s VSC based HVDC in terms of engineering technology,application and future development.It comprehensively analyzes the technical difficulties and future development orientation on the aspects of the main configurations of VSC based HVDC system,topological structures of converters,control and protection technologies,flexible DC cables,converter valve tests,etc.It introduces the applicable fields and current status of China’s VSC based HVDC projects,and analyzes the application trends of VSC based HVDC projects both in China and all over the world according to the development characteristics and demands of future power grids.

Robust current control design of a three phase voltage source converter

In this paper,a robust design method for current control is proposed to improve the performance of a three phase voltage source converter(VSC)with an inductorcapacitor-inductor(LCL)filter.The presence of the LCL filter complicates the dynamics of the control system and limits the achievable control bandwidth(and the overall performance),particularly when the uncertainty of the parameters is considered.To solve this problem,the advanced H?control theory is employed to design a robust current controller in stationary coordinates.Both control of the fundamental frequency current and suppression of the potential LC resonance are considered.The design procedure and the selection of the weight functions are presented in detail.A conventional proportional-resonant PR controller is also designed for comparison.Analysis showed that the proposed H∞ current controller achieved a good frequency response with explicit robustness.The conclusion was verified on a 5 kW VSC that had a LCL filter.

An Operational Test Method for VSC-HVDC Valves Based on Modular Multi-level Converters

为给基于模块化多电平换流器（modular multi-levelconverter,MMC）的电压源换流器高压直流输电（voltagesourced converter HVDC,VSC-HVDC）换流阀提供有效的检验手段,以验证其设计的正确性及其运行可靠性,对换流阀的运行特性和关键应力进行分析,提出基于原理等效的MMC阀运行试验方法,设计试验电路并阐明其运行机理。搭建的PSCAD电磁暂态仿真模型和实际的试验验证证明了试验方法的正确性和有效性。

Modeling framework of voltage-source converters based on equivalence with synchronous generator

Along with the increasing penetration of distributed generation with voltage-source converters(VSCs),there are extensive concerns over the potential virtual rotor angle stability, which is characterized by oscillations of power and frequency during the dynamic process of synchronization in the grid. Several control strategies have been developed for VSCs to emulate rotating inertia as well as damping of oscillations. This paper classifies these strategies and provides a small-signal modeling framework including all kinds of VSCs in different applications for virtual rotor angle stability. A unified perspective based on the famous Phillips–Heffron model is established for various VSCs. Thus, the concepts of equivalent inertia and the synchronizing and damping coefficients in different VSCs are highlighted, based on the similarities with the synchronous generator(SG) system in both physical mechanisms and mathematical models. It revealed the potentiality of various VSCs to achieve equivalence with the SG. This study helps promote the unity of VSCs and traditional SGs in both theories and methods for analyzing the dynamic behavior and enhancing the stability. Finally,future research needs and new perspectives are addressed.

Modeling method of sequence admittance for three-phase voltage source converter under unbalanced grid condition

The admittance is a strong tool for stability analysis and assessment of the three-phase voltage source converters(VSCs) especially in grid-connected mode.However, the sequence admittance is hard to calculate when the VSC is operating under unbalanced grid voltage conditions. In this paper, a simple and direct modeling method is proposed for a three-phase VSC taking the unbalanced grid voltage as a new variable for the system.Then coupling in the three-phase system can be calculated by applying the harmonic linearization method. The calculated admittance of three-phase VSCs is verified by detailed circuit simulations.

Enhanced control of DFIG-used back-to-back PWM VSC under unbalanced grid voltage conditions

This paper presents a unified positive-and negative-sequence dual-dq dynamic model of wind-turbine driven doubly-fed induction generator(DFIG) under unbalanced grid voltage conditions. Strategies for enhanced control and operation of a DFIG-used back-to-back(BTB) PWM voltage source converter(VSC) are proposed. The modified control design for the grid-side converter in the stationary αβ frames diminishes the amplitude of DC-link voltage ripples of twice the grid frequency,and the two proposed control targets for the rotor-side converter are alternatively achieved,which,as a result,improve the fault-ride through(FRT) capability of the DFIG based wind power generation systems during unbalanced network supply. A complete unbalanced control scheme with both grid-and rotor-side converters included is designed. Finally,simulation was carried out on a 1.5 MW wind-turbine driven DFIG system and the validity of the developed unified model and the feasibility of the proposed control strategies are all confirmed by the simulated results.