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-lateral 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-lateral 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.

Locating Sources of Oscillations Induced by Control of Voltage Source Converters Based on Energy Structure and Nonlinearity Detection

The oscillation phenomena associated with the control of voltage source converters(VSCs)are concerning,making it crucial to locate the sources of such oscillations and suppress the oscillations.Therefore,this paper presents a location scheme based on the energy structure and nonlinearity detection.The energy structure,which conforms to the principle of the energy-based method and dissipativity theory,is developed to describe the transient energy flow for VSCs,based on which a defined characteristic quantity is implemented to narrow the scope for locating the sources of oscillations.Moreover,based on the self-sustained oscillation characteristics of VsCs,an index for nonlinearity detection is applied to locate the VSCs that produce the oscillation energy.The combination of the energy structure and nonlinearity detection distinguishes the contribu-tions of different VSCs to the oscillation.The results of a case study implemented by the PSCAD/EMTDC simulation validate theproposed scheme.

Overview on Fault-Tolerant Four-Switch Three-Phase Voltage Source Converters

With the rapid development and widespread applications of power electronic converters,strong fault-tolerant capability of power electronic converters is required since they play important roles in power systems.In this paper,a review of one of the most promising fault-tolerant topologies for semiconductor open-circuit fault,called four-switch three-phase(FSTP)topology,is presented in terms of modeling analysis,modulation techniques,and control strategies.The configuration of FSTP voltage source converter(VSC)is illustrated.To minimize the negative effects caused by the innate drawbacks of this fault-tolerant converter topology,considerable research has been carried out regarding modulation techniques and control strategies.The modulation principle for FSTP topology is explained in detail,since the performance of FSTP VSCs relies on it.This paper aims to illustrate current research progress on this fault-tolerant FSTP VSC topology.

Research on Active and Reactive Power Decoupling Control Based on VSC-HVDC

Voltage Source Converter-based High Voltage Direct Current(VSC-HVDC)transmission technology represents a groundbreaking approach in high voltage Direct Current(DC)transmission,offering numerous technical advantages and broad application prospects.However,in the d-q synchronous rotating coordinate system,the VSC-HVDC exhibits the coupling effect of active power and reactive power,so it needs to be decoupled.This paper introduces the basic principle and mathematical model of the VSC-HVDC transmission system.Through the combination of coordinate transformation and variable substitution,a feedforward decoupling control method is derived.Then the VSC-HVDC simulation model is designed,and the simulation analysis is carried out in the MATLAB environment.The simulation results demonstrate that the method effectively achieves decoupling control of active and reactive power,exhibiting superior dynamic performance and robustness.These findings validate the correctness and effectiveness of the control strategy.

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.

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.

An Adaptive Wireless Power Sharing Control for Multiterminal HVDC

Power sharing among multiterminal high voltage direct current terminals(MT-HVDC)is mainly developed based on a priority or sequential manners,which uses to prevent the problem of overloading due to a predefined controller coefficient.Furthermore,fixed power sharing control also suffers from an inability to identify power availability at a rectification station.There is a need for a controller that ensures an efficient power sharing among the MT-HVDC terminals,prevents the possibility of overloading,and utilizes the available power sharing.A new adaptive wireless control for active power sharing among multiterminal(MT-HVDC)systems,including power availability and power management policy,is proposed in this paper.The proposed control strategy solves these issues and,this proposed controller strategy is a generic method that can be applied for unlimited number of converter stations.The rational of this proposed controller is to increase the system reliability by avoiding the necessity of fast communication links.The test system in this paper consists of four converter stations based on three phase-two AC voltage levels.The proposed control strategy for a multiterminal HVDC system is conducted in the power systems computer aided design/electromagnetic transient design and control(PSCAD/EMTDC)simulation environment.The simulation results significantly show the flexibility and usefulness of the proposed power sharing control provided by the new adaptive wireless method.

SINAMICS—the high efficiency drive technology for rolling mills

In the Metals Industry,interruption-free processes to produce high-quality end products are a prerequisite. The main drives together with the mill stands play a key role in the success of rolling mills.The crucial demands placed on the drive system are:high-dynamic performance of drive and its control system,ability to handle the process related overloads,smooth running,high availability,high efficiency,easy serviceability and ability to integrate seamlessly with the automation systems.With numerous reference installations and many years of experience Siemens VAI has the wider expertise and the portfolio to provide the right drive for every application. This paper examines the latest innovation,SINAMICS drive technology,for both new drive applications as well as for modernizing the existing drives in rolling mill applications like hot strips mills,plate mills,cold rolling mills,and long rolling mills.

Derivation of Multilevel Voltage Source Converter Topologies for Medium Voltage Drives

Multilevel voltage source converters(MLVSCs)have been widely applied in the medium voltage drive(MVD)industry.The performance of a MVD system is strongly dependent on the utilized topology.As of today,many interesting topologies have been proposed and evaluated in literature.In addition to proposing new topologies,another important research topic is the MLVSC topology derivation.In this paper,two topology derivation principles,i.e.horizontal conformation principle and vertical conformation principle,are proposed from the standpoint of modularity.In both principles,a MLVSC topology can be considered as a certain combination of one base switching cell and several module switching cells.With the proposed principle,the derived topology will naturally have modularity,which is favorable in practical applications.In addition,voltage level extension based on cascaded H-bridge building blocks(HBBBs)is also introduced.The challenging issues faced by the emerging topologies for MVD applications are also discussed.It is hoped that this paper can provide a new perspective on the MLVSC topology derivation and inspire new topologies in the future.

FPGA-Based Real-Time Simulation of Modular Multilevel Converter HVDC Systems

AC-HVDC-AC energy conversion systems using MMC （modular multilevel converters） are becoming popular to integrate distributed energy systems to the main grid. Such multilevel converters pose a serious problems for HIL （hardware in the loop） simulators required for control, protection design and testing due to the large number of cells that must be simulated individually using very small time steps. This paper demonstrates the advantages of using a very small time step to simulate a MMC topology. The MMC is implemented on FPGA （fiel-programmable gate array） to simulate fast transient with a time step of 250 ns. The AC network and HVDC bus is simulated on the PC, with a slower time step of 10 μs to 20 μs. The simulator architecture and the components simulated on the FPGA and on the PC will be discussed, as well as the method allowing the interconnection of this slow and fast system.

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.

Research on the control method for voltage-current source hybrid-HVDC system

The hybrid-HVDC topology,which consists of line-commutated-converter(LCC)and voltage source converter(VSC)and combines their advantages,has extensive application prospects.A hybrid-HVDC system,adopting VSC on rectifier side and LCC on inverter side,is investigated,and its mathematic model is deduced.The commutation failure issue of the LCC converter in the hybrid-HVDC system is considered,and a novel coordinated control method is proposed to enhance the system commutation failure immunity.A voltage dependent voltage order limiter(VDVOL)is designed based on the constant DC voltage control on the rectifier side,and constant extinction angle backup control is introduced based on the constant DC current control with voltage dependent current order limiter(VDCOL)on the inverter side.The hybrid-HVDC system performances under normal operation state and fault state are simulated in the PSCAD/EMTDC.Then,system transient state performances with or without the proposed control methods under fault condition are further compared and analyzed.It is concluded that the proposed control method has the ability to effectively reduce the probability of commutation failure and improve the fault recovery performance of the hybrid-HVDC system.

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.

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.

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.

Distributed Virtual Inertia Based Control of Multiple Photovoltaic Systems in Autonomous Microgrid

The large inertia of a traditional power system slows down system's frequency response but also allows decent time for controlling the system.Since an autonomous renewable microgrid usually has much smaller inertia,the control system must be very fast and accurate to fight against the small inertia and uncertainties.To reduce the demanding requirements on control,this paper proposes to increase the inertia of photovoltaic(PV) system through inertia emulation.The inertia emulation is realized by controlling the charging/discharging of the direct current(DC)-link capacitor over a certain range and adjusting the PV generation when it is feasible and/or necessary.By well designing the inertia,the DC-link capacitor parameters and the control range,the negative impact of inertia emulation on energy efficiency can be reduced.The proposed algorithm can be integrated with distributed generation setting algorithms to improve dynamic performance and lower implementation requirements.Simulation studies demonstrate the effectiveness of the proposed solution.

Damping controller design based on FO-PID-EMA in VSC HVDC system to improve stability of hybrid power system

Wind energy sources have different structures and functions from conventional power plants in the power system.These resources can affect the exchange of active and reactive power of the network.Therefore,power system stability will be affected by the performance of wind power plants,especially in the event of a fault.In this paper,the improvement of the dynamic stability in power system equipped by wind farm is examined through the supplementary controller design in the high voltage direct current(HVDC)based on voltage source converter(VSC)transmission system.In this regard,impacts of the VSC HVDC system and wind farm on the improvement of system stability are considered.Also,an algorithm based on controllability(observability)concept is proposed to select most appropriate and effective coupling between inputs-outputs(IO)signals of system in different work conditions.The selected coupling is used to apply damping controller signal.Finally,a fractional order PID controller(FO-PID)based on exchange market algorithm(EMA)is designed as damping controller.The analysis of the results shows that the wind farm does not directly contribute to the improvement of the dynamic stability of power system.However,it can increase the controllability of the oscillatory mode and improve the performance of the supplementary controller.

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 multibranch 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.

A Review of Series Voltage Source Converter with Fault Current Limiting Function

The series voltage source converter(SVSC)is widely used in the power electronic equipment,such as series active power filter,dynamic voltage restorer,unified power flow controller and so on.However,while the SVSC is more vulnerable to the impact of fault current,its applications are increasing,bringing huge challenges to the safe operation of the grid.In recent years,the topology and control strategy of the series voltage source converter with fault current limiting(SVSC-FCL)are a research hotspot.In this paper,it suggests classifying SVSC-FCL based SVSC into two groups:the control scheme optimization group and the existing topology improvement group.The research challenges and perspectives of the SVSC-FCL are introduced in detail.This paper aims to illustrate current research progress on SVSC-FCL and enrich the available pool of the multi-functional power electronic equipment.

A novel technology for control of variable speed pumped storage power plant

Variable speed pumped storage machines are used extensively in wind power plant and pumped storage power plant. This paper presents direct torque and flux control(DTFC) of a variable speed pumped storage power plant(VSPSP). By this method both torque and flux have been applied to control the VSPSP. The comparison between VSPSP's control strategies is studied. At the first, a wind turbine with the capacity 2.2 k W and DTFC control strategies simulated then a 250 MW VSPSP is simulated with all of its parts(including electrical, mechanical, hydraulic and its control system) by MATLAB software. In all of simulations, both converters including two-level voltage source converter(2LVSC) and three-level voltage source converter(3LVSC) are applied. The results of applying 2LVSC and 3LVSC are the rapid dynamic responses with better efficiency, reducing the total harmonic distortion(THD) and ripple of rotor torque and flux.