Analysis and synchronization controller design of dual-port gridforming voltage-source converters for different operation modes

Grid-tie voltage source converters(VSCs)can operate in three distinct modes:AC-dominant,DC-dominant,and balanced,depending on the placement of the stiff voltage sources.The distinct operation modes of the VSCs traditionally demand different synchronization control techniques,leading to heterogeneous VSCs.It is challenging for the power system to accommodate and coordinate heterogeneous VSCs.A promising universal synchronization control technique for VSCs is the DC-link voltage synchronization control(DVSC)based on a lead compensator(LC).The LC DVSC stabilizes both the DC and AC voltages of a VSC while achieving synchronization with the AC grid.This results in a dual-port grid-forming(DGFM)characteristic for the VSC.However,there has been very limited study on the stability and synchronization controller design of the VSCs with the LC DVSC operating in various modes.To bridge this gap,the paper presents a quantitative analysis on the stability and steady-state performance of the LC DVSC in all three operation modes of the DGFM VSC.Based on the analysis,the paper provides step-by-step design guidelines for the LC DVSC.Furthermore,the paper uncovers an instability issue related to the LC DVSC when the DGFM VSC operates in the balanced mode.To tackle the instability issue,a virtual resistance control is proposed and integrated with the LC DVSC.Simulation results validate the analysis and demonstrate the effectiveness of the DGFM VSC with the LC DVSC designed using the proposed guidelines in all three operation modes.Overall,the paper demonstrates the feasibility of employing the DGFM VSC with the LC DVSC for all three possible operation modes,which can help overcome the challenges associated with accommodating and coordinating heterogeneous VSCs in the power system.

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.

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.

Vector Control of Three-Phase Solar Farm Converters Based on Fictive-Axis Emulation

In this paper,a new method for adjusting the current of three-phase voltage source DC-AC converter in orthogonal(DQ)reference frame is presented.In the DQ reference system,AC variable appears in the constant form of DC,making the controller design the same as the DC-DC converter[1].It provides controllable gain benefits at the steady-state operating point,and finally realizes zero steady-state error[2].In addition,the creative analytical model is dedicated to building up a series of virtual quantities orthogonal to the actual single-phase system.In general,orthogonal imaginary numbers get the reference signal by delaying the real quantity by a quarter period.However,the introduction of such time delay makes the dynamic response of the system worse.In this paper,orthogonal quantities are generated from a virtual axis system parallel to the real axis,which can effectively improve the dynamic performance of traditional methods without increasing the complexity of controller structure.Through PSCAD simulation,the ideal experimental results are obtained.

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.

Harmonic Domain Modelling of Space Vector Based STATCOM

This paper presents frequency domain method for harmonic analysis of space vector based STATCOM. Space Vector Pulse Width Modulation (SVPWM) method is an advanced PWM method. It is a best method among all the PWM techniques. It provides a freedom in a switching cycle for placement space vector. In this paper, the SVPWM is used for switching of STATCOM. The harmonic (or frequency) domain is a steady-state form of harmonic analysis method, which represents converters to their harmonic spectra. This paper presents harmonic analysis by means of harmonic domain for space vector based Static shunt converter (STATCOM). Performance of the STATCOM is evaluated in harmonic domain simulation studies in MATLAB environment.

Dynamic Harmonic Domain Modelling of Space Vector Based SSSC

This paper presents analytical frequency domain method for harmonic modeling and evaluation of Space Vector Pulse Width Modulation (SVPWM) based static synchronous series converter (SSSC). SVPWM is the best among all the PWM techniques. It gives a degree of freedom of space vector placement in a switching cycle. Dynamic modeling technique is used for space vector modulation (SVM) based voltage source converter that is adapted as a static synchronous series converter (SSSC) for harmonic analysis using dynamic harmonic domain. Performance of the SSSC is evaluated in dynamic harmonic domain simulation studies in MATLAB environment. The switching function spectra are necessary for harmonic transfer matrix which is calculated using Fourier series. This paper presents the analysis of harmonics for space vector based SSSC during steady state and dynamic condition.

Synchronization Stability Analysis of PLL-based Grid-connected VSC System by Voltage Space Vectors

This paper focuses on synchronization stability analysis of the power system,in which power electronics are synchronized by the phase-locked loop(PLL).It provides new insight into the synchronization stability of power electronics from the voltage perspective.The synchronization stability analysis based on space vector is carried out by establishing a simplified model of the grid-connected voltage source converter(VSC)system.Without complex mathematical calculation,the existence criterion of equilibrium points and the criterion of transient instability dominated by the unstable equilibrium point(UEP)are derived,respectively.With the proposed method,synchronization stability can be determined by the voltage space vectors,which are more observable in potential engineering applications.At the end of this study,the steps of the synchronization stability determination by voltage space vectors are summarized,and the effectiveness and applicability of the proposed method are demonstrated by numerical simulations performed on the PSCAD/EMTDC platform.

Power Flow Calculation for VSC-based AC/DC Hybrid Systems Based on Fast and Flexible Holomorphic Embedding

The power flow(PF)calculation for AC/DC hybrid systems based on voltage source converter(VSC)plays a crucial role in the operational analysis of the new energy system.The fast and flexible holomorphic embedding(FFHE)PF method,with its non-iterative format founded on complex analysis theory,exhibits superior numerical performance compared with traditional iterative methods.This paper aims to extend the FFHE method to the PF problem in the VSC-based AC/DC hybrid system.To form the AC/DC FFHE PF method,an AC/DC FFHE model with its solution scheme and a sequential AC/DC PF calculation framework are proposed.The AC/DC FFHE model is established with a more flexible form to incorporate multiple control strategies of VSC while preserving the constructive and deterministic properties of original FFHE to reliably obtain operable AC/DC solutions from various initializations.A solution scheme for the proposed model is provided with specific recursive solution processes and accelerated Padéapproximant.To achieve the overall convergence of AC/DC PF,the AC/DC FFHE model is integrated into the sequential calculation framework with well-designed data exchange and control mode switching mechanisms.The proposed method demonstrates significant efficiency improvements,especially in handling scenarios involving control mode switching and multiple recalculations.In numerical tests,the superiority of the proposed method is confirmed through comparisons of accuracy and efficiency with existing methods,as well as the impact analyses of different initializations.

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.

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.

Adaptive droop control for better current-sharing in VSC-based MVDC railway electrification system

The share of voltage source converter(VSC)technology is increasing in conventional power systems,and it is penetrating into specific transportation systems such as electric vehicles,railways,and ships.Researchers are identifying feasible methods to improve the performance of railway electrification systems(RESs)by utilizing VSC-based medium-voltage direct current(MVDC)railways.The continuous motion of electric trains makes the catenary resistance a variable quantity,as compared to the traction substation(TSS),and affects the currentsharing behavior of the system.A modified droop control technique is proposed in this paper for VSC-based MVDC RES to provide more effective current-sharing while maintaining catenary voltages above the minimum allowable limit.The droop coefficient is selected through an exponential function based on the ratio between the concerned TSS current and the system average current.This enables small adjustments of droop values in less concerning marginal current deviations,and provides higher droop adjustments for large current deviations.Meanwhile,the catenary voltages are regulated by considering the voltage data at the midpoint between two TSSs,which experiences the lowest voltages owing to the larger distance from the TSSs.The proposed techniques are validated via simulations and experiments.

Modeling and Stability Issues of Voltage-source Converter-dominated Power Systems:A Review

With the substantive increase in the proportion of voltage-source converter(VSC)-based equipment,traditional power systems that primarily constituted of synchronous generators(SGs)gradually evolved into VSC-dominated ones.At the same time,there is an urgent need for modeling and stability assessment of such systems,since low inertia and weak damping features impair the ability of the systems to resist random disturbances.Existing works model the system dynamic processes from various domains(i.e.,time,frequency and energy),and analyze/determine the system stability under small or large disturbances.Among them,small-signal stability assessments mainly adopt the time-domain analysis based on the state-space model while frequency-domain methods include the impedance model,phase-amplitude dynamics model,and static synchronous generator model.Large-signal stability assessments mainly exploit the time-domain simulation with detailed models(i.e.,continuous/discrete-time mixed model with differentialdifference-algebraic equations),and the energy-domain analysis is based on energy function models.This paper presents a comprehensive review of existing modeling and stability analysis methods for VSC-dominated power systems,including their basic principles,key features,application scenarios and development tendencies.Key technical issues related to modeling and stability analysis are also summarized.

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.

Power hardware in the loop validation of fault ride through of VSC HVDC connected offshore wind power plants

This paper presents the power hardware in the loop(PHIL)validation of a feed forward DC voltage control scheme for the fault ride through(FRT)of voltage source converter(VSC)high voltage DC(HVDC)connected offshore wind power plants(WPPs).In the proposed FRT scheme,the WPP collector network AC voltage is actively controlled by considering both the DC voltage error and the AC current from the WPP AC collector system which ensures fast and robust FRT of the VSC HVDC connected offshore WPPs.The PHIL tests were carried out in order to verify the efficacy of the proposed feed forward DC voltage control scheme for enhancing the FRT capability of the VSC HVDC connected WPPs.The PHIL test results have demonstrated the proper control coordination between the offshore WPP and the WPP side VSC and the efficient FRT of the VSC HVDC connected WPPs.

Improved virtual synchronous control for grid-connected VSCs under grid voltage unbalanced conditions

This paper presents an improved virtual synchronous control(VSynC) for the grid-connected voltage source converter(VSC) so as to continuously operate under the grid voltage with steady unbalance.The improved VSynC introduces the negative sequence power controls on basis of conventional VSynC.The improved VSynC is capable of regulating the negative sequence internal voltage to reduce the negative-sequence injected currents and oscillated powers of the VSC aroused by the negative-sequence grid voltage.Three alternative local control objectives for the VSC itself under steady state unbalanced grid conditions and their corresponding power references are deduced and computed.Simulated and experimental results are presented to validate the correctness and effectiveness of the proposed improved VSynC to enhance the continuous operation performance of VSynC-based VSCs during grid voltage steady-state unbalance.

Sequential auto-reclosing strategy for hybrid HVDC breakers in VSC-based DC grids

The use of overhead lines for power transmission in the future high-voltage and large-capacity voltagesource converter(VSC)-based direct current(DC) grid will significantly increase the probability of temporary faults.To eliminate potential adverse impacts such as erroneous protection, line-insulation failure, and even damage to power electronic devices resulting from a DC breaker reclosing operation with the traditional sequential autoreclosing strategy, a new sequential auto-reclosing strat-egy for hybrid HVDC breakers(HHBs) in VSC-based DC grids is proposed. This strategy is based on the step-by-step operation of the transfer branch in the HHB. As a result,du/dt resulting from the HHB reclosing operation is greatly reduced, and therefore those potential negative impacts can be eliminated. Several other advantages are also presented.The feasibility and validity of the proposed strategy are verified in a four-terminal annular VSC-based DC grid electromagnetic transient model.

Adaptive Reference Power Based Voltage Droop Control for VSC-MTDC Systems

Featuring low communication requirements and high reliability,the voltage droop control method is widely adopted in the voltage source converter based multi-terminal direct current(VSC-MTDC)system for autonomous DC voltage regulation and power-sharing.However,the traditional voltage droop control method with fixed droop gain is criticized for over-limit DC voltage deviation in case of large power disturbances,which can threaten stable operation of the entire VSCMTDC system.To tackle this problem,this paper proposes an adaptive reference power based voltage droop control method,which changes the reference power to compensate the power deviation for droop-controlled voltage source converters(VSCs).Besides retaining the merits of the traditional voltage droop control method,both DC voltage deviation reduction and power distribution improvement can be achieved by utilizing local information and a specific control factor in the proposed method.Basic principles and key features of the proposed method are described.Detailed analyses on the effects of the control factor on DC voltage deviation and imbalanced power-sharing are discussed,and the selection principle of the control factor is proposed.Finally,the effectiveness of the proposed method is validated by the simulations on a five-terminal VSC based high-voltage direct current(VSC-HVDC)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 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.

Virtual Difference Voltage Scheme for Fault Detection in VSC-HVDC Transmission Systems

In this paper,a fast fault detection scheme for voltage source converter based high-voltage direct current(VSCHVDC)transmission systems is proposed.Based on Bergeron model equations,the remote terminal voltage of an adopted transmission system is calculated in terms of the local measured current and voltage signals.Subsequently,the computed voltage of the remote terminal is compared with the corresponding actual measured-communicated value.Provided that the considered transmission system is functioning well,the difference between the computed and measured voltages is almost zero.However,a considerable virtual voltage arises for fault conditions.When the voltage difference exceeds a predetermined threshold,a fault condition can be detected.Although a reliable communication link is required,the delay for detecting the fault is not caused by the communication time.For evaluation purpose,a detailed simulation is developed using PSCAD/EMTDC with various fault locations,including the cases near the inside or outside of the protected transmission system.The results corroborate a fast detection scheme depending on a moderate sampling/processing frequency level.A high security level is verified even with the worst external faults,or with the misaligned measured samples at the terminals.This corroborates the suitability of the proposed scheme for protecting multi-terminal HVDC systems.