查看更多>>摘要：To secure power system operations,practical dis-patches in industries place a steady power transfer limit on crit-ical inter-corridors,rather than high-dimensional and strong nonlinear stability constraints.However,computational com-plexities lead to over-conservative pre-settings of transfer limit,which further induce undesirable and non-technical congestion of power transfer.To conquer this barrier,a scenario-classifica-tion hybrid-based banding method is proposed.A cluster tech-nique is adopted to separate similarities from historical and gen-erated operating condition dataset.With a practical rule,trans-fer limits are approximated for each operating cluster.Then,to-ward an interpretable online transfer limit decision,cost-sensi-tive learning is applied to identify cluster affiliation to assign a transfer limit for a given operation.In this stage,critical vari-ables that affect the transfer limit are also picked out via mean impact value.This enables us to construct low-complexity and dispatcher-friendly rules for fast determination of transfer lim-it.The numerical case studies on the IEEE 39-bus system and a real-world regional power system in China illustrate the effec-tiveness and conservativeness of the proposed method.
查看更多>>摘要：To avoid the secondary frequency dip caused by the steep drop of the electrical power of wind turbines(WTs)at the end of frequency support stage,the torque limit-based iner-tial control(TLIC)method sets the power reference as a linear function of rotor speed,rather than the step form for the step-wise inertial control.However,the compensation effect on the frequency nadir(FN)caused by the load surge is weakened as the TLIC power is no longer in the step form.Specifically,the maximum point of the frequency response component(FRC)contributed by TLIC occurs earlier than the minimum point of FRC corresponding to the load surge,so that the FN cannot be adequately raised.Therefore,this paper first investigates the re-lation between the peak and nadir times of FRCs stimulated by the TLIC and load power.On this basis,with the compensation principle of matching the peak and nadir times of FRCs,the improved TLIC method based on delayed support is proposed.Finally,the effectiveness of the proposed method is validated via the experiments on the test bench of wind-integrated power system.
查看更多>>摘要：Medium-voltage distribution systems(MVDSs)mainly consist of a feeder head,lines,distribution transformers,and the equivalent load or power supply interfaced with the dis-tribution transformers.The information of such load or power supply can be measured via the three-wattmeter method(THM)and the two-wattmeter method(TWM).The measurements can be used to perform the control of the power supply and simu-late the characteristics of the load,so the models of the load and the power supply need to consider the measurement charac-teristics.Existing research works on three-phase power flow(PF)just consider the measurement characteristics of THM.Hence,the PF equation of the bus measured via TWM is firstly built.Based on conventional measurements,an accurate and general model of the grounded and ungrounded slack bus is proposed.Furthermore,the influence arising from the connec-tion type and angle shift of distribution transformers on the ad-mittance matrix is considered,and thus a general three-phase transformer model is summarized,which is applicable for all the transformers mentioned herein.Finally,Newton's method is adopted to solve the PF calculation,and the performance of the proposed PF model is demonstrated through designed tests.
查看更多>>摘要：Power converters and their interfacing networks are often treated as modular state-space blocks for small-signal stability studies in microgrids;they are interconnected by matching the input and output states of the network and con-verters.Virtual resistors have been widely used in existing mod-els to generate a voltage for state-space models of the network that require voltage inputs.This paper accurately quantifies the adverse impacts of adding the virtual resistance and proposes an alternative method for network modelling that eliminates the requirement of the virtual resistor when interfacing convert-ers with microgrids.The proposed nonlinear method allows ini-tialization,time-domain simulations of the nonlinear model,and linearization and eigenvalue generation.A numerically linear-ized small-signal model is used to generate eigenvalues and is compared with the eigenvalues generated using the existing modelling method with virtual resistances.Deficiencies of the existing method and improvements offered by the proposed modelling method are clearly quantified.Electromagnetic tran-sient(EMT)simulations using detailed switching models are used for validation of the proposed modelling method.
查看更多>>摘要：This paper introduces a distributed secondary con-trol scheme for achieving current sharing and average voltage regulation objectives in a DC microgrid.The proposed scheme employs a dynamic diffusion algorithm(DDA)instead of the consensus algorithm to enable distributed communication among converters.To help understand DDA,the relation of DDA and other diffusion algorithms is discussed in detail and its superiority is shown by comparison with diffusion and con-sensus algorithms.Furthermore,considering the discrete nature and different sampling time of the digital controller and com-munication network,a z-domain model of the entire DC mi-crogrid is established.The influence of communication and sec-ondary control parameters on the system stability is investigat-ed.Based on the established model,the tolerable communica-tion rates are obtained.Real-time simulations conducted on the OPAL-RT platform validate the effectiveness of the proposed scheme,showcasing its advantages in terms of convergence speed and stability.
查看更多>>摘要：There is a general concern that the increasing pene-tration of electric vehicles(EVs)will result in higher aging fail-ure probability of equipment and reduced network reliability.The electricity costs may also increase,due to the exacerbation of peak load led by uncontrolled EV charging.This paper pro-poses a linear optimization model for the assessment of the ben-efits of EV smart charging on both network reliability improve-ment and electricity cost reduction.The objective of the pro-posed model is the cost minimization,including the loss of load,repair costs due to aging failures,and EV charging expenses.The proposed model incorporates a piecewise linear model rep-resentation for the failure probability distributions and utilizes a machine learning approach to represent the EV charging load.Considering two different test systems(a 5-bus network and the IEEE 33-bus network),this paper compares aging fail-ure probabilities,service unavailability,expected energy not supplied,and total costs in various scenarios with and without the implementation of EV smart charging.
查看更多>>摘要：To reduce the difficulty and enhance the enthusi-asm of private-owned electric vehicles(EVs)in participating in frequency regulation ancillary service market(FRASM),a deci-sion aid model(DAM)is proposed.This paper presents three options for EV participating in FRASM,i.e.,the base mode(BM),unidirectional charging mode(UCM),and bidirectional charging/discharging mode(BCDM),based on a reasonable sim-plification of users'participating willingness.In BM,individual EVs will not be involved in FRASM,and DAM will assist users to set the optimal charging schemes based on travel plans un-der the time-of-use(TOU)price.UCM and BCDM are two modes in which EVs can take part in FRASM.DAM can assist EV users to create their quotation plan,which includes hourly upper and lower reserve capabilities and regulation market mileage prices.In UCM and BCDM,the difference is that only the charging rate can be adjusted in the UCM,and the EVs in BCDM can not only charge but also discharge if necessary.DAM can estimate the expected revenue of all three modes,and EV users can make the final decision based on their preferenc-es.Simulation results indicate that all the three modes of DAM can reduce the cost,while BCDM can get the maximum expect-ed revenue.
查看更多>>摘要：In recent years,sub-synchronous oscillation acci-dents 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-or-der 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 condi-tion monitoring and fault diagnosis.This study focuses on the hard limiters of the voltage source converter(VSC)control sys-tems in the wind farms and attempts to detect the nonlinear be-havior caused by bi-or uni-lateral saturation hard limiting us-ing the HOS analysis.First,the conventional describing func-tion is extended to obtain the detailed frequency domain infor-mation on the bi-and uni-lateral saturation hard limiting.Fur-thermore,the bi-and tri-spectra are introduced as the HOS,which are extended into bi-and tri-coherence spectra to elimi-nate the effects of the linear parts on the harmonic characteris-tics 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.
查看更多>>摘要：In recent years,high-frequency resonance(HFR)events occurred in several modular multilevel converter based high-voltage direct current(MMC-HVDC)projects.The time delay of an MMC-HVDC system is the critical factor that in-duces HFR.The frequency coupling affects the impedance char-acteristics of an MMC and further deteriorates system stability.Therefore,in this paper,a multi-input multi-output admittance model of an MMC-HVDC system is developed to analyze its fre-quency characteristics.The effects of current loop,power loop,phase-locked loop,and operating point on the MMC frequency coupling degree are analyzed in detail.Meanwhile,to further suppress HFR in the MMC-HVDC system,an enhanced imped-ance reshaping control strategy based on the equivalent single-input single-output impedance model is proposed.Finally,the accuracy of the enhanced impedance model and the effective-ness of the impedance reshaping control are verified by electro-magnetic transient simulations in PSCAD.
查看更多>>摘要：Due to the fact that a high share of renewable ener-gy sources(RESs)are connected to high-voltage direct current(HVDC)sending-end AC power systems,the voltage and fre-quency regulation capabilities of HVDC sending-end AC power systems have diminished.This has resulted in potential system operating problems such as overvoltage and overfrequency,which occur simultaneously when block faults exist in the HVDC link.In this study,a steady-state voltage security-con-strained optimal frequency control method for weak HVDC sending-end AC power systems is proposed.The integrated vir-tual inertia control of RESs is employed for system frequency regulation.Additional dynamic reactive power compensation de-vices are utilized to control the voltage of all nodes meet volt-age security constraints.Then,an optimization model that si-multaneously considers the frequency and steady-state voltage security constraints for weak HVDC sending-end AC power sys-tems is established.The optimal control scheme with the mini-mum total cost of generation tripping and additional dynamic reactive power compensation required is obtained through the optimization solution.Simulations are conducted on a modified IEEE 9-bus test system and practical Qing-Yu line commutated converter based HVDC(LCC-HVDC)sending-end AC power system to verify the effectiveness of the proposed method.
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