Deadbeat predictive current control (DPCC) can track the current command with a two-sample step response. However, voltage insufficiency is common during abrupt torque changes, and the current command tracking performance is inevitably degraded. To address this issue, an optimized overmodulation method is developed in this paper. By predicting the current vector hexagon in the subsequent switching period, the crossing point between the predicted current hexagon boundary and the expected current trajectory is determined as an optimized current reference. The purpose is to mitigate the current trajectory drift in the overmodulation zone, thus ensuring optimal dynamic trajectory tracking performance and voltage utilization. The solution of the crossing point is solved by the dichotomy due to the randomness of the expected trajectory. By combining DPCC with the proposed method, dynamic current reference optimization and deadbeat response in the overmodulation zone can be achieved, and accurate current trajectory tracking can be realized. Taking $i_{d}=0$ and $i_{d}=-i_{q}$ trajectory tracking as validation examples, the proposed method is compared with the traditional methods. Both simulation and experimental results have verified the effectiveness of the proposed method.
The modular multilevel converter(MMC)topology has been widely used in medium/high voltage high-power transmission and distribution and motor drive fields.The full-bridge submodule(FB-SM)MMC topology with fault handling capability is currently receiving increasing attention and application.However,in order to suppress the SM capacitor voltage ripple,the usage of SM capacitors with larger capacitance significantly increases the hardware cost and volume of the system.In this paper,a modified FB-SM with active power decoupling circuit(APD-SM)is introduced,it combines the ability of capacitor voltage ripple suppression and DC fault traversal through device reuse without changing the external output characteristics of the topology.Compared to traditional MMC,the SM capacitor voltage ripple of this topology can be suppressed significantly in the full power factor range.The deduction rules,operating principles,modulation methods,and control strategies of this topology are introduced in this artical.In addition,key parameters in the topology structure are designed and the topology is compared with traditional FB-SM topology from multiple aspects.Finally,the simulation and experimental platform of MMC with APD-SM and FB-SM(Abbreviated as APD-MMC and FB-MMC,respectively)are built,and experimental verification was conducted based on the prototype model.Simulations and experimental results verify the validity of the APD-MMC topology and control strategy.
Power system is an important field to achieve the goal of carbon neutrality in the future.With the aggravation of source and load uncertainties,the low-carbon economic dispatching method based on the deterministic model cannot accurately describe the impact of uncertain factors on carbon emissions.In view of the above problems,a low-carbon robust optimization model considering uncertainty is constructed at the level of power system,in which,the uncertainties of source and load are modeled using the probability model,and the significance level of the target meeting the expectation is described by chance constraint.The robust dispatching scheme under the risk aversion strategy can be obtained by maximizing the confidence level of the uncertainty.Then,an event-driven low-carbon response model is constructed at the level of power users.According to the calculation results of the level of power system,the excess carbon emission event is defined by setting the carbon emission threshold,and the low-carbon power consumption behavior of users is guided in the form of price.Finally,through the case analysis,the results show that the proposed model can effectively quantify the uncertainty level of the low-carbon economic dispatching results,give full play to the carbon reduction ability of the user side,and realize the coordination of the carbon reduction goals of both sides of the source and load.
Supercritical carbon dioxide (sCO2) is a promising working medium for coal-fired power plants, high-temperature solar power systems, nuclear reactor systems, and fuel cells owing to its high efficiency and inertness. In these cases, the sCO2 is in a round tube under high temperature and pressure, far from the pseudo-critical point. Here, the thermophysical properties of sCO2 changes, resulting in large prediction errors. In this study, a conjugate heat transfer model with the k-ε turbulent model was created to analyze the tube performance of the sCO2, and the effects of turbulent Prandtl number (Prt) on the sCO2 turbulent flow with high heat flux were examined through experimental and numerical investigations. Prt had a considerable effect on the performance of the sCO2 turbulent flow far from the pseudo-critical point. The mean relative errors of the inner wall temperature and convective heat transfer coefficient were reduced by approximately 13% and 27%, respectively, when Prt was decreased from 1 to 0.55. For the sCO2 turbulent flow with a high heat flux, constant Prt values of 0.55–0.6 were applicable. The findings afford key insights for the development of sCO2 turbulent flow with high heat flux.
The large-scale integration of renewable energy into the power grid leads to the decrease of system voltage support strength,which is usually characterized by its short circuit ratio(SCR).This results in stability issues such as static voltage stability problems and sub/super synchronous oscillation.Grid-forming(GFM)converters have effective voltage support capability,and thus equipping renewable energy stations with a certain proportion of GFM converters can improve the stability of the system.However,the analytical relationship between the capacity of GFM converters and the stability margin is unclear,which makes it difficult to estimate the required proportions of GFM converters theoretically.To fill this gap,this paper discusses the estimation method for the required capacity ratios between GFM converters and grid-following(GFL)converters to maintain small signal stability.First,based on the voltage-source equivalent analysis of GFM converters,the influence of the capacity ratios on the system strength and stability margin is analyzed through the generalized short circuit ratio(gSCR)index.Then,considering the practical engineering scenario of updating some wind turbines to GFM control or installing new GFM converters,the typical values of the capacity ratios are discussed based on typical parameters of step-up transformers and the related industry standards as boundary conditions.The validity of the conclusion is verified by simulation results of a typical multi-wind-farm system.
This article proposes a dual-negative-objective coordinated control strategy for brushless doubly fed induction generator (BDFIG) based wind power generation system under unbalanced grid voltage. To alleviate the mechanical stress and impaction on rotating shaft, the negative control objective (NCO) of machine side converter (MSC) is set to suppress the ripple of electromagnetic torque. While the NCO of grid side converter (GSC) is selected to suppress the oscillation of total output active power or the unbalanced degree of total output current for BDFIG generation system. In comparison with traditional single converter control scheme of the MSC or GSC, dual NCOs can be satisfied at the same time due to the enlarged freedom degree in the proposed improved coordinated control system for back-to-back converters. The effectiveness of proposed control strategy is validated by simulation and experimental results on a dual-cage-rotor BDFIG (DCR-BDFIG) prototype.
The modular multilevel matrix converter(M3C)is the core equipment of the low frequency transmission system(LFTS).The two AC systems with different frequency that M3C is connected to are directly coupled in the converter arms.And the arm electrical quantities contain not only the integer harmonics of the fundamental frequencies of both AC sides,but also the non-integer order harmonics generated by their interactive coupling(called differ-frequency coupling harmonics).Ignoring the dynamic DC voltage of the submodule and the differ-frequency coupling harmonics and taking the fundamental frequency as the frequency interval,the multi-harmonic linearization modeling method is difficult to accurately describe the harmonic characteristics of M3C,thus reduces the accuracy of the impedance model.Moreover,it cannot reveal the mechanism how the DC voltage dynamic of the submodule and the harmonics affect the impedance characteristics.In this paper,based on the harmonic analysis of M3C and considering the differ-frequency coupling harmonics,a multi-harmonic vector model in frequency domain is established firstly.Through analyzing the phase sequence and transfer relations of the multi-harmonic small signals of the M3C 9 arms,the M3C power stage model can be reduced to that of one arm,so the impedance modeling complexity is reduced.On this basis,the impedance model of M3C is developed including the effects of control loops coupling that can accurately reflect the impact of DC voltage dynamic and harmonic characteristics.Finally,the accuracy of the M3C impedance model is verified through RT-Lab real-time simulations and sweeping experiments,and the key factors affecting the impedance characteristics are analyzed.
The Ron/Roff model provides high accuracy in real-time simulation of high switching frequency power electronic converters. However, the iterative operations limit the application of the Ron/Roff model in high switching frequency domains. This paper presents a novel non-iterative method for determining the switch statuses of half-bridge arms. By predicting the state variables and correcting the switch status combinations in the natural commutation process with the status switching principle, accurate switch status can be obtained. Besides, this method adopts the backward Euler method to ensure the system stability and realizes the decoupling between the half-bridge arms, which greatly reduces the modeling complexity and calculation amount when judging the switch status. Finally, an ac-dc-ac topology with 100 kHz switching frequency and an LLC resonant converter with 250 kHz switching frequency are implemented on FPGA. The parallel solving architecture enables simulation time step as low as 25 ns. The results derived from the real-time simulation and hardware experiment corroborate low FPGA resource consumption and high precision of the proposed method.
There are many impedance-based methods for analyzing oscillation of a grid-connected converter system(GCS)via dq frame,sequence frame and polar frame,which can be transformed into each other and are equivalent in assessing the stability of the system.However,the discrepancy and adaptation of these methods have not been discussed theoretically.Therefore,the impedance models of GCS are established in above three kinds of coordinates respectively.Then,the mathematical and physical characteristics of them are analyzed.Next,the decoupling of impedance models in different frames is analyzed,and the differences of the impedance criteria in stability analysis and mechanism interpretation are given.Furthermore,the sub-synchronous oscillation caused by the phase-locked loop considering the converter's four quadrant operation scenario are illustrated.The existence of open-loop transfer function right-half-plane poles and its sensitivity to uncertain parameters are analyzed,focusing on the nominal performance and robustness of the impedance criteria.On this basis,the adaptation of several impedance criteria of GCS is analyzed.Theoretical analysis and simulations confirm the validity of the proposed conclusion.
When adjusting the transmission power of the dual-bridge series resonant converter (DBSRC) under the phase-shift modulation, significant step changes in phase-shift angles may occur. These changes stimulate a long-time oscillation process in the resonant tank with a large amplitude appearing in inductor current and capacitor voltage. During this transient process, the converter's dynamic performance deteriorates considerably, and the circuit components may experience severe overvoltage and overcurrent. In view of that, this paper develops a transient process calculation model (TPCM) for the DBSRC using the fundamental harmonic approximation to analyze the oscillations during the transient process. Subsequently, the peak capacitor voltage, the peak inductor current, and the settling time during the transient process are estimated. Further, based on the TPCM, this paper proposes a control method to make the converter reach a steady state after one switching cycle, thus avoiding the risk of overvoltage and overcurrent and greatly improving the dynamic performance of the converter. Finally, the results of the theoretical analysis and the proposed control method are verified by simulations and experiments.
Model-free predictive current control (MFPCC) methods based on look-up tables (LUTs) have been widely applied in voltage source inverters (VSIs) due to their simple implementation and excellent robustness. However, the performance of the VSIs is affected by the voltage vector used, accuracy of current gradients and sampling noise. In order to improve these, a new double-vector MFPCC method is proposed in this paper. First, a synthetic voltage vector composed of double vectors is applied in each control period to reduce current ripples, where the durations of the double vectors are calculated using predicted current gradients. Next, an extended LUT is designed by using the current gradients of the applied synthetic voltage vectors to update all current gradients of eight basic voltage vectors, so as to increase the accuracy of current gradients. Then, the effect of the sampling noise on MFPCC is analyzed in detail and suppressed by a sliding-mode observer (SMO) for the VSIs. Finally, simulation and experimental results have verified the effectiveness of the proposed method.
Silicon/Silicon carbide hybrid switch have been proven to have the advantages of high efficiency and low cost. However, existing research of hybrid switches only focuses on reducing device losses or ensuring reliable operation. In this article, a novel switching strategy based on the driving voltage and switching sequence for the hybrid switch is proposed. Experimental results show that compared with the existing switching strategy of the fixed driving voltage, the proposed switching strategy cannot only ensure a high reliability of the hybrid switch, but also improve the efficiency. Then, the effectiveness of the proposed switching strategy is verified in a single-phase inverter. Further, the efficiency of the single-phase inverter operating with the proposed switching strategy is increased by 0.79% compared with the existing switching strategy.
Under the dual-carbon goal, the operation environment and form of the urban power grid have new characteristics, and the urban power grid will be transformed into a new urban power grid (NUPG), which puts forward higher requirements for flexibility. It is urgent to establish a flexible dispatching system to meet the operation of the NUPG. This paper first introduces the eight major new operation characteristics and challenges of the NUPG. Secondly, a flexible dispatching architecture with the load as the main body is constructed, and the types of dispatching objects, dispatching means and media, and internal and external dispatching tasks are introduced. On this basis, four basic dispatching key technologies of the NUPG are pointed out: load response capacity assessment and distribution technology, load reserve configuration and power plan formulation technology, load participation in power grid frequency regulation/peak shaving/voltage regulation technology, and internal and external fault handling technology based on load resources. Finally, the effectiveness of the NUPG dispatching architecture and key technologies is verified by simulation analysis of the IEEE39-bus system and the actual power grid in East China.
The magnetic circuit theory stands as a pivotal theory for electromagnetic devices like electrical machines and transformers,furnishing indispensable tools for addressing intricate electromagnetic challenges and optimizing the performance of electromagnetic apparatus.In this paper,the historical development of magnetic circuit theory is first traced,providing insights into the valuable contributions made by earlier researchers towards refining magnetic circuit theories,including magnetic circuit parameters and their respective theoretical development.Additionally,their limitations and the challenges they encounter in practical applications are analyzed.On this basis,this paper pioneers the definition of magductance and hysteretance from the fundamental physical properties of magnetic circuit,establishes the vector magnetic circuit theory encompassing three core components(reluctance,magductance,and hysteretance),systematically and completely characterizes the three basic properties of magnetization,eddy current and hysteresis in the magnetic circuit,reveals the intrinsic connection between the virtual magnetic power and the electric power,and puts forward the magnetoelectric power law.An exhaustive comparative analysis with other magnetic circuit theories or modeling methods is conducted to illustrate their interconnections and differences.In order to show the applicative value of the vector magnetic circuit theory in science and engineering,four practical application scenarios are presented.Finally,the unique features of the proposed vector magnetic circuit theory are highlighted and the future research directions are prospected.
Large-scale grid integration of renewable energy sources has led to a continuous reduction in the inertia of the power system, and the spatial distribution characteristics of the system inertia resources are becoming increasingly significant. Most studies on the spatial-temporal distribution of system inertia (SDSI) have focused on inertia estimation. Meanwhile, the representation of the concept of system inertia is not unified and lack the in-depth analysis of its characteristics and the systematic sorting and analysis of related concepts. This paper attempts to explore the representation form of the SDSI characteristics under a major disturbance. The concept of system nodal inertia is proposed and its characteristic description parameters are given. Based on the three-machine equivalent system frequency response (TM-SFR) model, the analytical formula of the nodal inertia is derived. The WSCC 9 bus system is used to verify the analysis and judgment of the SDSI characteristics, which can be characterized comprehensively and normally by the proposed representation form.
This article focuses on the one-phase open-circuit fault in a dual three-phase synchronous reluctance motor (DTP-SynRM). Firstly, a mathematical model of the DTP- SynRM is established, Then the influence of the open-circuit fault on the voltage and current of the DTP-SynRM is analyzed. Afterwards, a fault-tolerant control method based on rotating coordinate transformation is proposed, which converts the secondary current harmonics through rotating coordinate transformation, so that the second current harmonics can be directly controlled by using a PI controller in the new coordinate system. Finally, the feasibility and effectiveness of the proposed fault-tolerant strategy are verified by simulation experiments.
Because the output voltage of the existing high-frequency instrument for the defect detection of long-scale transmission cable is too low, it is necessary to output both high-frequency and high-voltage excitation. A diagnosis and location method of cable defects based on pseudotrapezoidal pulse frequency modulation excitation and impedance spectrum digital reconstruction is proposed. A high-capacity pseudotrapezoidal excitation system based on SiC high-speed inverter is designed to measure the impedance spectrum up to the highest frequency of 7 MHz. Through digital reconstruction, the impedance spectrum with different cable defects can be obtained by the bandwidth extension within the range of 100 kHz–63 MHz. Compared with traditional low-voltage sinusoidal excitation, the location results using the proposed time-space conversion function have a more minor interval oscillation and a faster convergence speed. The accuracy of defect location is improved by 80%, which verifies the effectiveness and accuracy of the proposed method.
When the high-voltage transformerless battery energy storage system system operates at a high proportion of reactive power compensation, the structure of the battery cluster connected to the single-phase H-bridge converter results in that the battery current will reverse in a cycle of twice the fundamental frequency. The battery is charged and discharged with high frequency, which will have a great impact on battery life and battery status monitoring. To solve this problem, this paper proposes a high-proportion reactive power compensation control based on inner-phase active power transferring and the charged and discharged current with twice the fundamental frequency is removed.
The flexible peak regulation of gas-fired units increases the consumption of renewable energy but the resultant sharp gas-pressure fluctuations introduce new security risks to the natural gas network.This paper proposes a natural gas network dynamic security region to represent a set of peak regulation amounts of gas-fired units that satisfy dynamic pipe flow security constraints.First,impractical reasons for the dynamic security region with time-continuous gas loads'mass flow rates(MFRs)as the injection space are analyzed.To this end,the inlet MFR of each gas-fired unit is converted into a function of the active-power adjustment.And the dynamic security region is constructed by taking the discrete active-power adjustment as the injection space referring to the initial operating point.For balancing computational accuracy and efficiency,the continuous variables in the security region boundary search optimization model are discretized by the space-time orthogonal collocation.The gas network steady-state security region is revealed to be a reduced form of the dynamic security region,for it can be obtained by slacking some constraints in the optimization model.Simulation results show that the proposed method can overcome the over optimism of the gas network steady-state security region for intraday security analysis,thus enhancing coordinated situation awareness of electricity-gas coupling systems.
