| The fast-growing electric power demand has raised the necessity of transmitting power through very long distances from power plants to load centers.Half-wavelength transmission(HWLT)system is a kind of three-phase AC power transmission that the line length between its sending-end(SE)and receiving-end(RE)is about 3000km for 50 Hz or 2500km for 60 Hz.Half-wavelength transmission system has attracted considerable attention with advantages for not requiring reactive compensation or intermediate substations between the terminals.In extreme long distance transmission scenarios half-wavelength technology has been considered as another alternative except for high-voltage direct current(HVDC)technology.Half-wavelength transmission technology has been studied for over half a century.Some existing research results have found particular characteristics and behaviors based on simulation analysis.Low power consumption demand and technology bottle-neck discourages the further development of this technology and there are lack of in-depth analysis and mechanism explanation.This paper focuses on the mechanism analysis of unique operating characteristics of the half-wavelength transmission system and the interaction between lines and systems.It attempts to find the key technical factors which restricting its development,and proposes some solutions.In this paper,the characteristic voltage problem of half-wavelength transmission line is taken as the main issues,and the steady-state voltage and fault voltage characteristics are analyzed in detail.An improved secondary voltage control method suitable for half-wavelength transmission systems is put forward.Considering the interactive influence of different transmission scenarios,corresponding voltage control strategies are proposed respectively for "point-to-grid","grid-to-grid"transmission modes.Based on transient characteristics analysis results,the transient power angle stability mechanism and its influencing factors are explained,and then the transient stability detection indexes are proposed and verified in half-wavelength transmission system.The main research contents and contributions of this paper are listed as follows:(1)The steady-state and fault voltage characteristics of half-wavelength lines under ideal conditions are explained based on wave process method.Considering the distribution parameters of long transmission line,the steady-state voltage distribution of the half-wavelength line and its influencing factors are analyzed.Then,the distribution regularities of charging power and reactive power loss are derived.In addition,fault voltage characteristics are analyzed with load-rejection or short-circuit conditions based on fault equivalent circuit.The series resonance overvoltage will happen when short-circuit faults occurs at certain position and its peak value is influenced by grid strength or fault type.Furthermore,a refined transmission line model is presented to consider non-ideal factors and simulation results demonstrate that geomorphic conditions and electromagnetic environments influence line voltage characteristics in a certain extent.The analysis results show that the voltage distributions along the half-wavelength line are significantly affected by the transmitted active power and power factor.A low power factor can cause the steady-state overvoltage,which brings new challenges for voltage control.An internal short-circuit fault occurring at a distance of about 100 to 400 km from terminals may cause a series resonance overvoltage.The most severe overvoltage occurs at 1500km away from the fault point.The electric strengths of two sides systems have a significant effect on the fault characteristics.The resistance component of the equivalent impedance has a significant effect on the overvoltage value.It shows the feature that smaller equivalent resistance leads to the greater overvoltage.The reactance component has a more significant effect on the severe fault location.The smaller equivalent reactance of equivalent system,the severe fault location is closed to line terminals.(2)An improved secondary voltage control method for "point-to-grid" UHV half-wavelength transmission mode is proposed suitable for large-scale clean energy transmission scenarios.In order to avoid the obvious time delay and signal noise caused by long-distance control signal propagation,an independent voltage control mode for both sending-end and receiving-end is proposed as the basic control method.The control logic is completely based on the real-time electrical information at terminals of half-wavelength line.Aiming at "point-to-grid" transmission structure.the maintenance of the load node voltage level and the suppression of half-wavelength line steady-state overvoltages are both considered.The sending-end control zone and receiving-end control zone are properly appointed for implementing the different control objectives.Besides,in specific operating conditions.the cooperative control mode for the sending and receiving control zones is addressed to make full use of voltage/reactive power supporting ability of the sending-end generators.An over-voltage risk assessment method for half-wavelength transmission lines is put forward.Then,the differentiated voltage control mode switching and specific voltage control activation conditions are proposed based on the over-voltage risk level.The simulation results demonstrate that the improved voltage control method is adaptable to different wind power penetration operating conditions,and it also has a good control effect in one typical day.(3)An improved secondary voltage control method for "grid-to-grid" UHV half-wavelength transmission mode is proposed.In order to ensure the reliability and effectiveness of voltage control,the independent voltage control logic at the sending and receiving ends is continued with "point-to-grid" system.Variable pilot bus voltage reference calculation method is given according to terminal electrical quantity information such as voltage amplitude and power values,which makes contributuions to prevent the voltage oscillation caused by the unreasonable voltage reference value.By setting specific voltage control starting conditions,only one side of the sending and receiving ends is allowed to start voltage control at one moment to avoid control direction errors.A multi-objective optimization model is established to realize the limitation of reactive power at terminals of half-wavelength transmission line and tracking the pilot bus voltage as reference value at the same time.The priority of different objectives is determined according to the violation situation of controlled variables.Simulation results of a 7-machine-32-bus test system demonstrate that voltages of pilot buses for both control zones can be regulated at the acceptable level and the voltage profiles along half-wavelength line become flatter in different disturbance conditions.(4)The transient power angle stabilization mechanism of half-wavelength power transmission system is analyzed.The influencing factors of transient stability such as fault position and fault clearing time are quantitatively calculated,and the range of severe fault areas that resulting in instability is given.The phase-plane trajectory convexity-concavity is adopted as a basis for power angle stability dectection of half-wavelength power transmission system,which avoiding the influence of a large initial angle difference of a half-wavelength line.The simulation analysis is based on the electromechanical transient calculation model considering the detailed model of generators.Simulation results show that there is a 50?100km fault area that may cause power angle instability near the most severe fault point,and the fast fault clearing may improve the system stability after disturbance and reduce the instability fault range.The transient stability judgment indexes based on real-time response phase-planes of generators can accurately determine the system stability status.This paper focuses on the most special voltage problems of half-wavelength transmission lines as the starting point.The operating characteristics and steady-state voltage control problem have been studied.Then,it extends to the consideration of transient angle stability.The contributions of this paper can provide valuable theoretical and design basis for the development and application of UHV half-wavelength power transmission technology. |
PDF Full Text Request