Study On On-line Power-tapping Method Of HV Overhead Transmission Line

Safe,reliable,economic and efficient operation of power grid relies on on-line state monitoring,and the application and development of on-line monitoring technology require economic and reliable powering approach for the monitoring devices.Currently the devices are powered mainly with method of“solar energy plus battery”,or through bus-type induction coil getting power from conductor.However,the former method has deficiencies such as large size and unstablity of output power owing to environment performance,and the latter is only limited to applications at high-voltage side.With development of strong smart state grid,the powering technology has increasingly become one of the bottlenecks restricting the development of on-line monitoring technology of the transmission line.In view of the key to the feasibility of online power tapping（PT）lies in the power and engineering applicabilility,the electromagnetic field energy distribution around line are analized theoretically and systematically,and in combination with engineering feasibility,PT approaches oriented to on-line monitoring aiming at transmission line are proposed.Through building equivalent calculation circuit or PT model,equivalent circuit etc.,PT powers and concerned inluential factors of these methods are analyzed.And aiming at different line conditions and power demands,selection and control strategy of these PT methods is proposed for typical AC transmission line,while for DC line PT strategy is proposed mainly for promoting power.Then,the analysis for PT methods are verified with PT devices produced through optimized design and successfully applied to actual line,or through laboratory simulation test.The main research works and innovation points of the dissertation are listed as follows:First,from perspectives of electrostatic field,vortex field and magnetic field around the line,PT theories and methods are systematically analyzed aiming at typical AC overhead transmission line,coming out with points:in consideration of quite high current,PT from conductor with current transformer（PTCTC）is available;according to electrical topology of segmented groundwire（SG）,PT from SG respectively through vortex electric field and electrostatic field are available;and according to the electrical topology of continuously grounded groundwire（CG）,PT from CG with CT（PTCTCG）is also available under certain condition.Among them the groundwire-related ones are proposed for the first time.Based on the equivalent circuit model of the methods,the power maximum and influential factors are analyzed,and the selection and control strategy of the ground wire related approaches are proposed.Finally,PT devices are made via optimized design and applied successfully on an 500 kV Double Circuit line,which verifies the PT methods from SG respectively through vortex electric field and electrostatic field.Besides,PTCTCG is also verified through simulation test.Results show that the power maximum of these PT methods varies distinctly.For a220 kV line with load current being 200 A,span 400 m and segment length 10 km,the maximum power can reach up to 100 watts for PTCTC,56 watts or 10 watts for that from SG respectively based on static and vortex electric field（with PT circuit covering line of 400 m long）,and about 1 watt for CTPTCG（CT parameters:inner diameter=40mm,outer diameter=65 mm,height=65 mm,μ≈1.65×10⁴μ₀）.However,it’s inappropriate to pursue a PT approach with power higher than needed because the method with lower power has higher operation reliability and engineering feasibility,etc.Besides,besides load current and line voltage,line structure and location also play important role on the PT power（maximum change may above 50%）,which varies with PT methods.Therefore,on-line PT device design should be refined to maximize the PT power according to effects of line structure and PT location.Seeing the PT device is prone to lightning strike,the lightning protection scheme is optimized and key components parameters as well as their match are analyzed through both theoretic and simulation analysis,and then simulation test is conducted to verify the optimized scheme.Results show that PT device could be protected against lightning strike through a scheme of step-by-step discharge of lightning current.And in this scheme high-low combination of reference voltage of surge protection devices performs best,where the gap between their reference voltage being moderate.Then,aiming at on-line PT of HVDC transmission line,it is proposed to obtain power from insulator leakage current from the perspective of avoiding large device size based on the performance of the static electric field around line.Then the PT scheme is optimized to obtain reasonable energy and output voltage.Calculations indicate that the proposed PT method could obtain about 173 J energy from the line within 24 hours（insulator current takes 10μA）,which could power the on-line monitoring devices periodically at least 35 times（5 J consumed per time）,namely allow the device to sample and transmit data every about 40 minutes.Besides,In comprehensive considerations of stable voltage,charge transferring speed and efficiency,the selection and cooperation of capacitor parameters are analyzed,results show that the capacitance of the primary capacitor should be far greater than of the secondary capacitor.On this basis,aiming at promoting power or energy,PT strategy is analyzed which shows that the voltage of both the primary capacitor and the secondary capacitor should be as close to their upper limit as possible.Finally,simulation test are conducted in the artificial climate lab aiming for the basis and the key loop of the scheme.It turned out that the proposed PT method could obtain energy repeatedly and effectively from the line.Meanwhile,the insulator leakage current is consistent with actual line basically which further proved that the proposed PT approach of DC transmission line is feasible.To summarize,for both AC and DC overhead transmission line with different operation environments and various structures,online power tapping can be realized through systematic analysis and optimized design,and the equipment power,safety and reliability can be ensured under propriate lightning protection measures.