| With the fast development of domestic economic and growth of power load, therequirement of improving the transmission capacity of power grid becomes more and moreurgent. However, the contradiction between construction of transmission corridor andprotection of arable land resources is very serious, and it has been the main limitation of thedevelopment of power gird. Hence, it is of important economic value and socialsignificance to improve the capacity of transmission lines without adding transmissioncorridor. In recent years, due to its advantage in large transmission capacity and saving oftransmission corridor, parallel transmission lines on same tower have been widely adoptedin the domestic power gird. The parallel transmission lines on same tower have thefollowing characteristics: small distance between lines, complicated coupling condition, agreat variety of fault types and various operation modes. These characteristics make thetraditional parameter calculation method, digital simulation method, physical modelingmethod, current differential protection and auto-reclosure scheme which are applied onsingle transmission line and parallel transmission lines can not be applied on paralleltransmission lines on same tower directly. Moreover, limited by transmission corridor,incomplete-journey parallel transmission lines on same tower with different structures existin power transmission system. Diversification of the construction mode of transmissionlines brings new challenge for fault location. Therefore, it is of important significance tostudy the relay protection and fault location technology for parallel transmission lines onsame tower specifically. Thus, the key technical problems of parallel transmission lines onsame tower, like parameter calculation, symmetry analysis, digital simulation model,construction of physical model, current differential protection, adaptive reclosure, faultlocation and so on, are studied and discussed in this dissertation.According to the characteristics of parallel transmission lines on same tower, aparameter calculation method based on “single conductor-ground” circuit is proposed. Thismethod effectively overcome the disadvantages that existing parameter calculation methodsfor single transmission line and parallel transmission lines have in reflecting complicated coupling condition and change of operation condition, and it is applicable for paralleltransmission lines on same tower with different construction modes. The comparison of theline parameters calculated by this method and the measured line parameters shows that thismethod is flexible and convenient and its result has high accuracy which can satisfy therequirements of engineering application. Based on it, the influences of three classicalphase-sequence arrangement modes on symmetry of sequence impedance is analyzed andcompared. Then, some suggestions about the phase-sequence arrangement mode andtransposition of parallel transmission lines on same tower are proposed.A simulation modeling method of parallel transmission lines on same tower based ontower structure parameters is proposed. The proposed method solves the problem oftraditional parallel transmission lines model that it can not reflect the asymmetry of lineparameter. And the method can simulate the simple fault conditions and inter-line faultconditions of parallel transmission lines on same tower accurately. With the combination oftheoretical analysis and digital simulation, a physical modeling method of paralleltransmission lines on same tower based on the principle of minimum mutual impedance isproposed. Based on the coupling between different conductors of the two lines, the physicalmodeling method can reflect the difference of mutual inductance between different phasescompletely and effectively. It overcomes the disadvantage of existing physical models thatthey can only reflect zero-sequence mutual inductance between lines. This method isapplied to develop a physical model for a practical parallel transmission lines on sametower. The results of digital simulation and dynamic analog test show that the model cansatisfy the design requirements. And it can simulate the fault transient characteristics ofparallel transmission lines on same tower. Hence, it can be used for the test of relayprotection principle and equipments for parallel transmission lines on same tower.A novel differential protection scheme for parallel transmission lines on same towerbased on on-line calculation of positive-sequence capacitor is proposed. This schemeeffectively solve the problem of inaccurate capacitor current compensation caused by theinfluence of operation environment and operation mode on distributed capacitor of paralleltransmission lines on same tower. The whole performance is optimized by the coordinationof multi differential criteria. An appropriate higher threshold is adopted by segregatedcurrent differential protection to avoid the influence of the unbalanced current caused by the change of zero-sequence distributed capacitor, to reflect all kinds of internal severe fault.The accuracy of capacitor current compensation of fault component current differentialcriterion and negative-sequence current differential criterion is not affected by operationenvironment and operation mode. Hence, the compensation effect is good and the securityof criteria is high. Because both criteria consist of fault component, they are not affected byload current, have good sensibility of internal fault and can effectively make up thedisadvantage of segregated current differential protection that its sensibility of internalminor fault is not good enough. Results of digital simulation and dynamic analog test verifythe effectiveness and correctness of the novel scheme.A thorough adaptive auto-reclosure scheme for parallel transmission lines on sametower without shunt reactors is proposed. The scheme consists of adaptive auto-reclosurecriterion based on voltage phase, adaptive auto-reclosure criterion based on voltagemagnitude, arc extinction criteria of secondary arc and phase by phase reclosure method. Itovercomes the disadvantages of single auto-reclosure criterion in application, and it canidentify the fault characteristics of parallel transmission lines on same tower and arcextinction time of secondary arc when single phase grounded fault and inter-line fault ofsingle phase to single phase happen. Furthermore, for parallel transmission lines on sametower with shunt reactors, the characteristics of switched-off phase current and currentthrough the small reactor of neutral point are analyzed. Learning from the identificationmethod of fault characteristics of single transmission line, an identification scheme ofinstantaneous and permanent fault on parallel transmission lines on same tower with shuntreactors is proposed. In the end, the effectiveness and correctness of the proposed adaptiveauto-reclosure scheme for parallel transmission lines on same tower is verified by digitalsimulation and dynamic analog test.The error of the existing fault location scheme when it is applied onincomplete-journey parallel transmission lines on same tower is analyzed. And atwo-terminal fault location method which is suitable for incomplete-journey paralleltransmission lines on same tower is proposed. Based on the characteristics ofincomplete-journey parallel transmission lines on same tower, the proposed method dividesthe line into different sections in accordance with construction mode, and calculates thevoltage along transmission line from the two end of line with the parameter matrixes of different sections. The method is not affected by the line symmetry and construction mode,and it overcomes the disadvantage of existing fault location schemes for paralleltransmission lines on same tower that they can only be used on symmetrical line. Digitalsimulation results verify that this scheme is applicable for incomplete-journey paralleltransmission lines and it has high accuracy.In the end, the dissertation gives systematical summary and points out the furtherwork. |
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