| With development of economic in the country, the electric power network is built towards long distance, large scale and EHV level. Therefore, as an important electric power transmission mode, the double circuit lines on the same tower have been used, because it may increase the transmission capacity of each way, reduce used ground and save cost. The protective relay is very important factor to avoid unselected trip under complex faults between double circuit lines, and to enhance the reliability of power supply. Because of limited protective principle and poor communication technology, earlier protection used for double circuit lines is insufficiency in characteristic, often needs to use all voltages and currents of two circuits, thus it often causes inconvenience when it is used. In this thesis, the research on pilot distance protection for double circuit lines on the same tower using multiphase channel is presented, and type CSL-IOIC(D) digital pilot distance protection devices using voltages and currents collected at only the local end of a single-circuit are implemented. After the performances of some usual fault phase selectors under cross country faults occurred in double circuit lines are analyzed, a new phase selection scheme is proposed. It is also mentioned that the short time conversion fault is identified, which is based on fuzzy set theory, in order to avoid incorrect action of relay in such fault condition. For the correct trip of distance protection affected directly by impedance characteristic, its behavior under faults in double circuit lines on the same tower is analyzed in detail, and some novel schemes are introduced. The advancement of digital communication technology may make the channel mode of pilot protection diversify, and make the characteristic of channel enhance. Therefore, a novel channel scheme of protection for double circuit lines on the same tower using direct digital communication technology based on asynchronous communication is offered. The main contents of this thesis are:1. The fault phase selection is a key factor whether the fault line may be cut off according to selected phase by pilot protection. The research on the behavior of the phase current difference suddenly-change and the sequence current elements under various cross country faults in double circuit lines making use of the test results in Real Time Digital Simulator is provided. The error phase may be selected by above two elements under such fault conditions, due to effect of fault current of adjacent circuit. However, based on analysis of these faults, a new fault phase identification method isproposed for the first time, which is based on comparing the difference of fault voltages and judging the phasor condition between the fault currents. The results of analysis and test indicate that it is effective to pick up fault phase, and exact fault phase can be selected at one side at the least. The fault phase selector of new protection in this thesis is integrated by three components mentioned above.2. A new impedance calculation method based on identification for non-ground cross country faults is offered and realized firstly. Because the single phase fault with non-ground between double circuit lines is not real phase-ground fault in deed, there will not be right behavior for traditional ground distance relay under such fault conditions. However, the new method can make the ground distance relay trip correctly under above faults. Furthermore, the research on characteristics of ground distance and phase distance under various cross country faults is presented, and its setting volumes can be set according to normal standard of setting, the zero-sequence current from adjacent circuit should not be introduced for several reasons.3. A short time conversion fault identification method based on fuzzy set theory is proposed, in order to resolve the problem that the distance relay may cut off three phases incorrectly under outside fault shifted to inside single fault by short...
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