| In order to economize corridors and increase transmission capacity,the regional power grid began to promote the erection of four transmission lines on the same tower,which also brings new challenges to relay protection.Unlike traditional overhead transmission lines,the electromagnetic coupling relationship between four-circuit transmission lines on the same tower is extremely complicated.There is a competitive relationship between electrical connection and magnetic coupling during the fault.As the fault condition changes,the electromagnetic competition state will also change,which makes the setting work of protection more difficult.According to accident statistics,zero-sequence mutual inductance and cross-line faults are the two major factors that affect the incorrect operation of protection.On the one hand,since the existing four-circuit transmission lines on the same tower in regional power grid are mostly reconstructed lines,its zero-sequence mutual inductance parameters are difficult to obtain.So zero-sequence mutual inductance are usually ignored or roughly estimated,thus reducing the accuracy of protection setting,which is one of the weak links in current protection work.On the other hand,there are many fault types in the four-circuit transmission lines on the same tower,and most of the existing relay protection devices refer to the single-circuit and single terminal electrical quantity to act,so it cannot accurately meet the challenges brought by the cross-line fault,which is another weak link in the current protection work.In engineering,distance protection(phase-to-phase distance,grounding distance)is very important and widely used in overhead transmission networks,but the measured impedance felt by the protection is easily deviated from the actual line impedance under the influence of zero-sequence mutual inductance and cross-line faults,which might cause incorrect actions.In addition,there is no professional evaluation system to grade the security of the protection system comprehensively,which increases the difficulty to discover the potential problems.Therefore,it is urgent to conduct research on the weak links of the protection of the four-circuit transmission system on the same tower,in response to the common requirements of rapid economic development and safety of the power grid.In order to get its accurate value in a convenient way,the zero-sequence parameters of the transmission lines on the same tower when soil resistance,tower structure,height,and sag changes are studied.The results show that the effect of tower structure on mutual zero-sequence inductance is significant,except the accepted soil resistance.In the multi-circuit transmission lines,those circuits which near the earth,whose self-inductance are significantly larger than those which near the overhead ground.Based on that,the law of mutual-inductance/self-inductance ratio influenced by soil resistivity is also studied,and a method of zero-sequence mutual-inductance estimation based on voltage level and tower structure is proposed.PSCAD simulation verifies the robustness of using subdivision ratio to estimate the zero-sequence mutual-inductance of the same tower,and the method's accuracy satisfies the engineering requirement.Focusing on the safety of interphase distance protection in the transmission lines on the same tower,the particularity of the cross-line faults occurred in the common-teminal lines is analyzed firstly.Then,the expression of interphase measured impedance is deduced in combination with the fault zero-sequence additional network.Furthermore,the operation characteristic of interphase distance protection is discussed based on the expression.The study shows that when the sound phase is the same as the fault of the neighboring line,it will exhibit the "interphase short-circuit" fault characteristic together with the fault pahase of its own line,and the interphase measured impedance between the sound phase and the fault phase is a liner function of the fault position and passes through the origin,which can easily make the interphase distance protection to malfunction in the sound phase,causing the serious consequence of "one phase failure,one line being cut off".The PSCAD simulation results of real 220/110 kV four-circuit transmission lines on same tower show that the theoretical analysis is correct,and the corresponding protection measures are given in the end.In order to study the grounding distance protection,this paper firstly derives the expression of measured impedance in the four-circuit line of the same tower,and points out that the measured impedance has nonlinear characteristics with the change of the fault location,and the nonlinear phenomenon is explored.Based on that,the generation mechanism and change rule of nonlinear phenomenon is analyzed,and the surging of measured impedance under certain fault condition is explained.In the end,from the perspective of engineering practicality,the main factors influencing the nonlinear variation rule are analyzed,including power supply combination,line current and the ratio of four lines on the same tower are studied.A protection performance evaluation system for four-circuit transmission lines on the same tower is proposed in this paper.The main structure of the system covers all the erection forms of the current four-circuit transmission lines in the same tower and can be flexibly adjusted to suit the actual line structure.After inputting data such as line structure parameters and grid information,the system automatically runs fault scanning,calculates the corresponding evaluation values of protection performance according to the protection actions under various fault conditions,and provides the evaluation result in the worst case.According to the evaluation result,the rationality of the protection setting,the practicality of the new type of protection principle or countermeasure,the safety of the system operation and the way of overhauling the line,etc,can all be verified. |
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