Fault location method for a parallel two-circuit transmission line with n terminals

A method of locating where a fault lies in a parallel, two-circuit transmission line in an n-terminal system, having at least a sending end and a receiving end a plurality of circuits and a plurality of branches connected to the transmission line at branch points, the method comprising the steps of:

• detecting physical current signals of each phase of each end of said circuit of the parallel, two-circuit transmission line;
• converting each detected physical current signal into digital data;
• storing the digital data in a memory;
• transmitting the digital data stored in said memory to a predetermined main station;
• storing the transmitted digital data of currents of all the ends in a memory in said main station which also contains connection information of the n-terminal system and data about the lengths of different portions of the transmission line;
• determining, by the main station, a distance to the fault from one of the ends of the transmission line based on the connected information of the system and the data about lengths of the transmission line portions, the distance determining step including the substeps of:
• (a) performing equivalent transformation with respect to a branch point b.sub.k of said parallel two-circuit transmission line, to transform said transmission line into a T three-terminal parallel two-circuit transmission line circuit having three terminals T.sub.k-1, T.sub.k, and T.sub.k+1, said equivalent circuit indicative of a distance from a branch point b.sub.k to a first terminal connected to said branch point b.sub.k,
• (b) setting a value x.sub.k as a length on the equivalent circuit from a terminal connected to said branch B.sub.k to the fault on the equivalent circuit, on the basis of said branch lengths and said detected current signals flowing out of terminals T.sub.k-1, T.sub.k, and T.sub.k+1,
• (c) if x.sub.k is less than the length on the equivalent circuit of said branch B.sub.k, identifying the value of x.sub.k as the closest distance on the parallel two-circuit transmission line from said first terminal connected with said branch point b.sub.k to the fault,
• (d) is x.sub.k is not less than the length on the equivalent circuit of said branch B.sub.k, setting a value x.sub.k-1 as a length on the equivalent circuit from a terminal connected to a branch B.sub.k-1 to the fault on the basis of said branch lengths and said detected current signals flowing out of terminals T.sub.k-1, T.sub.k, and T.sub.k+1,
• (e) if x.sub.k-1 is less than the length on the equivalent circuit of branch B.sub.k-1 and if the branch point b.sub.k is connected to a second terminals with no branch points therebetween, identifying x.sub.k-1 as the closes distance from said second terminals to the fault,
• (f) if x.sub.k-1 is less than the length on the equivalent circuit of branch B.sub.k-1 and if the branch point b.sub.k is not connected to a second terminal with no branch points therebetween, performing equivalent transformation of the parallel two-circuit transmission line with respect to a branch point b.sub.k-1, to transform said transmission line into a T three-terminal parallel two-circuit transmission line circuit having three terminals T.sub.k-2, T.sub.k-1, and T.sub.k, said equivalent circuit including at least one branch B.sub.k-1 having a length on the equivalent circuit indicative of a distance from said branch point b.sub.k-1 to a first terminal connected with said branch point b.sub.k-1, this substep including the additional substeps of
• (f1) setting a value x.sub.k-1 representing a length in the equivalent circuit from a terminal connected to said branch B.sub.k-1 to the fault on the equivalent circuit, on the basis of said branch lengths and said detected signals flowing out of terminals T.sub.k-2, T.sub.k-1, and T.sub.k,
• (f2) is x.sub.k-1 is less than the length on the equivalent circuit of said branch B.sub.k-1, identifying the value x.sub.k-1 as the closest distance on the parallel two-circuit transmission line from said first terminal, connected with said branch point b.sub.k-1, to the fault,
• (f3) if x.sub.k-1 is not less than the length on the equivalent circuit of said branch B.sub.k-1, setting a value x.sub.k as a length on the equivalent circuit from a terminal connected to branch B.sub.k to the fault on the basis of difference currents,
• (f4) is x.sub.k is less than the length on the equivalent circuit of said branch B.sub.k, calculating the distance from said branch point b.sub.k-1 to said fault on the basis of said branch lengths and said detected signals flowing out of terminals T.sub.k-2, T.sub.k-1, and T.sub.k,
• (f5) if x.sub.k is not less than the length on the equivalent circuit of said branch B.sub.k, setting a value x.sub.k-2 representing a length on the equivalent circuit from a terminal connected to a branch B.sub.k-2 to the fault on the basis of said difference currents,
• (f6) if x.sub.k-2 is less than the length on the equivalent circuit of branch B.sub.k-2, and if the branch point b.sub.k-1 is connected to a second terminal with no branch points therebetween, identifying x.sub.k-2 as the closest distance from said second terminal to the fault,
• (f7) if x.sub.k-2 is less than the length on the equivalent circuit of branch B.sub.k-2 and if the branch point b.sub.k-1 is not connected to a second terminal with no branch nodes therebetween, performing equivalent transformation of the parallel two-circuit transmission line with respect to the branch point b.sub.k-2, to transform said transmission line into a T three-terminal parallel two-circuit transmission line circuit having three terminals T.sub.k-3, T.sub.k-2, and T.sub.k-1, said equivalent circuit including at least one branch B.sub.k-2 having a length on the equivalent circuit indicative of a distance from said branch point b.sub.k-2 to a first terminals connected to branch point b.sub.k-2, and
• (f8) decrementing k by one and repeating steps (f1)-(f8) until the closest distance to the fault is identified,
• (g) if x.sub.k-1 is not less than the length on the equivalent circuit of branch B.sub.k-1 and if the branch point b.sub.k is connected to a second terminals with no branch points therebetween, setting a value x.sub.k+1 as a length on the equivalent circuit from a terminal connected to branch B.sub.k+1 to the fault on the basis of said branch lengths and said detected signals flowing out of terminals T.sub.k-3, T.sub.k-2, and T.sub.k-1 and identifying x.sub.k+1 as the closest distance from said second terminal to the fault;
• and
• (h) if x.sub.k-1 is not less than the length on the equivalent circuit of branch B.sub.k-1 and if the branch point b.sub.k is not connected to a second terminal with no branch points therebetween, incrementing k by one;
• and
• locating the fault point using the closest distance identified in the distance determining step.