UA76360C2 - Method for detecting single-phase ground short circuit in a cable multiwire line of an alternating-current isolated neutral system - Google Patents

Abstract

The proposed method for detecting single-phase ground short circuit in a cable multiwire line of an alternating-current isolated neutral system consists in measuring zero-sequence currents in the cables of the line, comparing the current values, and detecting the faulty cable as the cable in which the zero-sequence current is the most distinctive from the zero-sequence currents in the other cables.

Description

Description of the invention

The proposed invention relates to the field of power supply and protection of multi-cable lines and can be applied in alternating current electrical networks with a voltage of 6-35 kV with an isolated neutral.

A well-known and used method in the industry is the detection of a cable line damaged by a single-phase earth fault (033), in which the zero-sequence current of each line cable is measured. If this current exceeds a predetermined threshold (setpoint), then an element intended for this, for example, a current relay, is triggered, and a signal is given about damage to the cable and the line as a whole. This signal can also 70 be used to disconnect the damaged cable line from the electrical network. This method has been covered many times in the literature, for example: 1. Krivenkov V.V. Novella V.N. Relay protection and automation of electrical supply systems. 1981, M.,

Znergoizdat 2. Shabad M.A. Calculations of relay protection and automation of distribution networks. 3rd ed., L., 1985, 19 Znergoatomizdat.

Note that a multi-cable line consists of two or more cables that are connected in parallel, that is, the cores of the same phases of the cables at the beginning of the cable line and at its end are connected to a common electrical contact. Each line cable has its own zero-sequence current transformer (NST). The primary winding of the TSNP is the cable itself, on which the TSNP is "dressed", that is, the cable passes through the hole of the TSNP. Secondary windings of TSNP cables of the line are connected in series or in parallel. If one line consists of four or more cables, then the secondary windings of their TSNP can also be connected in parallel-series, for example, two windings are connected in series, and then two such pairs of windings are connected in parallel. The electrical signal obtained from such a connection of the secondary windings of the TCNP is used to signal about 033 on this particular cable line or is also used to disconnect this damaged line from the electrical network. with

The main disadvantages of the considered method of protection against OZ are as follows: Ge) 1. The appearance of a false (incorrect) signal about 033 on a multi-cable line in the case when this line feeds a substation of a lower level and 033 happened on a line departing from the buses of a lower level substation. 2. The appearance of a false signal about OZ3Z on the mentioned line in the event that the substation busbars are powered by means of multi-cable communication lines from two or more substations, each of which has its own power source - a transformer (for example, 110/b6kV) or a generator. In this case, 033 on any line that Ge) deviates from the 6-10kV buses of the mentioned substations, connected by communication lines, leads to the appearance of a false signal about 033 on one, two or even all communication lines between substations . She

Z. Unreliable operation, i.e. insufficient signal level at OZ3Z on a multi-cable line feeding the final current receiver, in the case of a long line length (0.5...0.7 km). 3o 4. Low-lower than necessary-level of the signal about the OZZ when it appears at the very end of the multi-cable in the line feeding the powerful current collector.

So, as a prototype, we choose the above-mentioned method of protection against single-phase grounding of a multi-cable line in an alternating voltage electrical network with an isolated neutral, in which the zero-sequence currents of the cables of the line being protected are measured and a signal about 033 is issued, which happened only at C 70 exceeding a predetermined threshold (setpoint) by this current. c The main drawback of the known prototype method is that it is based on the measurement of the current

From" the zero sequence that flows through the cable of the line that is protected against 033. But, as can be seen from the above disadvantages 1, 2, the zero sequence current of increased value flows through the cables of a multi-cable line and in the case when none of its cables is damaged, and 033 takes place altogether beyond this line.

It is also known that the total capacitive current (zero sequence) of the power supply area (of one section, i.e. 6-10 kV busbar of a substation or a group of substations) can vary from the smallest to the largest in the range

Ge") approximately 1:4. This means that during OZ3Z in the mode of the lowest current of the zero sequence, the signal level of the secondary windings of the TSNP may be lower than the specified (as set) and the protection system will not work. o As for disadvantages 3.4, the requirement for the value of the coefficient is very often not met here

Gee! 20 sensitivity of relay protection K, which should be K 1.25. This means the unreliability, and sometimes even the failure of the known method of protecting cables from OZ33. c Therefore, the main drawback of the known method of protection against 033 - the measurement of the value of the zero-sequence current of a multi-cable line - does not ensure its reliable and correct operation of the protection of cables against 033.

The invention is based on the task of increasing the reliability of protection of multi-cable lines from OZ3Z3 by obtaining reliable information about OZ on one of the cables of the multi-cable line. The task is set

HF) is solved by the fact that in the method of protection against OZZ3 of a multi-cable line, which is based on measuring the zero-sequence currents of the cables of the protected line and issuing a signal about 033, what happened, according to the invention, the zero-sequence currents of each cable of the multi-cable line are compared and therefore, the current that is most different from other currents is defined as the damaged cable, on which a single-phase 60 short circuit occurred, and the damaged line as a whole.

The essence of the claimed method is explained by the drawing, where Fig. shows a three-phase scheme for replacing a 6kV electrical network with an insulated neutral according to the claimed method.

In Fig. 1 bkV buses of the main substation (GP) with phases A, B, C and the same buses of the 2nd substation of the lower level (PNR) are shown. Buses 1 HP are powered from the secondary winding of a transformer of significant power (for example, 31 MVA, 110/bkV). Cable lines 4.1, 4.2,... 4.p depart from busbars 71 GP, and cable lines 5.1, 5.2,..., 5.p depart from busbars 2 PNR. Each phase A, B, C of lines 4 and 5 has a capacity of C relative to ground 6.

This capacity exists physically, but not materially. Outgoing lines of 4 buses and 1 GP mainly feed substations of a lower level, and partially - own current receivers. Outgoing lines of 5 buses 2 of the PNR feed mostly current receivers and only partially - substations of an even lower level and distribution points (these current receivers and buses of the mentioned substations and points are not shown in Fig.).

Lines that feed substations are usually multi-cable, that is, they consist of two or more cables connected in parallel. One such line 4.t is shown in Fig. This is a communication line, it consists of four cables 4.t.1, 4.t.2, 4.t.3, 4.t.4 connected in parallel through buses 1 and 2 of the GP and PNR. To simplify the capacity of 70 phases of cables 4.t relative to ground 6 in Fig. not shown.

Each phase of cables 4.t is characterized by electrical resistance 7 - 7.1, 7.2, 7.3, 7.4 for cables 4.t.1,..., 4.t.4. This resistance has an active-inductive character. Each cable of the outgoing line - single-cable or multi-cable - is "dressed" with a zero-sequence current transformer TSNP 8. In Fig. such TOSNP 8.1... 8.4 are shown only on the communication line 4.t, on other lines they are not shown for simplicity (lack of space in Fig. 75). Transformers 8 marked "v" ("upper") are installed on cables departing from buses 1 of the upper level, and marked "n" ("lower") - on cables departing from buses 2 of the lower level. Resistor 9 is connected to the secondary winding of each TSNP.

Signal 10 about the appearance of a zero-sequence current, which indicates the appearance of a short circuit in the bkV network, is removed from the resistance part of the resistors 9 and fed to the input of the comparison blocks 11 of buses 1 and 12 of buses 2. For buses 1 of the GP, these are signals 10.1.v, 10.2.v, 10.3.c. 10.4.v6, which are taken from resistors according to 9.1.8. 9.2.v, 9.3.v, 9.4.6 and are fed to the comparison unit 11. For bus 2 PNR, these are signals 10.i.n,..., 10.4.n. which are taken from resistors 9.1.H.,..., 9.4.n and are fed to block 12. Output signal 13 of block 11 and output signal 14 of block 12 appear in case of occurrence of 033 in one of the cables of this particular line, i.e. in one of the cables 4.t.1...4.tp.4.

Let's consider two variants of 033: 15 on the cable of line 5.n of bus 2 and 16 on cable 4.GP.4 of the communication line 4.t between substations (buses) 1 and 2.

In case of occurrence of 033 15 on phase C of cable 5.n of the substation (bus), 2 currents of zero sequence flow i) through the capacitors of the cores of phases A and B of all cable lines 5.1...5.n. Capacitive current sums up on buses 2 6kV and along the cores of phases A and B of communication line 4.t flows to the secondary winding C of transformer bus 1 (phases A and B). Next in phase

C (on which 033 occurred in the network) of this winding and along the cores of phase C of line 4.t the current ОЗ3З3 flows to phase C of bus 2, then to the core of phase C of the damaged line 5.p and finally to the place of shorting - to the point ОЗЗ 15. Part of the capacitive current from the location O3Z 15 flows through the capacitances of the cores of phases A and B of lines 4.1...4.p, which are fed from busbars 1, to the winding ISE) of transformer 3, then through phase C of the winding Z and the communication line 4.t to phase C of line 5.n and to place ОЗ3З 15. The own capacitive current of line 4.t flows along these same paths.

Note that the vast majority of multi-cable lines consist of cables with the same core cross-section, for example, cables with a cross-section of 315 square mm. The length of all cables of one line is the same, therefore the resistance of one core of the same phase of each line cable between buses 1 and 2 is the same. For this reason, the zero-sequence capacitive current (current 033) is distributed evenly along the wires of the same phase of cables 4.t.1, 4.t.2, 4.t.3, 4.t.4. If, for example, the total current OZ3Z of one phase on buses 2 has a value of 40A, then a current of 10A flows through each core of phase A of cables 4.t.1...4.t1.4. The current of phase C (damaged) will be greater than 40A, because "the current of the OZZ lines 4.1...4.2 of bus 1 is added to it, say 20A. Thus, a current of 1.73"60A will flow through phase C of line 4.t with c to place ОЗ33З 15, and a current of 1.783715A will flow through each core of phase C of cables 4.t.1...4.t1.4. ;" Through the hole of TSNP transformers 8.1...8.4 of cables of line 4.t, the difference in the currents of phases A, B - one direction - and phase C - the opposite direction - at each moment of time flows. According to the given figures 40 and 20A, the resulting current of zero sequence through each TSNP 8.1...8.4 has a value of 1.73720:4-8.65 A. - And since the resulting zero-sequence current through each TSNP of the communication line is the same, then the signals on resistances 9.1...93.4 will be the same, as well as the same there will also be signals 10.1...10.4 at the inputs of the comparison units 11, 12. Signals 13, 14 at the output of the units 11, 12 will be zero, which indicates 2) the absence of an OZZ on cables 4.t. 1...4. t.4 of the communication line. The presence of 033 on the 5.p line must be recorded by the 5p Protection against OZZ system of this particular line using the signal of the secondary winding of the TSNP cable (cables) of the 5.p line.

Me, Let's now consider case 033 16 on cable 4.t.4 of the four-cable line 4.t. Capacitive currents from point 33 o 16 - earth b flow through the capacitors of phases A and B of all lines as discussed above, and then sum up on bus C of substation (buses) 1. From this bus C, most of the earth fault current flows through part of resistance 7.4 ("top" in Fig.) of phase C of line 4.t directly into the ground 6 through place OZ33Z 16, and its second part through parallel connected supports 7.1, 7.2, 7.3 of phase C of cables 4.t.1, 4. t.2, 4.t.3 of the same communication line 4.t flows to bus C of bus 2, and then through the second part ("lower" in Fig.) resistance 7.4 of phase C of the damaged cable 4.t.4 flows

Ф) to ground 6 through place 033 16. The signal of the secondary windings of TSNP 8 will be directly proportional to the resulting current of the zero sequence of its primary winding. The components of this current through the holes of TSNP 8 flow in different directions. Let's consider this aspect in more detail. in Capacitive currents on undamaged phases A and B flow from bus 2 from bottom to top in Fig. at some selected moment in time. In the same direction, the own capacitive currents of cables 4.t.1...4.t.4 flow along phases A and B. The resultant current ОЗ3З3 of the damaged phase C from buses 1 flows through the "upper" TSNP 8 from top to bottom (in Fig. .). As mentioned above, part of the effective current of phase C flows through the resistance part 7.4 of the cable 4.t.4 (phase C) directly to the ground 6.

The rest of it is evenly distributed between the resistances of phase C of cables 4.t.1, 4.t.2, 4.t.3 and also flows 65 From top to bottom (in Fig.) to bus C of bus 2, where these currents are added again and flow from the bottom up through the TSNP 8.4.n and the "lower" part of the resistance 7.4 of the damaged phase C of the cable 4.t.4 to the ground 6 through the place of damage 16.

From this consideration, it can be seen that the resulting zero-sequence currents through TSNP 8.1.v, 8.2.v, 8.3.8v are the same. They are also the same for TSNP 8.1.n, 8.2.n 8.3.n, although they have a slightly different value. The resulting zero-sequence current through TSNP 8.4.v differs from other "upper" TSNPs - it is smaller than in TSNPs 8.1.v...8.3.v. This difference is even greater for TSNP 8.4.n, because here the zero-sequence currents of phases A, B, and C flow in the same direction (from bottom to top), while for other TSNPs, the zero-sequence currents of phases A,

B, on the one hand, and phase C, on the other, flow in opposite directions, and therefore the effective zero-sequence current in them is much smaller than in TSNP 8.4.n. Therefore, the signal 10.4.v on the resistor 9.4.v (damaged cable 4.t.4) will be smaller than the signals 10.1.v...10.3.v. on resistors 9.1.v...9.3.v (undamaged cables), and the 7/0 signal 10.4.n on resistor 9.4.n will be significantly larger than the same signals 10.1.n...10.3.n on resistors 9.1.n. ..9.3.n. This difference is fixed on comparison units 11 and 12, after which information about 033 appears on their outputs 13 and 14, specifically on cable 4.t.4 of communication line 4.t.

Note that the specified difference increases if bus 2 also has its own power source: a generator or the secondary winding of a powerful transformer (110/6kV, 220/6kV, etc.).

We also note that the uneven resistance of the cores of different cables of a multi-cable line can be easily compensated by scaling the output signals 10, which are removed from the resistors 9 connected to the secondary windings of the TSNP 8.

Experimental testing on real electrical equipment of the described method of protection fully confirmed the positive effect achieved by the claimed method.

Claims (1)

The formula of the invention is a method of determining a single-phase short-circuit to the ground in multi-cable lines of an alternating voltage electrical network with an isolated neutral, which includes measuring the zero-sequence currents of the line cables and issuing a signal about a single-phase short-circuit to the ground that has occurred, which differs in that the currents are compared (about ) of the zero sequence of each cable of a multi-cable line and by the current that is most different from other currents, both the damaged cable, on which a single-phase short circuit occurred, and the damaged line as a whole are determined. o (Se) (ze) (Se) - - and? -i (o) (95) (o) (42) name) 60 b5