RU2788519C1 - Method for disconnecting a damaged connection with a single-phase earth fault in networks with an isolated neutral - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to the field of electrical engineering. According to the invention, the values of the active resistances of three ballast elements are determined, which are connected to the three-phase switch of the backup cell, connected to a star, the neutral of which is connected through the active resistance to the ground loop of the switchgear. The zero sequence current relay protection settings of all connections are formed, which generate a signal to disconnect the connection with a single-phase ground fault after the backup cell switch is turned on when a zero sequence voltage signal appears on the busbar section from the voltage transformer through the relay. The relay has a time delay, which allows to turn off the damaged connection after a specified time after the occurrence of the SEF.

EFFECT: increasing the reliability of determining the connection with a single-phase earth fault (SEF) in electrical networks of 6-35 kV with an isolated neutral.

1 cl, 3 dwg, 3 tbl

Description

The invention relates to electrical engineering, in particular to methods for determining the connection with a single-phase earth fault (OSZ) in electrical networks 6-35 kV with isolated neutral.

The task of determining the connection with SPZ in 6-10-35 kV networks currently does not have an effective solution despite the dynamic development of relay protection technology and an increase in the installed fleet of microprocessor devices at power facilities.

In real conditions at power facilities, until now, when a fault occurs on the bus sections, a non-selective alarm is triggered by an increase in the zero sequence voltage. At the same time, there is no clear definition of a feeder with a ground fault. If the busbar section is fed from an upstream substation, then the same non-selective signaling is triggered on several 6-10-35 kV switchgears. The search for SPZ is carried out by the method of successive switching off / on of all 6-10-35 kV feeders at all switchgears. These switchings affect the operation of consumer technological equipment, load the operating personnel with unproductive work and increase the time to eliminate the circuit.

A known method for determining the connection with a single-phase earth fault (OSZ) based on the measurement of the zero sequence current (CCR) of each line (Krivenkov V.V., Novella V.N. Relay protection and automation of power supply systems. 1981, M., Energoizdat; Shabad MA Calculations of relay protection and automation of distribution networks, 3rd ed., L., 1985, Energoatomizdat). The selectivity of such protections is ensured by detuning from the own capacitive current of the connection. The disadvantage of this method is that in some cases it is impossible to select the protection setting from the condition of the relay failure with an external short circuit fault and the condition of operation (sensitivity) at the protected connection. In practice, this leads to a “roughening” of the setpoint and, as a result, to false trips of serviceable protections.

A known method for determining a damaged connection in case of a single-phase ground fault in electrical systems with isolated or compensated neutral, implemented by the device according to the patent of the Russian Federation No. Why are they connected in the backup (standard) cell of the switchgear, it is also the switching cell of the device according to the specified patent, a high-voltage ballast resistance (betel resistor) and an ammeter, and at certain points of the outgoing power lines, the clamps of the measuring cell portable cable are connected. One of the two disconnectors of the measuring cell is switched, namely the one that is necessary for making measurements on the grounded phase. This prepares the circuit for a limited short-circuit current to flow through the earth, a high-voltage ballast, an ammeter and two phases of the supply line connected in parallel, one of which is the one on which the connection to earth is fixed, and the second is one of the other two phases. Then, at the consumer, one of two specially mounted disconnectors between phases A and C or B and C is turned on, the switch of the switching cell is activated. As a result of a perfect, time-limited single-phase short-circuit currents will flow through the place of a ground fault in two phases of the line, which are recorded by the recording system. After a time delay of 0.2-0.5 seconds, the switching cell switch is turned off.

The main disadvantage of the known method is the need to turn off the power transmission line of the consumer, as well as a large amount of organizational and technical measures to implement this.

A known method for determining a damaged connection in case of a single-phase ground fault in electrical systems with an isolated or compensated neutral, implemented in a method for determining the location and distance to a single-phase ground fault in 6-35 kV electrical networks with an isolated or compensated neutral (RF Patent No. 2293342, G01R31/08, publ. 10.02.2007, bull. No. 4), consisting in the fact that the ballast resistance is connected to the reserve cell of the busbar section of the switchgear, in which there is a ground fault. The high-voltage switch of the reserve cell is switched on, ensuring the flow through the ballast resistance of a two-phase short circuit current limited in magnitude and time. At the time of switching on, the readings of the devices for visual fixation of current surges are monitored and, by the presence or absence of current surges on visual fixation devices, the place of a single-phase short circuit to ground is judged.

The disadvantage of this method is the need to manually turn on the ballast resistance and visual fixation of current surges in a short period of time (no more than 1 second), which may be difficult due to the inertia of the ammeter components.

A known method for determining a damaged connection in case of a single-phase ground fault in electrical systems with isolated or compensated neutral, implemented in the method for determining the location and distance to the site of a single-phase ground fault in 6-35 kV electrical networks with isolated or compensated neutral (RF Patent No. 2685746 G01R31 /08, publ. 10.02.2007, bull. No. 4), consisting in the fact that a relay protection device is installed with the ability to register the phase currents of each connection and the backup cell of the switchgear, as well as with the possibility of fixing single-phase earth faults in each connection of the distribution devices, creating by supplying to the switch of the backup cell signals of a two-phase short-circuit current limited in magnitude and time, using a relay protection device, they determine the connection with a single-phase short circuit to earth and a damaged phase, connect the ballast resistance to the backup cell s busbars of the switchgear, in which there is a ground fault, turn on the high-voltage switch of the reserve cell, ensuring the flow through the ballast resistance of a two-phase short-circuit current limited in magnitude and time, during switching on, the values of the current are recorded through the ballast resistance and in the damaged phase of the connection with a single-phase circuit to the ground.

The disadvantages of this method are the complexity of organizational and technical measures and the need to form a two-phase short circuit, which can cause the development of an accident in other connections.

The prototype is a method for determining the location and distance to the location of a single-phase ground fault in electrical networks 6-35 kV with isolated or compensated neutral (RF Patent No. 2685747, G01R 31/08, publ. in that the ballast resistance is connected to the reserve cell of the busbar section of the switchgear, in which there is a ground fault, the high-voltage circuit breaker of the reserve cell is turned on, ensuring the flow of a short circuit current limited in magnitude and time through the ballast resistance, during switching on, the current values \u200b\u200bare recorded in the damaged connection phase with a single-phase earth fault. According to the method, a relay protection device is installed with the possibility of registering phase currents and phase voltages of each connection, as well as with the possibility of fixing single-phase earth faults in each connection of the switchgear, creating, by supplying a backup cell switch, signals of a short circuit current limited in magnitude and time in connection with a single-phase earth fault and a damaged phase, determine the phase currents of the undamaged phases of the connection with a single-phase earth fault and the phase voltage of the damaged phase using a relay protection device, determine the phase currents and voltage of the damaged phase of the connection with a single-phase earth fault registered during the flow of a time-limited short-circuit current of the undamaged phase, form a signal to turn off the damaged connection.

The method is implemented as follows. The occurrence of a short circuit fault on a damaged connection of a switchgear is determined using a relay protection device that is capable of implementing known technical solutions (methods) for detecting a short circuit fault, as described, for example, in [Shuin V.A., Gusenkov A.V. Ground fault protection in electrical networks 6-10 kV. - M.: NTF Energoprogress, 2001.]. Automatically, upon fixing the SPE, the relay protection device generates a signal to turn on the switch of the switchgear backup cell to ensure the flow through the ballast resistance of a short circuit current limited in time and magnitude in the undamaged phase. The choice of the flow time and the value of the short circuit current in the undamaged phase is determined by the parameters of the thermal resistance of the power transmission line wire. In this case, the relay protection device may contain one or more relay protection terminals. The necessary composition of the relay protection device, as well as the functions of its individual terminals, are selected based on the capabilities and functions of its individual terminals in the line of a specific relay protection manufacturer. For example, the determining design parameters can be: the number of analog and discrete inputs/outputs, the speed and performance of microprocessors, the possibilities and amount of memory for registering emergency events, etc.

The disadvantages of this method are the complexity of the organizational and technical measures to identify connections with SPZ and the need to form a short circuit, which can cause the development of an accident in other connections.

The method allows you to determine the accession with OZZ.

The disadvantage is that in order to determine the connection with SPZ, it is required to determine the damaged phase using a relay protection device, then turn on the switch in the undamaged phase in the reserve cell with ballast resistance, then the damaged connection is determined by the values of the phase currents of the connections.

Signs of the prototype, coinciding with the essential features of the invention are as follows:

• Automatically upon fixation of the SPE by the relay protection device, a signal is issued to turn on the switch of the backup cell of the switchgear;

• according to the magnitude of the current, the connection with the OZZ is determined;

• disconnect connection with OZZ.

The technical result of the invention is the simplification of operation and increased reliability in the method for determining the connection with SPZ in networks with isolated neutral.

The technical result of the claimed invention consists in preliminary determination of the value of active resistances of four ballast resistances, three of which are connected to a three-phase switch of the backup cell, connected to a star, the neutral of which is connected through the fourth resistance to the ground loop of the switchgear, the settings of the current relay protections of the zero sequence of connections are formed , which form a signal to disconnect connections with a single-phase earth fault when a zero-sequence voltage signal appears on the busbar section after the backup cell switch is turned on.

The main differences of the proposed method are as follows:

• preliminarily determine the values of active resistances of four ballast resistances, which are connected to the three-phase switch of the reserve cell;

• connect three ballast resistors into a star, the neutral of which is connected through the fourth ballast resistor to the ground loop of the switchgear;

• form the current relay protection settings of the zero sequence of connections, which generate a signal to disconnect the connection with a single-phase short to ground after the backup cell switch is turned on when a zero sequence voltage signal appears on the busbar section.

The presence of distinctive features allows us to conclude that the claimed invention complies with the condition of patentability "novelty".

No sources containing a set of distinctive features of the claimed invention have been identified from the prior art, and the influence of distinctive features on the achieved technical result has not been established, which allows us to conclude that the claimed invention complies with the patentability condition "inventive step".

The essence of the proposed method is illustrated by drawings, where:

figure 1 shows a three-phase equivalent network with a supply connection, with three consumer connections and with disconnected ballast resistors in the backup cell,

figure 2 shows a three-phase equivalent circuit of a network with a supply connection, with three connections of consumers, with an OZZ on one of the connections and with disconnected ballast resistances in the backup cell,

figure 3 shows a three-phase equivalent circuit of the network with a supply connection, with three consumer connections, with a ground fault at one of the connections and with connected ballast resistors in the backup cell.

The diagram of Fig.1 shows a three-phase source of electrical power 1 with a neutral 2 connected to the ground loop of the substation 3, which is connected to a three-phase power transformer 4 with a star-delta winding connection. Star neutral 5 of transformer 4 is connected to the ground loop of substation 3. Delta outputs of transformer 4 are connected to three-phase busbar system 6, to which three three-phase power lines 7, 8 and 9 are connected, with consumer loads 10, 11 and 12. K the busbar system 6 is connected to a reserve cell containing a three-phase switch 13 in the off position with three phase ballast resistances 14, 15 and 16 connected in a star, the neutral of which is connected through the resistance 17 to the ground loop of the substation 3. At each connection 7, 8, 9 and 4 zero sequence current protection 18, 19, 20, 21 is installed. A voltage transformer 22 with relay 23 is connected to the busbar system 6.

In the diagram of Fig. 2 shows everything the same as in the diagram of figure 1 and additionally shows a single-phase earth fault in phase "B" of the first connection 7.

In the diagram of Fig. 3 shows everything the same as in the diagram of figure 2 and additionally shows the on position of the switch 13, which ensures that the switch 13 connects the ballast elements 14, 15, 16 and 17 to the busbar system 6.

The method is implemented as follows:

Pre-determine the values of the active resistances of the three ballast resistances 14, 15, 16, 17, for example, as the ratio of the phase voltage to the rated phase current of the most loaded connection. By calculation, it was found that in a 10 kV network it is enough to install a resistance of 300 ohms. The resistances 14, 15, 16 are connected to the three-phase switch 13 of the reserve cell, they are connected to a star, the neutral of which is connected through the resistance 17 to the ground loop 3 of the switchgear. The settings of the current relay protections of the zero sequence 18, 19, 20, 21 of all connections are formed in accordance, for example, with [Mechanotronics organization standard STO DIVG-059-2017, Relay protection of distribution networks 6-10 kV, Calculation of settings, Guidelines, St. Petersburg, 2017]. These protections form a signal to turn off their connection when a single-phase ground fault occurs in it. However, if the neutral in the network is isolated, then the zero-sequence current in the earth-fault connection is very small and does not always provide the setting value, so the protection does not work stably. To ensure reliable and stable operation of protection, we create a temporary neutral in a network with an isolated neutral. When a single-phase ground fault occurs in any connection 4, 7, 8, 9, a zero-sequence voltage signal appears on the bus section 6 from the voltage transformer 22. Through the relay 23, the signal from the voltage transformer 22 briefly switches on the switch 13 of the backup cell. Ballast resistors 14, 15, 16 and 17 are connected to the network, creating a resistively compensated neutral. In the connection, a short circuit mode is created, controlled by the magnitude of the current and by time. The zero sequence current in the connection with a single-phase earth fault becomes greater than the value of the protection setting, respectively, the protection generates a signal to disconnect the connection. At the same time, in the remaining connections, the zero sequence current does not increase. Relay 23 has a time delay, which allows you to turn off the damaged connection after a specified time after the occurrence of the fault.

To test the operability of the proposed method, computational experiments were performed on the equivalent circuit of an electrical network with an isolated neutral shown in Fig. 1, 2, 3. The calculations were carried out in the software-computer complex "Calculation of modes in phase coordinates", developed at the department "Power plants, networks and systems" of IRNITU. The network equivalent circuit is set in a three-phase form, taking into account the mutual inductance and capacitance between the wires of the line, the capacitance between the wires and the ground, taking into account the mutual inductance between the windings of power transformers.

Table 1 shows the values of the currents in phases A, B, and C and the zero sequence current (3I ₀ ) connections 4, 7, 8, 9 (module, angle) in normal operation of the network with isolated neutral (figure 1). In connections 7 and 8 the loads in the phases are not symmetrical. Shown are the voltages in phases A, B, C and the zero sequence voltage (3U ₀ ) on the bus section 6.

Table 1

Phase A Phase B Phase C Zero sequence Module Corner Module Corner Module Corner Module Corner Current pr. 7 (A) 50.92 218.7 50.41 77.5 33.68 328.9 0.017 126.43 Current pr. 8 (A) 62.14 197.3 63.15 97.9 81.06 327.1 0.015 -44.42 Current etc. 9 (A) 55.19 208.1 55.18 88.1 55.19 328.0 0.0032 -100.16 Current pr. 4 (A) -166.3 207.3 -167.07 88.6 -169.97 327.7 -6.74E-09 142.74 Section voltage (V) 5879.1 209.6 5717.1 90.9 5683.8 328.8 363.9 -159.1

Table 2 shows the values of currents in phases A, B, and C and the zero sequence current (3I ₀ ) of connections 4, 7, 8, 9 (module, angle) in single-phase earth fault mode through a resistance of 5 Ohm in phase "B" the first connection in a network with isolated neutral (Fig. 2). Shown are the voltages in phases A, B, C and the zero sequence voltage (3U ₀ ) on the bus section 6.

table 2

Phase A Phase B Phase C Zero sequence Module Corner Module Corner Module Corner Module Corner Current pr. 7 (A) 50.88 218.8 50.46 78.3 33.75 329.0 0.503 165.26 Current pr. 8 (A) 62.07 197.4 63.15 97.8 81.15 327.1 0.258 -14.91 Current etc. 9 (A) 55.13 208.1 55.16 87.9 55.26 328.1 0.245 -13.92 Current pr. 4 (A) -166.18 207.3 -167.18 88.7 -170.21 327.7 -8.04E-09 125.77 Section voltage (V) 9864.1 238.6 222.5 119.1 9753.4 299.8 16700.4 -91.3

Table 2 shows that the zero sequence voltage (3U ₀ ) has appeared, and the zero sequence current (3I ₀ ) in the first line (connection 7) is greater than the currents in the second (connection 8) and third (connection 9) lines and is shifted by 180 degrees. However, the magnitude of the zero sequence current in the first line is not sufficient for reliable operation of the protection.

Table 3 shows the values of currents in phases A, B, and C and the zero sequence current of connections 7, 8, 9, 4 (modulus, angle) in single-phase ground fault mode through a resistance of 5 Ohm in phase "B" of the first connection in the network with a resistively grounded neutral created by a three-phase switch and ballast resistors 14, 15, 16 and 17 (Fig. 3). Shown are the voltages in phases A, B, C and the zero sequence voltage (3U ₀ ) on the bus section 6.

Table 3

Phase A Phase B Phase C Zero sequence Module Corner Module Corner Module Corner Module Corner Current pr. 7 (A) 50.87 218.5 63.69 80.1 33.63 329.1 13.736 89.82 Current pr. 8 (A) 62.07 197.3 63.14 97.8 81.13 327.1 0.253 -15.76 Current etc. 9 (A) 55.13 208.1 55.16 87.9 55.25 328.1 0.239 -14.80 Current pr. 4 (A) -187.69 208.7 -195.16 88.7 -191.55 326.7 -1.52E-09 -177.78 Section voltage (V) 9800.6 237.9 366.9 120.3 9607.1 299.7 16339.3 -92.2

Table 3 shows that the value of the zero-sequence current in connection 7 has increased significantly to a value sufficient to trigger the protection.

Claims (1)

A method for disconnecting a damaged connection in networks with an isolated neutral in case of a single-phase ground fault, in which current relay protection devices installed on the connections are used with the possibility of using the currents of each connection, turn on the high-voltage switch of the backup cell, ensuring flow through the sections of the switchgear busbars connected to the backup cell, in which there is a ground fault, ballast resistances of currents limited in magnitude and time, determine and disconnect, using a relay protection device, a connection with a single-phase ground fault, characterized in that the active resistance values of four ballast resistances are preliminarily determined, three of which are connected to a three-phase reserve cell switch, connect them into a star, the neutral of which is connected through the fourth resistance to the ground loop of the switchgear, the settings of the zero-sequence current relay protection are formed number of connections that generate a signal to disconnect the connection with a single-phase earth fault when a zero-sequence voltage signal appears on the busbar section after the backup cell switch is turned on.

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