RU2806893C1 - Method for compensating capacitive currents in electrical networks with isolated neutral - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention is aimed to increase the efficiency of voltage reduction at the point of a single-phase short circuit (SSC). It is achieved by the fact that in the method of compensating capacitive currents in electrical networks with an isolated neutral, a single-phase active resistance is first briefly connected to the bus section, the current in the resistance and the zero-sequence voltage on the section in the voltage transformer are measured, the network resistance set point is formed relative to the section buses, the set point is stored in electronic control unit of the compensating device. When a short-circuit fault occurs at one of the connections of the section, the zero-sequence voltage is measured, and a current in the neutral is automatically formed in the current source as the ratio of the module of the measured zero-sequence voltage to the set point, lagging behind the zero-sequence voltage by 90 degrees. The changed zero-sequence voltage is measured again, a new value of the total current in the neutral is formed in the current source as the ratio of the measured complex zero-sequence voltage to the setting, then the process of forming the total current in the neutral is iteratively repeated until the difference between the modules of the measured zero-sequence voltage and the previous measured zero-sequence voltage will not be less than a specified value, for example 1%.

EFFECT: increase the efficiency of voltage reduction at the point of a single-phase short circuit.

2 cl, 2 dwg

Description

The invention relates to the field of electrical engineering and can be used for automatic adjustment of compensation for capacitive ground fault currents in electrical networks with an isolated neutral.

The problem of reducing the current and extinguishing the electric arc at the site of a single-phase ground fault (SGC) in 6-35 kV networks is traditionally solved using arc suppression reactors (inductors) connected between the network neutral and the ground [Wilheim R., Waters M. Grounding the neutral in high voltage systems. M.: Gosenergoizdat, 1959. 416 p.]. However, this solution, based on compensation of only the capacitive component of the SGC current to the oppositely directed inductive current component created by the arc suppression reactor (AHR), gives good practical results only at one frequency and in the absence of significant losses. In real networks, where the SGC currents are large and exceed, for example, 100 A, the residual, not compensated by the GDR current at the SGC location, due to losses, harmonics and non-ideal settings of the SGC, can exceed 5 A and support the burning of an electric arc at the SGC location. In this case, a detuning of the DGR (5% of the rated current of the DGR is allowed) can cause a residual current capable of maintaining an arc in the area of the residual current. The current value at which stable arcing at the fault point is possible (5 A) was determined as a result of comprehensive research and is recorded in regulatory documents [Likhachev F.A. Instructions for selecting, installing and operating arc suppression coils. M.: Energy, 1971. 106 p. and Rules for the technical operation of consumer electrical installations: approved. By Order of the Ministry of Energy of Russia dated January 13, 2003 No. 6. M.: Energoservis, 2003]. Thus, in networks with a current of 100 A or more, the DGR is not a guaranteeing means of extinguishing the arc, since the residual current can exceed 5 A. This means that high-power DGRs turn out to be ineffective, because they do not fully fulfill their purpose. the main function of extinguishing the arc at the site of the short-circuit fault.

There are known methods for setting up compensation for capacitive ground fault currents based on extreme or phase methods of regulating the inductance of an arc suppression reactor (ARR), implemented by continuous monitoring in normal mode of the network voltage or current of the zero sequence circuit (ZNC) of the network and indirect analysis of its amplitude or phase characteristics [ Obabkov V.K. Improving the phase method for automatically maintaining the conditions for compensation of capacitive currents in cable networks 6-35 kV // Electricity, 1989, No. 1].

A system based on these tuning methods is operational only near the equilibrium point corresponding to the resonance of the controlled circuit, when the most favorable conditions for the formation of an information coordinate occur. In order to ensure operability in an extended range of changes in circuit parameters, an artificial neutral offset is used and measures are taken to stabilize the overall transmission coefficient. Since during operation the parameters of the network’s control parameters can significantly change in one direction or another, in order to avoid loss of controllability, periodic verification of capacitive currents and adjustment of the accuracy of system settings is provided, usually done manually and with a short-term interruption of power supply.

With all these measures, this system remains sensitive to the quality factor of the controlled circuit. With a low quality factor of the network control signal, a complete loss of controllability is possible due to the low level and high noise of the control signal.

Another drawback is determined by the difficulties of ensuring the stability of a servo system with a clearly nonlinear drive link in the control circuit, which manifests itself in multiple switching of the DGR when reaching the equilibrium point. If there is an unfavorable combination of parameters in the controlled circuit, self-oscillations may occur, leading to accelerated wear of the electromechanical system, which is unacceptable under the operating conditions of the DGR.

There is a known tuning method by which the voltage and current in the circuit of the PSC network, created artificially using a reference current source, are controlled, and a characteristic value proportional to the change in the controlled parameter is isolated [A.s. 1443077, RU. Gumin M.I. A method for setting compensation for capacitive ground fault current in electrical networks. Publ. in BI45, 1988]. This method is less dependent on the parameters of the controlled circuit. It provides stable operation in an extended range of changes in the inductance of the DGR and assumes reaching a given compensation mode with a smaller number of drive switchings. However, this resolution is achieved only if an energy-intensive reference current source is used, operating in a continuous mode in order to provide favorable conditions for isolating the information coordinate. This drawback is aggravated when this method is used in networks with a combined neutral grounding mode with a reduced quality factor of the network's QNP, since the installed power of the reference source turns out to be commensurate with the power of the installed regulated object. For this reason, this method has limited use.

There is a known method implemented in a device for automatically adjusting an arc suppression reactor, where one of the controlled parameters, based on the measurement results of which the reactor inductance is regulated, is its current [Likhachev F.A. Ground faults in networks with an isolated neutral and compensation of capacitive currents. - M.: Energia, 1971, p. 107-108].

However, this regulation principle is not applicable in the case of an intermittent arc, when the channel for measuring reactor parameters is blocked and a period of regulation occurs using a reactor tuning unit in an intermittent arc using parameters from the measuring voltage transformer.

The disadvantage of this method is also the need to select control logic in the ground fault mode and the associated insufficient performance, for example, during short-term saturation of arc suppression reactors, when due to the presence of a free component of the magnetic flux at the moment of the next arc ignition, the setting shifts to the overcompensation region. Saturation of reactors can also occur as a result of the appearance at the neutral of a voltage that is increased relative to the nominal values. Saturation-related detuning can lead to a significant increase in current at the fault site during a steady arc and to a significant increase in overvoltages during an intermittent arc. Moreover, the required setup time in these cases is desirable to be as short as possible. Thus, dangerous overvoltages can occur as early as 0.02 s after the next arc extinction, i.e. for the time required for known devices only to identify the nature of the arc and switch from one control logic to another.

There is a known method [patent RU No. 2025016, publ. 12/15/1994] in which, in the ground fault mode, the current parameter of the arc suppression reactor is measured and the compensation current is changed depending on this parameter. As a current parameter, the instantaneous value of the reactor current is used, which is compared with a setting proportional to the amplitude value of the reactor current in an unsaturated state, and if the current exceeds the setting, the compensation current is changed in proportion to the comparison result.

The proposed method differs from the known ones by introducing into the known regulator an additional control channel for the current of the arc suppression reactor, which provides high-speed control of the compensation setting in both intermittent and steady-state arc burning modes.

The increase in performance is explained by the fact that the control parameter is the value that directly determines the detuning, the compensation current, and not the inertially measured degree of compensation detuning. Since the response to the action of a strictly defined disturbance pulse is more predictable than to a signal of arbitrary shape created when switching connections, it is advisable to provide for artificial disturbance in all cases of measuring the natural frequency. This method can also be effectively used in networks with a reduced quality factor of the PSC, in particular in networks with a combined grounding mode, where compensation systems based on extreme and phase control methods are fundamentally ineffective.

The disadvantage of this method is the need to use artificial disturbance, which significantly complicates the technical implementation.

There is a known method adopted as a prototype [Patent RU 2663823, publ. 08/10/2018, Bulletin. No. 22, NPP Bresler LLC], in which, in normal network operation, the neutral bias voltage relative to the ground is measured, compared with a given threshold level, and the neutral bias voltage is changed. When the neutral bias voltage exceeds a given threshold level, a current is supplied to the network neutral, it is regulated by equalizing the neutral bias voltage with the ground potential, the current is measured, stored and reproduced. The specified technical solution involves connecting a controlled source to the neutral with an appropriate current control algorithm, ensuring complete neutralization of the source of asymmetry.

The neutral bias voltage is regulated independently of the quality factor of the zero-sequence circuit of the network. In this case, the component of the ground fault current caused by the source of network asymmetry is eliminated.

The disadvantage of this method is the need to measure the neutral bias voltage and change it to the threshold value in normal mode using a controlled DGR, the need to use an adjustable DGR in parallel with the current source in the OZZ mode and the possibility of application only in networks with a compensated neutral.

The features of the prototype that coincide with the essential features of the proposed invention are as follows:

• preliminary measure the displacement of the neutral relative to the ground (zero sequence voltage);

• compare the neutral displacement with a given threshold level;

• when the neutral bias voltage exceeds a specified threshold level, current is supplied to the network neutral;

• measure the current in the neutral.

The technical result of the proposed invention is to increase the efficiency of reducing the voltage at the point of protection in networks with an isolated neutral.

The technical result of the claimed invention is that a neutral is formed on the bus section, a compensating device is connected between the neutral and the ground, current is supplied to the neutral, it is regulated, according to the invention, a single-phase active resistance is first briefly connected to the section, the current in the resistance and the zero-sequence voltage are measured on the section, form the network resistance setting relative to the section buses as the ratio of the zero-sequence voltage module to the current module in the resistance, store the setting in the electronic control unit of the compensating device - the current source. If a single-phase ground fault occurs at one of the connections of the section, the zero-sequence voltage is measured, a current in the neutral is automatically generated in the current source as the ratio of the module of the measured zero-sequence voltage to the setting, lagging behind the zero-sequence voltage by 90 degrees, then the changed zero-sequence voltage is re-measured sequences, form a new value of the total current in the neutral in the current source, as the ratio of the measured complex zero-sequence voltage to the setting, then iteratively repeat the process of forming the total current in the neutral until the difference between the modules of the measured zero-sequence voltage and the previous measured zero-sequence voltage becomes less than the specified one values, for example 1%.

If there are several bus sections electrically connected to each other, first simultaneously briefly connect a single-phase active resistance to each section, measure the current in each resistance and the zero-sequence voltage on each section, and form a network resistance setting relative to the buses of each section.

The main differences of the proposed method are as follows:

• first simultaneously briefly connect a single-phase active resistance to each section, measure the current in each resistance and the zero-sequence voltage in each section, form a network resistance setting relative to the buses of each section;

• remember the setting in the electronic control unit of the compensating device - the current source;

• if a single-phase ground fault occurs at one of the section connections, measure the zero-sequence voltage;

• the current in the neutral is automatically generated in the current source as the ratio of the module of the measured zero-sequence voltage to the setting, lagging behind the zero-sequence voltage by 90 degrees;

• re-measure the changed zero-sequence voltage, form a new value of the total current in the neutral in the current source, as the ratio of the measured complex zero-sequence voltage to the set point;

• iteratively repeat the process of forming the total current in the neutral until the difference between the magnitudes of the measured zero-sequence voltage and the previous measured zero-sequence voltage becomes less than a specified value;

• if there are several bus sections electrically connected to each other, first simultaneously briefly connect a single-phase active resistance to each section, measure the current in each resistance and the zero-sequence voltage on each section, and form a network resistance setting relative to the buses of each section.

The presence of distinctive features allows us to draw a conclusion about the compliance of the claimed invention with the patentability condition of “novelty”.

From the prior art, no sources were identified containing a set of distinctive features of the claimed invention, and the influence of the distinctive features on the achieved technical result was not known, 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 Fig. 1 shows a three-phase equivalent circuit of a network with a supply connection, with three consumer connections, with a neutral transformer (NOT), a current source and a single-phase load in a backup cell, Fig. 2 shows a current source diagram.

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 diagram. The neutral of the "star" 5 of the transformer 4 is connected to the grounding circuit of the substation 3. The outputs of the "triangle" of the transformer 4 are connected to a three-phase bus system 6, to which three three-phase power lines 7, 8 and 9 are connected, with consumer loads 10, 11 and 12. To The bus system 6 is connected to a neutral-forming transformer 24, in the neutral 13 of which a current source 14 is connected, connected to the ground loop of the substation 3. A voltage transformer 15 is connected to the bus system 6. A pulse shaper 16 is connected to the current source 14, connected to a control unit 17, to which the voltage transformer circuit 15 and the setpoint generating unit 18 are connected. An active resistance 19 is connected to one phase of the bus system 6 through a switch 20 connected to the ground loop 3 of the substation. The active resistance circuit 19 includes a current transformer 21, the output of which is connected to the setpoint generation unit 18.

Figure 2 shows a diagram of an adjustable current source 14. The signal from the control unit 17 is supplied to the pulse shaper 16. In the pulse shaper 16, control signals are created that are supplied to bipolar transistors 22 of the current source 14. Transistors 22 instantly respond to control signals and, using reactor 23, create a compensating current of the current source.

The method is implemented as follows:

First, a single-phase active resistance 19 is briefly connected to the bus section 6, the current in the resistance 19 and the zero-sequence voltage in section 6 in the voltage transformer 15 are measured, the network resistance setting Z relative to the section buses is formed as the ratio of the zero-sequence voltage module 3U0 to the current module I in the resistance 19, setpoint Z is stored in the electronic control unit 17 of the compensating device - current source 14.

If a single-phase ground fault occurs at one of the connections 7, 8, 9 of section 6, the zero-sequence voltage 3U0 is measured in the voltage transformer 15, control actions are automatically generated in the control unit 17, which are sent to the pulse shaper 16. The pulses are transmitted to the transistors 22 of the source current 14. Controlled transistors 22 in the current source 14 generate a current In in the neutral 13 as the ratio of the module of the measured zero-sequence voltage to the setting In = 3U0/Z, lagging behind the zero-sequence voltage by 90 degrees,

Next, the changed zero-sequence voltage 3U0’ is re-measured, a new value of the total current In’ in neutral 13 is formed in the current source, as the ratio of the measured complex zero-sequence voltage to the setting In’=3U0’/Z.

Next, the process of generating the total current In” in the neutral is iteratively repeated until the difference between the magnitudes of the measured zero-sequence voltage 3U0” and the previous measured zero-sequence voltage 3U0’ becomes less than a specified value, for example (3U0”-3U0’)<1%.

If there are several sections of buses electrically connected to each other, first simultaneously briefly connect a single-phase active resistance to each section, measure the current in each resistance and the zero-sequence voltage in each section, and form a network resistance setting relative to the buses of each section.

To check the performance of the proposed method, calculation experiments were performed on the equivalent circuit of an electrical network with an isolated neutral, shown in Fig. 1. Calculations were carried out in the software-computing complex “Calculation of modes in phase coordinates”, developed at the Department of Electric Power Plants, Networks and Systems of INRTU. The network equivalent circuit is specified in 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.

The neutral displacement of the bus section in normal mode is 176.711 V, the angle is -114.952 degrees. The values of currents 3I ₀ in connections are shown in table. 1. Table 1 Currents 3I ₀ in connections

Power line no. 3I _0a
A 3I _0r
A 1 -0.00101 -0.00053 2 0.00049 0.00022 3 0.00050 0.00025

With a short-term (10 sec) connection of an active resistance of 300 Ohms between phase A and ground, we get 3U ₀ = 1 0495, angle = -23.42 degrees. and current in resistance I =46.17 A, angle = -112.93. We form the setting and remember Z=10495/46.17 =227.32 Ohm.

Next, we create the OZZ in the first line phase. We measure the voltage 3U ₀ on the bus section (column 5 in Table 2), form the current I _n in the neutral of the HOT (column 6) as In = 3U0/Z. When the current I _n is generated, the voltage 3U ₀ will change, accordingly we form a new current value, etc. iteratively.

In columns 3 and 4, we control the calculated values of voltage at the OZZ point and the OZZ current. In the case of OZZ at a real object, the indicated values are not available for observation.

The results of the iterations are shown in table. 2

Table 2. Iteration results. Iteration number U
at the OZZ point
IN I
OZZ
A 3U ₀
on the bus section
IN I _n
in neutral NOT
A Accuracy
By
3U ₀
% 1 2 3 4 5 6 7 1 module 2137 71 16383 72 corner 273 273 2.7 87.3 2 module 923 30.8 19432 85.5 15.69 corner 348 348 23.5 66.5 3 module 398 13.3 17590 77.4 -10.47 corner 64.6 64.6 31.3 58.7 4 module 179.7 6 16300 71.7 -7.91 corner 142.4 142.4 29.8 60.2 5 module 87.2 2.9 16358 72 0.35 corner 215 215 27.8 62.2 6 module 33.7 1.12 16610 73 1.52 corner 272 272 27.7 62.3 7 module 6.8 0.23 16653 73.25 0.26 corner 321 321 28 62

After 7 iterations, it is clear that with a current in the neutral I _n = 73.25 A, angle = 62 degrees, voltage 3U ₀ = 16653 V, angle = 28 degrees. In this case, the voltage at the GBZ point is 6.8 V, and the GBZ current is 0.23 A. At this current, the arc is guaranteed to go out.

Claims (2)

1. A method for compensating capacitive currents in electrical networks with an isolated neutral, in which a neutral is formed on a bus section, a compensating device is connected between the neutral and the ground, characterized in that a single-phase active resistance is first briefly connected to the section, the current in the resistance and the zero-sequence voltage are measured on the section, form the network resistance setting relative to the section buses as the ratio of the zero-sequence voltage module to the current module in the resistance, store the setting in the electronic control unit of the compensating device - the current source, and if a single-phase ground fault occurs at one of the section connections, measure the zero-sequence voltage , automatically generate a current in the neutral in the current source as the ratio of the module of the measured zero-sequence voltage to the setting, lagging behind the zero-sequence voltage by 90 degrees, then re-measure the changed zero-sequence voltage, form in the current source a new value of the total current in the neutral as the ratio of the measured complex zero-sequence voltage to the setting, then iteratively repeat the process of forming the total current in the neutral until the difference between the modules of the measured zero-sequence voltage and the previous measured zero-sequence voltage becomes less than a specified value, for example 1%. 2. The method according to claim 1, characterized in that if there are several sections of buses electrically connected to each other, a single-phase active resistance is first simultaneously briefly connected to each section, the current in each resistance and the zero-sequence voltage in each section are measured, the resistance setting is determined networks relative to the buses of each section.