RU2524347C2 - Device for earth fault current compensation in three-phase electrical networks (versions) - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention is used in electrical engineering. The device represents an earthing bipole - a tracking width-modulated voltage converter (a tracking PWM converter) attaching the neutral of the network feeding transformer to earth and forming lagging current for compensation of leading capacitance current of single-phase fault to earth, as well as a neutral current sensor, a network voltage transformer, a clock signal generator for measurement of network capacity and a calculation unit for determination of the required admittance of the above bipole. In addition to a common set of functional units, the tracking PWM converter is equipped with a current setting unit, the inputs of which are represented by neutral voltage and the required admittance, and the output of which is represented by current demand supplied to the current control input of the tracking PWM converter, and transfer function of the current setting device provides for lagging phase shift of about 90 degrees on network frequency.

EFFECT: improving quick action and enlarging functional capabilities.

9 cl, 46 dwg

Description

The claimed technical solution relates to electrical engineering and is intended primarily to compensate for earth fault current and arc extinction in three-phase electrical networks with isolated neutral.

In three-phase electric networks with insulated neutral, equipment is allowed to operate with a single-phase earth fault (moreover, single-phase earth faults account for up to 90% of the total number of faults), but the fault current is monitored. Based on the operating experience, the earth fault currents of not more than 5 ... 20 A (depending on the magnitude of the mains voltage) are permissible and do not require immediate power off.

In the event of a short circuit to ground of one of the phases of the electric network (medium voltage: 6, 10, 20 kV or high voltage: more than 20 kV), a capacitive current flows through the short circuit, which can be significant and maintain the burning of the short circuit arc. This phenomenon was studied by Petersen [1], and he also proposed a means to reduce the short-circuit arc current - by connecting the neutral of the supply transformer to the ground through an inductor (Petersen coil). The inductance of the coil (or its conductivity - admittance) is selected according to the resonance condition so that the lagging inductance current compensates for the leading capacitive current; when fine-tuned, the current at the fault location in steady state should become zero. Since then, the Petersen coil has been widely used in medium and high voltage electrical networks. In simple cases, an unregulated coil can be used. In branched networks, where the capacitance to the ground can vary, coils with taps switched by contactors [2, 3] or plunger reactors are used, the inductance of which is regulated by the movements of the magnetic shunt [4, 5, 6, 7]. The power of the arcing device Q _N depends on the mains voltage and the compensation current value.

The disadvantage of branch reactors is the difficulty in setting the compensation mode, due to the discreteness of the inductance change.

The disadvantages of plunger reactors are the large mass and dimensions, as well as the presence of rotating and moving parts, which adversely affects the reliability and durability of the reactor. The low speed of the moving part of the plunger type reactors imposes restrictions on their speed.

Reactors without moving parts, controlled by magnetization, are also known [8, 9, 10]. Devices have been developed that allow the tuning of controlled reactors automatically without operator intervention [5, 7, 9, 10]. For this, an auxiliary converter-generator of a test signal is applied, supplied to the network through an additional winding of the reactor. The response to the test signal determines the network capacitance to earth and then sets the corresponding value of the reactor inductance.

However, the use of an auxiliary converter significantly increases the cost of the extinguishing device as a whole.

раз. Однако сеть может продолжать работать некоторое время; немедленного прерывания питания потребителей удается избежать. Возможно также и другое, более благоприятное развитие событий, когда под действием снижения тока в промежутке короткого замыкания его электрическая прочность восстанавливается, и в результате восстанавливается и нормальная работа сети.In order to evaluate an arc suppressing device, a fundamental circumstance is that these devices must operate in two different classes of situations. The Petersen coil successfully operates in first-class situations when a short circuit to ground is stable (metallic short circuit). In these situations, in the steady-state short-circuit mode, a sinusoidal lagging current of the coil is set, which compensates completely or with some error ahead of the capacitive current and reduces current and heat generation in the short-circuit gap. The stresses of healthy phases relative to the earth turn out to be overestimated in

time. However, the network may continue to work for a while; immediate interruptions in consumer power are avoided. Another, more favorable development of events is also possible, when under the influence of a decrease in current in the short circuit period, its electric strength is restored, and as a result, the normal operation of the network is restored.

More typical, however, are situations of the second class with repeated breakdowns of the insulation gap, when a partial restoration of the electrical strength of the gap is followed by an increase in voltage across it, repeated breakdown, etc. The situations of this second class were considered by Peters and Slepyan [11], Juvarly Ch.M. [12], Belyaev N.M. [13]. In situations with repeated breakdowns, the use of the Petersen coil does not give a stable positive effect, which is to be expected, since its action is designed for steady state.

To improve the processes during repeated breakdowns, a combination device was proposed in which a resistor is connected to the Petersen coil. A study performed by Ilyin M. et al. [14] demonstrated the useful damping effect of a resistor in repeated breakdowns. At the same time, resistive damping is associated with a certain deterioration in the indices of steady-state processes. Thus, a contradiction arises. The resistor current is not compensated by anything and is added to the current flowing in the short circuit period. The possibilities for increasing the conductivity of the shunt resistor to improve dynamics are limited.

Closest to the claimed technical solution is a technical solution [10] in which a magnetically controlled electric reactor (UPER) with an automatic control system is used. UPER does not have moving parts and has a relatively high speed. The disadvantages of this technical solution are the overestimated consumption of electrical materials (copper, electrical steel), large mass and dimensions, the need to use a special device to generate test voltages on the transformer neutral. In addition, UPER does not provide damping of high-frequency oscillations that occur at the moment of ground fault during repeated breakdowns and contribute to the appearance of an arc. The UPER automatic control system does not have the ability to determine the moment of termination of a single-phase earth fault to exit the compensation mode. This leaves the possibility of resonant overvoltages during switching and other disturbances and reduces the level of network reliability.

The task to which the claimed technical solution is directed is to provide arc suppressing equipment, along with the main, additional useful functions, such as the ability to identify the network without the use of auxiliary equipment, the rapid suppression of transient current components, including during repeated breakdowns, accelerated current reduction arcs, while eliminating the above disadvantages inherent in the UPER [10].

When solving this problem, the technical result achieved is to reduce the material consumption of the extinguishing apparatus, in ease of use, and to increase the network's resistance to earth faults.

The essence of this technical solution is to use an active element as a pulse-width modulation (PWM) tracking transformer, as an arc suppressing device (instead of a reactor controlled in some way or a reactor combination with a shunt resistor), then referred to as PWM converter in the text. The current state of the art makes it possible to carry out such converters with the parameters required for suppression, and such converters, if properly controlled, can simulate an adjustable inductance. The use of the tracking PWM-converter corresponds to the trends of modern technology and gives some advantages by itself, with the simple reproduction of the same functions in a new way. However, the use of an active element - a PWM-tracking servo-converter - can give more - weaken the contradiction between the requirement of good accuracy of compensation for stable faults and the requirement of good damping for intermittent breakdowns.

In addition to the aforementioned, there is another useful property of the tracking PWM converter. In the previous consideration, it was considered as a device for obtaining a lagging current, i.e. as an analog of adjustable inductance. However, such converters can equally emit a leading current, so they should not be considered as an analog of inductance or capacitance, but as a universal controlled reactive element. A change in the reaction of such a link from inductive to capacitive corresponds to a change in the sign of the conductivity coefficient Yn of the converter as a two-terminal device. When the coefficient Yn passes through zero to the side of negative values of Yn, the PWM servo converter will give a leading current, i.e. will become an analogue of capacity. When working on the network, such a situation may occur if the conductivity of the reactor at the connection of another transformer is greater than required. The tracking PWM converter will automatically compensate for the excess, switching to a mode with a negative conductivity coefficient Yn and a leading current. No switching or reconfiguration is required to switch from inductive mode to capacitive mode. The ability of such a converter to bilateral regulation can be used when building an arcing device for price or other reasons.

In addition to the version of the arc suppression device based on the said converter, other options are possible, the choice of which is due to technical and economic considerations.

A good option is when an unregulated coil at half power Q _N / 2 and a follow-up PWM converter for the same half power Q _N / 2 are introduced into the structure of the suppression device. When adjusting the converter in the full range ± Q _N / 2, the total admittance is regulated in the full range - from zero to the nominal value.

Similarly, for price or other reasons (for example, to perform a converter for low voltage) it may be useful to use a matching transformer.

It may also be appropriate to design this transformer as a transformer-reactor, with an idle power equal to half the required rated power of the extinguishing device. The required installed power of the tracking PWM converter is also half the rated power.

The proposed PWM servo converter is similar in structure to STATCOM (Statkom - static reactive power compensation - a static compensator based on a voltage converter). The main property of STATCOM is the ability to generate current of any phase relative to the mains voltage. The proposed PWM servo converter can:

- work in a two-terminal mode with controlled impedance (admittance), which allows compensation of capacitive currents and arc quenching, and fast control system regulators in combination with practically inertia-free semiconductor power elements allow to obtain effective suppression of higher harmonics, favorable transient processes as with a stable circuit to the ground, and during intermittent breakdowns,

- generate test signals of the required power with any harmonic content required for accurate identification of the parameters of the three-phase network on the transformer neutral, while no additional equipment is required, except for the microprocessor program area in the control system, which, with the capabilities of modern powerful microprocessors, does not require additional material costs,

- work in a two-terminal mode, which has the property of an active load, that is, a resistor (but without absorption and dissipation of energy, with the exception of small losses in the circuit elements), which allows damping high-frequency oscillations that contribute to the appearance of overvoltages,

- combine during operation any of the above modes, for example, the compensation mode of capacitive currents and the damping mode of spurious oscillations and overvoltages, which is not feasible when using a passive element - a reactor - of any of the above types.

In accordance with the proposed technical solution, this problem is solved by the fact that in the known device for compensating the earth fault current in three-phase electric networks, containing a two-terminal network connecting the neutral of the supply network of the transformer to earth and forming a lagging current to compensate for the leading capacitive current of a single-phase earth fault, and also a neutral current sensor, a network voltage transformer, a test signal generator for measuring the network capacitance and a computing unit for determining the required hell according to the claimed technical solution, as a grounding two-terminal, a tracking pulse-width modulated voltage converter (tracking PWM converter) is used, equipped in addition to the usual set of functional blocks with a current sensing unit, the inputs of which are the neutral voltage and the required admittance, the output is the task current supplied to the input of the current regulator of the said PWM-converter, and the transfer function of the current regulator provides a lag phase shift colo 90 degrees at the network frequency.

In accordance with the proposed technical solution, this problem is also solved by the fact that the transfer function of the setter, in addition to the reactive (first) component, contains a damping component proportional to the input signal, which in the action of the converter appears as a shunt resistor (virtual resistor).

In accordance with the proposed technical solution, this problem is also solved by the fact that the constant voltage capacitors of the tracking PWM converter (storage capacitors) are made "suspended" (not connected to a voltage source or drain of comparable power), and an additional component is introduced into the transfer function of the setter (component balance) generated by the mismatch of the voltages of the storage capacitors of the tracking PWM converter and ensuring the maintenance of their voltages near the nominal level.

In accordance with the proposed technical solution, this problem is also solved by the fact that an unregulated coil is introduced into the grounding two-terminal to half the required reactive power of the grounding two-terminal (Q _N / 2), connected in parallel with the tracking PWM converter, which is also performed on the same half power (Q _N / 2); moreover, the regulation of the tracking PWM converter in the full range from -Q _N / 2 to + Q _N / 2 (from 90 degrees ahead of the currents to 90 degrees behind the currents) provides control of the total admittance in the full power range - from zero to the nominal value Q _N.

In accordance with the proposed technical solution, this problem is also solved by the fact that the tracking PWM converter is connected through a matching transformer.

In accordance with the proposed technical solution, this problem is also solved by the fact that the tracking PWM converter is connected through a matching transformer (transformer-reactor) with an open-circuit power Q _N / 2.

In accordance with the proposed technical solution, this problem is also solved by the fact that the tracking PWM converter simultaneously with the implementation of the main function (capacitive current compensation) injects (injects) the test current or test voltage into the neutral network, and the required admittance is calculated in the response control system test frequency in neutral voltage.

In accordance with the proposed technical solution, this problem is also solved by the fact that the transfer function of the setter, along with the reactive (compensation), damping and balance components, contains an additional component - a test signal with a frequency (ωtest) different from the network (ωs) (for example, ωtest = 0.5ωs) generated in the control system of the tracking PWM converter.

In accordance with the proposed technical solution, this problem is also solved by the fact that the test signal is the sum of two or more harmonic components of different frequencies, different from the network.

In accordance with the proposed technical solution, this problem is also solved by the fact that the control system of the tracking PWM converter, based on a powerful signal processor, combines the performance of functions specific to earth fault current compensation devices in three-phase electrical networks (such as: generating test signals , processing responses to test signals and calculating the required indicators), with non-specific (normal) control functions of the tracking PWM converter, thereby eliminating the need for hostname additional hardware.

The following illustrations are provided for clarification.

Figure 1 shows the equivalent circuit (Fig. 1 from the book [3]) of a three-phase electric network with an isolated neutral with an arcing reactor in the neutral.

On figa presents a well-known single-phase bridge circuit of a voltage converter with three levels of AC voltage.

On figb presents a well-known single-phase five-level circuit voltage Converter.

On figa and 3b presents the known options for the voltage Converter, made by a modular multi-level scheme.

Figure 4 presents a diagram of an active extinguishing device based on a single-phase servo PWM converter.

Figure 5 shows the structural diagram of a three-component DCB (D - damping, C - compensation, B - balance) of the current generator, the functional blocks of which are implemented on the basis of a powerful signal processor.

Figure 6 presents one of the possible options for the implementation of the block of the balance regulator, which is part of the current setter.

On figa shows a network with two power transformers with an arc suppression coil on the first of them and with a tracking PWM converter on the second.

On figb depicts a neutral equivalent circuit of the network shown in figa.

On Fig depicts a variant of an arcing device with parallel operation of an unregulated throttle and servo PWM-converter.

Figure 9 shows a variant of an extinguishing device with the connection of a tracking PWM converter through a matching transformer or transformer-reactor.

Figure 10 presents a diagram of an automatically adjusting arc suppression device with a tracking PWM converter.

On figa presents a functional diagram for determining the resistive Gn and capacitive Yn components of the conductivity of the network, as well as the desired coefficient of conductivity Yz compensation.

On figb, presents the equivalent circuit of the network at a test frequency f ^I different from the network frequency f.

On Fig presents a functional diagram for determining the resistive Gn and capacitive Yn components of the conductivity of the network, as well as the required compensation conductivity coefficient Yz for the case when the tracking PWM converter works in parallel with an unregulated arc suppression reactor in a circuit of the form of figa or in circuits of the form of fig. .8, Fig. 9.

The equivalent circuit of the network with an extinguishing reactor [3], presented in figure 1, illustrates the basic principle of compensation of the earth fault current and arc extinction. The extinguishing reactor is connected to the zero point of the network and to the ground. The stray capacitances of the phases relative to the earth are designated Ca, Cb, Cc.

The device of the claimed technical solution - tracking PWM-converter - in its static state can be described using the illustrations presented in figure 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12.

Tracking PWM converter - for use in an arc suppression device, it can be performed according to any of the known voltage converter circuits:

- on a single-phase bridge circuit with three levels of AC voltage (figa);

- according to a single-phase five-level scheme (figb);

- according to a modular multi-level scheme [15], (figa, 3b).

The choice of one or another of them is determined by the conditions of use. Latitudinal modulation should be sufficiently high frequency. The pulse width modulation is suppressed by the LC filter (not shown in FIGS. 2 and 3). The reactance X = ω _S L and admittance Y = ω _S C of the filter at the network frequency ω _{S are} negligible. The servo PWM converter is equipped with the usual set of blocks for the implementation of servo PWM: transistor drivers, sensors and a fairly powerful signal processor. The latter carries out the functions of a modulator and the required additional functions. The salient features of the PWM servo converter are as follows. Converter output voltage v _ae (t) with sufficient accuracy repeats the reference signal v _z (t); distracting from the scale can be written

In addition, the converter itself is a non-energy-accumulating and non-energy-dispersing unit (non-energetic; non-dissipativ); the power at the output of the converter P _ae (t) coincides with the power passing to the DC link Pd (t):

(v _d is indicated here, i _d - voltage and current in the DC link of the converter, i _ae - converter output current).

; при укрупненном рассмотрении вместо суммы можно рассматривать некий эквивалентный конденсатор. Для дальнейшего рассмотрения активный элемент - следящий PWM-конвертор - это управляемый четырехполюсник с двумя портами: переменного тока v_ae, i_ae и постоянного тока v_d, i_d, описываемый уравнениями (1, 2).For a modular multi-level circuit where there are several storage capacitors, the last expression will include the sum

; when enlarged, instead of the sum, we can consider a certain equivalent capacitor. For further consideration, the active element - the tracking PWM converter - is a controllable four-terminal with two ports: alternating current v _ae , i _ae and direct current v _d , i _d described by equations (1, 2).

An active arcing device is formed (see Fig. 4) by connecting the AC port of a single-phase PWM servo converter 1 between the neutral of the supply network of the transformer 2 and ground 3 in series with the current sensor 4. With respect to the three-phase AC network (in Fig. 4 single-line representation) it is a controllable two-terminal. The port (or ports) of the DC voltage when using the tracking PWM converter 1 in the arcing device is connected only to the storage capacitor 5 (capacitors). Connection to a source or drain of constant voltage of comparable power is not required; it is enough to have an attachment to the low-power charger for the preparatory charge of the capacitors 5. When used in an arcing device, the tracking PWM converter 1 contains, in addition to non-specific functional blocks, such as modulator 6 (mdl), current regulator 7 (regi), a specific function block - master 8 lag current iz (t), denoted by geniz. Figure 4 also shows the PWM converter 1 included in the composition:

- actually the voltage converter 9 (power unit, made in accordance with one of the options depicted in figure 2 and 3),

- LC filter 10, suppressing pulsations of latitudinal modulation,

- adder 11,

- auxiliary power supply unit 12 (bpsn), for powering the control system,

Figure 4 also shows the busbars 13 of a three-phase electric network with switching devices 14 both from the supply transformer 2 and from the consumers (consumers are not shown in Fig. 4) and the mains voltage transformer 15.

Figure 5 shows a structural diagram of a three-component DCB (D - damping, C - compensation, B - balance) of the current generator 8 iz (t), the functional blocks of which are implemented on the basis of a powerful signal processor.

The block diagram includes:

- adders 16, 17, 18,

- multiplication blocks 19, 20, 21,

- integrators 22, 23,

- block 24 of the balance regulator,

- block 25 of the voltage or energy regulator of the storage capacitors 5 of the voltage converter 9.

The input signals of the setter 8 of the current are the signals calculated in the control system:

- vn is the neutral bias voltage,

- Gdemp - damping conductivity coefficient,

- Yz is the required conductivity coefficient of compensation,

- εz is the nominal (calculated) energy level of the storage capacitor (s) 5,

- ud - current (measured) value of the voltage of the storage capacitor (s) 5.

Figure 6 presents one of the possible embodiments of the block 24 of the balance regulator. Block 24 of the balance controller contains a high-speed PID (proportional-integral-differential) controller 26, and a second harmonic filter 27. The PID controller includes integral 28, proportional 29 and differential 30 links with time constants tint, t1, tdif, respectively. The outputs of the links 28, 29, 30 are summed by the adder 31 with the corresponding coefficients 1, Kpr, Kdif. To reduce ripple, the second harmonic is additionally filtered using synchronous filtering. To this end, the output signal Gbal ^{I of the} adder 31 is fed to the input of the second harmonic filter 27. The output signal Gbal block 24 of the balance regulator is supplied to one of the inputs of the block 21 of the multiplication (see figure 5).

Fig. 7a shows a network with two supply transformers 32 and 2 with an arc suppression coil 33 on the first of them and with a tracking PWM converter 1 on the second. On figb depicts the neutral equivalent circuit used to identify the network parameters in the control system of the tracking PWM converter 1, grounding the neutral of the second transformer 2. The equivalent circuit (figb) consists of L, C, and R connected in parallel, the values of which are calculated in the control system (in and in vn - neutral current and voltage).

On Fig shows a variant of an extinguishing device with parallel operation of an unregulated choke 34, made at half the required reactive power of the grounding two-terminal (Q _N / 2) and tracking PWM converter 1 for the same power (Q _N / 2).

Figure 9 shows a variant of an extinguishing device with the connection of a tracking PWM converter 1 through a matching transformer 35.

Another possible option (the circuit is the same as in Fig. 9) is to make the matching transformer 35 in the form of a transformer-reactor, with an idle power equal to half the required rated power of the arcing device (Q _N / 2). The required installed power of the tracking PWM converter 1 is also half the rated power (Q _N / 2).

Figure 10 presents a diagram of an automatically adjusting arc suppression device with a tracking PWM converter 1. Auto-tuning functions are performed by the automatic tuning block 36 of the microprocessor control system, including:

- a subunit 37 for calculating the required addmittance (neutral conductivity coefficient) for an active two-terminal device - a tracking PWM converter 1,

- generator 38 test signal.

The control system for auto tuning has an adder 39.

On figa presents a functional diagram for determining the resistive Gn and capacitive Yn components of the conductivity of the network, as well as the desired coefficient of conductivity YZ compensation.

On figa designated:

a generator 40 (genθ) generating an orthogonal sinusoidal pair of variables cosθ and sinθ having a frequency f ^I different from the network frequency f,

- multiplication blocks 41, 42, 43, 44,

- adder 45,

- low pass filters 46, 47,

- unit 48 for calculating the resistive Gn ^I and capacitive Yn ^I components of the network conductivity at a test frequency f ^I different from the network frequency f,

On figb, presents the equivalent circuit of the network at a test frequency f ^I different from the network frequency f.

On Fig presents a functional diagram for determining the resistive Gn and capacitive Yn components of the conductivity of the network, as well as the required compensation conductivity coefficient YZ for the case when the tracking PWM converter 1 operates in parallel with an unregulated arc suppression reactor in a circuit of the form of Fig. 7a or in circuits of the form Fig.8, Fig.9. The circuit is constructed similarly to the circuit in Fig. 11a, differing from it by a double set of multipliers (multiplication blocks 49, 50, 51 are added) and low-pass filters (filters 52, 53 are added), as well as by the adder 54 introduced.

The device operates as follows.

As noted above, the tracking PWM converter 1 contains (see FIG. 4) the functional blocks commonly used in such devices: a modulator 6 and a current regulator 7. For use as an arc suppression device, a specific functional unit-setter 8 of the lagging current iz (t) is introduced into the PWM servo converter 1.

The current regulator 7, the construction of which at a sufficiently high modulation frequency does not cause difficulties, ensures that the output current of the tracking PWM converter 1 is equal to the task: i _ae = i _z ;

The current unit 8 generates a variable i _z (t), which in the steady state is behind the bias voltage vn by an angle of about 90º.

where the terms in parentheses are the mains phase voltages measured by the mains voltage transformer 15.

To obtain such a lagging variable, it is possible to use the function of admittance of the Petersen coil as the transfer function of the current generator 8

where Lo, Ro are the inductance and resistance of the coil. In this case, the PWM servo converter 1 would function as a passive two-terminal device. However, the use of the tracking PWM converter 1 allows you to take two steps forward. Firstly, the PWM servo converter 1 can simulate in the power circuit not only an inductor, but also a combined circuit composed of an inductance and a shunt resistor R ₀₀

These inductors and resistors are virtual; they are carried out no more than a couple of operators in the microprocessor control system of the tracking PWM converter 1, but in the power circuit they manifest themselves as a choke and a resistor. In the suppression devices described in [14], the damping resistor R _{00 of the} combined suppression circuit remains active even when it is no longer needed and spoils the characteristics of the mode. In the follow-up PWM converter 1, you can take the next improvement step, and remove the action of the virtual damping resistor as it becomes unnecessary. This step in a system with active suppression is not only possible, but also necessary. Energy absorbed by a virtual resistor

напряжения нейтрали v_n. Уровень воздействия определяется коэффициентом проводимости баланса Gbal:transmitted by the voltage converter 9 to the DC link (storage capacitor 5). In the proposed suppression circuit, the DC link is not connected to some source of comparable power, but only to the storage capacitors 5. To maintain the voltage (Ud) of the storage capacitors 5 in the vicinity of the rated voltage (Udz), the third component must be added to the current setting iz - balance component ibal. The balance component acts in the direction of the fundamental

neutral voltage v _n . The level of exposure is determined by the conductivity coefficient of the balance Gbal:

,

,

which in turn is determined by the voltage or energy regulator of the converter capacitors

where F (p) is the conversion function of the block 24 of the balance regulator,

Ez and Ed are the nominal (calculated) and measured (calculated) energy levels of the storage capacitor (s) 5, respectively.

Thus, a current setter system for an active arcing device is obtained in which the task current is composed of three components (Fig. 5):

- compensation component acting along the orthogonal direction of the neutral voltage with a conductivity coefficient of compensation Yz

- damping component acting on all components of the neutral voltage with a damping conductivity

is the balanced component acting in the direction of the main harmonic of the neutral voltage with the conductivity coefficient of the balance Gbal, which is determined by the voltage balance (energy) regulator of the storage capacitors (6, 7).

The current reference iz is the sum (adder 16 in FIG. 5)

и ортогональная составляющая v_n,ort могут быть получены с помощью фильтра второго порядка, изображенного на фиг.5 (блоки 17, 22, 23),Fundamental harmonic

and the orthogonal component v _{n, ort} can be obtained using the second-order filter shown in Fig.5 (blocks 17, 22, 23),

or in some other way.

An active arc suppression device made on the basis of a tracking PWM converter 1 with a three-component DCB current detector (D - damping, C - compensation, B - balance) provides damping no worse than the combined arcing circuit described in [14] (reactor with resistor) , but at the same time provides full compensation of capacitive current in steady-state circuit conditions.

As noted above, when used in branched networks, adjustable arc suppression devices are equipped with an automatic tuning system. Their action is based on the fact that the test signal, which has a frequency different from the mains frequency, is mixed with the main components of the neutral current or voltage. Based on the response to the test signal, the parameters of the neutral equivalent circuit of the network are determined, and the required conductivity coefficient of the grounding device is determined by these parameters. For this purpose, in arc suppression devices with plunger reactors or in bias-controlled reactors, a special additional winding and a special block of the test signal generator and response analyzer are used [5, 7, 9, 10]. When using the proposed device, the tasks of automatic tuning are greatly simplified. The tracking PWM converter 1 can inject test current or test voltage into the neutral network in parallel with performing the main function without any additions to it. The desired test signal itest is mixed with the current reference signal iz of the converter using adder 39:

,

,

as shown in FIG. 10. The current regulator 7 and voltage converter 9 provide a current equal to the reference to the neutral of the network, i.e. a component of the test frequency appears in the composition of the neutral current, mixed with the component of the network frequency.

,

,

Thus, the tracking PWM converter 1 simultaneously with the main function fulfills the function of a powerful low-frequency amplifier, which in arc suppression devices with inductors [5, 7, 9, 10] is a separate additional device. Receiving a test signal in the proposed system does not require any additional equipment at all; the itest signal is generated by the microprocessor control system of the tracking PWM converter 1 and is amplified by the tracking PWM converter 1 itself without any hardware additions. Analysis of the response to the test signal is also carried out by the microprocessor control system of the tracking PWM converter 1, again without requiring the use of dedicated additional devices in the suppression system. All the variables necessary for this in the control system of the tracking PWM converter 1 are already available for the execution of its main function, and the computational capabilities of modern signal processors easily combine the calculation of the response and their processing with the performance of basic functions. The task of automatically determining the required conductivity coefficient of the arcing device (blocks 37 and 38, figure 10) can be carried out by any of the known applied algorithms.

Figure 11 presents one of the possible functional schemes for determining the required conductivity coefficient Yz, constructed on the basis of synchronous filtering. Generator 40 (genθ) generates an orthogonal sinusoidal pair of variables cosθ and sinθ having a frequency f ^I different from the network frequency f:

.f ≠ f,

.

The value of the neutral voltage v _{n is} calculated by the adder 45 in accordance with the expression (3).

The test signal itest (t) is obtained using the multiplication block 41

itest = Itest × cosθ ′,

where Itest is the selected amplitude of the test current. In the system of orthogonal coordinates, the complex amplitude of the test current is

Itest = Id + jIq, Iq = 0.

Complex response amplitude

Vtest = Vd + jVq

is extracted from the neutral voltage v _n by the method of synchronous filtering by multiplying v _n by the reference variables cosθ and sinθ (multiplication blocks 42 and 43) and then by low-pass filters 46 and 47. The resistive Gn ^I and capacitive Yn ^I conductivity components are calculated by the conductivity component calculation unit 48 networks of complex amplitude components

The capacitive conductivity at the test frequency f ′ then determines the required conductivity coefficient Yz at the network frequency (multiplication unit 44)

All actions according to the scheme of Fig. 11 are performed by the microprocessor control system of the tracking PWM converter 1; no hardware additions required

When the converter is operating in parallel with an uncontrolled extinguishing reactor in a circuit of the type of FIG. 7a or in schemes of the type of FIG. 8, FIG. 9, the neutral equivalent circuit becomes a second-order circuit (FIG. 7b). The auto-tuning algorithm with a test signal of frequency f ′ ≠ f under these conditions is insufficient. However, this algorithm can be easily modified. To identify the parameters of a more complex equivalent circuit in the block 36 auto-tuning (figure 10) should be provided for the generation of a two-frequency test signal with frequencies f ′, f ″ that are different from each other and from the network frequency f

f ′ ≠ f ″ ≠ f.

The test signal is generated by the generator 40 and the adder 54 in the form:

itest = Id ₁ · cosθ ′ + Id ₂ · cosθ ″,

,

.

,

.

In the functional diagram shown in Fig. 12, a double set of multiplication blocks (41, 42, 43, 49, 50, 51) and low-pass filters (46, 47, 52, 53) are used to process the response, the outputs of which are the response components (per test signal) Vd ′, Vq ′, Vd ″, Vq ″. Based on these values, taking into account Iq ′ = 0, Iq ″ = 0, in block 48 (calc), L _n , C _n , R _n are calculated and then the required neutral conductivity coefficient Yz for the tracking PWM converter 1 is determined.

The operation of the inventive controllable semiconductor converter is illustrated by process graphs for capacitive current compensation and network identification obtained by mathematical modeling in the MathCad package (Fig. 14 - Fig. 23).

For the purposes of a comparative study of the possibilities of active arcing, the results of modeling a combined arcing system [14], in which an arcing coil with inductance Lo and a damping resistor with resistance Roo are used, are presented. A network with a voltage of 6.3 kV and a power of 5.6 MVA is considered, in which the steady-state capacitive current to earth is about 100 A. The nominal reactance of an arc suppression coil is

ω _s · Lon = 36.75Ω.

The actual Lo coil inductance may differ markedly from the required value, the range in question starts from

(overcompensation) and ends with

(undercompensation). The processes considered in this part provide the basis for the subsequent evaluation of a system with active suppression. The processes are considered in the simplest equivalent circuit (Fig.13).

The graphs of Arc 02 19 (Fig. 14) show the process with high resistance of the shunt resistor.

,

,

so only the arc suppression coil works. Arcing coil inductance taken with overcompensation

.

.

The upper graphs show the breakdown voltage vd (·) and the voltage at the earthing bipolar v0 (·). The critical stress graph vkrit is also derived, at which a gap breakdown occurs. The average graphs show the arc current id (·) and the current of the grounding two-terminal circuit i0 (·). The lower graphs show the phase voltages relative to the ground va (), vb (), vc ().

, a выделяющаяся в дуге энергия достигает Endu=5.2 кДж.The graphs show that with a large resistance of the shunt resistor, the process is completely unsatisfactory. In the interval t = 600 ms, repeated breakdowns follow one after another. Overvoltages reach multiplicity

, and the energy released in the arc reaches Endu = 5.2 kJ.

The graphs of Arc 02 20 (Fig. 15) show that the introduction of a shunt resistor with less resistance

corrects the process. After five successive breakdowns, the strength of the insulation gap is restored and the normal network mode is established. The maximum overvoltage decreases to = 2.45, and the energy released at the breakdown point decreases to 1.9 kJ.

The graphs of Arc 02 23 (Fig. 16) show the same processes as above, but with undercompensation

.

.

The results are similar to those described above: with an intermediate value of the conductivity of the shunt resistor, after several breakdowns, the normal operation of the network is restored.

The graphs Arc 02 26 (Fig) show the processes with fine-tuning compensation

The fine-tuning voltage rises slowly during fine tuning, and the shunt resistor under these conditions does not improve the process. Nevertheless, the use of a shunt resistor is useful because it mitigates the effect of unavoidable tuning errors.

The above (see Fig. 14, 15, 16, 17) consideration of the combined arcing circuit [14] is auxiliary. Its purpose is to provide a basis for evaluating the proposed active extinguishing device. The circuit of FIG. 4 is analyzed, in which the current controller 8 is made as in FIG. 5. The network equivalent circuit is the same (Fig. 13) that was used when considering the combined arcing device [14]. The equivalent circuit parameters and the arc model parameters are also saved.

, при недокомпенсации

и точной настройке

. Резистор Roo в активном дугогасящем устройстве является виртуальным. Его проводимость взята равной Roo=1.5×ω_SLon.The graphs Arc 05 01, 02, 03, (FIGS. 18, 19, 20) show the processes of an active suppressing device during overcompensation

under undercompensation

and fine tuning

. The Roo resistor in the active arc suppression device is virtual. Its conductivity is taken equal to Roo = 1.5 × ω _S Lon.

The variables and notation on the Arc 05 01, 02, 03 graphs are the same as above. A comparison of the graphs of the active arc suppressing device Arc 05 01, 02, 03 (Fig. 18, 19, 20) with the graphs of the combined arc suppressing device Arc 02 20, 23, 26 (Fig. 15, 16, 17) shows that each of these devices in considered conditions gives after a series of breakdowns the restoration of normal operation of the network. The quantitative indicators of the processes are also very close, as can be seen from the following table:

Value Detuning Combined circuit Active device

1.25 2.3 2.41 one 2.0 2.26 0.8 2.45 2.32 ε, kJ 1.25 2.19 2.33 one 0.62 1.52 0.8 1.92 1.47

Thus, an active suppression device in conditions of recovering breakdowns is equivalent to a combined suppression device [14], composed of a reactor and a resistor. The difference between the functional characteristics of these devices is manifested in another class of situations when a short circuit to ground is established and maintained. The energy balance system of the storage capacitors in these situations reduces the active component of the current to zero, thereby reducing the current in the short circuit period; with fine tuning, the latter is reduced to zero, as shown in the graphs Arc 05 04, 04 (Fig.21, 22). The combined circuit [14] with a steady short circuit injects the residual current into the circuit even with fine tuning (Arc 02 graphs 28 of Fig.23), which, of course, is its drawback. With inaccurate tuning, the advantage of an active arc suppressing device is somewhat reduced, since the adjustment error of the balance system is not compensated. Nevertheless, a certain decrease in current in the short circuit period is nevertheless obtained.

Everything described above equally applies to the version of the arc suppression device based on the tracking PWM converter 1 (Figs. 4, 7, 10), and to other variants (Figs. 8, 9), the choice of which is due to technical and economic considerations .

On Fig presents a variant of an extinguishing device with an unregulated choke 34, made at half the required reactive power of the grounding two-terminal network (Q _N / 2) and tracking PWM converter 1 for the same half power (Q _N / 2). When adjusting the converter in the full range ± Q _N / 2, the total admittance is regulated in the full range - from zero to the nominal value.

Similarly, for price or other reasons (for example, to perform a converter for low voltage) it may be useful to use a matching transformer 35 (Fig. 9). This transformer 35 can also be designed as a transformer-reactor, with an idle power equal to half the required rated power of the arcing device. The required installed power of the tracking PWM-converter 1 is also half the rated power.

So, as brief results, we can note:

- it is proposed to use an active element as a controlled extinguishing device - a tracking PWM converter 1, by itself or in combination with an unregulated choke 34 or transformer-reactor 35;

- a control algorithm is proposed with the generation of a three-component signal for setting the current of the tracking PWM converter 1 (DCB algorithm);

- a preliminary comparative study of the characteristics of an active arrester with a combined RL-type arrester [14] showed that in situations with intermittent breakdowns, their characteristics are close, and in situations with established short circuits, the active device - tracking PWM converter 1 has the advantage;

- the power equipment of the tracking PWM converter 1 and its microprocessor control system can be used as equipment for network testing and automatic tuning of the arcing device, thereby eliminating the need for any additional equipment for auto-tuning.

Thus, with the above performance of the claimed device, the basic functions are provided - compensation of capacitive current in case of a single-phase earth fault, as well as additional useful functions: the ability to identify the network without the use of auxiliary equipment, the rapid suppression of transient components of the current, including during repeated breakdowns, accelerated decrease in arc current, exit from compensation mode when the circuit breaks.

Based on the foregoing, the task of creating a device for compensating the earth fault current in three-phase electric networks based on a single-phase PWM servo converter, which along with the basic additional useful functions, such as the ability to identify the network, quickly suppresses transient components of the current, accelerates the reduction of the arc current, with the simultaneous elimination of the above disadvantages inherent in UPER [5] resolved.

Information sources:

1. Petersen W. Der aussetzende Erdschluss. ETZ, Bd 38, S.553-555; ETZ, Bd 47, S.564-566; ETZ, Bd48, 1917.

2. Likhachev F.A. Earth fault in networks with isolated neutral and with compensation of capacitive currents. M. Energy, 1971

3. Chernikov A.A. Compensation of capacitive currents in networks with non-grounded neutral .., M. Energia, 1974

4. Arc extinguishing reactors, technical information, www.energan.ru., LLC ENERGAN, 2007.

5. Gernot Druml, Olaf Seifert. Arc extinguishing reactors 6-35 kV. A new method for determining network parameters. Electrical Engineering News, No. 2 (44), 2007.

6. Koenigi R. Arc suppression coils - the key component of modern earth fault protection systems. Papp K. Transmission and Distribution Conference and Exposition: Latin America (T & D-LA), p. 366-371, 2010.

7. Arc Suppression Coils. Trench brochure. www.trenchgroup.com. TRENCH Group 2012.

8. Controlled magnetizable arc suppression reactors with automatic compensation of capacitive earth fault current for 6-35 kV networks. Bryantsev A.M., Lurie A.I., Dolgopolov A.G. et al. Electricity No. 7, 2000

9. Control and protection systems for arcing reactors controlled by bias. Bryantsev A.M., Dolgopolov A.G. Power plants No. 2, 2000

10. RF patent No. 2130677, H02J 3/26, H2N 3/17, publ. 05/20/1999 Authors: Bryantsev A.M., Dolgopolov A.G.

11. Peters I.F., Slepian I. Voltage Inducted by Arcing Grounds. Tr. AIEE, p. 478-489, 1923.

12. To the theory of overvoltage from grounding arcs in a network with isolated neutral. Juvarly Ch.M., Electricity No. 6, pp. 18-27, 1953

13. The study of overvoltage during arc short circuits to the ground in networks of 6 and 10 kV with isolated neutral. Belyakov N.M., Electricity No. 5, pp. 31-36, 1957

14. M. Ilyinykh, L. Sarin, A. Shirkovets, E. Buyanov. Compensated and combined grounded neutral. Operating experience of a 6 kV network of a metallurgical plant. Electrical Engineering News, No. 2 (44), 2007.

15. DE 10103031 B4, 2011.12.01, H02M 5/42. Marquardt Rainer. St romrichterschaltung mit verteilten Energiespeichern und Verfahren zur Steuerung einer derartigen Stromrichterschaltung

Claims (9)

1. A device for compensating the earth fault current in three-phase electric networks, comprising a two-terminal network connecting the neutral of the supply transformer network to the earth and generating a lagging current to compensate for the leading capacitive current of a single-phase earth fault, as well as a neutral current sensor, network voltage transformer, test signal generator for measuring the network capacity and a computing unit for determining the required admittance of said two-terminal network, characterized in that as a grounding two-terminal network a tracking pulse-width modulated voltage converter (tracking PWM converter) is used, equipped in addition to the usual set of functional blocks with a current sensing unit, the inputs of which are the neutral voltage and the required admittance, the output is a current reference supplied to the current regulator input of the said tracking PWM converter , and the transfer function of the current regulator provides a lag phase shift of about 90 degrees at the mains frequency. 2. The device for compensating the earth fault current in three-phase electric networks according to claim 1, characterized in that the transfer function of the setter, in addition to the reactive (first) component, contains a damping component proportional to the input signal, which in the action of the converter appears as a shunt resistor (virtual resistor). 3. The earth fault current compensation device in three-phase electric networks according to claim 1, characterized in that the DC voltage capacitors of the PWM-converter (storage capacitors) are made "suspended" (not connected to a voltage source or drain of comparable power), and in an additional component (balance component) has been introduced to the transfer function of the setter, which is generated by the voltage mismatch of the storage capacitors of the tracking PWM converter and ensuring the maintenance of their voltages OLO nominal level. 4. The device for compensating the earth fault current in three-phase electric networks according to claim 1, characterized in that an uncontrolled coil is introduced into the composition of the grounding two-terminal to half the required reactive power of the grounding two-terminal (Q _N / 2), connected in parallel with the tracking PWM converter, which it is also made at the same half power (Q _N / 2); moreover, the regulation of the tracking PWM converter in the full range from -Q _N / 2 to + Q _N / 2 (from 90 degrees ahead of the currents to 90 degrees behind the currents) provides control of the total admittance in the full power range - from zero to the nominal value Q _N. 5. The device for compensating the earth fault current in three-phase electric networks according to claim 1, characterized in that the tracking PWM converter is connected through a matching transformer. 6. The device for compensating the earth fault current in three-phase electric networks according to claim 1, characterized in that the PWM servo converter is connected through a matching transformer (transformer-reactor) with an open-circuit power Q _N / 2. 7. The device for compensating the earth fault current in three-phase electric networks according to claim 1, characterized in that the PWM servo converter, in parallel with performing the main function (capacitive current compensation), injects (injects) the test current or test voltage into the neutral network, and the required admittance is calculated in the control system by the response of the test frequency in the neutral voltage. 8. The device for compensating the earth fault current in three-phase electric networks according to claim 1, characterized in that the transfer function of the master along with the reactive (compensation), damping and balanced components contains an additional component - a test signal with a frequency (ωtest) other than the network (ωs) (for example, ωtest = 0.5ωs) generated in the control system of the tracking PWM converter, the test signal being the sum of two or more harmonic components of different frequencies, different from the network one. 9. The device for compensating the earth fault current in three-phase electric networks according to claim 1, characterized in that the control system of the tracking PWM converter, based on a powerful signal processor, combines the performance of functions specific to devices for compensating the earth fault current in three-phase electric networks, (such as generating test signals, processing responses to test signals and calculating the required indicators) with non-specific (normal) control functions of the tracking PWM converter, thereby eliminating the need for additional equipment.

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