SU754556A1 - Device for maximum current protection of three-phase load - Google Patents

Description

The invention relates to electrical engineering and can be used to protect an asynchronous electric motor supplied from a thyristor voltage regulator, for example, for electric drives of crane mechanisms in the event of a break in the thyristor circuit ⁵ or when the control pulse disappears on the thyristor.

A device for protection of three-phase electrical installations from asymmetry of phase currents is known, comprising a rectifier, a second harmonic filter, a smoothing filter, and a comparator unit. Filter the second harmonic of the variable component of the rectified voltage selects the voltage of the second harmonic, which is rectified, smoothed and then compared with the DC component of the rectified voltage and at their output to a specific ratio ustroy- _m-OPERATION appears a signal used for protection.

The disadvantages of the device are its inoperability when

2

in the current load of the constant component, as well as the inability to use to protect the three-phase circuit supplied from an unbalanced thyristor voltage regulator.

It is also known a device for monitoring valve failures. It uses a differential transformer, the magnetic flux of which is in good condition of the thyristors is compensated by an additional winding that receives power from an adjustable source ^ 2].

A disadvantage of this device is the impossibility of its use for a three-phase load connected to the power source by a three-wire line, as well as when adjusting the angle of unlocking the thyristors.

The closest to the proposed technical essence and the achieved effect is a device for current protection of an induction motor and thyristors from unacceptable values of current

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the stator. This device contains three current transformers, the primary windings of which are included in each phase of the load, and the secondary windings are shunted by resistors, connected in a star and connected!.! to the input of the three-phase rectifier, and the output of the rectifier through a smoothing filter is connected to the actuator and. The disadvantages of this device are its limited functionality and, in particular, inoperability in the presence of constant components of the phase load currents, which appear, for example, when the thyristor circuit is broken, as well as when the control pulse disappears on the thyristor and can lead to overload current elements of the load, which can be, for example, a three-phase asynchronous motor. An analysis of the operating modes of an asynchronous electric motor with a faulty thyristor voltage regulator shows that at lower speeds the losses in the stator windings increase by almost 3 times compared with the normal mode of operation.

This is the reason for the occurrence in practice of the failure of a motor operating with the thyristor voltage regulator indicated above.

The purpose of the invention is to expand the functionality of the device.

This goal is achieved by the fact that the device for overcurrent protection. a three-phase load containing a thyristor voltage regulator connected to the phases of the protected load, current transformers in each phase, the primary windings of which are connected to the phases of the protected load, and the secondary windings are loaded on the corresponding resistors, are connected in star and connected to the inputs of the three-phase rectifier, output which through a smoothing filter is connected to one input of the executive body, the output of which is connected to the control input of the thyristor: voltage regulator, additionally introduced three azny transformer unit, the single-phase full-wave rectifier and a smoothing filter, the three-phase primary windings of the transformer unit are connected to the phases of the protected load, and the secondary windings connected to open delta, connected through single-phase full-wave rectifier, and the newly introduced ·

smoothing filter to another input

the aforementioned executive body.

The three-phase transformer unit is designed as a three-phase voltage transformer, the primary windings of which are connected in a star.

The three-phase transformer unit is made in the form of additional secondary windings of the above-mentioned current transformers connected in an open triangle.

FIG. 1 shows a functional diagram of the device; in fig. 2 and 3 are respectively functional diagrams of the device when using a voltage transformer and additional secondary windings of current transformers as a three-phase transformer unit; in fig. 4 - experimental mechanical characteristics of an asynchronous electric drive with a thyristor voltage regulator. The solid lines show the experimentally obtained mechanical characteristics of an asynchronous motor type MT-1I-6 3.5 kW, 908 rpm with power from a healthy thyristor voltage regulator: in an open system with fully open controller, characteristic a and rigid mechanical characteristics b, c, g for a closed drive system. The dashed lines show the corresponding characteristics of d, e, g, s, in the absence of current in the circuit of one thyristor, which occurs, for example, in the absence of a pulse on the control electrode of this thyristor. The characteristics in FIG. 4 removed for the circuit in FIG. 2, where a three-phase load 1 uses a motor with slip rings.

The device contains a three-phase load 1, powered by a thyristor voltage regulator 2 (which can be made both symmetrically and asymmetrically), the transformers 3, 4 and 5 of the primary overlays are connected in series with the output terminals of the thyristor regulator 2 'voltage ^ a secondary the windings are bridged by resistors 6.7 and 8 and connected to a star, the outputs of which are connected to the input of a three-phase rectified bridge 9, the output of bridge 9 is connected via a smoothing filter 10 to one 5

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th input of the executive body 11, a three-phase transformer unit 12 primary windings of which 13, 14 and ^ are connected to the load phases, and the secondary windings 16, 17 and 18 are connected in an open triangle of the bridge and connected to the input of the rectifier bridge 19, the outputs of which are connected through a smoothing filter 20 to the second input of the Executive body 11, and the output of the latter is connected to the input of the control system of the voltage regulator 2.

In the device (see Fig. 2), a three-phase voltage transformer is used as a unit 12, the primary windings of which are connected in a star and connected to the terminals of a three-phase load 1, and the secondary windings are connected in an open triangle and connected to the input of a rectifier bridge 19.

In the device (see, fig. 3), current transformers 3, 4 and 5 contain additional secondary windings 16, 17 and 18, respectively, connected in an open triangle and connected to the input of a rectifier bridge 19.

The device works as follows. _h

If the phase currents of the load 1 have unacceptably large values and do not contain constant components, then at the first input of the actuator 11 connected to the three-phase rectifier 9, a voltage appears causing it to trigger, which causes the thyristor voltage regulator 2 to turn off (for example, by removing control pulses from control electrodes of thyristor regulator 2).

If there is no current in the circuit of one thyristor of the regulator 2 (for example, due to a broken contact of the conductor clamp suitable to the control electrode of the thyristor), then constant components of currents appear in the load circuit. In this case, due to the saturation of the current transformers 3, 4 and 5, the signal at the output of the rectifier 9 is insufficient for actuating the actuator 11, however, a voltage appears at its second input causing the actuator to operate, which causes the thyristor regulator 2 to turn off voltage. Suppose that the three-phase transformer unit 12 consists of three separate middle plates, one of which has windings 13 and 16, the other windings 14 and 17, and the third windings 15 and 18, Let's accept the primary windings 13, 14 and 15 are connected to phases

loads, and secondary windings 16, 17

5 and 18 are connected in an open triangle and connected to the input of the rectifier 19. It is also admissible that the resistance of the second input of the executive body 11 is sufficiently large. Wherein

10, the current flowing through the windings 16, 17, and 18 can be neglected, and in this case, it can be assumed that the three-phase transformer unit 12 is in idle mode. We will also accept

15 that the sum of currents, ΐ ₄₄ , flowing through the primary windings 13, 14

and 15 is zero, i.e.

ϊ, _Λ + 'ΐ, + 1 g. - 0. 43 44 45

(one)

20 If, under the specified conditions, the currents of the load 1 supplied by the voltage regulator 2 flow currents, then the secondary windings 16, 17 and 18 of the transformer unit 12 induce electric 25 driving forces, respectively

(2 /

47

? = - =

^ha <51

(3 /

thirty

35

'lb' /

(41

where ψ.

oi

- flux linkage of the windings 16, 17 and 18, respectively;

- mutual inductances between windings 13 and 16, 14 and 17, 15 and 18

accordingly.

In this case, the voltage on the input of the rectifier 19

⁴⁵ and -r »2. - ^ 1 ^КЛ ^ <з ^{+ м} а'лч ^{+ м} з'45 ') ·· ^υ βΜ9'V Cha - (51

If the phase currents of the load 1 do not contain constant components, then

50 " · · . ·

The currents ι, I ^ 5, respectively, in windings 13, 14 and 15 do not contain constant components, while all three-phase transformer flea cores 12 operate on linear sections of magnetization curves, and since these cores are made the same, then mutual inductances Μ, M ₂ ^and M are equal to each other, and the voltage

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K /., Is equal to zero, which also follows from the formula of expression (5) in view of the relation (I).

If the phase currents of the load 1 contain constant components, then the currents 1 ^ 3, '44, ϊ <5 in the primary windings of the three-phase transformer element also contain constant components, and the sum of the constant components of the currents in these primary windings according to (1) is zero. In this case, the constant components of currents of different values flow through the windings 13, 14 and 15. If the values of the constant components of currents are the same, then their sum does not equal zero, which contradicts the condition (ΐ). The presence of constant components of currents in the windings 13, 14 and 15 causes saturation of the cores of the three-phase transformer element, and the degree of saturation of the various cores is not the same, since the constant components of currents of equal magnitude flow through the windings 13, 14 and 15, as shown above. Consequently, the mutual inductances M, M 2, M3 are not equal between

a, AV this case, the voltage at the rectifier input ^ 19, and accordingly stvennoj ^A second input actuator 11 is not equal to zero.

Indeed, suppose that zero is the sum of the form

Μ ί + KA ι l Μ ι -O (345 (θ)

Then from the last expression, taking into account formula (1) it follows

ten

1M"

(7)

40

whence it is clear that between the values of currents (dd and '45 At every moment of time, there is a quantitative relationship determined only by the values of aerodynamic inductances, but this cannot be, since the currents ι 44 and variables change in time; for example, ^ at one moment time is equal to zero the current at another moment of time is equal to zero. Therefore, the sum of the form ( _MD » _4e + M2 × 4 + M _% » 4 ₉ ) with different MD, M ₃ cannot equal zero, and according to the formula (5) and voltage 9 ^^ is not zero.

/

Therefore, in the absence of constant components of the phase load currents 1 voltage at the input of the rectifier

15

20

19, and, accordingly, at the second input of the executive body 11, is zero for both the symmetric and asymmetric circuits of the voltage regulator 2, since the reasoning does not use the condition that the currents of the same amplitude pass through the phases, but take into account only the ratio ( one).

Three-phase transformer unit 12 can be made in the form of voltage transformers, and in the form of current transformers, since in both cases the formulas (2) - (5) are valid.

Three-phase transformer unit 12 can be performed not only on the basis of three identical cores, but also on the basis of a multi-phase magnetic system used, for example, for a three-phase transformer (see Fig. 2). Indeed, in the latter case, the flux linkages C> _l ,> Ψ3 secondary

windings of the transformer unit are equal

V ^ 44-46 ^ 4,5-46 ^ 5 ’

^W, 2 ^{= M} 2'44 ^{+ C} 43-41'43 ^{+ M} 45-41'45

(eight)

(9)

(ten)

thirty

’+ ΗΛ,

3 45 43-4 $ 43 44-П 44 ’

35

45

50

where m, ..... M, .. - weaimnoinduktiv44-10 44 "17

between primary cores and secondary windings located on different cores (for example, mutual inductance between windings 14 and 16).

If the currents flowing through the primary windings 13, 14 and 15 do not contain constant components, then the magnetic system of the three-phase transformer element is not saturated, and the mutual conductance between the primary and secondary windings located on the same core and between the primary and secondary windings located different cores are the same, i.e.

55

(and)

M ~ KA * KD L446 ^ 4-2 / (12)

where M <e and M42 are constant values.

9

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Under conditions (1'1) and (12), taking into account the relation (1), from (8) and (10) it follows

(13)

(14)

(15)

whence it is similar to formulas (2), (3), (4) and (5) with regard to relation (1)

ten

VM9 4 <a 47 LA

(Mi) '

four',

SI (16.)

those. in cases of absence in phase ₁₅

load currents 1 constant components of the voltage O ^ Dravko zero. If, however, the phase currents of load 1 contain constant components, then, similarly to the above, different cores 20 will be saturated to different degrees, mutual inductances between different windings will differ from each other and the voltage will be different from zero. 25

The principle of operation of the device in figure 2, where as a three-phase transformer unit voltage transformer is used is similar to the above. Since the primary windings are

the transformer is connected to a star, then the sum of the currents flowing through these windings is zero.

In the device of FIG. 3 as a three-phase transformer unit 35

additional secondary windings 16, 17 and 18 of current transformers 3, 4 and 5, respectively, were used. The principle of operation of the device in FIG. 3 is similar to that described. The primary current trays 40 transformers 3, 4 and 5 current flow directly phase currents 1 d,

, 1 _С load and in case of connection of a three-phase load into a star, the sum of these currents is zero, i.e. 45

\ 'ί ₀ *' ο · Ο

The flow of power, additional secondary windings 16, 17 and 18 of transformers 3, 4 and 5 of current 50 are respectively

(18) ’ΛΆν, (nineteen) (20)

where - mutual inductance between the primary and additional secondary windings of transformers 3,4 and 5 current, respectively;

m ₄ , * L ₅ , - mutual inductances between the secondary windings of current transformers 3,4 and 5, respectively;

¹ 4 ' ¹ б ^{1 1} 6 " ^{70X11 in the} secondary windings of the current transformers 3,4 and 5 respectively ^{resisted in} resistors, respectively.

If the phase currents of the load do not contain constant components, current transformers 3,4 and 5 operate on linear sections of the magnetization curves. At the same time, the relations

where Μθ and Md _b are constant values. In addition, under the specified conditions,

when the current transformers operate in linear mode, the currents shunted by resistors in the secondary windings of the current transformers are proportional to the respective currents of the primary windings, and in this case the flux 46 ^ - ^ψ <7> discordantly formula V18), (19) and {20) are equal

46 BUT ' (21) 'V ^g ' one with/ where K is a constant magnitude.

For the specified conditions, the voltage at the input of the rectifier 19 (similarly to the device in Fig. 1 - see formulas (1), (2), <3), (4) and (5) is zero.

In the case of the flow of currents containing constant components in the phases of the load, the cores of the transformers of currents 3,4 and 5 are saturated and, similarly to the device in FIG. 1 while the voltage I) ^ is set to non-zero, which causes the actuation of the actuator 11.

Three phase transformer unit

can be made as separate,

ιι

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additional current transformers, the primary windings of which are connected in series to the phases of the load, and the secondary windings are connected similarly to the windings in FIG. 3, however, in this case, such windings must be ea'shunted by resistors,

The device should be used for crane asynchronous electric drives with thyristor voltage regulators. This is determined by the fact that crane actuators operate in conditions of increased shaking and vibrations, and this may cause a breakdown of contact, for example, the control electrode of the thyristor with the supply conductor, which in the absence of adequate protection will lead to a violation of the thermal mode of the engine and its failure.

From the characteristics (see figure 4), it follows that when the induction motor is powered from a faulty thyristor voltage regulator (there is no control pulse of one thyristor), the engine can be operated at all reduced speeds in a wide range of load torque variation. Thus, the above-noted fault of the thyristor voltage regulator does not appear externally, therefore, the motor can operate for a long time in the specified mode. Protection made in accordance with the known does not respond to such a fault.

The use of the proposed device for maximum current protection makes it possible to protect an asynchronous motor from its failure if there are permanent components in the phase currents. The cost of the new engine, the cost of work on the βί ο installation and the damage caused by the forced stop of the crane arising from the described situation is much higher than the cost of the proposed protection device, therefore its use has a high economic effect.

This device has a simple circuit and low cost. Its implementation does not meet technical difficulties.

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Claims (3)

Claim 1. Device for maximum current protection of three-phase load, containing a thyristor voltage regulator connected to the phases of the protected load, current transformers in each phase, the primary windings of which are connected to the phases of the protected load, and the secondary windings are loaded on the corresponding resistors, are connected in star and connected to the inputs of a three-phase rectifier whose output through a smoothing filter is connected to a single input of the executive body, the output of which is connected to the thyristor control input voltage regulator, which is due to the fact that, in order to expand the functional capabilities of the device, a three-phase transformer unit was additionally introduced, a single-phase two a semi-period rectifier and a smoothing filter, the primary windings of a three-phase transformer unit are connected to the phases of the protected load, and the secondary windings are connected to an open triangle, connected through a single-phase two-wave rectifier and a newly introduced smoothing filter to another input of the aforementioned organ. 2. The device according to π, 1, about tl and c with the fact that the three-phase transformer unit is made in the form of a three-phase voltage transformer, the primary windings of which are connected in a star. 3. Device by item 1, that is, with the fact that the three-phase transformer unit is made in the form of additional secondary windings of the above-mentioned current transformers connected in an open triangle.