SU1145429A1 - Method of control of three-phase direct fraquency converter - Google Patents

Abstract

A METHOD OF CONTROL OF A THREE-PHASE DIRECT FREQUENCY CONVERTER OF A CURRENT WITH A NATURAL FACTOR OF THYRISTORS, containing three sets of gates, each consisting of two groups of gates, connected in an anti-parallel scheme, with the output outputs connected in a triangle of circular patterns. a three-phase load, the phases of which are interconnected according to the scheme a star without a neutral wire, consisting in the fact that they form a phase-shifted phase with a t-phase voltage These first system of pulse sequences form a second three-phase pulse sequence with a frequency equal to the output frequency of the converter, then compare the pulse sequences of the first and second systems and form uprapping pulses at the moments of coincidence of the pulses of the two sequences that compare with current forms in the phases of the load, the pulses of the second sequence are set to 4 equal to l / 2. SL 4i N) CO

Description

I The invention relates to converter equipment and can be used in traction and industrial frequency-controlled electric drives and in electrothermal installations. A known method of controlling a direct frequency converter with a t-phase output voltage containing m groups of anti-parallel valves, the root of which is that in each group during the period of the output voltage they turn on the vents of one direction, turn them off and turn on the valves of another directions, and each time the valve is turned on in one direction, a pause equal to 1/2 of the part of the output voltage period l 1 is delayed. The disadvantage of this method is that when powering The phase of the load connected according to the star without a neutral wire, and the rectangular shape of the valve rjiynn current, in the phases of the load, contain higher order harmonics, since the current curve differs from the sinusoidal one. The known method of controlling the direct converter, which consists in forming the first system of pulse sequences phased from phase 1, form a second, three-phase pulse sequence with a frequency equal to the output frequency of the converter, compare the pulse sequences of the first and second systems and form pulses of the valve groups at the moments of coincidence of the pulses of the two sequences 2. However, when the three-phase load is switched on, according to the star without a zero drive, the currents in the phases of the load contain harmonics of the highest, in the case of a rectangular form of the converter valve group current. The closest to the proposed method is a control of a three-phase direct frequency converter, the essence of which is in controlling a three-phase direct converter frequencies, containing three sets of valves, each of which contains two groups of valves, included 29 in an anti-parallel circuit, with low frequency leads in The central units are connected to each other in a triangle, to the vertices of which a three-phase load connected to a star is connected, form the first system of pulse sequences phased with the t-phase voltage of the power supply network, form a second three-phase pulse sequence with a frequency equal to the output frequency of the converter, compare the pulse sequences the first and second systems and form the control pulses of the valve groups at the moments of coincidence of the pulses of the two sequences, and itelnost pulses of the second sequence is set to be 2TG:. The disadvantage of this control method is the presence of higher harmonics in the phase load currents, since the shape of the current is different from the sinusoidal one. The purpose of the invention is to improve the shape of the current in the phases of the load. The goal is achieved by the fact that according to the method of controlling a three-phase direct frequency converter containing three sets of valves, each of which is made in the form of two groups connected in an anti-parallel scheme, the sets are connected by triangles with their low-frequency leads to which vertices are connected a three-phase load, the phases of which are interconnected according to the scheme a star without a neutral wire, consisting in the formation of a power supply phased with r-phase voltage ti first system pulse sequences forming the second three-phase pulse sequence with a frequency equal to the frequency of the output transducer, comparing pulse patterns of the first and second systems and the formation of control pulses at the instants of matching between two sequences of pulses, the pulse width of the second sequence is set to be 1/2. FIG. 1 shows the power scheme of the power part of the direct frequency converter, which implements the method proposed in FIG. 2 - timing diagrams explaining the principle of operation of the converter according to the proposed method; FIGS. 3a, b are replacement circuits of the converter for two operation modes (a, two valve groups work, b one valve group operates) i in FIG. 4 is a block diagram of a current frequency setting unit; in fig. 5 timing diagrams explaining the operation of the current frequency setting unit; in fig. 6 - time diagrams of the phase load current (and when controlling in a known manner, b when managing the proposed method). A direct frequency converter carrying out the proposed method contains a three-phase supply transformer with three secondary windings 1-3, to which a pole of three-phase rectifying bridges 49 constituting three sets are connected in pairs in parallel. The outputs of the counter-parallel bridges are connected in a closed triangle, to the vertices (phase terminals) of A, B, with which the load 10 is connected, the phases of which 11-13 are connected in a star. The converter control system contains an irreversible phase-shifting unit 14, which forms the first pulse sequence clocked with t, -power voltage of the power supply network, logic unit 15, Lorming the control pulses of the converter valves, unit 16 setting the output current frequency, which forms the second phase impulse p 1), Uj, (if,, Uj t Ug, with a frequency equal to the output frequency of the converter and a pulse duration equal to 7i / 2. The control pulses generated by the unit 15 are fed to the control electrodes of the bridge rectifier valves, which are currently The method of controlling a direct frequency converter is exemplified by its operation. A three-phase network frequency voltage is supplied to the input of the supply transformer, which from the secondary windings 1–3 is supplied to three-phase rectifier bridges 4–9. The pulse sequence defined by the output frequency of the converter, formed by block 16, controls the bridges 4–9 so that each of the bridge bridges operates 1/4 of the output frequency period. The control 294 pulses from the bridge axles, formed by the BaHFiue unit 14, are sent to the control electrodes of the valves in case of the control terminals across the bridges 4-9 of the block 15 to the control pulses of the phase-shifting unit 14. Thus, phase voltages applied to the load from the phase-shifting unit 14 are generated. given frequency. Unlike the prototype method, in the proposed method there are time intervals of duration / 6, when out of six rectifiers only one works, which leads to a change in the form of the current both in rectifiers and in load phases. This is explained using the dumming circuit of the converter shown in FIG. 3. In FIG. The diagram shows a replacement circuit corresponding to the time interval (O -) when two rectifiers of six 4 and 8 are in operation, where 4 PS 4 tons in SD voltage at the output of bridges 4-9; - i. - currents, bridges 4-9, - 2, 2, Zj- load phase resistance. The indicated equation is valid according to the law of loop currents 4 4 (z, + zp + igZj If o e j. с 4 e e. e "e; Ci-Ij e 31 Z; Fig. 3b shows the replacement circuit converter, corresponding to the interval (/ 6-) time, when one rectifier 4 is working. It is obvious that the current of rectifier 14 will increase by 1.5 pa, i 1-; -2 1.5, in this interval. As you can see of Lig.2, the currents of the rectifiers HH 7 8 5 have a stepped form, which in bed accounts improves the shape of the current of the phases of the load, the currents of which are derived from the expressions C) (H V (H 7) (.-C-LCH) with () - Phase-shifting unit 14 and the logical pulse shaping unit 15 are known devices and are described, for example, in C2J, the Output Current Frequency Setting Unit 16 can be easily implemented using the logical addition of two pulse sequences whose frequencies differ twice.

An example of the implementation of the unit setting the frequency of the output current 16 is given per u. 4. Block 16 contains master oscillator 17, frequency divider 18 per wa, two conversion rings 19 and 20, and six coincidence circuits 21-26. On ig. 5 shows time diagrams for the operation of block 16. The driving oscillator 17 forms the first pulse train with a frequency twice as large as the frequency of the output current of the converter (Fig. 5a).

At the output of the frequency divider 18, a second sequence of pulses is formed with a frequency equal to the frequency of the output current of the converters (Fig. 56). The scaling ring 20 creates three sequences of pulses tA, tB, tC with a frequency of the master oscillator 17 and a pulse duration equal to J (Fig. 5c, d, d).

The scaling ring 19 forms 30 second three sequences of pulses iA, IB, tC, the frequency of which is half as much. Folding logical impulse sequences, respectively, + A, + A i + B, + C, 35 + B; -A, + AH-B + C; -C + B, receive the output voltages of the transducer output frequency unit of the converter and., Uj, Uj, UY, Ug, Ug to control the corresponding three to 40 phase rectifiers (Fig. 5 and, k, l).

As can be seen from FIG. 5, duration) Co-setting the current frequency U.Ug is equal to J / 2.

The generator, frequency divider, scaling ring and coincidence circuit are widely known devices.

 Thus, the block 16 is built on well-known elements according to the well-known principle 50, but using two frequencies.

Estimating ef (1), the efficiency is proposed with respect to the control method as compared with the prototype method of FIG. 6. 55

The degree of approximation of the current curve to a sinusoidal estimate using the distortion factor

R

1AN

KV.

 An

where - the current value of fundamentals 1 AN

Noah harmonic current unloading - the effective value of current

loads;

The less the coefficient K differs from unity, the more the curve approaches the sine wave and the less it contains higher harmonics. The effective value of the load current is calculated by the formula

Then when managing a known method

AHW).,

where 1 is the amplitude of the current, and when controlling the proposed method

Ak (b) 0.661m

The actual value of the main harmonic in each case is determined by

Im-,

where 1 is the amplitude of the main harmonic

PC

current.

1, are found by decomposing in the Fourier series of curves (Fig. 6), which are symmetrical about the ordinate and have only a sinus component. For a current whose shape corresponds to FIG. 6a I-im,, and for a current whose form corresponds to FIG. 66 1 7,4-. In both cases x.

The distortion factor of the load current curves is determined. For FIG. 6a

7.7

-. G) Of, t

TO

and 0.71-8)

and for figs. 66

7.4

Thus, the proposed control method allows to improve the shape of the output current of the converter with a simple and reliable control scheme, while ensuring high energy performance of the converter, whose rectifiers can operate with the valves fully open.

fu.3

Fy

Q

.

t

Claims (1)

METHOD FOR CONTROL OF A THREE-PHASE DIRECT FREQUENCY CONVERTER with natural switching of thyristors, containing three sets of valves, each of which is made in the form of two groups of valves connected in an anti-parallel circuit, and with their output terminals the sets are connected to each other in a triangle, to the vertices of which load, the phases of which are interconnected according to the star circuit without a neutral wire, which consists in the fact that they form a phased voltage the system of pulse sequences, the second three-phase pulse sequence is formed with a frequency equal to the output frequency of the converter, then the pulse sequences of the first and second systems are compared and control pulses are generated at the moments of coincidence of the pulses of the two sequences, which, in order to improve the current shape in the phases of the load, the pulse duration of the second sequence is set equal to l / 2. >