RU2061994C1 - Dc=to-three-phase-ac voltage changer (options) - Google Patents

Abstract

FIELD: converter engineering; devices incorporating non-regulated dc voltage supply, dc voltage changer, LC filters at changer input and output, and three-phase bridge voltage inverter with forced thyristor switching assemblies and designed for power supply and frequency-starting of heavy-power induction motors. SUBSTANCE: there are two different circuit arrangements for attaining desired effect. Common for both and rather essential are the following features: double-stage switching of main thyristors of inverter, forced switching unit of isolating thyristors ensuring its off-line operation, use of switching voltage capacitor built around capacitor of dc changer input filter as additional charging source which provides for pre-charging switching capacitor by means of auxiliary thyristors, and voltage across it independednt of varying voltage in inverter power circuit, individual action on main thyristors. EFFECT: improved forced switching assembly of main thyristors of inverter to raise high switching stability of inverter at its constant output current with varying frequency and voltage at inverter output and wide voltage regulation range for inverter power circuit without using additional source of switching capacitor charge. 2 cl, 4 dwg

Description

 The invention relates to converter technology and is intended for power and frequency start-up of powerful (more than 50 kW) induction motors and can be used to power powerful synchronous motors and actively inductive loads with variable frequency and voltage. The invention allows the efficient use of thyristors in circuits of DC-to-three-phase AC converters, where the use of other semiconductor devices is difficult for technical or economic reasons.

The invention under consideration relates to devices, which include: unregulated DC power supply, input smoothing filter with a capacitive element, DC voltage regulator, output smoothing filter and bridge voltage inverter (hereinafter referred to as
inverter) with nodes of forced switching of thyristors.

 According to the control method, the invention relates to converters, the output voltage of which is changed by regulating the voltage along the DC voltage circuit, and the output voltage and frequency are generated by an inverter containing the main thyristor and diode in the arm, which are counterclockwise connected. The presence of smoothing LC or C filters, forced switching nodes of the main inverter thyristors in these circuits is mandatory.

 A feature of the operation of the converter in question is the preservation of a constant output current close to the nominal current with a deep change in the voltage at the output and the output frequency (in compliance with the ratio Uout./Rout.const.), Which creates special conditions for the operation of the forced switching unit of the main inverter thyristors, at which it is required to maintain the switching stability of the main thyristors at low voltage values of the constant voltage source at the inverter input.

 As an analogue and prototype, we select inverter circuits, the switching nodes of the main thyristors of which contain power sources of switching capacitors / 1,2 /.

 The circuit / 1 / contains an additional source, divided into two equal parts, which are directly included in the switching circuit of the main thyristors, two switching capacitors, the midpoint of which is connected to the midpoint of the filter capacitor at the inverter input. The drawback of the circuit is the inclusion of an additional power source in the switching circuit, the need for a balanced circuit for equalizing the potentials of the switching circuit, the inclusion of filter capacitors in the main thyristor circuit, which, due to the large capacity (selected from the conditions of the necessary rectified voltage filtering), delay the process of switching the main thyristors, decrease in switching stability of the main thyristors while reducing the voltage at the inverter input and maintaining the value in Khodnev current inverter at a level close to nominal.

 The prototype circuit / 2 / contains an additional power source, which charges the capacitors through a group of thyristors connected via a bridge circuit. The capacitors, in turn, are connected to a star, their common point is connected to the midpoint of the smoothing filter capacitors, the remaining capacitor plates are connected to the phase terminals of the above-mentioned bridge circuit and, through pairs of counter-parallel switched switching thyristors, are phase-connected to the main thyristors of the inverter. The disadvantage of the proposed switching method is the inclusion in the switching circuit of filter capacitors having a reverse polarity with respect to the polarity of the capacitor, which leads to a decrease in switching efficiency, as well as a special algorithm for charging capacitors.

 The purpose of the invention is the improvement of the forced switching unit of the main thyristors of the inverter in the direction of maintaining high switching stability of the inverter thyristors with a constant output current of the converter (at the calculated level) with changes in the frequency and voltage at the output of the converter and with a wide range of voltage regulation along the power supply circuit of the inverter (input voltage of the inverter) without using an additional switching capacitor charge source.

The specified goal is achieved by two different circuit solutions (options). Common and essential for the decisions are:
two-stage method of switching the main thyristors of the inverter;
the presence in the node of the forced switching of the main thyristors of the inverter separation thyristors, which create the conditions for the autonomous operation of the node of the forced switching of the main thyristors of the inverter;
the use of a voltage at the capacitive element of the input filter as an additional charge source for the switching capacitor, which provides a preliminary charge of the switching capacitor with the help of thyristors of the charge circuit and the independence of the voltage on it from the changing voltage at the inverter input, thereby achieving the inverter switching stability;
individual effects on the main thyristors of the inverter.

The application of the invention will provide the following advantages:
the independence of the operation of the forced switching unit from the changing voltage at the inverter input, which leads to an increase in the switching stability of the main thyristors while reducing the voltage at the inverter input and maintaining the load current at a level close to the nominal (rated) one;
the use of voltage on the capacitive element of the input filter for the preliminary charge of the switching capacitor;
the optimal choice of the capacitance of a switching capacitor, the value of which is determined only by the voltage on the capacitive element of the input filter and the turn-off time of the main thyristors;
the exclusion of a special additional source of charge of the switching capacitor;
reduced capacitance of switching capacitors;
the absence of energy storage in the switching capacitor, the compensation of losses in the switching circuit is carried out due to an unregulated source of constant voltage.

 Figure 1 presents the first variant of the Converter; figure 2 - distinctive from figure 1 fragment of the second variant of the Converter; figure 3 - diagrams of currents and voltages of the main elements of the Converter according to option 1 and 2 in the maximum voltage mode on the inverter; figure 4 diagrams of currents and voltages of the main elements of the Converter according to option 1 and 2 in the minimum voltage mode on the inverter.

 Common to the proposed options in figure 1, figure 2 is the connection of the inverter 1 to the series-connected output smoothing filter 2, the unit for regulating the constant voltage 3, the input smoothing filter 4, unregulated source of constant voltage 5. The following describes the general circuit solutions for the options for example one phase of figure 1.

 The main current path of the inverter 1 is formed in series by the main thyristor 6 by the anode to the inverter 1 plus, the main thyristor 7 by the anode to the main thyristor 6, by the cathode to the minus of the inverter 1. The thyristor 6 is shunted by the reverse diode 8, the thyristor 7 is bypassed by the reverse diode 9.

 The common node for phase switching of the main thyristors of the inverter consists of series-connected reactors 10,11, a switching capacitor 12. The inductor 11 is shunted by the diode 13. The anode of the diode 13 is connected to a common point of the switching capacitor 12 and the inductor 11. The inductor 10 (the end not being common with the inductor 11) is connected to the cathode of the separation thyristor 14, the cathodes of the switching thyristors 15,16,17 and the anode of the thyristor of the charge circuit 18. The anode of the separation thyristor 14 is connected to the plus of the inverter 1. The cathode of the thyristor circuit poison 18 is connected to the minus of the inverter 1 (or what is the same with the minus of the power source 5). The anode of the switching thyristor 15 is connected to the common point of the main thyristors 6 and 7, which is the output of phase A of the inverter 1 (or the output of phase A to the converters). Similarly, the anodes of the switching thyristors 16.17 are connected respectively to the phases B and C of the inverter 1 (phases B and C to the converters. The lining of the switching capacitor 12, which is not common with the inductor 11, is connected to the anode of the separating thyristor 19, the anodes of the switching thyristors 20,21,22 and the cathode of the thyristor of the charge circuit 23. The cathode of the separation thyristor 19 is connected to the minus of the inverter 1. The cathodes of the switching thyristors 20,21,22 are connected respectively to the anodes of the switching thyristors 15,16,17. The anode of the thyristor of the charge circuit 23 is connected with the plus of the capacitive element of the input filter 4 or its part (in order to reduce the voltage supplied to the switching circuit) .Thus, the switching thyristors 15,16,17,20,21,22 form a 3-phase bridge, the AC phase terminals of which are connected to the corresponding phase terminals of the inverter, and the DC terminals are connected to the extreme terminals of the sequential charging circuit 10-11 (13) -12.

 Option 2 (Fig. 2) differs from option 1 in that in the functional block 24 under consideration (see Fig. 1), instead of series-connected reactors 10 and inductor 11, shunted by diode 13, an inductor 11 is used, which at its beginning is connected to a switching capacitor 12, the other end is connected to the cathode of the separation thyristor 14. The inductor 10 is magnetically connected to the inductor 11 and connected with its end to the common point of the inductor 11 and the separation thyristor 14, and the beginning of the inductor 10 is connected to the anode of the thyristor of the charge circuit 18. The beginning and end of the throttle Rows 10 and 11 can be changed simultaneously.

 FIG. 3, Fig. 4 illustrate the operation of the converter with the parameters: Epit. 500 V, Env.max. 500 V, Emin. Min. 50 V, Pnag. 130 kW, Ed.ZOO V, Fout = 400 Hz, COSF = 0.7, L1O = 3 mH, L11 = 15 mH, CI2 = 50 μF.

The designations in the figures correspond to:
I1 the total current on the elements of 6.8 (the positive direction of the current corresponds to the current through the thyristor 6);
I4 total current on the elements 7.9 (the positive direction of the current corresponds to the current through the thyristor 7);
IA line current at the output of the converter;
Uab line voltage at the output of the converter;
Ua phase voltage at the output of the converter;
U12 voltage across the switching capacitor 12;
I10 for figure 3, the current of the inductor 10 according to the scheme of figure 1,
for figure 4, the current of the inductor 11 according to the scheme of figure 2;
I18 current through thyristor 18.

 We will consider the operating principles of the circuit using the example of switching of the main thyristor 6 (Fig. 1). The initial voltage (shown in Fig. 1 by plus and minus signs without brackets) on the switching capacitor 12 is formed at the moment of switching on the thyristors of the charge circuit 18.23. There is a charge of the switching capacitor 12 in the charge circuit: (capacitive element of the input filter 4 (Edop)) - -23-1-2-13-10-18. Voltage Edop. can be taken both from all capacitive elements of the filter 4, and from its part. The value of voltage Edop. is determined from the conditions for ensuring the necessary initial voltage at the switching capacitor 12, based on the conditions for ensuring switching of the main thyristors and reducing switching losses. For option 2, the charge of the switching capacitor 12 is carried out in the circuit: (capacitive element of the input filter 4) -23 -1.2-11-11-10-1.8. The magnitude of the inductance in the considered charge circuit of the switching capacitor 12 for option 1 and 2 is determined by the value of the inductance of the inductor 10 of option 1. At the end of the charging process of the capacitor 12, the thyristors of the charge circuit 18.23 are closed. The switching thyristor 20 and the separation thyristor 14 are opened. The switching capacitor 12 is recharged in the circuit: 12-20-6 (8) -14-10-11 (for option 2 in the circuit: 12-20-6 (8) -14-11 ) to the polarity indicated in parentheses. In the process of recharging the switching capacitor 12, the main thyristor 6 is turned off. As soon as the voltage at the switching capacitor 12 exceeds the voltage Einv. the separation thyristor 14 and the switching thyristor 20 are closed and the switching process ends.

 The total inductance of the charge circuit of the switching capacitor 12 is selected less than the total inductance of the switching circuit of the main inverter thyristors, which is shown in block 24. Here, for option 1, the inductance of the charge circuit of the switching capacitor is reduced by shunting the inductor 11 by diode 13. For option 2, the inductance of the charge circuit is reduced switching capacitor in comparison with the switching circuit of the main thyristors of the inverter is provided due to the on-line switching dross lei 10 and 11. This circuit solution for the charge of the switching capacitor 12 according to options 1 and 2 can reduce switching losses and the charging time of the switching capacitor 12.

 Separating thyristors 14 and 19 provide independent from the change (due to the operation of the constant voltage control unit 3) of the output voltage of the filter 2 (Einv.) The operation of the switching unit in terms of maintaining the required initial voltage on the switching capacitor 12. NO 2

Claims (2)

 1. A DC-to-three-phase DC converter, adjustable along the power supply circuit, comprising an input smoothing filter with a capacitive element, a DC voltage regulating unit, an output smoothing filter and a bridge voltage inverter, the phase outputs of which are connected in series and connected to the terminals for connecting an unregulated constant voltage source are output conclusions, in addition, the node for the forced switching of the main thyristors of a bridge voltage inverter containing a three-phase bridge of switching thyristors, a switching capacitor and a charging circuit of a switching capacitor with thyristors, characterized in that two isolation thyristors are introduced into the forced switching unit and two in series according to the connected inductors, one of which is shunted by a diode in the direction of charge of the switching capacitor, and forming a series charging circuit with a switching capacitor, and the extreme terminals of the series charging circuit through the thyristors of the circuit row connected to the terminals of the capacitive element of the input smoothing filter or part thereof, the separation thyristors are connected between the corresponding DC terminals of the bridge voltage inverter and the extreme terminals of the series charging circuit, which are connected to the DC terminals of a three-phase bridge from switching thyristors, the AC phase terminals of which are connected to corresponding phase outputs of the bridge voltage inverter.  2. A DC-to-three-phase DC converter, adjustable along the power supply circuit, comprising an input smoothing filter with a capacitive element, a DC voltage regulating unit, an output smoothing filter and a bridge voltage inverter, the phase outputs of which are connected in series and connected to the terminals for connecting an unregulated constant voltage source are output conclusions, in addition, the node of the forced switching of the main thyristors of the bridge voltage inventor, contains a three-phase bridge of switching thyristors, a switching capacitor and a charging circuit of a switching capacitor with thyristors, characterized in that two isolation thyristors are introduced into the forced switching unit and two inductors connected in series to the charge circuit, and the terminal ends of the serial circuit from series-connected inductors and a switching capacitor connected through the corresponding thyristors of the charge circuit with the findings of the capacitive element of the input smoothing f of a liter or part thereof, and the connection point of the chokes and the free output of the switching capacitor of the aforementioned series circuit are connected through separation thyristors to the corresponding DC terminals of the bridge voltage inverter and directly to the DC terminals of the three phase bridge from the switching thyristors, the AC phase terminals of which are connected to the corresponding phase outputs of a bridge voltage inverter.

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