RU2107984C1 - Regulated-power dc-to-three-phase inverter - Google Patents

Abstract

FIELD: converter engineering; power supply to and frequency starting of heavy-power induction motors; power supply to heavy-power synchronous motors and variable-frequency and variable-voltage resistive-inductive loads. SUBSTANCE: inverter has variable DC voltage supply, LC or C ripple filter, three-phase bridge-type voltage switch, forced switching units of main thyristors. Output current of inverter is maintained constant (at rated level) in case of its output voltage and frequency fluctuations; switching capacitor recharging circuit dispenses with filter capacitors; under rated operating conditions of inverter, additional power supply of switching capacitor does not consume power from supply mains. Two circuit arrangements (versions) are proposed. Common and essential for both of them is introduction of additional recharging circuit affording independent operation of forced switching unit in case of supply voltage fluctuations; two-step switching of main thyristors; introduction of isolating thyristors switch in forced switching unit of main thyristors ensuring off-line operation of unit; oscillatory recharging of switching capacitor; individual action on main thyristors of switch. EFFECT: simplified circuit arrangement; reduced switching loss. 2 cl, 2 dwg

Description

 The invention relates to a Converter equipment and is intended for power and frequency start-up of powerful (more than 10 kW) induction motors. It can be used to power powerful synchronous motors and active-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.

 A DC-to-three-phase AC voltage converter, regulated along the power supply circuit (hereinafter referred to as the converter), refers to devices that include: an adjustable DC voltage source, an LC or C smoothing filter, a three-phase bridge voltage switch (hereinafter referred to as the switch), main thyristor switching units .

 According to the control method, the invention relates to converters, the output voltage of which changes due to regulation along the DC voltage circuit, and the shape of the output voltage and frequency are formed by a switch containing in the arm counter-connected main thyristor and diode. The presence of an LC - or C-filter, forced switching nodes of the main thyristors with switching capacitors and sources of their charge in these circuits is mandatory.

A feature of the converter operation is the preservation of a constant output current close to the nominal current when the voltage and frequency at the output are deeply changed (observing the ratio U _o / F _o = const), which creates special conditions for the operation of the forced switching unit of the main thyristors of the switch, under which it is required to maintain the switching stability of the main thyristors at low voltage values of the constant voltage source at the input of the switch.

 Known devices that allow to solve the problem [1-4]. Schemes [1] and [2] are united by the presence of an oscillatory LC circuit included in the switching circuit of the main thyristors of the voltage inverter, the absence of additional sources of recharging of switching capacitors, but they have several disadvantages. Firstly, the charge current of the switching capacitor passes through the main thyristor; secondly, the voltage of the transient process is superimposed on the shape of the output voltage when the switching capacitor is charged; thirdly, switching nodes are necessary for each phase, which makes the circuit more complex; fourthly, the inability to turn off the main thyristor of the inverter when it is turned on for a short circuit in the load. When the voltage of the constant voltage source changes, additional sources of charging of the switching capacitors are needed. Devices [3, 4] require a commutation capacitor charge source to solve the problem, but have the disadvantages described in [5].

 As a prototype, a circuit [5] for converting a three-phase voltage to a three-phase variable, regulated by a power supply circuit, the switching unit of the main thyristors of which contains a charging source of a switching capacitor, which uses the voltage across the capacitor of the input filter or part thereof, is adopted. This converter with its own node for the forced switching of the main thyristors is implemented provided that the voltage of the power source is constant and in the presence of an intermediate DC-DC to DC converter with its own obligatory output filter. However, the circuit of such a converter is rather complicated due to the presence of an intermediate link for converting a constant voltage to an adjustable constant and an output filter. The disadvantages of the circuit include a large overcharge current of the switching capacitor, since the capacitance of the filter capacitors is several orders of magnitude higher than the capacitance of the switching capacitors.

 The purpose of the invention is to simplify the converter circuit by using an adjustable constant voltage source and improving the forced switching unit of the main thyristors of the switch as part of the converter, while maintaining high switching stability of the thyristors of the switch with constant output current of the converter (at the nominal level) with changes in the frequency and voltage at the output of the converter , reduction of switching losses due to exclusion from the circuit of the overcharge of the switching conde filter capacitors and the absence of power consumption from the network as an additional source of recharge of the switching capacitor in the nominal operating modes of the converter.

The specified goal is achieved by two different circuit solutions (options). The general and essential for the solution are:
the presence of an additional recharge circuit, which ensures the independence of the operation of the forced switching unit from the changing voltage of the power source;
two-stage switching method of the main thyristors of the switch;
the presence in the node of forced switching of the main thyristors of the switch of separation thyristors, which create the conditions for the autonomous operation of the node;
oscillatory recharge of a switching capacitor;
individual effects on the main thyristors of the switch.

The application of the invention allows to obtain the following advantages:
the independence of the operation of the forced switching unit from the changing voltage of the DC voltage source of the converter, which leads to an increase in the switching stability of the main thyristors while reducing the voltage at the input of the switch and maintaining the load current at a level close to the nominal (rated) one;
simplification of the converter circuit due to the absence of a constant voltage converter and an adjustable constant and output filter;
reduction of switching capacitor overcharge currents;
the optimal choice of the capacitance of a switching capacitor, the value of which is determined by the voltage on the additional source of recharge, the turn-off time of the main thyristors, the magnitude of the recharge current;
the absence of power consumption by an additional source of overcharge of the switching capacitor in the nominal mode, the compensation of losses in the switching circuit is carried out at the expense of the converter power source;
greater versatility of the forced switching unit as part of converters of different configurations.

 In FIG. 1 shows a first embodiment of a converter; in FIG. 2 - distinctive from FIG. 1 fragment of the second version of the Converter.

 Common to the proposed embodiments of FIG. 1 and 2 is a serial connection of an adjustable constant voltage source 1, a smoothing filter 2 and a switch 3. The following describes general circuit solutions for the options on the example of one phase of FIG. one.

 The main current path of the switch 3 is formed in series by the main thyristor 4 by the anode to the plus of the switch 3, the main thyristor 5 by the anode to the main thyristor 4, by the cathode to the minus of the switch 3. Thyristor 4 is shunted by the reverse diode 6, thyristor 5 is bypassed by the reverse diode 7. Common point of the main thyristors 4 and 5 forms the phase (for example, phase A) of the switch (or converter).

 The forced switching unit 8 of the main thyristors of the switch 3 consists of serially connected from the plus to the minus of the switch of the separation thyristor 9, the anode to the plus of the switch, inductor 10, the switching capacitor 11, the separating thyristor 12, the cathode to the minus of the switch. In parallel to the series circuit of the inductor 10 and the switching capacitor 11, a 3-phase thyristor bridge of switching thyristors 13-18 is connected. The cathode group of switching thyristors 13-15 is connected to the cathode of the separating thyristor 9, the anode group of switching thyristors 16-18 is connected to the anode of the separating thyristor 12. The common point of the switching thyristors 13 and 16 forms a phase (for example, phase A). The phase outputs of the 3-phase bridge from the switching thyristors are connected to the corresponding phase outputs of the switch 3.

 The overcharge circuit of the switching capacitor 19 is connected in parallel to the chain of the inductor 10 and the switching capacitor 11 and consists of a series 20 connected in the direction of overcharging (from the cathode of the thyristor 9 to the anode of the thyristor 12), a diode 3-phase (or 2-phase) bridge 21 (anode the group of which is connected to the inductor 20), the recharge thyristor 22. The diode bridge 21 is shunted by the capacitor 23.

 Option 2 (Fig. 2) differs from option 1 in that in the functional block 24 under consideration (Fig. 1), instead of a diode bridge 21, a recharge thyristor 22, a capacitor 23, a semi-controlled thyristor 2-phase bridge formed by thyristors 25-27 (anode group) and diodes 28-30 (cathode group). The anode group is connected to the inductor 20.

 We will consider the principles of the circuit using the example of switching the main thyristor 4 (Fig. 1). The initial voltage (shown in Fig. 1 by plus and minus signs without brackets) on the switching capacitor 11 is formed at the moment of switching on the thyristor recharge 22. There is a charge of the switching capacitor 11 in the overcharge circuit: 11-10-20-21 (and 23) -22- eleven. The magnitude of the voltage across the capacitor 23 is determined from the conditions for ensuring the necessary initial voltage at the switching capacitor 11, based on the conditions for ensuring switching of the main thyristors and reducing switching losses.

 For option 2, the charge of the switching capacitor 11 is carried out in the circuit: 11-10-20-semi-controlled 3-phase thyristor bridge 11. In the future, when the voltage on the switching capacitor 11 after the end of the switching process exceeds the voltage of the regulated power supply 1 in the nominal mode (source voltage 1 maximum), the switching capacitor is recharged only by switching on one of the thyristors 25, 26 or 27.

 At the end of the process of recharging the switching capacitor 11, the thyristors of the circuit recharge 22 or 25-27 are closed. The switching thyristor 16 and the separation thyristor 9 are opened. The switching capacitor 11 is recharged in the circuit: 11-16-6 (4) -9-10-11 to the polarity indicated in brackets. In the process of recharging the switching capacitor 11, the main thyristor 4 is turned off. As soon as the voltage on the switching capacitor 11 exceeds the voltage of the regulated power supply, the isolation thyristor 9 and the switching thyristor 16 are closed and the switching process ends.

 After the end of the switching process, the recharge circuit 19 (recharge thyristor 22, option 1, or recharge thyristors 25-27, option 2) is turned on, the voltage at the switching capacitor 11 is set with the signs shown in FIG. 1 without brackets. It should be noted that the recharging of the switching capacitor 11 according to option 1 occurs not only through the capacitor 23, but also through the series-connected diodes of the bridge 21, which allows to reduce the consumption of a 3-phase AC voltage from the network.

 If the converter operates in the nominal mode, then according to option 2, it is enough to turn on one of the thyristors 25, 26 or 27. For option 1, turning off the capacitor 23 also excludes energy consumption from the 3-phase network, since the switching capacitor is recharged when the overcharge thyristor 22 is turned on, through the series-connected diodes of the bridge 21.

 The magnitude of the inductance of the chokes 10 and 20 is determined from the conditions for ensuring the necessary recharge time, limiting the recharge current, and ensuring the oscillation of the recharge.

 Separating thyristors 9 and 12 provide the operation of the forced switching unit 8 independent of the changing voltage of the regulated power supply 1 in terms of maintaining the necessary initial voltage on the switching capacitor 11.

Claims (2)

 1. A DC-to-three-phase DC converter, adjustable along the power supply circuit, containing a series-connected adjustable DC voltage source, an LC filter, a three-phase bridge commutator, the main thyristors of which are shunted by reverse diodes, and a forced switching unit for the main thyristors of the switch, which includes a series circuit from the inductor and the switching capacitor connected between the poles of the switch, respectively, through the first and second isolation thyristors, switching thyristors connected in a three-phase bridge circuit, between the poles of which the indicated series circuit is connected, and a circuit for switching a switching capacitor with a thyristor for charging, connected in parallel to a series circuit from a choke and a switching capacitor, characterized in that the charge circuit is made in the form of a series circuit from a choke, a three-phase diode bridge, powered by a three-phase AC voltage source, with a capacitor connected between the poles of a three-phase diode bridge, and thyristor recharge.  2. A DC-to-three-phase DC converter, adjustable along the power supply circuit, containing a series-connected adjustable DC voltage source, an LC filter, a three-phase bridge switch, the main thyristors of which are shunted by reverse diodes, and a forced switching unit for the main thyristors of the switch, which includes a series circuit from the inductor and the switching capacitor connected between the poles of the switch, respectively, through the first and second isolation thyristors, switching thyristors connected in a three-phase bridge circuit, between the poles of which the indicated series circuit is connected, and a switching capacitor overcharge circuit connected in parallel to a serial circuit from a reactor and a switching capacitor, characterized in that the switching capacitor overcharge circuit is made in the form of a series-connected inductor and a three-phase semi-controlled thyristor bridge, powered by a three-phase AC voltage source.

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