RU2303851C1 - Multilevel static frequency converter for feeding induction and synchronous motors - Google Patents

Abstract

FIELD: electrical engineering; power supply to induction and synchronous motors from three- or single-phase mains.

SUBSTANCE: proposed multilevel static frequency converter designed for feeding induction and synchronous motors with sine-wave current of power factor close to unity from over 3-kV, 0.5- to 15-MW three- or single-phase mains and for power regeneration during their generator braking has power transformer, output filter, main control unit, and inverter-regeneration modules incorporating power transistors with built-in backward diodes forming two single-phase bridges connected in parallel over DC circuit through current sensor. Connected to first-bridge DC circuit are capacitor bank and voltage sensor. First-bridge AC circuit is connected to one of power-transformer secondary windings through current sensor and switch; the latter has two transistors connected in parallel opposition. Second-bridge AC circuit that functions as module output is connected in parallel with outputs of other modules to form three star-connected groups. Each module has local control unit for shaping power transistor control pulses in compliance with current operating mode of module and with control signals arriving from main control unit over optical channel.

EFFECT: enlarged range of frequency converters.

1 cl, 4 dwg

Description

FIELD OF THE INVENTION

The invention relates to a Converter equipment, in particular to frequency converters, the use for power, start-up and regulation of asynchronous and synchronous motors with voltage of 6.3 and 10 kV and a power of 0.5-15 MW.

State of the art

A known device for a similar purpose, which is widely used at present, is a frequency converter made according to the scheme with two transformers and a low voltage (up to 1000 V) frequency converter connected between them. The main parameters of such systems and the requirements for the transformers used in them are described in [1]. The use of a commercially available inverter module for a voltage of 380 V in this solution makes the converter quite simple and reliable in operation. The disadvantages of such a frequency converter are the presence of two transformers for the full power of the electric motor, the absence of the function of energy recovery into the network during forced braking of the electric motor, the low power consumption coefficient due to non-sinusoidal input current, the need for bulky filter-compensating devices at the input of the converter and a bulky output filter to provide electromagnetic compatibility.

Frequency converters constructed according to a transformerless circuit on series-connected semiconductor devices are also well known. Varieties of such schemes are described in [2] (electronic version on the Internet - http://news.elteh.ru/arh/2005/32/09.php). A significant drawback of such circuits is the uneven distribution of voltage between semiconductor devices due to the dispersion of their parameters in both statics and dynamics. This requires the use of special protective circuits that dissipate noticeable power, and reduces the maximum switching frequency. Switching high voltages leads to a high level of electromagnetic interference generated by the converter. To ensure electromagnetic compatibility, a bulky LC filter is required at the converter output, designed for the full amplitude voltage value and high PWM frequency. At the inlet of the converter, a filter-compensating device is required to ensure electromagnetic compatibility and a high-quality radio interference filter. The absence of the function of energy recovery in the network reduces the operational properties of the converter, since it excludes the converter in the modes of forced generator braking of the electric motor.

One of the circuitry solutions is a single-transformer circuit consisting of a power transformer with many secondary windings and the same rectifier-inverter cells connected in series to increase the output voltage. One example of such a solution is the device described in [3]. The device contains simple rectifier-inverter cells, but a complex power transformer with many secondary windings connected by a zigzag to reduce harmonics in the input current. Also, such a device does not provide energy recovery to the network during generator braking of the electric motor.

As the closest analogue of the claimed invention, you can specify the frequency converter described in [4]. The converter contains a power transformer having a primary and many insulated secondary windings. The design of the power transformer is simpler than in the device [3], but to reduce the harmonics of the input current, a complex three-phase bridge inverter at the input of each cell is required. In addition, the device does not provide for a smooth charge of the capacitor bank when the device is connected to the network. This can lead to voltage dips in the mains and other equipment malfunctioning. Also, shock currents that occur when turned on can disable semiconductor devices.

SUMMARY OF THE INVENTION

The task to which the claimed invention is directed is to expand the arsenal of frequency converters for powering asynchronous and synchronous electric motors with voltages exceeding 3 kV and a power of 0.5-15 MW.

Asynchronous electric motor is the simplest, most reliable and economical solution for building electric drives in various industries. A significant drawback of an induction motor is the inability to control the shaft speed only by changing the voltage, it is also necessary to change the frequency. To ensure optimal operation of the electric motor with a constant magnetic flux, it is necessary to fulfill the ratio U / f = const, where U is the voltage of the supply network; f is the frequency of the supply network.

When operating the electric motor in regenerative braking mode, it is also necessary to provide energy recovery to the supply network.

The synchronous electric motor has been widely used in industry due to the ability to work directly from the AC mains with a high power factor, and as a result it has lower overall dimensions than an induction motor. To regulate the speed of the synchronous motor, it is also necessary to fulfill the ratio U / f = const. In addition, an additional excitation device is required to power the synchronous motor.

The technical result of the claimed invention is the supply of an asynchronous or synchronous electric motor from a three-phase or single-phase AC network with a consumption of a sinusoidal input current with a power factor close to unity, and providing a sinusoidal voltage of variable frequency and amplitude in accordance with the required regulation law with the possibility of energy recovery in the network during generator braking of the electric motor.

Salient features of the claimed invention are: input power transformer, input voltage sensors, output voltage sensors, output current sensors, output LC filter, main control unit.

An essential distinguishing feature of the claimed invention, which provides a technical result, is that the converter contains inverter-recovery modules connected in series into three groups connected to a star. Each module consists of power transistors with built-in reverse diodes, forming two single-phase bridges connected in parallel through a DC circuit through a current sensor. A capacitor bank and a voltage sensor are connected to the DC circuit of the first bridge. The AC circuit of the first bridge is connected to one of the secondary windings of the power transformer through a current sensor and a key consisting of two thyristors connected in opposite-parallel mode. The alternating current circuit of the second bridge, which is the output of the module, is connected in series with the outputs of other modules in such a way that three groups are connected in a star. Each module contains a local control unit, which serves to process signals from the sensors of the module and generate control pulses of power transistors in accordance with the current mode of operation of the module and control signals received via the optical channel from the main control unit. Each module is powered by a single-phase voltage from one separate secondary winding of the power transformer. The number of inverter-recovery modules is equal to the number of secondary windings of the power transformer and is determined by the required output voltage of the converter and the operating voltage of one module.

The device can operate both in inverter and in recovery mode in accordance with the control signals from the main control unit. Moreover, the operation mode of power transistors in each specific module is determined individually by the local control unit.

In inverter mode, the first bridge of each module operates in rectification mode with the consumption of a sinusoidal current from the network. The second bridge inverts the DC voltage to the desired output voltage.

In the recovery mode, during generator braking of the engine, electric energy is supplied through the second bridge to the capacitor bank. The first bridge inverts the voltage of the capacitor bank and generates a sinusoidal current in the secondary winding of the power transformer entering back into the network.

In both modes, the device consumes and sends back to the network a sinusoidal current with a power factor close to unity. A three-phase sinusoidal voltage is generated at the device output by means of a multilevel PWM and an output LC filter to power the electric motor. A multi-level PWM can significantly reduce the size of the output LC filter. The power supply of the converter is possible both from a three-phase and from a single-phase alternating current network using an armored-type power transformer or a group of three single-phase transformers connected to a star. In the absence of the need for power from a single-phase network, it is advisable to use three-phase transformers of a rod structure to reduce weight and size indicators.

The listed set of features provides the specified technical result.

Comparative analysis with the prototype shows that the claimed device is characterized by the presence of new blocks and their relationships with other elements of the circuit. Thus, the invention is new. Comparison of the claimed solutions with other technical solutions from related fields of technology allows us to conclude that there are no significant distinguishing features of the claimed invention.

The list of figures of drawings and other materials.

Figure 1 and 2 presents an electrical schematic diagram of a static multilevel frequency converter for powering asynchronous and synchronous motors.

Figure 3 presents a diagram of the input voltage and input current of the inverter recovery module (IRM) with current consumption from the network.

Figure 4 presents a diagram of the output phase voltage of the Converter during operation of the three inverter recovery modules per phase (N = 3).

Information confirming the possibility of carrying out the invention

The electrical circuit diagram of the claimed invention "Static multilevel frequency converter for supplying asynchronous and synchronous electric motors" is shown in figures 1 and 2. The device includes: input power transformer 11, voltage sensors (DN) at the input 8, 9, 10, voltage sensors at the output 39, 40, 41, current sensors (DT) at the output 30, 31, 32, the output LC filter on the elements 33, 34, 35, 36, 37, 38, the main control unit 45, inverter-recovery modules (IRM ) 13, 15, 17, 19, 21, 23, 25, 27, 29, each of which consists of power transistors, about developing two single-phase bridges. The first single-phase bridge consists of transistors 48, 49, 57, 58. The second single-phase bridge consists of transistors 50, 51, 59, 60. The bridges are connected in parallel via a direct current circuit through a current sensor 47. The module also includes a capacitor bank 55, a sensor voltage 56, current sensor 52, thyristor switch, consisting of two thyristors 53, 54 connected counter-parallel, local control unit 46.

The three-phase mains voltage is connected to the terminals 1, 2, 3 and 4 of the primary winding 5, 6, 7 of the power transformer 11. When powered from a single-phase network, the input voltage is connected to the combined terminals 1, 2, 3 and neutral terminal 4 (the design of the power transformer should include such an inclusion). Further, from the secondary windings 12, 14, 16, 18, 20, 22, 24, 26, 28, the voltage is supplied to the inverter-recovery modules (IRM) 13, 15, 17, 19, 21, 23, 25, 27, 29 connected sequentially in three groups connected to a star, the output of which is connected to the output LC filter on the elements 33, 34, 35, 36, 37, 38 through the output current sensors 30, 31, 32. A thyristor switch on thyristors 53 is installed at the input of each module , 54, which serves to smoothly charge the capacitor bank 55 when the input voltage is applied and to protect the secondary winding of the power transformer from overcurrent and emergency mode. After the thyristor switch, the voltage is supplied to the AC input of the first bridge, consisting of transistors 48, 49, 57, 58, through the current sensor 52. From the DC output of the first bridge, the voltage is supplied to the capacitor bank 55, which serves to filter the alternating current component of the PWM and mains current. From the output of the capacitor bank 55, through the current sensor 47, the voltage is supplied to the DC input of the second bridge, consisting of power transistors 50, 51, 59, 60. The signals from the current sensors 47, 52 and the voltage sensor 56 connected to the output of the capacitor bank 55 enter the inputs of the local control unit 46, which generates control pulses of power transistors of the first and second bridge, as well as a thyristor key at the input of the inverter-recovery module.

The device operates as follows. When the device is connected to the network, the alternating voltage from the secondary windings of the power transformer 11 is supplied to the inverter recovery modules (IRM) 13, 15, 17, 19, 21, 23, 25, 27, 29. The main control unit, analyzing the signals from the input voltage sensors and control signals from an external control system, generates commands for switching on the modules transmitted via optical channels to local control units. Together with the commands, the parameters of the input network and synchronization pulses are transmitted. The local control unit, having received a command from the main control unit, provides a smooth charge of the capacitor bank 55 by gradually increasing the conductivity angle of the thyristors 53, 54 from 0 to 180 °. After charging the capacitor bank, the local control unit transmits readiness signals to the main control unit, as well as the current status parameters of the module. Upon receipt of signals about the readiness of all blocks, the main control unit generates control codes with synchronization pulses for each module. In accordance with the received codes, the local control unit of each module generates control pulses of power transistors. In this case, the local control unit analyzes the direction of energy transfer and sets the operation mode of the first module bridge. When energy is consumed by an electric motor, the local control unit generates control pulses of the power transistors of the first bridge according to the algorithm for generating a sinusoidal current using the PWM method. A diagram illustrating the process of generating a sinusoidal input current is shown in FIG. With a positive half-wave of the input voltage at the time t1 of the beginning of the next PWM period, the transistor 49 is turned on. The transistors 57, 58 are locked. The secondary winding of the power transformer through the open transistor 49 and the reverse diode of the transistor 48 under the influence of the input voltage begins to flow increasing current, limited by the leakage inductance of the winding of the power transformer. At time t2, transistor 49 closes and the reverse diode of transistor 58 opens. The secondary current of the power transformer, supported by the leakage inductance, charges the capacitor bank 55. Then, at time t3, transistor 49 opens and the process repeats. The magnitude of the ripple of the input current is determined by the frequency of the PWM and the magnitude of the leakage inductance of the winding of the power transformer. At a negative half-wave of the input voltage, the transistor 48 switches in a similar way. When the motor is in generator braking mode, the voltage on the capacitor bank rises, and the local control unit switches to recovery mode and generates control pulses of the first bridge according to the algorithm for generating a sinusoidal current to return to the network. Smoothing the input current is also carried out by the leakage inductance of the windings of the power transformer. The PWM frequency of the various modules is phase shifted to reduce the variable component of the input current in the primary winding of the power transformer. A diagram illustrating the process of generating the output voltage is shown in FIG. 4. The diagrams are shown for the case of the operation of three modules per phase (13, 19, 25). To ensure zero output voltage, the transistors 59, 60 of each module are closed, the transistors 50, 51 are open. When a positive half-wave of voltage is generated at time t0 in the module 13, the transistor 50 is locked and the transistor 59 is unlocked with a PWM frequency. By time t1, transistors 59, 51 are constantly open, and transistors 50, 60 are constantly closed. This provides an output voltage equal to the voltage of the first module. To further increase the output voltage, the transistor 50 is locked and the transistor 59 is turned on of the next module 19 at the PWM frequency until t2. At time t2, the voltage of the module 25 is connected. Thus, the output voltages of the modules are added up, and the resulting voltage is formed at the output, which is the sum of the voltage of the outputs of all the modules. When forming a negative half-wave of the output voltage, the modules operate in the same sequence, but with the formation of a negative voltage at the output of each module. For this, the transistors 51, 59 are locked and the transistors 50, 60 are unlocked.

Information sources

1. Zborovsky I.A. Transformers for frequency converters. - Electrical Engineering, 1999, No. 7, pp. 3-13.

2. Lazarev G.B. High voltage converters for variable frequency drive. The construction of various systems. - News of electrical engineering, 2005, No. 2, p.23.

3. US Patent No. 5625545, H02M 7/515, 1997, FIG. 1.

4. US patent No.US 2004240237, H02J 1/02, 2004, Fig.2.

Claims (1)

A static multilevel frequency converter for supplying asynchronous and synchronous electric motors, containing a power transformer, an output filter, a main control unit, characterized in that it contains inverter-recovery modules, each of which consists of power transistors with built-in reverse diodes, forming two single-phase bridges connected parallel to the DC circuit through the current sensor, and a capacitor bank is connected to the DC circuit of the first bridge, and the AC circuit the first bridge is connected to one of the secondary windings of the power transformer through a current sensor and a key consisting of two thyristors connected in opposite parallel, and the alternating current circuit of the second bridge, which is the output of the module, is connected in series with the outputs of other modules in such a way that three groups are formed connected to a star, the output of which is connected to an output filter through a current sensor, and each module contains a local control unit that serves to generate control pulses of power transistors in otvetstvii with the current parameters and the control signals being received on the optical channel from the master control unit.

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