Cascade multi-level converter for power supply of toothed stator windings of multi-phase electric motor

Abstract

FIELD: power electronics.SUBSTANCE: invention relates to control systems for electrical machines and is intended for use in industrial drives. The converter (1) connected to a three-phase power source (2) comprises power cells (3), combined into three groups (4) with n power cells (3) in each group, where n≥1 is an integer and defines the number of transformation levels. Each power cell (3) comprises a rectifier (5) connected to a DC link (6), to which an inverter (7) is connected, made with a pair of output terminals for connection to the stator tooth winding. Within each group (4), the rectifiers (5) of all power cells (3) are connected in series, forming a cascade rectifier, where the input terminals of the rectifiers (5) of the power cells (3) are connected to each other in a star pattern, and the first power cell (3) in the cascade is connected to the phase of the power source (2). The rectifier (5) can be made in the form of an active rectifier or in the form of an uncontrolled diode rectifier.EFFECT: increasing the control range and improving reliability.3 cl, 3 dwg

Description

[0001] The invention relates to the field of power electronics and control systems for electric machines and is intended for use in industrial drives.

[0002] The "System of a star-shaped multi-phase variable frequency drive with high-voltage to low-voltage conversion" is known (patent RU2687479C1, IPC H02P25 / 20, H02P27 / 08, H02M7 / 483, H02M7 / 539, H02M7 / 5395, published on 14.05.2019, Bulletin No. 14). This invention includes: a phase-shifting transformer with several secondary windings, each of which, through a three-phase rectifier, generates voltage on the DC link, to which a low-voltage inverter, which is a half-bridge, is connected. The inverter output is connected to the stator windings of a multi-phase motor, which are connected in a star.

[0003] The disadvantage of this technical solution is the need for a high-voltage phase-shifting transformer, which increases the weight, dimensions and cost of the system, and also complicates the manufacture and balancing of the secondary windings of the transformer.

[0004] A technical solution is known, described in the work "Multilevel inverter for a polyphase asynchronous motor with split windings with pole-phase modulated using dual inverter principle" (article B. P. Reddy and S. Keerthipati, "A Multilevel Inverter Configuration for an Open-End-Winding Pole-Phase-Modulated-Multiphase Induction Motor Drive Using Dual Inverter Principle," in IEEE Transactions on Industrial Electronics, vol. 65, no. 4, pp. 3035-3044, April 2018, doi: 10.1109 / TIE.2017.2750626), which presents a converter configuration that contains one nine-phase two-level inverter and three three-phase two-level inverters. All four inverters are connected to different ends of the corresponding group of stator windings and are powered by four isolated DC voltage sources. Vector pulse-width modulation with carrier shift is used to generate a four-level voltage in each phase.

[0005] The disadvantage of this technical solution is the need for four isolated power supplies, complex switching of separated windings, which leads to an increase in cost and a decrease in reliability when scaling.

[0006] The closest in technical essence to the claimed invention is the solution discussed in the article "A transformerless cascaded AC-DC-AC converter for multiphase propulsion drive application" (T. Xinghua, X. Lie, S. Yichao and S. Min, "A transformerless cascaded AC-DC-AC converter for multiphase propulsion drive application," 2011 International Conference on Electrical Machines and Systems, Beijing, China, 2011, pp. 1-5, doi: 10.1109 / ICEMS.2011.6073928), which shows a cascade structure from the grid side. Active rectifier modules based on a full bridge are connected in series and connected to the grid. Each rectifier generates voltage on the DC link, which is connected to a classic three-phase two-level inverter. The inverter generates output voltages on independent three-phase windings of a multiphase synchronous motor connected to it.

[0007] The disadvantage of this technical solution is its limited independent control of the windings of a multiphase motor. When powering groups of three-phase windings from common inverters, it is impossible to supply power to each winding individually, which reduces the flexibility of the electromagnetic process control system. In this solution, the use of this type of converter is focused on powering a permanent magnet synchronous machine and, therefore, is not a universal solution for various types of electrical machines.

[0008] The technical objective of the proposed invention is to provide independent power supply to each winding of the stator teeth of a multiphase electric motor with separated windings in order to create a high-voltage, scalable and fault-tolerant power system without the use of an input transformer.

[0009] The technical result consists in increasing the control range and improving reliability.

[0010] The technical result is achieved in that the converter, containing a rectifier connected to a power source, and inverters, according to the invention, the converter, connected to a three-phase power source, contains power cells, combined into three groups of n power cells in each group, where n ≥ 1, is an integer and determines the number of conversion levels, wherein each power cell contains a rectifier connected to a DC link, to which an inverter is connected, made with a pair of output terminals for connecting to the winding of a stator tooth, within each group, the rectifiers of all power cells are connected in series, forming a cascade rectifier, where the input terminals are connected to each other according to the "star" scheme, and the first power cell in the cascade is connected to the phase of the power source.

[0011] The rectifier can also be made as an active rectifier.

[0012] In addition, the rectifier can be designed as an uncontrolled diode rectifier.

[0013] The essence of the invention is explained by graphic material, where Fig. 1 shows a functional diagram of a cascade multi-level converter for powering the toothed windings of the stator of a multi-phase electric motor and the following designations are adopted:

[0014] 1 - multi-level converter;

[0015] 2 - power supply;

[0016] 3 - power cell;

[0017] 4 - power cell group;

[0018] 5 - rectifier;

[0019] 6 - DC link;

[0020] 7 - inverter.

[0021] Fig. 2 shows the topology of a power cell of a multi-level cascade converter with an active rectifier for controlling a switched-reluctance motor.

[0022] Fig. 3 shows the topology of a power cell of a multilevel cascade converter with an uncontrolled diode rectifier for controlling an asynchronous / synchronous motor.

[0023] In Fig. 2 - 3 the following designations are used:

[0024] 5 - rectifier;

[0025] 6 - DC link;

[0026] 7 - inverter.

[0027] The proposed solution utilizes a multi-level cascade converter topology directly powered from a medium- or high-voltage grid without a phase-shifting transformer. Each stator tooth winding is powered by its own independent inverter module, the type and switching level of which are determined based on the rated voltage and specific motor type. This optimizes the inverter's dimensions and takes into account the control features of a specific motor type. Each inverter module is connected to a DC link, the voltage on which is generated by an active rectifier or an uncontrolled diode bridge. The rectifier is directly connected to the grid in a star configuration, eliminating the need for a multiphase transformer.

[0028] The functional diagram (see Fig. 1) shows a multi-level converter 1 connected to a three-phase power source 2 and contains power cells 3 combined into three groups 4 with n power cells 3 in each group 4, where n ≥ 1 and is an integer, n determines the number of conversion levels. Today, there are multi-phase electric motors with the number of phases up to 48, and the number of converter levels n corresponds to the number of phases and is limited by the same number. Each such group 4 corresponds to one phase of a three-phase power source 2. Within each group 4, rectifiers 5 of all power cells 3 are connected in series, forming a cascade rectifier. In each group 4, the first power cell 3 in the cascade is connected to the phase of power source 2, namely the input terminal "+" of cell number 1, n+1 and 2n+1 (In 1+ , Вх n+1+, In 2n+1+). The input terminal "-" of power cell 3 is connected to the input terminal "+" of the next power cell 3, and the input terminals "-" of the last power cells 3 in the cascade of group 4 n, 2n and 3n (In n- , Вх 2n-, In 3n- ) are connected to each other, forming a "star" connection. In this case, each power cell 3 contains a rectifier 5, connected to a DC link 6, to which an inverter 7 is connected, made with a pair of output terminals (Output 1+ , Exit 1-etc.) for connection to the stator tooth winding. Each rectifier 5 generates a direct voltage on the DC links 6, containing a capacitor, of each power cell 3. Connected to each DC link 6 is an inverter 7 of the same power cell 3, which converts the direct voltage into an alternating voltage for its stator tooth winding, brought out to the corresponding pair of output terminals "+", "-" from power cell 3, which are directly connected to the terminals of the phase windings of the motor. Thus, 3n separate channels are formed at the converter output, each of which independently supplies one stator tooth winding.

[0029] The multi-level inverter for electric motor power supply works as follows.

[0030] Converter 1 is fed from a three-phase power source 2 and generates 3n independent voltages at the output to power the stator tooth windings of a multiphase motor. To power the isolated motor windings, each phase of the high-voltage three-phase power source is connected to the rectifier circuit 5 of converter 1. Each circuit contains a cascade of n rectifiers 5 connected in series. The rectifier circuits are connected in a star with a common neutral point. Each rectifier 5 in the cascade generates voltage on the corresponding DC link 6. Active single-phase rectifier 5 regulates the voltage on DC link 6. The control system (not shown in the diagram) of the converter generates pulse-width modulation (PWM) control signals for the power keys of rectifier 5. The network currents measured by the current sensors are converted from a three-phase system to a two-phase system with dq-coordinates. The current along the d-axis regulates the active power, and the current along the q-axis regulates the reactive power.Currents are maintained by PI controllers, and the controller outputs generate voltage references, which are converted back into the three-phase system and scaled to the level of individual low-voltage cells. A single-phase diode rectifier (rectifier 5) generates a fixed voltage on DC link 6, limited by the amplitude of the input AC voltage from power source 2, without the ability to control the voltage level or regenerate energy into the grid. To implement the energy regenerative mode, a controlled single-phase rectifier (rectifier 5) is used. It uses a bridge circuit and ensures bidirectional power exchange between DC link 6 and power source 2.

[0031] The invention shown in Fig. 1 can be used to control a multiphase switched-reluctance motor. In this case, each power cell 3 consists of a rectifier 5, a DC link 6, and an inverter 7 based on an asymmetrical half-bridge (Fig. 2). A single-phase rectifier 5 based on a full bridge (an example, not limited to this) is used to convert alternating voltage to direct voltage, as well as to operate in inverter mode during energy recovery into the grid. The full bridge (rectifier 5) consists of four power switches (based on IGBT or MOSFET), to which free-wheeling diodes are connected in parallel. At any given moment, two diagonally opposite pairs of switches are turned on, thus the current flows in one direction regardless of the polarity of the input voltage. Since the force of attraction of a ferromagnetic body to an electromagnet does not depend on the sign of the current, the motor phases are fed by unipolar current pulses generated from an asymmetric bridge (inverter 7).Connecting a pair of power switches in series with one winding supplies voltage and, consequently, current from DC link 6. This causes current to build up in the connected winding, toward which the rotor rotates. Switching to the other winding requires disconnecting the switches, which causes the diodes connected to that winding to conduct. This results in demagnetization of the phase winding being disconnected. This inverter topology simplifies the circuit and reduces losses, providing the necessary output voltages to power the phases of a switched-reluctance motor with minimal complexity and high reliability.

[0032] Similarly, the proposed cascade multi-level converter is also applicable to multiphase asynchronous and synchronous machines, where each power cell 3 generates independent power for the corresponding stator winding. In this case, currents in the asynchronous machine windings are controlled via full-bridge inverters 7 (Fig. 3), and rectifier 5 is based on a diode bridge (example, not limited to this). In this case, the full bridge (inverters 7) converts the DC voltage of DC link 6 to AC voltage by alternating the diagonal pairs of switches, which creates sinusoidal currents with uniform phase shifts and generates a smooth rotating magnetic field.

[0033] Rectifier 5 can be made either as an active rectifier (Fig. 2) to improve the quality of the current, or as an uncontrolled diode rectifier (Fig. 3) to simplify the design with less stringent requirements for harmonic distortion.

[0034] The DC link voltage 6 provided by the rectifier 5 is converted by the inverter 7 into AC voltage, the control of which depends on the type of motor that is fed from the converter 1.

[0035] Voltage-controlled reluctance motors have separate windings for each phase and operate using the alternate excitation principle. Independent voltage control on each phase allows torque to be generated based on rotor position. This reduces torque pulsation and makes the system resistant to individual phase failure.

[0036] For multi-phase synchronous and asynchronous machines with independent windings, inverter 7 is controlled. PWM is used to generate output voltages, with each inverter 7 cell generating a sinusoidal voltage with a specified phase shift. This ensures the creation of a multi-phase voltage system.

[0037] The main advantages of this multi-level cascade design include: no need for input power transformers, which reduces the system size; increased reliability due to modularity and the ability to operate with individual cell failures; and the ability to scale the system by adding cells to increase the output voltage or the number of phases; and universal application for various types of multi-phase electrical machines.

[0038] This technical solution provides voltage to separate windings of a multiphase motor, generated by a cascaded multi-level converter powered directly from the grid. This reduces the system's size and cost by eliminating the need for a multi-winding input transformer. It enables dynamically changing the number of pole pairs on the fly by individually shaping the flux on each stator tooth, expanding the control range and increasing reliability by enabling disconnection of any cell or tooth winding during operation in the event of a failure, resulting in a slight reduction in output power. The proposed topology is versatile and can be used for synchronous, asynchronous, and switched-reluctance electric machines.

[0039] For multiphase electric machines (with 6-24 phases or more), independently controlled phase windings allow the number of pole pairs to be varied. This enables the creation of multiple operating modes at high and low speeds. As a result, the speed control range for multiphase machines increases compared to traditional three-phase systems, which have a speed control range of 1:5-1:7, to 1:40 or more. When using individual phase control, as in switched-reluctance machines, it can reach up to 1:100 without the need for mechanically changing the number of pole pairs.

[0040] Reliability is increased through the modularity of the proposed solution. In the closest analogue, the failure of a single inverter or rectifier disrupts the entire three-phase section, whereas in the proposed solution, each winding is completely independently powered. If a single cell or phase fails, the system can continue to operate at partial power.

[0041] The transition between motor types is achieved by replacing the rotor and reconfiguring the control algorithm. The converter structure remains the same; only the inverter section may be modified. When using independent inverter modules for each phase, the number of pole pairs can be switched by changing the power switch control algorithm.

[0042] The use of the invention allows to increase the adjustment range and improve reliability.

Claims

1. A converter (1) comprising a rectifier (5) connected to a power source (2) and inverters (7), characterized in that the converter (1) connected to a three-phase power source (2) comprises power cells (3) combined into three groups (4) of n power cells (3) in each group, where n ≥ 1 is an integer and determines the number of conversion levels, wherein each power cell (3) comprises a rectifier (5) connected to a DC link (6) to which an inverter (7) is connected, made with a pair of output terminals for connection to the stator tooth winding, within each group (4) the rectifiers (5) of all power cells (3) are connected in series, forming a cascade rectifier, where the input terminals are connected to each other in a star pattern, and the first power cell (3) in the cascade is connected to a phase of the power source (2).

2. The converter (1) according to paragraph 1, characterized in that the rectifier (5) is made in the form of an active rectifier.

3. The converter (1) according to paragraph 1, characterized in that the rectifier (5) is made in the form of an uncontrolled diode rectifier.

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