High-power machine drive system based on modular multilevel converter

Abstract

The present invention discloses a high-power machine drive system based on a modular multilevel converter (MMC), which belongs to the technical field of power generation, power transformation, or power distribution. The high-power machine drive system consists of a modular multilevel converter and a multi-pulse cycloconverter. The MMC outputs k phases of high-frequency AC voltages with a phase difference of 2π/k, and the multi-pulse cycloconverter outputs a low-frequency voltage to drive a corresponding machine. According to the present invention, the MMC is combined with the multi-pulse cycloconverter, and by adopting the MMC that operates at a high frequency, the capacity, the volume, and the weight of the energy storage capacitor of the MMC are reduced, the voltage level at the DC side of the MMC is increased, and the capacity of the drive system is increased. By adopting the multi-pulse cycloconverter, quality of an output waveform at a machine side can be guaranteed, thereby implementing low frequency control on the machine. The present invention may be adapted to drive a high-power low-speed machine.

Claims

1. A high-power machine drive system based on a modular multilevel converter, comprising: a modular multilevel converter, arms of all phases being connected to a same DC bus, and the modular multilevel converter outputting k phases of high-frequency AC voltages with a phase difference of 2π/k; a multi-pulse cycloconverter whose thyristor modules of all phases consist of k groups of anti-parallel thyristors, input ends of the thyristor modules of all phases being connected to the k phases of high-frequency AC voltages with the phase difference of 2π/k, and the multi-pulse cycloconverter outputting a frequency-converted AC power to a to-be-driven high-power machine; a carrier generation module configured to generate carrier signals corresponding to the k groups of anti-parallel thyristors according to phase angles of the k phases of high-frequency AC voltages output by the modular multilevel converter, a speed vector controller configured to generate voltage modulation signals corresponding to the thyristor modules of all phases according to a machine speed and a reference value thereof, an AC current output from the multi-pulse cycloconverter to the machine, and the k phases of high-frequency AC voltages with the phase difference of 2π/k output by the modular multilevel converter; a multi-pulse modulator configured to generate trigger pulses of the thyristor modules of all phases of the multi-pulse cycloconverter by performing multi-pulse modulation on the carrier signals corresponding to the k groups of anti-parallel thyristors and the voltage modulation signals corresponding to the thyristor modules of all phases in combination with a current direction at an output side of the multi-pulse cycloconverter.

2. The high-power machine drive system based on a modular multilevel converter according to claim 1, wherein the modulation signals of the k phases of high-frequency AC voltages output by the modular multilevel converter are: y.sub.i=m.sub.1 sin[2πf.sub.mmct−2π(i−1)/k], wherein y.sub.i is a modulation signal of an i-phase high-frequency AC voltage, m.sub.1 is a modulation coefficient, and f.sub.mmc is a frequency at which the MMC outputs a voltage.

3. The high-power machine drive system based on a modular multilevel converter according to claim 1, wherein a carrier signal W.sub.i corresponding to an i.sup.th group of anti-parallel thyristors in the k groups of anti-parallel thyristors is: W.sub.i=sin[2πf.sub.mmct−+θ.sub.mmc−π(2i−3)/k], wherein f.sub.mmc is a frequency at which the MMC outputs a voltage, and θ.sub.mmc are phase angles of the k phases of high-frequency AC voltages output by the modular multilevel converter.

4. The high-power machine drive system based on a modular multilevel converter according to claim 1, wherein a method of performing multi-pulse modulation on the carrier signals corresponding to the k groups of anti-parallel thyristors and the voltage modulation signals corresponding to the thyristor modules of all phases in combination with the current direction at the output side of the multi-pulse cycloconverter comprises: locking all the groups of reversely connected thyristors when the current at the output side of the multi-pulse cycloconverter is in a forward direction, and locking all the groups of forward connected thyristors when the current at the output side of the multi-pulse cycloconverter is in a reverse direction; for all the unlocked groups of forward connected thyristors in the thyristor module, comparing falling edges of the carrier signals corresponding to the k groups of anti-parallel thyristors with the voltage modulation signals corresponding to the thyristor module of the phase, and continuously sending trigger pulses when the carrier signals are less than the voltage modulation signals, or otherwise skipping sending the trigger pulses; for all the unlocked groups of reversely connected thyristors in the thyristor module, comparing rising edges of the carrier signals corresponding to the k groups of anti-parallel thyristors with the voltage modulation signals corresponding to the thyristor module of the phase, and continuously sending trigger pulses when the carrier signals are greater than the voltage modulation signals, or otherwise skipping sending the trigger pulses.

5. The high-power machine drive system based on a modular multilevel converter according to claim 3, wherein the phase angles of the k phases of high-frequency AC voltages output by the modular multilevel converter are obtained through a phase-locked loop (PLL).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 (a) is a structural diagram of a high-power machine drive system according to the present invention, FIG. 1 (b) is a system structure diagram of phase A thyristor module, and FIG. 1 (c) is a structural diagram of the sub-module.

(2) FIG. 2 is a control block diagram of a high-power machine drive system according to the present invention.

REFERENCE NUMERALS

(3) 1. Modular multilevel converter, 1-1. Sub-module, 1-2. Arm inductor, 1-3. DC bus, 2. Multi-pulse cycloconverter, 2-1. Phase-A thyristor module, VT.sub.1_1a, VT.sub.1_2a, VT.sub.1_3a, and VT.sub.1_ka are the first group of forward connected thyristors, the second group of forward connected thyristors, the third group of forward connected thyristors, and the k.sup.th group of forward connected thyristors in the phase A thyristor module, and VT.sub.2_1a, VT.sub.2_2a, VT.sub.2_3a, and VT.sub.2_ka are the first group of reversely connected thyristors, the second group of reversely connected thyristors, the third group of reversely connected thyristors, and the k.sup.th group of reversely connected thyristors in the phase A thyristor module.

DETAILED DESCRIPTION

(4) In order to deepen the knowledge and understanding of the present invention, the technical solution is further introduced below with reference to the accompanying drawings and specific embodiments.

(5) As shown in FIG. 1 (a), a high-power machine drive system based on a modular multilevel converter (MMC) consists of a modular multilevel converter 1 and a multi-pulse cycloconverter 2. The MMC includes k phases of AC side outputs, and each phase of the converter includes upper and lower arms, and each of the arms includes n identical sub-modules 1-1 and an arm inductor 1-2. The sub-module is a half-bridge topology structure, or may be other topology structures such as a full-bridge sub-module, a clamp double sub-module, and the like. DC sides of all the phases are all connected to the same DC bus 1-3, and the k.sup.th phase of AC side output of the converter is connected to the k.sup.th phase of input of each of the multi-pulse cycloconverters. Moreover, in the above MMC topology, the number k of phases and the number n of sub-modules included in each of the arms are determined by design indicators such as the capacity of the drive system. The multi-pulse cycloconverter adopts the zero-type topology structure, which is composed of three-phase thyristor modules with the same internal topological structure. The phase A thyristor module 2-1 is shown in FIG. 1 (b). Each of the thyristor modules includes k groups of anti-parallel thyristors, and each group of anti-parallel thyristors includes thyristors VT.sub.1_i and VT.sub.2_i. The k.sup.th phase of input of the multi-pulse cycloconverter is connected to the k.sup.th group of anti-parallel thyristors in thyristor modules of all phases, and an output side of the multi-pulse cycloconverter is connected to corresponding three-phase machines 1-m.

(6) A high-power machine drive system based on a modular multilevel converter is shown in FIG. 2, including: a modular multilevel converter (MMC), a multi-pulse cycloconverter, a carrier generation module, a speed vector controller, and a multi-pulse modulator. The arms of all phases of the MMC are connected to a same DC bus, and the MMC outputs k phases of high-frequency AC voltages with a phase difference of 2π/k. All phases of thyristor modules of the multi-pulse cycloconverter consist of k groups of anti-parallel thyristors, input ends of the thyristor modules of all the phases are connected to the k phases of high-frequency AC voltages with the phase difference of 2π/k, and the multi-pulse cycloconverter outputs a frequency-converted AC power to a to-be-driven high-power machine. The carrier generation module generates carrier signals corresponding to the k groups of anti-parallel thyristors according to phase angles of the k phases of high-frequency AC voltages output by the modular multilevel converter. The speed vector controller generates voltage modulation signals corresponding to the thyristor modules of all phases according to a machine speed and a reference value thereof, an AC current output from the multi-pulse cycloconverter to the machine, and the k phases of high-frequency AC voltages with the phase difference of 2π/k output by the modular multilevel converter. The multi-pulse modulator generates trigger pulses of the thyristor modules of all phases of the multi-pulse cycloconverter by performing multi-pulse modulation on the carrier signals corresponding to the k groups of anti-parallel thyristors and the voltage modulation signals corresponding to the thyristor modules of all phases in combination with the current direction at an output side of the multi-pulse cycloconverter.

(7) The control method of the system is a coordinated control solution between the MMC, the multi-pulse cycloconverter, and the three-phase machine, including the following steps. The MMC outputs k phases of high-frequency AC voltages with a phase difference of 2π/k, and the multi-pulse cycloconverter outputs a low-frequency voltage to drive a corresponding machine. The method specifically includes the following steps.

(8) (1) The modular multilevel converter adopts a high frequency modulation mode. By comparing modulation signals y.sub.i (i=1, 2, . . . , k) with a high-frequency isosceles triangle carrier, the number of sub-modules n.sub.iu_on (n.sub.il_on) that need to be input by upper arms (lower arms) of all phases can be achieved. An i.sup.th phase of modulation signal is y.sub.i=m.sub.1 sin[2πf.sub.mmct−2π(i−1)/k+θ.sub.mmc]. m1 is a modulation coefficient, which is an allowed maximum value, so that the MMC can output the maximum voltage. f.sub.mmc is a frequency at which the MMC outputs a voltage, which is usually several hundred Hz. θ.sub.mmc is a phase angle of a first phase of high frequency AC voltage. Then capacitor voltages of the upper arm (lower arm) sub-modules of all phases are sorted in ascending order. When the arm current i.sub.iu (i.sub.il) is positive, the number n.sub.iu_on (n.sub.il_on) of sub-modules with the lowest capacitor voltage are input. When the arm current (iii) is negative, the number n.sub.iu_on (n.sub.il_on) of sub-modules with the highest capacitor voltage are input, so that the AC side of the MMC outputs k phases of high-frequency AC voltages with a phase difference of 2π/k.

(9) (2) A phase-locked loop (PLL) is used to obtain the phase angle θ.sub.mmc of the high-frequency AC voltage on the AC output side of the MMC, and carriers W1-Wk respectively corresponding to k groups of anti-parallel thyristors are generated. The carriers corresponding to the i.sup.th group of anti-parallel thyristors are W.sub.i=sin[2πf.sub.mmct−πi/k+θ.sub.mmc].

(10) (3) Three-phase low-speed operating machines 1-m all adopt speed vector control. The three-phase machine m is used as an example. The control method thereof is obtaining three-phase voltage modulation signals y.sub.maref, y.sub.mbref, and y.sub.mcref at the output side of the multi-pulse cycloconverter according to an actual speed n of the three-phase machine, a reference value n.sub.ref of the speed, three-phase currents i.sub.am, i.sub.bm, and i.sub.cm at a machine side, and high-frequency AC voltages u.sub.m1-u.sub.mk.

(11) (4) Multi-pulse cycloconverter modulation is performed on the carrier waves W1-Wk and the modulation signal y.sub.maref corresponding to a phase A thyristor module, to obtain trigger pulses S.sub.1_1a, S.sub.2_1a, . . . , S.sub.1_ka, S.sub.2_ka of phase A k groups of anti-parallel thyristors. Multi-pulse cycloconverter modulation is performed on the carrier waves W1-Wk and the modulation signal y.sub.mbref corresponding to a phase B thyristor module, to obtain trigger pulses S.sub.1_1b, S.sub.2_1b, . . . , S.sub.1_kb, S.sub.2_kb of phase B k groups of anti-parallel thyristors. Multi-pulse cycloconverter modulation is performed on the carrier waves W1-Wk and the modulation signal y.sub.mcref corresponding to a phase C thyristor module, to obtain trigger pulses S.sub.1_1c, S.sub.2_1e, . . . , S.sub.1_kc, S.sub.2_kc of phase C k groups of anti-parallel thyristors. Groups of forward connected thyristors and reversely connected thyristors in each phase of the thyristor module are driven by these trigger pulses, thereby outputting the low-frequency voltage to drive the machine m. The control method for the remaining three-phase machines is similar.

(12) The method of multi-pulse cycloconverter modulation in step (4) is analyzed by using the phase A thyristor module as an example (the other two-phase modules are similar), which is specifically as follows.

(13) (a) For all the groups of forward connected thyristors VT.sub.1_1a-VT.sub.1_ka in the phase A thyristor module, respective trigger pulses S.sub.1_1a-S.sub.1_ka are obtained by respectively comparing falling edges of the carriers W1-Wk with the modulation signal y.sub.maref If the carrier is less than the modulation signal, the trigger pulse is continuously sent, otherwise the trigger pulse is not sent.

(14) (b) For all the groups of reversely connected thyristors VT.sub.2_1a-VT.sub.2_ka in the phase A thyristor module, respective trigger pulses S.sub.2_1a-S.sub.2_ka are obtained by respectively comparing rising edges of the carriers W1-Wk with the modulation signal y.sub.maref. If the carrier is greater than the modulation signal, the trigger pulse is continuously sent, otherwise the trigger pulse is not sent.

(15) (c) If the current direction i.sub.am at the output side of the cycloconverter is greater than 0, all the groups of reversely connected thyristors VT.sub.2_1a-VT.sub.2_ka are locked; and if the current direction i.sub.am at the output side of the cycloconverter is less than 0, all the groups of forward connected thyristors VT.sub.1_1a-VT.sub.1_ka are locked.

(16) The present invention is especially suitable for high-power low-speed machine drive. Compared with the existing method, according to the present invention, the MMC is combined with the multi-pulse cycloconverter, and by adopting the MMC that operates at a high frequency, the capacity, the volume, and the weight of the energy storage capacitor of the MMC are reduced, the voltage level at the DC side of the MMC is increased, and the capacity of the drive system is increased. By adopting the multi-pulse cycloconverter, quality of an output waveform at a machine side can be guaranteed, thereby implementing low frequency control on the machine.

(17) It should be noted that the above embodiments are not used to limit the protection scope of the present invention, and equivalent changes or substitutions made on the basis of the above technical solutions fall within the protection scope of the claims of the present invention.