CN111934558A - Implementation scheme of novel medium-high voltage variable frequency speed control system without transformer isolation - Google Patents

Abstract

The invention relates to a novel middle-high voltage variable frequency speed control system implementation scheme without transformer isolation, which belongs to a middle-high voltage variable frequency speed control technology and a control technology implementation scheme thereof, and the scheme cancels a power frequency transformer used by the traditional middle-high voltage frequency converter and a high-frequency isolation DC/DC conversion link in a new generation of middle-high voltage frequency converter based on a non-power frequency transformer cascade multilevel converter, and each phase of cascade rectifier in the scheme forms a pair of common middle-high voltage direct current buses which can be directly connected with a diode clamping type or capacitor clamping type inverter circuit; and the frequency converter does not need electrical isolation, and the electrical isolation between three-phase inverters is realized by using a three-phase stator winding of the alternating current motor, so that the size and the weight of the frequency converter are greatly reduced, the system structure and the control complexity of the medium-high voltage frequency converter are greatly reduced, the loss in the operation process and the hardware cost are greatly reduced, the system reliability and the overall efficiency are also greatly improved, and the frequency converter is particularly suitable for the field of medium-high voltage high-power variable frequency speed regulation.

Description

A new type of medium and high voltage variable frequency speed regulation system implementation scheme without transformer isolation

technical field

The invention belongs to the technical field of medium and high voltage variable frequency speed regulation, and particularly relates to a novel medium and high voltage variable frequency speed regulation system implementation scheme composed of a high-power cascaded multi-level converter without an isolation transformer and an AC motor organically integrated.

Background technique

In recent years, "Multilevel Converter" (Multilevel Converter) has been used more and more in the fields of medium and high voltage and high power variable frequency speed regulation, active power filtering, high voltage direct current (HVDC) power transmission and power system reactive power compensation. successfully applied. The basic circuit topology of multi-level converters can be roughly divided into two categories: clamping type and unit cascade type. Flat medium and high voltage frequency converters, as well as cascaded H-bridge medium and high voltage frequency converters produced by Robinkang Company or Lidhuafu Company are typical representatives of these two types of products. In the two types of medium and high voltage inverters, no matter which type, in order to use low-voltage power electronic devices to complete high-voltage power conversion, it is necessary to use bulky, complicated and expensive power frequency shifters on the rectifier input side. The phase transformer achieves electrical isolation, which limits its application in many industrial applications.

Cascaded multi-level converters without power frequency transformers have received extensive attention in the field of power electronics technology in recent years, and are considered to be the realization of a new generation of medium and high voltage converters. This type of converter uses a high-frequency transformer to replace the power-frequency phase-shifting transformer in the traditional cascaded converter to achieve electrical isolation. When used for bidirectional power transmission, the intermediate stage uses a high-frequency isolated bidirectional DC/DC converter to transmit energy bidirectionally. Both sides or high-voltage side adopts cascaded fully-controlled H-bridge multi-level power converter structure. When used for unidirectional power transmission, the intermediate stage adopts a high-frequency isolated unidirectional DC/DC converter to transmit energy, and the rectifier side adopts a unidirectional cascaded multi-level power converter structure (including: cascaded diode + Boost rectifier circuit , cascade bridgeless rectifier circuit, cascade VIENNA rectifier circuit, etc.), the inverter side adopts cascade multi-level power converter structure (clamp type or unit cascade type). Compared with traditional medium and high voltage frequency converters, the implementation of this new generation of medium and high voltage frequency converters can effectively reduce system volume, weight and manufacturing cost. However, this type of converter also has obvious shortcomings, which are mainly manifested in: the DC/DC conversion stage composed of high-frequency isolation transformers makes the entire system structure complex and increases the loss during the operation process. The existence of this link hinders the further reduction of the entire system. Cost, improve efficiency, reduce volume and weight, and improve reliability are the key factors. Obviously, canceling the high-frequency isolation DC/DC conversion link in this new generation of medium and high voltage inverters will bring great benefits to the application of such inverters in practical industries. However, the reason why this new generation of medium and high voltage inverters must use high-frequency isolated DC/DC conversion links is because: 1) N cascaded modules in each phase will generate N groups of DC output terminals, and the N groups of DC output terminals It cannot be directly connected in series to form a pair of common DC output buses, but whether the clamped inverter circuit is a diode-clamped or a capacitor-clamped type, it needs to be powered by a common DC bus, so the N groups of DC output terminals cannot be connected with the clamp. The bit-type inverter circuits are directly connected, while the cascade inverter circuit composed of N H-bridge circuits requires N isolated and independent DC power supplies, and the N groups of DC output terminals are not isolated and independent N groups of DC power supplies. Therefore, it cannot be directly connected to the cascaded H-bridge inverter circuit, otherwise it will cause multiple short circuits in the circuit; 2) The three-phase cascaded rectifier does not have a common DC output bus. If there is no high-frequency isolation DC/DC conversion link, the cascade The three-phase inverter is directly connected to the stator winding of the star-connected or delta-connected AC motor, which will also cause multiple short circuits in the system. make the entire system inoperable.

In addition, in the clamped inverter circuit powered by an independent DC power supply, compared with the capacitor clamped circuit, the diode clamped multilevel inverter circuit does not need to use a lot of capacitors, and has a simple structure and high reliability. , easy to control and widely used in various fields of industry. However, the diode-clamped multi-level inverter circuit is difficult to achieve by itself due to the balanced control of the capacitor voltage on the input side. The imbalance will eventually degenerate into a three-level inverter circuit, which is why only a diode-clamped three-level inverter circuit has been successfully applied in the industry.

SUMMARY OF THE INVENTION

In order to solve the above problems, the present invention provides a new implementation scheme of medium and high voltage variable frequency speed regulation system, which can cancel the high voltage in the above-mentioned new generation of medium and high voltage frequency converters based on non-power frequency transformer cascaded multi-level converters. Frequency isolation DC/DC conversion link. Different from the traditional medium and high voltage inverters using power frequency transformers to achieve electrical isolation, and the above-mentioned new generation of medium and high voltage inverters using high frequency transformers to achieve electrical isolation, the present invention provides a new type of medium and high voltage variable frequency speed regulation with unidirectional energy transfer. System implementation plan, 1) Each phase cascade rectifier can form a common DC bus, so it can be directly connected with the clamping inverter circuit (diode clamping type or capacitor clamping type); 2) The inverter itself does not need Electrical isolation, using the three-phase stator windings of the AC motor to achieve electrical isolation between each phase of the three-phase inverter. Therefore, the volume, weight and cost of the medium and high voltage inverter based on the cascaded multilevel converter without a power frequency transformer can be greatly reduced, and the system efficiency is greatly improved.

The purpose of the invention of the present invention is to be achieved through the following technical solutions, a novel medium and high voltage variable frequency speed regulation system implementation solution without transformer isolation, which is characterized in that:

In order to achieve the above purpose, a new type of medium and high voltage variable frequency speed control system implementation scheme without transformer isolation is characterized in that: it includes three high-frequency filters and a three-phase AC-DC-AC converter circuit, and the three-phase AC-DC - The alternator circuit includes three single-phase AC-DC-AC inverter circuits, the single-phase AC-DC-AC inverter circuits include a single-phase rectifier stage circuit, and the single-phase rectifier stage circuit includes one A single-phase rectifier and N cascaded control module units, wherein N≥1 is a positive integer, the single-phase rectifier stage circuit provides a pair of common high-voltage output DC buses, and the single-phase AC-DC-AC converter The circuit includes a single-phase inverter stage circuit, the single-phase inverter stage circuit includes an N+1 level cascaded inverter unit, and the DC input end of the N+1 level cascaded inverter unit It is directly connected with a pair of common high-voltage output DC bus bars provided by the single-phase rectifier stage circuit. The single-phase rectifier stage circuit of the single-phase AC-DC-AC converter circuit has two AC input terminals, three The single-phase rectifier stage circuit of the single-phase AC-DC-AC converter circuit has a total of six AC input terminals, and the first three of the single-phase rectifier stage circuits of the single-phase AC-DC-AC converter circuit are the first. The AC input terminal constitutes one set of terminals, and the second AC input terminals of the single-phase rectifier stage circuits of the three single-phase AC-DC-AC converter circuits constitute another set of terminals, wherein one set of terminals is connected to On a common neutral point, the other group of terminals are respectively connected in series with the three high-frequency filters and connected to the three-phase power grid to form a star connection.

In order to achieve the above purpose, a new type of medium and high voltage variable frequency speed regulation system implementation scheme without transformer isolation is characterized in that: it includes a three-phase AC motor, and each phase of the stator winding of the three-phase AC motor is respectively connected with three of the single-phase AC motors. The AC output ends of the N+1 level cascaded inverter units in the AC-DC-AC converter circuit are connected to form an open-winding connection to achieve electrical isolation between three-phase power flows.

In order to achieve the above purpose, a new medium and high voltage variable frequency speed regulation system implementation scheme without transformer isolation is characterized in that: it includes a control strategy for the realization scheme of the new medium and high voltage variable frequency speed regulation system without transformer isolation, and the The control strategy steps of the realization scheme of the new medium and high voltage variable frequency speed regulation system with transformer isolation are as follows:

(1) In order to improve the DC voltage utilization rate of the inverter stage circuit and obtain the same control effect as the traditional space vector pulse width modulation, it is assumed that the three-phase sinusoidal reference voltage signal of each phase inverter stage circuit is given by the following formula:

where V ^* is the traditional space vector pulse width modulation command voltage signal, v _a ^* , _vb ^* , _vc ^* are the three-phase sinusoidal reference voltage signals, and ωt is the rotation angle of V ^* ;

(2) According to the traditional space vector PWM waveform and the area equivalent principle, the expressions of the PWM voltage waveforms v _a ^* , v _b ^* and v _c ^* of each phase inverter stage circuit in each sector are obtained, respectively: :

T _S =2(T ₀ +T _a +T _b )

In the formula, T ₀ is the zero vector action time, T _s is the switching period, V _d is the DC side voltage of each phase inverter, T _a and T _b are the vector action time of the lag command voltage signal V ^* and the lead command respectively. The vector action time of the voltage signal V ^* ;

(3) Assuming that the command voltage signal V ^* in step (1) is located in the fifth sector:

The above formula α is the angle of V ^* in each sector, and the above formulas T _a , T _b , ωt are brought into v _ao ^* , v _bo ^* , v _co ^* of the fifth sector in step (2), get the following formula:

When the command voltage signal V ^* in step (1) is located in the first, second, third, fourth and sixth sectors, the above expressions of Ta, _Tb , _ωt are brought into step in the same way The first, second, third, fourth, and sixth sectors in (2) can be calculated as v _ao ^* , v _bo ^* , v _co ^* in the first, second, third, fourth, and sixth sectors The expression for the sector is as follows:

(4) The v _ao ^* , v _bo ^* , v _co ^* of each sector in step (3) are synthesized to obtain the following formula:

(5) Periodically extend the three-phase PWM waveforms v _ao ^* , v _bo ^* , and v _co ^* in the range of 0≤ωt≤2π in step (4) as one cycle, and obtain the waveform within the range of 0≤ωt≤2kπ Three-phase pulse width modulation waveforms v _ao ^* , v _bo ^* , v _co ^* , k≥1 is a positive integer;

式中V_d为步骤(2)中每相逆变器的直流侧电压；(6) Suppose the peak value of the sinusoidal voltage to be inverted by the inverter stage circuit of each phase is V _ac , and the signals participating in the PWM modulation of the inverter stage circuit of each phase are v _am ^* , v _bm ^* , v _cm ^* , let

where V _d is the DC side voltage of each phase inverter in step (2);

(7) When g<1 in step (6), the signals v _am ^* , v _bm ^* and v _cm ^* participating in the PWM modulation of each phase inverter stage circuit are:

When g≥1 in step (6), the signals v _am ^* , v _bm ^* and v _cm ^* participating in the PWM modulation of each phase inverter stage circuit are:

v _ao ^* , v _bo ^* , v _co ^* in the above formula are the three-phase pulse width modulation waveforms v _ao ^* , v _bo ^* , v _co ^* in the range of 0≤ωt≤2kπ in step (5);

式中V_m为步骤(7)中每相逆变级电路PWM调制的信号v_am ^*、v_bm ^*、v_cm ^*的幅值；(8) Since each phase inverter stage circuit adopts N+1 level cascaded inverter units, N≥1 is a positive integer, and N different triangular carrier signals are required, and these N different triangular carrier signals have the same The frequencies and amplitudes of N are continuously and vertically distributed in space, symmetrically distributed on both sides of the time axis, and the phases of N different triangular carrier signals differ by 180° in turn, and the cycles of these N different triangular carrier signals are step (2) The switching period T _s in the amplitude is

where V _m is the amplitude of the signals v _am ^* , v _bm ^* and v _cm ^* modulated by the PWM modulation of each phase inverter stage circuit in step (7);

(9) Compare the PWM modulation signals v _am ^* , _vbm ^* , _vcm ^* of the inverter stage circuit of each phase in step (7) with the N triangular carrier signals in step (8), and obtain the inverse of each phase The PWM signal of the inverter circuit is used to control the power switch tube of each phase inverter circuit. This control strategy improves the utilization rate of DC voltage within a certain output voltage range, and achieves the same control effect as the traditional space vector pulse width modulation. .

The following will be described in detail with reference to the accompanying drawings in conjunction with the embodiments.

Description of drawings

Fig. 1 is a structure diagram of the realization scheme of a new type of medium and high voltage variable frequency speed regulation system without transformer isolation;

Fig. 2 is a traditional space vector pulse width modulation sector diagram;

Fig. 3 is the pulse width modulation waveform when the command voltage signal of traditional space vector control is located in the fifth sector;

Fig. 4 is a control block diagram of the realization scheme of a new type of medium and high voltage variable frequency speed regulation system without transformer isolation;

Figure 5 is a reference diagram of one cycle PWM modulation of each phase inverter stage circuit;

Detailed ways

Embodiments of the present invention and working principle are further described below in conjunction with the accompanying drawings:

Referring to Fig. 1, a new type of medium and high voltage variable frequency speed control system implementation scheme without transformer isolation includes three high-frequency filters and a three-phase AC-DC-AC converter circuit. The three-phase AC-DC-AC converter circuit includes Three single-phase AC-DC-AC converter circuits, the single-phase AC-DC-AC converter circuit includes a single-phase rectifier stage circuit and a single-phase inverter stage circuit, and the single-phase rectifier stage circuit includes a single-phase rectifier and N cascaded control module units, where N≥1 is a positive integer, the single-phase rectifier stage circuit provides a pair of common high-voltage output DC buses, and the single-phase inverter stage circuit includes an N+1 level cascaded inverter The DC input terminal of the N+1 level cascaded inverter unit is directly connected with a pair of common high-voltage output DC buses provided by the single-phase rectifier stage circuit, and the single-phase AC-DC-AC converter circuit The single-phase rectifier stage circuit has two AC input terminals, and the single-phase rectifier stage circuits of the three single-phase AC-DC-AC converter circuits have a total of six AC input terminals, and three single-phase AC-DC-AC The first AC input end of the single-phase rectifier stage circuit of the converter circuit constitutes a set of terminals, and the second AC input ends of the single-phase rectifier stage circuits of the three single-phase AC-DC-AC converter circuits constitute another A group of terminals, wherein one group of terminals is connected to a common neutral point, and the other group of terminals is respectively connected in series with the three high-frequency filters, connected to the three-phase power grid, to form a star connection.

Referring to Figure 1, a new type of medium and high voltage variable frequency speed regulation system implementation scheme without transformer isolation includes a three-phase AC motor, and the stator windings of each phase of the three-phase AC motor are respectively connected with three single-phase AC-DC-AC converter circuits. The AC output terminals of the N+1 level cascading inverter units in the system are connected to form an open-winding connection to achieve electrical isolation between the three-phase power flows.

Referring to Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5, a new medium and high voltage variable frequency speed regulation system implementation scheme without transformer isolation includes the control of the realization scheme of a new medium and high voltage variable frequency speed regulation system without transformer isolation strategy, the steps are as follows:

(1) In order to improve the DC voltage utilization rate of the inverter stage circuit and obtain the same control effect as the traditional space vector pulse width modulation, it is assumed that the three-phase sinusoidal reference voltage signal of each phase inverter stage circuit is given by the following formula:

where V ^* is the traditional space vector pulse width modulation command voltage signal, v _a ^* , _vb ^* , _vc ^* are the three-phase sinusoidal reference voltage signals, and ωt is the rotation angle of V ^* ;

(2) According to the traditional space vector PWM waveform and the area equivalent principle, the expressions of the PWM voltage waveforms v _ao ^* , v _bo ^* and v _co ^* of each phase inverter stage circuit in each sector are obtained as follows:

T _S =2(T ₀ +T _a +T _b )

In the formula, T ₀ is the zero vector action time, T _s is the switching period, V _d is the DC side voltage of each phase inverter, T _a and T _b are the vector action time of the lag command voltage signal V ^* and the lead command respectively. The vector action time of the voltage signal V ^* ;

(3) Assume that the command voltage signal V ^* in step (1) is located in the fifth sector:

The above formula α is the angle of V ^* in each sector, and the above formulas T _a , T _b , ωt are brought into v _ao ^* , v _bo ^* , v _co ^* of the fifth sector in step (2), get the following formula:

When the command voltage signal V ^* in step (1) is located in the first, second, third, fourth and sixth sectors, the above expressions of Ta, _Tb , _ωt can be brought into the same way For the first, second, third, fourth and sixth sectors in step (2), obtain v _ao ^* , v _bo ^* , v _co ^* in the first, second, third, fourth and sixth sectors The expression for the sector is as follows:

Synthesize v _ao ^* , v _bo ^* , v _co ^* of each sector in step (3) to obtain the following formula:

(5) Extend the three-phase pulse width modulation waveforms v _ao ^* , v _bo ^* , v _co ^* in the range of 0≤ωt≤2π in step (4) into one cycle, and obtain three-phase pulse width modulation waveforms in the range of 0≤ωt≤2kπ Phase PWM waveforms v _ao ^* , v _bo ^* , v _co ^* , k≥1 is a positive integer;

式中V_d为步骤(2)中每相逆变器的直流侧电压；(6) Suppose the peak value of the sinusoidal voltage to be inverted by the inverter stage circuit of each phase is V _ac , and the signals participating in the PWM modulation of the inverter stage circuit of each phase are v _am ^* , v _bm ^* , v _cm ^* , let

where V _d is the DC side voltage of each phase inverter in step (2);

(7) When g<1 in step (6), the signals v _am ^* , v _bm ^* and v _cm ^* participating in the PWM modulation of each phase inverter stage circuit are:

When g≥1 in step (6), the signals v _am ^* , v _bm ^* and v _cm ^* participating in the PWM modulation of each phase inverter stage circuit are:

v _ao ^* , v _bo ^* , v _co ^* in the above formula are the three-phase pulse width modulation waveforms v _ao ^* , v _bo ^* , v _co ^* in the range of 0≤ωt≤2kπ in step (5);

式中V_m为步骤(7)中每相逆变级电路PWM调制的信号v_am ^*、v_bm ^*、v_cm ^*的幅值；(8) Since each phase inverter stage circuit adopts N+1 level cascaded inverter units, N≥1 is a positive integer, and N different triangular carrier signals are required, and these N different triangular carrier signals have the same The frequencies and amplitudes of N are continuously and vertically distributed in space, symmetrically distributed on both sides of the time axis, and the phases of N different triangular carrier signals differ by 180° in turn, and the cycles of these N different triangular carrier signals are step (2) The switching period T _s in the amplitude is

where V _m is the amplitude of the signals v _am ^* , v _bm ^* and v _cm ^* modulated by the PWM modulation of each phase inverter stage circuit in step (7);

(9) Compare the PWM modulation signals v _am ^* , _vbm ^* , _vcm ^* of the inverter stage circuit of each phase in step (7) with the N triangular carrier signals in step (8), and obtain the inverse of each phase The PWM signal of the inverter circuit is used to control the power switch tube of each phase inverter circuit. This control strategy improves the utilization rate of DC voltage within a certain output voltage range, and achieves the same control effect as the traditional space vector pulse width modulation. .

The above-mentioned embodiments are only to describe the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. Various modifications and improvements made should fall within the protection scope of the present invention, and the technical content claimed in the present invention has been fully recorded in the claims.

Claims (3)

1. a novel medium and high voltage variable frequency speed regulation system realization scheme without transformer isolation is characterized in that: comprising three high-frequency filters and three-phase AC-DC-AC converter circuits, the three-phase AC-DC-AC The converter circuit includes three single-phase AC-DC-AC converter circuits, the single-phase AC-DC-AC converter circuit includes a single-phase rectifier stage circuit, and the single-phase rectifier stage circuit includes a single-phase rectifier and N cascaded control module units, where N≥1 is a positive integer, the single-phase rectifier stage circuit provides a pair of common high-voltage output DC bus bars, and the single-phase AC-DC-AC converter circuit includes a single-phase rectifier circuit. A phase inverter stage circuit, the single-phase inverter stage circuit includes an N+1 level cascaded inverter unit, and the DC input end of the N+1 level cascaded inverter unit is connected to the A pair of common high-voltage output DC buses provided by the single-phase rectifier stage circuit are directly connected, and the single-phase rectifier stage circuit of the single-phase AC-DC-AC converter circuit has two AC input ends, and three single-phase The single-phase rectifier stage circuit of the AC-DC-AC converter circuit has a total of six AC input terminals, and the three first AC input terminals of the single-phase rectifier stage circuit of the single-phase AC-DC-AC converter circuit A group of terminals is formed, and the second AC input terminals of the single-phase rectifier stage circuits of the three single-phase AC-DC-AC converter circuits constitute another group of terminals, wherein one group of terminals is connected to a common On the neutral point, another group of terminals is connected in series with the three high-frequency filters, respectively, and connected to the three-phase power grid to form a star connection. 2. The realization scheme of a novel medium and high voltage variable frequency speed regulation system without transformer isolation is characterized in that: it comprises a three-phase alternating current motor, and each phase stator winding of the three-phase alternating current motor is respectively matched with the three-phase alternating current motor described in claim 1. The AC output ends of the N+1 level cascaded inverter units in the AC-DC-AC converter circuit are connected to form an open-winding connection to achieve electrical isolation between three-phase power flows. 3. A realization scheme of a novel medium and high voltage variable frequency speed regulation system without transformer isolation is characterized in that: a control strategy of the realization scheme of a novel medium and high voltage variable frequency speed regulation system without transformer isolation is included, and the The control strategy steps of the realization scheme of the new medium and high voltage variable frequency speed regulation system are as follows: (1) In order to improve the DC voltage utilization rate of the inverter stage circuit and obtain the same control effect as the traditional space vector pulse width modulation, it is assumed that the three-phase sinusoidal reference voltage signal of each phase inverter stage circuit is given by the following formula:

where V ^* is the traditional space vector pulse width modulation command voltage signal, v _a ^* , _vb ^* , _vc ^* are the three-phase sinusoidal reference voltage signals, and ωt is the rotation angle of V ^* ; (2) According to the traditional space vector PWM waveform and the area equivalent principle, the expressions of the PWM voltage waveforms v _a ^* , v _b ^* and v _c ^* of each phase inverter stage circuit in each sector are obtained, respectively: :

(first sector)

(Second sector)

(third sector)

(fourth sector)

(Fifth sector)

(Sixth sector) T _S =2(T ₀ +T _a +T _b ) In the formula, T ₀ is the zero vector action time, T _s is the switching period, V _d is the DC side voltage of each phase inverter, T _a and T _b are the vector action time of the lag command voltage signal V ^* and the lead command respectively. The vector action time of the voltage signal V ^* ; (3) Assuming that the command voltage signal V ^* in step (1) is located in the fifth sector:

The above formula α is the angle of V ^* in each sector, and the above formulas T _a , T _b , ωt are brought into v _ao ^* , v _bo ^* , v _co ^* of the fifth sector in step (2), get the following formula:

When the command voltage signal V ^* in step (1) is located in the first, second, third, fourth and sixth sectors, the above expressions of Ta, _Tb , _ωt are brought into step in the same way The first, second, third, fourth, and sixth sectors in (2) can be calculated as v _ao ^* , v _bo ^* , v _co ^* in the first, second, third, fourth, and sixth sectors The expression for the sector is as follows:

(4) The v _ao ^* , v _bo ^* , v _co ^* of each sector in step (3) are synthesized to obtain the following formula:

(5) Periodically extend the three-phase PWM waveforms v _ao ^* , v _bo ^* , and v _co ^* in the range of 0≤ωt≤2π in step (4) as one cycle, and obtain the waveform within the range of 0≤ωt≤2kπ Three-phase pulse width modulation waveforms v _ao ^* , v _bo ^* , v _co ^* , k≥1 is a positive integer; (6) Suppose the peak value of the sinusoidal voltage to be inverted by the inverter stage circuit of each phase is V _ac , and the signals participating in the PWM modulation of the inverter stage circuit of each phase are v _am ^* , v _bm ^* , v _cm ^* , let

where V _d is the DC side voltage of each phase inverter in step (2); (7) When g<1 in step (6), the signals v _am ^* , v _bm ^* and v _cm ^* participating in the PWM modulation of each phase inverter stage circuit are:

When g≥1 in step (6), the signals v _am ^* , v _bm ^* and v _cm ^* participating in the PWM modulation of each phase inverter stage circuit are:

v _ao ^* , v _bo ^* , v _co ^* in the above formula are the three-phase pulse width modulation waveforms v _ao ^* , v _bo ^* , v _co ^* in the range of 0≤ωt≤2kπ in step (5); (8) Since each phase inverter stage circuit adopts N+1 level cascaded inverter units, N≥1 is a positive integer, and N different triangular carrier signals are required, and these N different triangular carrier signals have the same The frequencies and amplitudes of N are continuously and vertically distributed in space, and are symmetrically distributed on both sides of the time axis. The phases of N different triangular carrier signals differ by 180° in turn, and the cycles of these N different triangular carrier signals are step (2) The switching period T _s in the amplitude is

where V _m is the amplitude of the signals v _am ^* , v _bm ^* and v _cm ^* modulated by the PWM modulation of each phase inverter stage circuit in step (7); (9) Compare the PWM modulation signals v _am ^* , _vbm ^* , _vcm ^* of the inverter stage circuit of each phase in step (7) with the N triangular carrier signals in step (8), and obtain the inverse of each phase The PWM signal of the inverter circuit is used to control the power switch tube of each phase inverter circuit. This control strategy improves the utilization rate of DC voltage within a certain output voltage range, and achieves the same control effect as the traditional space vector pulse width modulation. .

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