Vector modulation method for three-phase two-level inverter to reduce common mode voltage on AC side

Abstract

A three-phase two-level inverter vector modulation method for reducing the common-mode voltage on the AC side belongs to the technical field of power electronic vector modulation. The present invention aims at the problem that the traditional modulation strategy of the three-phase two-level inverter will generate a common-mode voltage with high frequency and high amplitude. It includes: using three voltage vectors U1, U3 and U5 to synthesize a reference voltage vector in sectors I, III and V, and using three voltage vectors U2, U4 and U6 to synthesize a reference voltage vector in sectors II, IV and VI; adjusting the action and transmission order of the three voltage vectors within a carrier cycle so that the inverter three-phase bridge arm switch tube performs two switching actions within a carrier cycle; calculating the action time of the three voltage vectors in the sector; predicting the actual value of the three-phase current corresponding to the end or start time of each voltage vector, and performing dead zone compensation based on the time pulse width. The present invention is used for vector modulation of a three-phase two-level inverter.

Description

Technical Field

[0001] The invention relates to a three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side, and belongs to the technical field of power electronic vector modulation. Background Art

[0002] Common-mode voltage is the zero-sequence component in the output voltage of a voltage source PWM inverter. With the rapid development of science and technology in recent years, the performance of power electronic devices has become increasingly higher. In order to improve the performance of the system and achieve efficient use of electrical energy, the switching frequency and switching speed of power devices in the inverter have become increasingly higher. Some negative effects have begun to attract the attention of scholars, and electromagnetic interference (EMI) is one of the more serious problems. EMI includes common-mode electromagnetic interference and differential-mode electromagnetic interference, which will affect the normal operation of electronic and electrical equipment and hinder the development of electronic devices towards high speed.

[0003] At present, the research on differential mode electromagnetic interference suppression in EMI is quite mature, but the research on common mode electromagnetic interference suppression is not deep enough. The three-phase voltage source inverter adopts PWM technology, and its output voltage imbalance will lead to the generation of high-frequency common mode interference source - common mode voltage. The high-frequency common mode voltage acts on the common mode loop formed by the stray capacitance of the motor to the ground, which will generate common mode current and cause common mode electromagnetic interference. The continuous common mode current flowing through the insulation of the motor will cause long-term damage to the insulation of the motor, and the bearing current generated will also damage the motor bearing, reduce the life of the bearing and reduce the reliability of the motor operation. The leakage current generated by the high-frequency and high-amplitude common mode voltage is often large, which may cause the current relay protection device of the system grounding to malfunction, causing serious consequences to the system. If the space distance between the inverter and the motor is far, it needs to be connected through a long cable. The pulse voltage will be reflected when transmitted through the long cable. The superposition of the reflected wave and the incident wave will cause overvoltage at the motor end, thereby increasing the common mode voltage of the motor to the ground, aggravating the harm of the common mode voltage, and increasing the maintenance cost of the motor.

[0004] In order to reduce the overall EMI of the system, it is necessary to suppress the common-mode electromagnetic interference of the system; the common-mode voltage suppression methods mainly include hardware suppression and software suppression. Compared with hardware suppression, software suppression has strong portability, low cost and greater advantages. Summary of the invention

[0005] Aiming at the problem that the conventional modulation strategy of the three-phase two-level inverter generates a high-frequency and high-amplitude common-mode voltage, the present invention provides a three-phase two-level inverter vector modulation method for reducing the common-mode voltage on the AC side.

[0006] A three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side of the present invention comprises:

[0007] The voltage vector circle of the inverter is divided into six sectors. In sectors I, III and V, three voltage vectors U1, U3 and U5 are used to synthesize the reference voltage vector. In sectors II, IV and VI, three voltage vectors U2, U4 and U6 are used to synthesize the reference voltage vector.

[0008] Adjust the action and transmission order of the three voltage vectors in one carrier cycle so that the three-phase bridge arm switches of the inverter perform two switching actions in one carrier cycle to reduce switching losses;

[0009] According to the components of the reference voltage vector on the α and β axes, the sector where the current reference voltage vector is located is determined, and then combined with the volt-second balance principle, the action time of the three voltage vectors in the sector is calculated;

[0010] For each switch switching point in each switching cycle, the actual three-phase current value corresponding to the end or start time of each voltage vector is predicted according to the load voltage equation, and the polarity of the actual three-phase current value is obtained; dead zone compensation based on the time pulse width is performed according to the polarity of the actual three-phase current value; thereby achieving the suppression of common-mode voltage in the vector modulation process.

[0011] According to the three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side of the present invention, the corresponding relationship between the sector, the reference voltage vector angle and the voltage vector selection is as follows:

[0012] Sector Number Angle (rad) Voltage Vector Ⅰ [-π / 6,π / 6] <![CDATA[U1 U3 U5]]> Ⅱ [π / 6,π / 2] <![CDATA[U2 U4 U6]]> Ⅲ [π / 2,5π / 6] <![CDATA[U1 U3 U5]]> Ⅳ [5π / 6,7π / 6] <![CDATA[U2 U4 U6]]> Ⅴ [7π / 6,3π / 2] <![CDATA[U1 U3 U5]]> Ⅵ [3π / 2,11π / 6] <![CDATA[U2 U4 U6]]> .

[0013] According to the three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side of the present invention, the corresponding relationship between the action and transmission order of the three voltage vectors in one carrier cycle and the sectors is as follows:

[0014] Sector Number Voltage vector action sequence Ⅰ 51335 Ⅱ 62446 Ⅲ 13551 Ⅳ 24662 Ⅴ 35113 Ⅵ 46224

[0015] The numbers 1 to 6 in the table correspond to voltage vectors U1 to U6.

[0016] According to the three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side of the present invention, the method for determining the sector where the current reference voltage vector is located is:

[0017] Denote the reference voltage vector as U ref , use U α and U β Represents the reference voltage vector U ref The components on the α-axis and β-axis define three intermediate variables X, Y, and Z, and let:

[0018]

[0019] Define three intermediate variables A, B, and C, and set them as follows:

[0020] If X>0, then A=1, otherwise A=0;

[0021] If Y>0, then B=1, otherwise B=0;

[0022] If Z>0, then C=1, otherwise C=0;

[0023] Set the intermediate variable N = 4A + 2B + C, and get the corresponding relationship between N and sector:

[0024] N 4 6 2 3 1 5 Sector Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Ⅵ

[0025] The sector determined in the table is the sector where the current reference voltage vector is located.

[0026] According to the three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side of the present invention, the method for calculating the action time of the three voltage vectors in the sector is:

[0027] Assume that the current reference voltage vector U ref Located in sectors I, III, and V, according to the volt-second balance principle:

[0028]

[0029] Where T s represents the carrier cycle, T1 represents the action time of the voltage vector U1, T3 represents the action time of the voltage vector U3, and T5 represents the action time of the voltage vector U5;

[0030] Calculating formula (2) yields:

[0031]

[0032] Where U dc Indicates the bus voltage.

[0033] According to the three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side of the present invention, the method for predicting the actual value of the three-phase current corresponding to the end or start time of each voltage vector is:

[0034] According to the load voltage equation, the increment of the three-phase inductor current during each voltage vector action time is obtained, thereby predicting the actual value of the three-phase current.

[0035] Beneficial effects of the present invention: The method of the present invention is improved on the basis of the traditional space vector pulse width modulation (Space Vector Pulse Width Modulation, SVPWM), and an improved voltage vector-based modulation strategy (Modified Remote State Pulse Width Modulation, MRSPWM) is proposed, which uses different voltage vector combinations in different sectors to achieve the reduction of the common mode voltage amplitude and frequency. At the same time, in view of the additional number of switching actions introduced by the central symmetric vector action sequence, a method for improving the action vector sequence is proposed, which effectively reduces the number of switching actions of the power device in the inverter without changing the effect of suppressing the common mode voltage amplitude and frequency, thereby greatly improving the system efficiency. In view of the existence of the dead zone effect of the inverter, the effect of reducing the common mode voltage by the modulation strategy proposed by the present invention will be affected, and the problem of common mode voltage spikes will occur. A current prediction method is proposed to predict the polarity of the three-phase current at each dead zone in real time, and a dead zone compensation strategy based on the time pulse width is adopted to effectively solve the dead zone effect problem. Compared with the traditional current sampling method, the current prediction method of the present invention method greatly improves the accuracy of current polarity judgment. Therefore, the method of the present invention achieves the reduction of the common mode voltage amplitude and frequency while taking into account the inverter dead zone effect and switching loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a structural diagram of a three-phase two-level inverter; bus represents capacitor, S1-S6 represents six switch tubes, D1-D6 represents six diodes, PhaseA represents phase A, PhaseB represents phase B, PhaseC represents phase C, L represents inductance, R represents resistance, U represents A Indicates the A phase voltage, U B Indicates the B phase voltage, U C Indicates the C phase voltage;

[0037] Figure 2 The figure is a schematic diagram of a linear modulation area of ​​a vector modulation technology for performing inverter vector modulation using the method of the present invention; in the figure, U0 is a zero vector (000), U7 is a zero vector (111), and U1 to U6 are effective voltage vectors;

[0038] Figure 3 It is a schematic diagram of the switch state and common mode voltage of the first sector with the existing phase voltage being symmetrical at the center; Carrier in the figure represents the carrier signal during modulation; U a (t) represents the output voltage of phase A, U CM Represents common mode voltage;

[0039] Figure 4It is a schematic diagram of the switch state and common mode voltage of the first sector after the action and transmission sequence of three voltage vectors in one carrier cycle are adjusted by the method of the present invention;

[0040] Figure 5 The first sector voltage vector synthesis diagram is obtained after the method of the present invention is used to adjust the action and transmission order of three voltage vectors within a carrier cycle; in the diagram, θ is the reference voltage vector angle, O is the coordinate origin, and I represents the sector;

[0041] Figure 6 It is a common mode voltage diagram in the first sector obtained by the method of the present invention under the assumption that the three-phase current is positive, positive, and negative; Sector1 in the figure represents the situation when the first sector is in operation, T d Indicates dead time;

[0042] Figure 7 This is a schematic diagram of the common mode voltage under the dead zone effect of the traditional SVPWM modulation method; in the figure, Voltage represents voltage, and CMV represents common mode voltage;

[0043] Figure 8 It is a schematic diagram of the common mode voltage obtained by using the modulation method of the present invention when the dead zone effect is not compensated;

[0044] Fig. 9 It is a schematic diagram of the common mode voltage obtained when the dead zone effect is compensated by the modulation method of the present invention. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0047] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0048] Specific implementation method 1. Combination Figures 1 to 6 As shown, the present invention provides a three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side, comprising:

[0049] The voltage vector circle of the inverter is divided into six sectors. In sectors I, III and V, three voltage vectors U1, U3 and U5 are used to synthesize the reference voltage vector. In sectors II, IV and VI, three voltage vectors U2, U4 and U6 are used to synthesize the reference voltage vector.

[0050] Adjust the action and transmission order of the three voltage vectors in one carrier cycle so that the three-phase bridge arm switches of the inverter perform two switching actions in one carrier cycle to reduce switching losses;

[0051] According to the components of the reference voltage vector on the α and β axes, the sector where the current reference voltage vector is located is determined, and then combined with the volt-second balance principle, the action time of the three voltage vectors in the sector is calculated;

[0052] For each switch switching point in each switching cycle, the actual three-phase current value corresponding to the end or start time of each voltage vector is predicted according to the load voltage equation, and the polarity of the actual three-phase current value is obtained; dead zone compensation based on the time pulse width is performed according to the polarity of the actual three-phase current value; thereby achieving the suppression of common-mode voltage in the vector modulation process.

[0053] Modulation strategy introduction:

[0054] There are eight switching states for the traditional two-level inverter modulation strategy. Each switching state corresponds to a common-mode voltage value, except for the two zero switching states where the output common-mode voltage reaches 0 and U dc Except for the zero switching vector, the common mode voltage of other non-zero switching vector outputs is U dc / 3 and 2U dc / 3, so when performing reference voltage vector synthesis, avoiding the use of zero vector can effectively reduce the amplitude of the inverter output common-mode voltage.

[0055] In this implementation, in different sectors of the voltage vector, three different non-zero vectors are used to synthesize a reference voltage vector to simultaneously suppress the amplitude and frequency of the common mode voltage. Figure 1 The circular area in is the linear modulation area of ​​the method of the present invention, and the voltage vector circle is divided into six sectors according to the angle.

[0056] Further, combined with Figure 2 As shown, in order to suppress the amplitude and switching frequency of the common mode voltage, this embodiment uses three vectors U1, U3 and U5 to synthesize the reference voltage vector in odd sectors I, III and V, and three vectors U2, U4 and U6 to synthesize the reference voltage vector in even sectors II, IV and VI. The specific corresponding relationship between the sector, the reference voltage vector angle, the voltage vector selection and the corresponding common mode voltage is as follows:

[0057] Sector Number Angle (rad) Voltage Vector Common mode voltage Ⅰ [-π / 6,π / 6] <![CDATA[U1 U3 U5]]> <![CDATA[U dc / 3]]> Ⅱ [π / 6,π / 2] <![CDATA[U2 U4 U6]]> <![CDATA[2U dc / 3]]> Ⅲ [π / 2,5π / 6] <![CDATA[U1 U3 U5]]> <![CDATA[U dc / 3]]> Ⅳ [5π / 6,7π / 6] <![CDATA[U2 U4 U6]]> <![CDATA[2U dc / 3]]> Ⅴ [7π / 6,3π / 2] <![CDATA[U1 U3 U5]]> <![CDATA[U dc / 3]]> Ⅵ [3π / 2,11π / 6] <![CDATA[U2 U4 U6]]> <![CDATA[2U dc / 3]]> .

[0058] Because only three voltage vectors are used in each voltage sector, if the principle of phase voltage center symmetry is followed, there should be five switching states in one switching cycle. Take the first sector as an example. Figure 3 Figure 1 is the switching state and common mode voltage diagram of the first sector under the phase voltage center symmetry criterion. It can be seen that the B-phase bridge arm switch performs four switching operations in one carrier cycle, which increases the switching loss and reduces the system efficiency.

[0059] Therefore, considering the switching loss problem of the inverter power device, this implementation method modifies the action and transmission order of the voltage vector within a carrier cycle, thereby achieving the reduction of the switching times and the switching loss while suppressing the common mode voltage. Figure 4 The diagram is the switch state and common mode voltage diagram of the first sector after the modification by the method of the present invention. At this time, the three-phase bridge arm switch tubes perform two switching operations in one carrier cycle, thereby improving the system efficiency.

[0060] Going a step further, combined Figure 3 and Figure 4 As shown in the figure, the corresponding relationship between the order of action and transmission of the three voltage vectors in one carrier cycle and the sectors is as follows:

[0061] Sector Number Voltage vector action sequence Ⅰ 51335 Ⅱ 62446 Ⅲ 13551 Ⅳ 24662 Ⅴ 35113 Ⅵ 46224

[0062] The numbers 1 to 6 in the table correspond to the voltage vectors U1 to U6. By following the order of action of the voltage vectors in the table above, the switching loss can be reduced.

[0063] Going a step further, combined Figure 5 As shown, the vector action time is calculated:

[0064] Determine the reference voltage vector U ref The purpose of the sector is to determine the basic voltage space vector used in this switching cycle.

[0065] The method for determining the sector where the current reference voltage vector is located is:

[0066] Denote the reference voltage vector as U ref , use U α and U β Represents the reference voltage vector U ref The components on the α-axis and β-axis define three intermediate variables X, Y, and Z, and let:

[0067]

[0068] Then define three intermediate variables A, B, and C, and set them as follows through analysis:

[0069] If X>0, then A=1, otherwise A=0;

[0070] If Y>0, then B=1, otherwise B=0;

[0071] If Z>0, then C=1, otherwise C=0;

[0072] Set the intermediate variable N = 4A + 2B + C, and get the corresponding relationship between N and sector:

[0073] N 4 6 2 3 1 5 Sector Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Ⅵ

[0074] The sector determined in the table is the sector where the current reference voltage vector is located.

[0075] In actual use, the reference voltage vector U can be obtained by looking up the table. ref The sector where the device is located.

[0076] The method for calculating the action time of the three voltage vectors in the sector is:

[0077] Assume that the current reference voltage vector U ref Located in sectors I, III, and V, the reference voltage vector synthesis at this time is as follows Figure 5 As shown. According to the volt-second balance principle, we can get:

[0078]

[0079] Where T s represents the carrier cycle, T1 represents the action time of the voltage vector U1, T3 represents the action time of the voltage vector U3, and T5 represents the action time of the voltage vector U5;

[0080] Calculate formula (2) to get the action time of each vector:

[0081]

[0082] Where U dc Indicates the bus voltage.

[0083] Similarly, the calculation formula for the action time of the method of the present invention in other sectors can be obtained.

[0084] Going further, the suppression of the dead zone effect:

[0085] Combination Figure 6 As shown, the method for predicting the actual value of the three-phase current corresponding to the end or start time of each voltage vector is:

[0086] According to the load voltage equation, the increment of the three-phase inductor current during each voltage vector action time is obtained, thereby predicting the actual value of the three-phase current.

[0087] In the vector modulation method described in this embodiment, there are three switching points in one switching cycle, because at each switching point, two PWM drive signals are simultaneously actuated to ensure that the common-mode voltage remains unchanged. Therefore, when the dead time causes the two PWM drive signals that should be actuated simultaneously to actuate at different times, an undesirable common-mode voltage spike will be generated, and the waveform of the phase voltage will also be shifted and changed to a certain extent. Figure 6 It is a common mode voltage diagram in the first sector of the method of the present invention assuming that the three-phase current is positive, positive and negative.

[0088] Therefore, considering the dead zone effect is of great significance to vector modulation. This embodiment accurately predicts the three-phase current in a switching cycle at each switch switching moment to obtain the polarity of the three-phase current, thereby compensating the drive pulse width. The focus of current prediction is to derive the increment of the inductor current in each voltage vector action time according to the load voltage equation, so as to predict the actual phase current value at the end or start of each voltage vector. Because the end or start time of each voltage vector is exactly the dead zone effect action time, the predicted current values ​​at these moments can be used to perform a dead zone compensation strategy based on the time pulse width. The traditional dead zone compensation strategy is to use the three-phase current detected at the software program interrupt entry to approximate the phase current value at each dead zone moment, which has errors. The current prediction method of this embodiment can realize the real-time prediction of the phase current in a switching cycle, thereby improving the accuracy of the current polarity judgment and avoiding the wrong compensation at the current zero crossing to a certain extent.

[0089] Experimental verification: Figure 7 The common-mode voltage on the AC side of the inverter under the traditional SVPWM modulation strategy and its enlarged waveform are shown. It can be seen that the common-mode voltage jumps at a high frequency from 0 to 90V. Figure 8 and Fig. 9 The common mode voltage and its amplified waveform diagram are shown in the vector modulation technology without dead zone effect compensation and with dead zone effect compensation according to the method of the present invention. It can be seen that the improved vector modulation technology according to the method of the present invention combined with the dead zone effect compensation of current prediction can reduce the common mode voltage amplitude to jump from 30V to 60V, and the jumping frequency is greatly reduced to 6 times the fundamental frequency. This shows that the present invention greatly improves the system efficiency.

[0090] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that the features described in conjunction with a single embodiment may be used in other described embodiments.

Claims

1. A three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side, characterized in that include, The voltage vector circle of the inverter is divided into six sectors. In sectors I, III and V, three voltage vectors U1, U3 and U5 are used to synthesize the reference voltage vector. In sectors II, IV and VI, three voltage vectors U2, U4 and U6 are used to synthesize the reference voltage vector. Adjust the action and transmission order of the three voltage vectors in one carrier cycle so that the three-phase bridge arm switches of the inverter perform two switching actions in one carrier cycle to reduce switching losses; According to the components of the reference voltage vector on the α and β axes, the sector where the current reference voltage vector is located is determined, and then combined with the volt-second balance principle, the action time of the three voltage vectors in the sector is calculated; For each switch switching point in each switching cycle, the actual three-phase current value corresponding to the end or start time of each voltage vector is predicted according to the load voltage equation, and the polarity of the actual three-phase current value is obtained; dead zone compensation based on the time pulse width is performed according to the polarity of the actual three-phase current value; thereby achieving the suppression of common-mode voltage in the vector modulation process.

2. The three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side according to claim 1, characterized in that: The corresponding relationship between the sector, reference voltage vector angle and voltage vector selection is as follows: 。 3. The three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side according to claim 2, characterized in that: The corresponding relationship between the order of sending the three voltage vectors in one carrier cycle and the sectors is as follows: The numbers 1 to 6 in the table correspond to voltage vectors U1 to U6.

4. The three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side according to claim 3, characterized in that: The method for determining the sector where the current reference voltage vector is located is: Denote the reference voltage vector as U ref , use U α and U β Represents the reference voltage vector U ref The components on the α-axis and β-axis define three intermediate variables X, Y, and Z, and let: Define three intermediate variables A, B, and C, and set them as follows: If X>0, then A=1, otherwise A=0; If Y>0, then B=1, otherwise B=0; If Z>0, then C=1, otherwise C=0; Set the intermediate variable N = 4A + 2B + C, and get the corresponding relationship between N and sector: The sector determined in the table is the sector where the current reference voltage vector is located.

5. The three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side according to claim 4, characterized in that: The method for calculating the action time of the three voltage vectors in the sector is: Assume that the current reference voltage vector U ref Located in sectors I, III, and V, according to the volt-second balance principle: Where T s represents the carrier cycle, T1 represents the action time of the voltage vector U1, T3 represents the action time of the voltage vector U3, and T5 represents the action time of the voltage vector U5; Calculating formula (2) yields: Where U dc Indicates the bus voltage.

6. The three-phase two-level inverter vector modulation method for reducing the common mode voltage on the AC side according to claim 5, characterized in that: The method for predicting the actual value of the three-phase current corresponding to the end or start time of each voltage vector is: According to the load voltage equation, the increment of the three-phase inductor current during each voltage vector action time is obtained, thereby predicting the actual value of the three-phase current.

Images