A method for suppressing ripple voltage on the DC side of a unidirectional energy flow three-phase Y-connected rectifier

Abstract

The present invention discloses a method for suppressing ripple voltage on the DC side of a three-phase Y-connected rectifier with unidirectional energy flow. The method includes five parts: a method for obtaining a modulation wave voltage signal of a three-phase rectifier, a method for constructing a zero-crossing distortion compensation signal of a modulation wave voltage signal of a three-phase rectifier in an input current zero-crossing distortion interval, a method for constructing a harmonic compensation signal of a three-phase rectifier modulation wave voltage signal composed of a triple harmonic combination, a method for determining the amplitude of each harmonic in the harmonic compensation signal of the modulation wave voltage signal of the three-phase rectifier, and a method for processing the harmonic compensation signal of the modulation wave voltage signal of the three-phase rectifier in an input current zero-crossing distortion interval. Compared with the existing control method, the significant advantage of the present invention is that it does not need to increase the DC side filter capacitor, and only through optimized harmonic injection, the DC side voltage secondary ripple fluctuation amplitude can be effectively reduced without affecting the input current waveform quality and the system stability. The proposed method has a simple algorithm and is easy to implement.

Description

Technical Field

[0001] The invention belongs to the technical field of unidirectional energy flow multi-level rectifiers, and particularly relates to a method for realizing DC output voltage ripple suppression of a unidirectional energy flow three-phase Y-connected multi-level rectifier. Background Art

[0002] In recent years, multilevel power converters have been successfully applied more and more in the fields of high voltage and high power variable frequency speed regulation, active power filtering, high voltage direct current (HVDC) transmission and reactive power compensation of power systems. The basic circuit topology of multilevel converters can be roughly divided into two categories: clamped type and unit cascade type. The unidirectional energy flow diode clamped three-level medium and high voltage inverters that are currently most widely used in industry, and the unidirectional energy flow cascade H-bridge medium and high voltage inverters are typical representatives of these two types of products. In both types of medium and high voltage inverters, in order to use low withstand voltage power electronic devices to complete high voltage power conversion and suppress the harmonics of input current, a bulky, complex wiring and expensive power frequency phase shifting transformer is required on the input side of the rectifier. This limits its application in many industrial occasions. In order to eliminate the power frequency phase shifting transformer, academia and industry have tried to use high-frequency isolation DC-DC converters to connect the cascade rectifier stage and the cascade inverter stage to form a new type of medium and high voltage inverter. Compared with traditional medium and high voltage inverters, the rectifier stage implementation scheme of this new type of medium and high voltage inverter eliminates the bulky, complex wiring, and expensive power frequency phase-shifting transformer, thereby effectively reducing the system volume, weight, and manufacturing cost. However, this type of new type of medium and high voltage inverter also has obvious disadvantages, mainly manifested in: high power density, high efficiency, high power high frequency isolation DC / DC converter is very difficult to design and implement, which has become the main bottleneck for this type of new type of medium and high voltage inverter to truly become practical. For this reason, in recent years, a new generation of medium and high voltage inverters that use cascade rectifier stages and cascade inverter stages to connect directly have received more and more attention in the field of power electronics technology. Among them, a new generation of direct-connected medium and high voltage inverters composed of a Y-connected three-single-phase diode cascade Boost rectifier circuit + a three-single-phase multi-level inverter circuit and an open winding connection with the motor is one of the typical solutions. This new type of topology does not require power frequency transformer isolation, nor does it require the use of a high-frequency isolation DC conversion unit, thereby greatly reducing its volume, structure, cost, and control complexity, showing great practical advantages.

[0003] Although the new generation of direct-connected medium and high voltage inverters have shown obvious advantages, there are still some problems that must be solved in actual engineering applications. For example, the input power of the three single-phase rectifiers essentially has secondary power pulsation, which makes the output DC voltage of each single-phase rectifier superimposed with a large secondary ripple voltage. The large secondary ripple voltage on the DC side will not only increase the voltage stress of the power switch tube, but also affect the output voltage and output current of the subsequent inverter, and further affect the torque, speed and working efficiency of the motor and the entire transmission system. Although increasing the filter capacitor on the DC side can suppress this secondary ripple voltage to an acceptable level, the larger filter capacitor will increase the volume, weight and cost of the inverter and reduce the reliability of the work. Summary of the invention

[0004] In view of the problem that the secondary ripple voltage on the DC side of the above-mentioned direct-connected new generation medium and high voltage inverter is large, the purpose of the present invention is to provide a method for effectively suppressing the secondary ripple voltage on the DC side of the above-mentioned direct-connected new generation medium and high voltage inverter without adding a filter capacitor on the DC side. The purpose of the present invention is achieved through the following technical solution, which is characterized by comprising the following steps:

[0005] Step 1: Calculate the three-phase grid voltage e a 、e b 、e c , three-phase grid current i a 、i b 、i c , three-phase rectifier DC side voltage u dca 、u dcb 、u dcc Sampling is performed, and the modulation wave voltage signal u of the three single-phase rectifier is obtained through the traditional voltage and current double closed-loop control strategy based on Clark and Park coordinate transformation. * a 、u * b 、u * c ;

[0006] Step 2: According to the three single-phase rectifier input current sinusoidal reference signal i * a 、i * b 、i * c With the three-phase rectifier modulation wave voltage signal u * a 、u * b 、u * cThe polarity of u is used to determine whether a phase input current is in the zero-crossing distortion interval. The judgment basis is: if u * a with i * a The product of is less than zero, indicating that the current of phase A is in the zero-crossing distortion interval. Let the logic function Flag a =1, otherwise Flag a =0; if u * b with i * b The product of is less than zero, indicating that the current of phase B is in the zero-crossing distortion interval. Let the logic function Flag b =1, otherwise Flag b =0; if u * c with i * c The product of is less than zero, indicating that the C phase current is in the zero-crossing distortion range. c =1, otherwise Flag c =0;

[0007] Step 3: Construct the zero-crossing distortion compensation signal u of the modulation wave voltage signal of the three-phase rectifier according to formula (1): * comp_0 ;

[0008]

[0009] Step 4: Construct the harmonic compensation signal of the three-phase rectifier modulation wave voltage signal composed of three times harmonic combination according to formula (2) Among them, M ia 、M ib 、M ic are the amplitudes of the triple harmonics in the harmonic compensation signals of phase A, phase B, and phase C respectively;

[0010]

[0011] Harmonic compensation signal It can also be composed of a cubic square wave, because the cubic square wave signal is also a signal composed of multiple triple harmonics. Its characteristic is that the amplitude adjustment is simple, thereby reducing the calculation burden of the processor, and it can also effectively avoid the occurrence of over-modulation problems; the disadvantage is that the corresponding amplitude of each triple harmonic cannot be adjusted independently, and can only change in a fixed proportion with the change of the square wave amplitude; at this time, the harmonic compensation signal The amplitudes described by formula (3) are M ma 、M mb 、M mc The cubic wave composition is:

[0012]

[0013] Step 5: To keep the three-phase input current sinusoidal and operate at the specified power factor, the DC side voltage is stable and controllable, and the peak-to-peak value of the DC side voltage ripple is minimized, determine the amplitude M of each triple harmonic in the harmonic compensation signal in step 4 through an offline optimization method. ia 、M ib 、M ic When the three-phase harmonic compensation signal is composed of the cubic square waves described by equation (3), the amplitude of each cubic square wave is M ma 、M mb 、M mc It can be calculated by formula (4):

[0014]

[0015] in are the DC components of the three-phase DC side voltages A, B, and C respectively; V ma 、V mb 、V mc are the amplitudes of the three-phase grid voltages respectively;

[0016] Step 6: In the zero-crossing distortion region, if the triple harmonics are still injected, the zero-sequence components injected into the three phases will be unbalanced. Therefore, it is necessary to stop injecting the triple harmonics in the zero-crossing distortion region. For this purpose, the unified logic function Flag that represents the zero-crossing distortion interval can be constructed according to formula (5). zero , the construction method is to convert the logic signal Flag a , Flag b , Flag c After performing the OR operation, take the negation and convert the obtained Flag zero Multiply it with the harmonic compensation signal in step 4 to form a harmonic compensation signal that takes into account the zero distortion interval Unified logic function Flag zero When any phase current is in the zero-crossing distortion interval, its value is 0, otherwise it is 1. Therefore, the harmonic compensation signal in the zero-crossing distortion interval is considered so that the compensation signal only works in the normal working area and is zero in the zero-crossing distortion interval.

[0017]

[0018] Step 7: According to formula (6), the final modulation wave voltage signal u* of the three-phase single-phase rectifier is obtained. fa 、u* fb 、u* fc , change u* fa 、u* fb 、u* fc, after taking the absolute value, the amplitude is 1 and the frequency is f s The triangular carrier is intercepted to obtain the PWM wave driving signal required by the switching tubes of each phase of the three single-phase rectifier circuits;

[0019]

[0020] The above technical solution conceived by the present invention has the following significant advantages compared with the prior art: no need to increase the DC side filter capacitor, only through optimized harmonic injection, the DC side voltage secondary ripple fluctuation amplitude can be effectively reduced without affecting the input current waveform quality and system stability. The proposed method has a simple algorithm and is easy to implement.

[0021] The following is a detailed description with reference to the accompanying drawings in conjunction with examples. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a control flow chart of the present invention;

[0023] Figure 2 This is a circuit schematic diagram of a unidirectional energy flow three-phase Y-connected two-level rectifier according to an embodiment of the present invention;

[0024] Figure 3 The three-phase DC voltage waveform of the converter according to the embodiment of the present invention before the proposed ripple voltage suppression method is adopted;

[0025] Figure 4 : is a three-phase DC voltage waveform of the converter according to an embodiment of the present invention after adopting the proposed ripple voltage suppression method;

[0026] Figure 5 The A-phase grid current waveform and THD analysis of the converter according to the embodiment of the present invention before the proposed ripple voltage suppression method is adopted;

[0027] Figure 6 The A-phase grid current waveform and THD analysis of the converter according to the embodiment of the present invention after the proposed cubic square wave compensation signal is injected;

[0028] Figure 7 The A-phase grid current waveform and THD analysis of the converter according to the embodiment of the present invention after injecting the proposed cubic square wave compensation signal and adding the zero-crossing distortion compensation algorithm; DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not limited to the present invention.

[0030] Figure 1The present invention provides a control flow chart of a method for suppressing the ripple voltage on the DC side of a unidirectional energy flow three-phase Y-connected rectifier. Figure 2 A unidirectional energy flow three-phase Y-connected two-level Boost rectifier is shown as an example for description, and includes the following steps:

[0031] Step 1: Calculate the three-phase grid voltage e a 、e b 、e c Sampling is performed, and a phase-locked loop (PLL) is used to phase-lock the grid, obtain the grid voltage angle, and extract the amplitude V of the three-phase grid voltage. ma , V mb and V mc ; Perform Clark and Park coordinate transformation on the three-phase grid voltage to obtain the voltage sampling value v in the dq two-phase rotating coordinate system sd 、v sq ; For the three-phase grid current i a 、i b 、i c Sampling and extracting the amplitude I of the three-phase grid current ma , I mb and I mc ; Carry out Clark and Park coordinate transformation to obtain the sampling value i of the current in the dq two-phase rotating coordinate system d 、i q ; Sample the three-phase DC side voltage to obtain u dca 、u dcb 、u dcc , the DC side voltage command value v * dc The average value of the three-phase total DC link capacitor voltage u dc_ave The difference is passed through the PI controller to obtain the command current i in the d-axis coordinate system. * d At the same time, let the command current i in the q-axis coordinate system * q =0; the actual current sampling value i d 、i q With a given value i * d 、i * q The feedforward control method is used to control i d and i q Perform decoupling control to obtain the components u of the three-phase modulated wave voltage on the d-axis and q-axis * d 、u * q, and after iPark and iClark coordinate transformation, the three-phase rectifier modulation wave voltage signal u is obtained * a 、u * b 、u * c ;

[0032] Step 2: According to the three single-phase rectifier input current sinusoidal reference signal i * a 、i * b 、i * c With the three-phase rectifier modulation wave voltage signal u * a 、u * b 、u * c The polarity of u is used to determine whether a phase input current is in the zero-crossing distortion interval. The judgment basis is: if u * a with i * a The product of is less than zero, indicating that the current of phase A is in the zero-crossing distortion interval. Let the logic function Flag a =1, otherwise Flag a =0; if u * b with i * b The product of is less than zero, indicating that the current of phase B is in the zero-crossing distortion interval. Let the logic function Flag b =1, otherwise Flag b =0; if u * c with i * c The product of is less than zero, indicating that the C phase current is in the zero-crossing distortion range. c =1, otherwise Flag c =0;

[0033] Step 3: Construct the zero-crossing distortion compensation signal u of the modulation wave voltage signal of the three-phase rectifier according to formula (1): * comp_0 ;

[0034]

[0035] Step 4: Construct the harmonic compensation signal of the three-phase rectifier modulation wave voltage signal composed of three times harmonic combination according to formula (2) Among them, M ia , M ib , Mic They are the amplitudes of the triple harmonics in the harmonic compensation signals of phase A, phase B, and phase C respectively;

[0036]

[0037] Harmonic compensation signal It can also be composed of a cubic square wave, because the cubic square wave signal is also a signal composed of multiple triple harmonics. Its characteristic is that the amplitude adjustment is simple, thereby reducing the calculation burden of the processor, and it can also effectively avoid the occurrence of over-modulation problems; the disadvantage is that the corresponding amplitude of each triple harmonic cannot be adjusted independently, and can only change in a fixed proportion with the change of the square wave amplitude; at this time, the harmonic compensation signal The amplitudes described by formula (3) are M ma 、M mb 、M mc The cubic wave composition is:

[0038]

[0039] Step 5: To keep the three-phase input current sinusoidal and operate at the specified power factor, the DC side voltage is stable and controllable, and the peak-to-peak value of the DC side voltage ripple is minimized, determine the amplitude M of each triple harmonic in the harmonic compensation signal in step 4 through an offline optimization method. ia 、M ib 、M ic When the three-phase harmonic compensation signal is composed of the cubic square waves described by equation (3), the amplitude of each cubic square wave is M ma 、M mb 、M mc It can be calculated by formula (4):

[0040]

[0041] in They are the DC components of the three-phase DC side voltages A, B, and C respectively;

[0042] Step 6: In the zero-crossing distortion region, if the triple harmonics are still injected, the zero-sequence components injected into the three phases will be unbalanced. Therefore, it is necessary to stop injecting the triple harmonics in the zero-crossing distortion region. To this end, a unified logic function Flag representing the zero-crossing distortion interval is constructed according to formula (5). zero , the construction method is to convert the logic signal Flag a , Flag b , Flag c After performing the OR operation, take the negation and convert the obtained Flag zero Multiply it with the harmonic compensation signal in step 4 to form a harmonic compensation signal that takes into account the zero distortion interval Unified logic function Flag zero When any phase current is in the zero-crossing distortion interval, its value is 0, otherwise it is 1. Therefore, the harmonic compensation signal in the zero-crossing distortion interval is considered so that the compensation signal only works in the normal working area and is zero in the zero-crossing distortion interval.

[0043]

[0044] Step 7: According to formula (6), the final modulation wave voltage signal u* of the three-phase single-phase rectifier is obtained. fa 、u* fb 、u* fc , change u* fa 、u* fb 、u* fc , after taking the absolute value, the amplitude is 1 and the frequency is f s The triangular carrier is intercepted to obtain the PWM wave driving signal required by the switching tubes of each phase of the three single-phase rectifier circuits;

[0045]

[0046] Figure 3 This is a three-phase DC voltage waveform of the converter according to an embodiment of the present invention before the proposed ripple voltage suppression method is adopted, wherein the maximum value is about 519V, the minimum value is about 480V, and the ripple peak-to-peak value is 39V.

[0047] Figure 4 The three-phase DC voltage waveform of the converter according to the embodiment of the present invention after adopting the proposed ripple voltage suppression method has a maximum value of about 511V, a minimum value of about 488V, and a ripple peak-to-peak value of 23V, which is reduced by about 41% compared with before adopting the ripple voltage suppression method.

[0048] Figure 5 : The A-phase grid current waveform and THD of the converter according to the embodiment of the present invention before the proposed ripple voltage suppression method is adopted. Figure 6 The A-phase grid current waveform and THD of the converter of the embodiment of the present invention after the cubic square wave compensation signal is injected. It can be seen that after the cubic square wave signal mentioned in the patent of the present invention is injected into the modulation wave voltage signal of the three single-phase rectifier, the grid current can still maintain good waveform quality; Figure 7 The current waveform and THD of phase A grid after injecting cubic square wave and combining the proposed zero-crossing distortion suppression method are shown. It can be seen that the zero-crossing distortion of current is suppressed, and its THD is reduced from 2.02% to 1.79%, indicating that the proposed method helps to further improve the power quality on the grid side.

[0049] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. The present invention provides a method for suppressing the ripple voltage on the DC side of a unidirectional energy flow three-phase Y-connected rectifier, characterized in that: The following steps are involved: Step 1: Calculate the three-phase grid voltage e a 、e b 、e c Sampling is performed, and the three-phase grid voltage signal is converted to the dq coordinate system through abc-dq transformation to obtain the d-axis and q-axis grid voltage v sd and v sq ; Set the voltage e a 、e b 、e c The input phase-locked loop obtains the current grid voltage angular frequency ω and phase θ; the three-phase grid current i a 、i b 、i c Sampling is performed, and the three-phase signal is converted to the dq coordinate system through abc-dq transformation to obtain the d-axis and q-axis grid current i d and i q ; For the DC side voltage u of the three single-phase rectifier dca 、u dcb 、u dcc Sampling and calculating the average DC voltage u dc ; will u dc and DC voltage reference value v * dc The difference is compared through the voltage loop PI controller to obtain the d-axis current reference value i * d ; At the same time, set i * q =0; change i d and i * d The difference between the two values ​​is compared and then the active current PI controller is used to obtain I dout ;change i q and i * q The difference is compared and passed through the reactive current PI controller to obtain I qout ; v sd Add ω*L*i q Subtract I dout , get the modulation voltage signal u of the d-axis * d ; v sq Subtract ω*L*i d Subtract I qout , get the modulation voltage signal u of the q axis * q ; Where L is the inductance of the three-phase rectifier; for u * d and u * q The modulation wave voltage signal u of the three-phase rectifier is obtained by dq-abc transformation * a ,in * b ,in * c ; Step 2: According to the three single-phase rectifier bridge input current sinusoidal reference signal i * a 、i * b 、i * c With the three-phase rectifier modulation wave voltage signal u * a 、u * b 、u * c The polarity of u is used to determine whether a phase input current is in the zero-crossing distortion interval. The judgment basis is: if u * a with i * a The product of is less than zero, indicating that the current of phase A is in the zero-crossing distortion interval. Let the logic function Flag a =1, otherwise Flag a =0; if u * b with i * b The product of is less than zero, indicating that the current of phase B is in the zero-crossing distortion interval. Let the logic function Flag b =1, otherwise Flag b =0; if u * c with i * c The product of is less than zero, indicating that the C phase current is in the zero-crossing distortion range. c =1, otherwise Flag c =0; Step 3: Construct the zero-crossing distortion compensation signal u of the modulation wave voltage signal of the three-phase rectifier according to formula (1): * comp_0 ; Step 4: Construct the harmonic compensation signal of the three-phase rectifier modulation wave voltage signal composed of three times harmonic combination according to formula (2) Among them, M ia is the amplitude of the triple harmonics of phase A, M ib is the amplitude of the triple harmonics of phase B, M ic is the amplitude of each triple harmonic of phase C; Step 5: To keep the three-phase input current sinusoidal and operate at the specified power factor, the DC side voltage is stable and controllable, and the peak-to-peak value of the DC side voltage ripple is minimized, determine the amplitude M of each triple harmonic in the harmonic compensation signal in step 4 through an offline optimization method. ia , M ib and M ic ; Step 6: Construct a unified logic function Flag that represents the zero-crossing distortion interval according to formula (3) zero , the construction method is to convert the logic signal Flag a , Flag b , Flag c After performing the OR operation, take the negation and convert the obtained Flag zero Multiply it with the harmonic compensation signal in step 4 to form a harmonic compensation signal that takes into account the zero distortion interval Unified logic function Flag zero When any phase current is in the zero-crossing distortion interval, its value is 0, otherwise it is 1. Therefore, the harmonic compensation signal in the zero-crossing distortion interval is considered so that the compensation signal only works in the normal working area and is zero in the zero-crossing distortion interval. Step 7: According to formula (4), the final three-phase single-phase rectifier modulation wave voltage signal u* is obtained. fa 、u* fb 、u* fc , change u* fa 、u* fb 、u* fc , the absolute value is taken and the amplitude is 1 and the frequency is f s The triangular carrier is intercepted to obtain the PWM wave driving signal required by the switching tubes of each phase of the three single-phase rectifier circuits.

2. The method for suppressing the ripple voltage on the DC side of a unidirectional energy flow three-phase Y-connected rectifier according to claim 1, characterized in that: In step 4, the harmonic compensation signal It can also be composed of a cubic square wave, because the cubic square wave signal is also a signal composed of multiple triple harmonics. Its characteristics are simple amplitude adjustment, which reduces the calculation burden of the processor and effectively avoids the occurrence of overmodulation problems. The disadvantage is that the amplitude corresponding to each triple harmonic cannot be adjusted independently, but can only change in a fixed proportion with the change of the square wave amplitude. At this time, the harmonic compensation signal The amplitudes described by formula (5) are M ma 、M mb 、M mc The cubic wave composition.

3. The method for suppressing the ripple voltage on the DC side of a unidirectional energy flow three-phase Y-connected rectifier according to claim 1, characterized in that: In step 5, the three-phase input current is kept sinusoidal and operates at a specified power factor, the DC side voltage is stable and controllable, and the peak-to-peak value of the DC side voltage ripple is minimized. The amplitude M of each triple harmonic in the harmonic compensation signal in step 4 is determined by an offline optimization method. ia , M ib and M ic ; When the three-phase harmonic compensation signal is composed of the cubic square waves described by equation (5), the amplitude of each cubic square wave is M ma 、M mb 、M mc It can be calculated by formula (6): in are the DC components of the three-phase DC side voltages A, B, and C respectively; V ma 、V mb 、V mc are the amplitudes of the three-phase grid voltages respectively; Under the premise of maintaining high-quality input current waveform and stable and controllable DC side voltage, the fluctuation amplitude of DC side voltage ripple is effectively reduced through optimized harmonic injection.

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