A hybrid modulation method for current source rectifier

Abstract

The application discloses a current source rectifier hybrid modulation method, and belongs to the field of modulation control of power electronic converters. The method collects the line voltage of the AC side and reconstructs the three-phase phase voltage, determines the current sector according to the phase voltage relationship, and simultaneously identifies the sector switching area by judging whether the absolute value of the line voltage is less than a preset threshold. When the system is near the sector switching area, the superimposed time is introduced into the PWM comparison value, so that the to-be-conducted phase is conducted in advance before the original conduction-related break, thereby forming a short-time overlapping conduction area; when the system is far away from the sector boundary, the superimposed modulation is closed and the non-superimposed modulation is adopted. The method does not need to increase hardware, can inhibit the AC side current distortion caused by the sector switching of the non-superimposed modulation, and compared with the full-area superimposed modulation, can reduce the switching tube action times, reduce the switching loss and thereby improve the efficiency.

Description

Technical Field

[0001] This invention relates to the field of modulation control of power electronic converters, and particularly to a hybrid modulation method for a current source rectifier. Background Technology

[0002] Current-source PWM rectifiers have advantages such as continuous DC-side current, strong short-circuit withstand capability, and suitability for step-down rectification applications. They are valuable for applications in new energy power generation, electric vehicle charging, aviation power systems, energy storage systems, and high-voltage DC power supply systems.

[0003] In current-source PWM rectifiers, different switching sequences are typically selected based on the sector containing the AC side voltage vector to obtain a sinusoidal grid-side current. For a three-phase six-switch current-source PWM rectifier, the modulation process usually involves sector determination, duty cycle calculation, switching sequence allocation, and PWM comparison value generation.

[0004] However, in practical systems, an LC filter is typically installed on the AC input side. Due to the limited value of the filter capacitor, a switching frequency ripple exists in the input filter capacitor voltage. Near sector switching, the voltage relationship between adjacent phases changes, and the filter capacitor voltage ripple may cause the voltage relationship between the commutation phases to overlap or be misjudged in a short period of time, resulting in abnormal distribution of DC current between the commutation phases and thus causing AC current distortion.

[0005] While adding a large slack current time throughout the entire power grid cycle can improve converter reliability, it also introduces unnecessary overlapping conduction in non-sector switching intervals, increasing additional losses and reducing modulation performance. Therefore, a modulation method is needed that enables slack current time only near sector switching and maintains non-slack current modulation in other intervals. Summary of the Invention

[0006] The problem to be solved by the present invention is to provide a hybrid modulation method for a current source rectifier, which identifies the sector switching position by the line voltage threshold, adds superposition time near the sector switching, and uses non-superposition modulation in the remaining interval, thereby suppressing AC side current distortion during sector switching.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a hybrid modulation method for a current source rectifier, comprising the following steps:

[0008] Step 1: Acquire the AC line voltage e of the three-phase current source converter. ab e bc And calculate the third line voltage e based on the three-phase line voltage relationship. ca ;

[0009] Step 2, based on the line voltage e ab ebc e ca Calculate the three-phase phase voltage e a e b e c ;

[0010] Step 3: Based on the three-phase phase voltage e a e b e c The positive and negative signs and magnitude relationships are used to determine the sector where the current voltage vector is located;

[0011] Step 4: Determine the line voltage e ab e bc e ca Does the absolute value in the data exist that is less than a preset threshold V? set If the line voltage exists, it is determined that the current is near the sector switching and the superposition current modulation flag MdlHex is set; if it does not exist, it is determined that the current is far from the sector boundary and the superposition current modulation flag MdlHex is cleared.

[0012] Step 5: Calculate the duration of action of the three-phase bridge arm foundation based on the three-phase phase voltage amplitude and modulation ratio m;

[0013] Step 6: When the scrambling modulation flag MdlHex is set, the scrambling time t is introduced into the bridge arm comparison value corresponding to the current basic modulation sector. d Generate PWM comparison values ​​including the superimposed current time;

[0014] Step 7: When the slack current modulation flag MdlHex is cleared, no slack current time is introduced, and a non-slack current modulation PWM comparison value is generated.

[0015] Furthermore, the preset threshold V set The settings are based on the voltage change characteristics near the zero crossing of the input line voltage, sampling error, and the requirement to suppress input current distortion during sector switching, and are adjusted according to the input voltage amplitude and system control accuracy.

[0016] Furthermore, the superposition time t d The set superposition time d T and control period T s The result is obtained through conversion.

[0017] When the superposition modulation flag is At that time, the opening comparison value of at least one bridge arm is advanced by t relative to the corresponding foundation action time. d This allows the phase to be turned on before the original phase is turned off, thus creating a short-term overlapping conduction state.

[0018] When the superposition modulation flag is At that time, the comparison value used to generate early conduction is turned off, so that each phase bridge arm generates a PWM signal according to the basic action time corresponding to the non-overlapping current modulation.

[0019] Furthermore, the method of the present invention is applicable to three-phase six-switch current source PWM rectifiers, three-phase three-switch Buck PWM rectifiers, and other three-phase current source converters with AC-side filter capacitors and DC-side inductors.

[0020] Compared with the prior art, the present invention, employing the above technical solution, has the following technical effects:

[0021] 1. This invention identifies the area near sector switching by using line voltage threshold, and the judgment method is simple.

[0022] 2. Add cascading time t only near sector switching. d It can effectively improve the AC side current distortion caused by changes in the commutation phase voltage relationship and the voltage ripple of the input filter capacitor.

[0023] 3. When the sector boundary is far away, non-overlapping modulation is used, which does not introduce unnecessary overlapping time and helps to reduce additional losses.

[0024] 4. The method of the present invention can be directly implemented through PWM comparison value generation logic without the need for additional hardware circuitry. Attached Figure Description

[0025] Figure 1 This is the main circuit topology diagram of the current source type PWM rectifier of the present invention;

[0026] Figure 2 This is a sector 2 switching sequence diagram of the traditional modulation method;

[0027] Figure 3 This is a simulation waveform of the three-phase input current using a traditional modulation method.

[0028] Figure 4 This is a diagram of the sector 2 switching sequence of the modulation method proposed in this invention;

[0029] Figure 5 This is a simulation waveform of the three-phase input current of the modulation method proposed in this invention;

[0030] Figure 6 This is an experimental waveform diagram of the three-phase input current using a traditional modulation method;

[0031] Figure 7 This is an experimental waveform diagram of the three-phase input current of the modulation method proposed in this invention. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of the application will be further described in detail below with reference to the accompanying drawings. The described embodiments are only a part of the embodiments involved in this invention. All non-innovative embodiments based on these embodiments by other researchers in the art are within the protection scope of this invention. Furthermore, the step numbers in the embodiments of this invention are only set for ease of explanation and do not limit the order of the steps. The execution order of each step in the embodiments can be adaptively adjusted according to the understanding of those skilled in the art.

[0033] In one embodiment of the present invention, the local superposition modulation method is applied to a three-phase six-switch current source PWM rectifier, and the main circuit topology is as follows: Figure 1 As shown.

[0034] Specifically, the three-phase phase voltage e a e b e c Each through AC side filter inductor L F A filter capacitor C is connected in parallel on the AC side of the rectifier bridge arm input terminal. F It is used to filter out switching harmonics and improve current waveforms.

[0035] The rectifier bridge arm consists of upper bridge arm switches S1, S3, S5 and series diodes D1, D3, D5, and lower bridge arm switches S4, S6, S2 and series diodes D4, D6, D2, forming a three-phase six-switch structure to achieve controllable AC to DC conversion.

[0036] The output sides of the bridge arms are respectively connected to the DC side inductor L P L N The load R is connected, and a filter capacitor C is connected in parallel on the DC side. P C N and clamping diode D FP D FN It is used to maintain continuous DC current, suppress voltage ripple and realize device overvoltage protection.

[0037] Among them, i a i b i c For three-phase AC input current, i s,a i s,b i s,c For the phase current of the bridge arm, u P u N The positive and negative bus voltages on the DC side, u DC i is the DC-side bus voltage, and i is the DC-side inductor current. O For the load current, u O This is the output voltage of the rectifier.

[0038] Furthermore, by employing the hybrid modulation method of the present invention, the sector switching position is identified by the line voltage threshold, a superposition time td is added near the sector switching, and non-superposition modulation is used in the remaining interval, thereby suppressing AC side current distortion during sector switching.

[0039] The AC side-line voltage e is acquired through the controller. ab e bc The third line voltage is obtained from the three-phase line voltage relationship:

[0040] ;

[0041] Then the three-phase voltages are calculated:

[0042] ;

[0043] The controller is based on e a e b e c The polarity and magnitude of the phase voltage determine the current sector. Table 1 shows the relationship between the phase voltage magnitudes and the corresponding sector numbers in this embodiment.

[0044] Table 1. Relationship between phase voltage magnitudes and corresponding sector numbers

[0045]

[0046] Furthermore, to reduce the complexity of the modulation sequence, the twelve sectors are mapped to six basic modulation sectors. When the sector number Sector1 > 6, Sector2 = Sector1 − 6; otherwise, Sector2 = Sector1.

[0047] To determine whether the system is near a sector switch, a line voltage threshold V is set. set When the absolute value of the voltage on any line is less than this threshold, the following condition is met:

[0048] ;

[0049] The controller determines that it is currently near a sector switch and sets the superposition modulation flag MdlHex=1. When the absolute values ​​of the three line voltages are all greater than or equal to this threshold, the controller determines that it is currently far from the sector boundary and sets MdlHex=0.

[0050] Specifically, the preset threshold V set To prevent the algorithm from switching back and forth frequently, V is set. set The hysteresis method for value retrieval is as follows:

[0051] by For the condition to be established, The conditional return criterion; where, It is the peak line voltage, and it is required that... Otherwise, it is judged as undervoltage and the drive is blocked.

[0052] In this way, non-overlapping modulation can be achieved in 90% of the region, while overlapping modulation can be used in 10% of the region.

[0053] Then, the controller calculates the duration of the three-phase bridge arm foundation action. First, it calculates the voltage amplitude based on the three-phase phase voltages:

[0054] ;

[0055] Then, calculate the three-phase foundation action time based on the modulation ratio m and the PWM counting period PRD:

[0056] ;

[0057] Set the overlay time to d T And convert it into a PWM count value t d :

[0058] .

[0059] When currently near a sector switch, i.e. The controller generates a PWM comparison value including the superposition time based on the current basic modulation sector Sector2; when When the slack current modulation is disabled, the controller sets CMP1A, CMP2A, and CMP3A to zero, and the system adopts a non-slack current modulation mode. Table 2 below shows the PWM comparison value settings for different basic modulation sectors with slack current time.

[0060] Table 2: PWM Comparison Values ​​under Basic Modulation Sectors with Including Overlap Time

[0061]

[0062] Among the comparison values ​​above, -t is included. d The comparison value is used to enable the corresponding bridge arm to conduct in advance, thereby forming a short-time overlapping conduction interval between the commutation phases. This overlapping conduction interval enables the DC side current to transfer smoothly near the sector switching, avoiding abnormal distribution of current pulse area caused by input filter capacitor voltage ripple or sector switching.

[0063] When the current position is far from the sector switching position (MdlHex=0), the controller disables scrambling modulation, sets CMP1A, CMP2A, and CMP3A to zero, and retains only the comparison values ​​of CMP1B, CMP2B, and CMP3B corresponding to non-scrambling modulation. Finally, the controller rounds all PWM comparison values ​​to obtain comparison values ​​that can be directly fed into the PWM module of the DSP.

[0064] Through the above method, this invention achieves local slack current modulation based on sector boundary identification. This method effectively improves AC-side current distortion near sector switching while maintaining non-slack current modulation in other regions, thus balancing AC-side current quality, switching losses, and system operating efficiency.

[0065] To verify the superiority of the control method provided by this invention, simulation analysis was performed in MATLAB / Simulink, and experimental verification was also carried out on a 2.5kW experimental prototype. The system parameters are shown in Table 3.

[0066] Table 3 Key CSR Parameters

[0067]

[0068] Traditional modulation methods employ a sector-based arm clamping modulation approach. Within each sector, the arm with the smallest absolute value of the input phase voltage is selected for clamping conduction. The DC-side current is distributed among the AC-side phases through a combination of two active states and one freewheeling state. While this method reduces switching losses and DC-side current ripple, at sector switching boundaries, ripple in the input filter capacitor voltage can easily cause abnormal current distribution between commutation phases, resulting in distortion in the input AC current. The sector 2 switching sequence diagram is shown below. Figure 2 As shown, the simulation results are as follows: Figure 3 As shown.

[0069] The sector 2 switching sequence diagram of the modulation method proposed in this invention is shown below. Figure 4 As shown, the simulation results are as follows: Figure 5 As shown. The simulation settings are 0.1s half-load access and 0.2s full-load access. The experimental results of the traditional modulation method are as follows. Figure 6 As shown, the experimental results of the modulation method proposed in this invention are as follows: Figure 7 As shown in the figure, the comparison between simulation and experimental results clearly demonstrates that the modulation method proposed in this invention can effectively suppress the distortion generated in the input AC current during sector switching.

[0070] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A hybrid modulation method for a current source rectifier, characterized in that, Includes the following steps: Step 1, collect three-phase current source converter AC side line voltage e ab , e bc , and calculate the third line voltage e ca according to the relationship of three-phase line voltage; Step 2, the line voltage e ab , e bc , e ca is calculated a , e b , e c ; Step 3: Based on the three-phase phase voltage e a e b e c The positive and negative signs and magnitude relationships are used to determine the sector where the current voltage vector is located; Step 4: Determine the line voltage e ab e bc e ca Does the absolute value in the data exist that is less than a preset threshold V? set If the line voltage exists, it is determined that the current is near the sector switching and the superposition current modulation flag MdlHex is set; if it does not exist, it is determined that the current is far from the sector boundary and the superposition current modulation flag MdlHex is cleared. Step 5: Calculate the duration of action of the three-phase bridge arm foundation based on the three-phase phase voltage amplitude and modulation ratio m; Step 6: When the scrambling modulation flag MdlHex is set, the scrambling time t is introduced into the bridge arm comparison value corresponding to the current basic modulation sector. d Generate PWM comparison values ​​including the superimposed current time; Step 7: When the slack current modulation flag MdlHex is cleared, no slack current time is introduced, and a non-slack current modulation PWM comparison value is generated.

2. The hybrid modulation method for a current source rectifier according to claim 1, characterized in that, The third line voltage e ca The formula for calculation is: ； The formula for calculating the three-phase phase voltage is: ； ； 。 3. The hybrid modulation method for a current source rectifier according to claim 1, characterized in that, The judgment condition for the vicinity of the sector switching is: ； When the judgment condition is met, the superposition modulation flag MdlHex = 1; when none of the judgment conditions are met, the superposition modulation flag MdlHex = 0.

4. The hybrid modulation method for a current source rectifier according to claim 3, characterized in that, The preset threshold V set The hysteresis method is used to prevent the algorithm from switching back and forth frequently. by For the condition to be established, The conditional return criterion; where, It is the peak line voltage, and it is required that... Otherwise, it is judged as undervoltage and the drive is blocked.

5. The hybrid modulation method for a current source rectifier according to claim 1, characterized in that, The duration of the three-phase bridge arm foundation operation is determined by d. a d b d c The formula for its calculation is: ； in, The peak phase voltage is calculated as follows: 。 6. The hybrid modulation method for a current source rectifier according to claim 1, characterized in that, The superposition time t d The set superposition time d T and control period T s The conversion is as follows: 。 7. The hybrid modulation method for a current source rectifier according to claim 1, characterized in that, When the superposition modulation flag is At that time, the opening comparison value of at least one bridge arm is advanced by t relative to the corresponding foundation action time. d This allows the phase to be turned on before the original phase is turned off, thus creating a short-term overlapping conduction state.

8. The hybrid modulation method for a current source rectifier according to claim 1, characterized in that, When the superposition modulation flag is At that time, the comparison value used to generate early conduction is turned off, so that each phase bridge arm generates a PWM signal according to the basic action time corresponding to the non-overlapping current modulation.

9. The hybrid modulation method for a current source rectifier according to claim 1, characterized in that, The three-phase current source converter is a three-phase current source converter with AC-side filter capacitors and DC-side inductors, including but not limited to a three-phase six-switch current source PWM rectifier and a three-phase three-switch Buck PWM rectifier.

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