A three-level inverter and modulation method and device thereof

Abstract

The application relates to a three-level converter and a modulation method and device thereof, and belongs to the technical field of power electronics. The modulation method can realize mid-point potential control and zero-sequence circulating current suppression of the three-level converter under low common-mode voltage, the method is characterized in that a mid-point control zero-sequence component and a zero-sequence circulating current suppression component are superposed on a 13-vector modulation wave to generate a corresponding modulation wave, and the modulation wave is used for modulation, so that the mid-point potential control and the zero-sequence circulating current suppression of the three-level converter are realized while the common-mode voltage is suppressed, and the reliable operation of the converter is ensured.

Description

Technical Field

[0001] This invention relates to a three-level converter and its control method and device, belonging to the field of power electronics technology. Background Technology

[0002] With the widespread application of grid-connected power generation technology for new energy sources, the negative impacts of converters have received increasing attention, the most common of which is common-mode voltage. Due to parasitic parameters relative to ground in the DC-side equipment (batteries, photovoltaic arrays, etc.), AC-side equipment (transformers, loads, etc.), the converter itself, and primary cables, common-mode currents are generated on both the DC and AC sides during operation. This interferes with BMS and PCS controllers. Furthermore, voltage spikes from common-mode interference can cause aging of the insulation layers of batteries or photovoltaic arrays, and even dielectric breakdown, leading to safety accidents. Currently, a 13-vector modulation strategy can be used to suppress common-mode voltage. The principle is to select a switching vector with a low common-mode voltage, resulting in a smaller common-mode voltage amplitude and frequency of change, thereby suppressing common-mode current. However, the 13-vector modulation strategy lacks active adjustment capability when a three-level converter experiences a midpoint potential shift or generates zero-sequence circulating current. Summary of the Invention

[0003] The purpose of this invention is to provide a three-level converter and its modulation method and apparatus to solve the problem that the midpoint potential balance control cannot be actively achieved when using a 13-vector modulation strategy.

[0004] To achieve the above objectives, the beneficial effects of the technical solutions and corresponding solutions provided by this invention include:

[0005] The present invention provides a three-level converter modulation method. In scenarios where the common-mode current requirement is less than the common-mode current threshold, if the midpoint potential shift exceeds the midpoint potential threshold, then in a 13-vector modulation wave V′... x The final modulated wave V'' is obtained by controlling the zero-sequence component ΔV at the midpoint of the superposition. x , using V'' x Modulation is performed to achieve midpoint potential balance in the three-level converter; the value of ΔV is obtained based on the following principle: the change in midpoint charge ΔQ per unit time after injection ΔV. av It can balance the potential at the midpoint.

[0006] The beneficial effects of the above technical solution are as follows: It adds midpoint potential offset control to the three-level converter, calculates the zero-sequence component of the midpoint control to suppress the midpoint potential offset based on the offset, superimposes the zero-sequence component of the midpoint control onto the 13-vector modulation wave to obtain the midpoint control modulation wave, modulates the midpoint control modulation wave, and realizes the midpoint potential balance of the three-level converter, thereby solving the problems of increased low-order harmonic content and converter output voltage distortion caused by midpoint potential offset.

[0007] As a further improvement to the method, the calculation formula corresponding to the midpoint potential balance is:

[0008] C·V np +Q av +ΔQ av =0

[0009] V np =V dc2 -V dc1

[0010] Among them, V np V is the midpoint potential. dc1 V is the voltage across the upper bus. dc2 Where C is the lower bus voltage, C is the capacitance of the DC-side capacitor, and Q is the lower bus voltage. av When the modulated wave is V′ x The amount of charge flowing into the midpoint during a single switching cycle is calculated using the following formula:

[0011] Q av =i np0av T s

[0012] i np0av =(1-|V′) a |)i a +(1-|V′ b |)i b +(1-|V′ c |)i c

[0013] Among them, i np0av T is the average current over a unit switching cycle. s For the switching period, V′ a V′ b V′ c For a 13-vector modulated wave, i a i b i c This refers to the AC side current of the three-level converter.

[0014] The beneficial effects of the above technical solution are as follows: The main purpose of adding the zero-sequence component ΔV of the midpoint control is to balance the midpoint potential. Based on this principle, the DC upper bus voltage V in the three-level converter is used. dc1 DC bus voltage V dc2 The capacitance C of the DC-side capacitor and the amount of charge Q flowing into the midpoint during a single switching cycle. av The change in midpoint charge per unit time, ΔQ, can be calculated. av This provides a basis for calculating the zero-sequence component of midpoint control.

[0015] As a further improvement to the method, ΔQ av The calculation formula is:

[0016]

[0017] Where i a i b i c V′ is the AC side current of the three-level converter. a V′ b V′ c It is a 13-vector modulated wave.

[0018] The beneficial effects of the above technical solution are as follows: after adding the zero-sequence component ΔV of the midpoint control, the change in charge flowing into the midpoint per unit time ΔQ can be obtained by calculation. av Using ΔQ av The value of the zero-sequence component ΔV of the midpoint control is then obtained.

[0019] As a further improvement to the method, for a three-level converter system with AC and DC sides connected in parallel, at V′ x A zero-sequence circulating current suppression component ΔV′ is also superimposed on it to achieve midpoint potential control and zero-sequence circulating current suppression.

[0020] The beneficial effects of the above technical solution are as follows: a zero-sequence circulating current suppression component ΔV′ is superimposed on the 13-vector modulation wave, which suppresses the zero-sequence circulating current while realizing the midpoint potential control. This solves the problem of the converter's low harmonic content output rising due to the midpoint potential shift, resulting in converter output voltage distortion, and the problem of uneven current distribution in the converter caused by the zero-sequence circulating current in AC / DC parallel systems.

[0021] As a further improvement to the method, the value of ΔV′ is obtained by controlling the zero-sequence current using a proportional controller, and the calculation formula is: ΔV′=k·(i a +i b +i c )

[0022] Where k is the proportional coefficient of the proportional controller, i a i b i c This refers to the AC side current of the three-level converter.

[0023] The beneficial effects of the above technical solution are as follows: by using a proportional controller to control the AC side current of the three-level converter, the zero-sequence circulating current suppression component ΔV′ can be obtained. By superimposing the zero-sequence circulating current suppression component ΔV′ on the 13-vector modulation wave, the purpose of zero-sequence circulating current suppression can be achieved.

[0024] As a further improvement to the method, in scenarios where common-mode current is not required, the 13-vector modulation wave V′ can be directly applied. x The upper superposition midpoint controls the zero-sequence component ΔV.

[0025] The present invention provides a three-level converter modulation device, comprising a memory and a processor, wherein the processor is used to execute computer program instructions stored in the memory to implement the three-level converter modulation method described above.

[0026] The present invention provides a three-level converter, including the three-level converter modulation device described above.

[0027] The beneficial effects of the above technical solution are as follows: The modulation method adopted by the three-level converter is to generate a corresponding modulation wave by superimposing the midpoint control zero-sequence component and the zero-sequence circulating current suppression component on the basis of the 13-vector modulation wave. The three-level converter is modulated by the modulation wave to realize common-mode voltage suppression, midpoint potential control and zero-sequence circulating current suppression of the three-level converter. Attached Figure Description

[0028] Figure 1 This is a flowchart of the zero-sequence component selection process of the present invention;

[0029] Figure 2 This is the overall common-mode voltage waveform diagram of the n3V and 13 vector modulation strategies;

[0030] Figure 3 This is a partial waveform diagram of the common-mode voltage for the n3V and 13 vector modulation strategies;

[0031] Figure 4 This is the overall common-mode voltage waveform diagram after 13-vector modulation and midpoint control according to the present invention;

[0032] Figure 5 This is a partial waveform diagram of the common-mode voltage after 13-vector modulation and midpoint control according to the present invention;

[0033] Figure 6 This is a waveform diagram of the midpoint voltage after n3V, 13 vector modulation and the midpoint control of this invention are superimposed. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0035] The purpose of this invention is to provide a three-level converter, a three-level converter and its modulation method, and a three-level converter and its modulation device, so as to realize the midpoint potential control and zero-sequence circulating current suppression of the three-level converter under low common-mode voltage.

[0036] Example of a three-level converter:

[0037] like Figure 1 As shown, this embodiment uses the following modulation method to achieve modulation of the three-level converter:

[0038] 1) Obtain the 13-vector modulation wave V′ after superimposing the zero-sequence component on the original three-phase sinusoidal modulation wave. x (Abbreviated as 13-vector modulated wave).

[0039] 2) Based on the scene and the midpoint potential V np To determine whether the zero-sequence component ΔV of the midpoint control needs to be superimposed, the specific decision-making process is as follows:

[0040] In scenarios with strict common-mode voltage requirements (i.e., when the common-mode current requirement is less than 1A), the DC upper bus voltage is collected. dc1 With lower busbar V dc2 The voltage values ​​are calculated and the difference is used to obtain the midpoint potential V. np The midpoint potential is determined: if the midpoint potential exceeds the midpoint potential threshold, then at V′ x By superimposing the midpoint-controlled zero-sequence component ΔV, the midpoint-controlled modulation wave V'' is obtained. x , using V'' x Midpoint potential control is performed; if the midpoint potential does not exceed the midpoint potential threshold (i.e., the midpoint potential is less than or equal to the midpoint potential threshold), the zero-sequence component ΔV of the point control is not superimposed.

[0041] In scenarios where common-mode voltage requirements are not stringent (i.e., when common-mode current is not required), at V′ x By superimposing the midpoint-controlled zero-sequence component ΔV, the midpoint-controlled modulation wave V'' is obtained. x , using V'' x Perform midpoint potential control.

[0042] Specifically, the calculation process for the zero-sequence component ΔV of the midpoint control is as follows:

[0043] A. Utilizing the current i on the AC side a i b i c With 13 vector modulated wave V′ a V′ b V′ c The average current i within a unit switching cycle is calculated. np0av According to i np0av With switching period T s The charge force Q flowing into the midpoint during a single switching cycle is calculated. av The calculation formula is:

[0044] Q av =i np0av Ts

[0045] i np0av =(1-|V′) a |)i a +(1-|V′ b |)i b +(1-|V′ c |)i c ;

[0046] B. Without changing the 13-vector modulation wave V′ x Under the condition of the sign, the zero-sequence component ΔV of the injected midpoint control can be calculated to obtain the corresponding change in charge flowing into the midpoint per unit time ΔQ. av The calculation formula is:

[0047]

[0048] C. After injecting the zero-sequence component ΔV to control the midpoint, the midpoint potential is balanced, based on the upper bus voltage value V. dc1 Lower bus voltage value V dc2 Midpoint potential voltage value V np and Q av The following formula can be obtained:

[0049] V np =V dc2 -V dc1

[0050] C·V np +Q av +ΔQ av =0;

[0051] D. Substitute the results from processes A and B into process C to calculate the value of ΔV. The calculation formula is:

[0052]

[0053] E. Midpoint control modulation wave V x The formula for calculating '' is: V″ x =V′ x +ΔV.

[0054] 3) In a parallel system on the AC / DC side of the converter, at V′ x The zero-sequence circulating current suppression component ΔV′ is superimposed to suppress the zero-sequence circulating current (zero-sequence circulating current suppression is only applicable to AC-DC parallel systems where multiple converters are connected in parallel on both the DC and AC sides).

[0055] Specifically, the zero-sequence circulating current suppression component ΔV′ is obtained by controlling the zero-sequence current using a proportional controller, and its calculation formula is:

[0056] ΔV′=k·(i a +i b +i c )

[0057] Where k is the proportionality coefficient, i a i b i c For AC side current, the zero-sequence suppression component ΔV′ needs to be limited to ensure that the modulation wave does not exceed ±1.

[0058] 4) Add the midpoint control zero-sequence component ΔV calculated in step 2) and the zero-sequence suppression component ΔV′ calculated in step 3) to V′. x By doing so, a zero-sequence synthesized modulated wave V″′ that balances midpoint equilibrium and zero-sequence circulation suppression can be obtained. x As shown in the following formula:

[0059] V′″ x =V′ x +ΔV+ΔV′.

[0060] The feasibility of this method can be verified through the following data and analysis.

[0061] Specifically, a single branch of a 1500V / 2500kW energy storage converter was used for simulation analysis, with a switching frequency of 4kHz, AC voltage of 630V, DC voltage of 1300V, and upper and lower DC bus capacitances of 14mF.

[0062] Depend on Figure 2 , Figure 3 It can be seen that the common-mode voltage amplitude of the 13-vector modulation strategy is half that of n3V. Within one switching cycle, the common-mode voltage of n3V changes 6 times, while the 13-vector modulation strategy changes only 2 times. The 13-vector modulation strategy exhibits significant common-mode suppression. Therefore, a midpoint control zero-sequence component can be added to the 13-vector modulation strategy to achieve midpoint potential control and zero-sequence circulating current suppression under low common-mode voltage.

[0063] Depend on Figure 4 , Figure 5 It can be seen that when the power factor is 1 or 0, after superimposing the zero-sequence component of the midpoint control, the maximum common-mode voltage amplitude of the new modulation algorithm is the same as that of n3v. Within one switching cycle, the common-mode voltage changes a maximum of 6 times. The common-mode rejection effect of the 13-vector is somewhat affected, but it is still significantly better than that of n3v.

[0064] Specifically, when the power factor is 0, the midpoint balance control effect is the weakest. Comparing the simulation results of n3V and 13 vector modulation superimposed midpoint potential control under pure reactive power conditions, the results show that... Figure 6It can be seen that the midpoint potential fluctuation is the smallest in the n3V strategy and the largest in the 13-vector modulation strategy. After the 13-vector modulation strategy is superimposed with midpoint potential control, the midpoint offset can be effectively controlled and the midpoint fluctuation is improved.

[0065] In summary, the following conclusions can be drawn:

[0066] The 1.13 vector modulation strategy combined with midpoint potential control can achieve active midpoint potential balance. Compared with n3V, the common-mode voltage is significantly reduced. Compared with the 13 vector modulation strategy, the common-mode voltage is increased, but midpoint potential control can still be taken into account.

[0067] 2. In scenarios with strict common-mode voltage requirements (i.e., common-mode voltage less than 1A), a midpoint potential judgment can be added. When the midpoint potential deviation exceeds the threshold, a midpoint potential control strategy is added to achieve the minimum common-mode voltage and balanced midpoint potential.

[0068] 3. For AC / DC parallel systems, a 13-vector modulation strategy can be used to superimpose midpoint potential control and zero-sequence circulating current suppression to achieve common-mode voltage suppression, midpoint control and zero-sequence circulating current suppression.

[0069] Example of a three-level converter modulation method:

[0070] The main idea of ​​this three-level converter modulation method embodiment is to generate a corresponding modulation wave by superimposing a midpoint control zero-sequence component and a zero-sequence suppression component on a 13-vector modulation wave. This modulation wave is then used for modulation to achieve common-mode voltage suppression, midpoint potential control, and zero-sequence circulating current suppression. The specific implementation process has been detailed in the three-level converter embodiment and will not be repeated here.

[0071] Example of a three-level converter modulation device:

[0072] The three-level converter modulation device includes a memory and a processor, which exchange data via an internal bus. The processor executes computer program instructions stored in the memory to implement a three-level converter modulation method according to the present invention. The specific details of this method have been described in detail in the three-level converter embodiments and will not be repeated here. The memory can be selected from semiconductor memory, magnetic surface memory, read-only memory, sequential memory, etc., and the processor can be selected from Intel processors, MD processors, IBM processors, MediaTek processors, etc.

[0073] This invention suppresses the common-mode voltage, midpoint potential shift, and zero-sequence circulating current generated by a three-level converter by superimposing a midpoint control zero-sequence component and a zero-sequence suppression component on a 13-vector modulation wave.

[0074] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any modifications or changes made to the present invention by those skilled in the art after reading this application and referring to the above embodiments are within the scope of protection claimed in this patent application.

Claims

1. A modulation method for a three-level converter, characterized in that, In scenarios where the common-mode current requirement is less than the common-mode current threshold, if the midpoint potential shift exceeds the midpoint potential threshold, then in a 13-vector modulation wave... Upper superposition midpoint control zero sequence component Obtain the final modulated wave ,use Modulation is performed to achieve midpoint potential balance in the three-level converter; the Based on the following principle: Injection The corresponding change in midpoint charge per unit time This can balance the potential at the midpoint; among which, The calculation formula is: in , This refers to the AC side current of the three-level converter. , It is a 13-vector modulated wave. The switching cycle.

2. The three-level converter modulation method according to claim 1, characterized in that, The formula for calculating the midpoint potential equilibrium is: in, Midpoint potential This is the voltage of the upper bus. Where C is the lower bus voltage, and C is the capacitance value of the DC-side capacitor. When the modulated wave is The amount of charge flowing into the midpoint during a single switching cycle is calculated using the following formula: in, The average current per unit switching cycle. , , It is a 13-vector modulated wave. , This refers to the AC side current of the three-level converter.

3. The three-level converter modulation method according to claim 1, characterized in that, In scenarios where the common-mode current requirement is less than the common-mode current threshold, if the midpoint potential does not exceed the midpoint potential threshold, then the zero-sequence component of the point control is not superimposed. .

4. The three-level converter modulation method according to claim 1, characterized in that, For a three-level converter system with AC and DC sides connected in parallel, The zero-sequence circulation suppression component is also superimposed on it. This is to achieve midpoint potential control and zero-sequence circulating current suppression.

5. The three-level converter modulation method according to claim 4, characterized in that, The The value is obtained by controlling the zero-sequence current using a proportional controller, and the calculation formula is: The proportional coefficient of the proportional controller, , This refers to the AC side current of the three-level converter.

6. The three-level converter modulation method according to claim 1, characterized in that, In scenarios where common-mode current is not a requirement, the 13-vector modulation wave can be directly applied. Upper superposition midpoint control zero sequence component .

7. A three-level converter modulation device, characterized in that, Includes a memory and a processor, the processor being used to execute computer program instructions stored in the memory to implement a three-level converter modulation method. In scenarios where the common-mode current requirement is less than a common-mode current threshold, if the midpoint potential offset exceeds the midpoint potential threshold, then in a 13-vector modulation wave... Upper superposition midpoint control zero sequence component Obtain the final modulated wave ,use Modulation is performed to achieve midpoint potential balance in the three-level converter; the Based on the following principle: Injection The corresponding change in midpoint charge per unit time This can balance the potential at the midpoint; among which, The calculation formula is: in , This refers to the AC side current of the three-level converter. , It is a 13-vector modulated wave. The switching cycle.

8. The three-level converter modulation device according to claim 7, characterized in that, The formula for calculating the midpoint potential equilibrium is: in, Midpoint potential This is the voltage of the upper bus. Where C is the lower bus voltage, and C is the capacitance value of the DC-side capacitor. When the modulated wave is The amount of charge flowing into the midpoint during a single switching cycle is calculated using the following formula: in, The average current per unit switching cycle. , , It is a 13-vector modulated wave. , This refers to the AC side current of the three-level converter.

9. The three-level converter modulation device according to claim 7, characterized in that, In scenarios where the common-mode current requirement is less than the common-mode current threshold, if the midpoint potential does not exceed the midpoint potential threshold, then the zero-sequence component of the point control is not superimposed. .

10. The three-level converter modulation device according to claim 7, characterized in that, For a three-level converter system with AC and DC sides connected in parallel, The zero-sequence circulation suppression component is also superimposed on it. This is to achieve midpoint potential control and zero-sequence circulating current suppression.

11. The three-level converter modulation device according to claim 10, characterized in that, The The value is obtained by controlling the zero-sequence current using a proportional controller, and the calculation formula is: in, The proportional coefficient of the proportional controller. , This refers to the AC side current of the three-level converter.

12. The three-level converter modulation device according to claim 7, characterized in that, In scenarios where common-mode current is not a requirement, the 13-vector modulation wave can be directly applied. Upper superposition midpoint control zero sequence component .

13. A three-level converter, characterized in that, The device includes a three-level converter modulation apparatus, comprising a memory and a processor. The processor executes computer program instructions stored in the memory to implement a three-level converter modulation method. In scenarios where the common-mode current requirement is less than a common-mode current threshold, if the midpoint potential shift exceeds the midpoint potential threshold, then in a 13-vector modulation wave... Upper superposition midpoint control zero sequence component Obtain the final modulated wave ,use Modulation is performed to achieve midpoint potential balance in the three-level converter; the Based on the following principle: Injection The corresponding change in midpoint charge per unit time This can balance the potential at the midpoint; among which, The calculation formula is: in , This refers to the AC side current of the three-level converter. , It is a 13-vector modulated wave. The switching cycle.

14. The three-level converter according to claim 13, characterized in that, The formula for calculating the midpoint potential equilibrium is: in, Midpoint potential This is the voltage of the upper bus. Where C is the lower bus voltage, and C is the capacitance value of the DC-side capacitor. When the modulated wave is The amount of charge flowing into the midpoint during a single switching cycle is calculated using the following formula: in, The average current per unit switching cycle. , , It is a 13-vector modulated wave. , This refers to the AC side current of the three-level converter.

15. The three-level converter according to claim 13, characterized in that, In scenarios where the common-mode current requirement is less than the common-mode current threshold, if the midpoint potential does not exceed the midpoint potential threshold, then the zero-sequence component of the point control is not superimposed. .

16. The three-level converter according to claim 13, characterized in that, For a three-level converter system with AC and DC sides connected in parallel, The zero-sequence circulation suppression component is also superimposed on it. This is to achieve midpoint potential control and zero-sequence circulating current suppression.

17. The three-level converter according to claim 16, characterized in that, The The value is obtained by controlling the zero-sequence current using a proportional controller, and the calculation formula is: in, The proportional coefficient of the proportional controller. , This refers to the AC side current of the three-level converter.

18. The three-level converter according to claim 13, characterized in that, In scenarios where common-mode current is not a requirement, the 13-vector modulation wave can be directly applied. Upper superposition midpoint control zero sequence component .

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