Three-level half-wave symmetrical SHEPWM with midpoint potential low-frequency pulsation suppression capability

Abstract

The present invention provides a three-level half-wave symmetrical SHEPWM with the ability to suppress low-frequency pulsation of mid-point potential, comprising: S1. calculating the ratio k of the amplitude of the third harmonic to the amplitude of the fundamental wave for suppressing low-frequency pulsation of mid-point potential under different load power factors 3 And the phase θ of the third harmonic relative to the fundamental wave 3 The optimal value of S2. Construct a half-wave symmetrical SHEPWM solution model and set the ratio k 3 and the phase θ 3 Substitute into the solution model and use the Newton-Raphson iterative algorithm to solve the switching angle for eliminating harmonics; S3. Control the operation of the three-level converter based on the solved switching angle for eliminating harmonics. Through the above method, the low-frequency harmonics of the three-level converter can be effectively suppressed, thereby effectively ensuring that the converter output current has a good harmonic distribution, and can also effectively improve the adaptability of SHEPWM.

Description

Technical Field

[0001] The invention relates to a harmonic suppression method, in particular to a three-level half-wave symmetrical SHEPWM with midpoint potential low-frequency pulsation suppression capability. Background Art

[0002] With the continuous growth of medium voltage and high power power application scenarios, low switching frequency modulation technology is widely used due to its low switching loss. Among them, specific subharmonic elimination pulse width modulation technology (SHEPWM) is one of the many low switching frequency modulation technologies. It is highly favored because it can reduce the switching frequency, improve system efficiency and ensure good output performance of the converter.

[0003] For the low-frequency pulsation problem of the midpoint potential of the three-level converter under the specific harmonic elimination pulse width modulation technology (SHEPWM) under the traditional quarter-cycle symmetry, the current technical strategy is mainly to use different values ​​of the 3rd and 9th harmonic amplitudes to reduce the low-frequency pulsation. After the switching angle is calculated, this technical strategy can automatically suppress the low-frequency pulsation of the midpoint potential without any closed-loop control. However, the method is only effective in suppressing the low-frequency pulsation of the midpoint potential when the power factor is high. When the power factor is low, the suppression of low-frequency pulsation (i.e., low-frequency harmonics) is poor. For example, when the power factor is 0.5, the suppression rate of the low-frequency pulsation of the midpoint potential is only 80%; on the one hand, this will seriously affect the output current harmonic distribution of the inverter, and on the other hand, it will also limit the use scenario of SHEPWM.

[0004] Therefore, in order to solve the above technical problems, a new technical means is proposed to solve them. Summary of the invention

[0005] In view of this, the object of the present invention is to provide a three-level half-wave symmetrical SHEPWM with midpoint potential low-frequency pulsation suppression capability, which can effectively suppress the low-frequency harmonics of the three-level converter, thereby effectively ensuring that the converter output current has a good harmonic distribution, and can also effectively improve the adaptability of the SHEPWM.

[0006] The present invention provides a three-level half-wave symmetrical SHEPWM with midpoint potential low-frequency pulsation suppression capability, comprising the following steps:

[0007] S1. Calculate the ratio k of the amplitude of the third harmonic to the fundamental wave amplitude used to suppress the low-frequency pulsation of the midpoint potential under different load power factors 3 And the phase θ of the third harmonic relative to the fundamental wave 3 The optimal value of

[0008] S2. Construct a half-wave symmetrical SHEPWM solution model and set the ratio k 3and the phase θ 3 Substitute it into the solution model and use the Newton-Raphson iterative algorithm to solve the switching angle for eliminating harmonics;

[0009] S3. Control the operation of the three-level converter based on the solved switching angle for eliminating harmonics.

[0010] Further, in step S2, the half-wave symmetrical SHEPWM solution model is specifically:

[0011] in:

[0012]

[0013] The solution model also has the following constraints:

[0014] in:

[0015] f wthd is the optimization objective function of WTHD, m is the modulation ratio, the ratio of modulation wave amplitude to carrier amplitude, h represents the order of low-order harmonics, α i represents the i-th switching angle used to eliminate harmonics, 2N represents the number of switching angles in one cycle, and v h (pu) represents the value of the hth harmonic voltage amplitude relative to the fundamental wave, V 9cos and V 9sin Both indicate the magnitude of the 9th harmonic voltage amplitude compared to the fundamental amplitude.

[0016] Further, step S1 specifically includes:

[0017] Determine the midpoint potential v o and the current i flowing through the midpoint o :

[0018]

[0019] i u 、i v 、i w is the three-phase current, v u ,v v ,v w For the three-phase voltage:

[0020]

[0021] ψ is the impedance angle, v u1 、v u3 、v u9 are the voltage amplitudes of the fundamental wave, third harmonic, and ninth harmonic, respectively, where:

[0022]

[0023] Substituting the parameters obtained from formulas (2) and (3) into formula 1, we get:

[0024]

[0025]

[0026] Setting θ 3 The range is (-π,π), k 3 The value range of is set to (0,1 / 3); based on θ 3 and k 3 The range of θ is calculated by numerical solution. 3 and k 3 Substituting into formula (4) and (5) we can calculate the k when the converter midpoint voltage low-frequency fluctuation is the smallest. 3 and θ 3 as the optimal value.

[0027] Beneficial effects of the present invention: Through the present invention, the low-frequency harmonics of the three-level converter can be effectively suppressed, thereby effectively ensuring that the converter output current has a good harmonic distribution, and can also effectively improve the adaptability of SHEPWM. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0029] Figure 1 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below:

[0031] The present invention provides a three-level half-wave symmetrical SHEPWM with midpoint potential low-frequency pulsation suppression capability, comprising the following steps:

[0032] S1. Calculate the ratio k of the amplitude of the third harmonic to the fundamental wave amplitude used to suppress the low-frequency pulsation of the midpoint potential under different load power factors 3 And the phase θ of the third harmonic relative to the fundamental wave 3 The optimal value of

[0033] S2. Construct a half-wave symmetrical SHEPWM solution model and set the ratio k 3 and the phase θ 3 Substitute it into the solution model and use the Newton-Raphson iterative algorithm to solve the switching angle for eliminating harmonics;

[0034] S3. Based on the solved switching angle for eliminating harmonics, the three-level converter is controlled to work. Through the above method, the low-frequency harmonics of the three-level converter can be effectively suppressed, thereby effectively ensuring that the converter output current has a good harmonic distribution, and can also effectively improve the adaptability of SHEPWM.

[0035] In this embodiment, in step S2, the half-wave symmetrical SHEPWM solution model is specifically:

[0036] in:

[0037]

[0038] The solution model also has the following constraints:

[0039] in:

[0040] f wthd is the optimization objective function of WTHD, m is the modulation ratio, the ratio of modulation wave amplitude to carrier amplitude, that is, v 1 / (v dc / 2); h represents the order of low-order harmonics, α i represents the i-th switching angle used to eliminate harmonics, 2N represents the number of switching angles in one cycle, and v h (pu) represents the value of the hth harmonic voltage amplitude relative to the fundamental wave, that is, Vh / fundamental wave value, V 9cos and V 9sin They all represent the magnitude of the voltage amplitude of the 9th harmonic compared to the amplitude of the fundamental wave. The Newton-Raphson iteration algorithm is used to solve the above equation. The Newton-Raphson iteration algorithm is a prior art and will not be described in detail here. In the above model, the amplitude and phase of the harmonics are adjusted simultaneously, so that the converter operation is controlled based on the final determined switching angle, thereby achieving effective suppression of harmonics.

[0041] In this embodiment, step S1 specifically includes:

[0042] Determine the midpoint potential v o and the current i flowing through the midpoint o :

[0043]

[0044] i u 、i v 、i w is the three-phase current, v u ,v v ,v w For the three-phase voltage:

[0045]

[0046] ψ is the impedance angle, v u1 、v u3 、v u9 are the voltage amplitudes of the fundamental wave, the third harmonic, and the ninth harmonic respectively. The ellipsis in formula (2) indicates that only the fundamental wave, the third harmonic, and the harmonic are considered, and the other harmonics are omitted and not considered.

[0047]

[0048] Substituting the parameters obtained from formulas (2) and (3) into formula 1, we get:

[0049]

[0050]

[0051] Setting θ 3 The range is (-π,π), k 3 The value range of is set to (0,1 / 3); based on θ 3 and k 3 The range of θ is calculated by numerical solution. 3 and k 3 Substituting into formula (4) and (5) we can calculate the k when the converter midpoint voltage low-frequency fluctuation is the smallest. 3 and θ 3 As the optimal value

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A three-level half-wave symmetric SHEPWM with the ability to suppress low-frequency pulsation of the midpoint potential Characterized in that: It includes the following steps: S1. Calculate the ratio k of the amplitude of the third harmonic to the fundamental wave amplitude used to suppress the low-frequency pulsation of the midpoint potential under different load power factors 3 And the phase θ of the third harmonic relative to the fundamental wave 3 The optimal value of S2. Construct a half-wave symmetrical SHEPWM solution model and set the ratio k 3 and the phase θ 3 Substitute it into the solution model and use the Newton-Raphson iterative algorithm to solve the switching angle for eliminating harmonics; S3. Control the operation of the three-level converter based on the obtained switching angles for eliminating harmonics; In step S2, the specific half-wave symmetric SHEPWM solution model is: in: The solution model also has the following constraint conditions: in: f wthd is the optimization objective function of WTHD, m is the modulation ratio, the ratio of modulation wave amplitude to carrier amplitude, h represents the order of low-order harmonics, α i represents the i-th switching angle used to eliminate harmonics, 2N represents the number of switching angles in one cycle, and v h (pu) represents the value of the hth harmonic voltage amplitude relative to the fundamental wave, V 9cos and V 9sin Both indicate the magnitude of the 9th harmonic voltage amplitude compared to the fundamental amplitude; Specifically included in step S1: Determine the midpoint potential v o and the current i flowing through the midpoint o : i u 、i v 、i w is the three-phase current, v u ,v v ,v w For the three-phase voltage: ψ is the impedance angle, v u1 、v u3 、v u9 are the voltage amplitudes of the fundamental wave, third harmonic, and ninth harmonic, respectively, where: Substitute the parameters obtained from formulas (2) and (3) into formula 1 and after arrangement, we get: Setting θ 3 The range is (-π,π), k 3 The value range of is set to (0,1 / 3); based on θ 3 and k 3 The range of θ is calculated by numerical solution. 3 and k 3 Substituting into formula (4) and (5) we can calculate the k when the converter midpoint voltage low-frequency fluctuation is the smallest. 3 and θ 3 as the optimal value.

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