Dual-modulation wave reconstruction pulse width modulation strategy of three-level NPC

Abstract

The invention discloses a dual-modulation wave reconstruction pulse width modulation method for a three-level NPC, and relates to the technical field of converter control. The method comprises the following steps: by taking elimination of low-frequency ripples of neutral-point voltage as an optimization target, obtaining constraint conditions of duty ratios of various states, determining a corresponding modulation signal decomposition principle, and reconstructing a modulation signal; on the basis of the modulation strategy, in order to suppress common-mode voltage, voltage vectors used in the modulation process are analyzed, the switching sequences are optimized and arranged again, and an optimized switching sequence is obtained; and performing pulse width modulation on the three-level NPC by using the modulation wave reconstruction principle and the optimized switch sequence. The dual-modulation wave reconstruction pulse width modulation method is applied to the converter, the common-mode voltage of the inverter can be suppressed within 1 / 6 of the input voltage, the converter is controlled based on modulation wave decomposition and the optimal switching sequence, neutral-point potential balance and common-mode voltage cooperative suppression are achieved, and the neutral-point potential balance and common-mode voltage cooperative suppression are achieved. The method has the advantages of no need of parameter setting, low output current harmonic content and the like.

Description

Technical Field

[0001] The invention relates to the technical field of converter control, and in particular to a dual modulation wave reconstruction pulse width modulation strategy of a three-level NPC. Background Art

[0002] One of the traditional control strategies of the three-level neutral point clamped inverter (NPC) is carrier modulation (CBPWM). In the traditional modulation, a carrier stacking method is used, in which each phase corresponds to only one modulation wave. By comparing the size of the modulation wave and the carrier, different voltage vectors are selected to change the direction and size of the output voltage. However, the oscillation amplitude of the midpoint potential generated by it is large. For this reason, the method of superimposing the zero-sequence component on the modulation signal is usually used to suppress the low-frequency ripple of the midpoint voltage. However, this method has two obvious disadvantages: the calculation process of the superimposed zero-sequence component is complicated and the low-frequency ripple of the midpoint voltage cannot be eliminated under all working conditions. At the same time, due to the existence of distributed capacitance to the ground, the common-mode voltage will cause the generation of common-mode current, and the high-frequency common-mode voltage will generate high-frequency electromagnetic interference, affecting the normal operation of surrounding equipment. Applying a converter with a high common-mode voltage before the rotating equipment will cause irreversible damage to the equipment and affect the service life of the equipment. Therefore, it is necessary to achieve the midpoint potential balance and the suppression of the common-mode voltage under all working conditions at the same time, but there is an antagonistic relationship between the midpoint potential balance and the suppression of the common-mode voltage, which is difficult to meet at the same time. Summary of the invention

[0003] The purpose of the present invention is to provide a dual modulation wave reconstruction pulse width modulation strategy for a three-level NPC, which can control the converter based on modulation wave decomposition and optimized switching sequence, achieve midpoint potential balance and common mode voltage coordinated suppression, and reduce the output current harmonic content.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] The dual modulation wave reconstruction pulse width modulation strategy of three-level NPC includes:

[0006] Taking the elimination of low-frequency ripple of midpoint voltage as the optimization goal, the constraint conditions of duty cycle of each state are obtained, the corresponding modulation signal decomposition principle is determined, and the modulation signal is reconstructed; on the basis of this modulation strategy, in order to suppress the common-mode voltage, the switching sequence is rearranged by analyzing the voltage vector used in the modulation process to obtain the optimized switching sequence; the three-level NPC is pulse-width modulated by using the modulation wave reconstruction principle and the optimized switching sequence.

[0007] A three-level NPC dual modulation wave reconstruction pulse width modulation method, characterized by comprising:

[0008] Taking the elimination of low-frequency ripple of midpoint voltage as the optimization goal, the constraint conditions of duty cycle of each state are obtained, the corresponding modulation signal decomposition principle is determined, and the modulation signal is reconstructed;

[0009] On the basis of this modulation strategy, in order to suppress the common mode voltage, the switching sequence is rearranged by analyzing the voltage vector used in the modulation process to obtain the optimized switching sequence;

[0010] The three-level NPC is pulse-width modulated by utilizing the modulation wave reconstruction principle and the optimized switching sequence.

[0011] The dual modulation wave reconstruction pulse width modulation method is characterized in that the optimization goal is to eliminate the low-frequency ripple of the midpoint voltage, obtain the constraint conditions of the duty cycle of each state, determine the principle of corresponding modulation signal decomposition, and reconstruct the modulation signal, which specifically includes:

[0012] To eliminate the low-frequency ripple of the midpoint voltage, it is first necessary to obtain the mathematical model of the midpoint current generated by the connection of the three-phase bridge arm output point to the neutral point under the three-level NPC topology; then, it is analyzed that if the requirement of midpoint potential balance under all working conditions needs to be met, the zero-state duty cycle of the three-phase switch tube of the three-level NPC topology needs to be completely equal; according to the principle that the zero-state duty cycle of the three-phase switch tube of the three-level NPC topology needs to be completely equal, the duty cycle of the PON state of the three-phase dual modulation wave is recalculated; based on the re-formed duty cycle of each state of the three-phase dual modulation wave, the full time domain mathematical model of the three-phase dual modulation wave is reconstructed respectively.

[0013] According to the above-mentioned dual modulation wave reconstruction pulse width modulation method, on the basis of this modulation strategy, in order to suppress the common mode voltage, by analyzing the voltage vector used in the modulation process, the switching sequence is rearranged to optimize the switching sequence, which specifically includes:

[0014] By comparing the formed dual modulation wave with the triangular carrier, the voltage space vector formed by the switching state of each phase is obtained, and its common-mode voltage amplitude is obtained. With the optimization goal of suppressing the common-mode voltage to 1 / 6 of the bus voltage, the switching sequence is optimized and sorted to establish an optimized switching sequence.

[0015] Based on the reconstructed dual modulation wave and the optimized switch sequence, midpoint potential balancing and common mode voltage suppression are performed on the three-level NPC, specifically including:

[0016] By using the reconstructed dual modulation wave and the optimized switch sequence, when it is necessary to generate a desired voltage vector, the switch is controlled to be on and off according to the optimized switch sequence to meet different input-output voltage ratios and complete modulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the basic principle, technical solution and performance advantages of the present invention, the present invention is further described in detail below with reference to the accompanying drawings.

[0018] Figure 1 It is a schematic flow chart of the dual modulation wave reconstructing pulse width modulation method of the present invention;

[0019] Figure 2 This is a topological diagram of a three-level inverter according to the present invention;

[0020] Figure 3 Schematic diagram of the modulation signal after normalization in the present invention; wherein, part (a) is the original modulation signal diagram, and part (b) is the duty cycle of each state of phase a;

[0021] Figure 4 A diagram showing the construction process of the modulation signal in the present invention;

[0022] Figure 5 is a positive / negative modulation signal diagram corresponding to phase a in the present invention;

[0023] Figure 6 is a switching sequence diagram of each phase in the present invention;

[0024] Figure 7 The simulated waveforms of the input capacitor voltage under different modulation strategies of the present invention; wherein, part (a) is the midpoint voltage of the traditional algorithm, and part (b) is the midpoint voltage output by the proposed dual modulation wave reconstruction algorithm;

[0025] Figure 8 The simulated waveforms of the common-mode voltage under different modulation strategies of the present invention; wherein, part (a) is the common-mode voltage of the traditional algorithm, and part (b) is the common-mode voltage output by the proposed dual modulation wave reconstruction algorithm;

[0026] Fig. 9 The harmonic analysis of the output current of the present invention under different modulation strategies; wherein, part (a) is the harmonic of the output current of the traditional algorithm, and part (b) is the harmonic of the output current output by the proposed dual modulation wave reconstruction algorithm; DETAILED DESCRIPTION

[0027] In order to illustrate the basic principle, technical solution and performance advantages of the present invention, the present invention is further described in detail below with reference to the accompanying drawings.

[0028] like Figure 1 As shown, the present invention provides a three-level NPC dual modulation wave reconstruction pulse width modulation method, comprising:

[0029] Step 100: The constraints of the duty cycle of each state are obtained, and the corresponding modulation signal decomposition principle is determined, which specifically includes:

[0030] For a three-phase three-wire three-level topology system, such as Figure 2 As shown, the sum of the three-phase output current is 0, that is, i a +i b +i c =0, if the zero-state duty ratio of each phase can be controlled to be equal, that is, d ao =d bo =d co , then the midpoint current can be guaranteed to be constant at 0. In one switching cycle, the average voltage output of each phase is d xp -d xn , so different modulation strategies need to satisfy the following constraint equations:

[0031]

[0032] Therefore, the basic principle of eliminating the low-frequency ripple of the midpoint voltage can be summarized as follows: Under the condition of satisfying the above constraint equation, the zero-state duty cycle of each phase is controlled to be equal, that is, d ao =d bo =d co , the midpoint voltage oscillation can be completely eliminated.

[0033] A commonly used modulation wave decomposition method decomposes the single modulation wave in the traditional modulation strategy into two modulation waves, and in one switching cycle, the output of each phase has three states. If this dual modulation wave modulation strategy is adopted, the positive state duty cycle and negative state duty cycle of each phase can be configured more flexibly.

[0034] It is still assumed that the zero-state duty cycle of each phase is equal, which is recorded as: d o =d ao =d bo =d co , then the negative state duty cycle of each phase can be expressed as:

[0035]

[0036] At the same time, the constraint equations are satisfied:

[0037]

[0038] The solution to the above equation can be expressed as:

[0039]

[0040] Where d off is the bias value. Through the equation, the values ​​of the positive state duty cycle, negative state duty cycle and zero state duty cycle of each phase can be listed.

[0041] Then we need to solve the bias value d off and zero state duty cycle do .

[0042] Manually set the offset value d off and zero state duty cycle d o , d off and d o The general expression is as follows:

[0043]

[0044] After transformation by the above method, the positive / negative / zero state duty cycle of each phase can be obtained, as shown in Table 1.

[0045] Table 1 Expression of duty cycle of each phase in positive / negative / zero state

[0046]

[0047] Taking phase a as an example, when the modulation ratio m=1, the original modulation signal is as follows Figure 3 After the above transformation, the duty ratio of each phase in positive / negative / zero state can be obtained, where the duty ratio of each state of phase a is as follows: Figure 3 (b) as shown.

[0048] The construction process of the modulation signal is as follows Figure 4 As shown, in Figure 4 In the figure, line A represents the positive modulation signal u xp The output state of line B represents the negative modulation signal u xn The output state of the positive / negative modulation signal u is the combination of the two modulation signal output states. It can be seen that it is completely consistent with the duty cycle of each state. xp 、u xn It can be expressed as:

[0049]

[0050] According to the duty cycle of each state of phase a, the positive / negative modulation signal corresponding to phase a can be drawn as follows: Figure 5 shown.

[0051] Step 200: Based on the modulation strategy, by analyzing the voltage vector used in the modulation process, the switch sequence is rearranged to obtain an optimized switch sequence, which specifically includes:

[0052] The switching sequence is nopon, which is used in many literatures, but it is necessary to determine whether it is an optimal switching sequence. In order to reduce the common mode voltage, based on the dual modulation wave strategy proposed above, the optimal ordering method of the switching sequence is explored.

[0053] There are 27 switching states in the three-level NPC inverter, corresponding to different output connection methods. For example, the switching state opn corresponds to the a-phase output connected to the input midpoint o, the b-phase output connected to the input positive bus terminal p, and the c-phase output connected to the input negative bus terminal n. These switching states define a total of 19 different space vectors, which can be classified into: zero vector (V o ), small vector (V Si ), mid vector (V Mi ) and large vector (V Li ), where i = 1, 2,…, 6.

[0054] Through the above modulation signal construction process, the switching sequence of each phase in a switching cycle can be obtained, where:

[0055]

[0056] In the previous dual modulation wave strategy deduction process, it has been explained that u max The corresponding phase does not contain output state n, u min The corresponding phase does not contain the output state p.

[0057] Figure 6 There are small voltage vectors in the switching sequence shown, which will generate ±U dc / 3 of the common mode voltage.

[0058] To be able to eliminate these voltage vectors, the switching sequence needs to be rearranged. For three phases and two carriers, six different combinations can be expected: comparing two of the phases to carrier wave 1, comparing the remaining phase to carrier wave 2 (three combinations), or comparing only one phase to carrier wave 1 and the other two to carrier wave 2. Note that all six cases effectively reduce the CMV to ±U due to the use of two carriers. dc / 6.

[0059] Step 300: performing pulse width modulation on the three-level NPC by using the modulation wave reconstruction principle and the optimized switching sequence, specifically comprising:

[0060] By using the reconstructed dual modulation wave and the optimized switch sequence, the switch is controlled to be on and off when the desired voltage vector needs to be generated to meet different input-output voltage ratios and complete the modulation. The algorithm is verified by simulation comparison, where Figure 7 Part (a) is the midpoint voltage of the traditional algorithm, and part (b) is the midpoint voltage of the output of the proposed dual modulation wave reconstruction algorithm; Figure 8 Part (a) is the common mode voltage of the traditional algorithm, and part (b) is the common mode voltage output by the proposed dual modulation wave reconstruction algorithm; Fig. 9Part (a) shows the harmonics of the output current of the traditional algorithm, and part (b) shows the harmonics of the output current output by the proposed dual modulation wave reconstruction algorithm. Figure 7 , Figure 8 and Fig. 9 By comparison, it can be found that the coordinated control method of the midpoint potential and common-mode voltage of the three-level NPC inverter proposed in the present invention can effectively suppress the common-mode voltage, reducing the common-mode voltage amplitude to 1 / 6 of the original, while the midpoint potential remains stable under all operating conditions, and this method can effectively reduce current harmonics.

Claims

1. A three-level NPC dual modulation wave reconstruction pulse width modulation method, characterized in that: include: Taking the elimination of low-frequency ripple of midpoint voltage as the optimization goal, the constraint conditions of duty cycle of each state are obtained, the corresponding modulation signal decomposition principle is determined, and the modulation signal is reconstructed; On the basis of this modulation strategy, in order to suppress the common mode voltage, the switching sequence is rearranged by analyzing the voltage vector used in the modulation process to obtain the optimized switching sequence; The three-level NPC is pulse-width modulated by utilizing the modulation wave reconstruction principle and the optimized switching sequence.

2. The dual modulation wave reconstruction pulse width modulation method according to claim 1, characterized in that: Taking the elimination of low-frequency ripple of midpoint voltage as the optimization goal, the constraints of duty cycle of each state are obtained, the corresponding modulation signal decomposition principle is determined, and the modulation signal is reconstructed, including: To eliminate the low-frequency ripple of the midpoint voltage, it is first necessary to obtain a mathematical model of the midpoint current generated by connecting the output point of the three-phase bridge arm to the neutral point under the three-level NPC topology; then, it is analyzed that if the requirement of midpoint potential balance under all working conditions needs to be met, the zero-state duty cycle of the three-phase switch tubes of the three-level NPC topology needs to be completely equal; based on the principle that the zero-state duty cycle of the three-phase switch tubes of the three-level NPC topology needs to be completely equal, the duty cycle of the PON state of the three-phase dual modulation wave is recalculated; based on the re-formed duty cycle of each state of the three-phase dual modulation wave, the full time domain mathematical model of the three-phase dual modulation wave is reconstructed respectively.

3. The dual modulation wave reconstruction pulse width modulation method according to claim 1, characterized in that: On the basis of this modulation strategy, in order to suppress the common mode voltage, the switching sequence is rearranged by analyzing the voltage vector used in the modulation process to obtain the optimized switching sequence, which includes: By comparing the formed dual modulation wave with the triangular carrier, the voltage space vector formed by the switching state of each phase is obtained, and its common-mode voltage amplitude is obtained. With the optimization goal of suppressing the common-mode voltage to 1 / 6 of the bus voltage, the switching sequence is optimized and sorted to establish an optimized switching sequence.

4. The dual modulation wave reconstruction pulse width modulation method according to claim 1, characterized in that: Using the reconstructed dual modulation wave and the optimized switch sequence to balance the midpoint potential and suppress the common mode voltage of the three-level NPC specifically includes: By using the reconstructed dual modulation wave and the optimized switch sequence, when the desired voltage vector is required to be generated, When measuring, the switch is controlled on and off according to the optimized switch sequence to meet different input-output voltage ratios. Modulation is complete.

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