A method for analyzing and suppressing capacitor voltage fluctuation of a hexagonal modular multilevel AC-AC converter

Abstract

The application discloses a kind of hexagonal modular multilevel AC-AC converter's capacitor voltage fluctuation analysis method and inhibiting method, comprising the following steps: step 1, hexagonal modular multilevel AC-AC converter modeling is carried out, and the capacitor voltage expression of sub-module is obtained by power conservation law and the volt-ampere characteristic of capacitor;Step 2: introduce neutral point voltage and circulating current, update the capacitor voltage expression of sub-module under the condition that neutral point voltage and circulating current exist;Step 3, the capacitor voltage expression of sub-module obtained in step 2 is analyzed, and the technical conditions that neutral point voltage and circulating current need to meet to inhibit capacitor voltage ripple are obtained.The application can control the capacitor voltage of sub-module within a certain range, while minimizing the switching loss of the converter.

Description

TECHNICAL FIELD

[0001] The application relates to a power voltage fluctuation suppression method, in particular to a capacitor voltage fluctuation analysis method and suppression method of a hexagonal modular multilevel AC / AC converter. BACKGROUND

[0002] The hexagonal modular multilevel AC / AC converter (Hexverter) is a novel topology for realizing AC / AC conversion, and can realize three-phase AC frequency conversion. The Hexverter has the advantages of modular multilevel matrix converter (M3C) such as modularity and large number of levels, and has fewer branches and requires fewer capacitors and inductors compared with the M3C. In a low-frequency operation environment, the Hexverter is a highly competitive solution that can reduce device size and cost and reduce power loss. Figure 1 As shown in the figure, the Hexverter converter and its submodule topology structure are as follows: the Hexverter topology structure has six branches, each of which is formed by a branch inductor L and a plurality of submodules in series; each submodule is formed by a full-bridge and a capacitor in parallel; and each full-bridge is composed of four power electronic switching devices (IGBT).

[0003] When the Hexverter operates, the capacitor voltage inevitably has periodic fluctuations due to the reactive power exchange between the AC side and the DC side of the branch. If the amplitude of the capacitor ripple voltage is too large, the withstand voltage requirement of the switching tube will be too high, and in severe cases, the power quality and system reliability will be significantly reduced, affecting the normal operation of the system. Therefore, how to suppress the capacitor voltage fluctuation of the Hexverter is of great significance to improve the power quality and safety of the system. SUMMARY

[0004] The application provides a capacitor voltage fluctuation analysis method and suppression method of a hexagonal modular multilevel AC / AC converter, which can solve the problem of excessive capacitor voltage fluctuation of the Hexverter when the input / output reactive power is unbalanced.

[0005] The technical scheme adopted by the application is a capacitor voltage fluctuation analysis method of a hexagonal modular multilevel AC / AC converter, which comprises the following steps:

[0006] Step 1, modeling the hexagon modular multilevel AC-AC converter, obtaining the sub-module capacitor voltage expression through the power conservation law and the capacitor volt-ampere characteristic; the sub-module is an H-bridge structure, and the power electronic switching device forms a bridge arm, and the capacitor is connected across the bridge arm; the sub-module capacitor voltage expression is: Wherein, each expression is shown in formula (1).

[0007]

[0008] In formula (1), n is the branch number, u c，n is the capacitor voltage of the sub-module in branch n, is the average power voltage component, is the capacitor voltage component with a frequency of 2f1, is the capacitor voltage component with a frequency of 2f2, is the capacitor voltage component with a frequency of f1-f2, is the capacitor voltage component with a frequency of f1+f2; C is the capacitor value, N is the number of sub-modules, U c is the capacitor voltage rating, P1 and Q1 are the input active and reactive power, P2 and Q2 are the output active and reactive power, is the amplitude of the input phase voltage and current, is the amplitude of the output phase voltage and current, ω1 and ω2 are the angular frequencies, and is the power factor angle, ψ represents the phase difference.

[0009] Step 2: Introducing the neutral point voltage and circulating current, updating the sub-module capacitor voltage expression under the condition of existing neutral point voltage and circulating current; the sub-module capacitor voltage expression under the condition of existing neutral point voltage and circulating current is: Wherein, each expression is shown in formula (2),

[0010]

[0011] In formula (2), u′ c，n is the updated sub-module capacitor voltage, is the average power voltage component under the condition of existing neutral point voltage and circulating current, is the newly added capacitor voltage component with a frequency of f1 and f2 under the condition of existing neutral point voltage and circulating current, V st is the neutral point voltage, I cir is the circulating current.

[0012] Step 3, analyzing the updated sub-module capacitor voltage expression obtained in step 2, obtaining the technical conditions that the neutral point voltage and circulating current need to meet to suppress the capacitor voltage ripple.

[0013] The technical condition that needs to be met by the neutral point voltage and the circulating current for suppressing the capacitor voltage ripple is: Wherein, Q1 is the input side reactive power, and Q2 is the output side reactive power.

[0014] If it is necessary to further determine the values of the neutral point voltage and the circulating current, step 4 is further included, and the neutral point voltage and the circulating current are calculated with the minimum converter switching loss as the target. The converter switching loss expression is: SW (|I cir |), wherein, P SW See formula (3),

[0015]

[0016] In formula 3, P SW is the switching loss of the converter, N is the number of sub-modules, is the amplitude of the input phase voltage and current, is the amplitude of the output phase voltage and current, I cir is the circulating current, Q1 is the input side reactive power, and Q2 is the output side reactive power.

[0017] Finally, the neutral point voltage and the circulating current are respectively:

[0018]

[0019] In the formula, “+” corresponds to Q1> Q2, “-” corresponds to Q1< Q2, V st is the neutral point voltage, I cir is the circulating current, is the amplitude of the input phase voltage and current, is the amplitude of the output phase voltage and current, Q1 is the input side reactive power, and Q2 is the output side reactive power.

[0020] Based on the above analysis method, the application further provides a capacitor voltage fluctuation suppression method of a hexagonal modular multilevel AC-AC converter. The capacitor average power voltage component is eliminated by introducing a neutral point voltage and a circulating current to suppress the ripple voltage. The technical condition that needs to be met by the neutral point voltage and the circulating current is: Wherein, Q1 is the input side reactive power, and Q2 is the output side reactive power.

[0021] Further, the neutral point voltage and the circulating current are calculated with the minimum converter switching loss as the target, and the neutral point voltage and the circulating current are as formula (4).

[0022] Beneficial effects: compared with the prior art, the application has the following advantages: a Hexverter capacitor voltage fluctuation analysis method is given, the specific expression obtained through the analysis method gives a starting point for further analyzing the source of power fluctuation; the application eliminates the average power voltage component in the capacitor voltage by controlling the neutral point voltage and the circulating current, controls the sub-module capacitor voltage within a certain range, and application of the method can minimize the switching loss of the Hexverter. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a topology diagram of a hexagonal modular multilevel AC-AC converter;

[0024] Figure 2 is a waveform diagram of the sub-module capacitor voltage before and after application of the suppression method according to the application;

[0025] Figure 3 is a comparison diagram of the simulation value and the theoretical value of the sub-module capacitor voltage after application of the suppression method according to the application;

[0026] Figure 4 is a change of the switching loss when the circulating current value changes according to the application. DETAILED DESCRIPTION

[0027] The technical solutions of the application will be further described below in combination with the drawings and examples.

[0028] The capacitor voltage fluctuation analysis method of the hexagonal modular multilevel AC-AC converter according to the application comprises the following steps:

[0029] Step 1: model the converter according to the input and output voltage and current of the system, and obtain the expression of the sub-module capacitor voltage by using the power conservation law and the volt-ampere characteristic of the capacitor;

[0030] The topology structure of the Hexverter is shown in Figure 1 , which contains 6 branches, each branch includes N sub-module units and a branch inductance, each sub-module unit includes a full-bridge and a capacitor C. Each full-bridge is a full-bridge structure composed of 4 power electronic switching devices (IGBT).

[0031] The input voltage and current are defined as:

[0032]

[0033]

[0034]

[0035] The output voltage and current are:

[0036]

[0037]

[0038]

[0039] where v 1，k , i 1，k are input voltage and current, v 2，k , i 2，k are output voltage and current, k = 1, 2, 3 represent three phases. are the amplitude of input phase voltage and current, are the amplitude of output phase voltage and current, ω1 and ω2 are angular frequencies, and is the power factor angle, ψ represents the phase difference between two systems.

[0040] Based on the above definitions, the active power and reactive power of input / output side systems are respectively:

[0041]

[0042]

[0043]

[0044]

[0045] where P1, Q1 are input side active and reactive power, P2, Q2 are output side active and reactive power.

[0046] According to Kirchhoff's voltage and current law:

[0047]

[0048]

[0049] where v b，n , i b，n are branch voltage and current, i 1，12 , i 1，23 , i 1，31 , i 2，12 , i 2，23 , i 2，31 represent the line current between input side and output side nodes a-b, b-c, c-a, u-v, v-w, w-u, respectively, the specific expression of this set of line currents is:

[0050]

[0051]

[0052]

[0053]

[0054]

[0055]

[0056] Thus, the voltage and current of branch n (n = 1...6) are:

[0057]

[0058]

[0059] Further, the power of branch n is:

[0060]

[0061]

[0062]

[0063]

[0064]

[0065] Since the energy stored in the inductance of the branch is much smaller than the capacitance, the inductance of the branch can be ignored. The capacitor current of the sub-module is calculated as:

[0066]

[0067] According to the volt-ampere characteristic of the capacitor element, the capacitor ripple voltage is

[0068]

[0069] In the formula: The component is integrated from the average power of the branch and is linearly related to time. In this paper, it is recorded as the average power voltage component; The component is integrated from the AC component of the branch power.

[0070]

[0071] Step 2, introduce constant neutral point voltage and circulating current, update the sub-module capacitor voltage expression under the condition of existing neutral point voltage and circulating current.

[0072] According to the expression of the capacitor voltage fluctuation of the sub-module, we can analyze the main factors causing large fluctuations.

[0073] From the expression of the capacitor voltage fluctuation of the sub-module, we can see that when the average power voltage component is not 0, the capacitor voltage will change linearly with time, showing a continuous rising / falling trend, which endangers the safe operation of the system.

[0074] Analysis of the generation mechanism of the average power voltage component: From the expression of the average power voltage, we can see that the size of the average power voltage component is related to the active / reactive power on the input / output side. Generally, the active power on the input / output side of the converter is balanced, i.e. P1+P2=0. When the reactive power on the input / output side is balanced, Q1-Q2=0, the average power of the branch is 0, and the average power voltage component in the capacitor voltage fluctuation is 0, and the overall capacitor voltage shows a periodic change trend; when the reactive power on the input / output side is unbalanced, Q1-Q2≠0, the average power of the branch is not 0, and the average power voltage component in the capacitor voltage fluctuation is not 0, and the overall capacitor voltage shows a continuous increasing / decreasing trend. Note that the average power of the adjacent branch at this time is equal in size and opposite in direction, which indicates that the imbalance of the system reactive power leads to a power transmission between adjacent branches with a size of , causing the energy change of the branch, which is manifested as the continuous rise of the capacitor voltage on one side and the continuous decline of the capacitor voltage on the other side.

[0075] The expressions of the branch voltage and current after introducing the constant neutral point voltage and circulating current are:

[0076]

[0077]

[0078] wherein the neutral point voltage V st is the voltage value of the input side neutral point relative to the output side neutral point, and the circulating current I cir represents 1 / 6 of the sum of the branch currents.

[0079] The expression of the branch power is

[0080]

[0081] Since the new branch voltage and branch current have added a direct current component, compared with before, the direct current component of the corrected branch power will change and a new alternating current component with a frequency of input / output frequency will be added.

[0082]

[0083] The expression of the capacitor voltage fluctuation is

[0084]

[0085]

[0086] Step 3, analyze the updated average power voltage expression, and propose a ripple suppression method for input / output reactive imbalance: use the constant neutral point voltage and circulating current elimination capacitor to average power voltage components that meet certain conditions.

[0087] Analyzing the updated average power voltage expression, when the capacitor average power voltage is eliminated, and the suppression of the capacitor voltage ripple can be achieved.

[0088] Step 4, calculate the neutral point voltage and circulating current with the minimum switch loss of the converter as the target.

[0089] Only is not enough to calculate the specific values of the two variables V st and I cir , an additional limiting condition is needed. Switching loss P SW is an important indicator in the inverter that determines the cost of power semiconductors, and the design of V st and I cir parameters in this invention is based on the minimization of switching loss.

[0090] The maximum switching loss of a single semiconductor device is defined by the product of the maximum blocking voltage and the maximum forward current. For Hexverter, it can be estimated according to the maximum branch voltage v b，max and the maximum branch current i b，max . The maximum branch voltage and the maximum branch current of Hexverter can be expressed as

[0091]

[0092] The total switching loss is the sum of the switching losses of all semiconductor devices in the system. For Hexverter, there are 6 branches, each branch has N sub-modules, and each sub-module is composed of 4 switches, so the maximum total switching loss can be expressed as

[0093]

[0094] It can be seen that when the function P SW (|I cir |) is minimized, the circulating current is

[0095]

[0096] Therefore, assuming that the circulating current reference value is greater than zero, the circulating current and neutral point voltage reference values corresponding to the minimum switching loss are

[0097]

[0098] In the formula: "+" corresponds to Q1>Q2, and "-" corresponds to Q1Q2.

[0099] This embodiment builds a simulation model of the Hexverter capacitor voltage fluctuation suppression system in MATLAB / Simulink to verify the effectiveness of the application. The simulation parameters of this embodiment are shown in Table 1.

[0100] Table 1: MATLAB simulation model parameters of the Hexverter capacitor voltage fluctuation suppression system

[0101]

[0102]

[0103] To verify the correction effect of the ripple suppression strategy on the capacitor voltage, the capacitor voltage is analyzed in the time domain. Set t1=0.3s, t2=1s. The system starts running at t1, and the ripple suppression strategy is not used in the period from t1 to t2; the ripple suppression strategy is put into use at t2. The overall capacitor voltage waveform before and after the capacitor voltage ripple suppression strategy is put into use is shown in Figure 2 It can be seen from Figure 2 that when the input / output side reactive power is unbalanced, the capacitor voltage first continuously increases / decreases with time until the ripple suppression strategy is put into use, and then the capacitor voltage gradually returns to the rated value and finally stabilizes around the rated value. This shows that there is a non-zero average power voltage component in the capacitor voltage, and the ripple suppression strategy effectively eliminates this component, proving the effectiveness of the ripple suppression strategy in this paper.

[0104] Figure 3 The local waveform graph of the capacitor voltage after the ripple suppression strategy is put into use is given. It can be seen from Figure 3 that the theoretical calculation and simulation results of each component of the capacitor voltage are consistent within the allowable error range, verifying the correctness of the capacitor voltage formula.

[0105] To analyze the influence of different circulating current reference values on the switching loss, the change of the switching loss when the circulating current value changes is observed, as shown in Figure 4 It can be seen from Figure 4 that the method of the application can minimize the switching loss of the system.

Claims

1. A method for analyzing capacitor voltage fluctuations in a hexagonal modular multi-level AC-AC converter, characterized by: The following steps are involved: Step 1: Modeling a hexagonal modular multi-level AC-AC converter, and obtaining a sub-module capacitance-voltage expression based on the power conservation law and the volt-ampere characteristics of the capacitor; the sub-module is an H-bridge structure, with power electronic switching devices forming bridge arms and capacitors connected across the bridge arms; Step 2: Introduce the neutral point voltage and circulating current, and update the submodule capacitor voltage expression under the condition of the neutral point voltage and circulating current; Step 3: Analyze the updated submodule capacitor voltage expression obtained in step 2 to obtain the technical conditions that the neutral point voltage and circulating current need to meet to suppress the capacitor voltage ripple; the technical conditions that the neutral point voltage and circulating current need to meet to suppress the capacitor voltage ripple are: ,in is the reactive power on the input side, is the reactive power on the output side; Step 4: Calculate the neutral point voltage and circulating current with the goal of minimizing the converter switching loss. When the converter switching loss is minimized, the neutral point voltage and circulating current are: ， Where: "+" corresponds to ,"-"correspond , is the neutral point voltage, For circulation, 、 are the amplitudes of the input phase voltage and current, 、 are the amplitudes of the output phase voltage and current, is the reactive power on the input side, is the reactive power on the output side.

2. The method for analyzing capacitor voltage fluctuations of a hexagonal modular multi-level AC-AC converter according to claim 1, characterized in that: The submodule capacitor voltage expression in step 1 is: ,in, ， Where n is the branch number, is the capacitor voltage of the submodule in branch n, is the average power voltage component, The frequency is The capacitor voltage component, The frequency is The capacitor voltage component, The frequency is The capacitor voltage component, The frequency is The capacitor voltage component; C is the capacitance value, N is the number of submodules, is the capacitor voltage rating, 、 is the active and reactive power on the input side, 、 is the active and reactive power on the output side, 、 are the amplitudes of the input phase voltage and current, 、 are the amplitudes of the output phase voltage and current, and is the angular frequency, and is the power factor angle, Represents the phase difference.

3. The method for analyzing capacitor voltage fluctuations of a hexagonal modular multi-level AC-AC converter according to claim 2, characterized in that: The updated submodule capacitor voltage expression under the conditions of neutral point voltage and circulating current described in step 2 is: ， Where, is the updated submodule capacitor voltage, is the average power voltage component under the conditions of neutral point voltage and circulating current, The frequency added under the condition of neutral point voltage and circulating current is The capacitor voltage component, is the neutral point voltage, For circulation.

4. The method for analyzing capacitor voltage fluctuations of a hexagonal modular multi-level AC-AC converter according to claim 1, characterized in that: The converter switching loss expression is: ,in, ， Where, is the switching loss of the converter, N is the number of submodules, 、 are the amplitudes of the input phase voltage and current, 、 are the amplitudes of the output phase voltage and current, For circulation, is the reactive power on the input side, is the reactive power on the output side.

5. A method for suppressing capacitor voltage fluctuations using the capacitor voltage fluctuation analysis method of a hexagonal modular multi-level AC-AC converter according to claim 1, characterized in that: The neutral point voltage and circulating current are introduced to eliminate the average power voltage component of the capacitor to suppress the ripple voltage. The technical conditions that the neutral point voltage and circulating current need to meet are: ,in is the reactive power on the input side, is the reactive power on the output side.

6. The method for suppressing capacitor voltage fluctuation according to claim 5, wherein: The neutral point voltage and circulating current are calculated with the goal of minimizing the converter switching loss. The neutral point voltage and circulating current are: ， Where: "+" corresponds to ,"-"correspond , is the neutral point voltage, For circulation, 、 are the amplitudes of the input phase voltage and current, 、 are the amplitudes of the output phase voltage and current, is the reactive power on the input side, is the reactive power on the output side.

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