A Loss Reduction Control Method for Medium-Voltage MMC Converter Valves under CPS-PWM Modulation

Abstract

The present invention belongs to the technical field of MMC converter valve loss control, and particularly relates to a loss reduction control method for a medium-voltage MMC converter valve under CPS-PWM modulation, comprising the following steps: S1. Calculate the analytical formula of the arm current considering the second harmonic component for each arm; S2. Based on the arm current analytical formula obtained in S1, construct the loss calculation formula P of each sub-module of each arm SM,i,j ; S3. Based on the loss calculation formula P of the sub-module obtained in S2 SM,i,j , construct a circulating current injection optimal function; the circulating current injection optimal function is used to solve the current reference value of each arm; S4. Solve the circulating current injection optimal function to obtain the current reference value of each arm; S5. Extract the second harmonic component of the current reference value of each arm obtained in S4 as the input quantity of the second harmonic circulating current PR regulator, and inject a second harmonic circulating current compensation amount into the reference voltage of each arm of the MMC converter valve. This method can further reduce the converter valve loss during the operation of the MMC

Description

Technical Field

[0001] The present invention belongs to the technical field of MMC converter valve loss control, and particularly relates to a loss reduction control method for a medium-voltage MMC converter valve under CPS-PWM modulation. Background Technique

[0002] The modular multilevel converter (MMC) has the advantages of good output characteristics, low switching frequency, and easy expansion, and has been widely used in the field of high-voltage direct current (HVDC) at home and abroad. At the same time, it also has broad application prospects in medium-voltage scenarios such as DC distribution networks, photovoltaic energy storage and direct current flexibility, locomotive traction, and ship power systems.

[0003] Reducing the loss of the MMC converter valve is of significant importance for improving the economic operation of the system. During normal operation, its loss depends on the switching frequency of the semiconductor switching device and its arm current. At present, the research focus of the MMC converter valve loss reduction control method mainly focuses on studying new voltage equalization modulation algorithms to reduce the switching frequency of its sub-modules, thereby reducing its switching loss. When the operating conditions of the MMC converter valve are certain, the average switching frequency caused by modulation is a fixed value, and the average switching frequency caused by voltage equalization depends on the voltage equalization strategy of the sub-modules, which is mainly divided into the voltage equalization strategy based on pulse width modulation and the voltage equalization strategy based on the nearest level approximation modulation. However, in medium- and low-voltage application scenarios, the MMC converter valve usually adopts the voltage equalization strategy based on pulse width modulation. Due to the very high carrier frequency of pulse width modulation, it usually causes a large amount of switching loss, and its switching frequency is a fixed value. Another method for the MMC converter valve loss reduction control method is to optimize the arm current. Some literature uses three different sorting algorithms under different power factors to calculate the losses of the MMC converter valve with and without the double-frequency harmonic circulating current elimination strategy. The calculation results show that eliminating the double-frequency harmonic circulating current can reduce its loss. Some literature proposes a closed-loop control strategy based on a proportional-integral controller to eliminate the double-frequency harmonic circulating current. Some literature introduces a repetitive controller to improve the suppression effect of the harmonic circulating current. Some literature proposes a proportional-resonant controller, which can achieve the control of the double-frequency harmonic circulating current in the stationary coordinate system.

[0004] However, the existing literature only qualitatively states that eliminating the second-harmonic circulating current can reduce the losses of the MMC converter valve, without a detailed analysis of the mathematical relationship between its losses and the second-harmonic circulating current. Some literature has proposed a control strategy for injecting the second-harmonic circulating current to reduce the capacitor voltage fluctuation of the sub-modules of the MMC converter valve, but it does not involve reducing its losses by injecting a specific second-harmonic circulating current. Some literature has proposed an efficiency optimization strategy based on injecting the second-harmonic circulating current and derived the analytical expressions for the optimal phase and amplitude of the second-harmonic circulating current. However, there is an error between the theoretical calculated values and the actual values of the optimal amplitude and phase of the second-harmonic circulating current, and a fixed correction value needs to be superimposed on the basis of the theoretical calculated values. This correction value cannot be flexibly adjusted according to the actual operating conditions of the MMC. Therefore, although this technical solution can reduce the losses of the MMC converter valve to a certain extent, the effect of reducing losses is not ideal.

[0005] Therefore, how to further reduce the losses of the converter valve during the operation of the MMC has become an urgent problem to be solved. Summary of the Invention

[0006] In view of the above deficiencies of the existing technology, the present invention provides a loss reduction control method for a medium-voltage MMC converter valve under CPS-PWM modulation, which can further reduce the losses of the converter valve during the operation of the MMC.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A loss reduction control method for a medium-voltage MMC converter valve under CPS-PWM modulation, wherein the MMC converter valve includes three phases a, b, and c, and each phase includes an upper arm p and a lower arm n; each arm includes a plurality of sub-modules; the sub-modules are all half-bridge sub-modules HBSM.

[0009] The method includes the following steps:

[0010] S1. Calculate the analytical formula of the arm current considering the second-harmonic component for each arm;

[0011] S2. Based on the arm current analytical formula obtained in S1, construct the loss calculation formula P SM,i,j for each sub-module of each arm; where i = a, b, c, and j = p, n;

[0012] S3. Based on the loss calculation formula P SM,i,j of the sub-module obtained in S2, construct an optimal function for circulating current injection; the optimal function for circulating current injection is used to solve the current reference value of each arm;

[0013] S4. Solve the optimal function for circulating current injection to obtain the current reference value of each arm;

[0014] S5. Extract the second - harmonic components of the current reference values of each arm obtained in S4 as the input of the second - harmonic circulating - current PR regulator, and inject the second - harmonic circulating - current compensation amount into the reference voltages of each arm of the MMC converter valve.

[0015] Preferably, the upper arm and the lower arm of each phase of the MMC converter valve are symmetric.

[0016] Preferably, by the same method, calculate the analytical expressions of the arm currents considering the second - harmonic components for each arm respectively; among them, the calculation process of the analytical expression of the arm current of the a - phase arm considering the second - harmonic component includes:

[0017] According to Kirchhoff's current law, construct the arm - current equation of phase a, and the arm - current equation includes the current equations of the upper arm and the lower arm.

[0018] Based on the charging and discharging process of the sub - module capacitors, obtain the output - voltage equations of the upper arm and the lower arm of phase a.

[0019] Substitute the current equations of the upper arm and the lower arm of phase a into the corresponding output - voltage equations to obtain the detailed output - voltage equations of the upper arm and the lower arm of phase a, and process to obtain the common - mode voltage in phase a: the common - mode voltage only contains the second - harmonic component, and the fundamental - frequency component and the third - harmonic component are 0.

[0020] Based on the common - mode voltage in phase a, after correcting the arm - current equation of phase a, substitute the corrected arm - current equation of phase a into the output - voltage equations of the upper arm and the lower arm of phase a; and repeat the above process, and calculate by the iterative method to obtain the analytical expression of the arm current of phase a considering the second - harmonic component.

[0021] Preferably, in S1, the constructed current equation of phase a is:

[0022]

[0023] where, I dc is the DC current, I ac is the AC - current amplitude, ω is the fundamental - frequency angular frequency, is the current phase angle and let the current phase angle be 0.

[0024] Preferably, in S1, the output - voltage equations of the upper arm and the lower arm of phase a are:

[0025]

[0026] where, U dc is the DC - bus voltage, N is the number of sub - modules in the arm, C SM is the sub - module capacitance value; j = p, n; S a,j is the average switching function of the upper arm and the lower arm of phase a.

[0027]

[0028] Wherein, m is the modulation ratio.

[0029] Preferably, in S1, after substituting the current equations of the upper and lower arms of phase a into its output voltage equation, the detailed output voltage equations of the upper and lower arms of phase a are as follows:

[0030]

[0031] Wherein, u diff,a,1 is the fundamental frequency component of the differential mode voltage, u diff,a,3 is the triple frequency component of the differential mode voltage, u com,a,2 is the double frequency component of the common mode voltage, and its analytical expression is:

[0032]

[0033] Preferably, in S1, the common mode voltage in phase a obtained by processing is:

[0034]

[0035] Based on the common mode voltage in phase a, the phase a bridge arm current is corrected to:

[0036]

[0037] Wherein, represents the phase a bridge arm current before correction; and:

[0038]

[0039] Wherein, L arm is the arm reactance.

[0040] Preferably, in S2, by the same method, the loss calculation formula P SM,i,j of each sub-module is constructed respectively; among them, the loss calculation formula of the sub-module of the phase a bridge arm is:

[0041]

[0042] Wherein, I i,j,RMS is the effective value of the bridge arm current, I i,j,ABSAVE is the average value of the device current; the coefficients K2, K1 and K0 are respectively:

[0043]

[0044] Wherein, r is the larger value of the on-state resistances of the IGBT and the anti-parallel diode, and U0 is the larger value of the on-state voltage drops of the IGBT and the anti-parallel diode; a on,2 、a on,1 、a on,0 are the polynomial fitting coefficients of the IGBT turn-on energy; a off,2 、a off,1 、a off,0 are the polynomial fitting coefficients of the IGBT turn-off energy; a rec,2 、a rec,1 、a rec,0 are the polynomial fitting coefficients of the diode reverse recovery energy; f c is the carrier frequency; U SM is the sub-module voltage value, and U CE,ref is the reference cut-off voltage.

[0045] Preferably, in S3, the constructed optimal function of the circulating current injection is:

[0046]

[0047] Wherein:

[0048]

[0049] In the formula, i i,j,c (t) is the arm current after injecting the circulating current.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] 1. When the MMC operates normally, its loss depends on the switching frequency of the semiconductor switching device and its arm current. Reducing the switching frequency of the semiconductor switching device can reduce the switching loss, but under CPS-PWM modulation, its switching frequency is a fixed value, while optimizing the arm current can reduce both the on-state loss and the switching loss. The inventor of the present invention studied the mathematical relationship between the loss of the MMC converter valve and its second-harmonic circulating current component based on the simplified loss calculation model of the MMC converter valve. And on this basis, based on the loss calculation formula P SM,i,j of the sub-module obtained in S2, an optimal function of the circulating current injection is constructed; then, the reference values of the arm currents are obtained by solving the optimal injection function, and the second-harmonic component of the current reference value is extracted as the input of the second-harmonic circulating current PR regulator, and a second-harmonic circulating current compensation amount is injected into the reference voltage of each arm of the MMC converter valve. In this way, the injected second-harmonic circulating current compensation amount directly comes from the mathematical relationship between the loss of the MMC converter valve and its second-harmonic circulating current component. Compared with the prior art, it is more flexible to use and has better effects.

[0052] In summary, based on the existing technology, this method can further reduce the commutation valve losses during the operation of the MMC.

[0053] 2. Compared with the existing technology that superimposes a fixed correction value on the basis of the theoretical calculation value, since the injected double-frequency circulating current compensation amount of this method directly comes from the mathematical relationship between the MMC commutation valve loss and its double-frequency circulating current component, it can be used under different operating conditions, and can maximize the reduction of its loss when the switching frequency of the MMC commutation valve is fixed under various conditions, improve the overall operating efficiency of the MMC, and enhance the economic efficiency and reliability of the system operation.

[0054] 3. This method provides a specific solution method for the arm current analytical formula considering the double-frequency component of each arm. Using this method, the arm current analytical formula considering the double-frequency component of each arm calculated by this method can be accurately and stably solved. Based on this arm current analytical formula, the effectiveness of the reference value of the arm current to be solved subsequently can be ensured, thereby ensuring the effectiveness of the injected double-frequency circulating current compensation amount. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to make the objectives, technical solutions, and advantages of the invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings, where:

[0056] Figure 1 is a schematic circuit topology diagram of the MMC commutation valve in the embodiment;

[0057] Figure 2 is a flow chart in the embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0058] The following will be further described in detail through specific embodiments:

[0059] Embodiment:

[0060] The schematic circuit topology diagram of the MMC commutation valve is as Figure 1 shown. The MMC commutation valve includes three phases a, b, and c (i.e., Figure 1 Phase A, Phase B, and Phase C in it). Each of the three phases includes an upper arm p and a lower arm n, and the upper arm and the lower arm of each phase of the MMC commutation valve are symmetric; each arm includes a plurality of sub-modules, and the sub-modules are all half-bridge sub-modules HBSM; each sub-module includes a diode, a capacitor, and an IGBT. In this embodiment, the object of use of this method is the MMC using CPS-PWM modulation in medium and low voltage scenarios. It should be noted that in the embodiment, the individual a, b, and c in the lower subscript of the parameter are the corresponding phases, and the individual p and n in the subscript of the parameter are the corresponding arms.

[0061] As Figure 2As shown in the figure, the method includes the following steps:

[0062] S1. Calculate the analytical expressions of the arm currents considering the second harmonic components for each arm.

[0063] In specific implementation, the analytical expressions of the arm currents considering the second harmonic components are calculated for each arm by the same method. Among them, the calculation process of the analytical expression of the arm current of the a-phase arm considering the second harmonic component includes:

[0064] According to Kirchhoff's current law, when the upper and lower arms of the MMC three-phase unit are symmetric, a current equation for the a-phase is constructed:

[0065]

[0066] In the formula, I dc is the DC current, I ac is the amplitude of the AC current, ω is the fundamental angular frequency, is the current phase angle, and the current phase angle is set to 0;

[0067] Based on the charging and discharging process of the sub-module capacitor, the output voltage equations of the upper and lower arms of the a-phase are obtained:

[0068]

[0069] In the formula, U dc is the DC bus voltage, N is the number of sub-modules in the arm, C SM is the sub-module capacitance value (j = p, n); S a,j is the average switching function of the upper and lower arms of the a-phase;

[0070]

[0071] In the formula, m is the modulation ratio;

[0072] Substitute the current equations of the upper and lower arms of the a-phase into their output voltage equations to obtain the detailed output voltage equations of the upper and lower arms of the a-phase as:

[0073]

[0074] Among them, u diff,a,1 is the fundamental component of the differential-mode voltage, u diff,a,3 is the third harmonic component of the differential-mode voltage, u com,a,2 is the second harmonic component of the common-mode voltage, and its analytical expression is:

[0075]

[0076] Based on the above formula, the common-mode voltage in the a-phase is obtained:

[0077]

[0078] Based on the common-mode voltage in phase a, the phase-a leg current is corrected to:

[0079]

[0080] In the formula, represents the phase-a leg current before correction; and:

[0081]

[0082] In the formula, L arm is the leg reactance;

[0083] Substitute the corrected phase-a leg current into the output voltage equations of the upper and lower legs of phase a, and repeat the above process. Through the iterative method, the analytical formula of the leg current containing only the double-frequency circulating current in phase a is obtained. The fundamental-frequency component and the triple-frequency component of the circulating current in the phase-a leg current equation are 0, and only the double-frequency component is included (neglecting the high-order harmonics of the fourth order and above).

[0084] S2. Based on the analytical formula of the leg current obtained in S1, construct the loss calculation formula P of each sub-module of each leg SM,i,j ; where, i = a, b, c, j = p, n.

[0085] In specific implementation, through the same method, the loss calculation formulas P of each sub-module are respectively constructed SM,i,j . Specifically, adopt the simplified calculation model of the MMC loss with CPS-PWM modulation. For the sake of conservative reliability analysis, the on-state loss calculation coefficients of IGBT and diode take the larger value of the fitting coefficients of the two. Among them, the loss calculation formula of the sub-module of the phase-a leg constructed is:

[0086]

[0087] In the formula, I i,j,RMS is the effective value of the leg current, and I i,j,ABSAVE is the average value of the device current; the coefficients K2, K1, and K0 are respectively:

[0088]

[0089] In the formula, r is the larger value of the on-state resistances of IGBT and the anti-parallel diode, and U0 is the larger value of the on-state voltage drops of IGBT and the anti-parallel diode; a on,2 , a on,1 , a on,0 are the polynomial fitting coefficients of the IGBT turn-on energy; a off,2 , a off,1 , a off,0is the polynomial fitting coefficient of the IGBT turn-off energy; a rec,2 、a rec,1 、a rec,0 are the polynomial fitting coefficients of the diode reverse recovery energy; f c is the carrier frequency; U SM is the sub-module voltage value, U CE,ref is the reference cut-off voltage.

[0090] S3. Based on the sub-module loss calculation formula P SM,i,j obtained in S2, construct the optimal function of circulating current injection; the optimal function of circulating current injection is used to solve the current reference values of each arm.

[0091] Specifically in implementation, the constructed optimal function of circulating current injection is:

[0092]

[0093] Where:

[0094]

[0095] In the formula, i i,j,c (t) is the arm current after injecting the circulating current.

[0096] S4. Solve the optimal function of circulating current injection to obtain the current reference values of each arm.

[0097] S5. Extract the second harmonic component of the current reference values of each arm obtained in S4 as the input of the second harmonic circulating current PR regulator, and inject the second harmonic circulating current compensation amount into the reference voltage of each arm of the MMC converter valve.

[0098] When the MMC operates normally, its loss depends on the switching frequency of the semiconductor switching device and the current of its arm. Reducing the switching frequency of the semiconductor switching device can reduce the switching loss, but under CPS-PWM modulation, its switching frequency is a fixed value, while optimizing the arm current can reduce both the conduction loss and the switching loss. The inventor of the present invention studied the mathematical relationship between the loss of the MMC converter valve and its second harmonic circulating current component based on the simplified loss calculation model of the MMC converter valve. And on this basis, based on the sub-module loss calculation formula P SM,i,j obtained in S2, construct the optimal function of circulating current injection; then, solve the optimal injection function to obtain the current reference values of each arm, and extract the second harmonic component of the current reference values as the input of the second harmonic circulating current PR regulator, and inject the second harmonic circulating current compensation amount into the reference voltage of each arm of the MMC converter valve. In this way, the injected second harmonic circulating current compensation amount directly comes from the mathematical relationship between the loss of the MMC converter valve and its second harmonic circulating current component. Compared with the prior art, it is more flexible to use and has better effects.

[0099] In addition, compared with the prior art where a fixed correction value is superimposed on the theoretically calculated value, since the injected double-circulation compensation amount in this method directly comes from the mathematical relationship between the MMC converter valve loss and its double-frequency circulation component, it can be used under different operating conditions. Moreover, under various conditions, when the switching frequency of the MMC converter valve is fixed, it can minimize the loss to the greatest extent, improve the overall operating efficiency of the MMC, and enhance the operating economy and reliability of the system. Furthermore, this method provides a specific solution method for the arm current analytical formula considering the double-frequency component of each arm. By using this method, the arm current analytical formula considering the double-frequency component of each arm calculated by this method can be accurately and stably solved. Based on this arm current analytical formula, the effectiveness of the reference value of the arm current to be solved subsequently can be ensured, thereby ensuring the effectiveness of the injected double-frequency circulation compensation amount.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Those of ordinary skill in the art should understand that any modifications or equivalent replacements made to the technical solutions of the present invention without departing from the purpose and scope of the present technical solution shall be covered by the scope of the claims of the present invention.

Claims

1. A loss reduction control method for a medium - voltage MMC converter valve under CPS - PWM modulation, characterized in that, The MMC converter valve includes three phases a, b, and c, and each phase includes an upper arm p and a lower arm n; each arm includes a plurality of sub-modules; the sub-modules are all half-bridge sub-modules HBSM; The method includes the following steps: S1. Calculate the analytical formula of the arm current considering the second harmonic component for each arm; S2. Based on the bridge arm current analytical formula obtained in S1, construct the loss calculation formula \(P\) of each sub-module of each bridge arm SM,i,j ; where \(i = a, b, c\) and \(j = p, n\); S3. Based on the loss calculation formula P of the sub-module obtained in S2 SM,i,j , construct an optimal function for circulating current injection; the optimal function for circulating current injection is used to solve the current reference value of each bridge arm; S4. Solve the optimal function of the circulating current injection to obtain the current reference values of each arm; S5. Extract the second harmonic components of the current reference values of each arm obtained in S4 as the input of the second harmonic circulating current PR regulator, and inject the second harmonic circulating current compensation amount into the reference voltage of each arm of the MMC converter valve; Among them, in S2, by the same method, the loss calculation formula P of each sub-module is constructed respectively SM,i,j ; among them, the loss calculation formula of the sub-module of the a-phase bridge arm is: Where, I i,j,RMS is the effective value of the arm current, and I i,j,ABSAVE is the average value of the device current; the coefficients K2, K1, and K0 are respectively: where r is the larger value of the on-state resistances of the IGBT and the anti-parallel diode, and U0 is the larger value of the on-state voltage drops of the IGBT and the anti-parallel diode; a on,2 、a on,1 、a on,0 are the polynomial fitting coefficients of the IGBT turn-on energy; a off,2 、a off,1 、a off,0 are the polynomial fitting coefficients of the IGBT turn-off energy; a rec,2 、a rec,1 、a rec,0 are the polynomial fitting coefficients of the diode reverse recovery energy; f c is the carrier frequency; U SM is the sub-module voltage value, and U CE,ref is the reference cut-off voltage; In S3, the constructed optimal function of the circulating current injection is: Where: where i i,j,c (t) is the arm current after injecting the circulating current.

2. The loss reduction control method for a medium - voltage MMC converter valve under CPS - PWM modulation according to claim 1, characterized in that: The upper arms and lower arms of each phase of the MMC converter valve are symmetrical.

3. The loss reduction control method for a medium - voltage MMC converter valve under CPS - PWM modulation according to claim 2, characterized in that: In S1, by the same method, calculate the analytical formula of the arm current considering the second harmonic component for each arm respectively; among them, the calculation process of the analytical formula of the arm current of phase a considering the second harmonic component includes: According to Kirchhoff's current law, construct the arm current equation of phase a, and the arm current equation includes the current equations of the upper arm and the lower arm; Based on the charging and discharging process of the sub-module capacitor, obtain the output voltage equations of the upper arm and the lower arm of phase a; Substitute the current equations of the upper arm and the lower arm of phase a into the corresponding output voltage equations to obtain the detailed output voltage equations of the upper arm and the lower arm of phase a, and process to obtain the common-mode voltage in phase a: the common-mode voltage only contains the second harmonic component, and the fundamental frequency component and the third harmonic component are 0; Based on the common-mode voltage in phase a, after correcting the arm current equation of phase a, substitute the corrected arm current equation of phase a into the output voltage equations of the upper arm and the lower arm of phase a; and repeat the above process, and calculate by the iterative method to obtain the analytical formula of the arm current of phase a considering the second harmonic component.

4. The loss reduction control method for a medium - voltage MMC converter valve under CPS - PWM modulation according to claim 3, characterized in that: In S1, the constructed current equation of phase a is: Where, I dc is the direct current, I ac is the amplitude of the alternating current, ω is the fundamental angular frequency, is the current phase angle and let the current phase angle be 0.

5. The loss reduction control method for a medium - voltage MMC converter valve under CPS - PWM modulation according to claim 4, characterized in that: In S1, the output voltage equations of the upper arm and the lower arm of phase a are: Where, U dc is the DC bus voltage, N is the number of sub-module in the bridge arm, C SM is the capacitance value of the sub-module; j = p, n; S a,j is the average switching function of the upper and lower bridge arms of phase a; In the formula, m is the modulation ratio.

6. The loss reduction control method of the medium-voltage MMC converter valve under CPS-PWM modulation according to claim 5, wherein: In S1, after substituting the current equations of the upper arm and the lower arm of phase a into their output voltage equations, the detailed output voltage equations of the upper arm and the lower arm of phase a obtained are: where u diff,a,1 is the fundamental frequency component of the differential-mode voltage, u diff,a,3 is the third harmonic component of the differential-mode voltage, u com,a,2 is the second harmonic component of the common-mode voltage, and its analytical expression is:

7. The loss reduction control method of the medium-voltage MMC converter valve under CPS-PWM modulation according to claim 6, wherein: In S1, the common-mode voltage in phase a obtained by processing is: Based on the common-mode voltage in phase a, the arm current of phase a is corrected to: Wherein, represents the phase-a bridge arm current before correction; And: where L arm is the arm reactance.

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