MMC (Modular Multilevel Converter) system circulating current suppression technology optimization method

Abstract

The invention relates to an MMC system circulation suppression technology optimization method, the MMC system comprises a direct current module, a current converter and an alternating current module, the current converter comprises a plurality of sub-modules with half-bridge structures, and each half-bridge structure is composed of an IGBT and a diode; according to the MMC circulating current suppression technology optimization method, the circulating current suppression technology of an MMC system is evaluated based on electric energy quality and junction temperature data of an IGBT and a diode in an MMC sub-module, and the circulating current suppression technology is optimized according to an evaluation result. Compared with the prior art, the method comprehensively considers the influence of a plurality of influence factors on circulation suppression, thereby providing a more comprehensive optimization scheme.

Description

Technical Field

[0001] The invention relates to the field of power electronics, and in particular to an optimization method for circulating current suppression technology of an MMC system. Background Art

[0002] The circulating harmonics of modular multilevel converter (MMC) deteriorate the power quality and affect the stability of system operation. To this end, a series of circulating harmonic suppression technologies have been developed, but how to scientifically evaluate the suppression effect and select the adaptation technology still needs further research. The reason is that the existing evaluation methods mostly use the degree of improvement in power quality as an indicator. However, as the power of the converter increases, the heat loss of the insulated-gate bipolar transistor (IGBT) increases, becoming another important factor affecting the stability of system operation. The existing harmonic suppression technology only focuses on power quality, ignoring the impact of other important factors on the circulating harmonic suppression technology, resulting in incomplete circulating harmonic suppression technology. Summary of the invention

[0003] The purpose of the present invention is to provide a method for optimizing the circulation suppression technology of an MMC system in order to overcome the defects of the above-mentioned prior art.

[0004] In view of the demand for circulating current harmonic suppression technology and the requirement to master the influence of harmonic suppression on IGBT heat loss, it is urgent to develop a prediction method for the influence of circulating current suppression on IGBT junction temperature fluctuation in MMC system, so as to provide technical support for the comprehensive evaluation of circulating current harmonic suppression effect of modular multilevel converter. The establishment and development of this technology will promote the scientific formulation of circulating current suppression strategy and improve the efficient and stable operation capability of converter under the trend of high proportion of power electronics penetration in new power system.

[0005] The above-mentioned MMC system includes a DC module, a current converter and an AC module. The current converter includes a plurality of submodules with a half-bridge structure, and the half-bridge structure is composed of an IGBT and a diode.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A method for optimizing circulating current suppression technology of an MMC system is disclosed. The method optimizes the circulating current suppression technology of the MMC system based on power quality and junction temperature data of IGBTs and diodes in MMC submodules.

[0008] As an optimal technical solution, the method includes: constructing an MMC circulating current model and performing circulating current suppression on the MMC circulating current model; observing the harmonic distribution of the bridge arm current and calculating the total harmonic distortion value of the MMC bridge arm current based on the power quality; calculating the power losses of the MMC sub-module IGBT and diode respectively, and calculating the junction temperature data of the IGBT and diode based on the calculated power losses; and improving the circulating current technology in combination with the power quality and junction temperature data.

[0009] As a preferred technical solution, the MMC circulation model is:

[0010]

[0011] Where j∈{a,b,c} represents the three phases in the MMC system, i cirj represents the j-phase circulation, i jp represents the upper bridge arm current of phase j, i jl represents the lower bridge arm current of phase j, u cirj represents the j-phase circulating current voltage drop, L0 represents the bridge arm inductance, R0 represents the bridge arm resistance, U dc is the DC side voltage, u jp represents the voltage of the upper bridge arm of phase j, u jl Represents the j-phase lower bridge arm voltage.

[0012] As a preferred technical solution, the method for observing the harmonic distribution of the bridge arm current is to perform fast Fourier analysis on the MMC bridge arm current after circulating current suppression, and the expression is:

[0013]

[0014] Among them, i cirj (n) is the discrete bridge arm current of phase j in the MMC after sampling; N is the number of sampling points of bridge arm current in time domain; n is the sequence number of sampling points of bridge arm current in time domain; I cirj (k) is the discrete bridge arm current of phase j in the frequency domain MMC; k is the discrete bridge arm current point number in the frequency domain; is the rotation factor.

[0015] As a preferred technical solution, the method for calculating the total harmonic distortion value of the MMC bridge arm current is specifically: calculating the total harmonic distortion of the MMC bridge arm current based on the effective value of the current and the effective value of the current fundamental wave respectively.

[0016] As a preferred technical solution, the method for calculating the power loss of the MMC sub-module IGBT and the diode is specifically: calculating the power loss of the MMC sub-module IGBT based on the MMC sub-module conduction loss and the switching loss; calculating the diode power loss based on the diode conduction loss and the diode reverse recovery loss.

[0017] As a preferred technical solution, the calculation of the junction temperature data of the IGBT and the diode specifically includes the following steps:

[0018] A multi-order Foster thermal network model of the MMC sub-module IGBT is constructed to calculate the junction temperature data of the IGBT. The expression of the multi-order Foster thermal network model is:

[0019]

[0020] Among them, Z th_IGBT (t) is the thermal impedance, P is the total order of the Foster thermal network model, R th_i_IGBT is the equivalent resistance, C th_i_IGBT is the equivalent thermal capacitance; T c (t) is the ambient temperature, P IGBT is the total power loss of the MMC submodule IGBT, T j_IGBT (t) is the IGBT junction temperature;

[0021] A multi-order Foster thermal network model of the MMC sub-module diode is constructed to calculate the junction temperature data of the diode. The expression of the multi-order Foster thermal network model is:

[0022]

[0023] Among them, Z th_Diode (t) is thermal impedance, R th_i_Diode is the equivalent resistance, C th_i_Diode is the equivalent thermal capacitance, T c (t) is the ambient temperature, P Diode is the total power loss of the diode, T j_Diode (t) is the diode junction temperature.

[0024] As an optimal technical solution, the method for optimizing the circulating current technology is: statistically analyzing the junction temperature data of the IGBT and the diode to obtain the average junction temperature amplitude; obtaining the total harmonic distortion value of the MMC bridge arm current; if the total harmonic distortion value of the MMC bridge arm current is greater than or equal to the value specified by the grid-connected requirement, continue to optimize the circulating current suppression technology with the power quality as the optimization index; if the total harmonic distortion value of the MMC bridge arm current is less than the value specified by the grid-connected requirement, then the circulating current suppression technology is thermally balanced optimized with the average junction temperature amplitude as the main optimization index, and during the optimization, the total harmonic distortion value of the MMC bridge arm current needs to be maintained less than the value specified by the grid-connected requirement.

[0025] As a preferred technical solution, the grid connection requirement is specified to be no greater than 3%.

[0026] As a preferred technical solution, the MMC system circulating current suppression technology optimization method is applied to the flexible direct current transmission scenario.

[0027] Compared with the prior art, the present invention quantifies the relationship between circulating current suppression and IGBT heat loss by calculating the junction temperature data of the IGBT and the diode in the MMC sub-module, and adds the junction temperature data of the IGBT and the diode in the MMC sub-module as an auxiliary optimization indicator on the basis of the traditional indicator of only power quality, providing a more comprehensive reference solution for the optimization of circulating current suppression technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a flow chart of the MMC system circulation suppression technology optimization method of the present invention;

[0029] Figure 2 This is a structural diagram of the MMC system of the present invention;

[0030] Figure 3 The effect diagram of circulating current suppression of the MMC system based on the optimization condition of power quality according to the present invention; (3a) is the effect diagram before circulating current suppression; (3b) is the effect diagram after circulating current suppression;

[0031] Figure 4 4 is a diagram showing the effect of MMC system circulation suppression based on the junction temperature data as the optimization condition of the present invention; (4a) a comparison diagram of the effect before and after IGBT circulation suppression; (4b) a comparison diagram of the effect before and after diode circulation suppression. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0033] Reference to "embodiments" in this application means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those of ordinary skill in the art that the embodiments described in this application may be combined with other embodiments without conflict.

[0034] Embodiment 1:

[0035] This embodiment provides a method for optimizing circulating current suppression technology of an MMC system. The method is used in a flexible direct current transmission scenario and optimizes the circulating current suppression technology of the MMC system based on power quality and junction temperature data of IGBTs and diodes in MMC submodules.

[0036] The above-mentioned MMC system includes a DC module, a current converter and an AC module. The current converter includes a plurality of submodules with a half-bridge structure, and the half-bridge structure is composed of an IGBT and a diode.

[0037] A method for optimizing circulating current suppression technology of MMC system, the flow chart of which is as follows: Figure 1 As shown, the following steps are included:

[0038] S1. Construct an MMC circulation model and suppress the circulation of the MMC circulation model, where the MMC circulation model is:

[0039]

[0040] Where j∈{a,b,c} represents the three phases in the MMC system, i cirj represents the j-phase circulation, i jp represents the upper bridge arm current of phase j, i jl represents the lower bridge arm current of phase j, u cirj represents the j-phase circulating current voltage drop, L0 represents the bridge arm inductance, R0 represents the bridge arm resistance, U dc is the DC side voltage, u jp represents the voltage of the upper bridge arm of phase j, u jl Represents the j-phase lower bridge arm voltage.

[0041] S2. Observe the harmonic distribution of the bridge arm current and calculate the total harmonic distortion value of the MMC bridge arm current based on the power quality:

[0042] S21. Perform fast Fourier analysis on the MMC bridge arm current to observe the bridge arm current harmonic distribution, and the expression is:

[0043]

[0044] Among them, i cirj (n) is the discrete bridge arm current of phase j in the MMC after sampling; N is the number of sampling points of bridge arm current in time domain; n is the sequence number of sampling points of bridge arm current in time domain; I cirj (k) is the discrete bridge arm current of phase j in the frequency domain MMC; k is the discrete bridge arm current point number in the frequency domain; is the rotation factor;

[0045] S22. Based on the result obtained in S21 and the power quality, calculate the total harmonic distortion value THD of the MMC bridge arm current. The expression is:

[0046]

[0047] Among them, Q j Indicates the effective value of the j-phase bridge arm current in the MMC; Q j1 It represents the fundamental effective value of the j-phase bridge arm current in the MMC.

[0048] S3. Calculate the junction temperature data of IGBT and diode:

[0049] S31, respectively calculating the power losses of the MMC submodule IGBT and the diode, wherein the power loss of the MMC submodule IGBT is calculated based on the conduction loss and the switching loss of the MMC submodule, and the power loss of the diode is calculated based on the conduction loss and the reverse recovery loss of the diode. The relevant expressions are:

[0050] P IGBT =P cond_I +P sw ,

[0051] P Diode =P cond_D +P rec ,

[0052] Among them, P IGBT Represents the total power loss of the MMC submodule IGBT, P Diode Represents the total power loss of the MMC submodule diode, P cond_I Represents the conduction loss of the MMC submodule, P sw represents the switching loss of the MMC submodule, P cond_D Represents the diode conduction loss, P rec Represents the diode reverse recovery loss.

[0053] S32. Based on the IGBT power loss, a 4th-order Foster thermal network model is constructed to calculate the junction temperature data of the IGBT, where the expression of the multi-order Foster thermal network model is:

[0054]

[0055] Among them, Z th_IGBT (t) is the thermal impedance R th_i_IGBT is the equivalent resistance, C th_i_IGBT is the equivalent thermal capacitance; T c (t) is the ambient temperature, P IGBT is the total power loss of the MMC submodule IGBT, T j_IGBT (t) is the IGBT junction temperature;

[0056] S33. Based on the diode power loss, a multi-order Foster thermal network model of the diode is constructed to calculate the junction temperature data of the diode. The expression of the 4th-order Foster thermal network model is:

[0057]

[0058] Among them, Z th_Diode (t) is thermal impedance, R th_i_Diodeis the equivalent resistance, C th_i_Diode is the equivalent thermal capacitance, T c (t) is the ambient temperature, P Diode is the total power loss of the diode, T j_Diode (t) is the diode junction temperature.

[0059] S4. Improve the circulation technology based on power quality and junction temperature data:

[0060] S41, statistically analyzing the junction temperature data of the IGBT (including the diode) chip before and after the circulating current suppression, and obtaining the average junction temperature amplitude;

[0061] S42, obtaining the total harmonic distortion value of the MMC bridge arm current;

[0062] S43. If the total harmonic distortion value of the MMC bridge arm current is greater than or equal to 3%, continue to optimize the circulating current suppression technology with the power quality as the optimization index, and repeat steps S2-S42;

[0063] S44. If the total harmonic distortion value of the MMC bridge arm current is less than 3%, the circulating current suppression technology is thermally optimized with the average junction temperature amplitude as the main optimization indicator, and during the optimization, the total harmonic distortion value of the MMC bridge arm current must be maintained less than the value specified by the grid-connected requirements.

[0064] Embodiment 2:

[0065] This embodiment uses an MMC system circulation suppression technology optimization method provided in Example 1 to target Figure 2 The MMC system shown in the figure performs circulating current suppression technology evaluation and optimization, and in this embodiment, the MMC system specifically includes: a modular multilevel converter, a dual closed-loop controller, a circulating current suppressor and a capacitor voltage balancing controller, wherein the system capacity is 3MW, the number of bridge arm sub-modules is 22, the AC frequency is 50Hz, the AC measuring voltage is 6.6kV, the DC side voltage is 11kV, and the sub-module IGBT model is TIM1200DDM17.

[0066] The method flow of the above embodiment is used to perform circulation technology evaluation and optimization, including the following steps:

[0067] S1. Obtain the MMC circulation mathematical model and put the circulation suppressor into use;

[0068] S2. Observe the harmonic distribution of the bridge arm current and calculate the total harmonic distortion value of the MMC bridge arm current based on the power quality, which can be obtained as follows: Figure 3 The effect diagram shown, where Figure (3a) is the effect diagram before circulation suppression, and Figure (3b) is the effect diagram after circulation suppression:

[0069] S21. Perform fast Fourier analysis on the MMC bridge arm current before and after circulating current suppression to observe the harmonic distribution of the bridge arm current. The effect is shown in Figure (3a) and the part framed by solid lines in Figure (3b):

[0070] S22, based on the result obtained in S21 and the power quality, calculate the total harmonic distortion value THD of the MMC bridge arm current before and after circulating current suppression;

[0071] Referring to Figure (3a) and the dotted-line framed portion in Figure (3b), it can be seen that before the circulating current suppression, THD = 19.3% and its frequency distribution is uneven, and after the circulating current suppression, THD = 1.42%, which is 92.64% better than the THD before the circulating current suppression. It can be concluded that the circulating current suppressor effectively improves the power quality of the MMC system;

[0072] S3. Calculate the junction temperature data of the IGBT and diode before and after circulating current suppression:

[0073] S31. Calculate the power loss of MMC submodule IGBT and diode respectively:

[0074] S32, constructing a 4th-order Foster thermal network model based on IGBT power loss to calculate the junction temperature data of the IGBT corresponding to each time point in a certain time period, and generating an effect diagram as shown in FIG (4a), wherein the solid line represents after the circulating current is suppressed, and the dotted line represents before the circulating current is suppressed;

[0075] S33, constructing a fourth-order Foster thermal network model of the diode based on the diode power loss to calculate the junction temperature data of the diode corresponding to each time point in a certain time period, and generating an effect diagram as shown in FIG (4b), wherein the solid line represents after the circulating current is suppressed, and the dotted line represents before the circulating current is suppressed;

[0076] S34. Based on the calculation structure of S32 and S33, the average junction temperature fluctuation values ​​of the IGBT and the diode are obtained respectively, among which the temperature fluctuation of the upper tube of the IGBT before circulation suppression is 8.22℃, and the temperature fluctuation of the lower tube is 40.84℃; the temperature fluctuation of the upper tube of the IGBT after circulation suppression is 4.54℃, and the temperature fluctuation of the lower tube is 47.95℃; the temperature fluctuation of the upper tube of the diode before circulation suppression is 15.23℃, and the temperature fluctuation of the lower tube is 1.87℃; the temperature fluctuation of the upper tube of the diode after circulation suppression is 13.42℃, and the temperature fluctuation of the lower tube is 0.69℃; it can be seen that after the circulation suppressor is put into operation, the temperature fluctuation of the upper tube of the MMC sub-module IGBT and the upper and lower tubes of the diode is reduced, but the temperature fluctuation of the lower tube of the IGBT is increased.

[0077] In summary, although the circulating current suppressor effectively improves the power quality, it imposes a burden on the IGBT in the MMC sub-module. Therefore, the thermal balance optimization of the circulating current suppressor should be carried out with the average junction temperature amplitude as the main optimization indicator, and the total harmonic distortion value of the MMC bridge arm current should be maintained less than the specified value required by the grid during the optimization.

[0078] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for optimizing circulating current suppression technology of an MMC system, wherein the MMC system comprises a DC module, a current converter and an AC module, wherein the current converter comprises a plurality of submodules having a half-bridge structure, wherein the half-bridge structure is composed of an IGBT and a diode, and wherein: This method optimizes the MMC system circulating current suppression technology based on the power quality and the junction temperature data of the IGBT and the diode in the MMC submodule.

2. The MMC system circulation suppression technology optimization method according to claim 1 is characterized in that: The method includes: Construct the MMC circulation model and perform circulation suppression on the MMC circulation model; Observe the harmonic distribution of the bridge arm current and calculate the total harmonic distortion value of the MMC bridge arm current based on the power quality; Calculate the power loss of the IGBT and diode of the MMC submodule respectively, and calculate the junction temperature data of the IGBT and diode according to the calculated power loss; Improve the circulation technology by combining power quality and junction temperature data.

3. The MMC system circulation suppression technology optimization method according to claim 2 is characterized in that: The MMC circulation model is: Where j∈{a,b,c} represents the three phases in the MMC system, i cirj represents the j-phase circulation, i jp represents the upper bridge arm current of phase j, i jl represents the lower bridge arm current of phase j, u cirj represents the j-phase circulating current voltage drop, L0 represents the bridge arm inductance, R0 represents the bridge arm resistance, U dc is the DC side voltage, u jp represents the voltage of the upper bridge arm of phase j, u jl Represents the j-phase lower bridge arm voltage.

4. The MMC system circulation suppression technology optimization method according to claim 2 is characterized in that: The method for observing the harmonic distribution of the bridge arm current is to perform fast Fourier analysis on the MMC bridge arm current after circulating current suppression, and the expression is: Among them, i cirj (n) is the discrete bridge arm current of phase j in the MMC after sampling; N is the number of sampling points of bridge arm current in time domain; n is the sequence number of sampling points of bridge arm current in time domain; I cirj (k) is the discrete bridge arm current of phase j in the frequency domain MMC; k is the discrete bridge arm current point number in the frequency domain; is the rotation factor.

5. The MMC system circulation suppression technology optimization method according to claim 2 is characterized in that: The method for calculating the total harmonic distortion value of the MMC bridge arm current is specifically as follows: the total harmonic distortion of the MMC bridge arm current is calculated based on the effective value of the current and the effective value of the fundamental wave of the current.

6. The MMC system circulation suppression technology optimization method according to claim 2 is characterized in that: The method for calculating the power loss of the MMC submodule IGBT and the diode is specifically as follows: the power loss of the MMC submodule IGBT is calculated based on the conduction loss and the switching loss of the MMC submodule; and the power loss of the diode is calculated based on the conduction loss and the reverse recovery loss of the diode.

7. The MMC system circulation suppression technology optimization method according to claim 2 is characterized in that: The calculation of the junction temperature data of the IGBT and the diode specifically includes the following steps: A multi-order Foster thermal network model of the MMC sub-module IGBT is constructed to calculate the junction temperature data of the IGBT. The expression of the multi-order Foster thermal network model is: Among them, Z th_IGBT (t) is the thermal impedance, P is the total order of the Foster thermal network model, R th_i_IGBT is the equivalent resistance, C th_i_IGBT is the equivalent thermal capacitance; T c (t) is the ambient temperature, P IGBT is the total power loss of the MMC submodule IGBT, T j_IGBT (t) is the IGBT junction temperature; A multi-order Foster thermal network model of the MMC sub-module diode is constructed to calculate the junction temperature data of the diode. The expression of the multi-order Foster thermal network model is: Among them, Z th_Diode (t) is thermal impedance, R th_i_Diode is the equivalent resistance, C th_i_Diode is the equivalent thermal capacitance, T c (t) is the ambient temperature, P Diode is the total power loss of the diode, T j_Diode (t) is the diode junction temperature.

8. The MMC system circulation suppression technology optimization method according to claim 2 is characterized in that: The method for optimizing circulation technology is: Statistically analyze the junction temperature data of IGBT and diode to obtain the average junction temperature fluctuation; Get the total harmonic distortion value of the MMC bridge arm current; If the total harmonic distortion value of the MMC bridge arm current is greater than or equal to the value specified by the grid connection requirement, the circulating current suppression technology will continue to be optimized with power quality as the optimization index; If the total harmonic distortion value of the MMC bridge arm current is less than the value specified by the grid-connected requirement, the circulating current suppression technology is thermally optimized with the average junction temperature amplitude as the main optimization indicator, and the total harmonic distortion value of the MMC bridge arm current must be maintained less than the value specified by the grid-connected requirement during the optimization.

9. The MMC system circulation suppression technology optimization method according to claim 8 is characterized in that: The grid connection requirement is specified to be no greater than 3%.

10. A method for optimizing circulating current suppression technology of an MMC system according to any one of claims 1 to 9, characterized in that: The MMC system circulating current suppression technology optimization method is applied in flexible direct current transmission scenarios.

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