A method for determining the loss of a commutation valve of an MMC converter for optical energy storage and direct current flexible use

Abstract

The present application relates to the technical field of power system transmission and distribution, in particular to a kind of commutation valve loss determination method of light storage direct flexible MMC converter, comprising: S1, the on-state loss of the commutation valve of MMC converter is calculated;S2, the switching loss of the commutation valve of MMC converter is calculated;S3, the on-state loss and switching loss are added, and the commutation valve loss of MMC converter is obtained;Wherein, S1 includes: S11, the average on-state loss of switching device in sub-module based on fundamental frequency cycle is calculated;S12, the average on-state loss of switching device based on switching cycle is calculated;S13, the on-state fitting parameter of IGBT and diode is equivalent, and after taking the larger value as the calculation parameter respectively, the on-state loss of commutation valve is calculated by calculation parameter.The present application can be under CPS-PWM modulation, and the commutation valve loss of MMC converter is simply, quickly and accurately evaluated.

Description

Technical Field

[0001] This invention relates to the field of power system transmission and distribution technology, specifically to a method for determining the converter valve loss of a photovoltaic-storage-DC-flexible MMC converter. Background Technology

[0002] Modular Multilevel Converters (MMCs) possess advantages such as good output characteristics, low switching frequency, and ease of expansion. They have been widely used in high-voltage direct current (HVDC) transmission both domestically and internationally, and also show great promise in medium-voltage applications such as DC distribution networks, photovoltaic-storage-DC-flexible systems, locomotive traction, and marine power systems. Loss calculation is fundamental to the economic efficiency, safety, reliability, and system heat dissipation design of MMCs. Among these, converter valve losses are the main source of losses in MMCs and represent a key focus and challenge in analyzing their loss characteristics. These losses primarily originate from the switching devices, specifically the Insulated Gate Bipolar Transistor (IGBT) modules, including conduction losses and switching losses.

[0003] Domestic and international scholars have conducted extensive research on the calculation of switching device losses and the modeling of converter valve losses. Some literature has established physical calculation models for IGBT modules, accurately providing the transient voltage, current waveforms, and temperature field distribution characteristics of the switching devices. However, most device parameters are difficult to obtain, and the computational workload is enormous. Other literature, based on loss test charts provided by IGBT module manufacturers, uses curve fitting to obtain mathematical expressions for the switching losses, conduction losses, diode conduction losses, reverse recovery losses, and the current flowing through them. Regarding converter valve loss modeling, for medium-voltage MMC converter applications with relatively few sub-modules, carrier phase-shift pulse width modulation (CPS-PWM) with a fixed switching frequency is typically used. Some literature introduces the effective operating range of sub-module switching devices, derives the average and effective values ​​of the switching device current, analyzes the junction temperature fluctuation characteristics of each switching device, and proposes a junction temperature estimation method. Other literature proposes calculation methods for the conduction and switching losses of half-bridge sub-modules and establishes a converter valve thermal model, providing data support for the reliability analysis and full-cycle life assessment of MMC converters.

[0004] In summary, research on the calculation of switching device losses and the modeling of converter valve losses in MMC converters is relatively mature. However, considering the complexity of the analytical expression for the zero-crossing point of the MMC converter arm current with the second harmonic circulating current component, existing converter valve loss calculation models, while possessing high calculation accuracy, are extremely complex and slow. To improve the calculation speed of converter valve losses, some literature has established a method that accumulates the maximum switching energy loss with the estimated average number of switching operations. However, this method is relatively conservative, introducing significant errors into the design of the MMC converter cooling system and the selection of circuit component parameters, and increasing its hardware cost. Some literature has proposed a simplified loss calculation model based on the effective and absolute values ​​of the arm current; however, the coefficients for loss calculation in this model are all derived from engineering experience, making it difficult to guarantee calculation accuracy and thus hindering practical application.

[0005] Therefore, how to achieve a simple, fast, and accurate assessment of the converter valve losses of MMC converters under CPS-PWM modulation has become an urgent problem to be solved. Summary of the Invention

[0006] To address the shortcomings of the existing technology, this invention provides a method for determining the converter valve loss of an MMC converter for photovoltaic-storage-DC-flexible applications. This method enables a simple, fast, and accurate assessment of the converter valve loss of the MMC converter under CPS-PWM modulation.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A method for determining the converter valve loss of a photovoltaic-storage-DC-flexible MMC converter, wherein the converter valve of the MMC converter includes multiple bridge arms, each bridge arm including multiple identical sub-modules; each sub-module includes switching devices, the switching devices including diodes D and IGBTs; wherein the IGBTs include IGBT1 and IGBT2, and the diodes D include diodes D1 and D2;

[0009] The method includes the following steps:

[0010] S1. Calculate the on-state losses of the converter valves in the MMC converter;

[0011] S2. Calculate the switching losses of the converter valves in the MMC converter;

[0012] S3. Add the on-state loss and the switching loss to obtain the converter valve loss of the MMC converter;

[0013] S1 includes:

[0014] S11. Calculate the average conduction loss between switching devices in the submodule based on the fundamental frequency period;

[0015] S12. Calculate the average on-state loss of the switching device based on the switching cycle;

[0016] S13. Equivalently match the on-state fitting parameters of IGBT and diode, and take the larger value as the calculation parameter, then calculate the on-state loss of the converter valve through the calculation parameter.

[0017] Preferably, in S11, the average conduction loss of the switching device based on the fundamental frequency period is calculated as follows:

[0018]

[0019]

[0020]

[0021] In the formula, P T1,cond (i T1 ) represents the average on-state loss of IGBT1 based on the fundamental frequency period, P D1,cond (i D1 ) represents the average conduction loss of diode D1 based on the fundamental frequency period, P T2,cond (i T2 ) represents the average on-state loss of IGBT2 based on the fundamental frequency period, P D2,cond (i D2 ) represents the average conduction loss of diode D2 based on the fundamental frequency period, P T,cond For IGBT transient power conduction loss, P D,cond For diode D, the transient power conduction loss is τ; for i, the integral variable is i. T1 (τ) represents the current flowing through IGBT1 at time τ, i D1 (τ) represents the current flowing through diode D1 at time τ, i T2 (τ) represents the current flowing through IGBT2 at time τ, i D2 (τ) represents the current flowing through diode D2 at time τ;

[0022] T0 is the fundamental frequency period, T0 = 1 / f0; f0 is the modulation wave frequency; f c t is the carrier frequency; k,on t k,off The switching times of each switching cycle submodule; t k This represents the initial time of each switching cycle.

[0023] Preferably, in S12, the formula for calculating the average conduction loss of the switching device based on the switching cycle is as follows:

[0024]

[0025] In the formula, P cond(i) represents the average on-state loss of the switching device based on the switching cycle, i(t) k ) for t k The current flowing through the switching device at all times, δ(t) k ) for t k The duty cycle of the switching device at any given time.

[0026] Preferably, the duty cycle of each switching cycle of the IGBT and diode is:

[0027]

[0028] In the formula, δ T1 (t k ) for t k Duty cycle and δ of IGBT1 at each switching cycle D1 (t k ) for t k Duty cycle of diode D1 at each switching cycle, δ T2 (t k ) for t k Duty cycle and δ of IGBT2 at each switching cycle D2 (t k ) for t k The duty cycle of diode D2 at each switching cycle is given by time, m is the modulation ratio, and ω is the fundamental angular frequency.

[0029] Preferably, in S13, the step of equivalence of the on-state fitting parameters of the IGBT and diode, and taking the larger value of each as the calculation parameter, includes: fitting the IGBT and diode, assuming that the on-state voltage drop U of the IGBT in the submodule is... CE0 and the on-state voltage drop U of the diode f0 They are equal, and the on-state resistance r of the IGBT is equal. CE With the on-state resistance r of the diode f After they are equal, take U from the fitted results. CE0 and U f0 The larger value in the equation is used as the on-state resistance r in the calculation parameters, and U is taken as... CE0 and U f0 The larger value in the calculation is used as the on-state pressure drop U0 in the calculation parameters.

[0030] Preferably, in S13, the on-state loss of the converter valve is calculated using the following formula:

[0031]

[0032] In the formula, P MMC,cond Let r be the on-state loss of the converter valve, r be the on-state resistance in the calculation parameters, U0 be the on-state voltage drop in the calculation parameters, and i be the on-state voltage drop. i,j N represents the bridge arm current of the MMC converter, N represents the number of submodules in each bridge arm of the converter valve, and P represents the current in the bridge arm of the converter.T1,cond For the on-state loss of IGBT1, P D1,cond For the conduction loss of diode D1, P T2,cond For the on-state loss of IGBT2, P D2,cond This represents the conduction loss of diode D2.

[0033] Preferably, in S2, the bridge arm currents of the IGBT and diodes in each switching cycle are substituted into the formula for calculating the average switching loss based on the switching cycle to obtain the switching loss of the converter valve:

[0034]

[0035] In the formula, a on,2 a on,1 a on,0 Powering up the IGBT (E) on The polynomial fitting coefficients; a off,2 a off,1 a off,0 Energy E for IGBT turn-off off The polynomial fitting coefficients; a rec,2 a rec,1 a rec,0 The reverse recovery energy E of the diode rec The polynomial fitting coefficients;

[0036] The formula for calculating the average switching loss of the switching device based on the switching cycle is:

[0037]

[0038] In the formula, i(t) k ) for t k The current that flows through the switching device at all times;

[0039] The IGBT turn-on energy E on IGBT turn-off energy E off and diode reverse recovery energy E rec The polynomial fitting calculation formula is:

[0040] E dev =a2i 2 +a1|i|+a0;

[0041] In the formula, E dev Represents the IGBT's activated energy E on IGBT turn-off energy E off Or diode reverse recovery energy E rec a0, a1, and a2 are the energy fraction polynomial fitting coefficients of the IGBT or diode, respectively; i is the current flowing through the IGBT or diode.

[0042] Preferably, the IGBT turn-on energy E on IGBT turn-off energy E off and diode reverse recovery energy E rec Before performing polynomial fitting, E was also adjusted based on junction temperature feedback and voltage regulation coefficient. dev The following corrections are made:

[0043]

[0044] In the formula, E dev,125 The switching energy E of the switching device at 125°C is obtained from the switching energy fitting parameters and calculations. dev,150 K represents the switching energy of the switching device at 150°C, calculated from the switching energy fitting parameters; T represents the feedback junction temperature; K represents the switching energy of the switching device. V U is the voltage regulation coefficient. CE,ref U is the reference cutoff voltage. SM This represents the voltage value of the submodule.

[0045] Preferably, in S3, after adding the on-state loss and switching loss of the converter valve, the converter valve loss is:

[0046]

[0047] In the formula, the coefficients b2, b1, and b0 are respectively:

[0048]

[0049] Preferably, in S3, the calculation formula for the converter valve loss is further simplified through mathematical transformation as follows:

[0050]

[0051] In the formula, c2I i,j,RMS I represents the effective value of the bridge arm current. i,j,ABSAVE This represents the average value of the device current.

[0052] The coefficients c2, c1, and c0 are respectively:

[0053]

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] 1. This invention proposes a method for determining the converter valve loss of a photovoltaic-storage-DC-flexible MMC converter, which simplifies the calculation of valve loss under CPS-PWM strategy. Based on the loss distribution characteristics of a sub-module topology photovoltaic-storage-DC-flexible MMC converter, this invention proposes a simplified calculation method for converter valve loss. The on-state fitting parameters of IGBTs and diodes in the equivalent sub-module solve the problem that existing methods cannot easily, quickly, and accurately calculate the on-state loss under CPS-PWM modulation when considering the second harmonic circulating current component. Experiments show that this invention can easily, quickly, and accurately calculate the MMC valve loss under different operating conditions, both with and without circulating current suppression.

[0056] In summary, this invention enables a simple, rapid, and accurate assessment of the converter valve losses of MMC converters under CPS-PWM modulation.

[0057] 2. Considering the complexity of the analytical formula for the zero-crossing point of the MMC converter arm current containing the second harmonic circulating current component, the on-state fitting parameters of the IGBTs and diodes in the MMC converter submodule are equivalent. Therefore, the on-state loss of this invention no longer needs to consider the analytical formula for the zero-crossing point of the MMC converter arm current containing the second harmonic circulating current component. It only needs to be calculated based on the continuous conduction of one switching device. The relative calculation error caused by the equivalence of the on-state fitting parameters of the switching device is very small. Therefore, this invention can calculate the on-state loss of the converter valve of the MMC converter very simply and accurately. Then, combined with the calculation method of the switching loss in this invention, the total loss of the converter valve can be obtained. Attached Figure Description

[0058] To make the objectives, technical solutions, and advantages of the invention clearer, the invention will now be described in further detail with reference to the accompanying drawings, wherein:

[0059] Figure 1 Flowchart for an embodiment;

[0060] Figure 2 A schematic diagram of the circuit topology of an MMC converter for optical storage DC-DC flexible circuitry.

[0061] Figure 3 This is a schematic diagram of the loss distribution of the sub-modules in the embodiment. Detailed Implementation

[0062] The following detailed explanation illustrates the specific implementation methods:

[0063] Example:

[0064] like Figure 1 As shown, this embodiment discloses a method for determining the converter valve loss of a photovoltaic-storage-DC-flexible MMC converter. To facilitate understanding of the photovoltaic-storage-DC-flexible MMC converter, its structure is briefly described as follows: Figure 2 As shown, the converter valve of the MMC converter includes multiple bridge arms, each bridge arm includes multiple sub-modules SM, and each sub-module SM includes switching devices, namely diodes D and IGBTs; wherein, the IGBTs include IGBT1 and IGBT2, and the diodes D include diodes D1 and D2; in a specific implementation, the type of sub-module is HBSM.

[0065] This method includes the following steps:

[0066] S1. Calculate the on-state losses of the converter valves in the MMC converter.

[0067] Specifically, S1 includes:

[0068] S11. Calculate the average conduction loss between switching devices in the submodule based on the fundamental frequency period; in specific implementation, such as... Figure 3 As shown in Figure S11, the formula for calculating the average conduction loss of the switching device based on the fundamental frequency period is:

[0069]

[0070]

[0071]

[0072] In the formula, P T1,cond (i T1 ) represents the average on-state loss of IGBT1 based on the fundamental frequency period, P D1,cond (i D1 ) represents the average conduction loss of diode D1 based on the fundamental frequency period, P T2,cond (i T2 ) represents the average on-state loss of IGBT2 based on the fundamental frequency period, P D2,cond (i D2 ) represents the average conduction loss of diode D2 based on the fundamental frequency period, P T,cond For IGBT transient power conduction loss, P D,cond For diode D, the transient power conduction loss is τ; for i, the integral variable is i. T1 (τ) represents the current flowing through IGBT1 at time τ, i D1 (τ) represents the current flowing through diode D1 at time τ, i T2 (τ) represents the current flowing through IGBT2 at time τ, i D2 (τ) represents the current flowing through diode D2 at time τ;

[0073] T0 is the fundamental frequency period, T0 = 1 / f0; f0 is the modulation wave frequency; f c t is the carrier frequency; k,on t k,offThe switching times of each switching cycle submodule; t k This represents the initial time of each switching cycle.

[0074] S12. Calculate the average conduction loss of the switching device based on the switching cycle; in specific implementation, the formula for calculating the average conduction loss of the switching device based on the switching cycle is as follows:

[0075]

[0076] In the formula, P cond (i) represents the average on-state loss of the switching device based on the switching cycle, i(t) k ) for t k The current flowing through the switching device at all times, δ(t) k ) for t k The duty cycle of the switching device at any given time.

[0077] The duty cycle of each switching cycle for the IGBT and diode is as follows:

[0078]

[0079] In the formula, δ T1 (t k ) for t k Duty cycle and δ of IGBT1 at each switching cycle D1 (t k ) for t k Duty cycle of diode D1 at each switching cycle, δ T2 (t k ) for t k Duty cycle and δ of IGBT2 at each switching cycle D2 (t k ) for t k The duty cycle of diode D2 at each switching cycle is given by time, m is the modulation ratio, and ω is the fundamental angular frequency.

[0080] S13. Equivalently match the on-state fitting parameters of IGBT and diode, and take the larger value as the calculation parameter, then calculate the on-state loss of the converter valve through the calculation parameter.

[0081] In specific implementation, the equivalent on-state fitting parameters of the IGBT and diode, and taking the larger value as the calculation parameter, includes: fitting the IGBT and diode, assuming the on-state voltage drop U of the IGBT in the submodule. CE0 and the on-state voltage drop U of the diode f0 They are equal, and the on-state resistance r of the IGBT is equal. CE With the on-state resistance r of the diode f After they are equal, take U from the fitted results. CE0 and U f0The larger value in the equation is used as the on-state resistance r in the calculation parameters, and U is taken as... CE0 and U f0 The larger value in the calculation is used as the on-state pressure drop U0 in the calculation parameters.

[0082] The on-state losses of the converter valve are calculated using the following formula:

[0083]

[0084] In the formula, P MMC,cond Let r be the on-state loss of the converter valve, r be the on-state resistance in the calculation parameters, U0 be the on-state voltage drop in the calculation parameters, and i be the on-state voltage drop. i,j N represents the bridge arm current of the MMC converter, N represents the number of submodules in each bridge arm of the converter valve, and P represents the current in the bridge arm of the converter. T1,cond For the on-state loss of IGBT1, P D1,cond For the conduction loss of diode D1, P T2,cond For the on-state loss of IGBT2, P D2,cond This represents the conduction loss of diode D2. It should be noted that the conduction loss of a switching device only needs to be calculated based on the conduction loss of a single switching device continuously in operation.

[0085] S2. Calculate the switching losses of the converter valves in the MMC converter.

[0086] In practical implementation, the bridge arm currents of the IGBT and diodes in each switching cycle are substituted into the formula for calculating the average switching loss based on the switching cycle to obtain the switching loss of the converter valve:

[0087]

[0088] In the formula, a on,2 a on,1 a on,0 Powering up the IGBT (E) on The polynomial fitting coefficients; a off,2 a off,1 a off,0 Energy E for IGBT turn-off off The polynomial fitting coefficients; a rec 2. a rec,1 a rec,0 The reverse recovery energy E of the diode rec The polynomial fitting coefficients.

[0089] Similar to the calculation of conduction loss, the switching loss of a switching device can be calculated by simply taking the average switching loss of a single switching action.

[0090] The formula for calculating the average switching loss of the switching device based on the switching cycle is:

[0091]

[0092] In the formula, i(t) k ) for t k The current that flows through the switching device at all times;

[0093] The IGBT turn-on energy E on IGBT turn-off energy E off and diode reverse recovery energy E rec The polynomial fitting calculation formula is:

[0094] E dev =a2i 2 +a1|i|+a0;

[0095] In the formula, E dev Represents the IGBT's activated energy E on IGBT turn-off energy E off Or diode reverse recovery energy E rec a0, a1, and a2 are the energy division polynomial fitting coefficients for the IGBT or diode, respectively; i is the current flowing through the IGBT or diode. In actual fitting, those skilled in the art can obtain the required fitting parameters by consulting the manufacturer's datasheet, which will not be elaborated upon here.

[0096] Among them, the IGBT turn-on energy E on IGBT turn-off energy E off and diode reverse recovery energy E rec Before performing polynomial fitting, E was also adjusted based on junction temperature feedback and voltage regulation coefficient. dev The following corrections are made:

[0097]

[0098] In the formula, E dev,125 The switching energy E of the switching device at 125°C is obtained from the switching energy fitting parameters and calculations. dev,150 K represents the switching energy of the switching device at 150°C, calculated from the switching energy fitting parameters; T represents the feedback junction temperature; K represents the switching energy of the switching device. V U is the voltage regulation coefficient. CE,ref U is the reference cutoff voltage. SM This represents the voltage value of the submodule.

[0099] S3. Add the on-state loss and the switching loss to obtain the converter valve loss of the MMC converter.

[0100] Specifically, after adding the on-state loss and switching loss of the converter valve, the converter valve loss is:

[0101]

[0102] In the formula, the coefficients b2, b1, and b0 are respectively:

[0103]

[0104] In practical implementation, the calculation formula for the above-mentioned converter valve loss can be simplified through mathematical transformation as follows:

[0105]

[0106] In the formula, c2I i,j,RMS I represents the effective value of the bridge arm current. i,j,ABSAVE This represents the average value of the device current.

[0107] The coefficients c2, c1, and c0 are respectively:

[0108]

[0109] This invention proposes a method for determining the converter valve loss of a photovoltaic-storage-DC-flexible MMC converter, which simplifies the calculation of valve losses under CPS-PWM strategy. Based on the loss distribution characteristics of a submodule-topology photovoltaic-storage-DC-flexible MMC converter, this invention proposes a simplified calculation method for converter valve losses; the on-state fitting parameters of IGBTs and diodes in the equivalent submodule solve the problem that existing methods cannot easily, quickly, and accurately calculate the on-state loss under CPS-PWM modulation when considering the second harmonic circulating current component. Considering the complexity of the analytical formula for the zero-crossing point of the MMC converter arm current including the second harmonic circulating current component, the on-state fitting parameters of the IGBTs and diodes in the MMC converter submodule are equivalent. Therefore, the on-state loss of this invention no longer needs to consider the analytical formula for the zero-crossing point of the MMC converter arm current including the second harmonic circulating current component; it only needs to be calculated based on the continuous conduction of one switching device. The relative calculation error caused by the equivalent on-state fitting parameters of the switching device is very small. Therefore, this invention can calculate the on-state loss of the converter valve of the MMC converter very simply and accurately. Then, combined with the calculation method for switching losses in this invention, the overall loss of the converter valve can be obtained. Experiments show that, with and without circulating current suppression, this invention can easily, quickly, and accurately calculate the MMC valve loss under different operating conditions.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for determining the loss of a commutation valve of an MMC converter for optical energy storage and direct current flexible transmission, the commutation valve of the MMC converter comprising a plurality of bridge arms, each bridge arm comprising a plurality of identical sub-modules;each sub-module comprising a switching device, the switching device comprising an IGBT and a diode D;wherein, The IGBTs include IGBT1 and IGBT2, and the diodes D include diode D1 and diode D2; The method comprises the following steps: S1, calculating the on-state loss of the converter valve of the MMC converter; S2, calculating the switching loss of the converter valve of the MMC converter; S3, adding the on-state loss and the switching loss to obtain the converter valve loss of the MMC converter; In S1, the following steps are included: S11, calculating the average on-state loss of the switching devices in the sub-module based on the fundamental frequency period; S12, calculating the average on-state loss of the switching devices based on the switching period; S13, fitting the parameters of the on-state of the IGBT and the diode, taking the larger value as the calculation parameter respectively, and then calculating the on-state loss of the converter valve through the calculation parameter.

2. The MMC converter loss determination method for an optical storage and transmission flexible fiber according to claim 1, characterized in that: In S11, the calculation formula of the average on-state loss of the switching devices based on the fundamental frequency period is as follows: In the formula, P T1,cond (i T1 ) represents the average on-state loss of IGBT1 based on the fundamental frequency period, P D1,cond (i D1 ) represents the average conduction loss of diode D1 based on the fundamental frequency period, P T2,cond (i T2 ) represents the average on-state loss of IGBT2 based on the fundamental frequency period, P D2,cond (i D2 ) represents the average conduction loss of diode D2 based on the fundamental frequency period, P T,cond For IGBT transient power conduction loss, P D,cond For diode D, the transient power conduction loss is τ; for i, the integral variable is i. T1 (τ) represents the current flowing through IGBT1 at time τ, i D1 (τ) represents the current flowing through diode D1 at time τ, i T2 (τ) represents the current flowing through IGBT2 at time τ, i D2 (τ) represents the current flowing through diode D2 at time τ; T0 is a fundamental frequency period, T0 = 1 / f0; f0 is a modulated wave frequency; f c is a carrier frequency; t k,on , t k,off is a time when each switch period sub-module is switched; t k is an initial time of each switch period.

3. The method of claim 2, wherein the MMC converter of the optical storage and direct current flexible power system is a valve of the MMC converter. In S12, the calculation formula of the average on-state loss of the switching devices based on the switching period is as follows: where P cond (i) is the average on-state loss of the switching device based on the switching period, i(t k ) is the current flowing through the switching device at time t k , and δ(t k ) is the duty cycle of the switching device at time t k .

4. The method of claim 3, wherein the MMC converter is a light storage flexible MMC converter.

3. The duty cycle of each switching period of the IGBT and the diode is as follows: where δ(t) is the duty ratio of IGBT1 at time t, δ(t) is the duty ratio of diode D1 at time t, δ(t) is the duty ratio of IGBT2 at time t, and δ(t) is the duty ratio of diode D2 at time t. T1 k k D1 k k T2 k k D2 k k where δ(t) is the duty ratio of IGBT1 at time t, δ(t) is the duty ratio of diode D1 at time t, δ(t) is the duty ratio of IGBT2 at time t, and δ(t) is the duty ratio of diode D2 at time t.​​​​​​​​​​​ 5. The method of claim 4, wherein the MMC converter is a light storage flexible MMC converter. In S13, the on-state fitting parameters of the IGBT and the diode are equivalent, and the larger value is taken as the calculation parameter, comprising: fitting the IGBT and the diode, assuming that the on-state voltage drop U CE0 of the IGBT in the sub-module is equal to the on-state voltage drop U f0 of the diode, and the on-state resistance r CE of the IGBT is equal to the on-state resistance r f of the diode, then taking the larger value of U CE0 and U f0 in the fitting result as the on-state resistance r in the calculation parameter, and taking the larger value of U CE0 and U f0 as the on-state voltage drop U0 in the calculation parameter. ​ 6. The method of claim 5, wherein the MMC converter of the optical storage and direct current flexible power system is a valve of the MMC converter. In S13, the on-state loss of the converter valve is calculated through the following calculation formula: wherein P MMC,cond is the on-state loss of the converter valve, r is the on-state resistance in the calculation parameter, U0 is the on-state voltage drop in the calculation parameter, i i,j is the bridge arm current of the MMC converter, N is the number of sub-modules of each bridge arm of the converter valve, P T1,cond is the on-state loss of the IGBT1, P D1,cond is the on-state loss of the diode D1, P T2,cond is the on-state loss of the IGBT2, P D2,cond is the on-state loss of the diode D2.

7. The method of claim 6, wherein the MMC converter of the optical storage and direct current flexible power system is a valve of the MMC converter. In S2, the bridge arm current of each switching period of the IGBT and the diode is substituted into the calculation formula of the average switching loss based on the switching period to obtain the switching loss of the converter valve: ​ wherein a on,2 , a on,1 , a on,0 are polynomial fitting coefficients of the IGBT turn-on energy E on ; a off,2 , a off,1 , a off,0 are polynomial fitting coefficients of the IGBT turn-off energy E off ; a rec,2 , a rec,1 , a rec,0 are polynomial fitting coefficients of the diode reverse recovery energy E rec ; The calculation formula of the average switching loss of the switching devices based on the switching period is as follows: where i(t) is the current flowing through the switching device at time t k k t​ The IGBT turn-on energy E on The IGBT turn-off energy E off The diode reverse recovery energy E rec The polynomial fitting calculation formula is: E dev = a2i 2 + a1|i| + a0; wherein E dev represents the IGBT turn-on energy E on , the IGBT turn-off energy E off , or the diode reverse recovery energy E rec ; a0, a1, and a2 are energy polynomial fitting coefficients of the IGBT or diode, respectively; and i is the current flowing through the IGBT or diode.

8. The method of claim 7, wherein the MMC converter of the optical storage and direct current flexible power system is a valve of the MMC converter. IGBT turn-on energy E on IGBT turn-off energy E off and diode reverse recovery energy E rec Before polynomial fitting, the following correction is made to E dev according to junction temperature feedback and voltage regulation coefficient: wherein E dev,125 is the switching energy of the switching device at 125 °C calculated from the switching energy fitting parameters and dev,150 is the switching energy of the switching device at 150 °C calculated from the switching energy fitting parameters and T is the feedback junction temperature; K V is the voltage regulation coefficient, U CE,ref is the reference cut-off voltage; U SM is the submodule voltage value.

9. The method of claim 8, wherein the MMC converter of the optical storage and direct current flexible power system is a valve of a converter. In S3, after the on-state loss and the switching loss of the converter valve are added, the converter valve loss is as follows: ​ In the formula, the coefficients b2, b1 and b0 are respectively as follows:

10. The method of claim 9, wherein the MMC converter of the optical storage and direct current flexible power system is a valve of the MMC converter. In S3, the calculation formula of the converter valve loss is also simplified through mathematical transformation as follows: wherein c2I i,j,RMS is the bridge arm current effective value; I i,j,ABSAVE is the device current average value; The coefficients c2, c1 and c0 are respectively as follows:

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