Inverting mode open-circuit fault diagnosis method for modular multilevel converter of flexible direct-current converter station

Abstract

The invention discloses an open-circuit fault diagnosis method for an inversion mode of a modular multilevel converter of a flexible direct current converter station. An energy storage capacitor symbol in an FBSM circuit is replaced by a fractional order capacitor; establishing an FBSM fractional order equivalent circuit model considering the fractional order characteristic of the energy storage capacitor; based on the fractional order equivalent circuit model of the FBSM, calculating the theoretical value vc, x, y (t) of the end voltage of the energy storage capacitor in the FBSM at the t moment; calculating a capacitance correction value Ceq of an energy storage capacitor in the FBSM by using a fractional order expression of the capacitor in the FBSM; comparing the theoretical value vc, x, y (t) and the measured value vctest, x, y (t) of the capacitor end voltage in the FBSM, and calculating the difference value epsilon c (t) between the theoretical value vc, x, y (t) and the measured value vctest, x, y (t) at the t moment; and finally, the open-circuit fault state of the FBSM module is judged according to the deviation degree of the difference value epsilon c (t) at the moment t. The fault real-time diagnosis method can accurately describe the electrical signal characteristics of the inverter-mode modular multilevel converter when the open-circuit fault occurs, thereby providing a reference basis for the design and reliability analysis of a modular multilevel converter device using a half-bridge sub-module and a flexible DC converter station system.

Description

Technical Field

[0001] The present invention relates to the technical field of open circuit fault analysis of modular multilevel converters in flexible DC converter stations, and in particular to an open circuit fault diagnosis method for inverter mode of modular multilevel converters in flexible DC converter stations based on electrical signal analysis of fractional-order capacitors of half-bridge submodules. Background Art

[0002] With the accelerated construction of large wind and solar power base projects in the three northern regions of my country, the construction of large transmission channel projects is also in full swing. High-voltage flexible direct current transmission technology has been widely used due to its advantages in long-distance large-capacity transmission and grid interconnection, while modular multilevel converters (MMC) are widely used in related scenarios due to their strong scalability, high degree of modularization, and strong fault tolerance.

[0003] Among the common modular multilevel converters, there are usually typical structures such as half-bridge sub-module (HBSM), full-bridge sub-module (FBSM), clamping double sub-module (CDSM), etc. Among them, the full-bridge sub-module FBSM has the advantages of using fewer power electronic devices and high operating efficiency. Therefore, the MMC-based flexible direct current converter stations currently put into operation generally use half-bridge MMC as the basic unit.

[0004] like Figure 1 As shown in FIG. , it is a three-phase inverter MMC system with 6N FBSMs. Figure 2 As shown, it is a typical switching mode of FBSM. References [1]: Zhu Yubo. Research on fault diagnosis and redundant operation of MMC submodules based on packet detection [D]. Hunan University, 2021.

[0005] With the increase of the capacity of flexible DC system, the number of full-bridge submodules FBSM in modular multi-level converter increases accordingly, and IGBT devices and capacitors are widely used in such circuits. The working mode of full-bridge submodule FBSM is not only related to the switching state and characteristic parameters of IGBT devices, but also to the characteristic parameters of capacitors, and is directly related to the fault phenomenon of the system. Affected by the time-varying electrical signals borne by various components, many face various fault risks during operation, such as submodule open circuit, single-pole grounding, bipolar short circuit and broken line. The failure of a single submodule will affect the normal operation of the flexible DC converter station. For example, when a single or a few submodules have an open circuit fault, if the faulty submodule cannot be detected and removed in time, the submodule capacitor voltage will be abnormally charged and discharged, and in severe cases, the entire system will be shut down. Therefore, it is necessary to study the diagnosis method of the open circuit fault of the full-bridge submodule FBSM.

[0006] However, as represented by reference [1], in past research, components were regarded as ideal elements, and the influence of component microscopic characteristics on the working state of the circuit system was usually ignored. Reference [2]: Zehao Liu et al. Open-Circuit Fault Diagnosis for MMC Based on Event-Triggered and Capacitor Current State Observation [J]. IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, 2022, 69 (2): 534-538. It points out that in the process of fault analysis of MMC based on full-bridge submodule FBSM, the influence of measurement error and the deviation between the actual value and the nominal value of capacitor parameters on the fault analysis results is particularly important. The energy storage capacitor in the full-bridge submodule FBSM is a device whose characteristic parameters are easily affected by many factors and prone to aging failure. Therefore, it is necessary to consider the influence of component microscopic parameter characteristics on the fault cause and fault analysis results in the process of fault analysis. Summary of the invention

[0007] The purpose of the present invention is to provide a method for diagnosing open-circuit faults in inverter mode of a modular multilevel converter of a flexible DC converter station based on electrical signal analysis of fractional-order capacitors of half-bridge submodules. The method does not require the addition of additional control and analysis circuits, but only requires the detection and comparison of capacitor voltages to quickly and accurately determine the submodule where an open-circuit fault occurs, thereby helping to improve the reliability of the modular multilevel converter based on the FBSM.

[0008] The technical solution adopted by the present invention is:

[0009] A method for diagnosing an open-circuit fault in inverter mode of a modular multilevel converter in a flexible DC converter station comprises the following steps:

[0010] Step 1: Replace the energy storage capacitor symbol in the FBSM circuit with a fractional-order capacitor;

[0011] Step 2: Based on Kirchhoff's current and voltage laws and the switching modes of FBSM, a fractional-order equivalent circuit model of FBSM is established taking into account the fractional-order characteristics of the energy storage capacitor;

[0012] Step 3: Based on the FBSM fractional-order equivalent circuit model, calculate the theoretical value v of the voltage at the energy storage capacitor in the FBSM c,x,y (t); Step 4: Calculate the capacitance correction value C of the energy storage capacitor in FBSM using the fractional expression of the capacitor in FBSM; Step 5: Compare the theoretical value v of the voltage at the energy storage capacitor terminal in FBSM c,x,y (t) and the measured value v c_test,x,y (t), calculate the difference ε between the two at time t c (t);

[0013] Step 6: According to the difference ε at time t c (t) Deviation threshold δ, used to determine the open circuit fault state of the FBSM circuit.

[0014] In step 2, according to the Euler formula, the impedance expression of the fractional-order energy storage capacitor in the FBSM in the frequency domain is:

[0015]

[0016] Among them, C is the nominal value of the energy storage capacitor in the FBSM, α is the order of the energy storage capacitor, ranging from 0 to 1, which is used to correct the nominal value of the energy storage capacitor, j is the imaginary sign, and ω is the operating angular frequency.

[0017] The step 3 comprises the following steps:

[0018] Step 3.1. Based on Kirchhoff's current law and the switching mode of FBSM, write the function of the current flowing through the energy storage capacitor in FBSM:

[0019] i c,x,y (t) = s y (t)i x,y (t) (3); where i c,x,y (t) is the current flowing through the energy storage capacitor in the FBSM at time t; the subscript symbol x contains the subset {a, b, c}, which is used to indicate that the FBSM is in the a, b, and c three-phase branches of the modular multilevel converter MMC; the subscript symbol y contains the subset {p, n}, where p and n are used to indicate that the FBSM is in the upper and lower bridge arms of the MMC respectively; i x,y(t) is the current flowing through the FBSM at time t. The subscript symbol x contains the subset {a, b, c}, which is used to indicate that the FBSM is in the a, b, and c three-phase branches of the MMC respectively; the subscript symbol y contains the subset {p, n}, which is used to indicate that the FBSM is in the upper and lower bridge arms of the MMC respectively.

[0020] In normal working mode, the upper and lower bridge arms of the modular multilevel converter MMC are symmetrical. The upper and lower bridge arm currents in formula (3) are:

[0021]

[0022] Among them, I x is the phase current amplitude of the MMC AC side, I circ,dc is the DC component of the MMC circulating current,

[0023] In addition, in formula (3), s y (t) is the switching function of the upper and lower bridge arms of the MMC:

[0024]

[0025] The subscript symbol y contains the subset {p,n}, where p and n are used to indicate that the FBSM is in the upper and lower bridge arms of the MMC respectively; when the FBSM is in the upper bridge arm of the MMC, When FBSM is in the lower arm of MMC, m is the modulation ratio of MMC, m=U x / 2U dc , U x Indicates the amplitude of the three-phase voltage a, b, c on the AC side of the MMC, 2U dc is the DC side voltage of MMC.

[0026] Step 3.2: Substitute equations (4a), (4b), and (5) in step 3.1 into equation (3) in step 3.1, and based on the capacitor-voltage balance condition, eliminate the DC current component to obtain the expression of the energy storage capacitor in the FBSM:

[0027]

[0028] Step 3.3, based on the voltage-current relationship of the fractional-order capacitor, write the terminal voltage function of the energy storage capacitor in the FBSM:

[0029]

[0030] Among them, v c,x,y(t) is the voltage at the energy storage capacitor terminal at time t. The subscript symbol x contains the subset {a, b, c}, which is used to indicate that the FBSM is in the a, b, and c three-phase branches of the MMC respectively; the subscript symbol y contains the subset {p, n}, which is used to indicate that the FBSM is in the upper and lower bridge arms of the MMC respectively.

[0031] In step 3, according to the Grünwald-Letnikov definition, the fractional-order integral operation is performed on equation (7) in step 3.3 to obtain the theoretical value v of the voltage at the energy storage capacitor terminal in the recursive form of the FBSM: c,x,y The expression of (t) is:

[0032]

[0033] Where: v c,x,p (t) is the voltage value of the upper bridge arm capacitor in the three-phase branch of MMC a, b, c at time t, v c,x,n (t) is the voltage value of the lower bridge arm capacitor in the three-phase branches of MMC a, b, and c at time t.

[0034] v c,x,p (t-1) is the voltage value of the upper bridge arm capacitor in the three-phase branch of MMCa, b, and c at time t-1, v c,x,n (t-1) is the voltage value of the lower bridge arm capacitor in the three-phase branch of MMC a, b, c at time t-1;

[0035] v c,x,p (t), v c,x,n (t) is v c,x,y The subscript symbol y in (t) represents the symbols p and n when in the upper and lower bridge arms, respectively, see step 3.1 for explanation.

[0036] h is the time interval length for sampling the voltage at the energy storage capacitor terminal, N is the number of times the voltage at the energy storage capacitor terminal is sampled within a fixed time period, and in formula (8a) and formula (8b) The expression is:

[0037]

[0038] in, j=1,2,…,N, α is the capacitor order;

[0039] In step 4, the capacitance correction value C of the energy storage capacitor eq That is the imaginary part of equation (8a) and equation (8b), that is:

[0040]

[0041] In step 5, the theoretical value v of the capacitor terminal voltage in the FBSM at time t is c,x,y (t) and the measured value vc_test,x,y The difference ε of (t) c (t) is:

[0042] ε c (t) = v c,x,y (t)-v c_test,x,y (t) (10).

[0043] In step 6, considering that the actual capacitance of the capacitor generally has a deviation of less than 20% from its nominal value, the deviation threshold δ of the difference between the theoretical value and the measured value of the FBSM capacitor terminal voltage at the power frequency cycle time interval (20ms) is set to:

[0044]

[0045] When the threshold exceeds 5%, it is determined that an open circuit fault occurs in the inverter mode modular multilevel converter.

[0046] The present invention provides a real-time diagnosis method for open-circuit faults in inverter mode of modular multi-level converters in flexible DC converter stations based on half-bridge submodule fractional-order capacitor electrical signal analysis, and the beneficial effects are as follows:

[0047] Since the fractional-order parameter characteristics of the energy storage capacitor are taken into consideration, the open-circuit fault diagnosis method proposed in the present invention can more accurately describe the electrical signal characteristics of the modular multilevel converter in the inverter mode when an open-circuit fault occurs, thereby providing a reference for the design and reliability analysis of the modular multilevel converter device and the flexible DC converter station system using the half-bridge submodule. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0049] Figure 1 It is the topology diagram of a three-phase inverter MMC with 6N FBSMs.

[0050] Figure 2 This is a typical switching mode diagram of FBSM.

[0051] Figure 2 In the figure, the light red part of the circuit indicates that the circuit is in an open circuit state, and the light red part of the component symbol indicates that the device is in a cut-off state.

[0052] FIG3( a ) is a circuit diagram of a fractional-order capacitor;

[0053] Figure 3(b) is the fractional-order equivalent circuit diagram of the half-bridge MMC.

[0054] Figure 4 This is the measured curve of the voltage change at the energy storage capacitor terminal during normal operation.

[0055] Figure 5This is a comparison chart of the positioning analysis results of the fault occurrence time after an open circuit fault occurred in the first FBSM switch tube V1 of the upper bridge arm of phase a at 1.45 seconds. DETAILED DESCRIPTION

[0056] A real-time diagnosis method for open-circuit faults in inverter mode of modular multilevel converters in flexible DC converter stations based on electrical signal analysis of fractional-order capacitors in half-bridge submodules is used to diagnose open-circuit faults in modular multilevel converters in flexible DC converter stations working in inverter mode. It includes the following steps:

[0057] 1) The energy storage capacitor symbol in the FBSM circuit is replaced with a fractional-order capacitor having a circuit structure as shown in FIG3(a). The form before and after the replacement is shown in FIG3(b);

[0058] Figure 3(a) is the fractional-order equivalent circuit model of the energy storage capacitor in the FBSM;

[0059] FIG3( b ) shows the replacement of the integer-order ideal capacitor symbol of the energy storage capacitor in the FBSM circuit with the fractional-order capacitor model shown in FIG3( a );

[0060] 2) Using the fractional order expression of capacitance in FBSM, calculate the capacitance correction value C of the energy storage capacitor in FBSM eq ;

[0061] 3) Based on Kirchhoff's current and voltage laws and the switching modes of FBSM, a fractional-order equivalent circuit model of FBSM is established taking into account the fractional-order characteristics of the energy storage capacitor;

[0062] 4) Based on the fractional-order equivalent circuit model of FBSM, calculate the theoretical value v of the voltage at the energy storage capacitor in FBSM c,x,y (t);

[0063] 5) Compare the theoretical value v of the capacitor terminal voltage in FBSM c,x,y (t) and the measured value v c_test,x,y (t), calculate the difference between the two at time t;

[0064] 6) According to the difference ε c (t) Determine the open circuit fault status of the FBSM module.

[0065] The open circuit fault diagnosis method proposed in the present invention does not require the addition of additional control and analysis circuits, and only needs to detect and compare capacitor voltages to quickly and accurately determine the submodule where the open circuit fault occurs, thereby improving the reliability of the FBSM-based modular multi-level converter.

[0066] The present invention is further described below by taking a three-phase inverter MMC system based on FBSM as an example.

[0067] A three-phase inverter MMC circuit with 6*4 FBSMs, a DC side voltage of 1000V, a three-phase output of 50Hz, and a phase voltage of 380V, 24 FBSMs all use non-solid aluminum electrolytic capacitors with a nominal value of 10mF. A flexible DC converter station modular multilevel converter inverter mode open circuit fault diagnosis method based on half-bridge submodule fractional-order capacitor electrical signal analysis according to the present invention comprises the following steps:

[0068] S1. Replace the energy storage capacitor symbol in the FBSM circuit with a fractional-order capacitor having a circuit structure as shown in FIG3(a).

[0069] S2. Using the fractional order expression of capacitance in FBSM, taking the fractional order correction value α = 0.95, calculate the capacitance correction value C of the energy storage capacitor in FBSM eq =8.25μF;

[0070] S3, based on Kirchhoff's current and voltage laws and the switching mode of FBSM, establish a FBSM fractional-order equivalent circuit model taking into account the fractional-order characteristics of the energy storage capacitor, the step S2 includes:

[0071] S31. Based on Kirchhoff's current law and the switching mode of FBSM, write the function of the current flowing through the energy storage capacitor in FBSM:

[0072] i c,x,y (t) = s y (t)i x,y (t) (3); where i c,x,y (t) is the current flowing through the energy storage capacitor in the FBSM at time t, the subscript symbol x contains the subset {a, b, c}, which is used to indicate that the FBSM is in the a, b, and c three-phase branches of the MMC, and the subscript symbol y contains the subset {p, n}, where p and n are used to indicate that the FBSM is in the upper and lower bridge arms of the MMC, respectively; i x,y (t) is the current flowing through the FBSM at time t. The subscript symbol x contains the subset {a, b, c}, which is used to indicate that the FBSM is in the a, b, and c three-phase branches of the MMC respectively; the subscript symbol y contains the subset {p, n}, which is used to indicate that the FBSM is in the upper and lower bridge arms of the MMC respectively. In the normal working mode, the upper and lower bridge arms of the MMC are symmetrical. The upper and lower bridge arm currents in formula (3) are:

[0073]

[0074] Among them, I x is the phase current amplitude of the MMC AC side, I circ,dc is the DC component of the MMC circulating current,

[0075]

[0076] In addition, s in formula (1) y (t) is the switching function of the upper and lower bridge arms of the MMC:

[0077]

[0078] The subscript symbol y contains the subset {p,n}, where p and n are used to indicate that the FBSM is in the upper and lower bridge arms of the MMC respectively; when the FBSM is in the upper bridge arm of the MMC, When FBSM is in the lower arm of MMC, m is the modulation ratio of MMC, m=U x / 2U dc , U x Indicates the amplitude of the three-phase voltage a, b, c on the AC side of the MMC, 2U dc is the DC side voltage of MMC.

[0079] S32, substituting equations (4a), (4b), and (5) in step S31 into equation (3) in step S21, and based on the capacitor-voltage balance condition, eliminating the DC current component, the expression of the energy storage capacitor in the FBSM is obtained:

[0080]

[0081] S33. Based on the voltage-current relationship of fractional-order capacitors, write the terminal voltage function of the energy storage capacitor in FBSM:

[0082]

[0083] Among them, v c,x,y (t) is the voltage at the energy storage capacitor terminal at time t. The subscript symbol x contains the subset {a, b, c}, which is used to indicate that the FBSM is in the a, b, and c three-phase branches of the MMC, and the subscript symbol y contains the subset {p, n}, which is used to indicate that the FBSM is in the upper and lower bridge arms of the MMC, respectively.

[0084] S4. Based on the fractional-order equivalent circuit model of FBSM, calculate the theoretical value v of the voltage at the energy storage capacitor in FBSM c,x,y (t);

[0085] S5. Compare the theoretical value v of the capacitor terminal voltage in FBSM c,x,y (t) and the measured value v c_test,x,y (t), calculate the difference ε between the two at time t c (t);

[0086] S6, according to the difference ε at time t c (t) Deviation threshold δ, used to determine the open circuit fault state of the FBSM module.

[0087] Figure 4 This is the measured curve of the voltage change at the energy storage capacitor terminals of the four FBSMs in the upper bridge arm of phase a. Figure 4 From top to bottom in the figure, there are four measured curves of the energy storage capacitor voltages of the four FBSMs, blue, red, green and brown respectively. It can be seen that during normal operation, the voltage change trends of the four FBSM energy storage capacitors on the upper bridge arm of phase a are basically the same, and there is basically no deviation between the voltages.

[0088] Starting from 1.45 seconds, the first FBSM switch V1 on the upper bridge arm of phase a is set to have an open circuit fault. The voltage change curve of the energy storage capacitor terminal of the FBSM where the fault occurs is as follows: Figure 5 As shown, the red line represents the measured value, the green line represents the calculated value of the present invention, the blue line represents the analysis result without considering the fractional-order characteristics of the capacitor, and the black line is the switch tube V1 signal. It can be seen that within one power frequency cycle, the maximum relative error between the calculated value and the measured value of the FBSM capacitor terminal voltage by the method proposed by the present invention does not exceed 1.22%, indicating that the method proposed by the present invention is effective; while the maximum relative error of the traditional method without considering the fractional-order parameter characteristics of the capacitor is about 4.85%, indicating that the method proposed by the present invention can more accurately determine the occurrence of open circuit faults.

Claims

1. A method for diagnosing open-circuit faults in inverter mode of modular multilevel converters in flexible DC converter stations, characterized in that The following steps are involved: Step 1: Replace the energy storage capacitor symbol in the FBSM circuit with a fractional-order capacitor; Step 2: Based on Kirchhoff's current and voltage laws and the switching modes of FBSM, a fractional-order equivalent circuit model of FBSM is established taking into account the fractional-order characteristics of the energy storage capacitor; Step 3: Based on the FBSM fractional-order equivalent circuit model, calculate the theoretical value v of the voltage at the energy storage capacitor in the FBSM ,, (t); Step 4: Calculate the capacitance correction value C of the energy storage capacitor in the FBSM using the fractional order expression of the capacitor in the FBSM; Step 5: Compare the theoretical value v of the voltage at the energy storage capacitor in the FBSM ,, (t) and the measured value v _,, (t), calculate the difference ε(t) between the two at time t; Step 6: According to the deviation threshold δ of the difference ε(t) at time t, the open circuit fault state of the FBSM circuit is determined.

2. According to claim 1, the method for diagnosing open circuit faults in inverter mode of modular multilevel converters in flexible DC converter stations is characterized by: In step 2, the impedance expression of the fractional-order energy storage capacitor in the FBSM in the frequency domain is: Among them, C is the nominal value of the energy storage capacitor in the FBSM, α is the order of the energy storage capacitor, ranging from 0 to 1, which is used to correct the nominal value of the energy storage capacitor, j is the imaginary sign, and ω is the operating angular frequency.

3. The method for diagnosing open circuit faults in inverter mode of a modular multilevel converter in a flexible DC converter station according to claim 1, characterized in that: The step 3 comprises the following steps: Step 3.

1. Based on Kirchhoff's current law and the switching mode of FBSM, write the function of the current flowing through the energy storage capacitor in FBSM: i , , (t) = s(t)i , (t) (3); where i ,, (t) is the current flowing through the energy storage capacitor in the FBSM at time t; the subscript symbol x contains the subset {a, b, c}, which is used to indicate that the FBSM is in the a, b, and c three-phase branches of the modular multilevel converter MMC; the subscript symbol y contains the subset {p, n}, where p and n are used to indicate that the FBSM is in the upper and lower bridge arms of the MMC respectively; i , (t) is the current flowing through the FBSM at time t, the subscript symbol x contains the subset {a, b, c}, which is used to indicate that the FBSM is in the a, b, and c three-phase branches of the MMC respectively; the subscript symbol y contains the subset {p, n}, which is used to indicate that the FBSM is in the upper and lower bridge arms of the MMC respectively; In normal working mode, the upper and lower bridge arms of the modular multilevel converter MMC are symmetrical. The upper and lower bridge arm currents in formula (3) are: Where, I is the phase current amplitude of the MMC AC side, I +,,- is the DC component of the MMC circulating current, In addition, in formula (3), s(t) is the switching function of the upper and lower bridge arms of the MMC: The subscript symbol y contains the subset {p,n}, where p and n are used to indicate that the FBSM is in the upper and lower bridge arms of the MMC respectively; when the FBSM is in the upper bridge arm of the MMC, When FBSM is in the lower arm of MMC, m is the modulation ratio of MMC, m = U / 2U - , U represents the amplitude of the three-phase voltage a, b, c on the AC side of the MMC, 2U - is the DC side voltage of MMC; Step 3.2: Substitute equations (4a), (4b), and (5) in step 3.1 into equation (3) in step 3.1, and based on the capacitor-voltage balance condition, eliminate the DC current component to obtain the expression of the energy storage capacitor in the FBSM: Step 3.3: Based on the voltage-current relationship of the fractional-order capacitor, write the terminal voltage function of the energy storage capacitor in the FBSM: Among them, v ,, (t) is the voltage at the energy storage capacitor terminal at time t. The subscript symbol x contains the subset {a, b, c}, which is used to indicate that the FBSM is in the a, b, and c three-phase branches of the MMC respectively; the subscript symbol y contains the subset {p, n}, which is used to indicate that the FBSM is in the upper and lower bridge arms of the MMC respectively.

4. The method for diagnosing open circuit faults in inverter mode of a modular multilevel converter in a flexible DC converter station according to claim 3, characterized in that: In step 3, according to the Grünwald-Letnikov definition, the fractional-order integral operation is performed on equation (7) in step 3.3 to obtain the theoretical value v of the voltage at the energy storage capacitor terminal in the recursive form of the FBSM: ,, The expression of (t) is: Where: v ,,p (t) is the voltage value of the upper bridge arm capacitor in the three-phase branch of MMC a, b, c at time t; v ,,2 (t) is the voltage value of the lower bridge arm capacitor in the three-phase branch of MMCa, b, c at time t; v ,,p (t-1) is the voltage value of the upper bridge arm capacitor in the three-phase branch of MMCa, b, c at time t-1; v ,,2 (t-1) is the voltage value of the lower bridge arm capacitor in the three-phase branches of MMC a, b, and c at time t-1; h is the time interval length for sampling the voltage at the energy storage capacitor terminal, N is the number of times the voltage at the energy storage capacitor terminal is sampled within a fixed time period, and in formula (8a) and formula (8b) The expression is: in, j=1,2,…,N, α is the capacitance order.

5. The method for diagnosing open circuit faults in inverter mode of modular multilevel converters in flexible DC converter stations according to claim 1, characterized in that: In step 4, the capacitance correction value C of the energy storage capacitor is the imaginary part of equation (8a) and equation (8b), that is:

6. The method for diagnosing open circuit faults in inverter mode of modular multilevel converters in flexible DC converter stations according to claim 1, characterized in that: In step 5, the theoretical value v of the capacitor terminal voltage in the FBSM at time t is ,, (t) and the measured value v _,, The difference ε(t) of (t) is: ε(t)=v ,, (t)-v _,, (t)(10)。 7. The method for diagnosing open circuit faults in inverter mode of modular multilevel converters in flexible DC converter stations according to claim 1, characterized in that: In step 6, considering that the actual capacitance of the capacitor generally has a deviation of less than 20% from its nominal value, the deviation threshold δ of the difference between the theoretical value and the measured value of the FBSM capacitor terminal voltage at the power frequency cycle time interval is set to: When the threshold exceeds 5%, it is determined that an open circuit fault occurs in the inverter mode modular multilevel converter.

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