Heterogeneous MMC reliability modeling method considering multi-task profile and working fault continuity

Abstract

The invention provides a reliability modeling method of a heterogeneous modular multilevel converter (MMC) considering the continuity of a multi-task profile and a working fault, the common characteristics of the heterogeneous MMC comprise switching and operation of various topologies of a converter station, the multi-task profile is divided into different operation states of the MMC, and the multi-task profile is divided into different operation states of the MMC. The working fault continuity is to consider the influence of the branch circuit in the working state and the non-working state on the operation mode. The reliability modeling method comprises the following steps of: 1, respectively carrying out Markov modeling on each branch; 2, dividing a task profile and fault continuity of the MMC; and 3, establishing a Markov model of the MMC according to the task profile, and calculating the reliability. The multi-task profile reliability analysis method takes a bridge arm nested MMC (EFB-MMC) as an example for modeling, is not limited to the EFB-MMC, and is still suitable for other heterogeneous MMC involving circuit topology changes in the operation process. According to the method, the comprehensive reliability of the heterogeneous MMC for realizing multiple functions can be verified, and the method has relatively high universality.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a reliability modeling method suitable for heterogeneous MMCs taking into account multi-task profiles and working fault continuity. Background Art

[0002] Modular multi-level converter (MMC) is the main device for DC transmission, with the advantages of high modularity and good scalability. The reliability of a large number of power devices and capacitors contained in MMC is relatively weak. With the further evolution and development of MMC, the converter structure has become diversified, and the complexity of its operating conditions has increased. MMC faces more complex reliability issues.

[0003] The converter topologies used in engineering are mostly half-bridge MMC and semi-full hybrid MMC. Existing research on the reliability analysis of modular multilevel converters is mostly aimed at converters with independent sub-module structures. However, half-bridge MMC does not have the ability to ride through DC faults. Although semi-full hybrid MMC has the ability to ride through DC faults, the full-bridge sub-module accounts for a large proportion in engineering applications, which reduces the economic efficiency of the converter. Therefore, many heterogeneous MMCs with DC fault clearing capabilities and good economic efficiency have been proposed, such as bridge arm nested MMC. The flexibility of its multi-mode and operation mode determines that it has a variety of switch combination forms, different equivalent topologies under different operating conditions, and the failure of different components in different branches will lead to different operating states. For a variety of low-cost, multi-mode heterogeneous topologies, there is still a lack of effective reliability analysis methods. Therefore, reliability analysis of heterogeneous converters is of great significance for expanding their application boundaries. Summary of the invention

[0004] In order to overcome the problem of lack of effective analysis method for the reliability of the above-mentioned heterogeneous MMC, the present invention provides a heterogeneous MMC reliability modeling method considering multi-task profile and work fault continuity. Take the bridge arm nested MMC as an example for analysis. In the bridge arm nested MMC, each phase bridge arm includes a nested valve section and a half-bridge valve section, and the nested valve section includes bridge arms 1, 4 and bridge arms 2, 3 externally embedded in series HBSM. Bridge arms 1 and 4 are flow branches, which are turned on in steady state to realize the transmission of steady-state current inside the converter station, and the series HBSM outputs positive voltage to the outside. Bridge arms 2 and 3 are voltage-resistant branches, which realize bridge arm flipping in the event of a DC fault, and the nested valve section outputs a negative voltage to the outside to achieve non-locking fault crossing. The modulation method of the half-bridge valve section is consistent with that of the traditional half-bridge MMC. The bridge arm nested MMC controls the polarity of the converter output voltage by switching the bridge arm.

[0005] The present invention takes the bridge arm nested MMC as an example to provide a reliability modeling method applicable to heterogeneous MMC, and the modeling method comprises the following steps:

[0006] Step 1: Based on the failure rate of each component, reliability modeling is performed on bridge arms 1, 4, bridge arms 2, 3 and the half-bridge valve section to obtain the failure rate of each branch and determine the impact of the continuity of working faults on the failure rate of each branch.

[0007] Step 2: Divide the task profile based on the operating status of the nested MMC of the bridge arm, and analyze the continuity of the working faults of each branch according to the current flow path under different task profiles.

[0008] Step 3: Based on the division of the mission profile and the fault continuity of each branch, a multi-state Markov model of the nested MMC of the bridge arm is established to calculate the reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 This is a schematic diagram of the bridge arm nested MMC topology in Example 1 of the present invention;

[0010] Figure 2 This is a schematic diagram of the principle of the bridge arms 1 and 4 modules in Embodiment 1 of the present invention;

[0011] Figure 3 This is a schematic diagram of the Markov state transition of the bridge arm 1 and 4 modules in Example 1 of the present invention;

[0012] Figure 4 This is a schematic diagram of the principle of the bridge arms 2 and 3 modules in Embodiment 1 of the present invention;

[0013] Figure 5 This is a schematic diagram of the Markov state transition of the bridge arms 2 and 3 modules in Embodiment 1 of the present invention;

[0014] Figure 6 is a schematic diagram of a half-bridge submodule in Embodiment 1 of the present invention;

[0015] Figure 7 This is the impact of the failure of the bridge arm nested MMC component in Example 1 of the present invention;

[0016] Figure 8 This is the bridge arm nested MMC ten-state Markov model in Example 1 of the present invention.

[0017] Fig. 9 It is a reliability modeling method for heterogeneous MMC. DETAILED DESCRIPTION

[0018] The present invention provides a reliability modeling method suitable for heterogeneous MMCs that takes into account multi-task profiles and work failure continuity. The modeling steps of the present invention will be further described in detail below by taking a bridge arm nested MMC as an example.

[0019] Step 1: The topology of the bridge arm nested MMC is as follows Figure 1 As shown, there are three branches including bridge arms 1 and 4, bridge arms 2 and 3, and a half-bridge valve section, and reliability modeling is performed for the three branches respectively.

[0020] Bridge arms 1 and 4 are mainly composed of ultra-fast disconnector (UFD) and load commutation switch (LCS), where LCS is composed of several IGBTs connected in series. The module schematic is shown in Figure 2 When the number of LCS or UFD failures exceeds the number of redundancy, the branch is considered to be faulty and the Markov state transition diagram of bridge arms 1 and 4 is established as shown in Figure 3 shown.

[0021] Assume λ L , U They represent the failure rates of the series LCS and series UFD respectively. The Markov equations of the bridge arms 1 and 4 modules are solved as shown in equation (1).

[0022]

[0023] A large number of power electronic devices are connected in series on bridge arms 2 and 3. The module schematic diagram is shown in Figure 4 As shown in Figure 1, the structure of thyristor and diode in anti-parallel connection is regarded as module SD, the structure of IGBT and diode in anti-parallel connection is regarded as module ID, and module SD and module ID are connected in series to form bridge arms 2 and 3. When the number of faults in SD module or ID module exceeds the number of redundancy, it is considered that the branch is faulty, and the Markov state transition diagram of bridge arms 2 and 3 is established as shown in Figure 1. Figure 5 shown.

[0024] Assume λ S , I They represent the failure rates of the SD module and the ID module respectively. The Markov equations of the bridge arms 2 and 3 are solved as shown in equation (2).

[0025]

[0026] In the series-connected half-bridge valve section, the reliability of each HBSM is determined by the IGBT, diode, submodule capacitor, voltage-sharing resistor and submodule control system. The module schematic is shown in Figure 6 As shown, the reliability expression is shown in formula (3).

[0027]

[0028] Step 2: The bridge arms of the nested MMC are the flow branches, which are used to carry the normal working current and are composed of IGBT anti-series LCS and UFD; bridge arms 2 and 3 are blocking branches, which are used to carry and cut off the fault current and are composed of thyristors in series with IGBTs and anti-parallel series diodes. The operation status of the nested MMC includes the following four working conditions:

[0029] (1) Steady-state operating condition: bridge arms 1 and 4 in the EFB valve section are turned on, bridge arms 2 and 3 are turned off, and the EFB valve section outputs a positive voltage to the outside.

[0030] (2) Bridge arm switching condition: After a DC fault is detected, the HBSM in the bypass EFB valve section E , turning on bridge arms 2 and 3, and turning off the IGBT and UFD in bridge arms 1 and 4.

[0031] (3) Fault clearing condition: After UFD is reliably disconnected, HBSM is put into operation E At this time, bridge arms 2 and 3 are turned on, and the EFB valve section outputs a negative voltage to the outside, which is connected to the HBSM H The positive voltage at the output is offset, suppressing the fault current until the fault current drops to zero.

[0032] (4) Restoration of operation: After the fault line is removed, the system is restored and the HBSM is bypassed E , turn on the IGBT on bridge arms 1 and 4 and close the UFD, turn off bridge arms 2 and 3, and after the UFD is fully closed, re-enter the HBSM E , the valve section outputs positive voltage again, and the bridge arm nested MMC resumes stable operation.

[0033] According to the four working conditions of the bridge arm nested MMC, four task profiles are divided, such as Figure 7 As shown. Figure 7 The reasons for the division of working faults and non-working faults are as follows: when a branch in a working state fails, it is considered a working fault, represented by a solid line; when a branch in a non-working state fails, it is considered a non-working fault, represented by a dotted line.

[0034] Step 3: According to Figure 7 Establish the transition probability P(Δt) of the EFB-MMC Markov model. The Markov model is as follows: Figure 8 As shown in formula (4) (the specific expression is shown in formula (5), where P i.j (Δt) represents the probability of transition from state i to state j.

[0035]

[0036] According to the Markov process approximation principle, the coefficient matrix A is obtained. The specific expression of A is shown in formula (6).

[0037]

[0038] Assume that the probability of each state occurring is P i , the sum of the probabilities of all states occurring is 1, and a linear equation system is established:

[0039] [P1 P2 P3 P4 P5 P6 P7 P8 P9 P 10 ]A=0 (7)

[0040] P1+P2+P3+P4+P5+P6+P7+P8+P9+P 10 =1 (8)

[0041] Combining (7)-(8) to obtain the state probability of stable operation of each state of EFB-MMC, the reliability R EFB .

[0042] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Ordinary technicians in the relevant field can still modify or make equivalent substitutions to the specific implementation methods of the present invention with reference to the above embodiments. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the scope of protection of the claims of the present invention to be approved.

Claims

1. A reliability modeling method for heterogeneous MMCs considering multi-task profiles and work failure continuity, characterized in that: include: The common features of the heterogeneous MMC include that the multi-modality of the converter station is not limited to the sub-module structure and the operation mode after a fault involves branch switching. For the convenience of explanation, the article takes the bridge arm nested MMC as an example.

2. Each bridge arm in the nested bridge arm MMC is composed of a nested valve section and a half-bridge valve section in series; the nested valve section is composed of bridge arms 1 and 4, bridge arms 2 and 3 and a half-bridge sub-module (HBSM) in series; bridge arms 1 and 4 are current-carrying branches, which are used to realize the conduction of steady-state current in the converter station when the DC line is operating normally, and the sub-module operates in a normal half-bridge state, and the sub-module port outputs a positive level to reduce the conduction loss; bridge arms 2 and 3 are voltage-resistant branches, which are used to realize the hybrid full-bridge flipping in the event of a DC fault, and the valve section outputs a negative level to realize non-locking ride-through. The half-bridge valve section is used to realize the rectifier and inverter functions of the converter station, and the modulation method is consistent with the traditional half-bridge MMC.

3. According to the bridge arm nested MMC of claim 2, the nested valve section is characterized in that the bridge arms 1 and 4 are respectively connected in series with the bridge arms 2 and 3 to form common points a and b.

4. The bridge arm nested MMC according to claim 2, characterized in that: The on-state bypass includes an ultra-fast mechanical switch UFD and a load transfer switch LCS connected in series with the ultra-fast mechanical switch; the load transfer switch includes N IGBT units, which are combined in series and anti-series; the IGBT unit includes an IGBT and a diode anti-parallel to the IGBT.

5. The bridge arm nested MMC according to claim 2, characterized in that: The pressure-resistant valve section includes a single IGBT and multiple thyristors, and the IGBT and the thyristors are connected in series.

6. The bridge arm nested MMC according to claim 2, characterized in that: The series HBSM includes a capacitor, an IGBT, and a diode connected in anti-parallel with the IGBT.

7. The reliability modeling method considering multi-task profiles and work failure continuity according to claim 2 is characterized in that: include: The multi-task profile is characterized in that the components in the MMC involved in branch switching do not participate in the work in all task profiles, but all components participate in the work only at the moment of branch switching, and this situation only occurs on a millisecond time scale. Therefore, the task profile is divided according to the operating status of the MMC, and the bridge arm nested MMC includes four task profiles. The working fault continuity is characterized in that under a certain operating condition, the failure of the working component has no effect on the current operating state, but if it cannot be discovered and repaired in time, it will affect the normal operation of other conditions. The reliability modeling method is characterized in that reliability modeling is first performed on each branch in the MMC, and the reliability modeling of the converter as a whole is performed based on the task profile and working fault continuity of each branch. Taking the bridge arm nested MMC as an example, the method includes the following steps. Step 1: Reliability modeling for bridge arms 1, 4, bridge arms 2, 3 and half-bridge valve section respectively Step 2: Divide the task profile based on the operating status of the nested MMC of the bridge arm, and analyze the continuity of the working faults of each branch according to the current flow path under different task profiles. Step 3: Based on the division of the mission profile and the fault continuity of each branch, a multi-state Markov model of the nested MMC of the bridge arm is established to calculate the reliability.

Images