CN116482573A - Open-circuit fault diagnosis method for T-type three-level grid-connected converter - Google Patents

Open-circuit fault diagnosis method for T-type three-level grid-connected converter Download PDF

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CN116482573A
CN116482573A CN202310093221.7A CN202310093221A CN116482573A CN 116482573 A CN116482573 A CN 116482573A CN 202310093221 A CN202310093221 A CN 202310093221A CN 116482573 A CN116482573 A CN 116482573A
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fault
current
converter
phase
type
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金楠
樊武闯
朱妍
郭磊磊
申永鹏
方洁
马润致
贾凯阳
叶青阳
王正伟
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Zhengzhou University of Light Industry
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/50—Testing of electric apparatus, lines, cables or components for short-circuits, continuity, leakage current or incorrect line connections
    • G01R31/54—Testing for continuity
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/50—Testing of electric apparatus, lines, cables or components for short-circuits, continuity, leakage current or incorrect line connections
    • G01R31/56—Testing of electric apparatus
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0003—Details of control, feedback or regulation circuits
    • H02M1/0009—Devices or circuits for detecting current in a converter
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • H02M1/325—Means for protecting converters other than automatic disconnection with means for allowing continuous operation despite a fault, i.e. fault tolerant converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/493—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode the static converters being arranged for operation in parallel
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y04—INFORMATION OR COMMUNICATION TECHNOLOGIES HAVING AN IMPACT ON OTHER TECHNOLOGY AREAS
    • Y04S—SYSTEMS INTEGRATING TECHNOLOGIES RELATED TO POWER NETWORK OPERATION, COMMUNICATION OR INFORMATION TECHNOLOGIES FOR IMPROVING THE ELECTRICAL POWER GENERATION, TRANSMISSION, DISTRIBUTION, MANAGEMENT OR USAGE, i.e. SMART GRIDS
    • Y04S10/00—Systems supporting electrical power generation, transmission or distribution
    • Y04S10/50—Systems or methods supporting the power network operation or management, involving a certain degree of interaction with the load-side end user applications
    • Y04S10/52—Outage or fault management, e.g. fault detection or location

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Inverter Devices (AREA)

Abstract

The invention provides a method for diagnosing open-circuit faults of a T-type three-level grid-connected converter, which comprises the following steps: firstly, analyzing the operation condition of the T-type three-level grid-connected converter after an open-circuit fault occurs, and calculating the current flowing to each phase from the midpoint of the direct-current side capacitor according to the switch state; secondly, calculating the average value of the current flowing to each phase from the midpoint and the average value of the output current of the converter, comparing the average value of the current flowing to each phase from the midpoint and the average value of the output current of the converter with a set threshold value, and positioning the fault position; finally, classifying the faults into I type faults and II type faults according to the fault positions; and respectively establishing fault-tolerant structures of I type faults and II type faults, and respectively carrying out fault-tolerant control on the two types of faults based on a model predictive control strategy. The invention can rapidly position the position of the fault power device of the converter, realize fault tolerance continuous operation and improve the reliability of the converter.

Description

Open-circuit fault diagnosis method for T-type three-level grid-connected converter
Technical Field
The invention relates to the technical field of T-type converter fault diagnosis, in particular to a method for diagnosing open-circuit faults of a T-type three-level grid-connected converter.
Background
In order to accelerate the construction of a modern energy system with green low carbon and sustainable development, new energy power generation such as wind power, photovoltaic and the like is rapidly developed. The new energy grid-connected converter is used as key equipment for accessing new energy into a power grid, and the reliability of the new energy grid-connected converter directly influences the power quality of the power grid. The T-shaped three-level converter has the advantages of high efficiency, low output harmonic content and more balanced power consumption distribution, and is widely applied to the field of new energy power generation.
In the operation process of the photovoltaic power station, the failure rate of the converter accounts for 60% of common failures of the photovoltaic power station. Due to the influence of electromagnetic interference, pulse current, working environment and other factors, the failure rate of the T-type three-level grid-connected converter is increased. After the fault occurs, if the fault can be found and removed in time, the reliability of the new energy power generation grid connection can be improved. In order to ensure safe and reliable operation of the grid-connected power generation system, many researches are dedicated to improving the reliability of the grid-connected converter, so as to improve the stability of the new energy grid-connected system.
The power device faults of the grid-connected converter comprise open circuit faults and short circuit faults. Short circuit faults may be protected using a power device driver circuit. When an open circuit fault occurs, an overcurrent phenomenon generally does not occur, and protection is not easy to operate, so that open circuit fault detection is conducted on the grid-connected converter, and the method has important significance for improving the reliability of the system.
There are two main categories of open circuit fault diagnostic methods, including current-based and voltage-based diagnostic methods. Current-based methods typically utilize phase current, midpoint current, and current residuals to detect faults. Document [ U.Choi, J.Lee, F.Blaabjerg, K.Lee, et al, open-circuit fault diagnosis and fault-tolerant control for a grid-connected NPC inverter, IEEE Transactions on Power Electronics,2016,31 (10): 7234-7247 ] determines a fault phase based on an error in the actual output current of the converter from a reference current. Under different grid-connected conditions, the distortion degree of the output current is different, and the method can cause misdiagnosis. Document [ X.Ge, J.Pu, B.Gou, et al, open-circuit fault diagnosis approach for single-phase wire-level neutral-point-clamped converters [ J ]. IEEE Trans.Power Electron, 2018,33 (3): 2559-2570 ] uses converter output current error to locate faults based on the characteristics of the rate of change of the residual error. This approach requires the injection of specific control signals into the system, reducing the impact on the converter output current quality. The voltage-based approach obtains transient fault information through additional hardware circuitry or voltage observers. The literature [ C.Yang et al, voltage difference residual-based open-circuit fault diagnosis approach for three-level converters in electric traction systems [ J ]. IEEE Trans.Power Electron.,2020,35 (3): 3012-3028 ] proposes a DC bus voltage model for fault diagnosis using residual assessment functions and current similarity functions. Literature [ Wang, y.tang, and c. -j.zhang.open circuit fault diagnosis and tolerance strategy applied to four-wire T-type converter systems [ J ]. IEEE trans.power electron, 2019,34 (6): 5764-5778 ] decomposes models into positive, negative, zero sequence voltage models, and determines fault location from different voltage models. Similar fault characteristics caused by different power devices are fault diagnosis difficulties of a three-level converter, and in order to solve the problem, the literature [ J.Chen, C.Zhang, X.Xing, et al open-circuit fault diagnosis method for T-type wire-level semiconductors [ J ]. Proc.IEEE Energy converters.Congr.Expo ], 2019:5502-5506 ] positions the fault device according to the midpoint voltage change trend of the direct-current side capacitor, and midpoint voltage balance control is adopted to influence the midpoint voltage change trend, so that misdiagnosis is caused. Document [ Z.Li, B.Zhao, X.Zhang, et al, an IGBT open-circuit fault diagnosis method for grid-type T-type heat-level inverters [ J ]. Proc.IEEE Energy converters.Congr.Expo ], 2020:5324-5327 ] and document [ Z.Li, H.Ma, Z.Bai, et al, fast transmitter open-circuit faults diagnosis in grid-speed-phase VSIs based on average bridge arm pole-to-pole voltages and error-adaptive thresholds [ J ]. IEEE Trans.Power electronics,2018,33 (9): 8040-8051 ] propose a fault diagnosis method based on a voltage model, which is related to a modulation strategy, applicable only to Pulse Width Modulation (PWM) strategies.
The fault tolerance control method of the converter is mainly divided into two types of hardware fault tolerance and software fault tolerance. In literature [ KATEBI R, HE jiangbio, WEISE n.an advanced wire-level active neutral-point-clamped inverter with improved fault-tolerant capabilities [ J ]. IEEE Transactions on Power Electronics,2018, 33 (8): 6897-6909 ], [ Wang Yufeng, zhang Ying, li Shuang, etc.. A hardware fault-tolerant method is adopted in the multi-level cascade H-bridge inverter 3N+1 redundancy fault-tolerant strategy research [ J ]. High-voltage electric appliance, 2018,54 (02): 208-212 ], [ WANG Borong, LI Zhan, BAI Zhihong, et al A redundant unit to form T-type wire-level inverters tolerant of IGBT open-circuit faults in multiple legs [ J ]. IEEE Transactions on Power Electronics,2020,35 (1): 924-939 ], and a fourth redundancy bridge arm is used for fault-tolerant control instead of a fault bridge arm, which improves the reliability of the converter but increases the cost. The document [ AZER P, OUNI S, NARIMANI M.A non-fault-tolerant technique for active-neutral-point-clamped inverter using carrier-based PWM [ J ]. IEEE Transactions on Industrial Electronics,2020,67 (3): 1792-1803 ] proposes a fault-tolerant control strategy of an ANPC converter based on a carrier modulation algorithm, and a fault-tolerant function of a single or multiple devices can be realized, but the method does not consider capacitor midpoint voltage, and the midpoint potential of a direct current side is unstable after the fault.
Disclosure of Invention
Aiming at the defects in the background technology, the invention provides a diagnosis method for the open-circuit fault of a T-type three-level grid-connected converter, which solves the technical problem of low reliability of the grid-connected converter.
The technical scheme of the invention is realized as follows:
a method for diagnosing open-circuit faults of a T-type three-level grid-connected converter comprises the following steps:
step one: analyzing the operation condition of the T-type three-level grid-connected converter after the open-circuit fault occurs, and calculating the current flowing to each phase from the midpoint of the direct-current side capacitor according to the switch state;
step two: calculating the average value of the current flowing to each phase from the midpoint and the average value of the output current of the converter, comparing the average value of the current flowing to each phase from the midpoint and the average value of the output current of the converter with a set threshold value, and positioning the fault position;
step three: according to the fault position, classifying into a class I fault and a class II fault;
step four: and respectively establishing fault-tolerant structures of I type faults and II type faults, and respectively carrying out fault-tolerant control on the two types of faults based on a model predictive control strategy.
The open-circuit faults of the T-type three-level grid-connected converter comprise open-circuit faults of 4 power devices of an a-phase bridge arm, open-circuit faults of 4 power devices of a b-phase bridge arm and open-circuit faults of 4 power devices of a c-phase bridge arm.
The method for calculating the current flowing to each phase from the midpoint of the direct-current side capacitor according to the switch state comprises the following steps:
from kirchhoff's current law:
i o ＝i ao +i bo +i co (1)；
wherein i is o Indicating the sum of currents flowing to each phase at the midpoint of the direct-current side capacitor, i ao I is the current flowing to the a phase at the midpoint of the direct-current side capacitor bo I is the current flowing to the b phase at the midpoint of the direct-current side capacitor co A current flowing to the phase c for the midpoint of the direct-current side capacitor;
the converter switching state is defined as:
wherein x=a, b, c; p-state represents S x1 And S is x2 Conduction, S x3 And S is x4 Closing; o state represents S x2 And S is x3 Conduction, S x1 And S is x4 Closing; n states represent S x3 And S is x4 Conduction, S x1 And S is x2 Closing;
according to i xo The relationship corresponding to the switch state yields:
wherein i is a 、i b And i c The average converter outputs current;
according to formulas (1), (2) and (3), i is calculated xo ：
The fault location method comprises the following steps:
calculation of i xo Average current i of (2) xo_ave And will i xo_ave And a set diagnostic threshold I thr1 Comparing; when i xo_ave Less than-I thr1 When the fault is located in the upper half bridge arm; when i xo_ave Greater than I thr1 When the fault is located in the lower half bridge arm;
when the fault is located in the upper half bridge arm, calculating i a Average value i of (2) x_ave And will i x_ave And diagnostic threshold I thr2 Comparing; when i x_ave Less than-I thr2 At this time, the power device S x1 An open circuit fault occurs; conversely, power device S x2 An open circuit fault occurs;
when the fault is located in the lower half bridge arm, calculating i a Average value i of (2) x_ave And will i x_ave And diagnostic threshold I thr2 Comparing; when i x_ave Greater than I thr2 At this time, the power device S x4 An open circuit fault occurs; conversely, power device S x3 An open circuit fault occurs.
The I-type fault refers to a power device S x1 And S is x4 A failure occurs; class II failure refers to power device S x2 And S is x3 A failure occurs.
The model predictive control strategy is:
the mathematical model of the T-shaped three-level grid-connected converter is as follows:
wherein u is αβ ＝[u α ,u β ] T ，u α For the component of the converter output voltage in the alpha-axis, u β For the component of the output voltage of the converter in the beta axis, L is a filter inductance, R is a parasitic resistance of the filter inductance, i αβ ＝[i α ,i β ] T ，i α For the component of the converter output current in the alpha-axis, i β E, outputting a component of the current in the beta axis for the converter αβ ＝[e α ,e β ] T ，e α I is the component of the net side voltage on the alpha axis β Is the component of the net side voltage on the beta axis;
performing Euler discretization on the formula (5) to obtain:
wherein T is a control period, and k is a time;
and (3) reducing the simple formula (6) to obtain the predicted current at the moment k+1:
substituting available voltage vectors of different faults into formula (7) to obtain predicted current;
and constructing a cost function based on the predicted current and the predicted direct-current side capacitance voltage difference, and selecting a voltage vector with the minimum cost function as an optimal voltage vector and applying the optimal voltage vector to the next moment.
The converter voltage components after a class I fault are:
the converter voltage components after a class II fault are:
the expression of the cost function is as follows:
g＝|i αβref (k+1)-i αβ (k+1)|+λ|△u C (k+1)| (15)；
wherein i is αβref As the component of the reference current on the alpha beta axis, lambda is a weight coefficient, deltau C To predict the capacitive voltage difference.
Compared with the prior art, the invention has the beneficial effects that:
1) After the T-type three-level grid-connected converter fails, the current flowing to the failure phase from the midpoint of the direct-current side capacitor and the output current of the converter are lost. Calculating the average value of the current flowing to each phase at the midpoint of the capacitor at the direct current side and comparing the average value with a threshold I thr1 After comparison, the fault bridge arm is determined.
2) If the fault power device is positioned in the longitudinal bridge arm, the fault phase output current is absent; if the fault power device is positioned on the transverse bridge arm, the fault phase output current error is increased. By calculating the average value of the output current of the converter and comparing with a threshold I thr2 And comparing, determining the position of the fault device, and realizing the open-circuit fault detection of the converter.
3) According to the fault type and the fault tolerant structure, a model prediction fault tolerant control strategy is provided, fault tolerant continuous operation can be realized without adding extra hardware redundancy, and the operation reliability of the converter is improved.
Drawings
In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a diagram of a structure of a T-type three-level grid-connected inverter according to the present invention.
Fig. 2 is a voltage vector diagram of the T-type three-level grid-connected inverter of the present invention.
FIG. 3 is a current path diagram after an open circuit fault; wherein (a) P-state, S a1 Failure, i a >0 (b) O state, S a2 Failure, i a >0 (c) O state, S a3 Failure, i a <0 (d) N state, S a4 Failure, i a <0。
Fig. 4 shows the converter output current i a And a current i flowing to phase a from the midpoint ao Waveform (a) S a1 Failure (b) S a2 And (3) failure.
Fig. 5 is a flowchart of a fault diagnosis method of the present invention.
FIG. 6 is a fault tolerant architecture diagram of a class I fault T-type three level converter of the present invention.
FIG. 7 is a graph of voltage vector distribution for a phase a arm class I fault of the present invention.
FIG. 8 is a fault tolerant architecture diagram of a class II fault T-type three level converter of the present invention.
Fig. 9 is a voltage vector diagram of the a-phase arm of the present invention in class ii faults.
FIG. 10 shows a T-type three-level grid-connected converter fault tolerance hardware-in-the-loop experimental platform.
Fig. 11 shows a current waveform in which the midpoint O flows to the a phase.
FIG. 12 shows that the a phase is openBarrier-to-converter output current and fault diagnosis result (a) S a1 Failure (b) S a2 Failure (c) S a3 Failure (d) S a4 And (3) failure.
FIG. 13 shows a phase I failure (S a1 Fault) converter output current and midpoint voltage.
FIG. 14 shows a phase II failure (S a2 Fault) converter output current and midpoint voltage.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without any inventive effort, are intended to be within the scope of the invention.
The embodiment of the invention provides a method for diagnosing open-circuit faults of a T-type three-level grid-connected converter, which comprises the following specific steps:
step one: and analyzing the operation condition of the T-type three-level grid-connected converter after the open-circuit fault occurs, and calculating the current flowing to each phase from the midpoint of the direct-current side capacitor according to the switch state.
The structure of the T-type three-level converter is shown in fig. 1. U (U) dc Is the direct-current side voltage, i a 、i b And i c For the converter to output current, i ao 、i bo And i co The current flowing to each phase is the midpoint of the direct-current side capacitor. L is the filter inductance, and R is the parasitic resistance of the filter inductance. e, e a 、e b And e c Is the net side voltage. F (F) k1 And F k2 Is a fast fuse. Each phase bridge arm of the T-shaped three-level grid-connected converter consists of 4 power devices, and the power devices are respectively S xj (where x=a, b, c, j=1, 2, 3, 4). Each phase bridge arm has 3 switch states, which are P states (S x1 And S is x2 Conduction, S x3 And S is x4 Closed), O state (S x2 And S is x3 Conduction, S x1 And S is x4 Off) and N state (S x3 And S is x4 Conduction, S x1 And S is x2 Closing). Therefore, the T-type three-level grid-connected converter has 3 total 3 =27 switch states. Each switching state corresponds to a voltage vector, as shown in fig. 2, where the long arrow represents a large vector, the middle arrow represents a medium vector, and the short arrow represents a small vector.
To determine the location of an open circuit fault, the condition of the open circuit fault at different locations is analyzed. Assuming that an open circuit fault occurs in an a-phase bridge arm, the current flowing out of the converter is forward current, and the number of fault conditions is 4, namely S a1 ，S a2 ，S a3 And S is a4 . The specific case is as follows.
1) Power device S a1 Open circuit failure
As shown in FIG. 3 (a), when the switching state is P, the forward current passes through S a1 To the filter. When S is a1 The P-state is affected when an open circuit fails. If the voltage u from the midpoint O to the neutral point n of the capacitor of the converter on Greater than the output voltage u a (i.e. u OA >0) The current will pass through S a2 And D a3 Freewheeling until the forward current drops to 0, D a3 Is turned off by the reverse voltage, i ao No path is formed for forward current of (a); if u OA <And 0, the negative current output by the fault phase changes along with the reference value, and the positive current is absent. S is S a1 The occurrence of an open circuit failure does not affect the current flow paths in the O-state and N-state, and the P-state and N-state are not affected.
2) Power device S a2 Open circuit failure
As shown in FIG. 3 (b), when the switching state is O, the forward current passes through S a2 And D a3 To the filter. If S a2 The open circuit fault, the positive current flow path of the O state is affected, the negative current flow path of the O state is not affected, the P state and the N state can be realized, the T-shaped three-level converter has the effect similar to a two-level converter, and the current error is increased.
3) Power device S a3 Open circuit failure
As shown in FIG. 3 (c), when the switch state is O, and the normal operation is performed, a negative current passes throughS a3 And D a2 To the filter. If S a3 The open circuit fault, the negative current flow path of the O state is affected, the positive current flow path of the O state is not affected, the P state and the N state can be realized, the T-shaped three-level converter has the effect similar to a two-level converter, and the current error is increased.
4) Power device S a4 Open circuit failure
As shown in FIG. 3 (d), when the switching state is N, a negative current flows through S during normal operation a4 . If S a4 Open circuit failure, N-state is affected. If the voltage u from the midpoint O to the neutral point n of the capacitor of the converter on Less than the output voltage u a (i.e. u OA <0) The current passes through S a3 And D a2 Freewheel until the negative current is 0, D a1 Is turned off by reverse voltage, i ao No path is formed for negative current; if u OA >0, the fault phase output positive current varies with the reference value, but the negative current is missing. S is S a4 The occurrence of an open circuit failure does not affect the current flow paths of the O-state and the P-state, and the P-state and the N-state are not affected.
The open circuit fault is assumed to occur in the other two phases, the operation of which is the same as that of the open circuit fault occurring in the a phase, and detailed analysis is not repeated.
According to the analysis of the operation condition of the power device fault down-converter, the power device S x1 And S is x4 Is similar to the fault operation condition of S x2 And S is x3 Is similar to the fault operation condition of the system. To determine the position of the fault, the bridge arms of each phase are divided into an upper half bridge arm and a lower half bridge arm, wherein the upper half bridge arm comprises S x1 And S is x2 Two power devices, the lower half bridge arm comprises S x3 And S is x4 Two power devices (x=a, b, c).
Taking phase a as an example, assume that the fault location is located in the upper half leg of phase a. If power device S a1 An open circuit fault occurs and the inverter output a-phase current waveform is shown in fig. 4 (a). The P-state is affected and the fault phase forward current is lost. Due to u OA Greater than 0, the current will pass through S a2 And D a3 Freewheeling, the converter outputting a-phase current in positive half after failureThe wave still has a brief current present. Current i due to freewheel a The current i flows from the midpoint of the capacitor at the DC side of the converter to the fault phase a And i ao And (5) overlapping.
If power device S a2 An open circuit fault occurs and the inverter output a-phase current waveform is shown in fig. 4 (b). The forward current flow path in O state is affected, i ao Is completely absent. S is S a2 The open circuit fault does not affect the P state and the N state, and the forward current of the a phase output by the converter is not lost.
In summary, when the upper half arm of the a-phase fails (S a1 And S is a2 )，i ao Is missing, current i of the non-faulted phase bo And i co And will not be missing. Similarly, if the open fault location occurs in the lower half arm of the a-phase (S a3 And S is a4 )，i ao Is missing, current i of non-fault phase bo And i co And will not be missing.
Comparing the current i flowing to each phase at the midpoint of the direct-current side capacitor ao 、i bo And i co Further, the fault phase is determined by the following specific steps:
from kirchhoff's current law:
i o ＝i ao +i bo +i co (1)；
wherein i is o Indicating the sum of currents flowing to each phase at the midpoint of the direct-current side capacitor, i ao I is the current flowing to the a phase at the midpoint of the direct-current side capacitor bo I is the current flowing to the b phase at the midpoint of the direct-current side capacitor co A current flowing to the phase c for the midpoint of the direct-current side capacitor;
the converter switching state is defined as:
wherein x=a, b, c; p-state represents S x1 And S is x2 Conduction, S x3 And S is x4 Closing; o state represents S x2 And S is x3 Conduction, S x1 And S is x4 Closing; n states represent S x3 And S is x4 Conduction, S x1 And S is x2 Closing;
according to i xo The relationship corresponding to the switch state yields:
wherein i is a 、i b And i c The average converter outputs current;
according to formulas (1) and (2), i is calculated xo ：
Step two: calculating the average value of the current flowing to each phase from the midpoint and the average value of the output current of the converter, comparing the average value of the current flowing to each phase from the midpoint and the average value of the output current of the converter with a set threshold value, and positioning the fault position;
when the open-circuit fault is located in the upper half bridge arm, the forward current flowing to the fault phase from the midpoint of the direct-current side capacitor is absent. When the open-circuit fault is located in the lower half bridge arm, negative current flowing to the fault phase from the midpoint of the direct-current side capacitor is absent. By calculating i xo Average current i of (2) xo_ave And will i xo_ave And a set diagnostic threshold I thr1 Comparing; when i xo_ave Less than-I thr1 When the fault is located in the upper half bridge arm; when i xo_ave Greater than I thr1 And the fault is located in the lower half bridge arm.
When the upper half bridge arm power device S a1 When an open circuit fault occurs, the forward current of the failed phase is lost from the inverter output, as shown in fig. 4 (a). If the upper half bridge arm power device S a2 A fault occurs and the fault phase output current error increases as shown in fig. 4 (b). Calculation of i a Average value i of (2) x_ave And will i x_ave And diagnostic threshold I thr2 Comparing; the fault is located in the upper half bridge arm, when i x_ave Less than-I thr2 At this time, the power device S x1 Open circuit occursA barrier; conversely, power device S x2 An open circuit fault occurs; the fault is located in the lower half bridge arm, when i x_ave Greater than I thr2 At this time, the power device S x4 Open circuit failure occurs, otherwise, power device S x3 An open circuit fault occurs.
To sum up, will i x And i xo The comparison of the average value of (c) with the diagnostic threshold can achieve accurate localization of the faulty device. The flow chart of the proposed fault diagnosis method is shown in FIG. 5, wherein S nj Indicating a specific faulty switching device.
Step three: according to the fault position, classifying into a class I fault and a class II fault; by analyzing the operation condition of the a-phase power device after the open-circuit fault, the power device S can be known a1 And S is a4 The open circuit fault has similar influence on the converter and the power device S a2 And S is a3 The effect of an open circuit fault on the converter is similar. The fault types are divided into two types according to the fault positions, namely I type faults (power devices S x1 And S is x4 ) And type II faults (power device S x2 And S is x3 ) A failure occurs. And fault-tolerant control is carried out on the two types of faults respectively.
The mathematical model of the T-shaped three-level grid-connected converter is as follows:
wherein u is αβ ＝[u α ,u β ] T ，u α For the component of the converter output voltage in the alpha-axis, u β For the component of the output voltage of the converter in the beta axis, L is a filter inductance, R is a parasitic resistance of the filter inductance, i αβ ＝[i α ,i β ] T ，i α For the component of the converter output current in the alpha-axis, i β E, outputting a component of the current in the beta axis for the converter αβ ＝[e α ,e β ] T ，e α I is the component of the net side voltage on the alpha axis β Is the component of the net side voltage on the beta axis;
performing Euler discretization on the formula (5) to obtain:
wherein T is a control period, and k is a time;
and (3) reducing the simple formula (6) to obtain the predicted current at the moment k+1:
step four: and respectively establishing fault-tolerant structures of I type faults and II type faults, and respectively carrying out fault-tolerant control on the two types of faults based on a model predictive control strategy.
When the T-type converter has a class I fault, the fault phase passes through the power device S x2 And S is x3 Directly to the dc side capacitor midpoint. After the class I faults occur to the converter, the fault phase only has an O state, and the non-fault phase has three states respectively, and the total number of the states of the switch is 9. Post-fault voltage component u αβ As shown in formula (8).
Taking a phase a as an example, the fault-tolerant structure after a class I fault is shown in fig. 6. The voltage vectors available after a fault are shown in fig. 7 and in table 1.
Table 1a fault tolerant architecture voltage vector for phase arm class i faults
When the converter fails in class II, the O state of the failed phase is affected, the failed phase has only P state and N state, and the non-failed phase has three states respectively, and the total number of the non-failed phases is 18. Post-fault voltage component u αβ As shown in formula (9).
Taking a phase a as an example, a power device S after II type faults occur a2 And S is a3 Always kept in the off state, the post-fault voltage component u αβ When the a phase fails, S as in formula (9) a Taking 1 or-1, S b And S is c Taking 1, 0 or-1.
When the a-phase bridge arm II fails, available voltage vectors are shown in table 2, and available voltage vector distribution is shown in fig. 9.
Table 2a phase arm class ii fault tolerant architecture voltage vector
And substituting (7) available voltage vectors after different faults into the predicted current according to a limited control set model prediction control theory, evaluating a cost function, selecting the voltage vector with the smallest value as an optimal voltage vector, and applying the optimal voltage vector to the next moment.
After the fault, the direct-current side current directly flows into the fault phase through the midpoint of the capacitor, so that the problem of unbalanced voltage at the midpoint of the direct-current side is aggravated. The invention predicts the voltage difference of the upper capacitor and the lower capacitor at the direct current side through the switch state, and adds a prediction result into the cost function to achieve the purpose of balancing the midpoint voltage.
The expression of the DC side capacitor voltage is:
wherein I is C1 And I C2 Respectively, is to flow through the direct current side capacitor C 1 And C 2 Is set in the above-described range).
Discretizing (10) to obtain a capacitance voltage at the k+1th moment:
the two capacitance values of the direct current side are equal, and the capacitance voltage difference is:
wherein C is capacitance C 1 And C 2 Is a value of (2).
From (1) and (3), it is derived that:
i o ＝(1-|S a |)×i a +(1-|S b |)×i b +(1-|S c |)×i c (13)
substituting (13) into (12) to obtain a predicted capacitance voltage difference:
/>
the predicted current and the predicted DC-side capacitance voltage difference are substituted into a cost function (15).
g＝|i αβref (k+1)-i αβ (k+1)|+λ△u C (k+1)| (15)
Wherein i is abref ＝[i aref ,i bref ] T ，i aref And i bref The components of the reference current in the ab axis, respectively. λ is the weight coefficient.
Experimental analysis
In order to verify the effectiveness of the proposed fault diagnosis and fault tolerance control strategy of the T-type three-level grid-connected converter, a StarSim hardware-in-loop experimental platform shown in fig. 10 is built, a control circuit is completed by a rapid control prototype RCP (Rapid Control Prototyping), a hardware circuit part is completed by a hardware-in-loop HIL (Hardware In Loop) test platform, and experimental parameters are shown in table 3.
Table 3T type three-level grid-connected converter fault tolerance experimental parameter
FIG. 11 shows the current i flowing from the midpoint O of the converter capacitor to the a-phase ao Experimental results of actual values and calculated values of (a). When the converter operates normally, i ao Is consistent with the actual value. When the power device S a2 And when an open circuit fault occurs, the positive half wave of the current flowing to the a phase at the midpoint of the capacitor is missing, and the theoretical analysis is consistent. At this time, the value i is calculated ao The same is true. The experimental result verifies the correctness of the formula (4).
Fig. 12 shows the converter output current and the fault diagnosis result of the open-circuit fault of the a-phase arm of the T-type three-level converter. FIG. 12 (a) shows that when the power device S a1 After an open circuit failure occurs, due to u OA Greater than 0, a Xiang Zheng passing current through S a2 And D a3 Freewheeling until the current drops to D a3 Is turned off by the reverse voltage, and the output forward current of the fault phase is 0. After the fault occurs, the flag jumps from 0 to 1, indicating S a1 A failure occurs.
Fig. 12 (b) shows a power device S a2 And outputting current and fault diagnosis results by the converter after the open circuit fault occurs. When the power device S a2 After an open circuit failure occurs, the current ripple of the failed phase increases. After the fault occurs, the flag jumps from 0 to 2, indicating S a2 A failure occurs.
Fig. 12 (c) and 12 (d) show the power device S, respectively a3 And S is a4 The converter after the open circuit fault has occurred outputs a current waveform and a fault diagnosis result. Power device S a3 After open circuit fault occurs, the negative current ripple of the a phase increases, the flag jumps from 0 to 3, indicating S a3 A failure occurs. Power device S a4 After open circuit fault occurs, the negative current output by the fault phase is reduced to 0, the flag jumps from 0 to 4, indicating S a4 A failure occurs.
Diagnostic threshold I thr1 ＝0.35A、I thr2 =2a. The experimental result of fault diagnosis is consistent with theoretical analysis, and the proposed method can rapidly locate the position where the fault occurs.
Fig. 13 shows the output current of the converter and the upper and lower capacitor voltages after a phase I fault occurs. S is S a1 A phase forward current is lost after fault, and non-fault phase current is distorted. After the proposed fault-tolerant control strategy is adopted, the three-phase current approaches a sine wave, and THD is 4.02%. After the I-type fault, the upper capacitance and the lower capacitance are about 400V when a midpoint voltage balance strategy is not adopted, and the voltages of the upper capacitor and the lower capacitor are basically consistent after the midpoint voltage is balanced.
Fig. 14 shows the waveform of the output current of the inverter in the case of a phase II fault, with a waveform distortion rate of 4.12%. The T-type three-level grid-connected converter has certain self-fault-tolerant capability on II-type faults, and the output current waveform of the converter after the faults is good. After the II-type fault occurs, when a midpoint voltage balance strategy is not adopted, the voltage difference between the upper capacitor and the lower capacitor is about 500V, and the voltage of the two capacitors after the midpoint voltage balance is basically consistent.
The method comprises the steps of firstly analyzing the operation condition of the T-shaped three-level grid-connected converter after the open-circuit fault occurs, and calculating the current flowing to each phase from the midpoint of the direct-current side capacitor according to the switch state. The average value of the output current of the converter and the average value of the current flowing to each phase at the middle point are calculated, and compared with a set threshold value, so that the fault position is positioned. According to the fault location, the type I faults (vertical bridge arm faults) and the type II faults (horizontal bridge arm faults) are classified. And respectively establishing fault-tolerant structures of I type faults and II type faults, and respectively carrying out fault-tolerant control on the two types of faults based on a Model Predictive Control (MPC) strategy. And finally, building a T-type three-level grid-connected converter experimental platform, and verifying the effectiveness of the proposed fault diagnosis and fault tolerance control strategy.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, alternatives, and improvements that fall within the spirit and scope of the invention.

Claims (8)

1. A method for diagnosing open-circuit faults of a T-type three-level grid-connected converter is characterized by comprising the following steps:
step one: analyzing the operation condition of the T-type three-level grid-connected converter after the open-circuit fault occurs, and calculating the current flowing to each phase from the midpoint of the direct-current side capacitor according to the switch state;
step two: calculating the average value of the current flowing to each phase from the midpoint and the average value of the output current of the converter, comparing the average value of the current flowing to each phase from the midpoint and the average value of the output current of the converter with a set threshold value, and positioning the fault position;
step three: according to the fault position, classifying into a class I fault and a class II fault;
step four: and respectively establishing fault-tolerant structures of I type faults and II type faults, and respectively carrying out fault-tolerant control on the two types of faults based on a model predictive control strategy.
2. The method for diagnosing an open circuit fault of a T-type three-level grid-connected inverter according to claim 1, wherein the open circuit fault of the T-type three-level grid-connected inverter includes an open circuit fault of 4 power devices of a-phase bridge arm, an open circuit fault of 4 power devices of a b-phase bridge arm, and an open circuit fault of 4 power devices of a c-phase bridge arm.
3. The method for diagnosing an open circuit fault of a T-type three-level grid-connected inverter according to claim 1, wherein the method for calculating the current flowing to each phase from the midpoint of the capacitor on the direct current side according to the switching state is as follows:
from kirchhoff's current law:
i o ＝i ao +i bo +i co (1)；
wherein i is o Indicating the sum of currents flowing to each phase at the midpoint of the direct-current side capacitor, i ao I is the current flowing to the a phase at the midpoint of the direct-current side capacitor bo I is the current flowing to the b phase at the midpoint of the direct-current side capacitor co A current flowing to the phase c for the midpoint of the direct-current side capacitor;
the converter switching state is defined as:
wherein x=a, b, c; p-state represents S x1 And S is x2 Conduction, S x3 And S is x4 Closing; o state represents S x2 And S is x3 The electric conduction is carried out,S x1 and S is x4 Closing; n states represent S x3 And S is x4 Conduction, S x1 And S is x2 Closing;
according to i xo The relationship corresponding to the switch state yields:
wherein i is a 、i b And i c The average converter outputs current;
according to formulas (1), (2) and (3), i is calculated xo ：
4. The method for diagnosing an open circuit fault of a T-type three-level grid-connected inverter according to claim 3, wherein the method for locating the fault location is as follows:
calculation of i xo Average current i of (2) xo_ave And will i xo_ave And a set diagnostic threshold I thr1 Comparing; when i xo_ave Less than-I thr1 In the time-course of which the first and second contact surfaces,
the fault is located in the upper half bridge arm; when i xo_ave Greater than I thr1 When the fault is located in the lower half bridge arm;
when the fault is located in the upper half bridge arm, calculating i a Average value i of (2) x_ave And will i x_ave And diagnostic threshold I thr2 Comparing; when i x_ave Less than-I thr2 At this time, the power device S x1 An open circuit fault occurs; conversely, power device S x2 An open circuit fault occurs;
when the fault is located in the lower half bridge arm, calculating i a Average value i of (2) x_ave And will i x_ave And diagnostic threshold I thr2 Comparing; when i x_ave Greater than I thr2 At this time, the power device S x4 An open circuit fault occurs; conversely, power device S x3 An open circuit fault occurs.
5. The method for diagnosing an open circuit fault of a T-type three-level grid-connected converter as set forth in claim 3, wherein said I-type fault is a power device S x1 And S is x4 A failure occurs; class II failure refers to power device S x2 And S is x3 A failure occurs.
6. The method for diagnosing an open circuit fault in a T-type three-level grid-connected inverter according to claim 5, wherein the model predictive control strategy is:
the mathematical model of the T-shaped three-level grid-connected converter is as follows:
wherein u is αβ ＝[u α ,u β ] T ，u α For the component of the converter output voltage in the alpha-axis, u β For the component of the output voltage of the converter in the beta axis, L is a filter inductance, R is a parasitic resistance of the filter inductance, i αβ ＝[i α ,i β ] T ，i α For the component of the converter output current in the alpha-axis, i β E, outputting a component of the current in the beta axis for the converter αβ ＝[e α ,e β ] T ，e α I is the component of the net side voltage on the alpha axis β Is the component of the net side voltage on the beta axis;
performing Euler discretization on the formula (5) to obtain:
wherein T is a control period, and k is a time;
and (3) reducing the simple formula (6) to obtain the predicted current at the moment k+1:
substituting available voltage vectors of different faults into formula (7) to obtain predicted current;
and constructing a cost function based on the predicted current and the predicted direct-current side capacitance voltage difference, and selecting a voltage vector with the minimum cost function as an optimal voltage vector and applying the optimal voltage vector to the next moment.
7. The method for diagnosing an open circuit fault in a T-type three-level grid-connected inverter as claimed in claim 6, wherein the voltage component of the inverter after the class I fault is:
the converter voltage components after a class II fault are:
8. the method for diagnosing an open circuit fault in a T-type three-level grid-connected inverter according to claim 6, wherein the cost function is expressed as:
g＝|i αβref (k+1)-i αβ (k+1)|+λ|△u C (k+1)| (15)；
wherein i is αβref As the component of the reference current on the alpha beta axis, lambda is a weight coefficient, deltau C To predict the capacitive voltage difference.
CN202310093221.7A 2023-02-09 2023-02-09 Open-circuit fault diagnosis method for T-type three-level grid-connected converter Pending CN116482573A (en)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117783929A (en) * 2024-02-26 2024-03-29 西北工业大学 Switching tube fault diagnosis method of direct current converter
CN117783929B (en) * 2024-02-26 2024-05-03 西北工业大学 Switching tube fault diagnosis method of direct current converter

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