Open-circuit fault diagnosis method for three-phase interleaving three-level DC-DC converter

Abstract

The invention provides an open-circuit fault diagnosis method for a three-phase interleaving three-level DC-DC converter, which comprises the following steps of: establishing a mathematical model of the three-phase interleaving three-level DC-DC converter by taking the sum of three-phase inductive current iL1 + iL4, iL2 + iL5 and iL3 + iL6 of inductive current iL1-iL6 of the three-phase interleaving three-level DC-DC converter as state variables; on the basis, designing an improved sliding-mode observer of the three-phase interleaving three-level DC-DC converter, and combining an observation value output by the improved sliding-mode observer with an actual value of the sum of inductive currents of upper and lower bridge arms of the same phase to construct a fault detection variable Mx (x = A, B and C) based on a residual gamma (t); a fault detection variable Nj based on the difference value between the inductive current iL1-iL6 and the current mean value is constructed by utilizing the characteristic that the corresponding inductive current declines at the fastest speed after the open-circuit fault of the switching tube, and j is equal to 1-6; and judging whether the corresponding switch tube has an open-circuit fault or not by comparing the two fault detection variables with a set threshold value.

Description

Technical Field

[0001] The invention belongs to the technical field of fault diagnosis of power electronic converters, and in particular relates to an open circuit fault diagnosis method for a three-phase staggered parallel three-level DC-DC converter. Background Art

[0002] With the rapid development of DC microgrid and energy storage technology, the proportion of new energy power generation represented by photovoltaic and wind power in the new power system has gradually increased. In order to solve the problem of large fluctuations in the output of new energy, energy storage systems have become an important support for the grid connection and consumption of new energy. Among them, bidirectional DC-DC converters, as the core equipment for energy interaction between the power grid and the energy storage system, have become a research hotspot in recent years.

[0003] The three-phase interleaved parallel three-level DC-DC converter combines interleaved parallel and multi-level technologies. It has the advantages of interleaved parallel structure to improve converter power and three-level converter to reduce switch tube voltage stress, and can achieve high-power energy transmission. However, since the DC-DC converter in the energy storage microgrid is subjected to a large bus voltage and switches at high frequency between different operating states, the power switch tube is more likely to fail under high-frequency operating conditions. And because the three-phase interleaved parallel three-level DC-DC converter has twice the number of switch tubes as the two-level interleaved parallel DC-DC converter, the probability of failure is greater. Therefore, it is of great significance to study the fault diagnosis method of the three-phase interleaved parallel three-level DC-DC converter.

[0004] The faults of the switch tube in the DC-DC converter include short circuit faults and open circuit faults. The fault characteristic of short circuit faults in the DC-DC converter is the rise of inductor current, which will cause a large impact current in a short time. Therefore, the diagnosis of short circuit faults is usually achieved by detecting overcurrent or converting short circuit faults into open circuit faults. In recent years, the diagnosis of open circuit faults of switch tubes has attracted widespread attention, so the present invention mainly relates to a method for diagnosing open circuit faults of three-phase interleaved parallel three-level DC-DC converters. Summary of the invention

[0005] The purpose of the present invention is to consider the influence of the current sharing control strategy of the three-phase interleaved parallel three-level DC-DC converter on the circuit state under the open circuit fault of the converter switch tube, and propose a three-phase interleaved parallel three-level DC-DC converter open circuit fault diagnosis method based on an improved sliding mode observer.

[0006] A method for diagnosing an open circuit fault of a three-phase interleaved parallel three-level DC-DC converter comprises: L1 ~i L6 The sum of the three-phase inductor currents i L1+i L4 、i L2 +i L5 、i L3 +i L6 The mathematical model of the three-phase interleaved parallel three-level DC-DC converter is established as the state variable. On this basis, an improved sliding mode observer of the three-phase interleaved parallel three-level DC-DC converter is designed. Combining the observed value of the improved sliding mode observer output with the actual value of the sum of the upper and lower arm inductor currents of the same phase, a fault detection variable M based on the residual γ(t) is constructed. x , x = A, B, C; using the characteristic that the corresponding inductor current drops fastest after the switch tube opens the circuit, the inductor current i L1 ~i L6 The fault detection variable N is the difference between the current mean value and j , j = 1 ~ 6, j is the inductor number; the open circuit fault diagnosis method of the three-phase staggered parallel three-level DC-DC converter can be expressed as:

[0007] D j =(M x >T j )&(N j <W)

[0008] If the logical variable D j =1, it means the inductor current i Lj The corresponding switch tube has an open circuit fault; T j is an adaptive threshold, and W is a fixed negative threshold.

[0009] Preferably, the mathematical model of the three-phase interleaved parallel three-level DC-DC converter is:

[0010]

[0011] Among them, i(t)=[i L1 +i L4 i L2 +i L5 i L3 +i L6 ] T is the state variable; u(t)=[V b1 V b2 V o ] T is the input variable; F k (t) is the fault difference vector, and the corresponding model difference exists only in the fault phase; C is the third-order unit matrix, y is the output variable; the coefficient matrices A0 and B0 are:

[0012]

[0013]

[0014] L1~L6 are the inductances of each bridge arm, r L1 ~r L6 is the equivalent resistance of each inductor, d j is the duty cycle of the main control switch connected to the inductor Lj, j=1~6.

[0015] Preferably, the improved sliding mode observer of the three-phase interleaved parallel three-level DC-DC converter is:

[0016]

[0017] In the formula, is the observed value of the three-phase inductor current and; To improve the sliding surface of the sliding mode observer, it is also the residual between the observer output and the actual current sum; h is the observer gain; f(γ(t)) is the adaptive convergence rate, and the specific expression is:

[0018]

[0019] Among them, 0<ε<1, a>1.

[0020] Preferably, the fault detection variable

[0021]

[0022] In the formula, γ x (x=A, B, C) is the residual of the sum of the inductor currents of the upper and lower bridge arms of each phase.

[0023] Preferably, the adaptive threshold T j is based on the current i L1 ~i L6 Designed for average flow that deviates from the mean, average flow parameters z1~z6:

[0024]

[0025] In the formula, z j i represents the converter Lj The degree of balance between the inductor current and the mean value;

[0026] Adaptive threshold T j for:

[0027]

[0028] Adaptive threshold T j will quickly decrease to 0, where the parameter k is used to characterize the adaptive threshold T j rate of decrease.

[0029] Preferably, b=0.85, k=100.

[0030] Preferably, if there is no open-circuit fault in the upper and lower half bridges of a phase of the converter, the inductor current and residual γ of the phase x Close to 0, the fault detection variable M x is 0.5, the threshold value T corresponding to the inductor current j =2,M x <T j , determine T j The corresponding switch tube has no open circuit fault; if a single tube fault or a double tube fault occurs in one phase of the converter, the inductor current and residual γ of the fault phase x will deviate from 0, M x From 0.5 to 1, the fault inductor current corresponds to the threshold value T j =0,M x >T j , determine T j The corresponding switch tube has an open circuit fault.

[0031] Preferably,

[0032] The fixed negative threshold W is based on a single switching cycle T after an open circuit fault occurs. s The degree of fault current reduction within the switch cycle T after a single-tube open-circuit fault s The difference between the fault inductor current and the mean current is:

[0033]

[0034] When a double-tube fault occurs after a single-tube fault, the single switch cycle T s The difference between the internal fault inductor current and the current mean is:

[0035]

[0036] The fixed negative threshold W is designed as:

[0037]

[0038] Where D is the average duty cycle, V in is the input voltage. Lj is the inductance of each bridge arm, j=1-6. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a topological structure of a three-phase interleaved parallel three-level DC-DC converter;

[0040] Figure 2 In order to introduce a virtual voltage source V between the midpoint of the converter input capacitor and the midpoint of the output capacitor, GOAn equivalent topology diagram used to replace half of the topology;

[0041] Figure 3 is the virtual voltage source V GO Waveform diagram within a single cycle;

[0042] Figure 4 is the equivalent circuit diagram of mode 1;

[0043] Figure 5 is the virtual voltage source V in fault stage I GO0 Waveform;

[0044] Figure 6 This is the control block diagram of the average current sharing method;

[0045] Figure 7 S is the average current method A1 When an open circuit fault occurs, the circuit state in the fault stage II;

[0046] Figure 8 S is the average current method A1 When an open circuit fault occurs, the virtual voltage source V GO1 Waveform;

[0047] Fig. 9 This is the control block diagram of the master-slave current balancing method;

[0048] Fig.10 S under the master-slave current balancing method A1 When an open circuit fault occurs, the circuit state of the fault stage II; among them, Fig.10 (a) shows that in S A1 When the drive signal is high, the fault current i L1 S A1 The anti-parallel diode forms a reverse path; Fig.10 (b) shows that in S A1 When the driving signal is low, i L1 S A2 With virtual voltage source V GO4 Constitute a reverse path;

[0049] Fig.11 S under the master-slave current balancing method A1 When an open circuit fault occurs, the virtual voltage source V GO4 Waveform;

[0050] Fig.12 (a)-12(f) is the single tube fault (S A1 ) after a double tube failure (S A1 , S B1 ) is a schematic diagram of the diagnosis result; wherein, Fig.12 (a) is the inductor current, Fig.12 (b) is the switch tube S A1 Fault detection variable M A and threshold T1, Fig.12 (c) To close S B1 Fault detection variable M B and threshold T2, Fig.12 (d) To close S C1 Fault detection variable M C and threshold T3, Fig.12 (e) To manage S A1 —S C1 Fault detection variable N j and threshold W, Fig.12 (f) is the control S A4 —S C4 Fault detection variable N j and threshold W;

[0051] Fig.13 This is a schematic diagram of the diagnosis results of the switch tube fault at the same position in the simulation experiment under the master-slave current sharing method; Fig.13 (a) is the inductor current, Fig.13 (b) is the switch tube S A1 Fault detection variable M A and threshold T1, Fig.13 (c) To close S B1 Fault detection variable M B and threshold T2, Fig.13 (d) To close S C1 Fault detection variable M C and threshold T3, Fig.13 (e) To manage S A1 —S C1 Fault detection variable N j and threshold W, Fig.13 (f) is the control S A4 —S C4 Fault detection variable N j and threshold W. DETAILED DESCRIPTION

[0052] The embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

[0053] The topology of the three-phase interleaved parallel three-level DC-DC converter is as follows Figure 1 As shown. b1 、V b2 is the voltage divider capacitor C b1 , C b2 Voltage, V in is the input voltage, V o is the output voltage, L1~L6 are the inductances of each bridge arm, rL1 ~r L6 is the equivalent resistance of each inductor, C o is a voltage stabilizing capacitor. Each phase of the converter includes four switching tubes S x1 ~S x4 (x=A, B, C), two of which are used as main control switches, and their complementary switches act as freewheeling diodes. Taking phase A as an example, in Buck mode, S A1 , S A4 As the main control switch, S A2 , S A3 For the main control switch tube, the following analysis is based on the open circuit failure of the main control switch tube in Buck mode.

[0054] The switch tube of the same phase of the converter (such as: S A1 With S A4 ) The phase between the carriers is shifted by 180°, and the switch tube S A1 , S B1 , S C1 The carriers are 120° phase-shifted. A single switching cycle T s In the converter, there are 12 modes according to the combination of the on and off states of the corresponding switch tubes. However, circuit analysis based on each mode combination is more complicated. Therefore, a virtual voltage source V is introduced between the midpoint of the input capacitor and the midpoint of the output capacitor of the converter. GO Used to replace half of the topology, the equivalent topology is Figure 2 shown.

[0055] Figure 2 In the process, the switch bridge arm is controlled by a controlled voltage source V A1 、V A4 (V A1 =S A1 V b1 、V A4 =S A4 V b2 ) etc., according to the equivalent before and after i L1 +i L2 +i L3 =i L4 +i L5 +i L6 Keeping constant and Kirchhoff's law, we know that the virtual voltage source V GO The expression is as follows:

[0056]

[0057] In the formula, S A1 ~S C4 For the corresponding switch tube S A1 ~S C4 The driving signal, virtual voltage source V GOThe waveform in a single cycle is as follows Figure 3 As shown. Virtual voltage source V GO There are also 12 modes in a single cycle, corresponding to the 12 circuit modes in a switching cycle of the converter. GO It is equivalent to half of the topology, under its action, i L1 The waveform can be obtained in accordance with the existing direct analysis method. L1 waveform, thus verifying that V GO Equivalent correctness.

[0058] On the basis of topological equivalence, Figure 4 The equivalent circuit diagram taking mode 1 as an example is given.

[0059] Combination Figure 4 The corresponding circuit state equation can be written for the conduction state, and the duration of mode 1 is (d1 / 2+d2 / 2-2 / 3)T s , d j (j=1~6) is the switch tube S A1 ~S C4 The duty cycle of the converter is obtained by combining the circuit equations of the 12 modes and the corresponding durations:

[0060]

[0061] x(t)=[i L1 i L2 i L3 i L4 i L5 i L6 ] T (3)

[0062] u(t)=[V b1 V b2 V o ] T (4)

[0063]

[0064]

[0065] Among them, f k (t) represents the difference vector between the converter model caused by the open-circuit fault of the switch tube and the normal state model. In the normal state of the converter, f k (t)=0.

[0066] A phase S A1 Take an open circuit fault as an example. When S A1 After the circuit is opened, the inductor current of the upper half bridge arm of phase A is i L1Due to the loss of input voltage V b1 The support will gradually decay to 0. According to the virtual voltage source V GO The derivation process of i L1 When it decreases but is still greater than 0, i L1 +i L2 +i L3 =i L4 +i L5 +i L6 remains unchanged, formula (1) still holds, and is defined as fault stage I; i L1 After it decreases to 0, the circuit state is defined as fault stage II. Fault stage I is equivalent to Figure 3 Medium V A1 is always equal to 0, then the virtual voltage source V GO0 Waveform Figure 5 shown.

[0067] When the converter is operating normally, the input voltage V b1 =V b2 =V in / 2,V o =DV in , D is the average duty cycle. Then the fault stage I V GO0 In a cycle T s The average values ​​are as follows:

[0068]

[0069] Combination Figure 5 The controlled voltage source V A1 ~V C4 The cycle mean value can be used to calculate the fault inductor current cycle increment Δi in the fault stage I L1 ~Δi L6 :

[0070]

[0071] Formula (8) shows that in the fault stage I, the fault inductor current i L1 Rapidly decreases, the upper half topology inductor current i L2 ~i L3 Rising, lower half topology i L4 ~i L6 Decreases, but the current decreases faster than i L1 . i.e. i L1 Will decrease to 0 at the fastest speed. Figure 5 Mid-circuit mode and V GO0 The waveform can be used to write the corresponding circuit state equation. By combining the circuit equations of each mode with the duration, the difference vector f0(t) between the circuit mathematical model of the fault stage I and the normal state model can be obtained as follows:

[0072]

[0073] The typical bridge arm current active current sharing control strategies of staggered parallel DC-DC converters are average current sharing method and master-slave current sharing method. Due to the mutual influence of the upper and lower topologies, the fault switch duty cycle output by the two current sharing control strategies in the fault stage II is different, resulting in differences in the converter fault model. Therefore, it is necessary to establish the open circuit fault model of the fault stage II under different current sharing control strategies. The control block diagram of the average current sharing method is shown in the figure below. Figure 6 As shown in the figure, the current sharing control strategy is based on double closed-loop control, where the voltage outer loop generates the current inner loop reference value I Lref , inductor current i L1 ~i L6 The inner loop controller tracks I Lref And generate duty cycle d j (j=1-6), d j ∈[0,1], and finally output to the phase-shift PWM module to achieve interleaved parallel connection.

[0074] Figure 6 In any inductor current i Lj (j=1~6) The corresponding switch tube has an open circuit fault, then i Lj Rapidly decrease to 0, I Lref According to the current inner loop negative feedback controller, the corresponding switch duty cycle d j will quickly increase to 1, and the duty cycle of its complementary switch tube is 0. A1 Take an open circuit fault as an example. At this time, the circuit status in the fault stage II is as follows: Figure 7 shown.

[0075] Depend on Figure 7 It can be seen that L1 After decreasing to 0, S A1 Open circuit, its complementary switch tube S A2 The duty cycle is 0, so i L1 Unable to form an effective circuit, i L1 It will always remain at 0 during the fault phase II. At this time, the virtual voltage source V GO The equivalent prerequisite is given by i L1 +i L2 +i L3 =i L4 +i L5 +i L6 becomes i L2 +i L3 =i L4 +i L5 +i L6 , virtual voltage source V in fault stage II GO1 The expression is as follows:

[0076]

[0077] Virtual voltage source V GO1 Waveform Figure 8 As shown. Figure 8 Medium V GO1 The waveform and the corresponding circuit mode, combined with the circuit equations and duration of each mode, can be obtained under the average current sharing control strategy, S A1 The difference vector f1(t) between the fault phase II circuit mathematical model of the open circuit fault and the normal state model is:

[0078]

[0079] In S A1 Based on the open circuit fault, the double-tube open circuit fault model is further considered. According to the converter topology equivalent principle, the double-tube fault can be divided into the upper half topology (such as S A1 , S B1 Open circuit fault) and upper and lower part topology (such as S A1 , S A4 Open circuit fault). S A1 , S B1 In case of open circuit fault, the prerequisite for topological equivalence is i L3 =i L4 +i L5 +i L6 , virtual voltage source V GO2 The expression is as follows:

[0080]

[0081] S A1 , S A4 In case of open circuit fault, the prerequisite for topological equivalence is i L2 +i L3 =i L5 +i L6 , virtual voltage source V GO3 The expression is as follows:

[0082]

[0083] Similarly, under the average current sharing control strategy, S A1 , S B1 Open circuit fault and S A1 , S A4 The difference vectors f2(t) and f3(t) between the open circuit fault and normal state models are:

[0084]

[0085]

[0086] The control block diagram of the master-slave current sharing method is as follows: Fig. 9 As shown in the figure, the current sharing control strategy is also based on double closed-loop control, where the voltage outer loop generates a reference value I for the current inner loop. Lref . Inductor current i L1 ~i L6 The average value of I is tracked by the inner loop controller Lref And generate an average current duty cycle d PI The topology of the upper and lower parts is based on phase A as the main phase. The difference between the inductor current of the slave phase and the main phase current is used by the current sharing controller to generate the current sharing duty cycle d 12 d 13 d 45 d 46 , current sharing duty cycle and d PI Perform arithmetic operations to ensure that the inductor currents of each phase bridge arm are balanced.

[0087] In steady-state operation, the current sharing duty cycle d 12 d 13 d 45 d 46 Very small, the upper and lower limits of the limiting link are also small, generally d 12 d 13 d 45 d 46 ∈[-d lim ,d lim ],d lim =0.01. When any inductor current i Lj (j=1~6) When the corresponding switch tube has an open circuit fault, i Lj Rapidly decreases to 0, the current-sharing negative feedback controller will reach the limit, and the duty cycle d corresponding to the faulty switch tube j will become d PI -2d lim . A1 Take an open circuit fault as an example. At this time, the circuit status in the fault stage II is as follows: Fig.10 shown.

[0088] Depend on Fig.10 It can be seen that L1 After decreasing to 0, S A1 Open circuit, S A1 The duty cycle is d PI -2d lim , complementary switch tube S A2 The duty cycle is (1-d PI +2d lim ). Fault current i L1 In S A1 When the drive signal is high, the S A1The anti-parallel diode forms a reverse path such as Fig.10 (a) as shown; in S A1 When the driving signal is low, i L1 S A2 With virtual voltage source V GO4 The reverse path is formed as Fig.10 (b) as shown.

[0089] At the same time, in the fault stage II of the master-slave current sharing method, the voltage across the inductor L1 is consistent with the normal state, that is, i L1 The waveform trend is consistent with the normal state, only the current i L1 Reverse flow. Due to the fault switch S A1 The duty cycle d1 changes to d1-2d under the master-slave current sharing method. lim ,therefore Figure 3 The virtual voltage source V GO Will be at 4T s / 6 and 5T s / 6 adds a fault mode. Master-slave current sharing method S A1 Virtual voltage source V when open circuit GO4 Waveform Fig.11 The fault mode duration ΔT can be obtained by the volt-second balance principle under steady-state conditions, that is:

[0090]

[0091] Depend on Fig.11 Medium V GO4 The waveform and the corresponding circuit mode of each mode are combined with the circuit equation and duration of each mode to obtain S under the master-slave current sharing control strategy. A1 The difference vector f4(t) between the fault phase II circuit mathematical model of the open circuit fault and the normal state model is:

[0092]

[0093] In S A1 Based on the open circuit fault, the double-tube open circuit fault model under the master-slave current sharing control strategy is further considered. A1 , S B1 Open circuit fault) and upper and lower part topology (such as S A1 , S A4 When the fault is open circuit fault), the corresponding fault mode is added. Applying the superposition theorem, it can be obtained that under the master-slave current sharing control strategy, S A1 , S B1 Open circuit fault and S A1 , S A4 The difference vectors f5(t) and f6(t) between the open circuit fault and normal state models are:

[0094]

[0095]

[0096] According to the above analysis, after the switch tube open circuit fault occurs in the three-phase interleaved parallel three-level DC-DC converter, the average state equation of all inductor currents is different from that in the normal state. Therefore, combined with the fault difference vectors f0(t)~f6(t) of the above converter, it is proposed to establish the converter fault model based on the sum of the upper and lower bridge arm inductor currents of each phase.

[0097] Considering that the inductance values ​​Lj and L(j+3) of the upper and lower half bridges of the same phase of the converter are slightly different, the sum of the inductance current of the upper and lower bridge arms of each phase (such as i in phase A) is used. L1 +i L4 ) model, the average state equation of the non-fault phase inductor current and is the same as that of the normal state. Combining the difference between the average state equation of the fault phase inductor current and and the normal state model, the current model differences under different switch tube open circuit faults are shown in Table 1.

[0098] Table 1 Correspondence between fault type and current model difference

[0099]

[0100] Under fault conditions, the sum of the inductor currents of the upper and lower bridge arms of the same phase of the converter is used as the output of the observer to design a fault diagnosis method. Under normal operating conditions and without switch tube faults, the sum of the inductor currents is equal to the output of the observer, while the sum of the inductor currents of one phase with a switch tube open circuit fault will significantly deviate from the output of the observer. Based on the above principle, an open circuit fault diagnosis method for a three-phase interleaved parallel three-level DC-DC converter based on an improved sliding mode observer is proposed. The fault detection variable is constructed by the residual between the sliding mode observer output and the actual current sum, and an adaptive threshold based on the inductor current equalization is designed. Fault diagnosis is achieved by combining the deviation of the fault current from the inductor current mean.

[0101] Collect the inductor current i of the three-phase interleaved parallel three-level DC-DC converter L1 ~i L6 , the sum of the three-phase inductor current i L1 +i L4 、i L2 +i L5 、i L3 +i L6 As the state variable, the mathematical model of the three-phase interleaved parallel three-level DC-DC converter is described as follows:

[0102]

[0103] Among them, i(t)=[i L1 +i L4 i L2 +i L5 i L3 +i L6 ] T is the state variable, u(t)=[V b1 V b2 V o ] T is the input variable. k (t) is the fault difference vector, and the corresponding model difference exists only in the fault phase. C is the third-order unit matrix, and y is the output variable. The coefficient matrices A0 and B0 are:

[0104]

[0105]

[0106] On this basis, the improved sliding mode observer of the three-phase interleaved parallel three-level DC-DC converter is designed as follows:

[0107]

[0108] In the formula, is the observed value of the three-phase inductor current and; To improve the sliding surface of the sliding mode observer, it is also the residual between the observer output and the actual current sum; h is the observer gain; f(γ(t)) is the new adaptive approach rate designed by the present invention, and the specific expression is:

[0109]

[0110] Where 0<ε<1, a>1. Substituting the converter parameters into equation (23), the observed value of the sum of the inductor currents of the upper and lower bridge arms of each phase of the converter can be calculated:

[0111] Combining the observed value of the improved sliding mode observer output with the actual value of the sum of the upper and lower arm inductor currents of the same phase, a fault detection variable based on the residual γ(t) is constructed.

[0112] According to the analysis of the converter fault model, only the inductor current and model of the phase with the open-circuit fault of the switch tube differ from the normal state, and the inductor current and model of the phase without fault are still consistent with the normal state. Therefore, the residual γ(t) between the observed value and the actual value of the inductor current and is used to construct the fault detection variable M x (x=A、B、C):

[0113]

[0114] In the formula, γ x (x=A, B, C) is the residual of the sum of the inductor currents of the upper and lower bridge arms of each phase, such as

[0115] The fault detection variable M x Greater than the threshold is used as the fault judgment condition, and the adaptive threshold T is designed. j . Combined with the inductor current i L1 ~i L6 Based on the time domain characteristics of the current i L1 ~i L6 The adaptive threshold T of the average flow rate that deviates from the mean j , first define the average flow parameters z1~z6:

[0116]

[0117] In the formula, z j i represents the converter Lj Due to the existence of the current sharing control strategy, the i Lj Keeping in line with the mean, the average flow parameter z j Maintained near 1; the inductor current corresponding to the open circuit fault switch will decrease to 0 or a negative value, then z j Correspondingly decreases to 0 or a negative value. Therefore, the adaptive threshold T is designed j as follows:

[0118]

[0119] When there is no open circuit fault in the converter, z j =1,T j =2; when the switch tube has an open circuit fault, the corresponding inductor current balancing parameter z j Lower, when z j When it is less than parameter a, the adaptive threshold T j will decrease rapidly to 0, where the parameter k is used to characterize the threshold T in this process. j Combined with the current equalization requirement of the converter during normal operation, b=0.85 and k=100 are designed.

[0120] If there is no open-circuit fault in the upper and lower half bridges of a phase of the converter, the inductor current and residual γ x Close to 0, the fault detection variable M x is 0.5, the threshold value T corresponding to the inductor current j =2,M x <T j , determine T jThere is no open - circuit fault in the corresponding switching tube; if a single - tube fault or a double - tube fault occurs in one phase of the converter, the inductor current and the residual γ of the faulty phase x will deviate from 0, and M x will tend to 1 from 0. The threshold T corresponding to the faulty inductor current j = 0, and M x > T j , it is determined that T j corresponds to an open - circuit fault in the switching tube.

[0121] To prevent misjudgment, taking advantage of the characteristic that the inductor current corresponding to the open - circuit fault of the switching tube drops fastest, a fault - detection variable N based on the difference between the inductor current i L1 ~i L6 and the current mean value is constructed j (j = 1~6):

[0122]

[0123] In the normal state of the converter and for the non - faulty phases, the inductor current is consistent with the mean value, and the fault - detection variable N j is 0; the inductor current corresponding to the open - circuit fault switch tube will decrease to 0 or a negative value at the fastest rate of decline. At this time, the faulty inductor current is less than the mean value, and N j is negative. Therefore, a fixed negative threshold W is designed. When N j < W, it is determined that N j corresponds to an open - circuit fault in the switching tube.

[0124] To improve the speed of open - circuit fault diagnosis, the threshold W is designed based on the degree of fault - current decline within a single switching period T s after an open - circuit fault occurs. Within a single switching period T s after a single - tube open - circuit fault, the difference between the faulty inductor current and the current mean value can be expressed as:

[0125]

[0126] Similarly, when a double - tube fault occurs after a single - tube fault, the difference between the faulty inductor current and the current mean value within a single switching period T s can be obtained:

[0127]

[0128] Therefore, the threshold W is designed as:

[0129]

[0130] Combining the fault - detection variables M x and N j ​, the proposed open - circuit fault diagnosis method for the three - phase interleaved parallel three - level DC - DC converter can be expressed as:

[0131] D j =(M x >T j )&(N j <W)(32)

[0132] If the logic variable D j = 1, it means that the switch corresponding to the inductor current i Lj has an open - circuit fault.

[0133] Next, a simulation model is used to verify the effectiveness of the open - circuit fault diagnosis method based on the improved sliding - mode observer proposed in the present invention under the average current - sharing method and the master - slave current - sharing method. A simulation model is built according to the parameters shown in Table 2, and the present invention is used to diagnose the open - circuit fault of the switch of the three - phase interleaved parallel three - level DC - DC converter.

[0134] Table 2 Converter parameters

[0135]

[0136] Fig.12 The diagnosis results of double - switch faults (S A1 ) after single - switch fault (S A1 , S B1 ) are given under the average current - sharing method, Fig.13 and the diagnosis results of the switch faults at the same position under the master - slave current - sharing method are given. According to Fig.12 , when there is no switch fault in the converter, all the fault - detection variables M x <T j and N j >W, which do not meet the fault - determination conditions, and the diagnosis result is that there is no open - circuit fault of the switch. After S A1 has an open - circuit fault, its corresponding inductor current i L1 rapidly decreases to 0 and remains unchanged, and the other inductor currents still remain balanced with each other under the control of the average current - sharing method. The fault - detection variable M A of phase A rapidly increases after the fault of S A1 , the adaptive threshold T1 decreases, M A >T1, and at the same time, the fault - detection variable N1 changes from 0 to a negative value, less than the fixed threshold W. Combining M A >T1 and N1<W, it is determined that S A1 has an open - circuit fault.

[0137] To verify the effectiveness of double - switch fault diagnosis, an open - circuit fault of S A1 is set after the fault of S B1 , SB1 After the fault, its inductor current i L2 also decreases to 0 and remains unchanged. Only the inductor current i exists in the upper half topology of the converter L3 , and the inductor currents i L4 ~i L6 of the lower half topology still maintain current sharing. S B1 After an open-circuit fault occurs, the fault detection variable M of phase B B increases and is greater than the adaptive threshold T2, that is, M B >T2, and the fault detection variable N2 < W, it is determined that S B1 has an open-circuit fault. In addition, the fault detection variable of the non-fault switch tube S C1 always maintains M C <T3 and N3 > W during this stage, so it is determined that S C1 has no fault.

[0138] Fig.13 is the fault diagnosis result under the master-slave current sharing control method. According to the inductor current waveform in the figure, after an open-circuit fault occurs in S A1 , the corresponding inductor current i L1 rapidly decreases to 0 and circulates in the reverse direction, and the inductor currents corresponding to the remaining non-fault switch tubes maintain current sharing under the master-slave current sharing control method; after an open-circuit fault occurs in S B1 , i L2 also decreases to 0 and circulates in the reverse direction. Only the inductor current i exists in the upper half topology L3 , and the inductor currents i L4 ~i L6 of the lower half topology still maintain current sharing. After an open-circuit fault occurs in S A1 , the fault detection variable M of phase A A recovers to about 0.5 of the fault-free state after fluctuations. This is because the fault switch tube S A1 corresponding to the master-slave current sharing control method still has i L1 circulating in the reverse direction, and the difference between the converter fault model and the normal state is small. The output value of the improved sliding mode observer is still very close to the actual current value. However, since the adaptive threshold T1 corresponding to S A1 rapidly decreases to 0 after the fault, and N1 decreases to a negative value, satisfying M A >T1 and N1 < W, it is determined that S A1 has an open-circuit fault. Similarly, after an open-circuit fault occurs in S B1 , the fault detection variable M of phase B B remains unchanged, the adaptive threshold T2 rapidly decreases to 0, and N2 changes from 0 to a negative value, satisfying M B >T2 and N2 < W, it is determined that S B1 has an open-circuit fault. When double-switch faults of S A1 and S B1 occur, the non-fault switch tube SC1 The fault detection variable of C <T3 and N3>W, so it is determined that S C1 There is no fault.

[0139] The simulation results show that the open-circuit fault diagnosis method based on the improved sliding mode observer proposed in the present invention can effectively realize the single-switch and double-switch open-circuit fault diagnosis of the three-phase interleaved three-level DC-DC converter under the control of the average current sharing method and the master-slave current sharing method.

[0140] This embodiment is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A method for diagnosing an open circuit fault of a three-phase interleaved parallel three-level DC-DC converter, comprising: The inductor current i of the three-phase interleaved three-level DC-DC converter L1 ~i L6 The sum of the three-phase inductor currents i L1 +i L4 、i L2 +i L5 、i L3 +i L6 The mathematical model of the three-phase interleaved parallel three-level DC-DC converter is established as the state variable. On this basis, an improved sliding mode observer of the three-phase interleaved parallel three-level DC-DC converter is designed. Combining the observed value of the improved sliding mode observer output with the actual value of the sum of the upper and lower arm inductor currents of the same phase, a fault detection variable M based on the residual γ(t) is constructed. x , x = A, B, C; using the characteristic that the corresponding inductor current drops fastest after the switch tube opens the circuit, the inductor current i L1 ~i L6 The fault detection variable N is the difference between the current mean value and j , j = 1 ~ 6, j is the inductor number; the open circuit fault diagnosis method of the three-phase staggered parallel three-level DC-DC converter can be expressed as: D j =(M x >T j )&(N j <W) If the logical variable D j =1, it means the inductor current i Lj The corresponding switch tube has an open circuit fault; T j is an adaptive threshold, and W is a fixed negative threshold.

2. The method for diagnosing an open circuit fault of a three-phase interleaved parallel three-level DC-DC converter according to claim 1, characterized in that: The mathematical model of the three-phase interleaved parallel three-level DC-DC converter is: Among them, i(t)=[i L1 +i L4 i L2 +i L5 i L3 +i L6 ] T is the state variable; u(t)=[V b1 V b2 V o ] T is the input variable; F k (t) is the fault difference vector, and the corresponding model difference exists only in the fault phase; C is the third-order unit matrix, y is the output variable; the coefficient matrices A0 and B0 are: L1~L6 are the inductances of each bridge arm, r L1 ~r L6 is the equivalent resistance of each inductor, d j is the duty cycle of the main control switch connected to the inductor Lj, j=1~6.

3. The method for diagnosing an open circuit fault of a three-phase interleaved parallel three-level DC-DC converter according to claim 1, characterized in that: The improved sliding mode observer of the three-phase interleaved parallel three-level DC-DC converter is: In the formula, is the observed value of the three-phase inductor current and; To improve the sliding surface of the sliding mode observer, it is also the residual between the observer output and the actual current sum; h is the observer gain; f(γ(t)) is the adaptive convergence rate, and the specific expression is: Among them, 0<ε<1, a>1.

4. The method for diagnosing an open circuit fault of a three-phase interleaved parallel three-level DC-DC converter according to claim 1, characterized in that: The fault detection variable In the formula, γ x (x=A, B, C) is the residual of the sum of the inductor currents of the upper and lower bridge arms of each phase.

5. The method for diagnosing an open circuit fault of a three-phase interleaved parallel three-level DC-DC converter according to claim 1, characterized in that: The adaptive threshold T j is based on the current i L1 ~i L6 Designed for average flow that deviates from the mean, average flow parameters z1~z6: In the formula, z j i represents the converter Lj The degree of balance between the inductor current and the mean value; Adaptive threshold T j for: Adaptive threshold T j will quickly decrease to 0, where the parameter k is used to characterize the adaptive threshold T j rate of decrease.

6. A method for diagnosing open circuit faults of three-phase interleaved parallel three-level DC-DC converters according to claim 5, characterized in that: b=0.85, k=100.

7. The method for diagnosing an open circuit fault of a three-phase interleaved parallel three-level DC-DC converter according to claim 5, characterized in that: If there is no open-circuit fault in the upper and lower half bridges of a phase of the converter, the inductor current and residual γ x Close to 0, the fault detection variable M x is 0.5, the threshold value T corresponding to the inductor current j =2,M x <T j , determine T j The corresponding switch tube has no open circuit fault; if a single tube fault or a double tube fault occurs in one phase of the converter, the inductor current and residual γ of the fault phase x will deviate from 0, M x From 0.5 to 1, the fault inductor current corresponds to the threshold value T j =0,M x >T j , determine T j The corresponding switch tube has an open circuit fault.

8. The method for diagnosing an open circuit fault of a three-phase interleaved parallel three-level DC-DC converter according to claim 1, characterized in that: The fixed negative threshold W is based on a single switching cycle T after an open circuit fault occurs. s The degree of fault current reduction within the switch cycle T after a single-tube open-circuit fault s The difference between the fault inductor current and the mean current is: When a double-tube fault occurs after a single-tube fault, the single switch cycle T s The difference between the internal fault inductor current and the current mean is: The fixed negative threshold W is designed as: Where D is the average duty cycle, V in is the input voltage. Lj is the inductance of each bridge arm, j=1-6.

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