A method and device for locating faults in a MMC flexible direct current converter valve submodule

Abstract

The present invention discloses a method and device for locating faults in a submodule of an MMC flexible direct current converter valve, belonging to the field of direct current transmission technology. The method consists of two parts: fault monitoring and fault locating. The fault monitoring part relies on the voltage deviation value of the bridge arm of the MMC system to monitor whether a fault has occurred in the system and locate the fault at the bridge arm level. The fault locating part constructs a sliding mode observer for each submodule of the faulty bridge arm, estimates the capacitor voltage value of the submodule, compares it with the capacitor voltage measurement value to generate a residual sequence, and then compares it with the adaptive threshold to locate the fault to the specific submodule. The present invention provides a pre-condition for the subsequent MMC flexible direct current converter valve system to perform submodule fault removal, isolation, and fault-tolerant operation, thereby improving the reliability of the MMC flexible direct current converter valve system.

Description

Technical Field

[0001] The present invention relates to a method and device for locating a fault of an MMC flexible direct current (MMC) converter valve submodule, belonging to the technical field of direct current transmission. Background Art

[0002] The modular multilevel converter (MMC) was first proposed by Professor R. Marquardt in 2001. It consists of a cascade of multiple identical sub-modules (SMs). Sub-modules can be categorized into three types: half-H-bridge, full-H-bridge, and double-clamp. MMCs offer advantages such as low harmonic content, flexible control, low losses, high output power quality, and easy voltage and power scalability. They have attracted widespread attention and application in areas such as flexible direct current transmission (HVDC). However, MMC circuits often have numerous sub-modules, with each bridge arm containing a sub-module in high-voltage, high-power applications. This large number of sub-modules increases the probability of various failures. If a sub-module failure is not promptly detected and isolated, it can severely impact MMC system performance. Each half-bridge sub-module contains two IGBTs, a storage capacitor, and a voltage sensor, potentially subject to various component failures. For different types of faults, how to quickly and accurately locate the faults to facilitate subsequent MMC system fault removal, isolation and fault-tolerant operation has become a crucial issue. Summary of the Invention

[0003] The purpose of the present invention is to propose a method and device for locating faults of MMC flexible direct current converter valve submodules, which can realize arm-level positioning of multiple types of faults through a fault monitoring link, and realize submodule-level positioning of multiple types of faults through a fault location link.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] In a first aspect, the present invention provides a method for locating a fault in an MMC flexible direct current converter valve submodule, comprising:

[0006] Obtaining actual bridge arm voltage values and bridge arm voltage measurement values of the MMC three-phase six-bridge arm, and obtaining a bridge arm voltage deviation based on a difference between the actual bridge arm voltage values and the bridge arm voltage measurement values;

[0007] Calculating a bridge arm fault detection variable based on the bridge arm voltage deviation;

[0008] Determine whether the MMC has a fault based on the bridge arm fault detection variable, and locate the fault to the bridge arm level;

[0009] A capacitor voltage sliding mode observer is constructed for each submodule of the faulty bridge arm to obtain the estimated value of the submodule capacitor voltage.

[0010] For each submodule, the submodule capacitance voltage estimated value is compared with the submodule capacitance voltage measured value to generate a submodule capacitance voltage residual value;

[0011] Based on the submodule capacitance voltage residual value, it is determined whether a submodule fails, and the fault is located in a specific submodule.

[0012] Preferably, the obtaining of the actual values of the bridge arm voltages of the MMC three-phase six-bridge arms includes:

[0013]

[0014] Among them, U arm_real_pj is the actual value of the bridge arm voltage of the j-th phase upper bridge arm, U arm_real_nj is the actual value of the bridge arm voltage of the j-th phase lower bridge arm, U dc is the DC bus voltage, u phase_j represents the output phase voltage of the jth phase, L0 is the bridge arm inductance, i arm_pj is the bridge arm current of the j-th phase upper bridge arm, i arm_nj is the bridge arm current of the j-th phase lower bridge arm, j = a, b, c, representing the three phases A, B, and C.

[0015] Preferably, the bridge arm voltage measurement value is calculated as follows:

[0016]

[0017] Among them, U arm_meas_pj is the bridge arm voltage measurement value of the j-th phase upper bridge arm, U arm_meas_nj is the measured value of the bridge arm voltage of the lower bridge arm of phase j, S pjk is the switching function of the kth submodule on the upper bridge arm of the jth phase, S njk is the switching function of the kth submodule on the lower bridge arm of the jth phase, U c_meas_pjk is the capacitor voltage measurement value of the kth submodule of the jth phase upper bridge arm, U c_meas_njk is the capacitance voltage measurement value of the kth submodule of the lower bridge arm of the jth phase, and N is the number of submodules.

[0018] Preferably, the calculating of the bridge arm fault detection variable based on the bridge arm voltage deviation includes:

[0019]

[0020] Among them, F xj represents the fault detection variable of the xth bridge arm of the jth phase, x=p,n, p represents the upper bridge arm, n represents the lower bridge arm, U arm_Err_xj It represents the voltage deviation of the x-th bridge arm of the j-th phase, t is the time, and T is the fault monitoring period.

[0021] Preferably, judging whether an MMC fault occurs based on the bridge arm fault detection variable and locating the fault at the bridge arm level includes:

[0022] The sliding window method is used to calculate the arm fault detection variables of each of the three-phase six bridge arms window by window;

[0023] When |F xj |>ε F_D When it is established, let the count value of the fault monitoring counter C nt_Falut_D Start to automatically add 1, ε F_D is the bridge arm fault threshold;

[0024] If |F is satisfied within a continuous power frequency cycle xj |>ε F_D , it is determined that the MMC has a fault, and the fault location occurs in the jth phase and the xth bridge arm;

[0025] If |F xj |≤ε F_D , then let the count value C nt_Falut_D Reset to zero and restart counting.

[0026] Preferably, the fault monitoring period is set to one power frequency period;

[0027] The sliding window is set to 1 / 20 of the power frequency period.

[0028] Preferably, the bridge arm fault threshold ε F_D Set to U dc / 10% of N.

[0029] Preferably, constructing a capacitor voltage sliding mode observer for each submodule of the faulty bridge arm to obtain an estimated value of the submodule capacitor voltage includes:

[0030] Define the sliding surface

[0031] Based on the sliding mode surface, the submodule capacitance voltage sliding mode observer is established as follows:

[0032]

[0033] in, is the output value of the capacitor voltage sliding mode observer of the kth submodule of the xth bridge arm of the jth phase, U c_meas_xjk is the capacitance voltage measurement value of the kth submodule of the xth bridge arm of the jth phase, τ is the observation gain, sat(·) is the saturation function, and h is the width of the non-saturation region of the saturation function;

[0034] Select V = 0.5 × S TS is used as a Lyapunov function. By adjusting the observation gain τ, the submodule capacitor voltage sliding mode observer is controlled to a stable state of the sliding mode surface S=0. At this time, the output value of the capacitor voltage sliding mode observer is the submodule capacitor voltage estimation value.

[0035] Preferably, the determining whether a submodule fails based on the submodule capacitance voltage residual value and locating the fault to a specific submodule includes:

[0036] In each sampling period, the submodule capacitor voltage value measured by the voltage sensor is compared with the submodule capacitor voltage estimated value output by the capacitor voltage sliding mode observer to generate a residual value U c_Err ;

[0037] When|U c_Err |>ε C_Err When it is established, let the count value of the fault location counter C nt_Uc_Err Start to automatically add 1, ε C_Err is the submodule fault threshold;

[0038] If |U is satisfied within a continuous power frequency cycle c_Err |>ε C_Err , it is determined that the submodule is faulty;

[0039] If |U c_Err |≤ε C_Err , then let the count value C nt_Uc_Err Reset to zero and restart counting.

[0040] In a second aspect, the present invention provides an MMC flexible direct current converter valve submodule fault locating device, which is used to implement the MMC flexible direct current converter valve submodule fault locating method, and the device includes:

[0041] A first calculation module is used to obtain actual values of bridge arm voltages and measured values of bridge arm voltages of the three-phase six bridge arms of the MMC, and obtain bridge arm voltage deviations based on differences between the actual values of the bridge arm voltages and the measured values of the bridge arm voltages;

[0042] A second calculation module is used to calculate a bridge arm fault detection variable based on the bridge arm voltage deviation;

[0043] A first detection module is used to determine whether the MMC has a fault based on the bridge arm fault detection variable and locate the fault to the bridge arm level;

[0044] The third calculation module is used to construct a capacitor voltage sliding mode observer for each submodule of the fault bridge arm to obtain an estimated value of the submodule capacitor voltage;

[0045] a fourth calculation module, configured to compare, for each submodule, the submodule capacitance voltage estimation value with the submodule capacitance voltage measurement value to generate a submodule capacitance voltage residual value;

[0046] The second detection module is configured to determine whether a submodule fails based on the submodule capacitance voltage residual value, and locate the fault to a specific submodule.

[0047] The beneficial effects of the present invention are as follows:

[0048] The present invention provides a method for locating faults of submodules of an MMC flexible direct current converter valve. The method comprises two steps: fault monitoring and fault locating. The fault monitoring step locates the fault at the bridge arm level, and the fault locating step locates the fault at a specific submodule. This method provides a pre-condition for subsequent submodule fault removal, isolation, and fault-tolerant operation of the MMC flexible direct current converter valve system, thereby improving the reliability of the MMC flexible direct current converter valve system.

[0049] In the fault monitoring link of the present invention, the input quantities required in the calculation process, such as the output phase voltage, the bridge arm current, and the switch modulation signal, are all the sampling quantities required for MMC modulation and voltage balancing, so there is no need to add additional sampling circuits.

[0050] In the fault location link of the present invention, the fault location processes between the submodules are completely independent and autonomous, and parallel calculations can be performed to realize simultaneous location of multiple faults of different types.

[0051] The method of the present invention can locate various types of faults in the MMC flexible direct current converter valve submodule within 25ms. It is suitable for the rapid positioning of various types of faults in the MMC submodule, including the submodule IGBT upper switch open circuit fault, IGBT lower switch open circuit fault, voltage sensor disconnection fault, voltage sensor stuck fault, and voltage sensor gain fault. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 MMC topology diagram provided by an embodiment of the present invention;

[0053] Figure 2 for Figure 1 The MMC submodule topology diagram in the MMC topology diagram shown;

[0054] Figure 3 Schematic diagram of the sliding mode observer of the MMC submodule provided in an embodiment of the present invention;

[0055] Figure 4 A schematic flow chart of a method for locating faults in a MMC flexible DC converter valve submodule based on a sliding mode observer according to an embodiment of the present invention;

[0056] Figure 5This is a simulation waveform diagram of switch open circuit fault monitoring and fault location on the sub-module IGBT in an embodiment of the present invention;

[0057] Figure 6 This is a simulation waveform diagram of the open circuit fault monitoring and fault location of the lower switch of the sub-module IGBT in an embodiment of the present invention;

[0058] Figure 7 This is a simulation waveform diagram of the submodule voltage sensor gain fault monitoring and fault location in an embodiment of the present invention;

[0059] Figure 8 This is a simulation waveform diagram of submodule voltage sensor disconnection fault monitoring and fault location in an embodiment of the present invention;

[0060] Figure 9 This is a simulation waveform diagram of the submodule voltage sensor stuck fault monitoring and fault location in an embodiment of the present invention. DETAILED DESCRIPTION

[0061] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments and the accompanying drawings. Here, the exemplary embodiments of the present invention and their descriptions are used to explain the present invention, but are not intended to limit the present invention.

[0062] It should also be noted that, in order to avoid obscuring the present invention due to unnecessary details, the accompanying drawings only show structures and / or processing steps closely related to the solutions according to the present invention, while other details that are not closely related to the present invention are omitted.

[0063] It should be emphasized that the term "include / comprises" when used herein refers to the existence of features, elements, steps or components, but does not exclude the existence or addition of one or more other features, elements, steps or components.

[0064] It should be noted here that the step marks mentioned below do not limit the order of the steps, but it should be understood that the steps can be executed in the order mentioned in the embodiment, or in a different order from the embodiment, or several steps can be executed simultaneously.

[0065] The first embodiment of the present invention provides a sliding mode observer-based method for locating faults in an MMC flexible direct current converter valve submodule. This method can quickly and accurately locate multiple fault types, including two types of IGBT open-circuit faults and three types of voltage sensor faults in the MMC submodule. The method specifically includes:

[0066] Step S1: The actual value of the bridge arm voltage of the three-phase six-bridge arm of the MMC is obtained by calculating the DC bus voltage, the output phase voltage, the bridge arm current sampling value, etc.; the bridge arm voltage measurement value of the three-phase six-bridge arm of the MMC is obtained by calculating the bridge arm PWM switching sequence and the capacitor voltage sampling sequence; the deviation between the actual bridge arm voltage value and the bridge arm voltage measurement value is calculated; the fault detection variable is calculated by sliding to obtain the average value of the bridge arm voltage deviation of the three-phase six-bridge arm, and whether the MMC has a fault is determined based on the fault detection variable, and the fault is located at the bridge arm level.

[0067] Step S2: For the bridge arm determined to be faulty, a sliding mode observer is constructed for all submodules of the faulty bridge arm to estimate the capacitance and voltage values of all submodules. The residual sequence is compared with the capacitance and voltage measurement values of the submodules obtained by the voltage sensor to generate a residual sequence. The residual sequence is then compared with the adaptive threshold to locate the fault to the specific submodule.

[0068] In the embodiment of the present invention, whether an MMC fault occurs is determined based on the calculated bridge arm voltage deviation, and the fault is located at the bridge arm level. The specific implementation process is as follows:

[0069] See also Figure 1 The MMC topology shown in the figure is provided in an MMC flexible DC converter valve, wherein each phase includes two bridge arms, an upper arm and a lower arm, and each bridge arm includes N submodules (SM in the figure) and a bridge arm reactor.

[0070] u in the figure arm_xj and i arm_xj They are respectively represented as the sum of the voltage of all submodules on the x-th bridge arm of the j-th phase and the bridge arm current, j = a, b, c, representing the three phases A, B, and C, x = p, n, p represents the upper bridge arm, n represents the lower bridge arm, u phase_j and i phase_j represents the output phase voltage and phase current of phase j, U dc is the DC bus voltage, L0 is the bridge arm inductance, U sa 、U sb 、U sc are the AC side grid voltages of phases A, B, and C respectively, L g is the grid-side inductance.

[0071] Kirchhoff voltage equations for the upper and lower bridge arms are shown below:

[0072]

[0073] where u arm_xj and i arm_xj They are respectively expressed as the sum of the voltage of all submodules on the x-th bridge arm of the j-th phase and the bridge arm current. When the nearest level approximation modulation is adopted, the number of submodules put into operation in the upper and lower bridge arms of each phase is as follows:

[0074]

[0075] Among them, U ref_j is the reference modulation wave of the jth phase, n pj 、n nj are the number of submodules put into operation in the upper and lower bridge arms of the j-th phase, respectively. The round(·) function represents the rounding function on both sides.

[0076] After adopting the circulating current suppression strategy, the bridge arm current is:

[0077]

[0078] Among them, I d Indicates the DC bus current, i phase_j (t) represents the output phase current of the j-th phase at time t, and P is the transmission power of the MMC system.

[0079] The switch function of the MMC system submodule is defined as shown in formula (4), and the submodule schematic is shown in Figure 2 As shown, the submodule port voltage is defined as U sm_xjk , the current flowing through each submodule is the bridge arm current i arm_xj , the actual value of the submodule capacitor voltage is U c_real_xjk , the submodule capacitor voltage measurement value is U c_meas_xjk .

[0080]

[0081] Where k=1,2,3…N represents the kth submodule. xjk =1, indicating that the upper switch S1 of the kth submodule of the jth bridge arm of the MMC system is turned on and the lower switch S2 is turned off. xjk =0, it indicates that the upper switch S1 of the kth submodule of the xth bridge arm of the jth phase of the MMC system is turned off, and the lower switch S2 is turned on.

[0082] During normal operation, the submodule port voltage can be expressed as:

[0083] U sm_xjk =S xjk ·U c_real_xjk =S xjk ·U c_meas_xjk (5)

[0084] When there is no voltage sensor failure, the actual value of the submodule capacitor voltage is equal to the measured value:

[0085] U c_real_xjk =U c_meas_xjk (6)

[0086] When the upper switch S1 fails to open, if the bridge arm current i arm_xj <0 and the switching function S xjk =1. Since the upper switch S1 is open, the bridge arm current that should flow through S1 and discharge the capacitor is forced to flow to the anti-parallel diode D2 of S2. At this time, the capacitor current is 0, and the capacitor loses the discharge state. The voltage at its submodule port changes from formula (5) under normal working conditions to formula (7).

[0087]

[0088] When the lower switch S2 fails to open, if the bridge arm current i arm_xj >0 and the switch function S xjk =0. Since S2 is open, the bridge arm current that should flow through S2 is forced to flow to the anti-parallel diode D1 of S1 and charge the capacitor. At this time, the capacitor current is greater than 0, and the capacitor is forced to enter the charging state. The voltage at its submodule port changes from formula (5) under normal working conditions to formula (8).

[0089]

[0090] When a submodule voltage sensor disconnection fault occurs, when the bridge arm current i arm_xj > 0, if the number of submodules put into operation in the faulty bridge arm is ≠ 0, the faulty submodule will be in the charging state; if the number of submodules put into operation in the faulty bridge arm is =0, the faulty submodule will be in the bypass state; when the bridge arm current i arm_xj When <0, if the number of submodules put into operation in the faulty bridge arm ≠ N, the faulty submodule will be in the bypass state all the time. If the number of submodules put into operation in the faulty bridge arm = N, the faulty submodule will be in the discharge state all the time. The average value of the capacitance of the faulty submodule will continue to jump and increase. The measured capacitance voltage increment of the submodule will be 0 all the time. The port voltage of the faulty submodule is shown in formula (9), where & & Represents logical AND.

[0091]

[0092] When a submodule voltage sensor gain failure occurs, the gain coefficient after the gain failure is set to K gain Then when K gain <0.8, and when the bridge arm current i arm_xj >0, the submodule will be in charging state all the time; when the bridge arm current i arm_xj When K is less than 0, the submodule will be in the bypass state. In the early stage of the fault, the capacitor of the faulty submodule is only in the charging process, not the discharging process. The actual submodule capacitance average value continues to increase, and the capacitor voltage increment is only in the positive half cycle and is 0 in the negative half cycle. gainWhen the value is greater than 1.2, the charge and discharge process is reversed. At the end of the fault, the capacitor voltage measurement value is equal to the actual capacitor voltage value of other normal submodules. At this time, the charge and discharge process of the faulty submodule returns to normal, and the actual capacitor voltage value is 1 / K of the normal value. gain times. At the initial stage of the fault, the voltage at the fault submodule port is as shown in formula (10):

[0093]

[0094] From the above analysis, it can be seen that when a sub-module IGBT fault or a voltage sensor fault occurs, the sub-module port voltage will deviate from the port voltage in the normal operating mode, and the sum of the port voltages of all sub-modules on the bridge arm constitutes the bridge arm voltage. Therefore, this embodiment will construct a fault monitoring variable based on the measured value and actual value of the bridge arm voltage.

[0095] In this embodiment, the fault monitoring variable F xj It will be used to determine whether the MMC converter system has a fault and locate the fault at the bridge arm level. The fault monitoring variable F is defined. xj As shown in formula (11),

[0096]

[0097] Among them U arm_Err_xj It represents the deviation between the actual value of the bridge arm voltage of the x-th bridge arm of the j-th phase and the measured value of the bridge arm voltage, as shown in formula (12). T is the fault monitoring period, which is set to one power frequency period of 20ms.

[0098] Actual value of bridge arm voltage U arm_real_xj It can be deduced from formula (1), as shown in formula (13),

[0099] U arm_Err_xj =U arm_real_xj -U arm_meas_xj (12)

[0100]

[0101] The bridge arm voltage measurement value U measured by the voltage sensor arm_meas_xj As shown in formula (14),

[0102]

[0103] The input quantities required in the calculation process, such as the output phase voltage, bridge arm current, and switch modulation signal, are all sampling quantities necessary for MMC modulation and voltage balancing, so there is no need to add additional sampling circuits.

[0104] Assume that the i-th submodule fails, and the capacitor voltage value of the faulty submodule is U c_meas_xji , then the bridge arm voltage measurement value is:

[0105]

[0106] The bridge arm voltage deviation is shown in formula (16):

[0107] U arm_Err_xj =U sm_xji -S xji ×U c_meas_xji (16)

[0108] When an upper switch IGBT fault occurs, the bridge arm voltage deviation at this time is calculated by substituting it into formula (7), as shown in formula (17),

[0109]

[0110] When the lower switch IGBT fails, the bridge arm voltage deviation can be calculated by substituting the port voltage formula (8). Since the submodule capacitor voltage gradually increases when the lower switch IGBT fails, the bridge arm voltage deviation also gradually increases, as shown in formula (18).

[0111]

[0112] When a voltage sensor disconnection fault occurs, the bridge arm voltage deviation is calculated by substituting formula (9) into formula (19),

[0113]

[0114] When a voltage sensor stuck fault occurs, the voltage deviation of the fault bridge arm is as shown in formula (20), where U c_stuck is the capacitor voltage when the voltage sensor is stuck, U c_normal_xji It is the capacitor voltage value of the normal submodule in the same bridge arm when the voltage sensor is stuck.

[0115]

[0116] When a voltage sensor gain fault occurs, at the initial stage of the fault, the bridge arm voltage deviation is calculated by substituting the port voltage formula (10) into the formula (21) and the formula (22).

[0117] If K gain >1.2, then the bridge arm voltage deviation is obtained by formula (21),

[0118]

[0119] If K gain <0.8, then the bridge arm voltage deviation is obtained by formula (22):

[0120]

[0121] From equations (17) to (22), we can see that if a fault occurs, the fault monitoring variable F xj The value will deviate from zero and exceed the bridge arm fault threshold, thereby locating the fault to a specific bridge arm.

[0122] Fault detection variable F xj is the bridge arm voltage deviation U arm_Err_xj The average value within a power frequency cycle. In the actual MMC fault diagnosis system, the voltage deviation of each bridge arm of the three-phase six bridge arms is obtained by sliding, and each sliding window can be 1 / 20 power frequency cycle.

[0123] When the fault detection variable |F xj |>ε F_D When , it can be considered that the system has a fault, and the fault location occurs in the jth phase and the xth bridge arm.

[0124] It should be noted that the bridge arm fault threshold ε F_D Can be set to U dc / N is 10%, which not only takes into account the fault monitoring rate and accuracy, but also enhances the system's robustness to DC side voltage fluctuations and avoids the occurrence of false alarms and missed fault detection.

[0125] In the embodiment of the present invention, after the fault arm level is located in the fault monitoring link, the fault arm fault flag is set to 1 and the subroutine of the fault location link is started. The principle of the fault location link is as follows:

[0126] Considering that the switching period is much shorter than the fundamental period under the nearest level approach modulation, the bridge arm current i arm_xj The change within a switching cycle can be ignored. Therefore, the capacitor voltage can be averaged within a switching cycle and the switching function S can be defined. xjk The average value of the duty cycle d in one switching cycle xjk , as shown in formula (23),

[0127]

[0128] Among them, C sm is the capacitance value, U c_meas_xjk is the submodule capacitor voltage measurement value.

[0129] Establish the submodule capacitance voltage observation equation (24),

[0130]

[0131] in, is the output value of the submodule capacitor voltage observer, τ is the observation gain, and sat(·) is the saturation function.

[0132] Define the sliding surface Subtracting Equation (23) from the submodule capacitor voltage observation equation (24) yields the submodule sliding mode observer as follows: Equation (25):

[0133]

[0134] Where h represents the width of the unsaturated region of the saturation function. When it exceeds h or -h, the function output will be limited to the upper or lower bound.

[0135] Figure 3 Schematic diagram of the sliding mode observer of the MMC submodule. Select V = 0.5 × S T S is the Lyapunov function, and its derivative is formula (26). By adjusting the τ parameter, the observation system can be controlled to a stable state where the sliding surface S = 0. At this time, the sliding surface S will jump back and forth near 0. At this time, the observed value is equal to the measured value, that is, the estimated value of the submodule capacitance voltage is obtained; when a fault occurs, Will deviate from U c_meas_xjk This produces the residual:

[0136]

[0137] in, represents the derivative of V, represents the derivative of S.

[0138] As long as τ>0 is guaranteed, its derivative can be less than 0, which means that the sliding mode observer can converge to 0 when the system is operating normally. When the sliding mode observer observes a small disturbance, sat(S) = S / h. Substituting it into formula (25) yields:

[0139]

[0140] This formula shows that if the sliding mode observer deviates from the initial value S(0) due to a small disturbance, it decays exponentially, and the decay rate is determined by the parameter Decide.

[0141] The residual sequence is calculated by the sliding surface S, and the residual sequence is compared with the submodule fault threshold ε C_Err By comparing the two modules, the fault can be located in the specific sub-module.

[0142] Based on this, the embodiment of the present invention provides a method for locating multiple types of faults in the MMC flexible direct current valve submodule based on a sliding mode observer, see Figure 4 , including the following steps:

[0143] S1. Calculate the actual value of the bridge arm voltage based on the bridge arm current, output phase voltage, and switch modulation signal;

[0144] S2. Calculate the fault detection variable F based on the actual value of the bridge arm voltage and the measured value of the bridge arm voltage xj ;

[0145] S3, when |F xj |>ε F_D When it is established, let the count value of the fault monitoring counter C nt_Falut_D Start to automatically add 1, if |F appears in the middle xj |≤ε F_D If the case is, it is considered to be a jitter phenomenon, and the count value C nt_Falut_D Return to zero, otherwise loop calculation until the count value reaches 20 (i.e. within one consecutive power frequency cycle |F xj |are both greater than ε F_D ), then the system is considered to have a fault, and the fault location occurs in the jth phase and the xth bridge arm;

[0146] S4, set the fault bridge arm flag to position 1;

[0147] S5. Construct a capacitor voltage sliding mode observer for each submodule of the faulty bridge arm to obtain the estimated value of the submodule capacitor voltage; and compare it with the capacitor voltage value measured by the voltage sensor to generate a residual value U c_Err ;

[0148] S6, when | U c_Err |>ε C_Err When it is established, let the count value of the fault location counter C nt_Uc_Err Start to automatically add 1, if |U appears in the middle c_Err |≤ε C_Err If the case is, it is considered to be a jitter phenomenon, and the count value C nt_Uc_Err Return to zero, otherwise loop calculation until the count value reaches 20 (i.e. within one consecutive power frequency cycle |U c_Err |are both greater than ε C_Err ), it can be considered that the system has a fault, and the fault location occurs in the kth submodule, the fault submodule flag position is 1, and the fault is located in the specific submodule.

[0149] A simulation test is now conducted on the multi-type fault location method of the MMC flexible DC converter valve submodule based on the sliding mode observer provided in the embodiment of the present invention, as follows:

[0150] A Simulink simulation model was built with a DC bus voltage of 5000V. A half-bridge sub-module structure was adopted, with 20 sub-modules in each bridge arm, a sub-module bypass capacitor with a capacitance of 7mF, an initial sub-module voltage of 250V, a bridge arm inductance of 10mH, a DC side capacitor of 1mF, a sorting and equalizing algorithm for capacitor voltage balancing, and PIR control for circulating current suppression.

[0151] When writing the fault monitoring module program, under various types of faults, the DC bus voltage, output phase voltage, and bridge arm current sampling values are extracted to calculate the actual bridge arm voltage value; the bridge arm PWM switching sequence and capacitor voltage sampling sequence are extracted to calculate the bridge arm voltage measurement value; then the fault monitoring variable is calculated and compared with the threshold to locate the specific fault bridge arm. The results are as follows Figure 5 、 Figure 6 、 Figure 7 、 Figure 8 and Figure 9 As shown in the figure, (a) curve is the output phase current of the three phases, (b) curve is the deviation between the fault bridge arm current and the fault bridge arm voltage, (c) curve is the fault detection variable and threshold, (d) curve is the fault submodule capacitor voltage measurement value and the estimated value obtained by the sliding mode observer, (e) curve is the output residual sequence and positive and negative thresholds, U c_Fault_real Indicates the capacitor voltage measurement value of the faulty submodule, U c_Fault_obser represents the observed value of the capacitor voltage of the fault submodule, i arm_fault Indicates the fault arm current, U arm_fault_error The fault arm voltage is shown in curves (b) and (c) of the figure. Under the five different types of faults, the fault monitoring variable exceeds the threshold within 10 ms (half the power frequency cycle), accurately locating the fault arm.

[0152] Write a fault location program for each submodule in the fault bridge arm, build a sliding mode observer to observe the submodule capacitor voltage, and generate a residual sequence with the submodule capacitor voltage measurement value obtained by the voltage sensor, and then compare it with the threshold to locate the fault to a specific submodule, such as Figure 5 、 Figure 6 、 Figure 7 、 Figure 8 and Figure 9 As shown in the figure, it can be seen from curves (d) and (e) that the residual sequences of the faulty submodules all exceeded the threshold within 15ms, accurately locating the specific submodule and laying the foundation for the subsequent fault isolation and fault-tolerant operation of the MMC system.

[0153] Based on the same inventive concept, an embodiment of the present invention further provides an MMC flexible DC converter valve submodule fault locating device, which is used to implement the MMC flexible DC converter valve submodule fault locating method of the above embodiment. The device includes:

[0154] A first calculation module is used to obtain actual values of bridge arm voltages and measured values of bridge arm voltages of the three-phase six bridge arms of the MMC, and obtain bridge arm voltage deviations based on differences between the actual values of the bridge arm voltages and the measured values of the bridge arm voltages;

[0155] A second calculation module is used to calculate a bridge arm fault detection variable based on the bridge arm voltage deviation;

[0156] A first detection module is used to determine whether the MMC has a fault based on the bridge arm fault detection variable and locate the fault to the bridge arm level;

[0157] The third calculation module is used to construct a capacitor voltage sliding mode observer for each submodule of the fault bridge arm to obtain an estimated value of the submodule capacitor voltage;

[0158] a fourth calculation module, configured to compare, for each submodule, the submodule capacitance voltage estimation value with the submodule capacitance voltage measurement value to generate a submodule capacitance voltage residual value;

[0159] The second detection module is configured to determine whether a submodule fails based on the submodule capacitance voltage residual value, and locate the fault to a specific submodule.

[0160] It is worth noting that the device embodiment corresponds to the above-mentioned method embodiment, and the implementation methods of the above-mentioned method embodiments are applicable to the device embodiment and can achieve the same or similar technical effects, so they will not be repeated here.

[0161] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0162] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the steps in the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0163] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0164] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0165] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A method for locating a fault in an MMC flexible direct current converter valve submodule, characterized in that: include: Obtaining actual bridge arm voltage values and bridge arm voltage measurement values of the MMC three-phase six-bridge arm, and obtaining a bridge arm voltage deviation based on a difference between the actual bridge arm voltage values and the bridge arm voltage measurement values; Calculating a bridge arm fault detection variable based on the bridge arm voltage deviation, include: Among them, F xj represents the fault detection variable of the xth bridge arm of the jth phase, x=p,n, p represents the upper bridge arm, n represents the lower bridge arm, U arm_Err_xj represents the voltage deviation of the xth bridge arm of the jth phase, t is the time, and T is the fault monitoring period; Determining whether an MMC fault occurs based on the bridge arm fault detection variable and locating the fault at the bridge arm level includes: The sliding window method is used to calculate the arm fault detection variables of each of the three-phase six bridge arms window by window; When |F xj |>ε F_D When it is established, let the count value of the fault monitoring counter C nt_Falut_D Start to automatically add 1, ε F_D is the bridge arm fault threshold; If |F is satisfied within a continuous power frequency cycle xj |>ε F_D , it is determined that the MMC has a fault, and the fault location occurs in the jth phase and the xth bridge arm; If |F xj |≤ε F_D , then let the count value C nt_Falut_D Reset to zero and restart counting; A capacitor voltage sliding mode observer is constructed for each submodule of the faulty bridge arm to obtain the estimated value of the submodule capacitor voltage. For each submodule, the submodule capacitance voltage estimated value is compared with the submodule capacitance voltage measured value to generate a submodule capacitance voltage residual value; Determining whether a submodule fault occurs based on the submodule capacitance voltage residual value, locating the fault to a specific submodule, and determining the fault type includes: In each sampling period, the submodule capacitor voltage measurement value obtained by the voltage sensor is compared with the submodule capacitor voltage estimation value output by the capacitor voltage sliding mode observer to generate a residual value U c_Err ; When|U c_Err |>ε C_Err When it is established, let the count value of the fault location counter C nt_Uc_Err Start to automatically add 1, ε C_Err is the submodule fault threshold; If |U is satisfied within a continuous power frequency cycle c_Err |>ε C_Err , it is determined that the submodule has failed; If |U c_Err |≤ε C_Err , then let the count value C nt_Uc_Err Reset to zero and restart counting; Determining the fault type includes: like It is judged that the upper switch IGBT is faulty; like It is judged that the lower switch IGBT is faulty; like It is judged as a voltage sensor disconnection fault; like It is judged that the voltage sensor is stuck; If K gain >1.2, or K gain <0.8, It is judged to be a voltage sensor gain failure; Among them, U c_meas_xji is the capacitor voltage measurement value of the xth bridge arm of the jth phase of the faulty submodule i, U c_real_xji is the actual value of the capacitor voltage of the xth bridge arm of the jth phase of the faulty submodule i, i arm_xj is the bridge arm current of the xth bridge arm of the jth phase, n xj is the number of submodules put into the x-th bridge arm of the j-th phase, N is the number of submodules contained in each bridge arm, and U c_stuck is the capacitor voltage when the voltage sensor is stuck, U c_normal_xji K is the capacitor voltage value of the normal submodule in the same bridge arm when the voltage sensor is stuck. gain is the gain coefficient.

2. A method for locating a fault of an MMC flexible direct current converter valve submodule according to claim 1, characterized in that: The obtaining of the actual values of the bridge arm voltages of the MMC three-phase six-bridge arms includes: Among them, U arm_real_pj is the actual value of the bridge arm voltage of the j-th phase upper bridge arm, U arm_real_nj is the actual value of the bridge arm voltage of the j-th phase lower bridge arm, U dc is the DC bus voltage, u phase_j represents the output phase voltage of the jth phase, L0 is the bridge arm inductance, i arm_pj is the bridge arm current of the j-th phase upper bridge arm, i arm_nj is the bridge arm current of the j-th phase lower bridge arm, j = a, b, c, representing the three phases A, B, and C.

3. A method for locating a fault of an MMC flexible direct current valve submodule according to claim 2, characterized in that: The bridge arm voltage measurement value is calculated as follows: Among them, U arm_meas_pj is the bridge arm voltage measurement value of the j-th phase upper bridge arm, U arm_meas_nj is the measured value of the bridge arm voltage of the lower bridge arm of phase j, S pjk is the switching function of the kth submodule on the upper bridge arm of the jth phase, S njk is the switching function of the kth submodule on the lower bridge arm of the jth phase, U c_meas_pjk is the capacitor voltage measurement value of the kth submodule of the jth phase upper bridge arm, U c_meas_njk is the capacitor voltage measurement value of the kth submodule of the lower bridge arm of the jth phase.

4. A method for locating a fault of an MMC flexible direct current valve submodule according to claim 3, characterized in that: The fault monitoring period is set to one power frequency period; The sliding window is set to 1 / 20 of the power frequency period.

5. The MMC flexible direct current converter valve submodule fault location method according to claim 3, characterized in that: The bridge arm fault threshold ε F_D Set to U dc / 10% of N.

6. The MMC flexible direct current converter valve submodule fault location method according to claim 3, characterized in that: The method constructs a capacitor voltage sliding mode observer for each submodule of the fault bridge arm to obtain the estimated value of the submodule capacitor voltage, including: defining a sliding mode surface Based on the sliding mode surface, the submodule capacitance voltage sliding mode observer is established as follows: in, is the output value of the capacitor voltage sliding mode observer of the kth submodule of the xth bridge arm of the jth phase, U c_meas_xjk is the capacitance voltage measurement value of the kth submodule of the xth bridge arm of the jth phase, τ is the observation gain, sat(·) is the saturation function, and h is the width of the non-saturation region of the saturation function; Select V = 0.5 × S T S is used as a Lyapunov function. By adjusting the observation gain τ, the submodule capacitor voltage sliding mode observer is controlled to a stable state of the sliding mode surface S=0. At this time, the output value of the capacitor voltage sliding mode observer is the submodule capacitor voltage estimation value.

7. A fault location device for an MMC flexible direct current converter valve submodule, characterized in that: The device is used to implement the MMC flexible direct current converter valve submodule fault locating method according to any one of claims 1 to 6, comprising: A first calculation module is used to obtain actual values of bridge arm voltages and measured values of bridge arm voltages of the three-phase six bridge arms of the MMC, and obtain bridge arm voltage deviations based on differences between the actual values of the bridge arm voltages and the measured values of the bridge arm voltages; A second calculation module is used to calculate a bridge arm fault detection variable based on the bridge arm voltage deviation; A first detection module is used to determine whether the MMC has a fault based on the bridge arm fault detection variable and locate the fault to the bridge arm level; The third calculation module is used to construct a capacitor voltage sliding mode observer for each submodule of the fault bridge arm to obtain an estimated value of the submodule capacitor voltage; a fourth calculation module, configured to compare, for each submodule, the submodule capacitance voltage estimation value with the submodule capacitance voltage measurement value to generate a submodule capacitance voltage residual value; The second detection module is configured to determine whether a submodule fails based on the submodule capacitance voltage residual value, and locate the fault to a specific submodule.

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