An adaptive fault current limiting method based on current limiting contribution degree

Abstract

The present application relates to the field of flexible DC system fault, and particularly relates to a self-adaptive fault current limiting method based on current limiting contribution degree. The present application is based on the current change of the half-bridge MMC converter in the initial stage after the DC side fault, combined with the MMC voltage regulation control principle, only considering the fitting of the dynamic change of the fault current in the initial stage of the fault and the IGBT blocking of the MMC after the fault without special control, and equivalent the discharge model of the MMC converter station to the RLC discharge model as the MMC equivalent model. The influence law of the equivalent capacitor voltage according to the voltage regulation coefficient is considered under the condition of the change of the number of sub-module switching of the converter station. The present application uses the simplified model of the flexible DC converter station to analyze the DC side fault current. Since the dynamic change of the equivalent capacitor voltage of the converter station is considered, the present application provides a basis for the subsequent voltage reduction current limiting method and improves the accuracy of the fault current calculation and analysis model. The voltage regulation coefficient of the converter can be adaptively calculated according to the fault information.

Description

Technical Field

[0001] This invention relates to the field of flexible DC system faults, and specifically to an adaptive fault current limiting method based on current limiting contribution. Background Technology

[0002] Flexible DC transmission technology boasts advantages such as flexible and controllable DC voltage, no commutation failure, and high power quality, making it promising for applications in weak systems or isolated power supply, renewable energy grid integration, and urban power grid supply. Modular multilevel converters (MMCs) have enabled the development of flexible DC transmission towards high voltage and large capacity, leading to their widespread use. However, MMC-HVDC (MMC-based flexible DC transmission technology) exhibits characteristics of "weak damping" and "low inertia," resulting in rapid fault current rise and large current volumes. To prevent DC-side faults from causing converter station shutdown and subsequently expanding the fault's impact range, high demands are placed on the design of MMC-HVDC protection schemes.

[0003] Currently, DC circuit breakers (DCCBs) are mainly used in engineering to reliably interrupt fault currents. However, the corresponding high-speed, high-capacity DCCBs are very expensive, seriously affecting the economics of DC power grids. Therefore, the main purpose of using DC fault current limiting is to reduce the cost of DCCBs while avoiding overcurrent blocking by MMCs. Currently, fault current limiting is mainly achieved by changing the converter topology and adding current-limiting reactors. However, the above methods require additional current-limiting devices or modifications to the converter topology, increasing the construction cost of DC power grids.

[0004] The strategy of rapidly reducing the DC-side outlet voltage of the converter by changing the total number of MMC (Multi-Module Control) submodules has been proven to significantly suppress fault current. Compared to methods that modify the converter topology or add DC current limiters, this method limits current by changing the control strategy, without requiring the addition or modification of equipment, offering a cost advantage. The MMC fault equivalent RLC model is an important tool for analyzing instantaneous current and voltage changes after a fault. Currently, the equivalent RLC model is used for fault current analysis after DC-side faults in flexible DC transmission systems, exhibiting high accuracy within 10ms after the fault. Furthermore, due to the rapid development and large fault current of DC faults, traditional DC protection systems struggle to accurately reflect the fault occurrence time and predict the fault current development trend. Without precise fault location and transition resistance, the severity estimation of the fault in the flexible DC system is also difficult to guarantee accuracy, making it difficult to adaptively limit the fault current to the current-limiting target based on the fault severity.

[0005] Therefore, how to accurately assess the severity of a fault and adaptively calculate the converter voltage regulation coefficient based on the fault information to achieve current limiting has become an urgent problem to be solved in the fault analysis of flexible DC transmission systems. Summary of the Invention

[0006] To address the shortcomings of the existing technologies, this invention provides an adaptive fault current limiting method based on current limiting contribution, which can accurately assess the severity of the fault and adaptively calculate the converter voltage regulation coefficient based on the fault information to achieve the purpose of current limiting.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] An adaptive fault current limiting method based on current limiting contribution includes the following steps:

[0009] S1. Based on the current change of the half-bridge MMC converter in the initial stage after a DC side fault, combined with the MMC voltage regulation control principle, only the fitting of the dynamic change of the fault current in the early stage of the fault is considered, and the MMC does not take special control to block the IGBT after the fault. The discharge model of the MMC converter station is equivalent to the RLC discharge model, which is used as the MMC equivalent model.

[0010] S2, based on the current sampling data in the data window, determine whether the preset protection start condition has been met. If it has, start the protection after the fault and continue to collect data from the sampling points in the sampling window after the protection is started to obtain protection sampling information; based on the protection sampling information, obtain the restored fault initial fault current curve to locate the time of fault occurrence.

[0011] S3. Based on the fault current curve fitted in S2 and Kirchhoff's laws, calculate the distance D between the fault point and the busbar, and the transition resistance R considering the coupling effect of the virtual opposite end MMC2. g ;

[0012] S4, combined with the fault current curve fitted by S2 and the specified current limiting target, obtains the fault current after ideal suppression; and obtains the fault current after ideal suppression of the opposite end MMC2 using the same method.

[0013] S5, Substituting the ideally suppressed fault current into the current-limiting flux linkage equation, yields the result that the fault current is suppressed to the target value I. g The current-limiting magnetic flux λ required for MMC voltage regulation control de ;

[0014] S6. Substitute the calculation result of S5 into the formula for the actual current-limiting magnetic flux generated by the MMC voltage regulation control to obtain the MMC voltage regulation coefficient.

[0015] S7, based on the internal IGBT overcurrent capability of MMC, obtains the lower limit of the voltage regulation coefficient to prevent MMC lock-up, and uses this to adjust the modulation coefficient obtained in S6 to obtain the final output MMC voltage regulation coefficient.

[0016] S8. Based on the final output MMC voltage regulation coefficient obtained in S7, reduce the number of capacitors connected in the MMC submodule to adaptively limit fault current.

[0017] Preferably, in S1, in the MMC equivalent model, the values ​​of the equivalent resistance R1, equivalent inductance L1, and equivalent capacitance C1 of the MMC battery swapping station are as follows:

[0018]

[0019]

[0020]

[0021] Where, N SM The number of submodules for each bridge arm, ∑R ON The sum of the resistances of the conducting switching devices, R0 is the bridge arm resistance, L0 is the bridge arm inductance, and C0 is the submodule capacitance; after considering MMC voltage regulation control, the equivalent capacitance value is multiplied by the corresponding voltage regulation coefficient.

[0022] Preferably, in S2, the protection activation condition is: the rate of change k of the fault current exceeds a preset rate of change threshold;

[0023] The formula for calculating the rate of change k of the fault current is:

[0024]

[0025] In the formula, i j Let be the current data at the j-th sampling point, Δt be the sampling interval, and n be the number of sampling data points within the sampling data window.

[0026] Preferably, in S2, the process of locating the time of fault occurrence includes: obtaining the second-order differential value after the fault through protection sampling information; based on the assumption that the second-order differential value of the fault current remains unchanged in a short period of time during a DC grid fault, fitting the fault current change before the effective sampling point with the second-order differential value to obtain the fault current curve, and taking the intersection point of the fitted fault current curve and the current amplitude before the fault as the time of fault occurrence.

[0027] Preferably, in S3, the calculation process of the distance D between the fault point and the busbar includes:

[0028] Calculate the line resistance R from the fault point to the bus. 10 and line inductance L 10 :

[0029]

[0030] In the formula, R is the resistance per unit length of the transmission line, and L is the inductance per unit length of the transmission line.

[0031] R 10 and L 10 Substituting the equivalent circuit of the DC power grid fault instantaneously, fault location is achieved through KVL of the fault loop to obtain the distance D from the fault point to the bus:

[0032]

[0033] Among them, u c1 (t) represents the equivalent capacitor voltage of the converter station, L dc The reactance of CLR is represented; R1 and L1 represent the equivalent resistance and inductance of the MMC converter station, respectively, and i1(t) represents the fault current amplitude.

[0034] Preferably, in S3, the transition resistor R g The calculation process includes:

[0035] Using the fault current curve obtained from S2, the rate of change of fault current at the instant the fault occurs can be calculated.

[0036] Ignore the far-end feeder portion and replace all DC grid portions that feed fault current to the fault point through the line's peer MMC2 with a virtual peer MMC2; calculate the fault current i at the peer MMC2 based on the assumption that the transition resistance is zero. 2v (t), then the fault current i at the other end MMC2 2v (t) Substitute into the KVL of MMC1 at this end to calculate the transition resistance R. g :

[0037]

[0038] In the formula, i 10 (t) represents the magnitude of the near-end current at the fault point.

[0039] Preferably, in S4, the specified current limiting target includes the fault current target value I at time t2 when the DC circuit breaker breaks the current. g Where t2 is the fault clearing time;

[0040] Obtain the fault current i after ideal suppression id10 The process (t) includes: using the fault current curve fitted by S2, calculating the fault current amplitude after a fixed calculation time delay, and obtaining the initial control point [t1,i] on the fault current-time plane. p1[t1], where t1 represents the initial control time point; combined with the target value of the fault current I at time t2 when the DC circuit breaker breaks the current. g Obtain the target control point [t2, I g ]; then, the initial control point [t1,i p1 (t1)] and target control point [t2, I g Connecting them together, we obtain the ideal suppressed fault current i at this end of MMC1. id10 (t);

[0041] And using the same method, based on the fault current i at the other end of MMC2. 2v (t) The ideal suppressed fault current i at the opposite end MMC2 is calculated. idv20 (t).

[0042] Preferably, in S5, the fault current is suppressed to the target value I. g The current-limiting magnetic flux λ required for MMC voltage regulation control de The formula for calculation is:

[0043] λ de =λ C -λ R -λ L ;

[0044]

[0045] In the formula, λ de To suppress the fault current to I under the control timing t1. g The current-limiting magnetic flux required for MMC voltage regulation control; λ c The magnetic flux generated by MMC1 at this end; λ R For R ∑ and transition resistance R g The generated magnetic flux; λ L For L ∑ The generated magnetic flux; C1 represents the equivalent capacitance of the MMC battery swapping station, L ∑ and R ∑ These represent the sum of all inductances and all resistances in the faulty circuit, respectively.

[0046] Preferably, in S6, the current-limiting flux of the local MMC1 obtained in S5 is substituted into the formula for the actual current-limiting flux generated by the MMC voltage regulation control:

[0047]

[0048] The voltage regulation coefficient K of MMC1 at this end is obtained. S1 :

[0049]

[0050] In the formula λ de1 This indicates the current-limiting magnetic flux required for the voltage regulation control of MMC1 at this end;

[0051] The voltage regulation coefficient K of the MMC2 at the other end is obtained through the following calculation method. S2 :

[0052]

[0053] In the formula λ de2 C2 is the current-limiting magnetic flux required for voltage regulation control of the MMC2 at the other end; C2 is the equivalent capacitance of the MMC2 at the other end; u C2 This is the equivalent capacitor voltage of the opposite end MMC2.

[0054] Preferably, in S7, the process of obtaining the lower limit value of the voltage regulation coefficient to prevent MMC lockout includes:

[0055] Calculate the fundamental frequency component I of the AC circuit under the unlimited current measure. m,ac :

[0056]

[0057] In the formula, P is the active power transmitted by the system under steady-state operation, Q is the reactive power transmitted by the system under steady-state operation, and U is the reactive power transmitted by the system under steady-state operation. N AC rated voltage;

[0058] And calculate the AC current increment at the moment of fault clearing of MMC1 at this end.

[0059]

[0060] in,

[0061] And using the same method, the AC current increment Δi at the moment of fault clearing at the opposite end MMC2 was calculated. m2 ,ac,t2:

[0062]

[0063] in,

[0064] In the formula, ΔU m1 This represents the AC output voltage drop magnitude caused by the reduction of the bridge arm voltage by MMC1 at this end, ΔU. m2 U represents the magnitude of the AC output voltage drop caused by the reduction of the bridge arm voltage at the opposite end MMC2. dc Indicates the DC-side output voltage; m represents the converter modulation ratio; R0 is the bridge arm resistance, L0 is the bridge arm inductance; R ac For AC equivalent resistance, L acω is the AC equivalent inductance; ω is the current angular frequency; t2 is the fault clearing time; T is the common frequency period; K m Let K be the impact coefficient, and 1.8 ≤ K. m ≤1.9;

[0065] Next, calculate the maximum AC current I at the local terminal of MMC1 at time t2. ac1,max :

[0066]

[0067] And the maximum AC current I at the opposite end MMC2 at time t2 ac2,max :

[0068]

[0069] Then, based on the fact that the submodule capacitor discharge time is short and the voltage change is small during the fault clearing time, let the equivalent capacitor voltage u be... 10 (0) is a constant voltage source. Under current limiting conditions with source and grid coordination, the fault current i at this terminal MMC1 at time t2 is obtained. 10 (t2):

[0070]

[0071] In the formula, τ dc =L Σ / R ∑ I 10 (0) is the steady-state DC current of the system, U 10 (0) represents the DC voltage of the system in steady state;

[0072] And in the same way, the fault current i at time t2 of the opposite end MMC2 is obtained. 20 (t2);

[0073] Finally, calculate the maximum value of the MMC1 bridge arm current i at time t2. arm1,max :

[0074]

[0075] In conjunction with the condition that the local MMC1 is not latched during fault clearing to ensure the safety of IGBT devices, i arm1,max ≤2I N Calculate the lower limit of the voltage regulation coefficient of MMC1 at this end; where I N This refers to the rated current value of the IGBT.

[0076] Then, using the same method, the lower limit of the voltage regulation coefficient of the MMC2 at the other end was calculated.

[0077] Glossary:

[0078] Local MMC1 refers to the MMC converter side closest to the fault point.

[0079] The remote MMC2 refers to the MMC converter side that is connected to the local MMC1 via a transmission line.

[0080] Compared with the prior art, the present invention has the following beneficial effects:

[0081] 1. To address the challenges in existing technologies such as the difficulty in accurately predicting the timing of fault occurrence and the development trend of fault current, the difficulty in accurately estimating the severity of system faults, and the difficulty in adaptively limiting fault current to the current-limiting target, this invention is based on the current changes of a half-bridge MMC converter in the initial stage after a DC-side fault. Combining this with the MMC voltage regulation control principle, it only considers the fitting of the dynamic changes in fault current during the initial stage of the fault and does not employ special control to lock out the IGBTs after the fault. Therefore, the discharge model of the MMC converter station is equivalent to an RLC discharge model, which serves as the MMC equivalent model.

[0082] This invention considers the impact of changes in the number of switching submodules in the converter station on the equivalent capacitor voltage, which is adjusted according to the voltage regulation coefficient. It uses a simplified fault model of a flexible DC converter station to analyze the DC-side fault current. By considering the dynamic changes in the equivalent capacitor voltage of the converter station, it provides a foundation for subsequent voltage reduction and current limiting methods, improving the accuracy of the fault current calculation and analysis model.

[0083] In summary, this invention can accurately assess the severity of a fault and adaptively calculate the converter voltage regulation coefficient based on the fault information, thereby achieving adaptive current limiting.

[0084] 2. In calculating the voltage regulation control coefficient of the converter station, this invention uses a current-limiting contribution method to quantify the current-limiting requirements under different fault severity levels, thereby establishing a current-limiting contribution equation to meet the current-limiting target. Based on the converter's non-blocking principle, the lower limit of the voltage regulation coefficient is calculated in advance to prevent overcurrent in the converter's internal IGBTs. Using the current-limiting contribution method, the voltage regulation coefficient that the converter station needs to adjust under different fault conditions can be accurately calculated.

[0085] 3. This invention solves the problem of adaptive current limiting for DC faults in flexible DC systems and has high practical value for application in adaptive current limiting for DC side faults in flexible DC systems. Attached Figure Description

[0086] To make the objectives, technical solutions, and advantages of the invention clearer, the invention will now be described in further detail with reference to the accompanying drawings, wherein:

[0087] Figure 1 The flowchart in the embodiment is shown below;

[0088] Figure 2 This is a flowchart of the fault current adaptive suppression strategy in the embodiment;

[0089] Figure 3 This is a schematic diagram of the equivalent RLC discharge circuit for a DC fault in the embodiment. Detailed Implementation

[0090] The following detailed explanation illustrates the specific implementation methods:

[0091] Example:

[0092] like Figure 1 , Figure 2 As shown, this embodiment discloses an adaptive fault current limiting method based on current limiting contribution, including the following steps:

[0093] S1, based on the initial current change of a half-bridge MMC converter after a DC-side fault, and combined with the MMC voltage regulation control principle, only considering the fitting of the dynamic change of the fault current in the initial stage of the fault, and assuming that the MMC does not take special control to lock out the IGBTs after the fault, the discharge model of the MMC converter station is equivalent to an RLC discharge model, which is used as the MMC equivalent model. For ease of understanding, the simplified equivalent circuit of MMC considering the dynamic change of capacitor voltage is as follows: Figure 3 As shown.

[0094] Specifically, in the MMC equivalent model, the values ​​of the equivalent resistance R1, equivalent inductance L1, and equivalent capacitance C1 of the MMC battery swapping station are as follows:

[0095]

[0096]

[0097]

[0098] Where, N SM The number of submodules for each bridge arm, ∑R ON The sum of the resistances of the conducting switching devices, R0 is the bridge arm resistance, L0 is the bridge arm inductance, and C0 is the submodule capacitance; after considering MMC voltage regulation control, the equivalent capacitance value is multiplied by the corresponding voltage regulation coefficient.

[0099] S2, based on the current sampling data in the data window, determine whether the preset protection start conditions have been met. If they have been met, start the protection after the fault and continue to collect data from the sampling points in the sampling window after the protection is started to obtain protection sampling information. Based on the protection sampling information, obtain the restored fault current curve at the initial stage of the fault to locate the time when the fault occurred.

[0100] In specific implementation, the protection activation condition is: the rate of change k of the fault current exceeds a preset rate of change threshold;

[0101] The formula for calculating the rate of change k of the fault current is:

[0102]

[0103] In the formula, i j Let be the current data at the j-th sampling point, Δt be the sampling interval, and n be the number of sampling data points within the sampling data window.

[0104] The process of locating the time of fault occurrence includes: obtaining the second derivative value after the fault by means of protection sampling information; based on the assumption that the second derivative value of the fault current remains unchanged in a short period of time during a DC power grid fault, fitting the change in fault current before the effective sampling point with the second derivative value to obtain the fault current curve, and taking the intersection point of the fitted fault current curve and the current amplitude before the fault as the time of fault occurrence.

[0105] S3. Based on the fault current curve fitted in S2 and Kirchhoff's laws, calculate the distance D between the fault point and the busbar, and the transition resistance R considering the coupling effect of the virtual opposite end MMC2. g .

[0106] In practice, the calculation process for the distance D between the fault point and the busbar includes:

[0107] Calculate the line resistance R from the fault point to the bus. 10 and line inductance L 10 :

[0108]

[0109] In the formula, R is the resistance per unit length of the transmission line, and L is the inductance per unit length of the transmission line.

[0110] R 10 and L 10 Substituting the equivalent circuit of the DC power grid fault instantaneously, fault location is achieved through KVL of the fault loop to obtain the distance D from the fault point to the bus:

[0111]

[0112] Among them, u c1 (t) represents the equivalent capacitor voltage of the converter station, L dc The reactance of CLR is represented; R1 and L1 represent the equivalent resistance and inductance of the MMC converter station, respectively, and i1(t) represents the fault current amplitude.

[0113] The transition resistance R g The calculation process includes:

[0114] Using the fault current curve obtained from S2, the rate of change of fault current at the instant the fault occurs can be calculated.

[0115] Ignore the far-end feeder portion and replace all DC grid portions that feed fault current to the fault point through the line's peer MMC2 with a virtual peer MMC2; calculate the fault current i at the peer MMC2 based on the assumption that the transition resistance is zero. 2v (t), then the fault current i at the other end MMC2 2v (t) Substitute into the KVL of MMC1 at this end to calculate the transition resistance R. g :

[0116]

[0117] In the formula, i 10 (t) represents the magnitude of the near-end current at the fault point.

[0118] S4, combined with the fault current curve fitted by S2 and the specified current limiting target, obtains the fault current after ideal suppression; and obtains the fault current after ideal suppression of the opposite end MMC2 using the same method.

[0119] The specified current limiting target includes the fault current target value I at time t2 when the DC circuit breaker breaks the current. g , where time t2 is the fault clearing time.

[0120] Obtain the fault current i after ideal suppression id10 The process (t) includes: using the fault current curve fitted by S2, calculating the fault current amplitude after a fixed calculation time delay, and obtaining the initial control point [t1,i] on the fault current-time plane. p1 [t1], where t1 represents the initial control time point; combined with the target value of the fault current I at time t2 when the DC circuit breaker breaks the current. g Obtain the target control point [t2, I g ]; then, the initial control point [t1,i p1 (t1)] and target control point [t2, I g Connecting them together, we obtain the ideal suppressed fault current i at this end of MMC1. id10 (t);

[0121] And using the same method, based on the fault current i at the other end of MMC2. 2v (t) The ideal suppressed fault current i at the opposite end MMC2 is calculated. idv20 (t).

[0122] S5, Substituting the ideally suppressed fault current into the current-limiting flux linkage equation, yields the result that the fault current is suppressed to the target value I. gThe current-limiting magnetic flux λ required for MMC voltage regulation control de .

[0123] In practice, the fault current will be suppressed to the target value I. g The current-limiting magnetic flux λ required for MMC voltage regulation control de The formula for calculation is:

[0124] λ de =λ C -λ R -λ L ;

[0125]

[0126] In the formula, λ de To suppress the fault current to I under the control timing t1. g The current-limiting magnetic flux required for MMC voltage regulation control; λ c The magnetic flux generated by MMC1 at this end; λ R For R ∑ and transition resistance R g The generated magnetic flux; λ L For L ∑ The generated magnetic flux; C1 represents the equivalent capacitance of the MMC battery swapping station, L ∑ and R ∑ These represent the sum of all inductances and all resistances in the faulty circuit, respectively.

[0127] S6. Substitute the calculation result of S5 into the formula for the actual current-limiting magnetic flux generated by the MMC voltage regulation control to obtain the MMC voltage regulation coefficient.

[0128] In practical implementation, the current-limiting flux of the local MMC1 obtained from S5 is substituted into the formula for the actual current-limiting flux generated by the MMC voltage regulation control:

[0129]

[0130] The voltage regulation coefficient K of MMC1 at this end is obtained. S1 :

[0131]

[0132] In the formula λ de1 This indicates the current-limiting magnetic flux required for the voltage regulation control of MMC1 at this end;

[0133] The voltage regulation coefficient K of the MMC2 at the other end is obtained through the following calculation method. S2 :

[0134]

[0135] In the formula λde2 C2 is the current-limiting magnetic flux required for voltage regulation control of the MMC2 at the other end; C2 is the equivalent capacitance of the MMC2 at the other end; u C2 This is the equivalent capacitor voltage of the opposite end MMC2.

[0136] S7, based on the internal IGBT overcurrent capability of MMC, obtains the lower limit of the voltage regulation coefficient to prevent MMC lock-up, and uses this to tune the modulation coefficient obtained in S6 to obtain the final output MMC voltage regulation coefficient.

[0137] In practice, the process of obtaining the lower limit of the voltage regulation coefficient to prevent MMC lockout includes:

[0138] The process of obtaining the lower limit of the voltage regulation coefficient to prevent MMC lockout includes:

[0139] Calculate the fundamental frequency component I of the AC circuit under the unlimited current measure. m,ac :

[0140]

[0141] In the formula, P is the active power transmitted by the system under steady-state operation, Q is the reactive power transmitted by the system under steady-state operation, and U is the reactive power transmitted by the system under steady-state operation. N AC rated voltage;

[0142] And calculate the AC current increment at the moment of fault clearing of MMC1 at this end.

[0143]

[0144] in,

[0145] And using the same method, the AC current increment at the moment of fault clearing at the opposite end MMC2 was calculated.

[0146]

[0147] in,

[0148] In the formula, ΔU m1 This represents the AC output voltage drop magnitude caused by the reduction of the bridge arm voltage by MMC1 at this end, ΔU. m2 U represents the magnitude of the AC output voltage drop caused by the reduction of the bridge arm voltage at the opposite end MMC2. dc Indicates the DC-side output voltage; m represents the converter modulation ratio; R0 is the bridge arm resistance, L0 is the bridge arm inductance; R ac For AC equivalent resistance, L ac ω is the AC equivalent inductance; ω is the current angular frequency; t2 is the fault clearing time; T is the common frequency period; K mLet K be the impact coefficient, and 1.8 ≤ K. m ≤1.9;

[0149] Next, calculate the maximum AC current I at the local terminal of MMC1 at time t2. ac1,max :

[0150]

[0151] And the maximum AC current I at the opposite end MMC2 at time t2 ac2,max :

[0152]

[0153] Then, based on the fact that the submodule capacitor discharge time is short and the voltage change is small during the fault clearing time, let the equivalent capacitor voltage u be... 10 (0) is a constant voltage source. Under current limiting conditions with source and grid coordination, the fault current i at this terminal MMC1 at time t2 is obtained. 10 (t2):

[0154]

[0155] In the formula, τ dc =L ∑ / R ∑ I 10 (0) is the steady-state DC current of the system, U 10 (0) represents the DC voltage of the system in steady state;

[0156] And in the same way, the fault current i at time t2 of the opposite end MMC2 is obtained. 20 (t2);

[0157] Finally, calculate the maximum value of the MMC1 bridge arm current i at time t2. arm1,max :

[0158]

[0159] In conjunction with the condition that the local MMC1 is not latched during fault clearing to ensure the safety of IGBT devices, i arm1,max ≤2I N Calculate the lower limit of the voltage regulation coefficient of MMC1 at this end; where I N This refers to the rated current value of the IGBT.

[0160] In practical implementation, the lower limit of the local MMC1 voltage regulation coefficient can be determined in the following way:

[0161] During a fault, the system requires that the AC output voltage of the local MMC1 not be lower than 50% of the rated operating voltage, thus requiring the voltage regulation coefficient to be between 0.5 and 1. Substitute each system parameter into the calculation formula for the maximum value of the bridge arm current at time t2 to verify the value of the voltage regulation coefficient; if the local MMC1 non-blocking condition is not met... arm1,max ≤2I N Then, the voltage regulation coefficient is gradually increased according to the preset step size for recalibration until a suitable lower limit value for the voltage regulation coefficient is selected. The specific value of the preset step size can be set by those skilled in the art according to the specific adjustment accuracy requirements, and will not be elaborated here.

[0162] Then, using the same method, the lower limit of the voltage regulation coefficient of the MMC2 at the other end was calculated.

[0163] S8. Based on the final output MMC voltage regulation coefficient obtained in S7, reduce the number of capacitors connected in the MMC submodule to adaptively limit fault current.

[0164] In practical implementation, at time t1 after the fault, based on the final output MMC voltage regulation coefficient obtained from S7, the number of capacitors connected in the MMC submodule is reduced, thereby lowering the equivalent capacitor voltage, so that:

[0165]

[0166] Among them, T s For the time step, u c1s (t1) and u c2s (t1) represents the equivalent capacitor voltages after voltage regulation and current limiting of MMC1 at this end and MMC2 at the other end, respectively, and u c1 (t1-T s ) and u c2 (t1-T s ) represent the actual capacitor voltages of MMC1 at this end and MMC2 at the other end at the time t1.

[0167] The regulated equivalent voltage replaces the equivalent capacitor voltage u without current limiting control at time t1 after the fault. c1 (t) and u c2 (t); After the number of sub-modules connected changes following active current limiting on the source side, the equivalent capacitance of the converter is reduced. Substituting the equivalent capacitance into the voltage equation, we can solve for the bridge arm voltages after the fault. Since the initial currents at each terminal are different, the calculated voltage regulation controls may not all be activated. Taking the most severe fault condition where both voltage regulation controls are activated simultaneously, we obtain the fault current and the equivalent capacitance voltage as follows:

[0168]

[0169] To address the challenges of accurately predicting the timing of fault occurrence and the development trend of fault current in existing technologies, accurately estimating the severity of system faults, and adaptively limiting fault current to the current-limiting target, this invention addresses these issues. Based on the initial current changes of a half-bridge MMC converter after a DC-side fault, and combined with the MMC voltage regulation control principle, this invention only considers the fitting of the dynamic changes in fault current during the initial fault phase and does not employ special control to lock out IGBTs after the fault. The discharge model of the MMC converter station is thus equivalent to an RLC discharge model, serving as the MMC equivalent model. The invention also considers the impact of changes in the equivalent capacitor voltage based on the voltage regulation coefficient when the number of converter station submodules changes. This invention uses a simplified fault model of a flexible DC converter station to analyze the DC-side fault current. By considering the dynamic changes in the equivalent capacitor voltage of the converter station, it provides a foundation for subsequent voltage reduction and current limiting methods, improving the accuracy of the fault current calculation and analysis model. In addition, this invention uses a current-limiting contribution method to quantify the current-limiting requirements under different fault severity levels during the calculation of the converter station's voltage regulation control coefficient, thereby establishing a current-limiting contribution equation to meet the current-limiting target. Based on the converter's non-blocking principle, the lower limit of the voltage regulation coefficient is calculated in advance to prevent overcurrent in the converter's internal IGBTs. Using the current-limiting contribution method, the voltage regulation coefficient that the converter station needs to adjust under different fault conditions can be accurately calculated.

[0170] This invention solves the problem of adaptive current limiting for DC faults in flexible DC systems and has high practical value for application in adaptive current limiting for DC side faults in flexible DC systems.

[0171] 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 the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. An adaptive fault current limiting method based on current limiting contribution, characterized in that, Includes the following steps: S1. Based on the current change of the half-bridge MMC converter in the initial stage after a DC side fault, combined with the MMC voltage regulation control principle, only the fitting of the dynamic change of the fault current in the early stage of the fault is considered, and the MMC does not take special control to block the IGBT after the fault. The discharge model of the MMC converter station is equivalent to the RLC discharge model, which is used as the MMC equivalent model. S2, based on the current sampling data in the data window, determine whether the preset protection start condition has been met. If it has been met, start the protection after the fault and continue to collect data from the sampling points in the sampling window after starting the protection to obtain protection sampling information. Based on the protection sampling information, the restored initial fault current curve is obtained to pinpoint the time of fault occurrence. S3. Based on the fault current curve fitted by S2 and Kirchhoff's laws, calculate the distance D between the fault point and the busbar, and the transition resistance considering the virtual coupling effect of the opposite end MMC2. ; S4. Using the fault current curve fitted by S2 and the specified current limiting target, obtain the fault current after ideal suppression; and use the same method to obtain the fault current after ideal suppression at the other end MMC2. S5, Substituting the ideally suppressed fault current into the current-limiting flux linkage equation, yields the result that the fault current is suppressed to the target value I. g The current-limiting magnetic flux required for MMC voltage regulation control ; S6. Substitute the calculation result of S5 into the formula for the actual current-limiting magnetic flux generated by the MMC voltage regulation control to obtain the MMC voltage regulation coefficient. S7, based on the internal IGBT overcurrent capability of MMC, obtains the lower limit of the voltage regulation coefficient to prevent MMC lock-up, and uses this to adjust the modulation coefficient obtained in S6 to obtain the final output MMC voltage regulation coefficient. S8. Based on the final output MMC voltage regulation coefficient obtained in S7, reduce the number of capacitors connected in the MMC submodule to adaptively limit fault current.

2. The adaptive fault current limiting method based on current limiting contribution as described in claim 1, characterized in that: In S1, in the MMC equivalent model, the values ​​of the equivalent resistance R1, equivalent inductance L1, and equivalent capacitance C1 of the MMC converter station are as follows: ; ; ; Where, N SM The number of submodules for each bridge arm, ∑R ON The sum of the resistances of the conducting switching devices, R0 is the bridge arm resistance, L0 is the bridge arm inductance, and C0 is the submodule capacitance; after considering MMC voltage regulation control, the equivalent capacitance value is multiplied by the corresponding voltage regulation coefficient.

3. The adaptive fault current limiting method based on current limiting contribution as described in claim 2, characterized in that: In S2, the protection activation condition is: the rate of change of the fault current. The rate of change exceeded the preset threshold. The rate of change of the fault current The formula for calculation is: ； In the formula, For the first Current data at each sampling point The sampling interval is... This represents the number of sampled data points within the sampled data window.

4. The adaptive fault current limiting method based on current limiting contribution as described in claim 3, characterized in that: In S2, the process of locating the time of fault occurrence includes: obtaining the second derivative value after the fault through protection sampling information; based on the assumption that the second derivative value of the fault current remains unchanged in a short period of time during a DC grid fault, fitting the fault current change before the effective sampling point with the second derivative value to obtain the fault current curve, and taking the intersection point of the fitted fault current curve and the current amplitude before the fault as the time of fault occurrence.

5. The adaptive fault current limiting method based on current limiting contribution as described in claim 4, characterized in that: In S3, the calculation process for the distance D between the fault point and the busbar includes: Calculate the line resistance R from the fault point to the bus. 10 and line inductance L 10 : ； In the formula, R is the resistance per unit length of the transmission line, and L is the inductance per unit length of the transmission line. R 10 and L 10 Substituting the equivalent circuit of the DC power grid fault instantaneously, fault location is achieved through KVL of the fault loop to obtain the distance from the fault point to the bus. : ； Among them, u c1 (t) represents the equivalent capacitor voltage of the converter station, L dc The reactance of CLR is represented; R1 and L1 represent the equivalent resistance and inductance of the MMC converter station, respectively, and i1(t) represents the fault current amplitude.

6. The adaptive fault current limiting method based on current limiting contribution as described in claim 5, characterized in that: In S3, the transition resistor The calculation process includes: Using the fault current curve obtained from S2, the rate of change of fault current at the instant the fault occurs can be calculated. Ignore the far-end feeder portion and replace all DC grid portions that feed fault current to the fault point through the line's peer MMC2 with a virtual peer MMC2; calculate the fault current i at the peer MMC2 based on the assumption that the transition resistance is zero. 2v (t), then the fault current i at the other end MMC2 2v (t) Substitute into the KVL of MMC1 at this end to calculate the transition resistance. : ； In the formula, This indicates the magnitude of the near-end current at the fault point.

7. The adaptive fault current limiting method based on current limiting contribution as described in claim 6, characterized in that: In S4, the specified current limiting target includes the fault current target value I at time t2 when the DC circuit breaker breaks the current. g Where t2 is the fault clearing time; Obtain the fault current i after ideal suppression id10 The process (t) includes: using the fault current curve fitted by S2, calculating the fault current amplitude after a fixed calculation time delay, and obtaining the initial control point [t1,i] on the fault current-time plane. p1 [t1], where t1 represents the initial control time point; combined with the target value of the fault current I at time t2 when the DC circuit breaker breaks the current. g Obtain the target control point [t2, I] g ]; then, the initial control point [t1,i p1 (t1)] and target control point [t2, I g Connecting them together, we obtain the ideal suppressed fault current i at this end of MMC1. id10 (t); And using the same method, based on the fault current i at the other end of MMC2. 2v (t) The ideal suppressed fault current i at the opposite end MMC2 is calculated. idv20 (t).

8. The adaptive fault current limiting method based on current limiting contribution as described in claim 7, characterized in that: In S5, the fault current is suppressed to the target value I. g The current-limiting magnetic flux required for MMC voltage regulation control The formula for calculation is: ； ； In the formula, λ de To suppress the fault current to I under the control timing t1. g The current-limiting magnetic flux required for MMC voltage regulation control; λ c The magnetic flux generated by MMC1 at this end; λ R For R ∑ and transition resistance R g The generated magnetic flux; λ L For L ∑ The generated magnetic flux; L represents the equivalent capacitance of an MMC battery swapping station. ∑ R represents the sum of all inductances in the faulty circuit; ∑ This represents the sum of all resistances in the faulty circuit.

9. The adaptive fault current limiting method based on current limiting contribution as described in claim 8, characterized in that: In S6, the current-limiting flux of the local MMC1 obtained in S5 is substituted into the formula for the actual current-limiting flux generated by the MMC voltage regulation control: ； Obtain the voltage regulation coefficient of MMC1 at this end. : ； In the formula This indicates the current-limiting magnetic flux required for the voltage regulation control of MMC1 at this end; The voltage regulation coefficient K of the MMC2 at the other end is obtained through the following calculation method. S2 : ； In the formula This is to generate the current-limiting magnetic flux required for the voltage regulation control of the MMC2 at the other end; The equivalent capacitance of the MMC2 at the other end; This is the equivalent capacitor voltage of the opposite end MMC2.

10. The adaptive fault current limiting method based on current limiting contribution as described in claim 9, characterized in that: In S7, the process of obtaining the lower limit of the voltage regulation coefficient to prevent MMC lockout includes: Calculate the fundamental frequency component of AC under an infinite current measure. : ； In the formula, P is the active power transmitted by the system under steady-state operation, Q is the reactive power transmitted by the system under steady-state operation, and U is the reactive power transmitted by the system under steady-state operation. N AC rated voltage; And calculate the AC current increment at the moment of fault clearing of MMC1 at this end. : ； in, ; And using the same method, the AC current increment at the moment of fault clearing at the opposite end MMC2 was calculated. : ； in, ; In the formula, This indicates the magnitude of the AC output voltage drop caused by the reduction of the bridge arm voltage by MMC1 at this end. U represents the magnitude of the AC output voltage drop caused by the reduction of the bridge arm voltage at the opposite end MMC2. dc Indicates the DC-side output voltage; m represents the converter modulation ratio; R0 is the bridge arm resistance, L0 is the bridge arm inductance; R ac For AC equivalent resistance, L ac ω is the AC equivalent inductance; ω is the current angular frequency; t2 is the fault clearing time; T is the common frequency period; K m Let K be the impact coefficient, and 1.8 ≤ K. m ≤1.9; Next, calculate the maximum AC current of MMC1 at time t2. : ； And the maximum AC current of the MMC2 at the other end at time t2. : ； Then, based on the fact that the submodule capacitor discharge time is short and the voltage change is small during the fault clearing time, let the equivalent capacitor voltage u be... 10 (0) is a constant voltage source. The fault current of MMC1 at time t2 is obtained under current limiting conditions with the source and grid coordination. : ； In the formula, I 10 (0) is the steady-state DC current of the system, U 10 (0) represents the steady-state DC voltage of the system; And in the same way, the fault current of the opposite end MMC2 at time t2 is obtained. ; Finally, the maximum value of the local MMC1 bridge arm current at time t2 is calculated. : ； In conjunction with the condition that the local MMC1 is not latched during fault clearing to ensure the safety of IGBT devices, Calculate the lower limit of the voltage regulation coefficient of MMC1 at this end; where I N This refers to the rated current value of the IGBT. Then, using the same method, the lower limit of the voltage regulation coefficient of the MMC2 at the other end was calculated.

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