Method and system for determining overvoltage and overcurrent based on phase-to-ground fault

Abstract

This application proposes a method and system for determining overvoltage and overcurrent based on phase-to-phase grounding faults. The method includes: determining the maximum value of the negative-sequence fault current and the maximum value of the voltage of the non-faulty phase corresponding to the total impedance, positive-sequence voltage, and AC positive-sequence current of the system when a phase-to-phase grounding fault occurs; using the maximum value of the negative-sequence fault current as the negative-sequence overcurrent when a phase-to-phase grounding fault occurs, and using the maximum value of the voltage of the non-faulty phase as the overvoltage when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission. The technical solution proposed in this application can accurately determine the negative-sequence overcurrent and overvoltage values ​​when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission based on the total impedance, positive-sequence voltage, and AC positive-sequence current of the system, and control the stable operation of the wind power transmission system via flexible direct current transmission based on the determined negative-sequence overcurrent and overvoltage values.

Description

Technical Field

[0001] This application relates to the field of overcurrent and overvoltage, and in particular to a method and system for determining overvoltage and overcurrent based on phase-to-phase grounding faults. Background Technology

[0002] High-voltage direct current (HVDC) transmission systems based on voltage source converters (VSCs) provide an economical and efficient grid connection method for wind farms. Grid connection technology for wind power via flexible DC transmission systems has also become a key area of ​​focus. Wind farms typically use direct-drive turbines, and their grid-side converters (GSCs) usually employ two-level voltage source converters (2L-VSCs). Modular multilevel converters (MMCs) are gradually becoming the mainstream topology for VSC-HVDCs due to their unique advantages. Phase-to-phase faults are among the most common faults in power systems. When a phase-to-phase fault occurs in the AC system between the GSC and the sending-end MMC, the system generates significant transient overcurrents and overvoltages due to the influence of negative sequence currents. Because of the low overcurrent and low overvoltage characteristics of power electronic devices, fault ride-through is difficult, thus affecting the safe and reliable operation of the system. Therefore, studying the overcurrent and overvoltage characteristics under phase-to-phase faults is of great significance. Summary of the Invention

[0003] The method and system for determining overvoltage and overcurrent based on phase-to-phase grounding faults provided in this application aim to at least solve the technical problem in the prior art where the low overcurrent and low overvoltage characteristics of power electronic devices make it difficult for the system to achieve fault ride-through, thus affecting the safe and reliable operation of the system.

[0004] The first aspect of this application proposes a method for determining overvoltage and overcurrent based on phase-to-phase grounding faults, the method comprising:

[0005] When a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission, the total impedance value of the system, the positive sequence voltage output by the multilevel converter at the sending end, and the AC positive sequence current output by the wind turbine are obtained.

[0006] The maximum value of the negative sequence fault current and the maximum value of the voltage of the non-faulty phase are determined based on the total impedance value, the positive sequence voltage, and the AC positive sequence current when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct transmission.

[0007] The maximum value of the negative sequence fault current is taken as the negative sequence overcurrent when the wind power is connected to the flexible direct transmission system via a phase-to-phase grounding fault, and the maximum value of the voltage of the non-faulty phase is taken as the overvoltage when the wind power is connected to the flexible direct transmission system via a phase-to-phase grounding fault.

[0008] Preferably, the method further includes:

[0009] The positive sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system via flexible direct transmission is determined based on the total impedance value, the positive sequence voltage, and the AC positive sequence current when a phase-to-phase grounding fault occurs.

[0010] Obtain the reactance value corresponding to the wind turbine converter reactor in the system, and determine the negative sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system when a phase-to-phase grounding fault occurs, based on the reactance value corresponding to the wind turbine converter reactor, the total impedance value, the positive sequence voltage, and the AC positive sequence current.

[0011] Preferably, the formula for calculating the maximum value of the negative sequence fault current is as follows:

[0012]

[0013] In the formula, I fanmax U is the maximum value of the negative sequence fault current. mp X is the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. all I represents the total reactance of the wind power transmission system via flexible direct current, α represents the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current, and I represents the total reactance of the wind power transmission system via flexible direct current. wp This refers to the positive sequence AC current output by the fan.

[0014] Preferably, the formula for calculating the maximum voltage of the non-faulty phase is as follows:

[0015]

[0016] In the formula, U famax U is the maximum voltage of the non-faulty phase. mp X is the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. all I represents the total reactance of the wind power transmission system via flexible direct current, α represents the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current, and I represents the total reactance of the wind power transmission system via flexible direct current. wp This refers to the positive sequence AC current output by the fan.

[0017] Furthermore, the formula for calculating the positive sequence overvoltage corresponding to the common connection point of the wind turbine when a phase-to-phase grounding fault occurs is as follows: U wpmax :

[0018]

[0019] The formula for calculating the negative sequence overvoltage corresponding to the common coupling point of the wind turbine when a phase-to-phase grounding fault occurs is as follows:

[0020]

[0021] In the formula, U wpmax U is the positive-sequence overvoltage corresponding to the common connection point of the wind turbine when a phase-to-phase grounding fault occurs. wnmax U is the negative sequence overvoltage corresponding to the common connection point of the wind turbine during an indirect phase fault. mp I is the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. wp X is the positive sequence AC current output by the fan. all X is the total reactance value corresponding to the wind power transmission system via flexible direct current, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current, and X is the total reactance value. weq α is the reactance value corresponding to the wind turbine converter reactor, and α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current.

[0022] A second aspect of this application provides a system for determining overvoltage and overcurrent based on phase-to-phase grounding faults, comprising:

[0023] The acquisition module is used to acquire the total impedance value of the system, the positive sequence voltage output by the multilevel converter at the sending end, and the AC positive sequence current output by the wind turbine when a phase-to-ground fault occurs in the wind power transmission system via flexible direct current transmission.

[0024] The first determining module is used to determine the maximum value of the negative sequence fault current and the maximum value of the voltage of the non-faulty phase when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission, based on the total impedance value, the positive sequence voltage, and the AC positive sequence current, respectively.

[0025] The second determining module is used to take the maximum value of the negative sequence fault current as the negative sequence overcurrent when the wind power is connected to the flexible direct transmission system via a phase-to-phase grounding fault, and to take the maximum value of the voltage of the non-faulty phase as the overvoltage when the wind power is connected to the flexible direct transmission system via a phase-to-phase grounding fault.

[0026] Preferably, the determining system further includes:

[0027] The third determining module is used to determine the positive sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system via flexible direct current when a phase-to-phase grounding fault occurs, based on the total impedance value, the positive sequence voltage, and the AC positive sequence current.

[0028] The fourth determining module is used to obtain the reactance value corresponding to the wind turbine converter reactor in the system, and determine the negative sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system when a phase-to-phase grounding fault occurs, based on the reactance value corresponding to the wind turbine converter reactor, the total impedance value, the positive sequence voltage and the AC positive sequence current.

[0029] Preferably, the formula for calculating the maximum value of the negative sequence fault current is as follows:

[0030]

[0031] In the formula, I fanmax U is the maximum value of the negative sequence fault current. mp X is the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. all I represents the total reactance of the wind power transmission system via flexible direct current, α represents the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current, and I represents the total reactance of the wind power transmission system via flexible direct current. wp This refers to the positive sequence AC current output by the fan.

[0032] A third aspect of this application provides an electronic device, including: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the program, it implements the method described in the first aspect embodiment.

[0033] The fourth aspect of this application provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the method described in the first aspect.

[0034] The technical solutions provided by the embodiments of this application bring at least the following beneficial effects:

[0035] This application proposes a method and system for determining overvoltage and overcurrent based on phase-to-phase grounding faults. The method includes: acquiring the total impedance value of the system, the positive-sequence voltage output by the multilevel converter at the sending end, and the AC positive-sequence current output by the wind turbine when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission; determining the maximum value of the negative-sequence fault current and the maximum value of the voltage of the non-faulty phase when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission based on the total impedance value, the positive-sequence voltage, and the AC positive-sequence current; using the maximum value of the negative-sequence fault current as the negative-sequence overcurrent when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission, and using the maximum value of the voltage of the non-faulty phase as the overvoltage when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission. The technical solution proposed in this application can accurately determine the negative sequence overcurrent and overvoltage values ​​when a phase-to-phase ground fault occurs in the wind power transmission system via flexible direct current transmission, based on the total impedance value, positive sequence voltage, and AC positive sequence current of the system. The stable operation of the wind power transmission system via flexible direct current transmission can be controlled based on the determined negative sequence overcurrent and overvoltage values.

[0036] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0037] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

[0038] Figure 1 This is a flowchart of a method for determining overvoltage and overcurrent based on phase-to-phase grounding faults according to an embodiment of this application;

[0039] Figure 2 This is a schematic diagram of the control process of an MMC according to an embodiment of this application;

[0040] Figure 3 This is a schematic diagram of the control process of a GSC according to an embodiment of this application;

[0041] Figure 4 This is an equivalent circuit diagram of the system in full-load mode according to one embodiment of this application;

[0042] Figure 5 The present application provides an equivalent circuit diagram of a system in current-limited mode according to one embodiment of the present application.

[0043] Figure 6 This is an equivalent circuit diagram of a system during a phase-a ground fault according to an embodiment of this application;

[0044] Figure 7This is a schematic diagram showing that, according to an embodiment of this application, a fault occurs intermittently at point α;

[0045] Figure 8 This is a graph showing the relationship between the grounding coefficient and the fault point according to an embodiment of this application;

[0046] Figure 9 This is a graph showing the relationship between the negative sequence current coefficient and the fault point according to an embodiment of this application.

[0047] Figure 10 This is a graph showing the relationship between the positive sequence voltage of the PCC point and the fault point of a GSC according to an embodiment of this application.

[0048] Figure 11 An equivalent circuit diagram of phase-to-phase fault in current-limited mode provided according to an embodiment of this application;

[0049] Figure 12 This is a first structural diagram of a system for determining overvoltage and overcurrent based on phase-to-phase grounding faults according to an embodiment of this application;

[0050] Figure 13 This is a second structural diagram of a system for determining overvoltage and overcurrent based on phase-to-phase grounding faults according to an embodiment of this application. Detailed Implementation

[0051] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0052] This application proposes a method and system for determining overvoltage and overcurrent based on phase-to-phase grounding faults. The method includes: acquiring the total impedance value of the system, the positive-sequence voltage output by the multi-level converter at the sending end, and the positive-sequence AC current output by the wind turbine when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission; determining the maximum value of the negative-sequence fault current and the maximum value of the voltage of the non-faulty phases corresponding to the phase-to-phase grounding fault in the wind power transmission system via flexible direct current transmission based on the total impedance value, the positive-sequence voltage, and the positive-sequence AC current; using the maximum value of the negative-sequence fault current as the negative-sequence overcurrent when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission, and using the maximum value of the voltage of the non-faulty phases as the overvoltage when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission. The technical solution proposed in this application can accurately determine the negative sequence overcurrent and overvoltage values ​​when a phase-to-phase ground fault occurs in the wind power transmission system via flexible direct current transmission, based on the total impedance value, positive sequence voltage, and AC positive sequence current of the system. The stable operation of the wind power transmission system via flexible direct current transmission can be controlled based on the determined negative sequence overcurrent and overvoltage values.

[0053] The following describes a method and system for determining overvoltage and overcurrent based on phase-to-phase grounding faults according to embodiments of this application, with reference to the accompanying drawings.

[0054] Example 1

[0055] Figure 1 This is a flowchart illustrating a method for determining overvoltage and overcurrent based on phase-to-phase grounding faults according to an embodiment of this application. Figure 1 As shown, the method includes:

[0056] Step 1: Obtain the total impedance value of the system, the positive sequence voltage output by the multilevel converter at the sending end, and the AC positive sequence current output by the wind turbine when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission.

[0057] Step 2: Determine the maximum value of the negative sequence fault current and the maximum value of the voltage of the non-faulty phase when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission, based on the total impedance value, the positive sequence voltage, and the AC positive sequence current.

[0058] In this embodiment of the disclosure, the formula for calculating the maximum value of the negative sequence fault current is as follows:

[0059]

[0060] In the formula, I fanmax U is the maximum value of the negative sequence fault current. mp X is the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. allI represents the total reactance of the wind power transmission system via flexible direct current, α represents the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current, and I represents the total reactance of the wind power transmission system via flexible direct current. wp This refers to the positive sequence AC current output by the fan.

[0061] The formula for calculating the maximum voltage of the non-faulty phase is as follows:

[0062]

[0063] In the formula, U famax U is the maximum voltage of the non-faulty phase. mp X is the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. all I represents the total reactance of the wind power transmission system via flexible direct current, α represents the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current, and I represents the total reactance of the wind power transmission system via flexible direct current. wp This refers to the positive sequence AC current output by the fan.

[0064] Step 3: Take the maximum value of the negative sequence fault current as the negative sequence overcurrent when the wind power is connected to the flexible direct transmission system and take the maximum value of the voltage of the non-faulty phase as the overvoltage when the wind power is connected to the flexible direct transmission system.

[0065] In this embodiment of the disclosure, the method further includes:

[0066] Step 4: Determine the positive sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system when a phase-to-phase grounding fault occurs, based on the total impedance value, the positive sequence voltage, and the AC positive sequence current.

[0067] Step 5: Obtain the reactance value corresponding to the wind turbine converter reactor in the system, and determine the negative sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system when a phase-to-phase grounding fault occurs, based on the reactance value corresponding to the wind turbine converter reactor, the total impedance value, the positive sequence voltage, and the AC positive sequence current.

[0068] Furthermore, the formula for calculating the positive sequence overvoltage corresponding to the common connection point of the wind turbine when a phase-to-phase grounding fault occurs is as follows: U wpmax :

[0069]

[0070] The formula for calculating the negative sequence overvoltage corresponding to the common coupling point of the wind turbine when a phase-to-phase grounding fault occurs is as follows:

[0071]

[0072] In the formula, U wpmaxU is the positive-sequence overvoltage corresponding to the common connection point of the wind turbine when a phase-to-phase grounding fault occurs. wnmax U is the negative sequence overvoltage corresponding to the common connection point of the wind turbine during an indirect phase fault. mp I is the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. wp X is the positive sequence AC current output by the fan. all X is the total reactance value corresponding to the wind power transmission system via flexible direct current, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current, and X is the total reactance value. weq α is the reactance value corresponding to the wind turbine converter reactor, and α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current.

[0073] In this embodiment of the disclosure, to simplify the analysis, the wind farm can be equivalent to a GSC; the MMC and GSC adopt a dual closed-loop control strategy, including an outer voltage loop and an inner current loop. At the same time, in order to control the negative sequence current, negative sequence control is added to the MMC and GSC respectively.

[0074] Among them, such as Figure 2 The control process of MMC is shown below:

[0075] MMC adopts a traditional VF control strategy. First, the controller directly integrates the time using a 50Hz frequency to generate the phase angle θ of the MMC modulation voltage. MMC This phase angle is then fed into the Park transform (T) abc / dq ) and inverse Park transform (T dq / abc In the module, the current I of the MMC is collected. m and PCC point voltage U ms It is separated into positive and negative order components and d- and q-axis components, respectively I mdqp I mdqn U msdqp U msdqn The positive sequence component U of the voltage at point PCC msdqp The error value is compared with its reference value, and the error is input into the PI controller to obtain the command value of the positive sequence current. Then, the positive sequence current I0 is... mdqp The error is compared with its command value and input to the PI controller. The generated modulation voltage is then added to / subtracted from the cross-coupling term (ωL). eq I mdqp ) and voltage feedforward term (U msdqp This generates a positive-sequence modulation voltage, which is then passed through a positive-sequence modulation voltage limiter to generate the actual positive-sequence modulation voltage U. mcdqp Similarly, negative-sequence control only retains the inner-loop control strategy, controlling the negative-sequence current I. mdqnThe error is compared with its command value and input to the PI controller. The generated modulation voltage is then added to / subtracted from the cross-coupling term (ωL). eq I mdqn ) and voltage feedforward term (U msdqn This generates a negative-sequence modulation voltage U. mcdqn *. The positive and negative sequence modulated voltages are converted through an inverse Park transform (T). dq / abc After that, a three-phase modulated voltage is generated. In the diagram, U... mc U is the voltage at the AC output of the MMC. ms For its PCC point voltage, I m For MMC AC voltage, U wc U is the voltage at the GSC AC output. ws For its PCC point voltage, I w For GSC AC voltage, I dc u mdc These represent the DC-side current of the GSC and the DC-side voltage of the MMC, respectively, θ MMC θ VSC The phase angles of the PCC points on the MMC and GSC sides are X and X, respectively. line For the impedance of the AC transmission line, X w and X m These are the impedances from the fault point to the GSC output and from the fault point to the MMC output, respectively. weq and X meq These are commutator reactors for GSC and MMC, respectively.

[0076] Among them, such as Figure 3 The diagram shows the control process of the GSC. The GSC adopts a grid-following control strategy. First, it acquires the positive sequence voltage phase θ at the PCC point of the wind turbine through a phase-locked loop (PLL). VSC The phase angle is then fed into the Park transform (T). abc / dq ) and inverse Park transform (T dq / abc In the module, the current of GSC and the voltage of PCC point are collected, and the positive and negative sequence components and d-axis and q-axis components are separated, which are respectively I wdqp I wdqn U wsdqp U wsdqn Secondly, the DC voltage U of the GSC wdc The error value is compared with its reference value and input into the PI controller to obtain the command value of the positive sequence current, and then the positive sequence current I... wdqp The error is compared with its command value and input to the PI controller. The generated modulation voltage is then added to / subtracted from the cross-coupling term (ωLI). wdqp ) and voltage feedforward term (U wsdqp This generates a positive-sequence modulated voltage. Similarly, negative-sequence control retains only the inner-loop control strategy, controlling the negative-sequence current I...wdqn The error is compared with its command value and input to the PI controller. The generated modulation voltage is then added to / subtracted from the cross-coupling term (ωLI). wdqn ) and voltage feedforward term (U wsdqn This generates a negative-sequence modulated voltage. The positive and negative-sequence modulated voltages are then subjected to an inverse Park transform (T... dq / abc After that, a three-phase modulation voltage U is generated. wcp * and U wcn *

[0077] It should be noted that when a shallow fault occurs, the MMC output positive-sequence current is less than its current limit value. In this case, the MMC is in full-modulation mode and can be considered an equivalent voltage source. When a deep fault occurs, the MMC output positive-sequence current is limited to its limit value. The MMC is in current-limited mode and can be considered an equivalent current source, while the GSC can also be considered an equivalent current source. The equivalent circuit of the system in full-modulation mode, without considering negative-sequence control, is as follows: Figure 4 As shown, the equivalent circuit of the system in current-limited mode, without considering negative sequence control, is as follows: Figure 5 As shown in the figure, U mcplim and I mlim These are the MMC positive sequence voltage limit and the MMC total current limit, respectively. wc X is the voltage at the AC output of the MMC. w I is the impedance from the fault point to the GSC output. w For GSC AC current, I m For MMC AC current, X m f is the impedance from the fault point to the MMC outlet. (n) This is the point of failure.

[0078] The following is an introduction to the full-modulation mode:

[0079] The key to achieving phase-to-phase fault ride-through is to obtain the key influencing factors of system overvoltage and overcurrent. Therefore, this invention adopts an analysis method that decouples circuit characteristics and control characteristics, ignoring the negative sequence control of MMC and GSC to obtain the expressions for system overcurrent and overvoltage, and extracts the key influencing factors.

[0080] When a two-phase ground fault occurs in the system, taking a fault involving phases B and C as an example, and neglecting the system resistance, the equivalent circuit of the system is as follows: Figure 6 As shown, the formula for calculating the fault current can be... In the formula, I fap I is the positive sequence current of the system when a ground fault occurs in phases b and c. fan U is the negative sequence current of the system when phases b and c experience ground faults. mp This refers to the positive sequence voltage output from the multilevel converter at the sending end of the system. Xmn X represents the negative sequence component of the impedance from the fault point to the MMC outlet. wn This represents the negative-sequence component of the impedance from the fault point to the GSC outlet. In the system, the transformer is usually directly grounded, so this invention assumes that the positive, negative, and zero-sequence impedances in the system are the same. Figure 7 As shown, when the phase-to-phase fault occurs at point α, X mp X n 'Can be represented as In the formula, X mp X represents the positive-sequence impedance component from the fault point to the MMC outlet. all This represents all reactances from the GSC AC output to the MMC AC output, including the MMC converter reactor, line reactances, GSC filters, and their converter reactors. (The formula is...) Substitution It can be observed that when the GSC output current lags the MMC output voltage by 90°, i.e., when the wind turbine only outputs reactive power, the negative sequence current in the system reaches its maximum. Formula Substitution In the middle, the sorting results are obtained In the formula, k i The negative sequence current coefficient is expressed as follows:

[0081] Meanwhile, based on the characteristics of power grid operation, transient overvoltages may occur in non-faulty phases, while the overvoltage level is highest at the fault point. The PCC point of the GSC should also consider the problem of wind turbine disconnection due to overvoltage.

[0082] The voltage of the healthy phase (phase a) at the fault point is U. fa =U fap +U fan +U fa0 =2jI fap X n The amplitude of the healthy phase at the fault point is U. fa =k u (U mp +αI wp X all In the formula, ku is the phase-to-phase short-circuit grounding coefficient. It can be obtained that when the GSC output current lags the MMC voltage by 90°, that is, when the GSC only outputs reactive power, the voltage of the healthy phase at the fault point is the maximum, U. famax =k u (U mp +αI wp X all The positive-sequence voltage and negative-sequence voltage at the PCC point of the GSC are U. wp =-I fn X n '+I wpX wp U wn =αjI fn X weq It can be seen that when the GSC output current lags the MMC voltage by 90°, i.e., when the GSC only outputs reactive power, the maximum amplitude of the positive and negative sequence voltages is...

[0083] Furthermore, the relationship curve between the grounding coefficient and the fault point α is as follows: Figure 8 As shown, the relationship curve between the negative sequence current coefficient and the fault point α is as follows: Figure 9 As shown, the relationship curve between the positive sequence voltage at the PCC point of the GSC and the fault point α is as follows: Figure 10 As shown, it can be obtained that: when the GSC positive sequence current lags the MMC positive sequence voltage by 90°, the system's overvoltage and overcurrent levels are the highest; the closer the fault point is to the MMC, the higher the system's overvoltage and overcurrent levels; the system's negative sequence current level and overvoltage level are related to the GSC positive sequence current I. wp related.

[0084] The current-limiting mode is described below:

[0085] When a deep fault occurs in the AC system, the fault current reaches the MMC current limit. At this time, the MMC exhibits current source characteristics and enters current-limiting mode. In current-limiting mode, the system overvoltage is significantly lower than in full-modulation mode; therefore, system overvoltage levels are no longer analyzed in current-limiting mode. However, in current-limiting mode, the MMC output current equals the current limiter's limit and reaches the system protection upper limit, exceeding the current in full-modulation mode. This further increases the negative sequence current and exacerbates the overcurrent problem in the MMC bridge arm.

[0086] Equivalent circuit for phase-to-phase grounding faults in current-limited mode, such as Figure 11 As shown, the phase-to-phase fault negative sequence current is It can be observed that the magnitude of the system's negative sequence current is independent of the fault location, and only depends on the magnitude and phase of the output currents of the MMC and GSC. The system's negative sequence current is at its maximum when the GSC output current is in phase with the MMC output current, i.e., when the GSC only outputs reactive power. Similar to the full modulation mode, in the current-limiting mode, reducing the positive sequence current in the system can, to some extent, reduce the negative sequence current during phase-to-phase short-circuit faults.

[0087] In summary, the method for determining overvoltage and overcurrent based on phase-to-phase grounding faults proposed in this application can accurately determine the negative-sequence overcurrent and overvoltage values ​​when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission, based on the total impedance value, positive-sequence voltage, and AC positive-sequence current of the system. The method can then control the stable operation of the wind power transmission system via flexible direct current transmission based on the determined negative-sequence overcurrent and overvoltage values.

[0088] Example 2

[0089] Figure 12 According to one embodiment of this application, a system for determining overvoltage and overcurrent based on phase-to-phase grounding faults is provided, such as... Figure 12 As shown, it includes:

[0090] The acquisition module 100 is used to acquire the total impedance value of the system, the positive sequence voltage output by the multilevel converter at the sending end, and the AC positive sequence current output by the wind turbine when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct transmission.

[0091] The first determining module 200 is used to determine the maximum value of the negative sequence fault current and the maximum value of the voltage of the non-faulty phase when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission, based on the total impedance value, the positive sequence voltage, and the AC positive sequence current, respectively.

[0092] The second determining module 300 is used to take the maximum value of the negative sequence fault current as the negative sequence overcurrent when the wind power is connected to the flexible direct transmission system and take the maximum value of the voltage of the non-faulty phase as the overvoltage when the wind power is connected to the flexible direct transmission system.

[0093] In the embodiments disclosed herein, such as Figure 13 As shown, the determining system further includes:

[0094] The third determining module 400 is used to determine the positive sequence overvoltage corresponding to the common connection point of the wind turbine in the wind power transmission system via flexible direct transmission when a phase-to-phase grounding fault occurs, based on the total impedance value, the positive sequence voltage, and the AC positive sequence current.

[0095] The fourth determining module 500 is used to obtain the reactance value corresponding to the wind turbine converter reactor in the system, and determine the negative sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system when a phase-to-phase grounding fault occurs, based on the reactance value corresponding to the wind turbine converter reactor, the total impedance value, the positive sequence voltage and the AC positive sequence current.

[0096] In this embodiment of the disclosure, the formula for calculating the maximum value of the negative sequence fault current is as follows:

[0097]

[0098] In the formula, I fanmax U is the maximum value of the negative sequence fault current. mp X is the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. all I represents the total reactance of the wind power transmission system via flexible direct current, α represents the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current, and I represents the total reactance of the wind power transmission system via flexible direct current. wpThis refers to the positive sequence AC current output by the fan.

[0099] In this embodiment of the disclosure, the formula for calculating the maximum voltage of the non-faulty phase is as follows:

[0100]

[0101] In the formula, U famax U is the maximum voltage of the non-faulty phase. mp X is the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. all I represents the total reactance of the wind power transmission system via flexible direct current, α represents the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current, and I represents the total reactance of the wind power transmission system via flexible direct current. wp This refers to the positive sequence AC current output by the fan.

[0102] Furthermore, the formula for calculating the positive sequence overvoltage corresponding to the common connection point of the wind turbine when a phase-to-phase grounding fault occurs is as follows: U wpmax :

[0103]

[0104] The formula for calculating the negative sequence overvoltage corresponding to the common coupling point of the wind turbine when a phase-to-phase grounding fault occurs is as follows:

[0105]

[0106] In the formula, U wpmax U is the positive-sequence overvoltage corresponding to the common connection point of the wind turbine when a phase-to-phase grounding fault occurs. wnmax U is the negative sequence overvoltage corresponding to the common connection point of the wind turbine during an indirect phase fault. mp I is the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. wp X is the positive sequence AC current output by the fan. all X is the total reactance value corresponding to the wind power transmission system via flexible direct current, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current, and X is the total reactance value. weq α is the reactance value corresponding to the wind turbine converter reactor, and α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current.

[0107] In summary, the overvoltage and overcurrent determination system based on phase-to-phase grounding faults proposed in this application, and the technical solution proposed in this application, can accurately determine the negative-sequence overcurrent and overvoltage values ​​when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission, based on the total impedance value, positive-sequence voltage, and AC positive-sequence current of the system. Based on the determined negative-sequence overcurrent and overvoltage values, the stable operation of the wind power transmission system via flexible direct current transmission can be controlled.

[0108] Example 3

[0109] To implement the above embodiments, this disclosure also proposes an electronic device, including: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the program, it implements the method described in Embodiment 1.

[0110] Example 4

[0111] To implement the above embodiments, this disclosure also proposes a computer-readable storage medium storing a computer program thereon, characterized in that, when the program is executed by a processor, it implements the method described in Embodiment 1.

[0112] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0113] Any process or method description in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing custom logic functions or processes, and the scope of the preferred embodiments of this application includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the functions involved, as should be understood by those skilled in the art to which embodiments of this application pertain.

[0114] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method for determining overvoltage and overcurrent based on phase-to-ground fault, characterized in that, The method includes: When a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission, the total impedance value of the system, the positive sequence voltage output by the multilevel converter at the sending end, and the AC positive sequence current output by the wind turbine are obtained. The maximum value of the negative sequence fault current and the maximum value of the voltage of the non-faulty phase are determined based on the total impedance value, the positive sequence voltage, and the AC positive sequence current when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct transmission. The maximum value of the negative sequence fault current is taken as the negative sequence overcurrent when the wind power is connected to the flexible direct transmission system via a phase-to-phase fault, and the maximum value of the voltage of the non-faulty phase is taken as the overvoltage when the wind power is connected to the flexible direct transmission system via a phase-to-phase fault. The formula for calculating the maximum value of the negative sequence fault current is as follows: In the formula, This represents the maximum value of the negative sequence fault current. This refers to the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. This represents the total reactance value corresponding to the wind power transmission system via flexible direct current. The distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current is given. This refers to the positive sequence AC current output by the fan. The formula for calculating the maximum voltage of the non-faulty phase is as follows: In the formula, This represents the maximum voltage of the non-faulty phase. This refers to the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. This represents the total reactance value corresponding to the wind power transmission system via flexible direct current. The distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current is given. This refers to the positive sequence AC current output by the fan.

2. The method of claim 1, wherein, The method further includes: The positive sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system via flexible direct transmission is determined based on the total impedance value, the positive sequence voltage, and the AC positive sequence current when a phase-to-phase grounding fault occurs. Obtain the reactance value corresponding to the wind turbine converter reactor in the system, and determine the negative sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system when a phase-to-phase grounding fault occurs, based on the reactance value corresponding to the wind turbine converter reactor, the total impedance value, the positive sequence voltage, and the AC positive sequence current. The formula for calculating the positive-sequence overvoltage corresponding to the common connection point of the wind turbine when a phase-to-phase grounding fault occurs is as follows: The formula for calculating the negative sequence overvoltage corresponding to the common coupling point of the wind turbine when a phase-to-phase grounding fault occurs is as follows: In the formula, The positive-sequence overvoltage corresponding to the common connection point of the wind turbine in the event of a phase-to-phase grounding fault. The negative sequence overvoltage corresponding to the common connection point of the wind turbine in the event of a phase-to-phase grounding fault. This refers to the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. This refers to the positive sequence AC current output by the fan. This represents the total reactance value corresponding to the wind power transmission system via flexible direct current. The distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current is given. This is the reactance value corresponding to the wind turbine converter reactor.

3. A system for determining overvoltage and overcurrent based on phase-to-ground fault, characterized in that, include: The acquisition module is used to acquire the total impedance value of the system, the positive sequence voltage output by the multilevel converter at the sending end, and the AC positive sequence current output by the wind turbine when a phase-to-ground fault occurs in the wind power transmission system via flexible direct current transmission. The first determining module is used to determine the maximum value of the negative sequence fault current and the maximum value of the voltage of the non-faulty phase when a phase-to-phase grounding fault occurs in the wind power transmission system via flexible direct current transmission, based on the total impedance value, the positive sequence voltage, and the AC positive sequence current, respectively. The second determining module is used to take the maximum value of the negative sequence fault current as the negative sequence overcurrent when the wind power is connected to the flexible direct transmission system and take the maximum value of the voltage of the non-faulty phase as the overvoltage when the wind power is connected to the flexible direct transmission system. The formula for calculating the maximum value of the negative sequence fault current is as follows: In the formula, This represents the maximum value of the negative sequence fault current. This refers to the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. This represents the total reactance value corresponding to the wind power transmission system via flexible direct current. The distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current is given. This refers to the positive sequence AC current output by the fan. The formula for calculating the maximum voltage of the non-faulty phase is as follows: In the formula, This represents the maximum voltage of the non-faulty phase. This refers to the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. This represents the total reactance value corresponding to the wind power transmission system via flexible direct current. The distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current is given. This refers to the positive sequence AC current output by the fan.

4. The determination system of claim 3, wherein, The determining system further includes: The third determining module is used to determine the positive sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system via flexible direct current when a phase-to-phase grounding fault occurs, based on the total impedance value, the positive sequence voltage, and the AC positive sequence current. The fourth determining module is used to obtain the reactance value corresponding to the wind turbine converter reactor in the system, and determine the negative sequence overvoltage corresponding to the wind turbine common connection point in the wind power transmission system when a phase-to-phase grounding fault occurs, based on the reactance value corresponding to the wind turbine converter reactor, the total impedance value, the positive sequence voltage and the AC positive sequence current. The formula for calculating the positive-sequence overvoltage corresponding to the common connection point of the wind turbine when a phase-to-phase grounding fault occurs is as follows: The formula for calculating the negative sequence overvoltage corresponding to the common coupling point of the wind turbine when a phase-to-phase grounding fault occurs is as follows: In the formula, The positive-sequence overvoltage corresponding to the common connection point of the wind turbine in the event of a phase-to-phase grounding fault. The negative sequence overvoltage corresponding to the common connection point of the wind turbine in the event of a phase-to-phase grounding fault. This refers to the positive sequence voltage output from the multilevel converter at the sending end of the wind power transmission system via flexible direct current transmission. This refers to the positive sequence AC current output by the fan. This represents the total reactance value corresponding to the wind power transmission system via flexible direct current. The distance from the fault point to the multilevel converter at the sending end of the wind power transmission system via flexible direct current is given. This is the reactance value corresponding to the wind turbine converter reactor.

5. An electronic device, comprising: include: A memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor, when executing the program, implements the method as described in any one of claims 1 to 2.

6. A computer-readable storage medium having stored thereon a computer program, characterized in that, When the program is executed by the processor, it implements the method as described in any one of claims 1 to 2.

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