A hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method and system

Abstract

A hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method and system, comprising: injecting a protection detection signal by applying additional control in the single-pole single-end MMC converter trigger pulse modulation link; collecting the double-circuit grounding electrode line head voltage, current and tail current, calculating the line overall measurement equivalent impedance, judging whether the line fault criterion is met, if met, judging that the line has a fault; calculating the first and last end imbalance impedance modulus of the double-circuit grounding electrode line, taking the maximum value of the two to obtain the double-end maximum imbalance impedance modulus; when the double-end maximum imbalance impedance modulus is greater than the double-end maximum imbalance impedance modulus setting value, it is determined as single-line grounding fault, otherwise, it is determined as double-line grounding fault; based on the line overall theoretical equivalent impedance expression when various grounding faults occur and the line overall measurement equivalent impedance, a distance measurement equation set is constructed and solved to obtain the fault distance. The application can improve the comprehensive performance of the grounding electrode protection.

Description

Technical Field

[0001] This invention belongs to the field of power system line fault protection and control technology, and in particular relates to a hybrid MMC-HVDC grounding electrode line fault detection, identification and location method and system. Background Technology

[0002] With the increasing power generation from new energy sources and the development of power electronics technology, flexible DC transmission based on modular multilevel converter high voltage direct current (MMC-HVDC) technology has gradually taken a dominant position in long-distance, large-capacity power transmission, as well as cross-regional interconnection and new energy grid connection, due to its advantages such as high controllability of converter valves, low risk of commutation failure, and high output waveform quality. Compared with half-bridge and full-bridge MMC converters, hybrid MMC converters exhibit higher engineering cost-effectiveness and fault self-clearing capability, making them one of the mainstream topologies of MMC converters.

[0003] In high-voltage direct current transmission systems, compared to direct current transmission lines, grounding electrode lines are almost without pressure or current in the bipolar balanced operation state of the system. When a grounding fault occurs, the fault characteristics are hidden and it is difficult to achieve protection through conventional methods. If the fault is not investigated in time and it turns into a bipolar unbalanced operation or a unipolar-earth operation state, a large amount of current will flow into the fault point, threatening the safety of nearby personnel and equipment. Existing projects generally use impedance monitoring systems to detect faults and pulse ranging systems to locate faults, but both rely on corresponding external injection devices and isolation equipment, resulting in complex grounding electrode line structures, high maintenance difficulty, large differences in protection principles, and the existence of protection dead zones. Currently, existing grounding electrode protection has the following problems: (1) high structural complexity, many types of equipment, and high maintenance difficulty; (2) reliance on external injection devices and isolation equipment, high protection matching costs, and protection performance highly dependent on the performance of external devices; (3) large differences in protection principles between fault detection methods and fault location methods, and complex protection algorithms and devices required; (4) inability to identify specific fault types, existence of protection dead zones, and limited overall protection performance. In recent years, existing research has mostly improved upon existing protective structures and principles, but it has always been unable to break free from dependence on external devices and equipment, making it difficult to fundamentally solve its drawbacks.

[0004] Chinese patent application CN110470947B discloses a method for fault location on grounding electrode lines applicable to MMC DC systems. By modifying the number of MMC sub-modules, an AC harmonic component with a frequency of 1 / Tc is generated on the DC line. Then, the converter side voltage and converter side current are obtained. The same-direction component of the converter side voltage and the same-direction and opposite-direction components of the converter side current are calculated to obtain the expression of the fault point voltage and current with respect to the fault distance. Solving the location equation can accurately locate the fault. However, the following shortcomings still exist: (1) It cannot detect and identify the grounding fault type; (2) During signal injection, a negative additional DC component is generated on the DC side compared to the steady state, which interferes with the constant voltage control.

[0005] Chinese patent application CN113608069B discloses a method and system for identifying grounding faults in DC distribution networks using MMC-injected detection signals. This method employs an additional control strategy to inject detection signals into feeders after a fault occurs, identifying the faulty feeder based on the difference in response to the detection signals between the faulty and healthy feeders. However, it still has the following shortcomings: (1) it cannot pinpoint the fault distance; (2) the line uses an equivalent π model, resulting in lower accuracy under long-line broadband models.

[0006] As the location of grounding electrodes becomes increasingly remote, the length of grounding electrode lines increases accordingly, significantly increasing the difficulty of line protection. Existing protection methods are gradually failing to meet the operational reliability requirements of grounding electrode lines under the bipolar balanced operation conditions of the system. There is an urgent need to propose a new method for grounding electrode line protection suitable for new power systems. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method and system for detecting, identifying, and locating faults in hybrid MMC-HVDC grounding electrode lines.

[0008] The present invention adopts the following technical solution.

[0009] In a first aspect, this invention discloses a method for fault detection, identification, and location of a hybrid MMC-HVDC grounding electrode line, wherein the grounding electrode line is a dual-circuit grounding electrode line; the method includes the following steps: Step 1: In the hybrid MMC-HVDC system, additional control is applied to the trigger pulse modulation stage of the single-pole single-ended MMC converter to inject a protection detection signal, and then proceed to Step 2; Step 2: Collect the voltage and current at the beginning and end of the double-circuit grounding electrode line, calculate the overall equivalent impedance of the line, and determine whether the line fault criterion is met based on the overall equivalent impedance of the line. If it is met, the line is determined to be faulty and proceed to step 3. Step 3: Calculate the unbalanced impedance magnitudes at the beginning and end of the double-circuit grounding electrode line, and take the maximum value of the two to obtain the maximum unbalanced impedance magnitude at both ends; when the maximum unbalanced impedance magnitude at both ends is greater than the set value of the maximum unbalanced impedance magnitude at both ends, it is determined to be a single-line grounding fault; otherwise, it is determined to be a double-line grounding fault, and proceed to step 4. Step 4: Based on the determined ground fault type, and using the overall theoretical equivalent impedance expression of the line when the ground fault of that type occurs, as well as the overall measured equivalent impedance of the line, construct a set of distance measurement equations and solve them to obtain the fault distance.

[0010] More preferably, In step 1, when the number of hot-standby full-bridge submodules in both the upper and lower arms of the three-phase single-ended MMC converter is not less than... n ctr At that time, a protection detection signal injection is initiated, and the following additional control is simultaneously applied to the upper and lower arms of the three phases of the single-pole single-ended MMC converter: Selecting in each arm... n ctr Each hot-standby full-bridge submodule is deactivated from the minimum-level approximation modulation and capacitor voltage sorting algorithm, meaning it remains in its disconnected state and does not participate in the switching of subsequent submodules. It is then switched to a mode where its operation can be controlled independently, and simultaneously, it operates according to a preset injected signal frequency. f ctr Control the n ctr Each hot-standby full-bridge submodule periodically switches between positive and negative on / off states; among them, n ctr The preset number of modules.

[0011] More preferably, The step of selecting from each bridge arm n ctr The hot-standby full-bridge submodule is selected as follows: the current direction in the current bridge arm is detected; if the current is positive, the first submodule is selected in descending order of capacitor voltage. n ctr One hot-standby full-bridge submodule; if the current is negative, select the first capacitor in ascending order of capacitor voltage. n ctr A hot standby full-bridge submodule.

[0012] More preferably, When injecting the protection detection signal, the DC voltage fluctuation amplitude shall not exceed 10% of the rated DC voltage of the hybrid MMC-HVDC system, specifically satisfying the following: ,in, This refers to the rated capacitor voltage of the full-bridge submodule in the MMC converter. The rated DC voltage for the hybrid MMC-HVDC system.

[0013] More preferably, During the protection probe signal injection period, the modulation ratio of the MMC converter is controlled. Not exceeding 1, specifically satisfying: ,in, This refers to the rated capacitor voltage of the full-bridge submodule in the MMC converter. The rated DC voltage for the hybrid MMC-HVDC system; This represents the fundamental amplitude of the AC phase voltage of the MMC converter.

[0014] More preferably, The preset injection signal frequency f ctr The following conditions must be met simultaneously: 1) ,in, The sampling frequency of the converter station control and protection system; 2) ,in, This refers to the submodule switching frequency during normal operation of the MMC converter. , 3) is a positive integer. ,in, L The inductance per unit length of the grounding electrode line; C Capacitance per unit length of the grounding electrode line; For the total length of the grounding electrode line; 4) ,in, This indicates that the MMC converter operates at a certain frequency. The corresponding equivalent positive sequence impedance. It is the power frequency; This is for phase angle calculation.

[0015] More preferably, In step 2, the line fault criterion is specifically as follows: ,in, The equivalent impedance of the current line is measured as a whole. This is the overall theoretical equivalent impedance of the line during normal operation. The set value is used to determine the magnitude of the line impedance deviation.

[0016] More preferably, The line impedance deviation magnitude set value The tuning principle is as follows: ,in, K This is the reliability coefficient.

[0017] More preferably, In step 3, the maximum unbalanced impedance modulus setting is taken as the line impedance deviation modulus setting. .

[0018] More preferably, In step 3, the dual-circuit grounding electrode line includes line I and line II. The magnitude of the unbalanced impedance at the beginning of the dual-circuit grounding electrode line is the magnitude of the difference between the impedance at the beginning of line I and the impedance at the beginning of line II. The magnitude of the unbalanced impedance at the end of the dual-circuit grounding electrode line is the magnitude of the difference between the impedance at the end of line I and the impedance at the end of line II.

[0019] More preferably, The overall theoretical equivalent impedance expression for the line includes: The theoretical equivalent impedance expression for a line experiencing a single-wire ground fault is as follows:

[0020] in, The overall theoretical equivalent impedance of the line where a single-wire grounding fault occurs; β 0 and β 1 represents the zero-mode phase constant and the line-mode phase constant, respectively; x Distance to the fault; d This is the difference between the total length of the line and the distance to the fault; Z C0 and Z C1 These are the zero-mode impedance and the line-mode impedance, respectively. R d For transition resistance; As an intermediate variable; The theoretical equivalent impedance expression for a line experiencing a double-wire ground fault is as follows:

[0021] in, The overall theoretical equivalent impedance of a line experiencing a double-wire grounding fault.

[0022] More preferably, The ranging equations are constructed based on the principle that when complex numbers are equal, their real and imaginary parts are equal, and include real part equations and imaginary part equations. The real part equations make the real part of the overall theoretical equivalent impedance of the line equal to the real part of the overall measured equivalent impedance of the line, and the imaginary part equations make the imaginary part of the overall theoretical equivalent impedance of the line equal to the imaginary part of the overall measured equivalent impedance of the line.

[0023] More preferably, The protection detection signal has a signal injection duration of [duration missing]. Must meet:

[0024] in, It is a non-negative integer used to determine the frequency range of the protection detection signal relative to the power frequency; It is a positive integer; The upper limit critical time coefficient is determined based on the safe range of capacitor voltage in the sub-modules of the MMC converter in actual engineering. It is the power frequency.

[0025] Secondly, the present invention discloses a hybrid MMC-HVDC grounding electrode line fault detection, identification and location system based on the aforementioned method, including a protection detection signal injection module, a line fault detection module, a line fault type identification module and a line fault location module; The protection detection signal injection module is used to inject a protection detection signal in a hybrid MMC-HVDC system by applying additional control to the trigger pulse modulation stage of the single-pole single-ended MMC converter. The line fault detection module is used to collect the voltage and current at the beginning and end of the double-circuit grounding electrode line, calculate the overall measured equivalent impedance of the line, and determine whether the line fault criterion is met based on the overall measured equivalent impedance of the line. If it is met, the line is judged to have a fault. The line fault type identification module is used to calculate the magnitude of the unbalanced impedance at the beginning and end of the double-circuit grounding electrode line, and take the maximum value of the two to obtain the maximum unbalanced impedance magnitude at both ends; when the maximum unbalanced impedance magnitude at both ends is greater than the set value of the maximum unbalanced impedance magnitude at both ends, it is determined to be a single-line grounding fault; otherwise, it is determined to be a double-line grounding fault. The line fault location module is used to combine the determined ground fault type with the overall theoretical equivalent impedance expression of the line when the ground fault of that type occurs and the overall measured equivalent impedance of the line to construct a set of distance measurement equations and solve them to obtain the fault distance.

[0026] Thirdly, the present invention provides a terminal, including a processor and a storage medium; The storage medium is used to store instructions; The processor is configured to operate according to the instructions to perform the steps of the method according to any one of the first aspects of the present invention.

[0027] Fourthly, the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method described in any one of the first aspects of the present invention.

[0028] This invention simplifies the design of existing grounding electrode system protection structures. Based on this, it injects protection detection signals into the grounding electrode line by controlling the operation of the hot-standby full-bridge submodule in the hybrid MMC converter valve at a specific frequency. According to the line impedance and its differences, it achieves the detection, identification, and location of various types of grounding faults, thus constructing a complete grounding electrode line protection system. Compared with existing technologies, it has the following beneficial technical effects: (1) Eliminating dependence on external signal injection devices: The protection method proposed in this invention is applicable to bipolar balanced operation mode and does not rely on external signal injection devices. It can complete the injection of protection detection signals and the realization of protection functions by using only the system's own control capabilities and electrical quantities. Compared with the method that requires additional configuration of external signal injection devices, this invention can significantly reduce system complexity and equipment costs, solve the drawback of protection performance being highly dependent on the performance of external devices, and facilitate engineering implementation.

[0029] (2) Improved grounding electrode protection system: This invention can simultaneously achieve correct fault detection, accurate identification and precise location, forming a complete protection system, and there is almost no protection dead zone within the range of the studied faults. In addition, the proposed fault identification method can also be used as one of the main protections for single-line faults, further improving the reliability of the protection.

[0030] (3) Improve the overall performance of grounding electrode protection: Compared with existing engineering application methods and other research methods, which often only perform well in some aspects and ignore other aspects, this invention achieves a better level in multiple performance indicators such as the accuracy of fault detection, identification and location, dead zone, applicability and robustness. It can still achieve protection under extreme working conditions and has stronger comprehensive engineering applicability.

[0031] (4) Reduced complexity of grounding electrode protection principle: In this invention, fault detection, fault identification and fault location are all based on the impedance characteristics of grounding electrode measurement, without the need to use multiple heterogeneous principles for different functions. Compared with the combination of multiple methods such as impedance monitoring method and pulse ranging method, this invention can effectively reduce the differences in protection principles, simplify the algorithm design and parameter setting of protection devices, and facilitate integrated engineering implementation. Attached Figure Description

[0032] Figure 1 This is a flowchart illustrating a hybrid MMC-HVDC grounding electrode line fault detection, identification, and location method provided by the present invention. Figure 2 This is a schematic diagram of an existing grounding electrode structure provided in Embodiment 1 of the present invention; Figure 3 This is a simplified design structure diagram of the grounding electrode provided in Embodiment 1 of the present invention; Figure 4This is a schematic diagram of a single-phase topology of a hybrid MMC converter provided in Embodiment 1 of the present invention; Figure 5 This is an additional control schematic diagram provided in Embodiment 1 of the present invention; Figure 6 This is a schematic diagram of the additional control flowchart provided in Embodiment 1 of the present invention; Figure 7 This is a schematic diagram of the hybrid MMC-HVDC topology provided in Embodiment 2 of the present invention; Figure 8 This is a schematic diagram of the frequency-dependent parameter model of the receiving-end grounding electrode line provided in Embodiment 2 of the present invention; Figure 9 This is a schematic diagram of the impedance deviation magnitude during a single-wire grounding fault provided in Embodiment 2 of the present invention; Figure 10 This is a schematic diagram of the impedance deviation magnitude during a two-wire grounding fault provided in Embodiment 2 of the present invention; Figure 11 This is a schematic diagram of the single-ended unbalanced impedance magnitude during a single-wire grounding fault, provided in Embodiment 2 of the present invention. Figure 12 This is a schematic diagram of the maximum unbalanced impedance magnitude at both ends during a single-wire grounding fault, provided in Embodiment 2 of the present invention. Figure 13 This is a schematic diagram of a DC line waveform provided in Embodiment 2 of the present invention; Figure 14 This is a schematic diagram of the grounding electrode line waveform provided in Embodiment 2 of the present invention; Figure 15 This is a schematic diagram of the DC line control frequency component waveform provided in Embodiment 2 of the present invention; Figure 16 This is a schematic diagram of the grounding electrode line control frequency component waveform provided in Embodiment 2 of the present invention; Figure 17 This is a schematic diagram of the DC component waveform of the grounding electrode line current provided in Embodiment 2 of the present invention; Figure 18 This is a schematic diagram of the system power change waveform provided in Embodiment 2 of the present invention; Figure 19 This is a schematic diagram of the grid-side voltage waveform of the converter transformer provided in Embodiment 2 of the present invention; Figure 20 This is a schematic diagram of the AC grid-side A-phase current spectrum provided in Embodiment 2 of the present invention. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0034] like Figure 1 As shown, this invention discloses a method for detecting, identifying, and locating faults in a hybrid MMC-HVDC grounding electrode line, comprising: Step 1: In the hybrid MMC-HVDC system, additional control is applied to the trigger pulse modulation stage of the single-pole single-ended MMC converter to inject a protection detection signal, and then proceed to Step 2; Furthermore, when the number of hot-standby full-bridge submodules in both the upper and lower arms of the three-phase single-ended MMC converter is not less than [number missing], n ctr At that time, a protection detection signal injection is initiated, and the following additional control is simultaneously applied to the upper and lower arms of the three phases of the single-pole single-ended MMC converter: Selecting in each arm... n ctr Each hot-standby full-bridge submodule is deactivated from the minimum-level approximation modulation and capacitor voltage sorting algorithm, meaning it remains in its disconnected state and does not participate in the switching of subsequent submodules. It is then switched to a mode where its operation can be controlled independently, and a preset injection signal frequency is applied. f ctr Control the n ctr Each hot-standby full-bridge submodule periodically switches between positive and negative on / off states; among them, n ctr The preset number of modules.

[0035] The step of selecting from each bridge arm n ctr The hot-standby full-bridge submodule is selected as follows: the current direction in the current bridge arm is detected; if the current is positive, the first submodule is selected in descending order of capacitor voltage. n ctr One hot-standby full-bridge submodule; if the current is negative, select the first capacitor in ascending order of capacitor voltage. n ctr A hot standby full-bridge submodule.

[0036] More preferably, when injecting the protection detection signal, the DC voltage fluctuation amplitude does not exceed 10% of the rated DC voltage of the hybrid MMC-HVDC system, specifically satisfying: ,in, This refers to the rated capacitor voltage of the full-bridge submodule in the MMC converter. The rated DC voltage for the hybrid MMC-HVDC system.

[0037] More preferably, during the protection probe signal injection period, the modulation ratio of the MMC converter is controlled. Not exceeding 1, specifically satisfying: ,in, This refers to the rated capacitor voltage of the full-bridge submodule in the MMC converter. The rated DC voltage for the hybrid MMC-HVDC system; This represents the fundamental amplitude of the AC phase voltage of the MMC converter.

[0038] More preferably, the preset injection signal frequency f ctr The following conditions must be met simultaneously: 1) ,in, The sampling frequency of the converter station control and protection system; 2) ,in, This refers to the submodule switching frequency during normal operation of the MMC converter. , 3) is a positive integer. ,in, L The inductance per unit length of the grounding electrode line; C Capacitance per unit length of the grounding electrode line; For the total length of the grounding electrode line; 4) ,in, This indicates that the MMC converter operates at a certain frequency. The corresponding equivalent positive sequence impedance. It is the power frequency; This is for phase angle calculation.

[0039] More preferably, the protection detection signal has a signal injection duration of Must meet:

[0040] in, It is a non-negative integer used to determine the frequency range of the protection detection signal relative to the power frequency; It is a positive integer; The upper limit critical time coefficient is determined based on the safe range of capacitor voltage in the sub-modules of the MMC converter in actual engineering. It is the power frequency.

[0041] Step 2: Collect the voltage and current at the beginning and end of the double-circuit grounding electrode line, calculate the overall equivalent impedance of the line, and determine whether the line fault criterion is met based on the overall equivalent impedance of the line. If it is met, the line is determined to be faulty and proceed to step 3. The line fault determination criteria are as follows: ,in, The equivalent impedance of the current line is measured as a whole. This is the overall theoretical equivalent impedance of the line during normal operation. The set value is used to determine the magnitude of the line impedance deviation.

[0042] The line impedance deviation magnitude set value The tuning principle is as follows: ,in, K This is the reliability coefficient.

[0043] Step 3: Calculate the unbalanced impedance magnitudes at the beginning and end of the double-circuit grounding electrode line, and take the maximum value of the two to obtain the maximum unbalanced impedance magnitude at both ends; when the maximum unbalanced impedance magnitude at both ends is greater than the set value of the maximum unbalanced impedance magnitude at both ends, it is determined to be a single-line grounding fault; otherwise, it is determined to be a double-line grounding fault, and proceed to step 4. The dual-circuit grounding electrode line includes line I and line II. The magnitude of the unbalanced impedance at the beginning of the dual-circuit grounding electrode line is the magnitude of the difference between the impedance at the beginning of line I and the impedance at the beginning of line II. The magnitude of the unbalanced impedance at the end of the dual-circuit grounding electrode line is the magnitude of the difference between the impedance at the end of line I and the impedance at the end of line II.

[0044] The maximum unbalanced impedance modulus setting at both ends is taken as the modulus setting of the line impedance deviation. .

[0045] Step 4: Based on the determined ground fault type, and using the overall theoretical equivalent impedance expression of the line when the ground fault of that type occurs, as well as the overall measured equivalent impedance of the line, construct a set of distance measurement equations and solve them to obtain the fault distance.

[0046] The overall theoretical equivalent impedance expression for the line includes: The theoretical equivalent impedance expression for a line experiencing a single-wire ground fault is as follows:

[0047] in, The overall theoretical equivalent impedance of the line experiencing a single-line ground fault; β 0 and β 1 represents the zero-mode phase constant and the line-mode phase constant, respectively; x Distance to the fault; d This is the difference between the total length of the line and the distance to the fault;Z C0 and Z C1 These are the zero-mode impedance and the line-mode impedance, respectively. R d For transition resistance; As an intermediate variable; The theoretical equivalent impedance expression for a line experiencing a double-wire ground fault is as follows:

[0048] in, The overall theoretical equivalent impedance of a line experiencing a double-wire grounding fault.

[0049] The ranging equations are constructed based on the principle that when complex numbers are equal, their real and imaginary parts are equal, and include real part equations and imaginary part equations. The real part equations make the real part of the overall theoretical equivalent impedance of the line equal to the real part of the overall measured equivalent impedance of the line, and the imaginary part equations make the imaginary part of the overall theoretical equivalent impedance of the line equal to the imaginary part of the overall measured equivalent impedance of the line.

[0050] Example 1: like Figure 1 As shown, this invention discloses a method for detecting, identifying, and locating faults in a hybrid MMC-HVDC grounding electrode line, comprising: Step 1: In the hybrid MMC-HVDC system, additional control is applied to the trigger pulse modulation stage of the single-pole single-ended MMC converter to inject a protection detection signal, and then proceed to Step 2; This invention simplifies the design of the grounding electrode line structure by eliminating the external signal injection device and some of its isolation equipment, based on the existing grounding electrode structure; for example... Figure 2 As shown, a typical grounding electrode circuit structure diagram of an existing project containing a protection system is given; among which, U M This is the voltage at the beginning of the line. I MI and I MII These are the starting currents of line I and line II, respectively. I NI and I NIIThese represent the currents at the ends of Line I and Line II, respectively. During bipolar balanced operation, the impedance monitoring system injects a high-frequency signal into the line at the beginning of the grounding electrode line, calculating the equivalent impedance deviation modulus at that frequency point to detect faults. Upon fault detection, the pulse ranging system injects a pulse signal into the line through a coupling capacitor, calibrating the pulse reflection time and combining it with wave velocity to locate the fault. To prevent externally injected high-frequency and pulse signals from interfering with the neutral line and affecting its stable operation, a corresponding wave trap should be installed at the beginning of the grounding electrode line to confine the injected signal transmission within the grounding electrode system. Furthermore, to improve the protection sensitivity of the impedance monitoring system, a matching resistor should be connected in series at the end of the grounding electrode line, with a value approximately equal to half of the zero-mode impedance of the line at the injected signal frequency point. A wave trap with the same parameters as the beginning of the line should be connected in parallel across the matching resistor. This parallel structure ensures that the impedance approaches zero for the DC component, guaranteeing a low-impedance path for the DC current; while for the injected high-frequency signal, the impedance is approximately the value of the matching resistor, thus achieving a feedback-free termination. Figure 3 As shown, the present invention simplifies the design of the grounding electrode line, eliminating the need for any external injection device and its first-end wave trap, and only requiring the parallel structure at the end of the line to be retained; Furthermore, based on the hybrid MMC converter topology and switching principle, by applying additional control to the trigger pulse modulation stage of the single-pole single-ended converter, continuous and stable injection of the protection detection signal is achieved; based on the single-phase topology of the hybrid MMC converter ( Figure 4 If redundant and faulty submodules are not considered, the number of half-bridge submodules in a single-phase bridge arm is denoted as . N 1. The number of full-bridge submodules is N 2. The total number of submodules is N The modulation ratio is M The number of submodules in an active state at any given time during normal system operation. N in Number of submodules in the cut-off state N out Approximately:

[0051] In actual engineering, to improve system reliability, a certain margin is allowed in the number of converter valve submodules, with the number set per phase being [number missing]. N re Redundant submodules account for approximately 6% to 10% of the total number of submodules in this phase. To ensure that redundant submodules can replace faulty submodules at any time, they are typically used in a hot standby mode. This means they participate in submodule voltage sequencing and switching in real time to avoid current distortion caused by the long charging time required for submodule activation under cold standby mode. Assuming that all available submodules considering the redundant submodules are hot standby submodules under all disconnected states, the number of hot standby submodules in a single-phase bridge arm is...N le for:

[0052] If the current flowing through the upper and lower bridge arms is in a positive (negative) direction, it means that the submodule capacitor is in a charging (discharging) state. According to the output of the capacitor voltage sorting algorithm, the submodule with the lower (higher) capacitor voltage will be given priority, and the submodule with the higher (lower) capacitor voltage will be in a disconnected state. If it contains a full-bridge submodule, additional control can be activated. Figure 5 To inject a protective detection signal, and to describe the control strategy in more detail, an additional control flow diagram is provided. Figure 6 Additional control will select the highest (lowest) voltage among them. n ctr The hot-standby full-bridge submodule exits the minimum-level approximation modulation and capacitor voltage sorting algorithm, remaining in its disconnected state and not participating in the switching of subsequent submodules, transitioning to a mode where its operation can be controlled independently. Since this submodule does not participate in the synthesis of the DC and AC voltages when exiting the modulation algorithm, this process will not affect the system voltage stability. At this point, only the preset injection signal frequency needs to be used. f ctr By controlling VT1, VT4, VT2, and VT3 of the full-bridge submodule to conduct alternately, it can be periodically switched between positive and negative states at this control frequency. Since the output voltage is [voltage value missing] during positive operation... U C When negative input is applied, the output voltage is - U C When the upper and lower bridge arms are controlled synchronously, a frequency of [frequency value] will be generated on the DC side. f ctr Amplitude value 2 n ctr U C The periodic voltage signal.

[0053] Because hybrid MMC converters contain both full-bridge and half-bridge modules, there may be a shortage of hot-standby full-bridge modules during the initial stages of additional control. n ctr In this situation, the existing hot-standby full-bridge submodule can be deactivated from the modulation algorithm, waiting for subsequent switching to rotate out other hot-standby full-bridge submodules. This process is repeated until the required number of hot-standby full-bridge submodules in both the upper and lower arms of the three phases are reached before signal injection is initiated. The submodule switching frequency is only on the order of hundreds of microseconds; this process is very brief, but to meet the signal injection conditions as quickly as possible during protection startup, n ctrIt is not advisable to select an excessively large value, while the full-bridge MMC-HVDC does not have the above limitations. In addition, the MMC converter valve is equipped with submodule redundancy exhaustion protection, that is, when the number of bypasses of any bridge arm submodule in the MMC converter valve exceeds the limit, the converter valve will be directly locked. By minimizing the number of full-bridge submodules used for additional control, the reliability of the protection can also be improved under extreme conditions where there is a large-scale failure of submodules but the locking condition is not reached.

[0054] As can be seen from the above analysis, the injected protection detection signal is essentially an AC steady-state signal, which is easier to acquire and analyze than the traveling wave transient signal. It also places lower demands on the transient performance and sampling frequency of the measuring equipment and is easier to implement technically in engineering. Considering that the constant power control parameters are all acquired from the converter transformer grid side, and DC side voltage fluctuations have a relatively small impact on them, the additional control strategy proposed in this invention is applied to the constant power control side. Furthermore, this additional control is applied simultaneously to both the upper and lower bridge arms of the three phases. That is, during the signal injection process, the parameters of the upper and lower bridge arms are always consistent and the three phases are symmetrical, preventing the generation of DC bias current on the converter transformer valve side.

[0055] More preferably, Taking into account multiple factors such as Shannon's sampling theorem, signal sampling spectrum leakage, fault location coherence characteristics, equipment parameters, system constant voltage control characteristics, current-limiting reactor characteristics, and broadband oscillation risk, the protection detection signal frequency is selected. According to Shannon's sampling theorem, to accurately reconstruct the amplitude and phase of a continuous signal, the sampling frequency must be no less than twice the highest frequency of the original signal. Therefore, the injected signal frequency... f ctr Must meet:

[0056] In the formula: f kb This refers to the sampling frequency of the converter station control and protection system.

[0057] When the sampling frequency is out of sync with the signal frequency, the discontinuity in the phase of the periodic sampling signal at the start and end points may lead to measurement errors and spectral leakage. Therefore, the injected signal frequency should satisfy the following:

[0058] In the formula: f tq This refers to the switching frequency of the submodules during normal operation of the MMC converter. The submodule switching frequency during normal operation of the MMC converter refers to the switching frequency of the submodule under the original modulation and capacitor voltage equalization control when the additional control is not applied.

[0059] When the protection detection frequency is too high, there will be a significant co-modulation characteristic between the grounding electrode line impedance, fault distance, and transition resistance. That is, the same measured impedance corresponds to multiple combinations of fault distance and transition resistance. To obtain an accurate and unique location result, the zero-mode half-wavelength of the protection detection signal should be not less than the total length of the line, i.e., satisfying the following:

[0060] In the formula: λ 0 represents the zero-mode wavelength of the protected detection signal; L The inductance per unit length of the grounding electrode line; C Capacitance per unit length of the grounding electrode line; This is the total length of the grounding electrode line.

[0061] Regarding equipment parameters, the lower the signal frequency, the larger the required matching resistor, and the larger the inductance and capacitance of the wave trap. This leads to a super-linear increase in economic costs. Therefore, the protection detection signal frequency cannot be too low, meaning it must meet the following requirements:

[0062] In the formula: f cb The lower critical frequency, which takes into account economic costs, is determined based on the manufacturing cost constraints of the matching resistor and the passive components of the wave trap.

[0063] In hybrid MMC-HVDC systems, the constant DC voltage controller is typically designed with low-pass characteristics. To reduce the mutual interference between constant voltage control and protection detection signals, the frequency of the protection detection signal should be selected to be much higher than the bandwidth of the voltage controller, preventing the voltage controller from responding to such high-frequency fluctuations. Therefore, the injected signal frequency should satisfy the following:

[0064] In the formula: f dy The lower critical frequency, which takes into account the influence of constant voltage control, is determined based on the low-pass characteristics and bandwidth constraints of constant voltage controllers in actual engineering.

[0065] Meanwhile, since the proposed protection detection signal injection method adopts a signal injection mode with constant amplitude fluctuation based on steady-state voltage, the measured voltage parameter of constant voltage control input during the signal injection process remains the original steady-state voltage after passing through the low-pass circuit, and will not interfere with the injection of protection detection signal.

[0066] The current-limiting reactors connected in series on the MMC-HVDC pole bus and neutral bus exhibit high impedance to high-frequency currents. Therefore, while ensuring that DC-side voltage fluctuations meet the system's stable operating range, the frequency of the injected signal should not be too high to guarantee the injection amplitude effect of the protection detection signal.

[0067] In the formula: f dk The upper critical frequency, which takes into account the influence of the current-limiting reactor, is determined based on the constraints of the inductance value of the current-limiting reactor and the target injection amplitude effect in actual engineering.

[0068] When the frequency appears on the DC side is f ctr When the harmonic voltage is high, a frequency of will be generated in the differential mode voltage. f ctr - f The negative order components and frequencies of 0 are f ctr + f The zero-sequence component is generated, and a corresponding alternating current is induced. Based on impedance analysis... f ctr ± f The system resonance risk at the 0-frequency point is analyzed. According to the Nyquist stability criterion, the intersection of the amplitudes of the MMC impedance and the AC system impedance is considered the oscillation risk point. If the phase margin of the two impedances at the frequency corresponding to the oscillation risk point is negative, it indicates that the system is unstable. Since the phase range of AC systems in actual engineering is -90° to 90°, when the absolute value of the positive-sequence impedance phase of the MMC is greater than 90°, it is considered that the MMC impedance has a negative damping frequency band, and the system has an oscillation risk. Therefore, the following must be satisfied:

[0069] In the formula: Z MMC ( f ctr ± f 0) represents the MMC converter at a frequency The corresponding equivalent positive sequence impedance; To perform phase angle calculations; It is the power frequency.

[0070] More preferably, Taking into account multiple factors such as the DC voltage fluctuation range and the system operation modulation ratio requirements, the amplitude of the protection detection signal is selected. To avoid affecting the system's voltage stability, the DC voltage fluctuation amplitude should not exceed 10% of the rated DC voltage of the hybrid MMC-HVDC system, that is:

[0071] in, This refers to the rated capacitor voltage of the full-bridge submodule in the MMC converter. The rated DC voltage for the hybrid MMC-HVDC system; During normal operation, the submodule switching characteristics of a hybrid MMC converter are basically the same as those of a half-bridge MMC converter. Although the negative-level output capability of the full-bridge submodules allows for overmodulation operation, it also introduces the risk of capacitor voltage imbalance. Therefore, the modulation ratio should still be controlled to not exceed 1 during signal injection, i.e.:

[0072] in, This refers to the fundamental amplitude of the phase voltage on the AC side of the MMC converter. More preferably, Taking into account multiple factors such as the minimum sampling time required for the Discrete Fourier Transform (DFT), the group delay of the digital filter, and the voltage fluctuation of the submodule capacitor, the duration of the protection detection signal is selected. The entire process mainly includes the following steps: The proposed protection method uses DFT to extract the amplitude and phase of the injected signal, requiring a sampling time of at least one protection detection signal cycle. However, to improve the robustness of the DFT algorithm, under the condition of a fixed sampling frequency, the sampling time can be appropriately extended to expand the time window and obtain higher frequency domain resolution. Therefore, the signal injection duration... At least the following should be met:

[0073] While the DFT algorithm possesses some noise immunity, it lacks filtering capabilities. Therefore, this invention first employs an FIR digital filter to initially extract the signal; the signal injection time must not be less than the filter delay, i.e.:

[0074] In the formula: t lb This is the group delay of the digital filter; N lb The order of the digital filter; f lb This represents the sampling frequency of the digital filter.

[0075] To avoid potential voltage imbalance within the bridge arm capacitors due to differing charging and discharging states between the controlled full-bridge submodule and other submodules during additional control, the duration of the injected signal needs to be limited. During the charging and discharging of the submodule capacitors, other higher-order circulating current components are ignored, and the current flowing through the capacitors... I C ( t This can be represented as:

[0076] In the formula: floor(·) is the floor function; Idc This refers to the DC component of the direct current. I ac This refers to the amplitude of the AC phase current. ω 0 represents the power frequency angular frequency. ω 0 = 2π f 0, It is the power frequency; I ctr ( t () represents the AC current signal component injected into the DC side; This refers to the current component in the capacitor current generated by the original AC and DC components of the system. This refers to the current component in the capacitor current introduced by the additional control injection signal; To protect the injection start time of the detection signal; The initial phase angle is the phase current on the AC side.

[0077] for I C1 ( t For any non-negative integer, there exists k 0, satisfying when f ctr =( k 0+1 / 2) f At 0, its integral over two power frequency cycles is zero; when f ctr =2 k 0 f At time 0, its integral over one power frequency cycle is zero. And... I C2 ( t The integral over any time interval is always positive, and its value increases with the signal amplitude and the integration time interval. Therefore, to minimize the capacitor voltage deviation and potential limit exceedance during the controlled full-bridge submodule's operation, measures should be taken to ensure... I C1 ( t While the integral value is zero, the amplitude and duration of the injected signal are reduced, that is, when n ctr At a certain time, it should be reduced as much as possible. t ctr The signal injection duration should then satisfy the following:

[0078] In the formula: It is a non-negative integer used to determine the frequency range of the protection detection signal relative to the power frequency; It is a positive integer; k tThe upper limit critical time coefficient for submodule capacitor voltage offset is determined based on the safety range constraints of submodule capacitor voltage in actual engineering.

[0079] Step 2: Collect the voltage and current at the beginning and end of the double-circuit grounding electrode line, calculate the overall equivalent impedance of the line, and determine whether the line fault criterion is met based on the overall equivalent impedance of the line. If it is met, the line is determined to be faulty and proceed to step 3. The impedance at the beginning of the grounding electrode line is calculated by protecting the detection signal. Based on the characteristic that the impedance changes significantly when a fault occurs, fault detection is achieved. Fault detection can be performed using the impedance monitoring principle while maintaining a simplified grounding electrode structure. By adjusting the fault detection signal to the injected protection detection signal, the impedance measurement frequency becomes the frequency of this signal. f ctr The line fault detection criterion is as follows:

[0080] In the formula: Δ Z This represents the magnitude of the line impedance deviation. Z M The calculated overall equivalent impedance of the line is measured. Z Meq0 This is the overall theoretical equivalent impedance of the line during normal operation. Z set1 The setting principle for the modulus of line impedance deviation is as follows: Z set1 = K ·| Z Meq0 |, K The reliability coefficient is typically taken as 0.1 to 0.3.

[0081] The following is a derivation of the overall theoretical equivalent impedance of a specific line:

[0082]

[0083]

[0084] In the formula: Z Meq0 This is the overall theoretical equivalent impedance of the line during normal operation. Z Meq1 and Z Meq2 These are the overall theoretical equivalent impedances of the line under single-wire grounding fault and double-wire grounding fault, respectively. Z C0 and Z C1These are the zero-mode impedance and the line-mode impedance, respectively. R P For matching resistors; R d For transition resistance; l The total length of the line; x The distance to the fault. d = l - x ; γ 0 and γ 1 represents the zero-mode propagation coefficient and the linear-mode propagation coefficient, respectively; It is an intermediate variable.

[0085] Step 3: Calculate the unbalanced impedance magnitudes at the beginning and end of the double-circuit grounding electrode line, and take the maximum value of the two to obtain the maximum unbalanced impedance magnitude at both ends; when the maximum unbalanced impedance magnitude at both ends is greater than the set value of the maximum unbalanced impedance magnitude at both ends, it is determined to be a single-line grounding fault; otherwise, it is determined to be a double-line grounding fault, and proceed to step 4. By measuring the first and last impedances of the grounding electrode double-circuit line using the protection detection signal, the maximum unbalanced impedance between the two circuits is calculated. Based on the difference that the impedance magnitude is large when a single-wire grounding fault occurs, but almost zero when a double-wire grounding fault occurs, fault identification is achieved. Due to variations in the grounding electrode reference potential and the bipolar voltage-current coupling effect, the amplitude and phase of the protection detection signal injected into the grounding electrode line are not fixed. Using conventional voltage or current parameters as protection criteria has significant uncertainty and is difficult to set. Since line impedance is only related to line parameters and signal frequency, it is adopted as the fault identification criterion. Here, the expressions for the first and last impedances of line I and line II are defined as follows:

[0086] In the formula: and These are the impedances at the beginning of line I and line II, respectively. and These are the end impedances of line I and line II, respectively; This refers to the voltage at the beginning of the grounding electrode line; , These are the starting currents of line I and line II, respectively; , These are the end currents of line I and line II, respectively.

[0087] When a double-wire ground fault occurs on a line, the impedances of line I and line II are the same; however, when a single-wire fault occurs, the line parameters are no longer symmetrical, and the impedances of line I and line II will differ. Therefore, the magnitudes of the unbalanced impedance at the beginning and end are introduced:

[0088] In the formula: This represents the magnitude of the unbalanced impedance at the beginning. This represents the magnitude of the unbalanced impedance at the end. This is the overall equivalent impedance of the line when a ground fault occurs.

[0089] The theoretical derivation expression for the magnitude of the unbalanced impedance at the starting end when a single-wire connection fault occurs is given:

[0090] The theoretical derivation expression for the magnitude of the unbalanced impedance at the end of a single-wire connection fault is given below:

[0091] When a metallic ground fault occurs at the beginning of the line:

[0092] When a high-resistance ground fault occurs at the beginning of the line:

[0093] When a metallic ground fault occurs at the end of the line:

[0094] When a high-resistance ground fault occurs at the end of the line:

[0095] It is evident that if the single-ended unbalanced impedance magnitude at the beginning or end is used as the fault identification criterion alone, a protection dead zone will appear at the opposite end. However, the combination of the two can eliminate this dead zone.

[0096] Based on the above principles, the fault identification criterion is constructed as follows:

[0097] In the formula: Δ Z MN This represents the magnitude of the maximum unbalanced impedance at both ends. Z set2 The maximum unbalanced impedance magnitude setting is determined by referring to the impedance deviation magnitude setting principle in fault detection, and can be taken as follows: Z set2 = Z set1;max(·) is the operation to find the maximum value.

[0098] Since most grounding electrode line faults are single-wire grounding faults, the proposed fault identification method can also be used as the main protection for single-wire grounding faults, further improving the reliability of grounding electrode line protection.

[0099] Step 4: Based on the determined ground fault type, and using the overall theoretical equivalent impedance expression of the line when the ground fault of that type occurs, as well as the overall measured equivalent impedance of the line, construct a set of distance measurement equations and solve them to obtain the fault distance.

[0100] The impedance at the beginning of the grounding electrode line is calculated by protecting the detection signal. Based on the correlation and correspondence between this impedance and the fault distance, the fault location is achieved. During the fault detection and identification phase, the line impedance was calculated. This known impedance can be fully utilized to locate the fault based on its correlation with the fault distance. Ignoring the zero-mode attenuation constant in the high-frequency domain, the measured impedance expressions for single-wire grounding faults and double-wire grounding faults can be transformed based on the characteristics of hyperbolic functions to obtain:

[0101]

[0102] In the formula: The overall theoretical equivalent impedance of the line experiencing a single-line ground fault; β 0 and β 1 represents the zero-mode phase constant and the line-mode phase constant, respectively; x Distance to the fault; d This is the difference between the total length of the line and the distance to the fault; Z C0 and Z C1 These are the zero-mode impedance and the line-mode impedance, respectively. R d For transition resistance; It is an intermediate variable.

[0103] It is evident that the theoretical impedance expression for both types of faults can be represented as a combination of real and imaginary parts. Therefore, based on the correspondence between the real and imaginary parts of the actual impedance measurement and the theoretical impedance expression, the following set of distance measurement equations for ground faults can be constructed:

[0104] In the formula: Re(·) is the operation of extracting the real part; Im(·) is the operation of extracting the imaginary part; This is the theoretical equivalent impedance expression for the entire line; The equivalent impedance of the current line is measured as a whole.

[0105] Apart from the known line parameters, only x and R d Two unknowns, eliminate R d The fault distance can then be calculated. x .

[0106] Example 2: This embodiment provides a method for detecting, identifying, and locating faults in a hybrid MMC-HVDC grounding electrode line.

[0107] First, a hybrid MMC-HVDC electromagnetic transient simulation model including a grounding electrode system was built in PSCAD / EMTDC. Figure 7 The simulation data was processed and analyzed using MATLAB software, and the model parameter table (Table 1) and the frequency-dependent parameter model of the receiving-end grounding electrode line are given. Figure 8 ).

[0108] Table 1 Basic parameters of the hybrid MMC-HVDC model

[0109] Secondly, based on the proposed signal selection principles, additional control parameters are selected: n ctr =1, f ctr =625Hz, t ctr =0.04s. Substituting the grounding electrode system parameters into the line impedance expression during normal operation, we can see that... Z Meq0 =255.50∠-2.60°Ω, considering the protection capability against extreme high-resistance grounding faults, a reliability coefficient is taken. K 1=0.1, and the constant value of the impedance deviation magnitude can be calculated: Z set1 =25.55Ω. Combined Z Meq1 and Z Meq2 The theoretical derivation expression can be used to plot the impedance deviation magnitude Δ for single-wire grounding faults and double-wire grounding faults. Z Schematic diagram of distribution under different fault distances and transition resistances ( Figure 9 , Figure 10 Among them, Δ during a single-wire grounding fault. Z The ΔΩ value during a double-wire grounding fault is distributed between 143.16 and 3381.07 Ω. Z The resistance is distributed between 143.08 and 3313.01 Ω, all significantly higher than the protection setting, thus effectively detecting faults. According to... Z set1The setting value can be used to obtain the maximum unbalanced impedance modulus at both ends. The setting value is the same as that: Z set2 = Z set1 =25.55Ω. In the case of a single-wire grounding fault, substitute the grounding electrode system parameters into Δ. Z M and Δ Z N The expression can be used to draw a schematic diagram of the distribution of the two under different fault distances and transition resistances. Figure 11 ), where Δ Z M Distributed between 0.30 and 7271.61 Ω, it exhibits a protection dead zone at both high-resistance grounding and metallic grounding at the end of the line; Δ Z N The resistance ranges from 0.21 to 6210.95 Ω, and a protection dead zone exists when the line head is grounded at a high resistance. Based on the fault identification criteria, a Δ... Z MN Schematic diagram of distribution under different fault distances and transition resistances ( Figure 12 ), where Δ Z MN The Ω values ​​range from 365.03 to 7271.61 Ω, all of which are significantly greater than the protection setting. At this point, there is no protection dead zone, which can effectively detect single-line grounding faults and identify the fault type.

[0110] When the simulation system is running normally, additional control is initiated on the positive constant power control side after 3 seconds, based on the DC line voltage and current waveforms on that side. Figure 13 ), grounding electrode voltage and current waveforms ( Figure 14 ) and its corresponding control frequency component waveform ( Figure 15 , Figure 16 As can be seen, during the additional control period, the voltage and current of the DC line and the grounding electrode line continuously fluctuated at a control frequency of 625Hz. The DC line voltage fluctuation range was 292~308kV, and the amplitude change was only about 2.67% of the primary rated voltage, achieving stable injection of the protection detection signal. Simultaneously, the DC current component of the grounding electrode was extracted ( Figure 17 As can be seen, the DC current flowing into the grounding electrode never exceeds 5A, which is far less than 0.1pu, meeting the requirements of the grounding electrode operation regulations. During the injection of the protection detection signal, the waveform of the power change transmitted by the system ( Figure 18 ) and converter transformer grid-side voltage waveform ( Figure 19 As can be seen, the power fluctuation before and after the additional control is minimal, the AC side voltage waveform does not undergo significant distortion, and the system's power transmission is almost unaffected. Further analysis of the A-phase current spectrum on the AC grid side ( Figure 20It can be seen that when a 625Hz voltage signal is injected into the DC side, 575Hz and 675Hz current harmonic components will be coupled out on the AC side. By measuring the positive sequence impedance of the MMC at the above two frequencies by injecting voltage disturbance at the grid connection point, the impedance angles are found to be approximately 46.14° and 65.62°, respectively, which meet the stability constraint conditions, and there is no risk of system resonance. For flexible DC-DC projects, a filter stage can be added to the voltage feedforward stage when selecting the signal injection frequency to suppress the risk of broadband oscillations that may be induced by the selected frequency.

[0111] Furthermore, considering a fault distance of 2 to 98 km and a ground fault transition resistance of 0 to 200 Ω, the results of fault detection, identification, and location are shown in Table 2.

[0112] Table 2 Simulation results of protective actions

[0113] Note: "+" indicates a fault was detected; "-" indicates no fault was detected. The same applies to the following table.

[0114] As can be seen from the simulation results, the method proposed in this invention can effectively detect and identify faults, and the positioning error is within 1% of the total length of the line, which has high positioning accuracy and meets engineering requirements.

[0115] Example 3: Based on Example 2, the impact of noise interference on protection performance was considered. Gaussian white noise with a signal-to-noise ratio of 20dB was mixed into the injected signal. To reduce the influence of the randomness of noise interference on the conclusions, the maximum positioning error in 200 simulation results was taken and summarized in Table 3.

[0116] Table 3 Simulation results of protection actions at a signal-to-noise ratio of 20dB

[0117] As can be seen from the simulation results, the high-frequency interference can be effectively separated after processing by digital filters and DFT algorithm during signal extraction. Therefore, the method of the present invention is less affected under low signal-to-noise ratio conditions and can still accurately detect, identify and locate faults, which has good noise resistance and meets engineering requirements.

[0118] Example 4: Based on Example 2, the impact of extreme submodule failure conditions is considered. In Example 2, the number of full-bridge submodules used for protection detection signal injection in each phase single bridge arm is selected as 1. Therefore, the system always has a sufficient number of hot-standby full-bridge submodules for protection before the protection action results in blocking due to the exhaustion of submodule redundancy. There will be no situation where the system is still running but the protection cannot function due to insufficient hot-standby full-bridge submodules. Table 4 shows the simulation results of protection action under extreme conditions when there is only 1 redundant full-bridge submodule in each bridge arm.

[0119] Table 4 Simulation results of protection actions when the number of redundant full-bridge submodules is 1

[0120] The simulation results show that even if each bridge arm has only one redundant full-bridge submodule, the proposed method can still effectively inject protection detection signals and protect the entire length of the line, which meets the engineering requirements.

[0121] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0122] Computer-readable storage media can be tangible devices capable of holding and storing instructions for use by an instruction execution device. Computer-readable storage media can be, for example—but not limited to—electrical storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital multifunction disc (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or recessed protrusions storing instructions thereon, and any suitable combination of the foregoing. The computer-readable storage media used herein are not to be construed as transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses through fiber optic cables), or electrical signals transmitted through wires.

[0123] The computer-readable program instructions described herein can be downloaded from computer-readable storage media to various computing / processing devices, or downloaded via a network, such as the Internet, local area network, wide area network, and / or wireless network, to an external computer or external storage device. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards them to the computer-readable storage media in the respective computing / processing device.

[0124] Computer program instructions used to perform the operations of this disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, status setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The computer-readable program instructions may execute entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer may be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN)—or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, electronic circuitry, such as programmable logic circuitry, field-programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), is personalized by utilizing the status information of the computer-readable program instructions to implement various aspects of this disclosure.

[0125] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A hybrid MMC-HVDC grounding electrode line fault detection, identification and location method, the grounding electrode line is a double circuit grounding electrode line; characterized in that, The method comprises the following steps: Step 1: In the hybrid MMC-HVDC system, by applying additional control in the single-pole single-end MMC converter trigger pulse modulation link, a protection detection signal is injected, and step 2 is entered; Step 2: Collecting the voltage and current at the first end and the current at the end of the double-circuit grounding electrode line, calculating the overall line measurement equivalent impedance, and judging whether the line fault criterion is met based on the overall line measurement equivalent impedance, if met, judging that the line has a fault, and entering step 3; Step 3: Calculating the first and end unbalanced impedance modulus values of the double-circuit grounding electrode line, taking the maximum value of the two to obtain the double-end maximum unbalanced impedance modulus value; when the double-end maximum unbalanced impedance modulus value is greater than the double-end maximum unbalanced impedance modulus value setting value, it is determined as a single-line ground fault, otherwise, it is determined as a double-line ground fault, and step 4 is entered; Step 4: Combined with the determined ground fault type, based on the overall line theoretical equivalent impedance expression when the type of ground fault occurs and the overall line measurement equivalent impedance, a distance measurement equation set is constructed and solved to obtain the fault distance.

2. The hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method according to claim 1, characterized in that: In step 1, when the number of hot standby full-bridge submodules in the upper and lower bridge arms of the three-phase unipolar single-ended MMC converter is not less than n ctr , a protection detection signal is injected, and the following additional control is simultaneously applied in the three-phase upper and lower bridge arms of the unipolar single-ended MMC converter: in each bridge arm, select n ctr hot standby full-bridge submodules, and make them exit the minimum level approximation modulation and capacitor voltage sorting algorithm, that is, keep them in the cut-off state and do not participate in the subsequent switching of submodules, and switch to the mode of independently controllable operation, and periodically switch the f ctr hot standby full-bridge submodules between the positive and negative input states according to the preset injection signal frequency; wherein n ctr the n ctr is a preset number of modules.

3. The hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method according to claim 2, characterized in that: The selecting in each bridge arm is respectively n ctr The selecting mode is specifically: detecting the current direction in the current bridge arm, if the current is positive direction, selecting the front n ctr The selecting mode is specifically: detecting the current direction in the current bridge arm, if the current is positive direction, selecting the front If the current is in the negative direction, the preceding one is selected in the order of the capacitor voltage from low to high n ctr one hot-standby full-bridge sub-module.

4. The hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method according to claim 2, characterized in that: When the protection detection signal is injected, the fluctuation amplitude of the direct current voltage does not exceed 10% of the rated direct current voltage of the hybrid MMC-HVDC system, specifically meets: Wherein, is the rated capacitor voltage of the full-bridge sub-module in the MMC converter, is the rated direct current voltage of the hybrid MMC-HVDC system.

5. The hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method according to claim 4, characterized in that: Controlling a modulation ratio of an MMC converter during protection probe signal injection does not exceed 1, specifically satisfies: wherein, is a capacitor voltage rating of a full-bridge sub-module in the MMC converter, is a rated DC voltage of the hybrid MMC-HVDC system; is a fundamental amplitude of an AC side phase voltage of the MMC converter.

6. The hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method according to claim 2, characterized in that: The preset injection signal frequency f ctr , the following conditions must be met simultaneously: 1) , wherein, is the sampling frequency of the converter station control and protection system; 2) , wherein, is the sub-module switching frequency when the MMC converter is in normal operation, , is a positive integer; 3) , wherein, L is the unit length inductance of the ground electrode line; C is the unit length capacitance of the ground electrode line; is the full length of the ground electrode line; 4) , wherein, represents the equivalent positive sequence impedance of the MMC converter at the frequency , is the power frequency; is the phase angle operation.

7. The hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method according to claim 1, characterized in that: In step 2, the line fault criterion is specifically: wherein, is the current line overall measured equivalent impedance, is the line overall theoretical equivalent impedance when the line is in normal operation; and is the line impedance deviation modulus value.

8. The hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method according to claim 7, characterized in that: The line impedance deviation modulus value setting The setting principle is: Wherein, K Is a reliability coefficient.

9. The hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method according to claim 8, characterized in that: In step 3, the double-ended maximum imbalance impedance modulus value is set to the line impedance deviation modulus value .

10. The hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method according to claim 1, characterized in that: In step 3, the double-circuit grounding electrode line includes line I and line II, the first end unbalanced impedance modulus value of the double-circuit grounding electrode line is the modulus value of the difference between the first end impedance of line I and the first end impedance of line II, and the end unbalanced impedance modulus value of the double-circuit grounding electrode line is the modulus value of the difference between the end impedance of line I and the end impedance of line II.

11. The hybrid MMC-HVDC grounding electrode line fault detection, identification and positioning method according to claim 1, characterized in that: The overall line theoretical equivalent impedance expression includes: The overall line theoretical equivalent impedance expression when a single-line ground fault occurs: wherein, Zth is the overall theoretical equivalent impedance of the line to ground fault; β 0 and β 1 are the zero-mode phase constant and the line-mode phase constant, respectively; x Df is the fault distance; d D is the difference between the full length of the line and the fault distance; Z C0 and Z C1 Zo and Zm are the zero-mode wave impedance and the line-mode wave impedance, respectively; R d Rt is the transition resistance; Zth is the overall theoretical equivalent impedance of the line to ground fault; The overall line theoretical equivalent impedance expression when a double-line ground fault occurs: wherein, is the overall theoretical equivalent impedance of the line to ground fault.

12. The method for detecting, identifying, and locating faults in a hybrid MMC-HVDC grounding electrode line according to claim 11, characterized in that: The ranging equations are constructed based on the principle that when complex numbers are equal, their real and imaginary parts are equal, and include real part equations and imaginary part equations. The real part equations make the real part of the overall theoretical equivalent impedance of the line equal to the real part of the overall measured equivalent impedance of the line, and the imaginary part equations make the imaginary part of the overall theoretical equivalent impedance of the line equal to the imaginary part of the overall measured equivalent impedance of the line.

13. The method for detecting, identifying, and locating faults in a hybrid MMC-HVDC grounding electrode line according to claim 2, characterized in that: The protection detection signal, signal injection duration Must meet: wherein, is a non-negative integer, used to determine the frequency range of the protection detection signal relative to the power frequency; is a positive integer; is an upper limit critical time coefficient, determined based on the capacitor voltage safety range of the sub-module in the MMC converter in actual engineering; is the power frequency.

14. A hybrid MMC-HVDC grounding electrode line fault detection, identification, and location system based on the method of any one of claims 1-13, comprising a protection detection signal injection module, a line fault detection module, a line fault type identification module, and a line fault location module, characterized in that: The protection detection signal injection module is used to inject a protection detection signal in a hybrid MMC-HVDC system by applying additional control to the trigger pulse modulation stage of the single-pole single-ended MMC converter. The line fault detection module is used to collect the voltage and current at the beginning and end of the double-circuit grounding electrode line, calculate the overall measured equivalent impedance of the line, and determine whether the line fault criterion is met based on the overall measured equivalent impedance of the line. If it is met, the line is judged to have a fault. The line fault type identification module is used to calculate the magnitude of the unbalanced impedance at the beginning and end of the double-circuit grounding electrode line, and take the maximum value of the two to obtain the maximum unbalanced impedance magnitude at both ends; when the maximum unbalanced impedance magnitude at both ends is greater than the set value of the maximum unbalanced impedance magnitude at both ends, it is determined to be a single-line grounding fault; otherwise, it is determined to be a double-line grounding fault. The line fault location module is used to combine the determined ground fault type with the overall theoretical equivalent impedance expression of the line when the ground fault of that type occurs and the overall measured equivalent impedance of the line to construct a set of distance measurement equations and solve them to obtain the fault distance.

15. A terminal, comprising a processor and a storage medium; characterized in that: The storage medium is used to store instructions; The processor is configured to operate according to the instructions to perform the steps of the method according to any one of claims 1-13.

16. A computer readable storage medium having stored thereon a computer program, characterized in that When executed by a processor, the program implements the steps of the method according to any one of claims 1-13.

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