An active detection type fault location method for AC-DC hybrid power distribution network
By injecting interharmonic signals into modular multilevel converter stations and utilizing S-transform energy differences to locate faults, the problem of identifying weak fault characteristics in AC/DC hybrid distribution networks has been solved, achieving unified fault location and efficient response, and reducing system transformation costs.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- JIANGNAN UNIV
- Filing Date
- 2026-04-10
- Publication Date
- 2026-05-29
AI Technical Summary
In AC/DC hybrid distribution networks, traditional protection methods are difficult to effectively identify weak fault characteristics. Especially in the case of high penetration of inverter-type distributed power sources and pseudo-bipolar structures, the fault current is weak or changes rapidly, which increases the risk of false tripping and failure to trip. Existing active detection protection technologies are mostly aimed at single systems and have failed to form a unified fault location framework for AC/DC hybrid distribution networks.
A modular multilevel converter station is used as a signal injection device. Based on the topology characteristics, interharmonic signals are injected in a differentiated manner. The fault is located by using the S-transform energy difference. The current response signals are collected by the detection devices on the AC side and DC side to realize the fault location, avoid interference with non-faulty lines, and reduce the system transformation cost.
It realizes a unified solution for fault location in AC/DC hybrid distribution networks, enhances fault characteristic identification, reduces system transformation costs, has strong field adaptability and engineering practicality, and meets the requirements of millisecond-level response speed.
Smart Images
Figure CN122109727A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of relay protection technology for power system distribution networks, and in particular to an active detection-based fault location method for AC / DC hybrid distribution networks. Background Technology
[0002] With the large-scale integration of distributed power sources and the rapid growth of DC loads, AC / DC hybrid distribution networks have become an important development direction for modern distribution networks. AC / DC hybrid distribution networks interconnect AC and DC grids through modular multilevel converters (MMCs), combining the flexibility of AC systems with the efficiency of DC systems. They demonstrate significant advantages in improving power supply reliability, promoting the consumption of new energy sources, and reducing line losses. However, while AC / DC hybrid distribution networks offer greater flexibility in operation and control, they also bring new fault characteristics and protection challenges.
[0003] On the AC side, with the integration of numerous inverter-type distributed power sources, the distribution network is gradually evolving from a passive network with unidirectional power supply to an active network with bidirectional power flow. When the penetration rate of distributed power sources is high, the fault current amplitude is typically only 1.2 to 2 times the rated current due to the current-limiting control strategy of the inverter, far lower than the short-circuit current level provided by traditional synchronous generators. Simultaneously, bidirectional power flow renders the traditional three-stage current protection settings based on a single power direction ineffective, significantly increasing the risk of maloperation and failure to operate. Furthermore, the complex harmonic environment in the distribution network further interferes with the accurate extraction of fault characteristics, posing a severe challenge to AC-side fault detection.
[0004] On the DC side, medium-voltage DC distribution networks often employ a pseudo-bipolar structure, with the positive and negative poles connected to the neutral point via clamping resistors. When a unipolar ground fault occurs, the fault current amplitude is significantly suppressed due to the limitations of the system grounding method and the clamping resistor, exhibiting weak fault characteristics. Especially in high-resistance grounding cases, the fault current is almost undetectable, making it difficult for traditional overcurrent detection-based protection methods to accurately identify and locate the fault. Simultaneously, the DC-side fault current rises rapidly and lacks a natural zero-crossing point, placing millisecond-level demands on the response speed of protection devices. This contradiction between weak fault characteristics and the need for rapid action makes DC-side fault location particularly difficult.
[0005] In recent years, active detection protection technology has gained increasing attention due to the challenges posed by the aforementioned weak fault characteristics. This type of method actively injects specific signals into the system through power electronic converters, thereby enhancing fault characteristics and providing a new approach to solving AC / DC side protection problems. In the AC distribution network field, existing research has utilized inverter-type distributed generation to inject harmonic signals for fault location; in the DC distribution network field, research has also utilized converters to inject characteristic signals for fault localization. However, existing technologies are mostly developed independently for single AC or DC systems, and a unified active detection framework applicable to both sides of AC / DC hybrid distribution networks has not yet been formed. Summary of the Invention
[0006] This application addresses the aforementioned problems and technical requirements by proposing an active detection-based fault location method for AC / DC hybrid distribution networks. The technical solution of this application is as follows: An active detection-based fault location method for an AC / DC hybrid distribution network, the AC / DC hybrid distribution network including an AC grid, a DC grid, and... A modular multilevel converter station; the AC power grid includes multiple AC feeders, all connected to a common bus; the DC power grid includes... Each modular multilevel converter station has one DC feeder and adopts a pseudo-bipolar structure. Its AC output is connected to the common bus of the AC power grid, and its DC output is connected to one DC feeder in the DC power grid. (Integer parameters) Active detection-based fault location methods include: When an asymmetrical grounding fault is detected in the AC power grid based on the electrical quantities at the AC output of any modular multilevel converter station, all... Each modular multilevel converter station simultaneously injects power into the AC grid. Interharmonic signals of different frequencies; after the interharmonic signals are injected, the current response signals are synchronously collected by the AC detection devices deployed at the beginning of each AC feeder and at both ends of each section, and the faulty AC feeder, faulty phase and the section where the asymmetrical grounding fault occurs are determined. When according to the first The electrical quantity at the DC output of the modular multilevel converter station was detected by the first connected... When a single-pole ground fault occurs on the DC feeder, the first... The modular multilevel converter station to the first A preset frequency interharmonic signal is injected into the DC feeder; after the interharmonic signal is injected, it is transmitted through the first... DC detection devices deployed at both ends of each section of the DC feeder synchronously collect current response signals and determine the current response signal. The section of a DC feeder where a single-pole grounding fault occurs; integer parameters .
[0007] The further technical solution involves determining the faulty AC feeder, faulty phase, and section where the asymmetrical grounding fault occurs, including: The S-transform spectrum of the current response signal synchronously collected by the AC detection device deployed at the head end of each AC feeder is calculated to obtain the S-transform energy of each phase at the head end of each AC feeder under each inter-harmonic signal; based on the S-transform energy of each phase at the head end of each AC feeder under each inter-harmonic signal, the faulty AC feeder and faulty phase that have asymmetrical grounding fault are determined. The section of the AC feeder where the asymmetrical grounding fault point is located is determined by the current response signal synchronously collected by the AC detection devices deployed at both ends of each section in the faulty AC feeder.
[0008] A further technical solution involves determining the faulty AC feeder and faulty phase experiencing an asymmetrical grounding fault based on the S-transform energy of each phase at the beginning of each AC feeder under each interharmonic signal. When there is an AC feeder Head Phase satisfies under an interharmonic signal and At that time, determine the AC feeder. For the faulty AC feeder that experienced an asymmetrical grounding fault, and The phase is the faulty phase; in, It is an AC feeder Head Phase frequency S-transform energy under interharmonic signals It is an AC feeder Head Phase frequency S-transform energy under interharmonic signals It is an AC feeder Head Phase frequency S-transform energy under interharmonic signals; integer parameters , Indicates three phases of the feeder. Indicates a three-phase feeder and .
[0009] A further technical solution involves determining the faulty sections in the feeder based on the current response signals synchronously collected by detection devices deployed at both ends of each section. The S-transform spectrum of the current response signal synchronously collected by the AC detection devices deployed at both ends of each section of the faulty AC feeder with asymmetrical grounding fault is calculated to obtain the S-transform energy of the faulty phase at both ends of each section of the faulty AC feeder under each interharmonic signal. The difference of the S-transform energy of the faulty phase at both ends of each section under the same interharmonic signal is calculated, and the section with the largest difference is determined as the section where the asymmetrical grounding fault point is located. For the first single-pole ground fault The S-transform spectrum of the current response signal synchronously acquired by the DC detection devices deployed at both ends of each section of the DC feeder is calculated to obtain the first... The two ends of each section of the DC feeder are at the first The S-transform energy under the interharmonic signal injected into each modular multilevel converter station is calculated. The difference between the S-transform energy at both ends of each section is calculated, and the section with the largest difference is determined to be the section where the single-pole grounding fault point is located.
[0010] A further technical solution is that the active detection fault location method also includes: When the first Zero-mode voltage at the DC output of a modular multilevel converter station At that time, determine the first A single-pole grounding fault occurred at the positive terminal of a DC feeder. When the first Zero-mode voltage at the DC output of a modular multilevel converter station At that time, determine the first A single-pole grounding fault occurred at the negative terminal of a DC feeder.
[0011] A further technical solution involves synchronously injecting interharmonic signals into the AC power grid using each modular multilevel converter station, including: The d-axis modulation signal output by the inner loop current controller and q-axis modulation signal After performing dq / abc coordinate transformation to obtain the three-phase modulation signal, interharmonic signals of the corresponding frequency are superimposed on the three-phase modulation signal.
[0012] Its further technical solution is to utilize the first The modular multilevel converter station to the first The DC feeder injects interharmonic signals including: Inner loop current controller at d-axis current reference value Based on the addition of the interharmonic component to the d-axis component, and then compared with the d-axis current. The differential input PI controller obtains the d-axis voltage. and the q-axis current reference value Based on the addition of the interharmonic component q-axis component, and then compared with the q-axis current The q-axis voltage is obtained by a differential input PI controller. ; Alternatively, the inner loop current controller feeds forward voltage along the d-axis. Add feedforward decoupling term Based on this, the d-axis component of the interharmonic component is superimposed, and then the d-axis voltage is subtracted. Obtain the d-axis modulated signal and the q-axis feedforward voltage Add feedforward decoupling term Based on this, the q-axis component of the interharmonic component is superimposed, and then the q-axis voltage is subtracted. Obtain the q-axis modulated signal ;in, It is the equivalent inductance. It is angular frequency.
[0013] A further technical solution is that the frequency of the interharmonic signal injected by any modular multilevel converter station into the AC power grid or DC feeder is much greater than the fundamental frequency, and is different from both the power frequency integer multiples and the inherent background harmonic frequency of the system.
[0014] A further technical solution involves detecting whether an asymmetrical grounding fault has occurred in the AC power grid based on the electrical quantities at the AC output of any modular multilevel converter station, including: The zero-sequence current at the AC output of each modular multilevel converter station is monitored in real time. When the zero-sequence current at the AC output of any modular multilevel converter station exceeds the zero-sequence current over-limit threshold, an asymmetrical grounding fault is determined to have occurred in the AC power grid.
[0015] Its further technical solution is, according to the first The electrical quantity detection at the DC output of a modular multilevel converter station is connected to the first... Whether a single-pole ground fault has occurred on a DC feeder includes: Real-time detection The zero-mode current and positive and negative voltages at the DC output of a modular multilevel converter station are monitored. When the zero-mode current exceeds the zero-mode current over-limit threshold, or the positive voltage is less than the positive voltage over-limit threshold, or the negative voltage is less than the negative voltage over-limit threshold, the connection to the first... A single-pole grounding fault occurred on one of the DC feeders.
[0016] The beneficial technical effects of this application are: This application discloses an active detection-based fault location method for AC / DC hybrid distribution networks. This method reuses modular multilevel converter stations (MMCs) in the AC / DC hybrid distribution network as signal injection devices. When a fault is detected in the DC or AC grid based on the electrical quantities at the DC and AC outlets of the MMCs, a differentiated injection strategy based on the fundamentally different topological characteristics of the AC-side common bus structure and the DC-side independent feeder structure is proposed. In the event of an AC-side fault, all MMCs synchronously inject interharmonic signals of different frequencies into the AC grid, enhancing fault characteristics through the superposition of multiple signals. In the event of a DC-side fault, only the MMC corresponding to the faulty feeder initiates injection, avoiding interference with non-faulty feeders. Fault location is then performed by collecting current response signals from detection devices deployed on the feeders. This method fully leverages the inherent patterns of the AC and DC topologies, achieving optimal configuration of the injected signals, and eliminates the need for dedicated signal injection equipment on both the AC and DC sides, thus reducing system modification costs.
[0017] This method superimposes interharmonic signals onto the current component or voltage feedforward of the inner-loop current controller during DC-side faults, and onto the three-phase modulation signal after dq / abc coordinate transformation during AC-side faults. This achieves independent control and signal injection of the AC and DC output voltages of the converter. This multi-stage collaborative injection strategy enables the converter to flexibly generate the required interharmonic detection signals according to the fault type while operating normally in grid-connected mode. Furthermore, the injection functions on both the AC and DC sides do not interfere with each other, providing an integrated fault location solution for AC / DC hybrid distribution networks.
[0018] This method proposes a segment location method based on S-transform energy difference. It leverages the excellent time-frequency analysis capability of the S-transform for non-stationary signals to directly extract the energy values corresponding to each harmonic frequency. The fault segment is determined by the maximum energy difference between adjacent detection points. This method only requires comparing the relative magnitudes of the S-transform energy values at both ends of the segment, has low requirements for synchronization accuracy between detection devices, does not require precise line parameters, and has strong field adaptability and engineering practicality. Attached Figure Description
[0019] Figure 1 This is a schematic diagram of an AC / DC hybrid distribution network topology in one example of this application.
[0020] Figure 2 This is a flowchart of an active detection-based fault location method in one embodiment of this application.
[0021] Figure 3 This is the control block diagram of the dual closed-loop dq-axis vector control link used in existing modular multilevel converter stations.
[0022] Figure 4 This application is in Figure 3 A schematic diagram showing the superposition of interharmonic signals in the control link to enable the modular multilevel converter station to inject interharmonic signals into the AC power grid.
[0023] Figure 5 This application is in Figure 3 A schematic diagram showing how interharmonic signals are superimposed in the control link to inject interharmonic signals into the DC feeder from a modular multilevel converter station.
[0024] Figure 6 This application is in Figure 3 Another schematic diagram showing the superposition of interharmonic signals in the control link to inject interharmonic signals into the DC feeder by the modular multilevel converter station.
[0025] Figure 7 This is a waveform of the zero-mode voltage at the DC output of MMC1 detected in an example.
[0026] Figure 8 yes Figure 7 The zero-mode current waveform detected at the DC output of MMC1 in the example. Detailed Implementation
[0027] The specific embodiments of this application will be further described below with reference to the accompanying drawings.
[0028] This application discloses an active detection-based fault location method for AC / DC hybrid distribution networks. The structure of the AC / DC hybrid distribution network is first described below. Please refer to... Figure 1 The diagram shows a hybrid AC / DC distribution network topology. A hybrid AC / DC distribution network includes an AC grid, a DC grid, and... A modular multilevel converter station, integer parameters ,like Figure 1 by For example: The AC power grid consists of multiple AC feeders, all connected to a common busbar. Each AC feeder is divided into multiple sections by the busbar, and at least one AC feeder is connected to an inverter-type distributed generation source. For example... Figure 1 The AC power grid includes AC feeder 1, AC feeder 2, and AC feeder 3. AC feeder 1 connects to inverter-type distributed generation IIDG1 and is divided into three sections by the bus, denoted as Line 1, Line 2, and Line 3. AC feeder 2 connects to inverter-type distributed generation IIDG2 and is divided into three sections by the bus, denoted as Line 4, Line 5, and Line 6. AC feeder 3 connects to inverter-type distributed generation IIDG3 and is divided into three sections by the bus, denoted as Line 7, Line 8, and Line 9.
[0029] DC power grid includes It has one DC feeder and adopts a pseudo-bipolar structure, with each DC feeder also divided into multiple blocks. For example... Figure 1 The DC power grid includes DC feeder 1, DC feeder 2, and DC feeder 3. DC feeder 1 is divided into three sections, denoted as Line 10, Line 11, and Line 12. DC feeder 2 is divided into three sections, denoted as Line 13, Line 14, and Line 15. DC feeder 3 is divided into three sections, denoted as Line 16, Line 17, and Line 18.
[0030] Each modular multilevel converter station connects its AC output to the common busbar of the AC power grid and its DC output to a DC feeder in the DC power grid, thereby achieving interconnection between the AC and DC power grids. For example... Figure 1 In the process, the modular multilevel converter station MMC1 is connected to the common bus of the AC power grid and DC feeder 1, the modular multilevel converter station MMC2 is connected to the common bus of the AC power grid and DC feeder 2, and the modular multilevel converter station MMC3 is connected to the common bus of the AC power grid and DC feeder 3.
[0031] This application selects a modular multilevel converter station connecting the AC and DC grids in an AC / DC hybrid distribution network as a signal injection device to achieve active detection-based fault location. The active detection-based fault location method includes the following steps; please refer to [reference needed]. Figure 2 The flowchart shown: Step 210: Detect whether a fault has occurred in the AC / DC hybrid distribution network based on the electrical quantities at the AC and DC outputs of the modular multilevel converter station, including fault detection on both the AC and DC sides. (1) Detect whether an asymmetrical grounding fault has occurred in the AC power grid based on the electrical quantities at the AC output of each modular multilevel converter station. The electrical quantities used to detect asymmetrical grounding faults on the AC power grid side are zero-sequence currents, including: Real-time monitoring of the zero-sequence current at the AC output of each modular multilevel converter station; when the zero-sequence current at the AC output of any modular multilevel converter station... Greater than the zero-sequence current over-limit threshold When an asymmetrical grounding fault is detected in the AC power grid, the zero-sequence current at the AC output of all modular multilevel converter stations is determined. None of them are greater than the zero-sequence current over-limit threshold. If it is determined that no asymmetrical grounding fault has occurred in the AC power grid, monitoring continues. Among these measures, the zero-sequence current exceeding the limit threshold is... It is a pre-set experience value.
[0032] Since each modular multilevel converter station is connected to the common bus of the AC power grid rather than to a single AC feeder, it can only be determined that an AC feeder in the AC power grid has an asymmetrical grounding fault, but it is impossible to determine which AC feeder has the asymmetrical grounding fault.
[0033] (2) Detect whether a single-pole grounding fault has occurred in the DC grid based on the electrical quantities at the DC output of each modular multilevel converter station. The electrical quantities used to detect single-pole grounding faults on the DC grid side are zero-mode current and positive and negative pole voltages, including: Real-time detection Zero-mode current at the DC output of a modular multilevel converter station And positive and negative voltages, when zero-mode current is detected Greater than the zero-mode current over-limit threshold Or positive voltage Less than the threshold voltage Or negative voltage Less than the threshold voltage At that time, it was determined that a single-pole ground fault had occurred in the DC power grid. And because of the first... Each modular multilevel converter station is connected to only one DC feeder, therefore it can be directly identified as the first one. The first modular multilevel converter station connected to the first A single-pole ground fault occurred on one of the DC feeders. (Integer parameters) .
[0034] In addition, it will also monitor the first in real time. Zero-mode voltage at the DC output of a modular multilevel converter station And based on zero-mode voltage The polarity determination fault occurred in the first Whether the positive or negative terminal of the DC feeder is: specifically, when the first... Zero-mode voltage at the DC output of a modular multilevel converter station At that time, determine the first A single-pole ground fault occurred at the positive terminal of the DC feeder. When the fault was detected... Zero-mode voltage at the DC output of a modular multilevel converter station At that time, determine the first A single-pole grounding fault occurred at the negative terminal of a DC feeder.
[0035] Once an asymmetrical grounding fault is determined to have occurred in the AC power grid or a unipolar grounding fault is determined to have occurred in the DC power grid, a modular multilevel converter station is used as a signal injection device to inject interharmonic signals for active fault location. Different injection strategies are executed according to the fault type, including: when an asymmetrical grounding fault is determined to have occurred in the AC power grid, subsequent steps 220 and 230 are executed; when a unipolar grounding fault is determined to have occurred in the DC power grid, subsequent steps 240 and 250 are executed.
[0036] Step 220: When an asymmetrical ground fault is determined to have occurred in the AC power grid, all [systems / systems] are activated simultaneously. A modular multilevel converter station injects power into the AC grid. Several interharmonic signals of different frequencies. Since all AC feeders in the AC power grid are connected to a common bus, therefore all... The interharmonic signals injected into each modular multilevel converter station all propagate through the common bus, forming multi-frequency, multi-source interharmonic injection, which enhances the identifiability of fault characteristics.
[0037] For example in Figure 1 In one example, when an asymmetrical ground fault is detected in the AC power grid, MMC1, MMC2, and MMC3 are simultaneously activated to inject three interharmonic signals of different frequencies into the AC power grid. To avoid injecting... Interharmonic signals influence each other, and the frequencies of interharmonic signals injected into the AC grid by each modular multilevel converter station are different. Moreover, the frequency of the interharmonic signal injected into the AC grid or DC feeder by any modular multilevel converter station must meet the following requirements: it must be different from both the power frequency integer multiples and the inherent background harmonic frequency of the system, and be much greater than the fundamental frequency, at least 5 times the fundamental frequency.
[0038] When injecting interharmonic signals into the AC grid using any modular multilevel converter station, the interharmonic signal components are directly superimposed into the existing control link of the modular multilevel converter station. Through modulation, the output voltage of the modular multilevel converter station includes the interharmonic signal. The modular multilevel converter station employs a dual closed-loop dq-axis vector control link, and its control block diagram is shown below. Figure 3 As shown: The outer loop controller is responsible for the power / DC voltage closed loop and provides a reference value for the DC bus voltage. Actual value of DC bus voltage After differential calculation, input to PI controller and... The d-axis current reference value is obtained after limiting the amplitude within the range. Or the active power reference value and actual value of active power After differential calculation, input to PI controller and... The d-axis current reference value is obtained after limiting the amplitude within the range. Reference value for reactive power Actual value of reactive power After differential calculation, input to PI controller and... The q-axis current reference value is obtained after limiting within the range. .in, and the maximum value of the d-axis current, It is the maximum value of the q-axis current.
[0039] The inner loop current controller is responsible for tracking and It also outputs a three-phase modulation signal, and the inner loop current controller provides a reference value for the d-axis current. and d-axis current The d-axis voltage is obtained by inputting the difference into the PI controller. d-axis feedforward voltage Add feedforward decoupling term Subtract d-axis voltage Obtain the d-axis modulated signal The inner loop current controller references the q-axis current. and q-axis current The q-axis voltage is obtained by inputting the difference into the PI controller. q-axis feedforward voltage Add feedforward decoupling term Subtract q-axis voltage Obtain the q-axis modulated signal Modulation signal for the d-axis and q-axis modulation signal A dq / abc coordinate transformation is performed to obtain the three-phase modulation signal of the modular multilevel converter station, which is then used to output the required voltage to the DC or AC power grid. It is the equivalent inductance. It is angular frequency.
[0040] Based on this existing control link, when the modular multilevel converter station needs to inject interharmonic signals into the AC power grid, such as Figure 4 As shown, the d-axis modulation signal output by the inner loop current controller... and q-axis modulation signal After performing dq / abc coordinate transformation to obtain the three-phase modulation signal, interharmonic signals of the corresponding frequency are superimposed on the three-phase modulation signal, so that the AC side output voltage of the modular multilevel converter station contains interharmonic signals of the corresponding frequency.
[0041] Step 230: After injecting interharmonic signals into the AC power grid, the current response signals are synchronously collected by multiple AC detection devices deployed on each AC feeder, and the faulty AC feeder, faulty phase, and section where the asymmetrical grounding fault occurs are determined.
[0042] Each AC feeder is equipped with an AC detection device at its beginning and at both ends of each section, for example, in Figure 1 In this configuration, AC detection devices are deployed at the beginning of AC feeder 1 (CB1), the other end of section Line 1 (CB1'), both ends of section Line 2 (CB2 and CB2'), and both ends of section Line 3 (CB3 and CB3'), respectively. The other two AC feeders are configured similarly. Each AC detection device deployed on each AC feeder is synchronized and has a unified time reference. Each AC detection device synchronously acquires the current response signal after the injected signal, with a sampling frequency of 8kHz to meet the sampling requirements of interharmonic signals.
[0043] When locating faults based on the current response signals collected by various AC detection devices, the following steps are included: First, the S-transform spectrum of the current response signal synchronously collected by the AC detection device deployed at the beginning of each AC feeder is calculated to obtain the S-transform energy of each phase at the beginning of each AC feeder under each interharmonic signal.
[0044] The S-transform is a time-frequency analysis tool that combines the advantages of the short-time Fourier transform and the wavelet transform. It has both good time-frequency resolution and feature extraction capabilities. The current response signal at any location acquired by any detection device deployed on any feeder is defined as... Response signal to current Its time-frequency domain representation can be obtained by performing an S-transform. :
[0045]
[0046] in, The time shift factor, This represents the frequency. The transform uses a Gaussian window function to window the signal, with the window width varying with the harmonic frequency. Changes, harmonic frequencies At lower frequencies, the window is wider, resulting in higher frequency resolution; harmonic frequencies At higher speeds, the window is narrower, resulting in higher time resolution, thus enabling the simultaneous acquisition of current response signals. The S-transform directly processes the original current response signal, eliminating the need for complex preprocessing and spectral analysis algorithms, thus simplifying the signal processing flow and improving detection efficiency. Based on the S-transform results, the harmonic frequency of the feeder at the detection device can be calculated. S-transform spectrum of interharmonic signals As the S-transform energy, i.e., the harmonic frequency of the interharmonic signal. The energy of the component is integrated over the time axis to obtain the result at that harmonic frequency. The total energy below.
[0047] For ease of distinction, the AC feeder is defined here. In the AC testing device Place Phase frequency The S-transform energy of the interharmonic signal is , For integer parameters, Integer parameter and Indicates the beginning of the AC feeder. This indicates a three-phase feeder. First, based on the S-transform energy of each phase at the beginning of each AC feeder under each inter-harmonic signal, the faulty AC feeder and faulty phase experiencing the asymmetrical grounding fault are determined. u Because when the AC feeder... When an asymmetrical grounding fault occurs, the AC feeder The short-circuiting of the ground capacitance of the faulty phase creates a low-impedance path, allowing the injected interharmonic signals to preferentially flow into the ground through the faulty phase, causing the AC feeder to... The S-transform energy of the faulty phase at the first end increases significantly, exceeding that of the AC feeder. The non-faulty phases and non-faulty AC feeders, therefore: when an AC feeder exists Head Phase satisfies under an interharmonic signal and At that time, determine the AC feeder. For the faulty AC feeder that experienced an asymmetrical grounding fault, and The phase is the faulty phase. Among them, It is an AC feeder Head Phase frequency S-transform energy under interharmonic signals It is an AC feeder Head Phase frequency S-transform energy under interharmonic signals It is an AC feeder Head Phase frequency S-transform energy under interharmonic signals; integer parameters , Indicates three phases of the feeder. Indicates a three-phase feeder and . The "greater than" sign means that the "greater than" requirement is met when the difference between two parameters reaches a set threshold.
[0048] After locating the faulty AC feeder and the faulty phase, the section where the asymmetrical grounding fault point is located in the faulty AC feeder is further determined based on the current response signals synchronously collected by the AC detection devices deployed at both ends of each section in the faulty AC feeder. This includes: calculating the S-transform spectrum of the current response signals synchronously collected by the AC detection devices deployed at both ends of each section in the faulty AC feeder, obtaining the S-transform energy of the faulty phase at both ends of each section in the faulty AC feeder under each interharmonic signal, calculating the difference in the S-transform energy of the faulty phase at both ends of each section under the same interharmonic signal, and determining the section with the largest difference as the section where the asymmetrical grounding fault point is located.
[0049] Step 240, when the first When a single-pole ground fault occurs on a DC feeder, since each DC feeder operates independently, only the first one is utilized. The modular multilevel converter station is located at the faulty first A preset frequency interharmonic signal is injected into the DC feeder, and other modular multilevel converter stations do not need to be started, thus avoiding interference to non-faulty DC feeders.
[0050] For example in Figure 1 In the example, when the zero-mode current of MMC2 Greater than the zero-mode current over-limit threshold If a single-pole ground fault is detected in DC feeder 2 connected to MMC2, only MMC2 will be activated to inject a preset frequency interharmonic signal into DC feeder 2; MMC1 and MMC3 do not need to be activated. The modular multilevel converter station to the first The frequency of the interharmonic signal injected by the DC feeder can be set by the user, but it must also meet the following requirements: it must be different from both the integer multiples of the power frequency and the inherent background harmonic frequency of the system, and be much greater than the fundamental frequency, at least 5 times the fundamental frequency.
[0051] In utilizing the first The modular multilevel converter station to the first When interharmonic signals are injected into a DC feeder, they are also directly at the first harmonic. The modular multilevel converter station already has interharmonic signal components superimposed in the control link, including interharmonic signals of corresponding frequencies superimposed on the current component or voltage feedforward of the inner loop current controller, thereby causing the first The DC-side output voltage of a modular multilevel converter station contains interharmonic signals of corresponding frequencies. Specifically, there are two methods: Method 1, please refer to Figure 5 The inner loop current controller at the d-axis current reference value Based on the addition of the interharmonic component to the d-axis component, and then compared with the d-axis current. The differential input PI controller obtains the d-axis voltage. and the q-axis current reference value Based on the addition of the interharmonic component q-axis component, and then compared with the q-axis current The q-axis voltage is obtained by a differential input PI controller. .
[0052] Method 2, please refer to Figure 6 The inner loop current controller feeds forward voltage on the d-axis. Add feedforward decoupling term Based on this, the d-axis component of the interharmonic component is superimposed, and then the d-axis voltage is subtracted. Obtain the d-axis modulated signal and the q-axis feedforward voltage Add feedforward decoupling term Based on this, the q-axis component of the interharmonic component is superimposed, and then the q-axis voltage is subtracted. Obtain the q-axis modulated signal .
[0053] Step 250, in the direction of the first After the interharmonic signal is injected into the DC feeder, it passes through the first DC detection devices deployed at both ends of each section of the DC feeder synchronously collect current response signals and determine the current response signal. The section where the single-pole grounding fault point is located in a DC feeder.
[0054] Each section of the DC feeder is equipped with a DC detection device at both ends, for example, in Figure 1 In DC feeder 2, DC detection devices are deployed at both ends CB13 and CB13' of section Line 13, both ends CB14 and CB14' of section Line 14, and both ends CB15 and CB15' of section Line 15. The other two DC feeders are similarly configured. Each DC detection device deployed on each DC feeder performs synchronous time synchronization and has a unified time reference. When utilizing the... The modular multilevel converter station to the first After the DC feeder injects interharmonic signals, the first Each DC detection device deployed on the DC feeder synchronously acquires the current response signal after the injection signal, with a sampling frequency of 8kHz to meet the sampling requirements of interharmonic signals.
[0055] Since the DC feeder where the single-pole ground fault occurred and the pole where the fault occurred can be determined in step 210, this step only needs to further locate the first... The method for determining the fault section on the DC side is similar to that used for the section where the single-pole grounding fault occurred: For the section where the single-pole grounding fault occurred... The S-transform spectrum of the current response signal synchronously acquired by the DC detection devices deployed at both ends of each section of the DC feeder is calculated to obtain the first... The two ends of each section of the DC feeder are at the first The S-transform energy under the interharmonic signal injected into each modular multilevel converter station was calculated. Then, the difference in S-transform energy between the two ends of each section was calculated, and the section with the largest difference was determined to be the section where the single-pole grounding fault point was located.
[0056] To verify the effectiveness and adaptability of the method proposed in this application, a system was built in the MATLAB / Simulink environment as follows: Figure 1 The simulation model of the AC / DC hybrid distribution network shown is illustrated in Table 1. Table 1. Simulation model parameters for AC / DC hybrid distribution network
[0057] In a simulation example, A positive single-pole ground fault was injected at a distance of 3.2 km between Line 11 and CB11 of DC feeder 1. The zero-mode voltage waveform at the DC output of MMC1 was detected using the method described in this application, as shown below. Figure 7 As shown, the zero-mode current waveform is as follows: Figure 8 As shown, by Figure 7 and Figure 8 It can be seen that after the fault occurred, the positive voltage dropped rapidly, leading to zero-mode voltage. A negative offset occurs, zero-mode current. Simultaneously increase. Zero-mode current is detected. Exceeding the zero-mode current over-limit threshold Subsequently, it was determined that a single-pole ground fault occurred in DC feeder 1 connected to MMC1. Since the zero-mode voltage was less than 0, it was determined that the single-pole ground fault occurred on the positive pole. Then, a 410Hz interharmonic signal was injected into DC feeder 1 using MMC1, with the injection amplitude controlled within 5% of the rated modulation ratio. The current response was collected by DC detection devices at both ends of each section of DC feeder 1, and the S-transformation energy was converted. The calculated S-transformation energy difference between the two ends of section Line 10 was 714.8, the S-transformation energy difference between the two ends of section Line 11 was 18411.4, and the S-transformation energy difference between the two ends of section Line 12 was 10.6. This confirmed that a single-pole ground fault occurred on the positive pole of DC feeder 1, and the fault point was located in section Line 11, consistent with the actual fault injection results, thus verifying the effectiveness of this method.
[0058] In another simulation example, a phase A ground fault is injected at a distance of 1.95 km between Line 2 and CB2 of AC feeder 1, and the transition resistance is... When the zero-sequence current at the AC output of a modular multilevel converter station exceeds the zero-sequence current over-limit threshold, an asymmetrical grounding fault is determined to have occurred in the AC power grid, according to the method of this application. MMC1, MMC2, and MMC3 are simultaneously activated to inject interharmonic signals of 385Hz, 430Hz, and 478Hz into the AC power grid, respectively, with the injected amplitude controlled within 5% of the rated modulation ratio and varying in magnitude. The S-transform spectrum is calculated from the current response signals collected by the AC detection devices at the beginning of each AC feeder to obtain the S-transform energy distribution. The S-transform energy of the three phases at the beginning of each AC feeder under the 430Hz interharmonic signal is shown in Table 2 below. Table 2. S-transform energy of three phases at the head end of each AC feeder under 430Hz interharmonic signal.
[0059] As can be seen from the data in the table above, the S-transformation energy of phase A at the beginning of AC feeder 1 is much greater than that of phases B and C at the beginning of AC feeder 1, and the S-transformation energy of phase A at the beginning of AC feeder 1 is much greater than that of phase A at the beginning of AC feeder 2 and AC feeder 3. Therefore, it can be determined that an asymmetrical grounding fault has occurred in AC feeder 1 and the faulty phase is phase A.
[0060] Further calculations were performed on the S-transformation energy of phase A at both ends of different sections of AC feeder 1 under interharmonic signals. Taking the S-transformation energy under a 430Hz interharmonic signal as an example, the calculated S-transformation energy difference between phase A at both ends of section Line 1 of AC feeder 1 under a 430Hz interharmonic signal was 236.3, the S-transformation energy difference between phase A at both ends of section Line 2 under a 430Hz interharmonic signal was 13960.9, and the S-transformation energy difference between phase A at both ends of section Line 3 under a 430Hz interharmonic signal was 2.5. This determined that a phase A ground fault occurred in AC feeder 1, and the asymmetrical ground fault point was located in section Line 2, which was consistent with the actual fault injection results, verifying the effectiveness of this method.
[0061] The above descriptions are merely preferred embodiments of this application, and this application is not limited to the above embodiments. It is understood that other improvements and variations that can be directly derived or conceived by those skilled in the art without departing from the spirit and concept of this application should be considered to be included within the protection scope of this application.
Claims
1. A method for active detection-based fault location in an AC / DC hybrid distribution network, characterized in that, The AC / DC hybrid distribution network includes AC power grid, DC power grid, and... A modular multilevel converter station; the AC power grid includes multiple AC feeders, all connected to a common bus; the DC power grid includes... Each modular multilevel converter station has one DC feeder and adopts a pseudo-bipolar structure. Its AC output is connected to the common bus of the AC power grid, and its DC output is connected to one DC feeder in the DC power grid. (Integer parameters) ; The active detection-based fault location method includes: When an asymmetrical grounding fault is detected in the AC power grid based on the electrical quantities at the AC output of any modular multilevel converter station, all... Each modular multilevel converter station simultaneously injects power into the AC grid. Interharmonic signals of different frequencies; after the interharmonic signals are injected, the current response signals are synchronously collected by the AC detection devices deployed at the beginning of each AC feeder and at both ends of each section, and the faulty AC feeder, faulty phase and the section where the asymmetrical grounding fault occurs are determined. When according to the first The electrical quantity at the DC output of the modular multilevel converter station was detected by the first connected... When a single-pole ground fault occurs on the DC feeder, the first... The modular multilevel converter station to the first A preset frequency interharmonic signal is injected into the DC feeder; after the interharmonic signal is injected, it is transmitted through the first... DC detection devices deployed at both ends of each section of the DC feeder synchronously collect current response signals and determine the current response signal. The section of a DC feeder where a single-pole grounding fault occurs; integer parameters .
2. The active detection-based fault location method according to claim 1, characterized in that, The faulty AC feeder, faulty phase, and section where the asymmetrical grounding fault occurred were identified, including: The S-transform spectrum of the current response signal synchronously collected by the AC detection device deployed at the head end of each AC feeder is calculated to obtain the S-transform energy of each phase at the head end of each AC feeder under each inter-harmonic signal; based on the S-transform energy of each phase at the head end of each AC feeder under each inter-harmonic signal, the faulty AC feeder and faulty phase that have asymmetrical grounding fault are determined. The section of the AC feeder where the asymmetrical grounding fault point is located is determined by the current response signal synchronously collected by the AC detection devices deployed at both ends of each section in the faulty AC feeder.
3. The active detection-based fault location method according to claim 2, characterized in that, Based on the S-transform energy of each phase at the beginning of each AC feeder under each inter-harmonic signal, the faulty AC feeder and faulty phase that experienced an asymmetrical grounding fault are determined as follows: When there is an AC feeder Head Phase satisfies under an interharmonic signal and At that time, determine the AC feeder. For the faulty AC feeder that experienced an asymmetrical grounding fault, and The phase is the faulty phase; in, It is an AC feeder Head Phase frequency S-transform energy under interharmonic signals It is an AC feeder Head Phase frequency S-transform energy under interharmonic signals It is an AC feeder Head Phase frequency S-transform energy under interharmonic signals; integer parameters , Indicates three phases of the feeder. Indicates a three-phase feeder and .
4. The active detection-based fault location method according to claim 1 or 2, characterized in that, Based on the current response signals synchronously collected by the detection devices deployed at both ends of each section of the faulty feeder, the faulty sections in the feeder are determined to include: The S-transform spectrum of the current response signal synchronously collected by the AC detection devices deployed at both ends of each section of the faulty AC feeder with asymmetrical grounding fault is calculated to obtain the S-transform energy of the faulty phase at both ends of each section of the faulty AC feeder under each interharmonic signal. The difference of the S-transform energy of the faulty phase at both ends of each section under the same interharmonic signal is calculated, and the section with the largest difference is determined as the section where the asymmetrical grounding fault point is located. For the first single-pole ground fault The S-transform spectrum of the current response signal synchronously acquired by the DC detection devices deployed at both ends of each section of the DC feeder is calculated to obtain the first... The two ends of each section of the DC feeder are at the first The S-transform energy under the interharmonic signal injected into each modular multilevel converter station is calculated. The difference between the S-transform energy at both ends of each section is calculated, and the section with the largest difference is determined to be the section where the single-pole grounding fault point is located.
5. The active detection-based fault location method according to claim 1, characterized in that, The active detection-based fault location method also includes: When the first Zero-mode voltage at the DC output of a modular multilevel converter station At that time, determine the first A single-pole grounding fault occurred at the positive terminal of a DC feeder. When the first Zero-mode voltage at the DC output of a modular multilevel converter station At that time, determine the first A single-pole grounding fault occurred at the negative terminal of a DC feeder.
6. The active detection-based fault location method according to claim 1, characterized in that, Synchronously injecting interharmonic signals into the AC grid using each modular multilevel converter station includes: The d-axis modulation signal output by the inner loop current controller and q-axis modulation signal After performing dq / abc coordinate transformation to obtain the three-phase modulation signal, interharmonic signals of the corresponding frequency are superimposed on the three-phase modulation signal.
7. The active detection-based fault location method according to claim 1, characterized in that, Using the first The modular multilevel converter station to the first The DC feeder injects interharmonic signals including: Inner loop current controller at d-axis current reference value Based on the addition of the interharmonic component to the d-axis component, and then compared with the d-axis current. The differential input PI controller obtains the d-axis voltage. and the q-axis current reference value Based on the addition of the interharmonic component q-axis component, and then compared with the q-axis current The q-axis voltage is obtained by a differential input PI controller. ; Alternatively, the inner loop current controller feeds forward voltage along the d-axis. Add feedforward decoupling term Based on this, the d-axis component of the interharmonic component is superimposed, and then the d-axis voltage is subtracted. Obtain the d-axis modulated signal and the q-axis feedforward voltage Add feedforward decoupling term Based on this, the q-axis component of the interharmonic component is superimposed, and then the q-axis voltage is subtracted. Obtain the q-axis modulated signal ;in, It is the equivalent inductance. It is angular frequency.
8. The active detection-based fault location method according to claim 1, characterized in that, The frequency of the interharmonic signal injected by any modular multilevel converter station into the AC power grid or DC feeder is much greater than the fundamental frequency, and it is different from both the power frequency integer multiples and the inherent background harmonic frequency of the system.
9. The active detection-based fault location method according to claim 1, characterized in that, Detecting whether an asymmetrical grounding fault has occurred in the AC power grid based on the electrical quantities at the AC output of any modular multilevel converter station includes: The zero-sequence current at the AC output of each modular multilevel converter station is monitored in real time. When the zero-sequence current at the AC output of any modular multilevel converter station exceeds the zero-sequence current over-limit threshold, an asymmetrical grounding fault is determined to have occurred in the AC power grid.
10. The active detection-based fault location method according to claim 1, characterized in that, According to the The electrical quantity detection at the DC output of a modular multilevel converter station is connected to the first... Whether a single-pole ground fault has occurred on a DC feeder includes: Real-time detection The zero-mode current and positive and negative voltages at the DC output of a modular multilevel converter station are monitored. When the zero-mode current exceeds the zero-mode current over-limit threshold, or the positive voltage is less than the positive voltage over-limit threshold, or the negative voltage is less than the negative voltage over-limit threshold, the connection to the first... A single-pole grounding fault occurred on one of the DC feeders.