Islanding new energy power grid zero sequence current protection method and device

Abstract

The application provides an island new energy micro-grid adaptive zero sequence current protection method and device. The method is aimed at the problem that when a single-phase grounding fault occurs in a neutral point non-effective grounding micro-grid, the zero sequence current time-frequency characteristics are distorted and the sensitivity of traditional zero sequence protection is greatly reduced or even refused to act due to the influence of the new energy inverter low voltage ride through control strategy. The application collects the inverter DC side bus voltage and the micro-grid AC side zero sequence current signal, calculates the DC side bus voltage drop rate, and extracts the time-frequency energy spectrum of the AC side zero sequence current based on continuous wavelet transform. Through the collaborative construction of AC / DC cross-domain characteristic quantities, the adaptive zero sequence protection action criterion and setting condition are established, the protection sensitivity is dynamically adjusted during the low voltage ride through period, and the LVRT distortion and the real grounding fault are effectively distinguished. The application breaks through the limitation of traditional single AC side power frequency zero sequence current as the criterion, effectively improves the protection sensitivity and reliability of the island new energy grid in complex transition state.

Description

Technical Field

[0001] This invention relates to the field of power system relay protection technology, specifically a zero-sequence current protection method and device for an islanded new energy power grid. Background Technology

[0002] With the rapid development of new energy power generation technologies, distributed power sources based on power electronic inverter interfaces have become an important component of microgrids. In microgrids with a high proportion of new energy connections, when a short-circuit fault occurs in the system causing a voltage drop at the grid connection point, the new energy inverter needs to enter the Low Voltage Ride Through (LVRT) control mode in accordance with the grid connection guidelines to maintain grid-connected operation and provide reactive power support to the grid.

[0003] For microgrids with an ineffectively grounded neutral point (including ungrounded neutral, neutral grounded via an arc suppression coil, or neutral grounded via high resistance), zero-sequence protection is the primary means of detecting single-phase ground faults. However, during inverter LVRT control, the time-frequency characteristics of the AC-side zero-sequence current are significantly distorted due to the inverter's current-limiting control strategy. This distortion is mainly manifested in the suppression of the power frequency component amplitude, coupled with the superposition of higher harmonics and interharmonic components. This distortion is easily confused with the actual zero-sequence current of a ground fault, causing a significant reduction in the sensitivity of traditional zero-sequence overcurrent protection that relies solely on the single AC-side power frequency zero-sequence current as a criterion, or even complete failure to operate.

[0004] Existing zero-sequence protection methods are mostly based on steady-state quantities such as power frequency zero-sequence voltage, zero-sequence current amplitude, or zero-sequence power direction to construct criteria, which have the following shortcomings: (1) they fail to fully consider the time-frequency distortion characteristics of zero-sequence current during the LVRT control of new energy inverters; (2) they do not effectively utilize the coupling characteristics information between the DC and AC sides of the inverter; and (3) they lack a mechanism for adaptively adjusting protection settings according to different grounding methods and operating states of the microgrid. The above shortcomings make it difficult to meet the protection performance requirements under LVRT conditions, and it is easy to cause false tripping or failure to trip.

[0005] Therefore, there is an urgent need for a zero-sequence current protection method that can effectively distinguish between zero-sequence distortion caused by LVRT control and actual grounding faults, and can adaptively adjust the protection sensitivity according to the microgrid operating status. Summary of the Invention

[0006] To address the aforementioned problems in existing technologies, this invention provides an adaptive zero-sequence current protection method for isolated renewable energy microgrids. This invention aims to overcome the limitations of traditional methods that rely solely on a single AC-side power frequency criterion. By collaboratively constructing AC / DC cross-domain characteristic quantities, it achieves adaptive adjustment of the zero-sequence protection action criterion and setting conditions, thereby effectively improving the protection sensitivity and reliability of isolated renewable energy microgrids under complex transient states.

[0007] The above-mentioned objective of this invention is achieved through the following technical solution:

[0008] An adaptive zero-sequence current protection method for isolated renewable energy microgrids is applied to microgrids containing renewable energy inverters and where the neutral point is not effectively grounded. This method is used to isolate faults when a single-phase ground fault occurs and the AC-side zero-sequence current characteristics are suppressed due to current limiting control during the inverter's low-voltage ride-through. The method includes the following steps:

[0009] S1: Construct a transient simulation model of a microgrid with a neutral point that is not effectively grounded. The model includes a new energy inverter, distributed power sources, loads, and lines. Simulate the control process of the inverter's low-voltage ride-through in the simulation platform and obtain simulation operation data.

[0010] S2: Based on the simulation data described in step S1, the AC side zero-sequence current signal and the DC side bus voltage signal are synchronously acquired during the low voltage ride-through control period. The time-frequency characteristics of the AC side zero-sequence current signal are extracted using continuous wavelet transform to obtain the time-frequency distortion characteristics of the zero-sequence current.

[0011] S3: Calculate the DC-side bus voltage drop rate based on the DC-side bus voltage signal collected in step S2, and calculate the AC-side zero-sequence energy spectrum based on the time-frequency distortion features extracted in step S2. After normalizing the DC-side bus voltage drop rate and the AC-side zero-sequence energy spectrum, construct the AC / DC cross-domain feature vector.

[0012] S4: Based on the AC / DC cross-domain feature vector constructed in step S3, establish an adaptive zero-sequence protection action criterion. The criterion calculates the comprehensive protection action criterion value based on the weighted coupling relationship between the DC side bus voltage drop rate and the AC side zero-sequence energy spectrum, and compares the comprehensive protection action criterion value with the adaptive setting threshold value.

[0013] S5: Based on the comparison results described in step S4, dynamically adjust the sensitivity of the zero-sequence protection action during the low-voltage ride-through period. When the zero-sequence distortion is determined to be caused by the inverter's low-voltage ride-through control, increase the action threshold. When the fault is determined to be a real ground fault, maintain or decrease the action threshold to achieve adaptive zero-sequence protection for microgrids with non-effectively grounded neutral points.

[0014] Furthermore, the continuous wavelet transform described in step S2 uses the complex Morlet wavelet as the mother wavelet function, and its expression is:

[0015] ;

[0016] Where fc is the center frequency and fb is the bandwidth parameter; the scale parameter is adaptively selected according to the main harmonic frequency band of the zero-sequence current signal; the time-frequency distortion characteristics include the wavelet coefficient amplitude distribution and energy concentration frequency band of the zero-sequence current at different scales.

[0017] Furthermore, the formula for calculating the DC-side bus voltage sag rate in step S3 is as follows:

[0018] ;

[0019] in, This is the rated value of the DC bus voltage. During low voltage ride-through The instantaneous value of the DC-side bus voltage at time 1;

[0020] Furthermore, the rate of change of the DC-side characteristic frequency voltage oscillation ripple excited by the instantaneous power imbalance caused by the single-phase ground fault in the system is extracted as a supplementary feature quantity of the AC / DC cross-domain feature vector.

[0021] Furthermore, the formula for calculating the AC-side zero-sequence energy spectrum in step S3 is:

[0022] ;

[0023] in, The zero-sequence current signal is transformed by continuous wavelet transform at scale ,time wavelet coefficients at the location, The time window for low voltage ride-through control;

[0024] By calculating the instantaneous amplitude and phase at characteristic frequencies, an AC-side zero-sequence energy spectrum matrix containing three dimensions of time, frequency, and energy is constructed.

[0025] Furthermore, the adaptive zero-sequence protection action criterion in step S4 is:

[0026] ;

[0027] in, To protect the overall judgment value of the action, This refers to the DC bus voltage sag rate. For the resonant scale The zero-order energy spectrum below, The zero-sequence energy spectrum is used as the reference value for tuning. These are the weighting coefficients for the characteristic quantities on the DC and AC sides, respectively, and satisfy the following conditions: ;

[0028] when The condition for a protective action is deemed met; when At that time, zero-sequence protection is locked out or the original sensitivity is maintained; among which This is for adaptive tuning of the threshold value.

[0029] Furthermore, the adaptive tuning threshold value The settings are determined in stages based on the microgrid grounding method:

[0030] For neutral point ungrounded systems: ;

[0031] For neutral point grounded via arc suppression coil: ;

[0032] For a neutral point grounded through a high-resistance system: ;

[0033] in, This is the benchmark threshold value; This represents the total capacitance to ground of the system. Reference capacitance to ground; To compensate for the arc suppression coil current; This refers to the system capacitor current; The neutral point grounding resistance; Reference grounding resistance; This is the corresponding correction factor.

[0034] Furthermore, the specific method for dynamically adjusting the sensitivity of the zero-sequence protection action in step S5 is as follows: during the initial stage of low-voltage ride-through, the AC / DC cross-domain characteristic vector is monitored in real time, and the following three judgment branches are executed:

[0035] Decision branch ①: If and If the power frequency component does not dominate in the zero-sequence current spectrum, it is determined to be a zero-sequence distortion caused by the inverter's low-voltage ride-through control, and the zero-sequence protection action threshold is automatically raised to [value missing]. ,in To prevent accidental activation of the protection mechanism;

[0036] Decision branch ②: If and However, if the power frequency component in the zero-sequence current spectrum recovers to its dominant position and the phase relationship between the zero-sequence voltage and zero-sequence current satisfies the ground fault criterion, then it is determined to be a superimposed real ground fault during low-voltage ride-through, and the original operating threshold is maintained. Unchanged, according to Determine whether to execute a protection action;

[0037] Decision branch ③: If If the zero-sequence voltage and zero-sequence current amplitudes simultaneously meet the traditional power frequency grounding fault criteria, then it is determined to be a true grounding fault under normal operating conditions, and the action threshold is adaptively lowered to [value missing]. ,in <1, accelerate fault clearance.

[0038] Furthermore, the neutral point non-effective grounding method includes neutral point not grounded, neutral point grounded through arc suppression coil, or neutral point grounded through high resistance.

[0039] An adaptive zero-sequence current protection device for isolated renewable energy microgrids is applied in microgrids containing renewable energy inverters and where the neutral point is not effectively grounded. It is used to isolate the fault when a single-phase ground fault occurs and the AC-side zero-sequence current characteristics are suppressed due to current limiting control during the inverter's low-voltage ride-through. The device includes:

[0040] The simulation modeling unit is used to construct a transient simulation model of a microgrid with a neutral point that is not effectively grounded. The model includes a new energy inverter, distributed power sources, loads, and lines. The control process of the inverter's low-voltage ride-through is simulated in the simulation platform to obtain simulation operation data.

[0041] The signal acquisition unit is used to synchronously acquire the AC side zero-sequence current signal and the DC side bus voltage signal during the low voltage ride-through control based on the simulation operation data.

[0042] The time-frequency feature extraction unit is used to extract the time-frequency features of the AC side zero-sequence current signal using continuous wavelet transform, and to obtain the time-frequency distortion features of the zero-sequence current.

[0043] The cross-domain feature quantity construction unit is used to calculate the DC-side bus voltage drop rate based on the DC-side bus voltage signal and the AC-side zero-sequence energy spectrum based on the time-frequency distortion characteristics. After normalizing the DC-side bus voltage drop rate and the AC-side zero-sequence energy spectrum, the AC-DC cross-domain feature vector is constructed.

[0044] The criterion comparison unit is used to establish an adaptive zero-sequence protection action criterion based on the AC / DC cross-domain feature vector, calculate the comprehensive protection action criterion value based on the weighted coupling relationship between the DC side bus voltage drop rate and the AC side zero-sequence energy spectrum, and compare the comprehensive protection action criterion value with the adaptive setting threshold value.

[0045] An adaptive adjustment unit is used to dynamically adjust the sensitivity of the zero-sequence protection action during low-voltage ride-through based on the comparison results. When the zero-sequence distortion is determined to be caused by inverter low-voltage ride-through control, the action threshold is increased. When the actual ground fault is determined, the action threshold is maintained or decreased, thereby realizing adaptive zero-sequence protection for microgrids with non-effectively grounded neutral points.

[0046] Furthermore, the cross-domain feature quantity construction unit includes:

[0047] The voltage sag rate calculation subunit is used to calculate the voltage drop rate according to... Calculate the DC-side bus voltage sag rate, where, This is the rated value of the DC bus voltage. During low voltage ride-through The instantaneous value of the DC-side bus voltage at time 1;

[0048] The zero-order energy spectrum calculation subunit is used to calculate according to... Calculate the zero-order energy spectrum on the AC side, where, The zero-sequence current signal is transformed by continuous wavelet transform at scale ,time wavelet coefficients at the location, The time window for low voltage ride-through control;

[0049] A normalized sub-unit is constructed to normalize the DC-side bus voltage drop rate and the AC-side zero-sequence energy spectrum to construct an AC / DC cross-domain feature vector.

[0050] The cross-domain feature quantity construction unit also includes a DC-side feature frequency extraction subunit, which is used to extract the rate of change of the DC-side feature frequency voltage oscillation ripple excited by the instantaneous power imbalance caused by the single-phase ground fault in the system, and input it into the normalization construction subunit as a supplementary feature quantity of the AC / DC cross-domain feature vector.

[0051] Compared with the prior art, the present invention has the following advantages:

[0052] (1) This invention extracts the time-frequency characteristics of the zero-sequence current during the low voltage ride-through period by continuous wavelet transform, revealing the time-frequency distortion mechanism of the zero-sequence current during the LVRT control period of the new energy inverter, and breaking through the limitation of the traditional method that only relies on the steady-state quantity of the power frequency.

[0053] (2) By constructing the AC-DC cross-domain characteristic vector of DC-side bus voltage drop rate and AC-side zero-sequence energy spectrum, this invention effectively utilizes the coupling characteristic information of the DC side and AC side of the inverter, and can accurately distinguish between zero-sequence distortion caused by LVRT control and real grounding fault.

[0054] (3) The adaptive zero-sequence protection action criteria and setting conditions proposed in this invention can be set in stages according to the microgrid grounding method, line-to-ground capacitance and inverter capacity, and dynamically adjust the protection sensitivity under different operating conditions, effectively preventing protection maloperation and failure to operate.

[0055] (4) When a single-phase ground fault occurs under normal operating conditions, the present invention can adaptively reduce the protection action threshold and accelerate fault clearing. It is especially suitable for high transition resistance fault scenarios and significantly improves the speed and sensitivity of protection. Attached Figure Description

[0056] Figure 1 This is a flowchart of the adaptive zero-sequence protection method described in this invention;

[0057] Figure 2 This is a schematic diagram of the microgrid topology described in this invention;

[0058] Figure 3 This is the zero-sequence current time-frequency distortion waveform diagram described in this invention;

[0059] Figure 4 This is the spectrum of the zero-sequence current during the low-voltage ride-through period as described in this invention, obtained through continuous wavelet transform.

[0060] Figure 5 This is the AC zero-sequence current energy spectrum diagram described in this invention;

[0061] Figure 6 This is a flowchart of the normalized construction process of the AC / DC cross-domain feature F described in this invention;

[0062] Figure 7 This is the hierarchical tuning logic diagram of the tuning value described in this invention;

[0063] Figure 8 This is a schematic diagram of the adaptive zero-sequence protection action criterion described in this invention. Detailed Implementation

[0064] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0065] like Figure 1 The diagram shown is a control flowchart of an adaptive zero-sequence current protection method for an islanded new energy microgrid in this invention. First, it constructs AC / DC cross-domain characteristic quantities based on the collected DC-side bus voltage drop rate and AC-side zero-sequence energy spectrum. Then, it determines whether the zero-sequence current distortion is caused by the inverter's low-voltage ride-through (LVRT) control. Based on different operating conditions, it dynamically adjusts the sensitivity and setting conditions of the zero-sequence protection action to achieve rapid fault isolation or prevent protection maloperation.

[0066] like Figure 2The diagram shown is a schematic of the microgrid topology for an islanded renewable energy microgrid adaptive zero-sequence current protection method according to the present invention, including a renewable energy power source (rated power 500kW), a DC bus capacitor ( Three-phase full-bridge inverter, LCL filter, step-up transformer (0.4kV / 10.5kV), AC line ( Equivalent circuit of type), concentrated load ( The microgrid uses a neutral point non-effective grounding method, and the zero-sequence protection device is installed at the line to collect the AC side zero-sequence current signal and the DC side bus voltage signal.

[0067] Compared to traditional power frequency zero-sequence overcurrent protection, the adaptive zero-sequence current protection method can use different judgment branches to distinguish between LVRT distortion and actual grounding faults during the low voltage ride-through of new energy inverters, ensuring the reliability of microgrid zero-sequence protection.

[0068] The main implementation steps of this invention are as follows:

[0069] Construct a transient simulation model of a microgrid with a neutral point that is not effectively grounded;

[0070] During low voltage ride-through control, the zero-sequence current signal on the AC side and the bus voltage signal on the DC side are acquired simultaneously, and the time-frequency characteristics are obtained by performing continuous wavelet transform on the zero-sequence current signal.

[0071] Based on the acquired signals, the DC-side bus voltage drop rate and AC-side zero-sequence energy spectrum are calculated respectively, and the time-frequency distortion feature extraction results are used to calculate and generate the AC-side zero-sequence energy spectrum, and construct the AC / DC cross-domain feature vector.

[0072] Establish an adaptive zero-sequence protection action criterion, calculate the comprehensive protection action criterion value and compare it with the setting threshold value;

[0073] Based on the action criteria and setting conditions, the sensitivity of the zero-sequence protection action is dynamically adjusted during the low voltage ride-through period to distinguish between LVRT distortion and actual grounding faults, thereby realizing adaptive zero-sequence protection for microgrids with non-effectively grounded neutral points.

[0074] Specifically, a transient simulation model of a microgrid with a neutral point that is not effectively grounded is first built using the MATLAB / Simulink platform. If the DC bus voltage drop rate is not detected to exceed the set value and the power frequency component of the zero-sequence current is dominant, judgment branch ③ is used to determine that it is a real ground fault under normal operating conditions. If the DC bus voltage drop rate is detected to exceed the set value and the power frequency component in the zero-sequence current spectrum is not dominant, judgment branch ① is used to determine that it is zero-sequence distortion caused by LVRT distortion, and the protection action threshold is raised to prevent false tripping. If the DC bus voltage drop rate is detected to exceed the set value and the power frequency component in the zero-sequence current spectrum recovers to the dominant position, judgment branch ② is used to determine that it is a real ground fault superimposed during LVRT, and the original action threshold is maintained to ensure protection sensitivity.

[0075] The new energy power source is connected in parallel with the DC bus capacitor, and the rated voltage of the DC bus is... Bus capacitor value A three-phase full-bridge inverter topology is adopted, with IGBTs as the switching devices and a switching frequency of 10 kHz. An LCL filter is connected to the inverter output side, where the converter-side inductor... Grid-side inductor Filter capacitor When the voltage drop at the grid connection point exceeds 20% of the rated voltage, the inverter automatically switches to LVRT control mode and initiates a current-limiting control strategy, limiting the AC output current to within 1.1 times the rated current. The microgrid has a rated voltage of 10 kV and is connected to the AC line via a step-up transformer. The line-to-ground capacitance... Line positive sequence impedance The line length is set at 5km. A single-phase ground fault point is set at the midpoint of the line, and the fault transition resistance is taken as follows: .

[0076] The specific control method of the present invention is as follows:

[0077] like Figure 3 The figure shown is a waveform diagram of zero-sequence current time-frequency distortion. The grid connection point voltage drops by 50%, with no grounding fault. Traditional power frequency zero-sequence overcurrent protection malfunctions due to zero-sequence current distortion; the method of this invention correctly identifies it as LVRT distortion, and the protection reliably does not operate.

[0078] like Figure 4 The image shows the spectrum of the zero-sequence current during the low-voltage ride-through period using continuous wavelet transform. The zero-sequence current signal on the AC side was acquired synchronously. and DC side bus voltage signal The sampling frequency was set to 10kHz. During the LVRT period of t=0.1~0.3s, the acquired zero-sequence current waveform showed significant distortion, with the amplitude of the power frequency (50Hz) component suppressed, and higher harmonics and interharmonics superimposed. A continuous wavelet transform was performed on the zero-sequence current signal, using the complex Morlet wavelet as the mother wavelet function, the expression of which is:

[0079]

[0080] Wherein, the center frequency f c =1Hz, bandwidth parameter f b =1.5.

[0081] The scaling parameter 'a' is set to cover the frequency band from 20Hz to 350Hz to fully capture the full-band time-frequency distortion characteristics of the zero-sequence current during LVRT. The wavelet transform coefficients are calculated using the following formula:

[0082]

[0083] By studying different scales Lower wavelet coefficient amplitude Analysis was performed to obtain the time-frequency spectrum of the zero-sequence current. During normal operation, the time-frequency energy is concentrated at the power frequency level; after entering the LVRT stage, the energy at the power frequency significantly weakens, and the energy shifts towards the resonant level near 150Hz. During migration, energy accumulation occurs in the 75Hz interharmonic frequency band.

[0084] like Figure 5 The image shows the energy spectrum of the zero-sequence current measured in AC. Based on the two acquired signals, characteristic quantities on the DC and AC sides are calculated respectively. In this embodiment, when the grid connection point voltage drops by 50%, the DC bus voltage drops synchronously. The measured value is approximately 32%–38% (the drop is slightly smaller than that on the AC side due to the support provided by the DC-side capacitor of the inverter). In addition, the rate of change of the DC-side characteristic frequency voltage oscillation ripple caused by the instantaneous power imbalance due to a single-phase ground fault is extracted as a supplementary characteristic quantity.

[0085] DC bus voltage sag According to the collected Calculate using the following formula:

[0086]

[0087] in, This is the rated voltage on the DC side. During LVRT The instantaneous value of the bus voltage at time t.

[0088] AC-side zero-sequence energy spectrum Based on CWT results, within the time window Internal resonant scale Integrating the wavelet coefficient energy at a given point yields the flow-side zero-order energy spectrum. :

[0089]

[0090] like Figure 6 The diagram shown is a flowchart of the normalized construction of the AC / DC cross-domain characteristic F. Resonance scale. The corresponding frequency is approximately 150Hz, determined by scanning the maximum energy value at various scales. Tuning reference value. Take the microgrid during normal operation The statistical maximum value of the zero-order energy spectrum at the scale. After normalizing the two characteristic quantities, an AC / DC cross-domain characteristic vector is constructed:

[0091]

[0092] This feature vector contains both DC-side operating status information and AC-side zero-sequence time-frequency feature information, providing a foundation for the establishment of subsequent adaptive protection criteria.

[0093] like Figure 7 The diagram shown is a hierarchical setting logic diagram for the setting value. Based on the AC / DC cross-domain feature vector F, a comprehensive protection action criterion is established:

[0094]

[0095] Among them, S op The comprehensive criterion value for protection action; α and β are the weighting coefficients of the characteristic quantities on the DC and AC sides, respectively, and satisfy α + β = 1. Adaptive tuning threshold value. The setting is determined according to the classification of microgrid grounding methods. For systems with an ungrounded neutral point, the setting formula is as follows:

[0096]

[0097] Among them, the benchmark threshold value Reference capacitance to ground Correction factor Substituting into the calculation, we get... .

[0098] The protection action logic is as follows: When When, it is determined that the conditions for a protective action are met; when At that time, the zero-sequence protection is locked or the original sensitivity is maintained.

[0099] In the typical simulation scenario of this embodiment, after entering the LVRT stage at t = 0.1s, since there is no grounding fault, Approximately 35%, Approximately 1.8, calculated as follows ,Exceed = 0.816. If this logic is applied directly, it will lead to false protection activation. Therefore, a dynamic adjustment mechanism is introduced in step S5 to differentiate between these factors.

[0100] like Figure 8 The diagram shown illustrates the adaptive zero-sequence protection action criteria. During the initial stage of LVRT, AC / DC cross-domain characteristic quantities are monitored in real time, and the protection sensitivity is dynamically adjusted according to the following three judgment branches:

[0101] Judgment branch ①, zero-sequence distortion caused by LVRT distortion (to prevent accidental operation): If and If the power frequency component does not dominate in the zero-sequence current spectrum, it is determined to be a zero-sequence distortion caused by the inverter LVRT control, rather than a real ground fault, and the zero-sequence protection action threshold is automatically raised to: .

[0102] In this embodiment, the reliability coefficient is taken. ,but Recalculate Moment It is still greater than 1.06, but based on the power frequency ratio, it is confirmed to be LVRT distortion, and the protection is reliably locked to prevent malfunction.

[0103] Determine branch ②, where a real ground fault is superimposed during LVRT: If and However, if the power frequency component in the zero-sequence current spectrum regains its dominance, and the phase relationship between the zero-sequence voltage and zero-sequence current satisfies the ground fault criterion, then it is determined to be a true ground fault superimposed on the LVRT operating condition. In this case, the original operating threshold is maintained. Unchanged, according to Determine whether to execute a protective action.

[0104] In this embodiment, after a single-phase ground fault is superimposed at t = 0.3 s ( The zero-sequence current at power frequency increases rapidly, and the power frequency proportion rises to 75%, satisfying the phase relationship condition. Recalculate. The protection system operates reliably within approximately 12ms, enabling rapid fault isolation.

[0105] Determine branch ③, the actual grounding fault under normal operation: If This indicates that the inverter has not entered LVRT mode, and the zero-sequence voltage and zero-sequence current amplitudes meet the traditional power frequency grounding fault criteria, thus it is determined to be a true grounding fault under normal operating conditions. At this time, the system adaptively lowers the action threshold (to 0.85 times the original threshold) to accelerate fault clearing; that is, the new action threshold value is updated to... Since inverter distortion is not prevented at this time, lowering the threshold not only accelerates fault clearing (the action time is shortened to about 10ms), but also effectively avoids protection failure under high transition resistance faults, significantly improving the speed and sensitivity of conventional minor faults or high-resistance grounding. Through the above three-branch adaptive adjustment mechanism (as shown in Table 1), in this embodiment, the protection of branch ① (LVRT distortion, no fault) reliably does not operate; branch ② (LVRT superimposed grounding fault) operates within about 12ms; and the action time of branch ③ (grounding fault under normal operation) is about 10ms. Under different operating conditions, it can correctly distinguish between LVRT distortion and real grounding faults, realizing reliable zero-sequence protection for microgrids with non-effectively grounded neutral points during low-voltage ride-through of new energy inverters.

[0106] Table 1. Comparison of the effects of the three-branch adaptive zero-sequence protection mechanism in isolated renewable energy microgrids.

[0107]

[0108] Therefore, by adopting the adaptive zero-sequence current protection method and based on its three judgment branches, the microgrid with a neutral point that is not effectively grounded can correctly distinguish between LVRT distortion and actual grounding fault during the low voltage ride-through of the new energy inverter, ensuring the reliability of the microgrid's zero-sequence protection, preventing protection maloperation and failure to operate, and improving the protection performance of the islanded new energy microgrid under complex transition states.

[0109] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for adaptive zero-sequence current protection in an isolated renewable energy microgrid, characterized in that, This method is applied to microgrids containing renewable energy inverters and where the neutral point is not effectively grounded. It is used to isolate faults when a single-phase ground fault occurs and the AC side zero-sequence current characteristics are suppressed due to current limiting control during the inverter's low-voltage ride-through. The method includes the following steps: S1: Construct a transient simulation model of a microgrid with a neutral point that is not effectively grounded. The model includes a new energy inverter, distributed power sources, loads, and lines. Simulate the control process of the inverter's low-voltage ride-through in the simulation platform and obtain simulation operation data. S2: Based on the simulation data described in step S1, the AC side zero-sequence current signal and the DC side bus voltage signal are synchronously acquired during the low voltage ride-through control period. The time-frequency characteristics of the AC side zero-sequence current signal are extracted using continuous wavelet transform to obtain the time-frequency distortion characteristics of the zero-sequence current. S3: Calculate the DC-side bus voltage drop rate based on the DC-side bus voltage signal collected in step S2, and calculate the AC-side zero-sequence energy spectrum based on the time-frequency distortion features extracted in step S2. After normalizing the DC-side bus voltage drop rate and the AC-side zero-sequence energy spectrum, construct the AC / DC cross-domain feature vector. S4: Based on the AC / DC cross-domain feature vector constructed in step S3, establish an adaptive zero-sequence protection action criterion. The criterion calculates the comprehensive protection action criterion value based on the weighted coupling relationship between the DC side bus voltage drop rate and the AC side zero-sequence energy spectrum, and compares the comprehensive protection action criterion value with the adaptive setting threshold value. S5: Based on the comparison results described in step S4, dynamically adjust the sensitivity of the zero-sequence protection action during the low-voltage ride-through period. When the zero-sequence distortion is determined to be caused by the inverter's low-voltage ride-through control, increase the action threshold. When the fault is determined to be a real ground fault, maintain or decrease the action threshold to achieve adaptive zero-sequence protection for microgrids with non-effectively grounded neutral points.

2. The adaptive zero-sequence current protection method for an islanded renewable energy microgrid according to claim 1, characterized in that, The continuous wavelet transform described in step S2 uses the complex Morlet wavelet as the mother wavelet function, and its expression is: ； Among them, f c f is the center frequency. b The bandwidth parameter is used; the scale parameter is adaptively selected based on the main harmonic frequency band of the zero-sequence current signal; the time-frequency distortion characteristics include the amplitude distribution of wavelet coefficients of the zero-sequence current at different scales and the frequency band of energy concentration.

3. The adaptive zero-sequence current protection method for an islanded renewable energy microgrid according to claim 1, characterized in that, The formula for calculating the DC-side bus voltage sag rate in step S3 is: ； in, This is the rated value of the DC bus voltage. During low voltage ride-through The instantaneous value of the DC-side bus voltage at time 1; Furthermore, the rate of change of the DC-side characteristic frequency voltage oscillation ripple excited by the instantaneous power imbalance caused by the single-phase ground fault in the system is extracted as a supplementary feature quantity of the AC / DC cross-domain feature vector.

4. The adaptive zero-sequence current protection method for an islanded renewable energy microgrid according to claim 1, characterized in that, The formula for calculating the AC-side zero-sequence energy spectrum in step S3 is: ； in, The zero-sequence current signal is transformed by continuous wavelet transform at scale ,time wavelet coefficients at the location, The time window for low voltage ride-through control; By calculating the instantaneous amplitude and phase at characteristic frequencies, an AC-side zero-sequence energy spectrum matrix containing three dimensions of time, frequency, and energy is constructed.

5. The adaptive zero-sequence current protection method for an islanded renewable energy microgrid according to claim 3, characterized in that, The adaptive zero-sequence protection action criterion in step S4 is: ； in, To protect the overall judgment value of the action, This refers to the DC bus voltage sag rate. For the resonant scale The zero-order energy spectrum below, The zero-sequence energy spectrum is used as the reference value for tuning. These are the weighting coefficients for the characteristic quantities on the DC and AC sides, respectively, and satisfy the following conditions: ; when The condition for a protective action is deemed met; when At that time, zero-sequence protection is locked out or the original sensitivity is maintained; among which This is for adaptive tuning of the threshold value.

6. The adaptive zero-sequence current protection method for an islanded renewable energy microgrid according to claim 5, characterized in that, The adaptive tuning threshold value The settings are determined in stages based on the microgrid grounding method: For neutral point ungrounded systems: ; For neutral point grounded via arc suppression coil: ; For a neutral point grounded through a high-resistance system: ; in, This is the benchmark threshold value; This represents the total capacitance to ground of the system. Reference capacitance to ground; To compensate for the arc suppression coil current; This refers to the system capacitor current; The neutral point grounding resistance; Reference grounding resistance; This is the corresponding correction factor.

7. The adaptive zero-sequence current protection method for an islanded renewable energy microgrid according to claim 6, characterized in that, The specific method for dynamically adjusting the sensitivity of the zero-sequence protection action in step S5 is as follows: During the initial stage of low-voltage ride-through, the AC / DC cross-domain characteristic vector is monitored in real time, and the following three judgment branches are executed: Decision branch ①: If and If the power frequency component does not dominate in the zero-sequence current spectrum, it is determined to be a zero-sequence distortion caused by the inverter's low-voltage ride-through control, and the zero-sequence protection action threshold is automatically raised to [value missing]. ,in To prevent accidental activation of the protection mechanism; Decision branch ②: If and However, if the power frequency component in the zero-sequence current spectrum recovers to its dominant position and the phase relationship between the zero-sequence voltage and zero-sequence current satisfies the ground fault criterion, then it is determined to be a superimposed real ground fault during low-voltage ride-through, and the original operating threshold is maintained. Unchanged, according to Determine whether to execute a protection action; Decision branch ③: If If the zero-sequence voltage and zero-sequence current amplitudes simultaneously meet the traditional power frequency grounding fault criteria, then it is determined to be a true grounding fault under normal operating conditions, and the action threshold is adaptively lowered to [value missing]. ,in <1, accelerate fault clearance.

8. The adaptive zero-sequence current protection method for an islanded renewable energy microgrid according to claim 1, characterized in that, The neutral point non-effective grounding methods include neutral point not grounded, neutral point grounded through arc suppression coil, or neutral point grounded through high resistance.

9. An adaptive zero-sequence current protection device for an isolated renewable energy microgrid, characterized in that, This device is applied in microgrids containing renewable energy inverters and where the neutral point is not effectively grounded. It is used to isolate faults when a single-phase ground fault occurs and the AC side zero-sequence current characteristics are suppressed due to current limiting control during the inverter's low-voltage ride-through. The device includes: The simulation modeling unit is used to construct a transient simulation model of a microgrid with a neutral point that is not effectively grounded. The model includes a new energy inverter, distributed power sources, loads, and lines. The control process of the inverter's low-voltage ride-through is simulated in the simulation platform to obtain simulation operation data. The signal acquisition unit is used to synchronously acquire the AC side zero-sequence current signal and the DC side bus voltage signal during the low voltage ride-through control based on the simulation operation data. The time-frequency feature extraction unit is used to extract the time-frequency features of the AC side zero-sequence current signal using continuous wavelet transform, and to obtain the time-frequency distortion features of the zero-sequence current. The cross-domain feature quantity construction unit is used to calculate the DC-side bus voltage drop rate based on the DC-side bus voltage signal and the AC-side zero-sequence energy spectrum based on the time-frequency distortion characteristics. After normalizing the DC-side bus voltage drop rate and the AC-side zero-sequence energy spectrum, the AC-DC cross-domain feature vector is constructed. The criterion comparison unit is used to establish an adaptive zero-sequence protection action criterion based on the AC / DC cross-domain feature vector, calculate the comprehensive protection action criterion value based on the weighted coupling relationship between the DC side bus voltage drop rate and the AC side zero-sequence energy spectrum, and compare the comprehensive protection action criterion value with the adaptive setting threshold value. An adaptive adjustment unit is used to dynamically adjust the sensitivity of the zero-sequence protection action during low-voltage ride-through based on the comparison results. When the zero-sequence distortion is determined to be caused by inverter low-voltage ride-through control, the action threshold is increased. When the actual ground fault is determined, the action threshold is maintained or decreased, thereby realizing adaptive zero-sequence protection for microgrids with non-effectively grounded neutral points.

10. The adaptive zero-sequence current protection device for an isolated new energy microgrid according to claim 9, characterized in that, The cross-domain feature quantity construction unit includes: The voltage sag rate calculation subunit is used to calculate the voltage drop rate according to... Calculate the DC-side bus voltage sag rate, where, This is the rated value of the DC bus voltage. During low voltage ride-through The instantaneous value of the DC-side bus voltage at time 1; The zero-order energy spectrum calculation subunit is used to calculate according to... Calculate the zero-order energy spectrum on the AC side, where, The zero-sequence current signal is transformed by continuous wavelet transform at scale ,time wavelet coefficients at the location, The time window for low voltage ride-through control; A normalized sub-unit is constructed to normalize the DC-side bus voltage drop rate and the AC-side zero-sequence energy spectrum to construct an AC / DC cross-domain feature vector. The cross-domain feature quantity construction unit also includes a DC-side feature frequency extraction subunit, which is used to extract the rate of change of the DC-side feature frequency voltage oscillation ripple excited by the instantaneous power imbalance caused by the single-phase ground fault in the system, and input it into the normalization construction subunit as a supplementary feature quantity of the AC / DC cross-domain feature vector.

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