Current-limiting and harmonic elimination device for electromagnetic voltage transformer based on PTC characteristic and optimization method thereof

Abstract

The application discloses an electromagnetic voltage transformer current-limiting and resonance-eliminating device based on PTC characteristics and an optimization method thereof, and relates to the technical field of power protection. The device comprises the following steps: adopting donor doping, acceptor doping and glass phase modification to regulate and control barium titanate-based positive temperature coefficient ceramics to form a positive temperature coefficient core; forming a conductive electrode layer on the positive temperature coefficient core; connecting the positive temperature coefficient core in series according to the number of multiple pieces and in series according to the insulation packaging parameters, and then connecting the positive temperature coefficient core in series between a primary neutral point of a voltage transformer and the ground to form a current-limiting and resonance-eliminating device; and arranging the collected zero sequence voltage and neutral point current according to a unified sampling time mark to form a joint discrimination signal. The application can improve the identification accuracy among frequency resonance, fundamental frequency ferromagnetic resonance, arc grounding and single-phase grounding by first screening abnormal states in layers and then further distinguishing power frequency dominant abnormalities, so that the misjudgment and misoperation are reduced.

Description

Technical Field

[0001] This invention relates to the field of power protection technology, and in particular to an electromagnetic voltage transformer current limiting and harmonic elimination device based on PTC characteristics and its optimization method. Background Technology

[0002] In distribution networks with ineffectively grounded neutral points, electromagnetic voltage transformers perform functions such as zero-sequence voltage detection, grounding monitoring, and protection measurements. The transient response characteristics of the primary neutral point branch of the voltage transformer directly affect the evolution of ferroresonance and grounding anomalies. Regarding overvoltage suppression of electromagnetic voltage transformers, existing technologies typically employ methods such as primary-side series harmonic suppression resistors, nonlinear resistors, open-delta harmonic suppression devices, zero-sequence voltage monitoring, and grounding anomaly identification. By changing the neutral point branch impedance, extracting zero-sequence voltage characteristics, or combining fault current information, these methods monitor and address sub-frequency resonance, fundamental frequency ferroresonance, arcing grounding, and single-phase grounding. With the development of ceramic-based positive temperature coefficient (PTC) materials in current-limiting protection, utilizing the low cold-state resistance, high hot-state resistance, and recoverable characteristics of PTC materials, connecting the PTC core in series between the primary neutral point and ground of the voltage transformer to balance normal operation conduction and current-limiting suppression under abnormal conditions has become one of the important technical directions for current-limiting and harmonic suppression in electromagnetic voltage transformers.

[0003] While existing technologies can suppress or monitor overvoltage anomalies in electromagnetic voltage transformers to some extent, they generally suffer from two shortcomings: First, some solutions rely mainly on fixed impedance adjustment or a single zero-sequence voltage criterion, making it difficult to effectively distinguish abnormal states under conditions where frequency division resonance, fundamental frequency ferroresonance, and grounding anomalies coexist. This can easily lead to problems such as an overly broad discrimination range, insufficient action targeting, and uncoordinated switching between monitoring and suppression. Second, even when using nonlinear resistors or positive temperature coefficient materials, some solutions often lack synergistic optimization between material parameters, series connection quantity, insulation encapsulation, and discrimination rules, making it difficult to balance the impact on normal operation, action sensitivity, withstand voltage stability, and thermal recovery capability. Summary of the Invention

[0004] In view of the aforementioned existing problems, the present invention is proposed.

[0005] Therefore, this invention provides an optimization method for current limiting and harmonic elimination of electromagnetic voltage transformers based on PTC characteristics to solve the problem of difficulty in accurately identifying and quickly coordinating the suppression of abnormal states at the neutral point of electromagnetic voltage transformers.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics, comprising,

[0008] Barium titanate-based positive temperature coefficient ceramics are formed by controlling donor doping, acceptor doping and glass phase modification to form a positive temperature coefficient core.

[0009] A conductive electrode layer is formed on the positive temperature coefficient core. Multiple electrodes are connected in series and encapsulated according to the insulation encapsulation parameters. The electrode layer is then connected in series between the primary neutral point and ground of the voltage transformer to form a current limiting and harmonic elimination device. The zero-sequence voltage and neutral point current are collected and arranged according to a unified sampling time scale to form a joint discrimination signal.

[0010] Joint sequential discrimination is performed on the joint discrimination signal. The joint sequential discrimination includes performing discrete spectrum analysis on the continuous sampling segment of zero-sequence voltage to obtain the dominant frequency component, and performing wave-current correlation discrimination on the anomaly of the dominant frequency maintaining the power frequency and the corresponding continuous sampling segment of neutral point current to form resonance suppression state and grounding monitoring state.

[0011] Under the resonance suppression state, the current limiting and harmonic elimination device changes from a low resistance state to a high resistance state as the neutral point current continues to increase, raising the resistance of the neutral point branch and weakening the excitation current to destroy the resonance condition. Under the ground monitoring state, it maintains a low resistance state and restores a low resistance state after the neutral point current falls back, thus forming the result of the harmonic elimination action.

[0012] The results of harmonic elimination are used to coordinately adjust the positive temperature coefficient core parameters, the number of multiple chips connected in series, the insulation packaging parameters, and the discrimination rules for the joint sequence, forming an optimized neutral point current limiting and harmonic elimination parameter set.

[0013] As a preferred embodiment of the current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics described in this invention, wherein: the formation of a positive temperature coefficient core specifically includes:

[0014] Barium titanate-based powder is mixed with donor-doped components, ball-milled and dried to form a pre-calcined material;

[0015] The pre-burned material is calcined and pulverized, then mixed with acceptor doping components and glass phase components to form modified powder.

[0016] The modified powder is pressed into shape and sintered into sheets to form a positive temperature coefficient ceramic chip, and a conductive electrode layer is formed on the surface of the positive temperature coefficient ceramic chip to form a positive temperature coefficient core.

[0017] As a preferred embodiment of the current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics described in this invention, the current limiting and harmonic elimination device specifically includes:

[0018] A positive temperature coefficient core with a conductive electrode layer formed on its surface is used as an electrode core unit. Multiple electrode core units are connected in series according to the number of multiple series-connected pieces to form a core series assembly.

[0019] The core series assembly is encapsulated in epoxy resin and filled with quartz sand according to the insulation encapsulation parameters, and equipped with a heat sink and an outer insulating sleeve to form a current limiting and harmonic elimination device.

[0020] As a preferred embodiment of the current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics described in this invention, the formation of the joint discrimination signal specifically includes:

[0021] The current limiting and harmonic elimination device is connected in series between the primary neutral point of the voltage transformer and ground. The zero-sequence voltage output by the open delta winding is collected, and the neutral point current flowing through the current limiting and harmonic elimination device is collected simultaneously to form a dual-channel detection signal.

[0022] The dual-channel detection signals are arranged according to a unified sampling time scale to form a continuous sampling signal group;

[0023] The zero-sequence voltage continuous sampling segment in the continuous sampling signal group is paired and associated with the corresponding neutral point current continuous sampling segment to form a joint discrimination signal.

[0024] As a preferred embodiment of the current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics described in this invention, the joint sequence discrimination includes frequency screening and wave current discrimination.

[0025] As a preferred embodiment of the current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics described in this invention, the frequency screening specifically includes:

[0026] Zero-sequence voltage continuous sampling segments are extracted from the joint discrimination signal, and discrete spectrum analysis is used to determine the dominant frequency components of each zero-sequence voltage continuous sampling segment to form a frequency feature group.

[0027] Based on the frequency characteristic group, anomalies with a dominant frequency lower than the power frequency are classified into the frequency sub-resonance group, anomalies with a dominant frequency that remains at the power frequency are classified into the power frequency dominant anomaly group, and anomalies with a dominant frequency higher than the power frequency are classified into the non-power frequency anomaly record group, thus forming the frequency grouping result.

[0028] The power frequency dominant anomaly group and the corresponding neutral point current continuous sampling segment in the retained frequency grouping results are used to form the wave-current discrimination object group.

[0029] As a preferred embodiment of the current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics described in this invention, the wave current discrimination specifically includes:

[0030] From the wave-current discrimination object group, retrieve the waveform sequence of the zero-crossing region of the zero-sequence voltage and the continuous change sequence of the neutral point current, extract the continuous and symmetrical change characteristics of the zero-crossing waveform of the zero-sequence voltage and the continuous change characteristics of the neutral point current, and form a wave-current correlation feature group;

[0031] Based on the wave-current correlation characteristic group, anomalies such as sudden changes, gaps, spikes, and asymmetrical changes in the zero-crossing region, and continuous fluctuations in the neutral point current, are classified into the arc grounding candidate group; anomalies with continuous and smooth zero-crossing regions and a sinusoidal zero-crossing trend are classified into the single-phase grounding candidate group; and anomalies where at least one of the continuity and symmetry of the zero-crossing region is disrupted and the neutral point current continues to increase are classified into the fundamental frequency ferroresonant candidate group.

[0032] The candidate groups for fundamental frequency ferromagnetic resonance are mapped to resonance suppression states, and the candidate groups for arc grounding and single-phase grounding are mapped to grounding monitoring states, thus forming state discrimination results.

[0033] As a preferred embodiment of the current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics described in this invention, the formation of the harmonic elimination action result specifically includes:

[0034] Read the resonance suppression state and ground monitoring state from the state discrimination results to form the action criteria;

[0035] Based on the action criteria, the current limiting and harmonic elimination device connected to the resonance suppression state switches from a low resistance state to a high resistance state as the neutral point current continues to increase, thereby raising the resistance of the neutral point branch and weakening the excitation current, thus forming a resonance suppression result.

[0036] Based on the action criteria, the connected current limiting and harmonic elimination device is kept in a low resistance state under grounding monitoring, and the neutral point current drop process and the positive temperature coefficient core cooling process are tracked to form the recovery action result.

[0037] The resonance suppression result and the recovery action result are merged to form the harmonic elimination action result.

[0038] As a preferred embodiment of the current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics described in this invention, the step of forming the optimized neutral point current limiting and harmonic elimination parameter set specifically includes:

[0039] The results of harmonic elimination actions and state determination are combined to form parameter evaluation results;

[0040] Based on the parameter evaluation results, when resonance suppression causes hysteresis, the Curie temperature range and resistance jump characteristics are corrected; when the impact of normal operation increases, the cold resistance is corrected; when the withstand voltage is insufficient, the number of multiple series chips and insulation package parameters are corrected; when the recovery process is prolonged, the heat dissipation path is corrected; when state discrimination causes hysteresis, the discrimination time period is corrected; when the state switching is unstable, the discrimination sequence is corrected, thus forming the parameter correction results.

[0041] The Curie temperature range, cold resistance, and resistance jump characteristics in the parameter correction results are matched with the number of multiple chips connected in series, insulation packaging parameters, and heat dissipation paths. The matched structural parameters are then linked with the discrimination time period and discrimination sequence in the joint order discrimination to form an optimized neutral point current limiting and harmonic elimination parameter set.

[0042] Secondly, the present invention provides an electromagnetic voltage transformer current limiting and harmonic elimination device based on PTC characteristics, including a core forming module, which uses donor doping, acceptor doping and glass phase modification to regulate barium titanate-based positive temperature coefficient ceramics to form a positive temperature coefficient core.

[0043] The signal coupling module forms a conductive electrode layer on the positive temperature coefficient core, connects multiple electrodes in series according to the number of electrodes connected in series and encapsulates them according to the insulation encapsulation parameters, and then connects them in series between the primary neutral point and ground of the voltage transformer to form a current limiting and harmonic elimination device. The zero-sequence voltage and neutral point current collected are arranged according to a unified sampling time scale to form a joint discrimination signal.

[0044] The sequence discrimination module performs joint sequence discrimination on the joint discrimination signal. The joint sequence discrimination includes performing discrete spectrum analysis on the continuous sampling segment of zero-sequence voltage to obtain the dominant frequency component, and performing wave-current correlation discrimination on the abnormality of the dominant frequency maintaining the power frequency and the corresponding continuous sampling segment of neutral point current to form resonance suppression state and grounding monitoring state.

[0045] In the resonance suppression state, the current limiting and harmonic elimination module changes from a low resistance state to a high resistance state as the neutral point current continues to increase, raising the resistance of the neutral point branch and weakening the excitation current to destroy the resonance condition. In the ground monitoring state, it maintains a low resistance state and returns to a low resistance state after the neutral point current falls back, thus forming the harmonic elimination action result.

[0046] The parameter optimization module uses the results of harmonic elimination to coordinately adjust the positive temperature coefficient core parameters, the number of multiple chips connected in series, the insulation encapsulation parameters, and the discrimination rules for the joint sequence, forming an optimized neutral point current limiting and harmonic elimination parameter set.

[0047] The beneficial effects of this invention are as follows: by integrating the material properties of the positive temperature coefficient core, the current limiting and harmonic elimination structure of the neutral point branch, and the joint sequence discrimination of zero-sequence voltage and neutral point current into an integrated design, the electromagnetic voltage transformer can respond to different response modes of low resistance connection, rapid resistance increase, and stable recovery under three working conditions: normal operation, resonance development, and grounding monitoring. Compared with existing technologies, this invention no longer relies solely on fixed impedance adjustment or a single zero-sequence voltage criterion. Instead, it first performs stratified screening of abnormal states and then further distinguishes between power frequency dominant abnormalities. This improves the identification accuracy between frequency division resonance, fundamental frequency ferroresonance, arc grounding, and single-phase grounding, reducing misjudgments and malfunctions. Simultaneously, the positive temperature coefficient core can quickly transition from a low-resistance state to a high-resistance state when the neutral point current continuously increases, promptly raising the neutral point branch resistance and weakening the excitation current. This helps to disrupt the resonance maintenance conditions and suppress the continuous amplification of overvoltage and overcurrent. After the abnormality weakens, it can return to a low-resistance state, which is beneficial for balancing the continuity of grounding monitoring and the reusability of the device. Through the coordinated correction of the positive temperature coefficient core parameters, the number of series connections, the insulation encapsulation parameters, and the discrimination rules, the material's operational capability, structural withstand voltage capability, heat dissipation recovery capability, and discrimination response timing are kept matched, thereby improving the operational reliability, operational stability, and engineering applicability of the current limiting and harmonic elimination device. Attached Figure Description

[0048] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 This is a flowchart of an optimization method for current limiting and harmonic elimination in electromagnetic voltage transformers based on PTC characteristics.

[0050] Figure 2 Flowchart for the preparation and testing of core materials with positive temperature coefficient.

[0051] Figure 3 This is a flowchart for determining the order of union.

[0052] Figure 4 A flowchart for the action response and parameter optimization of the current limiting and harmonic elimination device. Detailed Implementation

[0053] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0054] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0055] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0056] Reference Figures 1-4 As one embodiment of the present invention, this embodiment provides a current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics, including the following steps:

[0057] S1. A barium titanate-based positive temperature coefficient ceramic is formed by controlling the donation doping, acceptor doping and glass phase modification.

[0058] S1.1. Based on the application scenario of neutral point current limiting and harmonic elimination in electromagnetic voltage transformers, the material system and performance targets of barium titanate-based positive temperature coefficient ceramics are determined. The material system adopted is barium titanate-based ceramic system because it has high withstand voltage strength, relatively stable positive temperature coefficient effect and fast thermal response capability, making it suitable for direct neutral point connection conditions. The performance targets are limited to low resistance under normal conditions, resistance increase during operation, natural recovery, stable withstand voltage, and stable performance after repeated current flow. Among them, the room temperature resistivity is controlled above 1×10^5Ω·cm, the Curie temperature is controlled at 120±10℃, the positive temperature coefficient jump amplitude reaches more than 3 orders of magnitude, the breakdown field strength is higher than 7kV / mm, the thermal response time does not exceed 2s, and the change in room temperature resistivity and Curie temperature after cyclic aging and thermal shock is less than 20%. The above-mentioned requirements are used to form the barium titanate-based positive temperature coefficient ceramic formulation requirements.

[0059] S1.2. The requirements for the formulation of barium titanate-based positive temperature coefficient ceramics shall be implemented in the raw material preparation. The barium titanate-based powder shall be prepared using barium carbonate and titanium dioxide. The donor dopant shall be at least one of niobium pentoxide and yttrium oxide. The acceptor dopant shall be at least one of manganese carbonate and copper oxide. The glass phase component shall be silicon dioxide and... It is at least one of the glass phases.

[0060] In this embodiment, barium carbonate, titanium dioxide, and niobium pentoxide are added to a ball mill jar according to the formula ratio and deionized water is added for ball milling. After ball milling, the mixture is dried to form a pre-sintered material. The role of the donor doping component is to enable the barium titanate-based positive temperature coefficient ceramic to form semiconductor grains, thereby ensuring low-power operation under normal conditions. The content of the donor doping component is controlled within the range of 0.2 to 0.4 mol% to balance room temperature resistivity and subsequent resistance increase capability. After the above treatment, a pre-sintered material that meets the pre-sintering conditions is formed.

[0061] The pre-fired material is loaded into a crucible for pre-firing, so that barium carbonate and titanium dioxide can complete the solid-phase reaction and form the main crystalline phase. In this embodiment, the pre-firing temperature is 1200℃ and the holding time is 120min.

[0062] After pre-calcination, the pre-calcined product is pulverized and then mixed with the acceptor dopant and glass phase components to form modified powder. The acceptor dopant component increases the grain boundary barrier height and enhances the resistance surge capability. The acceptor dopant content is controlled within the range of 0.05–0.2 mol%, and in this embodiment, 0.05 mol% manganese carbonate is used. The glass phase component lowers the sintering temperature, increases density, improves mechanical strength, and enhances pressure resistance and thermal shock stability. In this embodiment, 1 mol% silicon dioxide is used, and in extended embodiments, 0.5–2.0 wt% can also be used. The glass phase, after the above treatment, forms a modified powder that meets the conditions for compression molding.

[0063] S1.3. After sieving, the modified powder is added to a pressing mold for pressing and molding. During the pressing and molding stage, a pre-pressure is first applied to expel air from the gaps between the powder particles, and then the pressure is gradually increased to form a green sheet. In this embodiment, the pre-pressure is 5 MPa and the holding time is 30 s. After the green sheet passes the appearance inspection, it is sent to a box furnace for sintering to further form grains and grain boundaries and obtain a dense ceramic structure. In this embodiment, an air atmosphere is used during the sintering stage, the heating rate is 5℃ / min, the sintering temperature is 1280℃, and the holding time is 1 h. When it is necessary to improve the density or further optimize the pressure resistance, the sintering temperature can also be adjusted to the range of 1340~1380℃. After sintering, it is naturally cooled to form a positive temperature coefficient ceramic chip.

[0064] Conductive silver paste is coated onto the surface of a positive temperature coefficient ceramic chip and then sintered to form a conductive electrode layer on the surface of the positive temperature coefficient ceramic chip. A stable ohmic contact is formed between the conductive electrode layer and the positive temperature coefficient ceramic chip. After the formation of the conductive electrode layer, a positive temperature coefficient core is constituted. In order to ensure that the positive temperature coefficient core can be directly used for subsequent device assembly, it is preferable to test the positive temperature coefficient core for room temperature resistivity, Curie temperature, withstand voltage strength, thermal response time and recovery time.

[0065] In this embodiment, the Curie temperature is 120°C, and the cold resistance is screened using three levels: 500Ω, 1000Ω, and 2000Ω. The recovery time is no more than 1 minute, the power frequency withstand voltage is above 700Vac, and a positive temperature coefficient core with a withstand voltage of 9kV / mm, a thermal response time of 0.82s, and a high-temperature resistance decay of about 17% after 100 cycles is used as the positive temperature coefficient core for subsequent device assembly.

[0066] S2. A conductive electrode layer is formed on the positive temperature coefficient core. Multiple electrodes are connected in series and packaged according to the insulation packaging parameters. The electrode layer is then connected in series between the primary neutral point of the voltage transformer and ground to form a current limiting and harmonic elimination device. The zero-sequence voltage of the voltage transformer and the neutral point current flowing through the current limiting and harmonic elimination device are collected to form a joint discrimination signal.

[0067] S2.1. Assemble the positive temperature coefficient core as an electrode core unit. First, check the continuity, adhesion, and ohmic contact of the conductive electrode layer. Then, connect multiple electrode core units in series one by one to form a core series assembly. During the series connection stage, use high-strength PCB board soldering technology to connect the ceramic electrodes to alleviate the connection stress caused by thermal expansion and contraction, and to ensure that the core series assembly maintains stable electrical connection under current-carrying conditions. The number of multiple units in series is selected according to the rated voltage and withstand voltage requirements, so that the core series assembly maintains low resistance during normal operation and has the ability to continuously withstand changes in neutral point current during the resonant triggering stage.

[0068] S2.2. The core series assembly is encapsulated according to the insulation encapsulation parameters. First, the core series assembly is placed in an epoxy cylinder, and then 50-60 mesh high-purity quartz sand is poured in to isolate the core series assembly and internal connectors from the external environment and form an insulating body. An aluminum alloy heat sink is tightly assembled on the outside of the insulating body so that the heat after the current limiting and harmonic elimination device is activated can be dissipated in time. An epoxy resin outer insulating sleeve or a silicone rubber composite sleeve is further configured on the outside of the insulating body so that the current limiting and harmonic elimination device can simultaneously meet the requirements of external insulation, mechanical protection, moisture resistance, dirt resistance and installation fixation. After encapsulation, the current limiting and harmonic elimination device is formed.

[0069] S2.3. Connect the current-limiting and harmonic-eliminating device in series between the primary neutral point and ground of the voltage transformer, and complete the terminal connection and fixed installation so that the current-limiting and harmonic-eliminating device is in the neutral point branch current-carrying position; the open delta winding serves as the zero-sequence voltage output position, and the conduction path flowing through the current-limiting and harmonic-eliminating device serves as the neutral point current output position. After wiring, the zero-sequence voltage output by the open delta winding and the neutral point current flowing through the current-limiting and harmonic-eliminating device are collected respectively to form a dual-channel detection signal; during normal operation, the voltage of the voltage transformer neutral point to ground is lower than the device's action threshold, the current flowing through the current-limiting and harmonic-eliminating device is within the leakage current monitoring range, the current-limiting and harmonic-eliminating device maintains a low-resistance state, and has little impact on the voltage transformer neutral point potential, ground capacitance current, and measurement accuracy; when the neutral point displacement or excitation current increases, the neutral point current flowing through the current-limiting and harmonic-eliminating device increases accordingly, and the current-limiting and harmonic-eliminating device can directly accept the change in neutral point branch current.

[0070] S2.4. Arrange the dual-channel detection signals according to a unified sampling time scale to establish a one-to-one correspondence between the zero-sequence voltage sampling value and the neutral point current sampling value at the same time position, forming a continuous sampling signal group; after the continuous sampling signal group is formed, pair and associate the continuous sampling segments of the zero-sequence voltage with the corresponding continuous sampling segments of the neutral point current, so that each segment of the zero-sequence voltage change process corresponds to a segment of the synchronous neutral point current change process; after pairing and association, a joint discrimination signal is formed, which simultaneously retains the zero-sequence voltage change information and the neutral point current change information.

[0071] S3. Perform joint sequential discrimination on the joint discrimination signal to form a resonance suppression state and a ground monitoring state.

[0072] S3.1. Expand the joint discrimination signal according to the continuous sampling time scale. First, extract the continuous sampling segment of zero-sequence voltage from the joint discrimination signal. Then, perform frequency analysis on the continuous sampling segment of zero-sequence voltage to determine the dominant frequency component in each continuous sampling segment of zero-sequence voltage and form a frequency feature group. The frequency analysis is performed using the existing discrete spectrum analysis method. In the discrete spectrum analysis stage, the non-DC frequency component with the largest amplitude is retained as the dominant frequency component to ensure that the continuous sampling segment of zero-sequence voltage can be clearly distinguished between power frequency anomalies and frequency division anomalies.

[0073] In this embodiment, the zero-sequence voltage continuous sampling segment covers four consecutive power frequency cycles, and the sampling frequency is consistent with the unified sampling time scale, thereby ensuring a one-to-one correspondence between the dominant frequency component extraction results and the neutral point current continuous sampling segment.

[0074] S3.2. The frequency feature group is used for frequency screening. The abnormal states in the joint discrimination signal are grouped for the first time according to the relationship between the dominant frequency component and the power frequency. The abnormalities with the dominant frequency lower than the power frequency are classified into the frequency division resonance group, and the abnormalities with the dominant frequency maintaining the power frequency are classified into the power frequency dominant abnormality group. The abnormalities with the dominant frequency higher than the power frequency are classified into the non-power frequency abnormality record group. The frequency division resonance group corresponds to the abnormal state where the zero-sequence voltage dominant frequency shifts to a lower frequency. In this embodiment, it is manifested as the frequency division component around 25Hz dominating. The power frequency dominant abnormality group corresponds to the abnormal state where the zero-sequence voltage dominant frequency still maintains 50Hz. The power frequency dominant abnormality group also retains the neutral point current continuous sampling segment corresponding to the timing of the zero-sequence voltage continuous sampling segment. After frequency screening, the frequency grouping result is formed. The power frequency dominant abnormality group and the corresponding neutral point current continuous sampling segment are retained from the frequency grouping result to form the wave current discrimination object group.

[0075] It should be noted that by first performing frequency screening based on the relationship between the dominant frequency component of zero-sequence voltage and the power frequency, and then retaining the power frequency dominant anomaly group and the corresponding neutral point current continuous sampling segment for subsequent wave current discrimination, the sub-frequency resonance can be independently screened out from the joint discrimination signal, and the discrimination focus can be concentrated on the power frequency dominant anomaly range. This reduces the mutual interference between sub-frequency resonance and arc grounding, single-phase grounding, and fundamental frequency ferroresonance, and improves the pertinence and coherence of subsequent anomaly state identification. Compared with the existing technology that directly relies on a single zero-sequence voltage waveform or simultaneously discriminates multiple types of anomalies, the above processing method adds a pre-frequency screening process and continues to retain the neutral point current continuous sampling segment corresponding to the timing of the zero-sequence voltage continuous sampling segment after the first grouping. This allows the subsequent wave current discrimination to be based on the narrowed anomaly range and the synchronously retained current information. Therefore, it can more stably distinguish between sub-frequency resonance and power frequency dominant anomaly, and provide a more accurate discrimination basis for the subsequent formation of resonance suppression state and grounding monitoring state.

[0076] S3.3. Use the wave-current discrimination object group for wave-current discrimination. First, retrieve the zero-sequence voltage zero-crossing region waveform sequence and the neutral point current continuous change sequence from the wave-current discrimination object group. Then, extract the continuity and symmetry changes of the zero-sequence voltage zero-crossing waveform and the continuous change characteristics of the neutral point current to form a wave-current correlation feature group. The zero-sequence voltage zero-crossing region waveform sequence refers to the adjacent sampled waveform when the zero-sequence voltage changes from positive to negative or from negative to positive. The zero-sequence voltage zero-crossing waveform change characteristics mainly reflect the continuity, symmetry, and distortion degree of the waveform before and after the zero-crossing. The neutral point current continuous change sequence refers to the neutral point current change sequence in the same sampling period as the zero-sequence voltage zero-crossing region waveform sequence. The continuous change characteristics of the neutral point current mainly reflect the peak value change and effective value change of the neutral point current in the continuous power frequency cycle.

[0077] It should be noted that by synchronously extracting the waveform sequence of the zero-sequence voltage in the zero-crossing region of the wave-current discrimination object group with the continuous change sequence of the neutral point current in the same sampling period, and further forming a wave-current correlation feature group, the continuity, symmetry, and distortion degree of the zero-sequence voltage in the zero-crossing region can be correlated with the peak value change and effective value change of the neutral point current in the continuous power frequency cycle. This makes the abnormal state identification no longer based on a single voltage signal or a single current signal, but on the synchronous correlation between voltage waveform characteristics and current change characteristics. Therefore, it can more accurately distinguish between arc grounding, single-phase grounding, and fundamental frequency ferroresonance, which exhibit similar behavior in the zero-crossing region but have different current characteristics. This is different from existing technologies that mainly rely on the amplitude, frequency component, or... Compared to the method of judging zero-crossing distortion by observing it alone, the above processing method adds a continuous change sequence of neutral point current that strictly corresponds to the waveform sequence of the zero-crossing region of the zero-sequence voltage. It also unifies the characteristics of the zero-crossing waveform change and the continuous change characteristics of the neutral point current into the wave-current correlation characteristic group. This allows the subsequent judgment to reflect both the waveform distortion in the zero-crossing region and the continuous current change in the neutral point branch. This improves the pertinence, stability, and anti-interference ability of the internal subdivision of the power frequency dominant anomaly, reduces misjudgments caused by relying solely on a single voltage waveform, enhances the ability to distinguish the repeated fluctuation characteristics of arc grounding, the smooth zero-crossing characteristics of single-phase grounding, and the continuous current increase characteristics of fundamental frequency ferroresonance, and provides a more sufficient and direct basis for the subsequent formation of resonance suppression state and grounding monitoring state.

[0078] In this embodiment, the waveform sequence of the zero-sequence voltage zero-crossing region is formed by selecting multiple adjacent sampling points before and after each zero-crossing point. The continuous change characteristics of the neutral point current are compared by comparing two or more consecutive power frequency cycles to ensure that the differences between arc grounding, single-phase grounding and fundamental frequency ferroresonance can be identified under the same judgment criteria.

[0079] S3.4. The wave-current correlation feature group is used for abnormal state identification. Any abnormality in the zero-crossing region, such as abrupt change, gap, spike, or asymmetric change before and after, and the neutral point current fluctuates continuously, is classified into the arc grounding candidate group. Any abnormality in the zero-crossing region that is continuous and smooth and maintains a sinusoidal zero-crossing trend is classified into the single-phase grounding candidate group. Any abnormality in the zero-crossing region where at least one of the continuity and symmetry is disrupted and the neutral point current continues to increase is classified into the fundamental frequency ferroresonant candidate group. After the above identification, the candidate grouping results are formed.

[0080] Among them, any one of the following in the zero-crossing region is a sudden change, gap, spike, or asymmetrical change before and after the zero crossing: any one of the following is a local sudden change, local gap, spike response, or asymmetrical amplitude before and after the zero crossing; continuous smoothness and maintaining a sinusoidal zero-crossing trend: the zero-sequence voltage maintains a continuous and smooth change before and after the zero crossing and continues the sinusoidal zero-crossing shape; at least one of continuity and symmetry is disrupted: the zero-sequence voltage experiences a continuity interruption or symmetry imbalance before and after the zero crossing; continuous fluctuation of neutral point current: the neutral point current fluctuates repeatedly within the continuous power frequency cycle; continuous increase of neutral point current: the peak value or effective value of neutral point current maintains an upward trend within the continuous power frequency cycle.

[0081] In this embodiment, the candidate group for arc grounding mainly corresponds to the sudden change, gap, spike, and asymmetrical change state near the zero-sequence voltage zero-crossing point. The candidate group for single-phase grounding mainly corresponds to the state where the zero-sequence voltage is continuous and smooth and maintains a sinusoidal zero-crossing trend. The candidate group for fundamental frequency ferroresonant mainly corresponds to the state where at least one of the continuity and symmetry of the zero-sequence voltage is destroyed and accompanied by a continuous increase in the neutral point current. After differentiation, the candidate grouping results are formed.

[0082] S3.5. Perform state mapping on the candidate grouping results, mapping the fundamental frequency ferromagnetic resonance candidate group to the resonance suppression state, and mapping the arc grounding candidate group and single-phase grounding candidate group to the grounding monitoring state. After state mapping, a state discrimination result is formed. The state discrimination result includes both the resonance suppression state and the grounding monitoring state. The resonance suppression state corresponds to the abnormal state that needs to enter the current limiting and harmonic elimination action, and the grounding monitoring state corresponds to the abnormal state that maintains low resistance monitoring.

[0083] S4. The current limiting and harmonic elimination device changes from a low resistance state to a high resistance state as the neutral point current continues to increase under the resonance suppression state, raising the resistance of the neutral point branch and weakening the excitation current to destroy the resonance condition. Under the ground monitoring state, it maintains a low resistance state and restores the low resistance state after the neutral point current falls back, thus forming the result of the harmonic elimination action.

[0084] S4.1. The resonance suppression state and ground monitoring state in the state discrimination results are respectively mapped to the operation path of the current limiting and harmonic elimination device to form the operation criteria. In the operation criteria, the resonance suppression state corresponds to the operation path of the current limiting and harmonic elimination device switching from a low resistance state to a high resistance state, and the ground monitoring state corresponds to the operation path of the current limiting and harmonic elimination device maintaining a low resistance state and tracking the neutral point current drop process. After the operation criteria are formed, the current limiting and harmonic elimination device continues to maintain the connection position between the primary neutral point of the series-connected voltage transformer and ground, and continuously receives the joint discrimination signal and the state discrimination results, thereby ensuring that the operation process of the current limiting and harmonic elimination device is consistent with the state discrimination results.

[0085] S4.2. The resonance suppression state in the action criterion is used in the resonance suppression process. When the neutral point current flowing through the current limiting and harmonic elimination device continues to increase, the positive temperature coefficient core gradually heats up due to the heat generated by the current. After the temperature of the positive temperature coefficient core reaches the Curie temperature, the resistance of the positive temperature coefficient core increases rapidly, and the current limiting and harmonic elimination device changes from a low resistance state to a high resistance state. After the current limiting and harmonic elimination device changes to a high resistance state, the resistance of the neutral point branch increases synchronously, the current carrying capacity in the primary neutral point branch of the voltage transformer decreases, the excitation current corresponding to the core of the voltage transformer decreases accordingly, the energy exchange process between the nonlinear inductor and the capacitance to ground is weakened, and the amplitude of the zero-sequence voltage and the amplitude of the neutral point current begin to fall back, forming the resonance suppression result.

[0086] In this embodiment, the Curie temperature of the core obtained by S1 screening is 120°C, the thermal response time after the current limiting and harmonic elimination device is activated does not exceed 2s, and the thermal response time corresponding to the formula reaches 0.82s, so that the current limiting and harmonic elimination device can complete the resistance value switching from low resistance state to high resistance state before the fundamental frequency ferromagnetic resonance continues to develop.

[0087] S4.3. The ground monitoring state in the action criterion is used in the ground monitoring process. The current limiting and harmonic elimination device maintains a low resistance state in the ground monitoring state, so that the neutral point current flowing through the current limiting and harmonic elimination device is continuously monitored. The zero-sequence voltage and neutral point current continue to be synchronously acquired in accordance with the method of forming a joint discrimination signal according to S2. During the continuous ground monitoring state, the focus is on tracking the neutral point current decline process and the positive temperature coefficient core cooling process. The neutral point current decline refers to the gradual decrease of the peak value and effective value of the neutral point current within the continuous power frequency cycle. The positive temperature coefficient core cooling process refers to the gradual decrease of the positive temperature coefficient core temperature from the region above the Curie temperature to the region below the Curie temperature. After the neutral point current has completed its decline and the positive temperature coefficient core temperature has dropped below the Curie temperature, the current limiting and harmonic elimination device returns to the low resistance state, forming a recovery action result.

[0088] In this embodiment, the recovery time of the positive temperature coefficient core obtained by S1 screening does not exceed 1 minute. Within the recovery time range, the current limiting and harmonic elimination device continues to maintain stable electrical connection, thereby ensuring that there is no interruption of conduction or malfunction when transitioning from ground monitoring state to normal operation state.

[0089] S4.4. Merge the resonance suppression results and the recovery action results to form the harmonic elimination action results. The resonance suppression results are used to characterize the process by which the current limiting harmonic elimination device completes the action of increasing the resistance, raising the neutral point branch resistance, and weakening the excitation current under the resonance suppression state. The recovery action results are used to characterize the process by which the current limiting harmonic elimination device completes the action of maintaining low resistance, tracking the neutral point current drop, and recovering the positive temperature coefficient core temperature under the ground monitoring state. After merging the resonance suppression results and the recovery action results, the harmonic elimination action results simultaneously include the change in neutral point current, the change in positive temperature coefficient core temperature, and the change in the resistance of the current limiting harmonic elimination device.

[0090] S5. The results of the harmonic elimination action are used to coordinately adjust the core parameters with positive temperature coefficient, the number of multiple chips connected in series, the insulation packaging parameters, and the discrimination rules for the joint sequence, so as to form an optimized neutral point current limiting harmonic elimination parameter set.

[0091] S5.1. The results of the harmonic suppression action and the state judgment results are combined to form the parameter evaluation results. The parameter evaluation results include at least the neutral point current drop process, the zero-sequence voltage drop process, the positive temperature coefficient core temperature change process, the current limiting harmonic suppression device resistance change process, and the low resistance maintenance process and recovery action process under the ground monitoring state. After the parameter evaluation results are formed, it is first determined whether there are situations such as slow resistance switching, slow zero-sequence voltage drop, slow neutral point current drop, insufficient withstand voltage margin, slow heat dissipation, or excessive impact on normal operation during the resonance suppression process. Then, the corresponding judgment results are used for subsequent parameter correction to ensure that the parameter correction direction is consistent with the harmonic suppression action results.

[0092] S5.2. Use the parameter evaluation results to correct the parameters of the positive temperature coefficient core. When the parameter evaluation results show that the resonance suppression process is slow, lower the Curie temperature range and enhance the resistance jump characteristic. When the parameter evaluation results show that the impact of normal operation is too large, raise the cold resistance value. The Curie temperature range is used to determine the temperature range in which the positive temperature coefficient core changes from a low resistance state to a high resistance state. The cold resistance value is used to determine the input resistance level during normal operation. The resistance jump characteristic is used to determine the resistance change range after the positive temperature coefficient core enters the Curie temperature range.

[0093] In this embodiment, the Curie temperature range is maintained within 120±10℃, and the cold resistance is selected from three levels: 500Ω, 1000Ω, and 2000Ω. The resistance jump characteristic is maintained with a resistance change range of more than three orders of magnitude. When the parameter evaluation results show that the impact of normal operation is too large, the cold resistance is increased while maintaining the resistance jump characteristic, so that the low resistance connection is maintained during the normal operation stage and the rapid resistance increase is maintained during the resonance suppression stage. After correction, the positive temperature coefficient core parameter correction result is formed.

[0094] S5.3. The parameter evaluation results are used to correct the number of multiple series components, insulation encapsulation parameters, and heat dissipation path. When the parameter evaluation results show insufficient withstand voltage margin, the number of multiple series components is increased and the insulation encapsulation strength is improved. When the parameter evaluation results show insufficient thermal stability or slow recovery, the continuity of the heat dissipation path is enhanced. The number of multiple series components is used to determine the withstand voltage sharing capacity of the core series assembly. The insulation encapsulation parameters are used to determine the epoxy cylinder encapsulation thickness, quartz sand filling state, and external insulation sleeve configuration state. The heat dissipation path is used to determine the continuity of heat transfer from the positive temperature coefficient core to the aluminum alloy heat sink and the external environment. In this embodiment, the number of multiple series components is reselected according to the rated voltage and withstand voltage requirements. The insulation encapsulation parameters continue to use a combination of epoxy cylinder encapsulation, high-purity quartz sand filling, and epoxy resin external insulation sleeve or silicone rubber composite sleeve. The heat dissipation path maintains the continuous thermal conductivity relationship between the positive temperature coefficient core, the core series assembly, the insulation body, and the aluminum alloy heat sink. When the parameter evaluation results show that the heat dissipation is slow, the heat dissipation path and insulation encapsulation parameters are corrected first.

[0095] S5.4. The parameter evaluation results are used to correct the discrimination time period and discrimination order in the joint sequential discrimination. When the parameter evaluation results show that the resonance suppression state is formed too slowly, the discrimination time period corresponding to the zero-sequence voltage continuous sampling segment is extended. When the parameter evaluation results show that the ground monitoring state discrimination is unstable, frequency screening is performed first and then wave current discrimination is performed, and the discrimination time period corresponding to the zero-sequence voltage zero-crossing region waveform sequence and the neutral point current continuous change sequence is corrected. The discrimination time period is used to determine the value length of the zero-sequence voltage continuous sampling segment, the neutral point current continuous sampling segment, the zero-sequence voltage zero-crossing region waveform sequence, and the neutral point current continuous change sequence. The discrimination order is used to determine the execution order of frequency screening and wave current discrimination.

[0096] In this embodiment, the order of discrimination is maintained: frequency screening is performed first, followed by wave current discrimination. The discrimination period is maintained so that the continuous sampling segment of zero-sequence voltage covers four consecutive power frequency cycles. The continuous change characteristics of neutral point current are still compared using two or more consecutive power frequency cycles. When the parameter evaluation results show that the frequency division resonance is unclear, the discrimination period corresponding to the continuous sampling segment of zero-sequence voltage is extended first. When the parameter evaluation results show that the distinction between the candidate groups of arc grounding, single-phase grounding, and fundamental frequency ferroresonant resonance is insufficient, the discrimination period corresponding to the waveform sequence of zero-crossing region of zero-sequence voltage and the continuous change sequence of neutral point current is corrected first. After the above corrections, the discrimination rule correction result is formed.

[0097] S5.5. Match the correction results of the positive temperature coefficient core parameters, structural parameters, and discrimination rules accordingly. First, match the Curie temperature range, cold resistance, and resistance jump characteristics with the number of multiple chips connected in series, insulation packaging parameters, and heat dissipation paths to ensure that the positive temperature coefficient core's operating temperature, normal connection resistance, resistance jump capability, withstand voltage sharing capability, and heat dissipation capability remain consistent. Then, link the matched results with the discrimination time period and discrimination sequence to ensure that the timing of the joint sequential discrimination to form the resonance suppression state and ground monitoring state is consistent with the operation process of the current limiting and harmonic elimination device transitioning from a low resistance state to a high resistance state and from a high resistance state back to a low resistance state. After matching and linking, an optimized neutral point current limiting and harmonic elimination parameter set is formed. The optimized neutral point current limiting and harmonic elimination parameter set is used for subsequent positive temperature coefficient core formation, current limiting and harmonic elimination device formation, and joint sequential discrimination execution.

[0098] This embodiment also provides an electromagnetic voltage transformer current limiting and harmonic elimination device based on PTC characteristics, including: a core forming module, which uses donor doping, acceptor doping and glass phase modification to control barium titanate-based positive temperature coefficient ceramics to form a positive temperature coefficient core;

[0099] The signal coupling module forms a conductive electrode layer on the positive temperature coefficient core, connects multiple electrodes in series according to the number of electrodes connected in series and encapsulates them according to the insulation encapsulation parameters, and then connects them in series between the primary neutral point and ground of the voltage transformer to form a current limiting and harmonic elimination device. The zero-sequence voltage and neutral point current collected are arranged according to a unified sampling time scale to form a joint discrimination signal.

[0100] The sequence discrimination module performs joint sequence discrimination on the joint discrimination signal. The joint sequence discrimination includes performing discrete spectrum analysis on the continuous sampling segment of zero-sequence voltage to obtain the dominant frequency component, and performing wave-current correlation discrimination on the abnormality of the dominant frequency maintaining the power frequency and the corresponding continuous sampling segment of neutral point current to form resonance suppression state and grounding monitoring state.

[0101] In the resonance suppression state, the current limiting and harmonic elimination module changes from a low resistance state to a high resistance state as the neutral point current continues to increase, raising the resistance of the neutral point branch and weakening the excitation current to destroy the resonance condition. In the ground monitoring state, it maintains a low resistance state and returns to a low resistance state after the neutral point current falls back, thus forming the harmonic elimination action result.

[0102] The parameter optimization module uses the results of harmonic elimination to coordinately adjust the positive temperature coefficient core parameters, the number of multiple chips connected in series, the insulation encapsulation parameters, and the discrimination rules for the joint sequence, forming an optimized neutral point current limiting and harmonic elimination parameter set.

[0103] In summary, this invention integrates the material properties of the positive temperature coefficient core, the current-limiting and harmonic-eliminating structure of the neutral point branch, and the joint sequence discrimination of zero-sequence voltage and neutral point current into a single design. This enables the electromagnetic voltage transformer to respond to different modes of low-resistance connection, rapid resistance increase, and stable recovery under three operating conditions: normal operation, resonance development, and grounding monitoring. Compared with existing technologies, this invention no longer relies solely on fixed impedance adjustment or a single zero-sequence voltage criterion. Instead, it first performs stratified screening of abnormal states and then further distinguishes between power frequency dominant abnormalities. This improves the identification accuracy between frequency division resonance, fundamental frequency ferroresonance, arc grounding, and single-phase grounding, reducing misjudgments and malfunctions. Simultaneously, the positive temperature coefficient core can quickly transition from a low-resistance state to a high-resistance state when the neutral point current continuously increases, promptly raising the neutral point branch resistance and weakening the excitation current. This helps to disrupt the resonance maintenance conditions and suppress the continuous amplification of overvoltage and overcurrent. After the abnormality weakens, it can return to a low-resistance state, which is beneficial for balancing the continuity of grounding monitoring and the reusability of the device. Through the coordinated correction of the positive temperature coefficient core parameters, the number of series connections, the insulation encapsulation parameters, and the discrimination rules, the material's operational capability, structural withstand voltage capability, heat dissipation recovery capability, and discrimination response timing are kept matched, thereby improving the operational reliability, operational stability, and engineering applicability of the current limiting and harmonic elimination device.

[0104] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A current-limiting and harmonic-eliminating optimization method for electromagnetic voltage transformers based on PTC characteristics, characterized in that: include, Barium titanate-based positive temperature coefficient ceramics are formed by controlling donor doping, acceptor doping and glass phase modification to form a positive temperature coefficient core. A conductive electrode layer is formed on the positive temperature coefficient core. Multiple electrodes are connected in series and encapsulated according to the insulation encapsulation parameters. The electrode layer is then connected in series between the primary neutral point and ground of the voltage transformer to form a current limiting and harmonic elimination device. The zero-sequence voltage and neutral point current are collected and arranged according to a unified sampling time scale to form a joint discrimination signal. Joint sequential discrimination is performed on the joint discrimination signal. The joint sequential discrimination includes performing discrete spectrum analysis on the continuous sampling segment of zero-sequence voltage to obtain the dominant frequency component, and performing wave-current correlation discrimination on the anomaly of the dominant frequency maintaining the power frequency and the corresponding continuous sampling segment of neutral point current to form resonance suppression state and grounding monitoring state. Under the resonance suppression state, the current limiting and harmonic elimination device changes from a low resistance state to a high resistance state as the neutral point current continues to increase, raising the resistance of the neutral point branch and weakening the excitation current to destroy the resonance condition. Under the ground monitoring state, it maintains a low resistance state and restores a low resistance state after the neutral point current falls back, thus forming the result of the harmonic elimination action. The results of harmonic elimination are used to coordinately adjust the positive temperature coefficient core parameters, the number of multiple chips connected in series, the insulation packaging parameters, and the discrimination rules for the joint sequence, forming an optimized neutral point current limiting and harmonic elimination parameter set.

2. The current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics as described in claim 1, characterized in that: The formation of the positive temperature coefficient core specifically includes: Barium titanate-based powder is mixed with donor-doped components, ball-milled and dried to form a pre-calcined material; The pre-burned material is calcined and pulverized, then mixed with acceptor doping components and glass phase components to form modified powder. The modified powder is pressed into shape and sintered into sheets to form a positive temperature coefficient ceramic chip, and a conductive electrode layer is formed on the surface of the positive temperature coefficient ceramic chip to form a positive temperature coefficient core.

3. The current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics as described in claim 2, characterized in that: The aforementioned current-limiting and harmonic-eliminating device specifically includes: A positive temperature coefficient core with a conductive electrode layer formed on its surface is used as an electrode core unit. Multiple electrode core units are connected in series according to the number of multiple series-connected pieces to form a core series assembly. The core series assembly is encapsulated in epoxy resin and filled with quartz sand according to the insulation encapsulation parameters, and equipped with a heat sink and an outer insulating sleeve to form a current limiting and harmonic elimination device.

4. The current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics as described in claim 3, characterized in that: The formation of the joint discrimination signal specifically includes: The current limiting and harmonic elimination device is connected in series between the primary neutral point of the voltage transformer and ground to collect the zero-sequence voltage output by the open delta winding, and simultaneously collect the neutral point current flowing through the current limiting and harmonic elimination device to form a dual-channel detection signal. The dual-channel detection signals are arranged according to a unified sampling time scale to form a continuous sampling signal group; The zero-sequence voltage continuous sampling segment in the continuous sampling signal group is paired and associated with the corresponding neutral point current continuous sampling segment to form a joint discrimination signal.

5. The current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics as described in claim 4, characterized in that: The joint sequence discrimination includes frequency screening and wave flow discrimination.

6. The current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics as described in claim 5, characterized in that: The frequency screening specifically includes: Zero-sequence voltage continuous sampling segments are extracted from the joint discrimination signal, and discrete spectrum analysis is used to determine the dominant frequency components of each zero-sequence voltage continuous sampling segment to form a frequency feature group. Based on the frequency characteristic group, anomalies with a dominant frequency lower than the power frequency are classified into the frequency division resonance group, anomalies with a dominant frequency that remains at the power frequency are classified into the power frequency dominant anomaly group, and anomalies with a dominant frequency higher than the power frequency are classified into the non-power frequency anomaly record group, thus forming the frequency grouping result. The power frequency dominant anomaly group and the corresponding neutral point current continuous sampling segment in the retained frequency grouping results are used to form the wave-current discrimination object group.

7. The current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics as described in claim 6, characterized in that: The wave-current discrimination specifically includes: From the wave-current discrimination object group, retrieve the waveform sequence of the zero-crossing region of the zero-sequence voltage and the continuous change sequence of the neutral point current, extract the continuous and symmetrical change characteristics of the zero-crossing waveform of the zero-sequence voltage and the continuous change characteristics of the neutral point current, and form a wave-current correlation feature group; Based on the wave-current correlation characteristic group, anomalies such as sudden changes, gaps, spikes, and asymmetrical changes in the zero-crossing region, and continuous fluctuations in the neutral point current, are classified into the arc grounding candidate group; anomalies with continuous and smooth zero-crossing regions and a sinusoidal zero-crossing trend are classified into the single-phase grounding candidate group; and anomalies where at least one of the continuity and symmetry of the zero-crossing region is disrupted and the neutral point current continues to increase are classified into the fundamental frequency ferroresonant candidate group. The candidate groups for fundamental frequency ferromagnetic resonance are mapped to resonance suppression states, and the candidate groups for arc grounding and single-phase grounding are mapped to grounding monitoring states, thus forming state discrimination results.

8. The current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics as described in claim 7, characterized in that: The resulting harmonic elimination action specifically includes: Read the resonance suppression state and ground monitoring state from the state discrimination results to form the action criteria; Based on the action criteria, the current limiting and harmonic elimination device connected to the resonance suppression state switches from a low resistance state to a high resistance state as the neutral point current continues to increase, thereby raising the resistance of the neutral point branch and weakening the excitation current, thus forming a resonance suppression result. Based on the action criteria, the connected current limiting and harmonic elimination device is kept in a low resistance state under grounding monitoring, and the neutral point current drop process and the positive temperature coefficient core cooling process are tracked to form the recovery action result. The resonance suppression result and the recovery action result are merged to form the harmonic elimination action result.

9. The current limiting and harmonic elimination optimization method for electromagnetic voltage transformers based on PTC characteristics as described in claim 8, characterized in that: The formation of the optimized neutral point current limiting and harmonic elimination parameter set specifically includes: The results of harmonic elimination actions and state determination are combined to form parameter evaluation results; Based on the parameter evaluation results, when resonance suppression causes hysteresis, the Curie temperature range and resistance jump characteristics are corrected; when the impact of normal operation increases, the cold resistance is corrected; when the withstand voltage is insufficient, the number of multiple chips in series and the insulation package parameters are corrected; when the recovery process is prolonged, the heat dissipation path is corrected; when state discrimination causes hysteresis, the discrimination time period is corrected; when the state switching is unstable, the discrimination sequence is corrected, thus forming the parameter correction results. The Curie temperature range, cold resistance, and resistance jump characteristics in the parameter correction results are matched with the number of multiple chips connected in series, insulation packaging parameters, and heat dissipation paths. The matched structural parameters are then linked with the discrimination time period and discrimination sequence in the joint order discrimination to form an optimized neutral point current limiting and harmonic elimination parameter set.

10. A current-limiting and harmonic-eliminating device for an electromagnetic voltage transformer based on PTC characteristics, wherein the current-limiting and harmonic-eliminating optimization method for an electromagnetic voltage transformer based on PTC characteristics is described in any one of claims 1 to 9, characterized in that: include, The core forming module uses donor doping, acceptor doping and glass phase modification to control barium titanate-based positive temperature coefficient ceramics to form a positive temperature coefficient core; The signal coupling module forms a conductive electrode layer on the positive temperature coefficient core, connects multiple electrodes in series according to the number of electrodes connected in series and encapsulates them according to the insulation encapsulation parameters, and then connects them in series between the primary neutral point and ground of the voltage transformer to form a current limiting and harmonic elimination device. The zero-sequence voltage and neutral point current collected are arranged according to a unified sampling time scale to form a joint discrimination signal. The sequence discrimination module performs joint sequence discrimination on the joint discrimination signal. The joint sequence discrimination includes performing discrete spectrum analysis on the continuous sampling segment of zero-sequence voltage to obtain the dominant frequency component, and performing wave-current correlation discrimination on the abnormality of the dominant frequency maintaining the power frequency and the corresponding continuous sampling segment of neutral point current to form resonance suppression state and grounding monitoring state. In the resonance suppression state, the current limiting and harmonic elimination module changes from a low resistance state to a high resistance state as the neutral point current continues to increase, raising the resistance of the neutral point branch and weakening the excitation current to destroy the resonance condition. In the ground monitoring state, it maintains a low resistance state and returns to a low resistance state after the neutral point current falls back, thus forming the harmonic elimination action result. The parameter optimization module uses the results of harmonic elimination to coordinately adjust the positive temperature coefficient core parameters, the number of multiple chips connected in series, the insulation encapsulation parameters, and the discrimination rules for the joint sequence, forming an optimized neutral point current limiting and harmonic elimination parameter set.

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