Transformer direct current magnetic bias tolerance evaluation method, system, medium and equipment

Abstract

The invention discloses a transformer direct current magnetic bias tolerance evaluation method, system, medium and equipment in an HEMP E3 environment, and the method comprises the steps: calculating the voltage and current data of a transformer under different levels of direct current magnetic bias based on a transformer structure through employing a UMEC model; constructing a finite element simulation model of the transformer according to the voltage and current data, obtaining a temperature change time constant of the transformer under direct current magnetic bias and steady-state temperature rise of the transformer under different levels of direct current magnetic bias, and obtaining a coefficient K representing a relationship between the steady-state temperature rise of the transformer and the direct current magnetic bias level; obtaining a transformer hot-spot temperature iterative calculation formula through the coefficient K and a transformer time constant, and obtaining transformer hot-spot overheating time under different direct current levels through the iterative calculation formula and finite element calculation; according to the characteristics of large amplitude, fast change and short duration of HEMP E3 type DC magnetic bias, the endurance capability of the DC magnetic bias of the transformer is divided into three grades.

Description

Technical Field

[0001] This invention relates to the field of transformer technology under HEMP E3 environment, and particularly to a method, system, medium and equipment for evaluating the DC bias magnetic tolerance of transformers under HEMP E3 environment. Background Technology

[0002] A high-altitude electromagnetic pulse (HEMP) is generated by a nuclear explosion at a depth of 30 km or higher. Its late-stage component (HEMP E3) lasts approximately 1000 seconds. The E3 waveform in the IEC 61000-2-9 standard is as follows: Figure 1 As shown, the peak value is approximately 38.67 V / km, with a duration of 1–1000 s. In contrast, the GIC (Geomagnetically Induced Current) waveform caused by unipolar-earth or double-polarity unbalanced operation, or by geomagnetic storms due to solar activity, exhibits no clear pattern, with amplitudes typically ranging from 1–10 V / km. Its rise time is difficult to calculate, and it can last for hours or even days. Therefore, in comparison, the transformer DC bias caused by the HEMPE3 component is characterized by high amplitude, rapid rise, and short duration.

[0003] Current methods for assessing DC bias tolerance typically only consider steady-state temperature rise to assign a critical DC tolerance value, neglecting the differences in tolerance between different transformer types. The UMEC model, a Steinmetz-based equivalent magnetic circuit model, considers inter-phase interactions in addition to classical methods, allowing for accurate simulation of the core magnetic circuits of three-phase three-limb, three-phase five-limb, and single-phase three-limb transformers. Traditional methods for assessing transformer DC bias tolerance ignore the time dimension of temperature rise, primarily considering the steady-state temperature rise caused by DC bias, without taking into account the characteristics of DC bias in the HEMP E3 environment. Furthermore, existing evaluation standards for transformer DC bias tolerance are inconsistent and largely based on empirical conclusions, making it difficult to differentiate the tolerance of different transformer types. Therefore, it is necessary to develop a DC limit standard that includes a time dimension for assessing the DC bias tolerance of transformers in the HEMP E3 environment.

[0004] The information disclosed in the background section is only for enhancing the understanding of the background of this invention, and therefore may contain information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0005] This invention provides a method, system, medium, and equipment for evaluating the DC bias magnetic withstand capability of transformers under HEMP E3 conditions. It can solve the problem that conventional transformer DC bias magnetic withstand current evaluation methods fail to consider the high amplitude and short duration of HEMP E3, and at the same time consider the differences in withstand capability of different transformer core structures, thereby giving accurate evaluation results to improve the operational reliability of the equipment.

[0006] In a preferred embodiment of the transformer DC bias magnetic withstand capability assessment method under HEMP E3 environment, a unified magnetic equivalent circuit (UMEC) model is adopted based on the transformer structure. This model is a Steinmetz-based magnetic circuit equivalent model that takes into account the inter-phase interaction of the transformer, enabling accurate simulation of the transformer. Furthermore, this model has the advantage of not requiring information on the number of winding turns, core length, or core cross-sectional area. In this model, the transformer's transient operating equations can be written using the principle of magnetic permeability and flux balance in various parts of the transformer, while the nonlinear characteristics of its excitation branch can be calculated based on the "no-load test" data from the transformer's factory test data.

[0007] To calculate the voltage and current data of a transformer under different levels of DC bias using the UMEC model, it is first necessary to establish the transformer's magnetic circuit. Any winding and core of the transformer can be equivalent to a unified magnetic circuit. The UMEC model distinguishes between transformer core types and simulates three-phase three-limb transformers, three-phase five-limb transformers, and single-phase three-limb transformers to obtain the transformer's voltage and current data. Figure 3 , Figure 4 , Figure 5 The patent first uses UMEC models for different types of transformers. Then, based on the magnetic circuit model, it derives the inductance and resistance matrices of the transformer. These matrices reflect the self-inductance and mutual inductance relationships between the transformer windings and the resistance characteristics of the windings, which can be used for electromagnetic transient simulation and analysis. Secondly, considering the saturation characteristics of the core, this patent uses the no-load test data from the transformer's factory test data to obtain the loss characteristic curve of the transformer under no-load conditions and combines it with the corresponding voltage and current data to finally obtain the nonlinear relationship reflecting the saturation characteristics of the transformer core. Finally, using the data obtained above, the voltage and current data of the transformer under different loads and different DC conditions are calculated.

[0008] Then, a finite element simulation model of the transformer was established to calculate the steady-state temperature rise of the transformer under different DC levels. Due to the nonlinear characteristics of the transformer core, the finite element simulation of the transformer magnetic field under DC bias is difficult to converge. In order to reduce the simulation difficulty and reduce the amount of calculation, the voltage and current data of the transformer under DC bias obtained by the UMEC model were directly imported, and only the electric field and temperature field were simulated by finite element simulation to obtain the hot spot temperature rise of each part of the transformer under different DC levels.

[0009] In a preferred embodiment of the transformer DC bias magnetism tolerance assessment method under HEMP E3 environment, the coefficient K is used to characterize the relationship between the transformer's steady-state temperature rise and the DC bias magnetism level, where K is not a constant. The hot spot temperature rise of a large power transformer undergoing half-cycle saturation with DC variations is nonlinear, and the value of K also changes with the DC current and transformer type. By combining voltage and current data for finite element simulation of the transformer, and fitting the transformer temperature rise time constant τ to the simulated transformer temperature rise data, the transformer hot spot temperature rise can be... The calculation yielded, where This represents the temperature rise value of the transformer hot spot. Let τ be the steady-state hot spot temperature rise value under a certain DC injection, and τ be the calculated transformer temperature rise time constant. Based on the nonlinear asymptotic behavior of the transformer, and using finite element simulations under DC with different time lengths, the coefficient K, characterizing the relationship between the transformer's steady-state temperature rise and the DC bias level, is obtained, and a K-value lookup table is created. The expression for the coefficient K is: ,in Steady-state hotspot temperature rise value The corresponding DC value of the injection transformer.

[0010] In a preferred embodiment of the transformer DC bias magnetic withstand capability assessment method under HEMP E3 environment, the calculated K value is substituted into the time iteration formula to obtain the temperature rise value within the corresponding time step. The time iteration formula is: .

[0011] Where Δt is the time step of the iteration. This represents the DC value at each time step. This represents the temperature rise at each time step. To determine the sequence number, the specific temperature of the hot spot is obtained by superimposing the temperature rise values. The iterative calculation formula for the transformer hot spot temperature is: , in The top oil temperature, This represents the temperature rise at each time step. This refers to the hot spot temperature of the transformer.

[0012] In a preferred embodiment of the transformer DC bias tolerance assessment method under HEMP E3 environment, temperature limits corresponding to different load types of the transformer are determined based on the current and temperature limits under loads exceeding the nameplate rating. The various limits of the transformer are typically based on continuous operating conditions under the designed rated operating conditions. When the load exceeds the nameplate rating, the transformer will be subject to a certain degree of danger and accelerated aging. Transformers operating under HEMP E3 can be considered to be operating under loads exceeding the nameplate rating; therefore, it is necessary to specify the parameter limits under this condition to reduce the degree of danger. Since HEMP E3 is characterized by large DC bias amplitude, rapid change, and short duration, it can be considered a short-term emergency load.

[0013] According to GB / T 1094.7—2008 Load Guidelines for Oil-Immersed Power Transformers, the current limits for distribution transformers, medium-sized transformers, and large transformers under short-term emergency loads are 2.0 (pu), 1.8 (pu), and 1.5 (pu), respectively. The winding hot spot temperature limits and the temperature limits for metal parts in contact with fiber insulation materials for medium-sized and large transformers are 160℃ and 160℃, respectively. The hot spot temperature limits for other metal parts of medium-sized and large transformers are 180℃ and 180℃, respectively. The top oil temperature limit for medium-sized and large transformers is 115℃ and 115℃, respectively. The top oil temperature and hot spot temperature limits for distribution transformers under short-term emergency loads are not specified because controlling the duration of emergency loads on distribution transformers is generally impractical. It should be noted that when the hot spot temperature exceeds 140℃, bubbles may form, thus reducing the insulation strength of the transformer. Furthermore, temperature and current limits do not apply simultaneously. The current can be slightly lower than the limits in the table to meet the temperature restrictions. Conversely, the temperature can be lower than the limit in the table to meet the current limit requirements.

[0014] In a preferred embodiment of the transformer DC bias magnetic withstand capability assessment method under HEMP E3 environment, the HEMP E3 environment is defined as the late-stage component of a high-altitude electromagnetic pulse generated by a nuclear explosion at a depth of 30 km or more above the Earth's surface, lasting approximately 1000 seconds. Transformers operating under normal periodic load, long-term emergency load, short-term emergency load, and rated load are classified into categories I, II, III, and IV, respectively. Based on the characteristics of HEMP E3 type DC bias magnetic amplitude, rapid change, and short duration, and according to the duration the transformer can withstand under different DC levels, the transformer's DC bias magnetic withstand capability is divided into three levels: fully withstand (A), relatively withstand (B), and relatively sensitive (C). Level A withstands for approximately 30 minutes or more under this DC level, Level B withstands for 10-30 minutes, and Level C withstands for 10 minutes or less.

[0015] The electromagnetic parameters of transformers under HEMP E3 environment are calculated using the UMEC model, and the temperature changes of transformers are rapidly calculated using the finite element model. Temperature is used as the evaluation standard to quickly and accurately determine the transformer's condition. This method fully considers the differences in DC bias magnetic withstand capability among different types of transformers, as well as the temperature limits of transformers under different overheating load conditions. It proposes a DC bias magnetic withstand capability evaluation standard that includes a time dimension, effectively compensating for the shortcomings of traditional evaluation strategies that ignore the high amplitude and short duration characteristics of the HEMP E3 environment, and significantly improving the reliability of equipment operation.

[0016] A system for implementing the method includes:

[0017] The UMEC module, based on the transformer structure, uses the UMEC model to calculate the voltage and current data of the transformer under different levels of DC bias.

[0018] The finite element simulation module constructs a finite element simulation model of the transformer based on the voltage and current data, obtains the temperature change time constant of the transformer under DC bias and the steady-state temperature rise of the transformer under different levels of DC bias, and obtains the coefficient K characterizing the relationship between the steady-state temperature rise of the transformer and the DC bias level.

[0019] The calculation module determines the temperature limit corresponding to different load types of the transformer based on the current and temperature limits under load exceeding the nameplate rated value. It obtains the iterative calculation formula for the transformer hot spot temperature through the coefficient K and the transformer time constant. It then obtains the transformer hot spot overheating time under different DC levels through the iterative calculation formula and finite element calculation.

[0020] The evaluation module, based on the characteristics of HEMP E3 type DC bias magnetization—large amplitude, rapid change, and short duration—classifies the transformer's DC bias magnetization tolerance into three levels: fully tolerant (A), relatively tolerant (B), and relatively sensitive (C).

[0021] A computer storage medium including computer instructions that, when run on a computer, cause the computer to perform the method.

[0022] An electronic device includes: a memory, a processor, and a computer program stored in the memory and executable on the processor; wherein the processor executes the program to implement the method.

[0023] Compared with existing technologies, this invention has the following advantages: This invention uses the UMEC model combined with the finite element method to accelerate the calculation speed and accuracy of the evaluation; while the DC bias magnetic tolerance assessment standard that includes the time dimension effectively makes up for the shortcomings of traditional assessment strategies that ignore the characteristics of high amplitude and short duration in the HEMP E3 environment. At the same time, it specifies the transformer hot spot temperature limit according to the transformer overheat load type, gives the DC limitation idea of ​​transformer with time dimension consideration and specific evaluation results, and proposes an effective standard for evaluating the DC bias magnetic tolerance of transformers in the HEMP E3 environment. Attached Figure Description

[0024] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Obviously, the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any creative effort. Furthermore, the same reference numerals denote the same parts throughout the drawings.

[0025] In the attached diagram:

[0026] Figure 1 This is the HEMP E3 waveform diagram specified in the IEC 61000-2-9 standard described in this invention;

[0027] Figure 2 This is an evaluation flowchart of a method for evaluating the DC bias magnetic tolerance capability of a transformer in a HEMP E3 environment, as described in this invention.

[0028] Figure 3 This is a UMEC model diagram of a three-phase, three-limb transformer used for evaluating the DC bias magnetic withstand capability of transformers in the HEMP E3 environment, as described in this invention. Let N1i1, N3i3, ​​and N5i5 represent the magnetomotive force (MOF) of the primary windings of the three phases, respectively; P1, P3, and P5 represent the permeability of the core portion of the primary windings of the three phases; N2i2, N4i4, and N6i6 represent the MOF of the secondary windings of the three phases, respectively; P2, P4, and P6 represent the permeability of the core portion of the secondary windings of the three phases; and P7~P... 12 P is the leakage permeability of the winding. 13 、P 14 The magnetic permeability of the yoke is represented; the magnetomotive force of each winding and the magnetic permeability of the part of the iron core column to which it is wound form a series magnetic circuit, which is then connected in parallel with the leakage magnetic permeability of the corresponding winding to form their respective winding magnetic circuits; the magnetic circuits of the high-voltage winding and the low-voltage winding in the same phase are further connected in series, and at the same time, they need to be connected in parallel with the magnetic permeability of the transformer side column to construct a complete magnetic circuit structure for each phase; finally, by connecting the three-phase magnetic circuits in parallel, the UMEC model of the three-phase three-column transformer can be obtained.

[0029] Figure 4 This is a schematic diagram of the core structure of a three-phase five-limb transformer using the UMEC model in the HEMP E3 environment, as described in this invention. The following represent the magnetic flux of each branch: N1i1, N3i3, ​​and N5i5 represent the magnetomotive force (MOF) of the three-phase primary winding; P1, P3, and P5 represent the permeability of the core portion of the three-phase primary winding; N2i2, N4i4, and N6i6 represent the MOF of the three-phase secondary winding; P2, P4, and P6 represent the permeability of the core portion of the three-phase secondary winding; P7 and P8 represent the permeability of the side columns; and P9~P... 14 P represents the leakage permeability of each winding. 15 、P 16 The magnetic permeability of the yoke is used; the magnetomotive force of each winding is connected in series with the magnetic permeability of the part of the iron core column to which it is wound, and the whole is connected in parallel with the leakage magnetic permeability of the corresponding winding to form their respective winding magnetic circuits; the magnetic circuits of the high-voltage windings and the low-voltage windings of the same phase are further connected in series and connected in parallel with the magnetic permeability of the side columns on both sides to form the magnetic circuit structure of each phase; then the three-phase magnetic circuits are connected in parallel to construct a complete three-phase magnetic circuit model; at the same time, considering the magnetic permeability of the yoke, according to the geometric layout between the iron core and the windings, the magnetic permeability of the yoke is introduced into the overall magnetic circuit in series, thus obtaining the complete three-phase five-column transformer UMEC model;

[0030] Figure 5 This is a diagram of the core structure of a single-phase three-limb transformer using the UMEC model in the HEMP E3 environment, as described in this invention. N1i1 represents the magnetomotive force (MOF) of the primary winding, P1 represents the permeability of the portion of the primary winding wound around the core column, N2i2 represents the MOF of the secondary winding, P2 represents the permeability of the portion of the secondary winding wound around the core column, P3 represents the combined permeability of the two side columns, and P4 and P5 represent the leakage permeability of the winding. The MOF of the primary winding is connected in series with the permeability of the portion of the core column it is wound around, and the MOF of the secondary winding is also connected in series with the permeability of the corresponding portion of the core column. These circuits are then connected in parallel with the corresponding leakage permeability to form the magnetic circuit structure of the primary and secondary sides. The magnetic circuits on both sides are then connected in parallel with the combined permeability of the two side columns to form the complete UMEC model of a single-phase three-column transformer. Detailed Implementation

[0031] The following will refer to the appendix. Figures 1 to 5 Specific embodiments of the invention will be described in more detail below. While specific embodiments of the invention are shown in the accompanying drawings, it should be understood that the invention may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0032] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0033] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0034] A method for evaluating the DC bias magnetic withstand capability of transformers in HEMP E3 environment, the process of which is as follows: Figure 2 As shown in the diagram, firstly, a UMEC model is established based on the transformer type, and the voltage and current responses under DC bias are quickly calculated using this model. Then, the finite element method is used to determine the transformer's temperature rise time constant, and subsequently, the steady-state temperature rise under different DC levels is calculated. Based on this, a K-value table characterizing the relationship between steady-state temperature rise and DC bias level is constructed. According to the transformer's overheat load type, hotspot temperature limits are determined, and appropriate temperature limit standards are selected. Next, thermal analysis is conducted, calculating the hotspot overheating time based on different DC bias levels, and assessing its impact on equipment lifespan. Considering both the transformer's structural characteristics and load type, the final evaluation results of the transformer's DC bias tolerance under HEMP E3 environment are obtained.

[0035] Example 1:

[0036] This embodiment discloses a method for evaluating the DC bias magnetic withstand capability of transformers in a HEMP E3 environment. It includes the following steps:

[0037] Step 1: Based on a 6300 kVA 110 kV three-phase three-limb transformer, establish its UMEC model, as follows: Figure 3 As shown, calculate the voltage and current of the transformer under DC bias at different DC levels;

[0038] Step 2: Based on the transformer model, construct its finite element simulation model. Combine the voltage and current data calculated by the UMEC model to perform finite element simulation of the transformer temperature rise. Calculate the transformer's time constant by fitting the temperature rise data. The time was 31 minutes. Then, the coefficient K value, which characterizes the relationship between the steady-state temperature rise of the transformer and the DC bias level, was obtained by finite element simulation under a series of different DC bias conditions and compiled into a table.

[0039] Step 3: Based on the national standard "Load Guidelines for Oil-Immersed Power Transformers", the winding hot spot temperature of a medium-sized power transformer under normal cyclic load shall not exceed 120 ℃ and the top oil temperature shall not exceed 105 ℃. Under long-term emergency load, the winding hot spot temperature shall not exceed 140 ℃ and the top oil temperature shall not exceed 115 ℃. Under short-term emergency load, the winding hot spot temperature shall not exceed 160 ℃ and the top oil temperature shall not exceed 115 ℃.

[0040] Step 4: Using the time constant and K value lookup table for this three-phase three-limb transformer, the transformer temperature rise calculation formula is obtained as follows: If the top oil temperature is considered constant within a sufficiently small time step, then the hot spot temperature iterative formula is: The overheating time of transformer hotspots under different DC levels and different load types can be obtained by using this formula and finite element calculation.

[0041] Step 5: Based on the hot spot temperature limits corresponding to the overheated load type of the three-phase three-limb transformer in Step 3, and considering the characteristics of the HEMP E3 type—large DC bias amplitude, rapid change, and short duration—the classification standard is as follows: Grade A withstands for more than 30 minutes under this DC load; Grade B, relatively withstands for 10-30 minutes under this DC load; and Grade C, relatively sensitive, withstands for 10 minutes or less under this DC load. The DC bias withstand capability of the transformer under different DC levels and load types is evaluated according to the standard. The evaluation results are shown in Table 1. This provides a corresponding decision-making basis for the safe temperature operation of the transformer.

[0042] Table 1. Withstand Capacity of Three-Phase Three-Column Transformers

[0043]

[0044] This three-phase, three-limb medium-sized transformer can withstand a GIC caused by HEMP E3 components with an amplitude of 0~549A for more than 90 minutes under all load conditions; it can withstand a GIC caused by HEMP E3 components with an amplitude of 550~1449A for 30~89 minutes; it can withstand a GIC caused by HEMP E3 components with an amplitude of 1500~3000A for 4~29 minutes; and it can only withstand a GIC caused by HEMP E3 components with an amplitude greater than 3000A for less than 4 minutes.

[0045] Example 2:

[0046] This embodiment discloses a method for evaluating the DC bias magnetic withstand capability of transformers in a HEMP E3 environment. It includes the following steps:

[0047] Step 1, based on a 180 MVA 220 kV three-phase five-limb transformer, using the UMEC model as follows: Figure 4 As shown, calculate the voltage and current of the transformer under DC bias at different DC levels;

[0048] Step 2: Based on the transformer model, construct a finite element simulation model of the transformer. Combine the voltage and current data obtained from the UMEC model to perform finite element simulation of the transformer temperature rise. Calculate the transformer's time constant by fitting the transformer temperature rise data. The time was 28 minutes. Then, the coefficient K value, which characterizes the relationship between the steady-state temperature rise of the transformer and the DC bias level, was obtained by finite element simulation under a series of different DC bias conditions and compiled into a table.

[0049] Step 3: Based on the national standard "Load Guidelines for Oil-Immersed Power Transformers", the top oil temperature of a large transformer shall not exceed 105 ℃ under normal cyclic load, 115 ℃ under long-term emergency load, and 115 ℃ under short-term emergency load.

[0050] Step 4: Using the time constant and K-value lookup table for this three-phase five-limb transformer, the formula for calculating the transformer temperature rise is obtained as follows: If the top oil temperature is considered constant within a sufficiently small time step, then the hot spot temperature iterative formula is: The overheating time of transformer hotspots under different DC levels and different load types can be obtained by using this formula and finite element calculation.

[0051] Step 5: Based on the hot spot temperature limits corresponding to the overheated load type of the three-phase five-limb transformer in Step 3, and considering the characteristics of the HEMP E3 type—large DC bias amplitude, rapid change, and short duration—the classification standard is as follows: Grade A withstands for more than 30 minutes under this DC load; Grade B, relatively withstands for 10-30 minutes under this DC load; and Grade C, relatively sensitive, withstands for 10 minutes or less under this DC load. The DC bias withstand capability of the transformer under different DC levels and load types is evaluated according to the standard. The evaluation results are shown in Table 2. This provides a corresponding decision-making basis for the safe temperature operation of the transformer.

[0052] Table 2 DC withstand capability of three-phase five-limb transformers

[0053]

[0054] The above steps can be used to evaluate the DC bias magnetic tolerance capability of transformers under HEMP E3 environment.

[0055] Transformers operating under Class I (normal cyclic load) can withstand GIC caused by HEMP E3 components with amplitudes of 0~246A for more than 36 minutes at this DC level; they can withstand GIC caused by HEMP E3 components with amplitudes of 241~501A for 10~35 minutes; and they can withstand GIC caused by HEMP E3 components with amplitudes greater than 501A for less than 10 minutes.

[0056] Transformers operating under Class II and III (long-term emergency load and short-term emergency load) conditions can withstand GIC caused by HEMP E3 components with an amplitude of 0~303A for more than 35 minutes at this DC level; they can withstand GIC caused by HEMP E3 components with an amplitude of 304~501A for a total of 14~35 minutes; and they can only withstand GIC caused by HEMP E3 components with an amplitude greater than 501A for less than 14 minutes.

[0057] Example 3:

[0058] Step 1, based on a 334 MVA 500 kV single-phase three-limb autotransformer, using the UMEC model as follows: Figure 5 As shown, calculate the voltage and current of the transformer under DC bias at different DC levels;

[0059] Step 2: Based on the transformer model, construct its finite element simulation model. Combine the voltage and current data calculated by the UMEC model to perform finite element simulation of the transformer temperature rise. Calculate the transformer's time constant by fitting the temperature rise data. The simulation was performed for 35 minutes. Then, the coefficient K, which characterizes the relationship between the steady-state temperature rise of the transformer and the DC bias level, was obtained by finite element simulation under a series of different DC bias conditions and the results were compiled into a table.

[0060] Step 3: Based on the national standard "Load Guidelines for Oil-Immersed Power Transformers", the top oil temperature of a large transformer shall not exceed 105 ℃ under normal cyclic load, 115 ℃ under long-term emergency load, and 115 ℃ under short-term emergency load.

[0061] Step 4: Using the time constant and K value lookup table for this three-phase three-limb transformer, the transformer temperature rise calculation formula is obtained as follows: If the top oil temperature is considered constant within a sufficiently small time step, then the hot spot temperature iterative formula is: The overheating time of transformer hotspots under different DC levels and different load types was obtained using this formula and finite element calculations. Wherein;

[0062] Step 5: Based on the hot spot temperature limits corresponding to the overheated load type of the single-phase three-limb transformer in Step 3, and considering the characteristics of the HEMP E3 type—large DC bias amplitude, rapid change, and short duration—the classification standard is as follows: Grade A withstands for more than 30 minutes under this DC load; Grade B, relatively withstands for 10-30 minutes under this DC load; and Grade C, relatively sensitive, withstands for 10 minutes or less under this DC load. The DC bias withstand capability of the transformer under different DC levels and load types is evaluated according to the standard. The evaluation results are shown in Table 3. This provides a corresponding decision-making basis for the safe temperature operation of the transformer.

[0063] Table 3 DC bias magnetic withstand capability of single-phase three-limb transformers under different operating conditions

[0064]

[0065] The above steps can be used to evaluate the DC bias magnetic tolerance capability of transformers under HEMP E3 environment.

[0066] Transformers operating under Class I (normal cyclic load) can withstand GIC caused by HEMP E3 components with an amplitude of 0~174A for more than 30 minutes at this DC level; they can withstand GIC caused by HEMP E3 components with an amplitude of 174~295A for 10~30 minutes; and they can withstand GIC caused by HEMP E3 components with an amplitude greater than 295A for less than 10 minutes.

[0067] Transformers operating under Class II (long-term emergency load) conditions can withstand GIC caused by HEMP E3 components with an amplitude of 0-262A for more than 30 minutes at this DC level; and can withstand GIC caused by HEMP E3 components with an amplitude of 262-430A for a total of 10-30 minutes. Transformers operating under Class III (short-term emergency load) conditions can withstand GIC caused by HEMP E3 components with an amplitude of 0-262A for more than 30 minutes at this DC level; and can withstand GIC caused by HEMP E3 components with an amplitude of 276-446A for a total of 10-30 minutes.

[0068] The beneficial effects of this embodiment are as follows: the UMEC model is used to distinguish the differences between transformers with different core types, which improves the accuracy of the evaluation calculation; the temperature simulation combined with the finite element method accelerates the evaluation calculation speed; at the same time, the transformer hot spot temperature limit is specified according to the transformer overheat load type, and an effective standard and specific evaluation results are given for the evaluation of the DC bias magnetic withstand capability of the transformer including the time dimension, which effectively makes up for the shortcomings of the traditional evaluation strategy that ignores the characteristics of high amplitude and short duration in the HEMP E3 environment.

[0069] This invention enables accurate assessment of transformer withstand capability under high-amplitude, rapidly changing, and short-duration DC bias conditions. The UMEC model is used to rapidly calculate voltage and current data of the transformer windings under different DC bias levels. This model considers the differences in electromagnetic characteristics between transformers with different core structures, such as three-phase three-limb, three-phase five-limb, and single-phase three-limb transformers, enabling more accurate simulation of the electromagnetic characteristics of various transformers. This provides necessary foundational data for subsequent finite element simulations, improving the accuracy of the assessment results. Based on the voltage and current data calculated by the UMEC model, a finite element thermal simulation model of the transformer is constructed to predict the temperature change process and steady-state temperature rise of the transformer under DC bias conditions. This method can analyze the internal heat distribution of the transformer in detail, providing more accurate temperature rise predictions than traditional methods, helping to identify potential hot spots and thus ensuring the safe operation of the transformer. According to the national standard "Load Guidelines for Oil-Immersed Power Transformers," combined with the current and temperature limits for loads exceeding the nameplate rating, the temperature limits of the transformer under different types of loads are determined. This measure ensures that the maximum temperature rise limit that the transformer may face under actual operating conditions is fully considered during the assessment process, guaranteeing equipment safety. By utilizing the coefficient K (characterizing the relationship between steady-state temperature rise and DC bias level) and the transformer's time constant, an iterative formula for calculating hotspot temperature is derived. This formula allows for the prediction of changes in transformer hotspot temperature within a sufficiently small time step, thereby assessing its withstand capability under different DC bias levels. This method improves computational efficiency and enables rapid response to changes in DC bias. The transformer's withstand capability to DC bias is classified into three levels: A (fully withstands), B (relatively withstands), and C (relatively sensitive), based on the amplitude, rate of change, and duration of DC bias. This classification method facilitates rapid assessment of the transformer's safety status, guiding maintenance personnel to take appropriate protective measures and effectively preventing faults caused by overheating.

[0070] This evaluation method not only focuses on the steady-state temperature rise of the transformer under DC bias conditions but also introduces a time dimension. Through an iterative hotspot temperature calculation formula, it predicts the time required for the transformer's hotspot to reach an overheated state under different DC bias levels. This approach more closely reflects actual operating conditions and can comprehensively reflect the long-term thermal stability and safety of the transformer under DC bias. Compared with traditional methods relying on single finite element simulation, this invention effectively combines the UMEC model with finite element simulation, significantly improving calculation speed and shortening analysis time while ensuring the accuracy of thermal state evaluation. This method is suitable for evaluating extra-large transformers, significantly reducing reliance on computational resources while maintaining high simulation accuracy, thus enhancing the feasibility and practical value of engineering applications.

[0071] Although embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments and application fields described above. The specific embodiments described above are merely illustrative and instructive, and not restrictive. Those skilled in the art can make many other forms based on the guidance of this specification and without departing from the scope of protection of the claims of the present invention, and all of these are within the scope of protection of the present invention.

Claims

1. A method for evaluating the DC bias magnetic tolerance of a transformer under HEMP E3 environment, characterized in that, The steps include: S1) Based on the transformer structure, the voltage and current data of the transformer under different levels of DC bias are calculated using the UMEC model; S2) Construct a finite element simulation model of the transformer based on the voltage and current data, obtain the time constant of temperature change of the transformer under DC bias and the steady-state temperature rise of the transformer under different levels of DC bias, and obtain the coefficient K characterizing the relationship between the steady-state temperature rise of the transformer and the DC bias level. S3) Based on the current and temperature limits when the load exceeds the nameplate rated value, determine the temperature limits corresponding to different load types of the transformer; S4) The iterative calculation formula for transformer hot spot temperature is obtained by using coefficient K and transformer time constant. The overheating time of transformer hot spot under different DC levels is obtained by using the iterative calculation formula and finite element calculation. S5) Based on the characteristics of the HEMP E3 type DC bias magnetization, such as large amplitude, rapid change and short duration, the DC bias magnetization tolerance of the transformer is divided into three levels: fully tolerant A, relatively tolerant B, and relatively sensitive C.

2. The method for evaluating the DC bias magnetic tolerance of a transformer under HEMP E3 environment according to claim 1, characterized in that, Preferably, the UMEC model distinguishes between transformer core types and simulates three-phase three-limb transformers, three-phase five-limb transformers, and single-phase three-limb transformers to obtain transformer voltage and current data.

3. The method for evaluating the DC bias magnetic tolerance of a transformer under HEMP E3 environment according to claim 1, characterized in that, Finite element simulation of the transformer was performed using voltage and current data. The transformer temperature rise time constant τ was fitted using the obtained temperature rise data. Then, finite element simulations under different DC bias conditions were used to obtain the coefficient K values ​​characterizing the relationship between the transformer's steady-state temperature rise and the DC bias level, and a lookup table of K values ​​was created. The expression for coefficient K is as follows: ,in It is DC Steady-state hot spot temperature rise value under injection.

4. In a preferred embodiment of the transformer DC bias magnetic withstand evaluation method under HEMP E3 environment according to claim 3, the calculated K value is substituted into the time iteration formula to obtain the temperature rise value within the corresponding time step. The time iteration formula is as follows: In this context, Δt represents the time step of the iteration. This represents the DC value at each time step. This represents the temperature rise at each time step. To determine the sequence number, the specific temperature of the hot spot is obtained by superimposing the temperature rise values. The iterative calculation formula for the transformer hot spot temperature is as follows: , in This refers to the top oil temperature.

5. The method for evaluating the DC bias magnetic tolerance of a transformer under HEMP E3 environment according to claim 1, characterized in that, The tolerance level A is more than 30 minutes under DC bias, the relatively tolerant level B is 10-30 minutes under DC bias, and the relatively sensitive level C is 10 minutes or less under the same DC bias.

6. The method for evaluating the DC bias magnetic tolerance of a transformer under HEMP E3 environment according to claim 1, characterized in that, The HEMP E3 environment is characterized by the late-stage component of an electromagnetic pulse generated by a nuclear explosion at a depth of 30 km above the Earth's surface, lasting approximately 1000 seconds.

7. The method for evaluating the DC bias magnetic tolerance of a transformer under HEMP E3 environment according to claim 1, characterized in that, The UMEC model is based on Steinmetz's equivalent magnetic circuit model.

8. A system for implementing the method of any one of claims 1-7, characterized in that, It includes: The UMEC module, based on the transformer structure, uses the UMEC model to calculate the voltage and current data of the transformer under different levels of DC bias. The finite element simulation module constructs a finite element simulation model of the transformer based on the voltage and current data, obtains the temperature change time constant of the transformer under DC bias and the steady-state temperature rise of the transformer under different levels of DC bias, and obtains the coefficient K characterizing the relationship between the steady-state temperature rise of the transformer and the DC bias level. The calculation module determines the temperature limit corresponding to different load types of the transformer based on the current and temperature limits under load exceeding the nameplate rated value. It obtains the iterative calculation formula for the transformer hot spot temperature through the coefficient K and the transformer time constant. It then obtains the transformer hot spot overheating time under different DC levels through the iterative calculation formula and finite element calculation. The evaluation module classifies the transformer's withstand capability under DC bias into three levels: fully withstand (A), relatively withstand (B), and relatively sensitive (C), based on the duration the transformer can withstand under different DC levels. Among them, the withstand level A is more than 30 minutes under this DC, the relatively withstand level B is 10-30 minutes under this DC, and the relatively sensitive level C is 10 minutes or less under this DC.

9. A computer storage medium, characterized in that, The storage medium includes computer instructions that, when executed on a computer, cause the computer to perform the method as described in any one of claims 1-7.

10. An electronic device, characterized in that, The electronic device includes: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the method as described in any one of claims 1-7.

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