A reliability assessment method for power electronic transformers taking into account insulation life

Abstract

The present invention discloses a reliability assessment method for a power electronic transformer taking into account insulation life, and relates to the technical field of transformers. The method is centered on the insulation degradation between HFT layers of a high-frequency transformer, and includes: constructing a life model and a reliability model of a single-layer insulation material in an HFT insulation unit under different working conditions, further analyzing the stress conditions of the insulation, and calculating the reliability model of the HFT insulation unit according to the reliability model of the single-layer insulation material subjected to the maximum electrical stress. According to the reliability of the HFT insulation unit and in combination with the failure rate of peripheral power electronic devices, the series-parallel reliability model of the isolation link is calculated. According to the overall topological structure of the PET, the reliability model of the PET is calculated in combination with the series-parallel reliability model, and the life of the entire PET system is evaluated accordingly. The method constructs an insulation medium-HFT-PET multi-level system reliability assessment model taking into account insulation degradation, which can accurately evaluate the reliability and life of the PET.

Description

Technical Field

[0001] The invention relates to the technical field of transformers, and in particular to a reliability evaluation method for power electronic transformers taking insulation life into account. Background Art

[0002] Power Electronic Transformer (PET) is a new type of electrical equipment that deeply integrates power electronic technology and high-frequency transformer (HFT). Medium-voltage PET not only has flexible reactive power compensation, DC equipment access, and port fault isolation functions, but also has a high level of intelligence. It also has the functions of self-detection and communication with other intelligent devices. It has a wide range of application needs in the fields of high-speed train traction transmission, smart grid, and new energy grid connection.

[0003] HFT is a key component for PET voltage level conversion and isolation, and its operating frequency is generally greater than 10kHz. Under PET working conditions, the excitation voltage borne by the HFT winding is no longer a sine wave, and its insulation unit faces harsh and complex pulse voltage working conditions. Especially when the insulation design is unreasonable or there are internal defects, the failure problem becomes more serious under the continuous action of electrothermal stress, making the HFT insulation unit the short board of the entire PET system life.

[0004] However, most existing studies have stopped at the analysis of the characteristics of insulating dielectric films, without considering the reliability of PET including HFT and power electronic devices, and lack models and solutions for evaluating the long-term usable life of PET devices and systems from the insulation perspective. Therefore, it is urgent to carry out research on typical insulation degradation and life assessment of HFT, explore the laws of insulation degradation under high-frequency pulse stress, establish insulation life models under full working conditions and multiple factors, and PET reliability assessment methods taking into account insulation degradation, so as to promote the research and development and upgrading of power electronic electrical equipment and the reliable operation of new power systems. Summary of the invention

[0005] In view of the above problems and technical requirements, the inventors have proposed a reliability assessment method for power electronic transformers taking into account insulation life. The technical solution of the present invention is as follows:

[0006] A reliability assessment method for a power electronic transformer taking into account insulation life comprises the following steps:

[0007] Construct the life model and reliability model of the single-layer insulation material in the HFT insulation unit of the high-frequency transformer under different working conditions;

[0008] The electrical stress between HFT winding layers is analyzed, and the reliability model of the HFT insulation unit is calculated based on the reliability model of the single-layer insulation material subjected to the maximum electrical stress;

[0009] According to the reliability model of the HFT insulation unit and the failure rate of the power electronic devices around the HFT, the series-parallel reliability model of the isolation link is calculated;

[0010] According to the overall topological structure of PET, the reliability model of PET is calculated in combination with the series-parallel reliability model of the isolation link;

[0011] The reliability model of PET is used to evaluate the life of PET.

[0012] The further technical solution is to construct a life model of the single-layer insulation material in the HFT insulation unit of the high-frequency transformer under different working conditions, including:

[0013] Under each set working condition, the high-frequency transformer HFT is subjected to insulation degradation test by step-by-step pressurization to determine the breakdown voltage of a single-layer insulation material under the working condition. The working conditions include voltage waveform parameters, operating temperature and thickness of a single-layer insulation material.

[0014] a%~100% of the breakdown voltage of the single-layer insulating material under the working condition is selected as the experimental applied voltage U, and the voltage step is b%, and the life L of the single-layer insulating material under different applied voltages U is obtained. The UL characteristic equation of the single-layer insulating material under different working conditions is obtained by fitting as the life model of the single-layer insulating material.

[0015] A further technical solution is to obtain the life L of a single-layer insulating material under different applied voltages U, including:

[0016] The insulation degradation experiment was repeated under the same working conditions, and the Weibull distribution statistical method was used to calculate the average operating time for the single-layer insulation material to reach breakdown under different applied voltages U to characterize the life L of the single-layer insulation material.

[0017] Its further technical solution is that the high-frequency transformer HFT includes an iron core and a concentrically wound low-voltage winding and a high-voltage winding, the low-voltage winding is in the inner layer, the high-voltage winding is in the outer layer, and insulating material is arranged between the layers, and the insulating material is one of polyimide, polyester film, Nomex, and epoxy resin materials.

[0018] The further technical solution is to construct a reliability model of the single-layer insulation material in the HFT insulation unit of the high-frequency transformer under different working conditions, including:

[0019] The life model MTTF of the single-layer insulation material in the HFT insulation unit of the high-frequency transformer under different working conditions is converted into the corresponding reliability model R(t). The conversion formula is expressed as follows:

[0020]

[0021] Where λ is the failure rate.

[0022] A further technical solution is to calculate the reliability model of the HFT insulation unit according to the reliability model of the single-layer insulation material subjected to the maximum electrical stress, including:

[0023] Assume that there are n single layers of insulating material subjected to the maximum electrical stress in HFT. H layer, reliability is R i , and i=1,2…,n H , then the reliability model of the HFT insulation unit is:

[0024]

[0025] A further technical solution is to calculate the series-parallel reliability model of the isolation link based on the reliability model of the HFT insulation unit and the failure rate of the power electronic devices outside the HFT, including:

[0026] In the power electronic transformer PET, the isolation link includes n D There are isolated converter modules connected in parallel, each of which includes an HFT and its peripheral power electronic devices. The series reliability model of the isolated converter modules is:

[0027] R con =R HFT ·R ele ;

[0028] In the formula, R HFT is the reliability model of HFT insulation unit; R ele is the reliability model of power electronic devices, which is obtained based on the failure rate of power electronic devices;

[0029] The series-parallel reliability model of the isolation link is calculated based on the series reliability model of the parallel-connected isolation converter module.

[0030] A further technical solution is to calculate the series-parallel reliability model of the isolation link according to the series reliability model of the isolation converter module in parallel form, including:

[0031] When not less than k D When the number of isolation converter modules is working normally, the isolation link is working normally. When the number of failed isolation converter modules is greater than n D -k DWhen the isolation link fails, the series-parallel reliability model of the isolation link is:

[0032]

[0033] In the formula, is the series reliability model of the i-th isolation converter module.

[0034] A further technical solution is to calculate the reliability model of the PET based on the overall topological structure of the PET and the series-parallel reliability model of the isolation link, including:

[0035] The overall topological structure of PET includes the rectifier link, isolation link and inverter link connected in sequence, so the reliability model of PET is:

[0036] R sys =R rec ·R iso ·R inv ;

[0037] In the formula, R rec , R iso , R inv They are respectively the reliability model of the rectification link, the series-parallel reliability model of the isolation link and the reliability model of the inverter link, wherein the reliability models of the rectification link and the inverter link are obtained according to the corresponding failure rates.

[0038] A further technical solution is to use the reliability model of PET to evaluate the life of PET, including:

[0039] The reliability model of PET is converted into the life of PET, which is expressed as:

[0040]

[0041] The beneficial technical effects of the present invention are:

[0042] The present application discloses a reliability assessment method for power electronic transformers taking into account insulation life. The method takes high-frequency transformer insulation degradation as the core, constructs a reliability model from the reliability of a single-layer insulation material, to the reliability of the HFT insulation unit, and then to the reliability model of the entire PET system, thereby evaluating the PET life.

[0043] This method establishes an insulation life model under all operating conditions and multiple factors and a PET reliability model taking into account insulation degradation, which can be converted into each other, thereby promoting the research and development and upgrading of power electronic electrical equipment and the reliable operation of new power systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1This is a flow chart of a reliability assessment method for power electronic transformers taking into account insulation life provided in this application.

[0045] Figure 2 This is a schematic diagram of the electrical stress analysis between high-frequency transformer winding layers provided in this application.

[0046] Figure 3 This is the overall topology of the power electronic transformer provided by this application. DETAILED DESCRIPTION

[0047] The specific implementation of the present invention will be further described below in conjunction with the accompanying drawings.

[0048] Please refer to Figure 1 As shown, the present application provides a method for evaluating the reliability of a power electronic transformer taking into account the insulation life, which specifically includes the following steps:

[0049] Step 1: Construct a life model and reliability model of a single-layer insulating material in the insulation unit of a high-frequency transformer HFT under different working conditions. The high-frequency transformer HFT includes an iron core and a concentrically wound low-voltage winding and a high-voltage winding. The low-voltage winding is in the inner layer, and the high-voltage winding is in the outer layer. Insulating materials are provided between layers and turns. The insulating material may be one of polyimide, polyester film, Nomex, and epoxy resin materials. In this embodiment, the rated frequency of the HFT is selected to be 20kHz, and the layers are wrapped with polyimide insulating film to improve the insulation strength. The turns ratio of the high-voltage winding to the low-voltage winding is 28:14. For the insulation unit of the above-mentioned HFT, the internal defect discharge of the HFT can be equivalent to a ball-plate discharge model, that is, the high-voltage winding is regarded as a ball model, the low-voltage winding is regarded as a plate model, and insulating films of different thicknesses are placed between the ball model and the plate model.

[0050] The life model of the single-layer insulation material in the insulation unit of the high-frequency transformer HFT under different working conditions is constructed specifically including: 1) Under each set working condition, the high-frequency transformer HFT is subjected to an insulation degradation experiment by step-by-step pressurization to determine the breakdown voltage of the single-layer insulation material under the working condition, and the working condition includes voltage waveform parameters (pulse width time, operating frequency, rise time), operating temperature and thickness of the single-layer insulation material. 2) 25% to 100% of the breakdown voltage of the single-layer insulation material under the working condition is selected as the experimental applied voltage U, and the pressurization step is 10%, and the life L of the single-layer insulation material under different applied voltages U is obtained, and the UL characteristic equation of the single-layer insulation material under different working conditions is obtained by fitting as the life model of the single-layer insulation material. In 2), obtaining the life L of the single-layer insulation material under different applied voltages U specifically includes: repeating the insulation degradation experiment under the same working condition, and using the Weibull distribution statistical method to count the average operating time (Mean time to failure, MTTF) of the single-layer insulation material to reach breakdown under different applied voltages U, and characterizing the life L of the single-layer insulation material. When the voltage frequency is 20kHz, the rise time is 50ns, the duty cycle is 50%, and the operating temperature is 95°C, the UL characteristic equations of single-layer insulation materials of different thicknesses are shown in Table 1.

[0051] Table 1. UL characteristic equations for single-layer insulation materials of different thicknesses under given operating conditions

[0052]

[0053] *R 2 Represents the fitting error

[0054] When the applied voltage U is fixed at 2 kV, the characteristic equation of the thickness and life of the single-layer insulation material can also be obtained through experiments, as shown in formula (1). This formula can predict that under the conditions of voltage 2 kV and operating temperature 95 ° C, the life of the single-layer insulation material with a thickness of 2 mm is 12.68 years.

[0055] L(d)=3.04·d 2.06 (year)(U=2kV)(1)

[0056] The reliability model of the single-layer insulation material in the high-frequency transformer HFT insulation unit under different working conditions is constructed. Specifically, the reliability of each component of the power electronic transformer PET can be expressed as R(t)=e -λt , where λ is the failure rate; and the MTTF of the insulating material is related to reliability and is expressed as Therefore, the MTTF and reliability R of the insulating material can be converted to each other, so that the life model MTTF of the single-layer insulating material in the high-frequency transformer HFT insulation unit under different working conditions can be converted into the corresponding reliability model R(t). In this embodiment, the reliability model of the single-layer insulating material under given working conditions is obtained according to formula (1):

[0057]

[0058] Step 2: Analyze the electrical stress between HFT winding layers, and calculate the reliability model of the HFT insulation unit based on the reliability model of the single-layer insulation material subjected to the maximum electrical stress.

[0059] The interlayer insulation with the greatest electrical stress in the HFT is damaged first, leading to transformer damage. Therefore, the life and reliability of the HFT insulation unit require analysis of the maximum electrical stress on the HFT interlayer insulation. Assume that there are n single layers of insulation material with the greatest electrical stress in the HFT. H layer, reliability is R i (i=1,2…,n H ), then the reliability model of the HFT insulation unit is:

[0060]

[0061] For the HFT provided in this embodiment, the electrical stress analysis between winding layers is as follows: Figure 2 As shown, assuming that the HFT primary voltage amplitude is U in According to the turns ratio and Z-type winding method of HFT, and simplifying the electrical stress between windings to a linear distribution, the maximum stress between high-voltage winding layers is U in / 2. Since the transformation ratio between the high voltage winding and the low voltage winding is 2:1, the maximum voltage of the HFT secondary winding is also U in / 2, the electrical stress on the insulation between the high-voltage winding and the low-voltage winding decreases from both ends to the inside, and the maximum electrical stress at both ends is also U in / 2. Since the maximum electrical stress between high-voltage winding layers is the same as that between high-voltage and low-voltage winding layers, and the reliability model of single-layer insulation materials is also shown in formula (2), the reliability model of the HFT insulation unit provided in this example can be determined according to formula (3):

[0062] R HFT =R 2 (4)

[0063] Step 3: Based on the reliability model of the HFT insulation unit and the failure rate of the power electronic devices around the HFT, calculate the series-parallel reliability model of the isolation link. Figure 3The life of the PET system of the 10kV distribution network shown in the figure is evaluated. The high-voltage side uses a modular multi-level converter as the rectifier input link, the medium-voltage side uses a dual active bridge (DAB) as the DC-DC isolation converter, and the low-voltage side is a three-phase inverter output. The rectifier link and DAB isolation link use IGBT modules of model 5SNA 0800N330100, with a maximum rated voltage of 3.3kV and a design voltage amplitude of 1.9kV. The failure rates of each link and component of PET can be obtained by referring to the literature, as shown in Table 2.

[0064] Table 2. Failure rates of various links and components of PET

[0065]

[0066] Isolation links include n D There are isolated converter modules connected in parallel, each of which includes an HFT and its peripheral power electronic devices. The series reliability model of the isolated converter modules is: R con =R HFT ·R ele , where R ele is the reliability model of power electronic devices, which is obtained according to the failure rate of power electronic devices. For the DAB isolation converter module provided in this example, it includes a high-voltage H-bridge (4 high-voltage side power devices H_IGBT), HFT and a low-voltage H-bridge (4 low-voltage side power devices L_IGBT) connected in sequence, as well as two identical voltage stabilizing capacitors C1 and C2, a resonant capacitor Cr and a resonant inductor Lr. According to the series form of R con Model expression, by incorporating the reliability of peripheral power electronic devices, the series reliability model of the DAB isolation converter module is calculated as:

[0067]

[0068] Finally, the reliability model of the isolation link is calculated based on the series reliability model of the parallel isolation converter module, which is named the series-parallel reliability model of the isolation link. Specifically, it includes: when no less than k D When the number of isolation converter modules is working normally, the isolation link is working normally. When the number of failed isolation converter modules is greater than n D -k D When the isolation link fails, the series-parallel reliability model of the isolation link is:

[0069]

[0070] In the formula, is the series reliability model of the i-th isolation converter module.

[0071] In this embodiment, the DAB isolation link adopts 5 DAB isolation converter modules in parallel. It is assumed that the DAB isolation link works normally when at least 2 DAB isolation converter modules are in normal operation. Combining equations (5) and (6), the series-parallel reliability model R of the DAB isolation link is: iso for:

[0072]

[0073] Step 4: Calculate the reliability model of PET based on the overall topology of PET and the series-parallel reliability model of the isolation link. The overall topology of PET includes the rectifier link, isolation link and inverter link connected in sequence. Failure of any link in the rectifier link, DAB isolation link and inverter link will cause the PET system to fail, which can be regarded as a series reliability system. Then the reliability model of PET is:

[0074] R sys =R rec ·R iso ·R inv (7)

[0075] In the formula, R rec , R iso , R inv They are respectively the reliability model of the rectifier link, the series-parallel reliability model of the isolation link and the reliability model of the inverter link, wherein the reliability models of the rectifier link and the inverter link are obtained according to the corresponding failure rates. In this embodiment, the reliability model of the rectifier link is: R rec =e (-0.0104t) , the reliability model of the inverter link is: R inv =e (-0.0623t) .

[0076] Step 5: Use the PET reliability model to evaluate the PET lifetime.

[0077] According to the conversion relationship between the MTTF and reliability R of the insulating material mentioned in step 1, the reliability model (7) of PET is converted into the life of PET, which is expressed as:

[0078]

[0079] The final calculation shows that when the thickness of the insulating material is 2 mm, the life of the PET system is 5.87 years.

[0080] The above is only a preferred embodiment of the present application, and the present invention is not limited to the above embodiments. It is understood that other improvements and changes directly derived or associated by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included in the protection scope of the present invention.

Claims

1. A reliability assessment method for power electronic transformers taking into account insulation life, characterized in that: The method comprises: Construct the life model and reliability model of the single-layer insulation material in the HFT insulation unit of the high-frequency transformer under different working conditions; The electrical stress between HFT winding layers is analyzed, and the reliability model of the HFT insulation unit is calculated based on the reliability model of the single-layer insulation material subjected to the maximum electrical stress; According to the reliability model of the HFT insulation unit and in combination with the failure rate of the power electronic devices around the HFT, a series-parallel reliability model of the isolation link is calculated; According to the overall topological structure of the PET, the reliability model of the PET is calculated in combination with the series-parallel reliability model of the isolation link; evaluating the life of the PET using the reliability model of the PET; Among them, the life model of the single-layer insulation material in the HFT insulation unit of the high-frequency transformer under different working conditions is constructed, including: Under each set working condition, the high-frequency transformer HFT is subjected to insulation degradation test by step-by-step pressurization to determine the breakdown voltage of a single-layer insulation material under the working condition. The working conditions include voltage waveform parameters, operating temperature and thickness of a single-layer insulation material. The a%~100% of the breakdown voltage of the single-layer insulation material under the working condition is selected as the experimental applied voltage U, and the voltage step length is b%, and the life L of the single-layer insulation material under different applied voltages U is obtained. The UL characteristic equation of the single-layer insulation material under different working conditions is obtained by fitting, which is used as the life model of the single-layer insulation material; Among them, the reliability model of the single-layer insulation material in the HFT insulation unit of the high-frequency transformer under different working conditions is constructed, including: The life model MTTF of the single-layer insulation material in the HFT insulation unit of the high-frequency transformer under different working conditions is converted into the corresponding reliability model R ( t ), the transformation formula is expressed as: ， ； In the formula, λ is the failure rate; The step of calculating the reliability model of the HFT insulation unit according to the reliability model of the single-layer insulation material subjected to the maximum electrical stress includes: Assume that the single layer of insulating material subjected to the maximum electrical stress in HFT is n H layer, the reliability is R i , and i=1,2…, n H , then the reliability model of the HFT insulation unit is: According to the reliability model of the HFT insulation unit and the failure rate of the power electronic devices around the HFT, the series-parallel reliability model of the isolation link is calculated, including: In the power electronic transformer PET, the isolation link includes n D There are isolated converter modules connected in parallel, each of which includes an HFT and its peripheral power electronic devices. The series reliability model of the isolated converter modules is: ； In the formula, R HFT It is the reliability model of HFT insulation unit; R ele is the reliability model of power electronic devices, which is obtained based on the failure rate of power electronic devices; Calculate the series-parallel reliability model of the isolation link according to the series reliability model of the isolation converter module in parallel form; According to the overall topological structure of PET, the reliability model of PET is calculated in combination with the series-parallel reliability model of the isolation link, including: The overall topological structure of the PET includes a rectifier link, an isolation link and an inverter link connected in sequence, and the reliability model of the PET is: ； In the formula, R rec , R iso , R inv They are respectively the reliability model of the rectifier link, the series-parallel reliability model of the isolation link and the reliability model of the inverter link, wherein the reliability models of the rectifier link and the inverter link are obtained according to the corresponding failure rates; The use of the reliability model of the PET to evaluate the life of the PET includes: The reliability model of the PET is converted into the life of the PET, which is expressed as: 。 2. The reliability assessment method of power electronic transformer taking into account insulation life according to claim 1 is characterized in that: The life L of a single-layer insulating material under different applied voltages U is obtained, including: The insulation degradation experiment was repeated under the same working conditions, and the Weibull distribution statistical method was used to calculate the average operating time for the single-layer insulation material to reach breakdown under different applied voltages U to characterize the life L of the single-layer insulation material.

3. The reliability assessment method of power electronic transformer taking into account insulation life according to claim 1, characterized in that: The high-frequency transformer HFT includes an iron core and a concentrically wound low-voltage winding and a high-voltage winding, wherein the low-voltage winding is in an inner layer, the high-voltage winding is in an outer layer, and insulating material is arranged between the layers, wherein the insulating material is one of polyimide, polyester film, Nomex, and epoxy resin materials.

4. The reliability assessment method of power electronic transformer taking into account insulation life according to claim 1, characterized in that: The series-parallel reliability model of the isolation link is calculated based on the series reliability model of the isolation converter module in parallel form, including: When not less than k D When the isolation converter module is working normally, the isolation link is working normally. When the failure isolation converter module is greater than n D -k D When the isolation link fails, the series-parallel reliability model of the isolation link is: ； In the formula, For the i Reliability model of series connection of 3 isolated converter modules.

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