Method for obtaining an improved transformer design for a power plant

Abstract

A method for obtaining an improved transformer design for a power plant, the method comprising the steps of: defining one or more critical temperatures (θ hw normal,θ hw contingency, θh, θo), determining the limit of at least one of these critical temperatures (θ h w,max normal,θ hw,max contingency, θh ,max ,θ o,max ), and obtaining an adjusted design of the transformer by using a data set of transformer loads, the data set comprising an amount of time (H) at a specific load level (κ) at a specific ambient temperature (θa). The method further comprises producing a transformer having design parameters (Δθ hr , Δθ or , R, x, y) and / or using a transformer with design parameters (Δθ hr , Δθ or , R, x, y) of the adjustment value of the transformer.

Description

Technical Field

[0001] The present disclosure relates to a method for obtaining an improved transformer design for a power plant. The transformer may be utilized in a renewable energy grid connection. As an example, the transformer may be utilized in a wind power plant. Background Art

[0002] Traditional transformers are designed for a power rating that represents a typical steady load and the ability to handle higher loads in emergency situations. Transformers are typically designed with a nameplate rating for maximum apparent power at a maximum ambient temperature.

[0003] However, renewable energy power plants are characterized by highly variable loads and a limited maximum load corresponding to the installed power generation. When using conventionally designed transformers for renewable energy grid connection, the transformer load is much smaller than its load capacity. As a result, these transformers are oversized, i.e. larger and more expensive than they need to be.

[0004] There are several reasons for oversizing. First, the nameplate rating is usually set to include a budget for emergency loads that do not occur in the case of a renewable energy collection grid. In addition, renewable energy generation depends on weather conditions that improve cooling performance. As an example, higher wind power generation is provided at low ambient temperatures. In addition, load variations and transformer thermal inertia mean that the temperature is always far from the steady-state limit. In addition, the maximum active power does not necessarily coincide with the maximum reactive power. In addition, the capacity factor is much lower compared to conventional transformers. In addition, there is limited value in maintaining a high insulation life after the end of the renewable energy plant life.

[0005] "Cost-optimized dynamic design of offshore wind power plant transformers with reliability and contingency measures" published by SHHKazmi et al. (International Journal of Power and Energy Systems, 2021) discloses a method for optimizing the design of offshore wind power plant transformers. Summary of the invention

[0006] Embodiments of the present disclosure are directed to a method for obtaining an improved transformer design.

[0007] According to one aspect of the present disclosure, a method for obtaining an improved transformer design for a power plant is disclosed. The method may include producing a transformer having an improved design. Specifically, a manufacturing machine may be controlled so that a transformer having an improved design is obtained. The manufacturing machine may be controlled by a processor configured to run an algorithm for obtaining improved design parameters of the transformer. The method may include using a transformer having an improved transformer design in a power plant.

[0008] In a method for obtaining an improved transformer design for a power plant, one or more critical temperatures of the transformer in operation are defined. For at least one of the critical temperatures, a limit is defined. For example, the limit can be defined by material parameters and expected life of the transformer.

[0009] In addition, a data set of transformer loads is provided. The data set may be an input to the processor. The data set may include the amount of time that one or more transformers or power plants are at a specific load level at a specific ambient temperature. For example, the data set may be provided by an operator or developer of a power plant. The data set may be data obtained over a representative number of years during operation of the power plant.

[0010] In a further method step, an expression for one or more of these critical temperatures dependent on the load level and dependent on design parameters related to the geometrical and / or electrical design of the transformer is provided. The expression may be an input to the processor or may be a value stored in the processor.

[0011] Then, the geometric and electrical design of the transformer is adjusted and adjusted values ​​of one or more of these design parameters are obtained so that the expression of at least one of these critical temperatures is closer to the determined limit than the previous design of the transformer. The design can be adjusted so that the value obtained from the expression is equal to the determined limit or differs from the determined limit by at most 50° C. As an example, the difference can be at most 10° C.

[0012] As an example, adjusting the geometric design may include changing the length, width, height, cross-section of components such as the internal transformer, including windings, core, tanks, and cooling equipment. Different materials may also be selected. For example, adjusting the electrical design may include changing the winding type from a disc winding to a layer winding. Since parameters may be related to each other, adjusting a particular parameter may require adjusting more parameters. For example, the values ​​of the design parameters may be obtained using standard simulation tools such as circuit models, finite element models, empirical relationships, etc.

[0013] By adjusting specific geometrical and electrical design parameters, adjusted characteristic design parameters of the transformer are also obtained.

[0014] When the adjustment value has been reached, a transformer with an improved design can be produced. A transformer according to the improved design can be selected from a plurality of transformers of different designs and used in a power plant. The method can also be used to verify whether the transformer meets the design criteria for use in the power plant when the transformer load level for a specific power plant is provided.

[0015] The method allows to obtain a transformer design in which critical temperature limits can be observed. Before building a power plant, the expected power generation and the limited life of components such as generators are usually known. By using this information in the design process of the transformer for grid connection, cheaper and more compact transformers can be manufactured. The expected load and ambient temperature are used as input data to appropriately size the transformer.

[0016] The improved transformer design may make the size and / or rating of the transformer as small as possible. The limit of the critical temperature calculated according to the improved design of the transformer may be as close as possible to the determined limit. As an example, the limit of the critical temperature rise relative to the environment of the improved design may differ from the determined limit by at most 10°C.

[0017] In the case where only one critical temperature with one limit is defined, the adjusted design can be accepted as an improved design. Other criteria can also be checked before accepting the design, and in the case that further criteria are not met, the design is further adjusted and the method is iterated until all criteria are met. Iterations are started by providing expressions and setting the design parameters to the adjusted design parameters.

[0018] In case several critical temperatures with temperature limits are defined, the method may include the step of checking whether all other limits among these critical temperatures are observed by the adjusted design, i.e., checking whether all other limits are lower than or equal to their corresponding limits. Likewise, other criteria may be checked. If a criterion is not met, the design may be further adjusted until all limits are observed, and the method is iterated until all criteria are met. The iteration is started by providing an expression and setting the predefined design parameters to the adjusted design parameters. When all criteria are met, the design is accepted.

[0019] As an example, the critical temperature may include at least one of a weighted hot spot temperature under normal load and a weighted hot spot temperature under emergency load. Both of these critical temperatures may be defined in the method. Operating the transformer under emergency load means that one of the other transformers in the power plant is out of operation, so that the full load of the power plant is redistributed to the remaining transformers.

[0020] Additionally, the critical temperature may include at least one of a top layer liquid temperature and a hot spot temperature at a maximum load and a maximum ambient temperature.

[0021] The data set for the transformer load may be provided as time quantities for the number of ambient temperature discretization steps and the number of load discretization steps. The sum of the time quantities may be equal to the number of hours in a year. Time intervals other than hours may be used.

[0022] The obtained improved transformer design can be configured for use in renewable energy power plants.As an example, the transformer can be used in a wind power plant.

[0023] According to another aspect, a transformer with an improved design obtainable by a method according to any of the preceding claims is disclosed. As an example, at least one critical temperature calculated by the expression in the step taking as input data the load level of the transformer or the load level of a power plant comprising the transformer and using the design parameters of the transformer is equal to the determined limit or differs therefrom by at most 50°C. The difference may be at most 10°C.

[0024] The present disclosure comprises several aspects and embodiments. Each feature described with respect to one of these aspects and these embodiments is also disclosed herein with respect to other aspects and embodiments, even if the corresponding feature is not explicitly mentioned in the context of the other aspects and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Further features, improvements and conveniences will become clear from the following description of exemplary embodiments in conjunction with the accompanying drawings. In the accompanying drawings, elements with the same structure and / or function may be represented by the same reference numerals. It should be understood that the embodiments shown in the accompanying drawings are illustrative representations and are not necessarily drawn to scale.

[0026] Figure 1 is a flow chart of a method for obtaining an improved transformer design,

[0027] Figure 2 is an example of a table of transformer load data. DETAILED DESCRIPTION

[0028] Figure 1 A flow chart of a method for obtaining an improved transformer design is shown. The method can be used for transformers in wind power applications. As an example, the method can be used for offshore wind, offshore wind, or submarine wind applications. The method can also be used for other applications, particularly renewable energy grid connections, such as solar energy.

[0029] The method may allow selection of a suitable rating of the transformer for dynamic overload. In this context, dynamic load means that the transformer is subjected to a varying load exceeding that according to the transformer rating. For dynamic overload, the overload temperature limit is higher than the temperature limit for normal cyclic load.

[0030] In this approach, absolute temperature limits are defined, especially for dynamic overloads. These temperature limits can be defined based on long-term emergency temperature limits in national and international industry standards such as IEC 60076-7 and IEC 60076-14. IEC 60076-7 states that long-term emergency loads can last for weeks or months, and lists negative effects as: aging of conductor insulation, aging of other insulation, increased contact resistance of tap changers, and embrittlement of gasket materials.

[0031] It is assumed that the tap changer and gasket materials are selected based on the temperature environment of the tap changer and gasket materials when the conductor insulation reaches its long-term emergency temperature limit, while the other insulation materials have a lower temperature compared to the conductor insulation. Therefore, the size of the dynamic overload is based only on the aging of the conductor insulation and the absolute temperature limit.

[0032] For applications with limited experience in high temperature operation, a lower temperature limit than the long-term emergency limit may be selected as such experience is gained.

[0033] By this method an improved transformer with a suitable rating can be obtained. In the following, the meaning of the term "rating of the transformer" is explained.

[0034] In a power plant (e.g. a wind power plant), the load level κ of the wind power plant can be defined as

[0035]

[0036] In the above equation, s is the apparent power output of the wind farm at a particular time, and S=max(s) is the maximum apparent power output of the wind farm.

[0037] During operation, the load level κ will vary between 0 and 1. Assume that this load is shared among a total of N transformers in the power plant, each with a power rating r. The rating of the transformer may be considered in accordance with IEC or any other relevant standard. The power rating r may be defined as the power output that the transformer can continuously deliver at a specified rated voltage without exceeding the internal temperature limit.

[0038] The load level K of each transformer, that is, the ratio of the apparent power output of each transformer at a specific time to the maximum apparent power output of each transformer during normal operation, is

[0039]

[0040] The maximum load level for a transformer with rating r is

[0041]

[0042] If the transformer rating r is chosen such that r = S / N, the load level K will vary from 0 to 1, but this is not generally the case. A typical grid connection will be designed with (N-1) redundancy to ensure that even if one transformer fails, the output of the entire grid connection can continue to be dispatched without exceeding the transformer rating. The rating of a transformer with (N-1) redundancy is

[0043]

[0044] As obtained from equations (4) and (5), the maximum load level of the transformer with (N-1) redundancy during normal operation is:

[0045]

[0046] The dynamic overload factor λ can be defined as the maximum load level of a transformer with a rating r and the load level of a transformer with a rating r (N-1) The ratio of the maximum load level of the transformer

[0047]

[0048] The load level K during normal operation of a transformer with a rating r as given in equation (2) is normal It can be expressed by the overload factor λ as

[0049]

[0050] During emergency operation, i.e. when one transformer stops operating, the full load of the wind farm is distributed to the remaining (N-1) transformers. The load level K of a transformer with rating r during emergency operation is contingency for

[0051]

[0052] It can be seen from equation (6) that, when the number of transformers and the maximum apparent power output of the power plant are known, determining the rating of the transformer is equivalent to determining λ.

[0053] In the following, reference Figure 1 The routine shown describes the method. In step A), the parameters related to the specific transformer design are defined. The characteristic parameters can be as follows:

[0054] Δθ hr : Gradient from hot spot to top liquid at rated current

[0055] Δθ or : Top liquid temperature rise at rated loss

[0056] R: Ratio of load loss to no-load loss under rated conditions

[0057] x: Liquid index depending on cooling mode

[0058] y: Winding index depending on cooling mode

[0059] In this context, the top liquid temperature or top oil temperature is the liquid temperature measured at the top of the transformer, which corresponds to the maximum liquid temperature in the transformer (this is due to the upward movement of hot liquid due to buoyancy). The top liquid temperature rise means subtracting the ambient temperature from the measured liquid temperature to obtain a value that is independent of the ambient temperature. Usually thermal tests are performed without applying voltage, Δθ or It is provided at rated losses because there are additional losses (no-load losses) which are voltage-dependent and also cause a temperature rise in the liquid.

[0060] The hot spot temperature is the maximum temperature in the transformer winding. This value can be measured or estimated. The hot spot to top liquid gradient means subtracting the top liquid temperature from the measured or estimated hot spot temperature. Δθ hr It is provided at rated current because there are no voltage induced losses in the windings.

[0061] The loss ratio R is the ratio of the load loss (based on current) to the no-load loss (based on voltage). x and y are modeling coefficients that describe Δθ hr and Δθ or How does it change for loads other than the rated power.

[0062] In addition, one or more critical temperature limits are defined during transformer operation. The critical temperature can be calculated in both normal operation and emergency operation. As an example, a weighted hot spot temperature θ for normal load is defined. hw normal and the weighted hot spot temperature θ of the emergency load hw contingency As an example, these limits can be calculated based on the aging model of Annex A of IEC60076-7 as follows:

[0063]

[0064] In the above equation, DP start is the initial degree of polymerization, and DP end The initial degree of polymerization depends on the manufacturing process, but can usually be taken as D start = 1000. The end-of-life criterion is usually defined as DP end= 200. In general, the degree of polymerization describes the aging of solid insulation materials of transformers. Solid insulation materials can include cellulose consisting of long polymer chains, where the mechanical strength decreases over time due to polymerization. Aging is accelerated at high temperatures or in the presence of moisture or oxygen.

[0065] In addition, A and E A are material parameters. Specifically, A and E A is an aging parameter based on the type of solid insulation material, humidity level and oxygen level. Assuming the humidity of the thermally modified paper is 1.5% and has low oxygen, then A = 3.0 10 4 h -1 , and E A =86 kJ / mol. L is the total life of the transformer, which is specified by the operator of the grid connection. As an example, L = 36 years. A subset of L is denoted C, which is the number of years the transformer is assumed to operate at emergency level. As an example, L = 1 year. The rest of the time the transformer is assumed to operate at normal level. In this example, LC = 35 years.

[0066] Seasonal variations can be quite large. Therefore, it can be helpful to express L and C as integer years. In particular, winter and summer weather conditions are different, resulting in different stresses on the transformer in winter and summer. When providing C as an integer, the difference between winter and summer can be ignored in the parameter.

[0067] Different values ​​of weighted hot spot temperature are provided for emergency load and normal load.

[0068] Assuming that 20% of the transformer life can be consumed under emergency load, the maximum weighted hot spot temperature is:

[0069]

[0070] For normal load, the maximum weighted hot spot temperature is:

[0071]

[0072] These values ​​are independent of the load data and geometry of the transformer. They depend only on the specified life, the specified operation at emergency level and the material parameters.

[0073] It is noteworthy that the obtained weighted average of the hot spot temperature at normal load is lower than 98°C, which is the arithmetic mean of the hot spot temperature of the transformer at a temperature rise of 78K according to IEC and an annual average ambient temperature of 20°C. This is because the weighted average gives more weight to higher temperatures. In addition, according to IEC 60076-7 Annex A, the life of thermally modified paper at a continuous temperature of 98°C and a humidity of 1.5% is less than 20 years, so the temperature must be reduced to achieve an expected life of the wind power plant of more than 20 years.

[0074] For different combinations of liquid and solid insulation materials in the transformer, further temperature limits may be defined according to standards such as IEC and IEEE. As an example, a temperature limit θ for the hot spot temperature may be defined as h,max and the temperature limit of the top liquid temperature and θ o,max .

[0075] Then, in step B) of the method, a data set of transformer loads is provided as input. The data set specifies the amount of time that one or more transformers or power plants are at a specific load level at a specific ambient temperature. For example, the data set may be provided by an operator or developer of a power plant. The data set may be data obtained over a representative amount of time, such as a representative number of years.

[0076] Figure 2 An example of a corresponding data set is shown. The data set is in the form of a two-dimensional histogram, which represents the annual power plant at or expected to be at a specific ambient temperature θ a The data set can also be the load of only one transformer or several transformers in a power plant.

[0077] The number of load discretization steps is denoted as n K , and the number of steps for the ambient temperature discretization is represented by n θa The sum of the hours must equal the number of hours in a year, that is,

[0078]

[0079] For example, the number of load discretization steps and the number of ambient temperature discretization steps can be selected in accordance with the customer. Figure 2 In the specific example shown, an ambient temperature discretization step size of 1°C and a load discretization step size of 0.05 are used. The maximum and minimum temperatures depend on the load data. In the data set shown, the ambient temperature varies between -13°C and 29°C. This makes n K =20 and n θa =42.

[0080] In a further step C), one or more critical temperature limits are set for the transformer during operation. These limits can be set for both normal operation and emergency operation.

[0081] The critical temperature is then expressed in dependence on the load data and parameters related to the design of the transformer. In certain cases, the expression further depends on parameters related to the rating of the transformer.

[0082] In the case where the expression contains a parameter related to the rated value of the transformer, the critical temperature in the expression is set to its limit, whereby the critical value of the parameter related to the rated value of the transformer can be obtained for each temperature.

[0083] In particular, in step C), the weighted hot spot temperatures θ of the normal load and the emergency load hw It can be calculated depending on the load data as follows:

[0084]

[0085] In this equation, E A is the activation energy, which is equal to 86×10 3 J / mol, and R g is the gas constant, equal to 8.314 J / mol / K.

[0086] In the expression, θ h is the transformer hot spot temperature, which can be calculated for each load level and ambient temperature level according to a dynamic thermal model (such as the IEC 60076-7 dynamic thermal model) as follows:

[0087]

[0088] In this equation, θ a is the ambient temperature, Δθ or is the top liquid temperature rise at rated current, K is the load level of the transformer, and R is the ratio of the load loss to the no-load loss at rated conditions. Therefore, θ h and θ hw Depends on the load level and design parameters of the transformer.

[0089] It is worth noting that the hotspot temperature for each load level is calculated as a steady-state value, and the expression including the hotspot temperature in equation (13) is multiplied by the number of hours at that load level. This method ignores thermal inertia, that is, the temperature of the transformer depends not only on the current load but also on the previous load. However, the analysis is done over a longer period of time and includes both load increases and load decreases. Assuming that the processes of load increase and load decrease are similar enough, the errors caused by ignoring thermal inertia will cancel out.

[0090] Other methods such as thermal hydraulic network models or computational fluid dynamics (CFD) can also be used to evaluate the hot spot temperature for each load level and ambient temperature level. If accuracy is the only concern, then CFD is the most appropriate. However, since CFD is time consuming and the aging at lower temperatures will be smaller and therefore less relevant, it is sufficient to examine the CFD results at higher temperature levels and generally use the IEC dynamic thermodynamic method.

[0091] For normal loads, the load level κ of the input data can be multiplied by a factor To represent the transformer load level κ in equation (14), where, according to equation (7), N is the number of transformers in the power plant and λ is the dynamic overload factor. λ is related to the transformer rating according to equation (6), where,

[0092] Then, the transformer hot spot temperature for the adjusted designed normal load can be expressed as:

[0093]

[0094] For emergency loads, the load level K of the transformer in equation (14) can be expressed by multiplying the load level κ of the input data by λ according to equation (8).

[0095] Therefore, the transformer hot spot temperature of the adjusted designed emergency load can be expressed as:

[0096]

[0097] In the above equations (15) and (16), the transformer temperature θ h normal and θ h contingency It is expressed as a function of the load κ and ambient temperature θ obtained from the input data set. a The dynamic overload factor λ is a function of . According to equation (13), when θ h normal and θ h contingency Replace the transformer temperature θ h When the normal load θ is obtained h normal The weighted hot spot temperature and emergency load θ h contingency The weighted hot spot temperature is taken as a function of the dynamic overload factor λ.

[0098] Then, when we put the θ in these equations hw normal and θ hwcontingency When set to the temperature limit obtained from equations (10) and (11), in step D), the critical value λ of the dynamic overload factor λ can be determined n and λ c , at these critical values, the weighted hot spot temperatures of the normal load and the emergency load are at predefined limits. This is equivalent to obtaining the critical values ​​r for the corresponding rated values n and r c .

[0099] Similarly, the top liquid temperature θ at maximum load O and the hot spot temperature θ under maximum load h It can be expressed in dependence on λ as follows:

[0100]

[0101]

[0102] The parameters are defined as follows:

[0103] θ a,max : Maximum ambient temperature

[0104] K max : Maximum load level of the transformer

[0105] Δθ hr : Gradient from hot spot to top liquid at rated current

[0106] Δθ or : Top liquid temperature rise at rated loss

[0107] R: Ratio of load loss to no-load loss under rated conditions

[0108] x: Liquid index depending on cooling mode

[0109] y: Winding index depending on the cooling mode.

[0110] In a further method step E), a critical value λ of the dynamic overload factor λ is compared c , n , o and λ h , and set the dynamic overload factor λ to the lowest critical value among these critical values.

[0111] Then, according to the selected critical value of the dynamic overload factor λ, in step F), the design of the transformer is adjusted. h , the design is adjusted to the maximum hot spot limit for maximum load and maximum ambient temperature. When the critical value is λ o, the design is adjusted to the maximum top liquid limit for maximum load and maximum ambient temperature. When the critical value is λ n , the design is adjusted to the maximum hot spot limit of the nameplate rating. When the critical value is λ c , the design is tuned to the maximum hot spot limits for maximum load and maximum ambient temperature. Tuning the design means determining the geometric and electrical parameters that meet critical criteria while allowing other criteria to exceed their corresponding limits.

[0112] As an example, a critical value λ may be used h The appropriate rating is calculated and then the design process is used to ensure that the hot spot temperature is as close as possible to the limit of the rating. As an example, the limit of the critical temperature rise relative to the environment of the improved design can be equal to the determined limit or differ from it by at most 10°C. It is sufficient that the obtained value is closer to the determined limit than that of the previous design.

[0113] It is also possible to use the load level K of the normal load and / or emergency load of the individual transformers directly as input data and to express the critical temperature in dependence on the load level K without the factor λ. Also in this case, the design can be adjusted so that at least one critical temperature obtained according to the equation is as close as possible to its temperature limit.

[0114] Then, in step G), it is checked whether the other criteria are met in the adjusted design, ie the temperature does not exceed its corresponding limit. Likewise, other criteria can be checked.

[0115] In step H), when all temperature criteria are met, the process ends and a suitable design characterized by relevant design parameters is provided in step I). A transformer according to the design is produced, tested and used in a power plant. It is also possible to select a transformer according to an improved design from a plurality of transformers of different designs and use it in a power plant.

[0116] If all temperature criteria are not met in step H), the design is further modified in step J) until all criteria are met. This means that the critical temperature is not exceeded at its limit.

[0117] Then, in step K), the parameter Δθ is redefined according to the modified design hr , Δθ or , R, x, and y. Here, Δθ is defined based on the nameplate rating hr , Δθ or and R, define x based on the same contingency load top liquid as the nameplate environment, and define y based on the same contingency load hot spot as the nameplate environment.

[0118] Specific examples are given below.

[0119] A transformer having a design characterized by the following characteristic parameters is provided:

[0120] Δθ hr : 18K

[0121] Δθ or : 60K

[0122] R: 10

[0123] x: 0.8

[0124] y:1.3

[0125] The value of the characteristic parameter can be selected as a standard value. hr and Δθ or Selected to comply with the rated temperature rise limits according to IEC 60076-2. R, x and y are the standard values ​​listed in IEC 60076-7.

[0126] Provide the basis Figure 2 The load data and the aging parameters of the values ​​obtained according to equations (10) and (11). The maximum permissible top oil temperature is 115° C. and the maximum permissible hot spot temperature is 140° C. The critical temperature is defined as in IEC 60076-7, for example.

[0127] The dependence of the critical temperature on the transformer load level and characteristic design parameters is provided by expressions (13)-(16). When the critical temperature is set to the determined limit and the transformer load level and characteristic design parameters are used, the critical value of the dynamic overload factor λ can be obtained. Then, the transformer rating is determined by the lowest one of the dynamic overload factors in expressions (13)-(16).

[0128] In the example, the following critical values ​​of the dynamic overload factor λ are obtained:

[0129] Criteria: Critical value of λ:

[0130] Aging during normal operation λ n =1.4216

[0131] Aging during emergency operation c =1.3234

[0132] Maximum top oil temperature during emergency operation λ o =1.2748

[0133] Maximum hot spot temperature during emergency operation λ h =1.2773

[0134] By the minimum dynamic overload factor (i.e. λ o) determines the rating. The geometric and electrical design of the transformer is adjusted to achieve a match between the calculated critical temperature and the critical temperature limit under rated conditions. Thus, the dynamic overload factor and rating of the transformer with the adjusted design is determined by the lowest dynamic overload factor.

[0135] In a particular embodiment, the adjusted values ​​of the design parameters are obtained so that the top oil temperature during emergency operation obtained according to expression (16) is equal to the determined limit. It is sufficient that the top oil temperature is closer to the determined limit than in a previous design of the transformer, or such an expression, or differs from the determined limit by at most 50°C.

[0136] Some parameters (e.g. length, width) can be adjusted incrementally, while other parameters (e.g. choice of material) can take only discrete values. Since these parameters may be interrelated (e.g. tank height and transformer core height), adjusting one specific parameter may require adjusting another specific parameter. The adjustment can be done by using well-known simulation tools.

[0137] After reaching the adjusted design, new characteristic design parameters can be obtained based on the adjusted electrical and geometric parameters.

[0138] Finally, it is checked whether all limits of critical temperature are observed with the adjusted design. In this step, the adjusted characteristic design parameters can be used in equations (13)-(16).

[0139] When all other criteria are met and the iteration is completed, the appropriate rating can be calculated. Assuming that the apparent power of the wind farm is 800MVA, with N=4 transformers, the appropriate rating can be obtained from equation (6) as 800MVA / 1.2748 / 3=209.2MVA. Accordingly, a rating of about 210MVA is appropriate.

[0140] In case all limits of critical temperature are not observed, the method is iterated by setting the characteristic parameters to their adjusted values.

[0141] As an example, the characteristic parameters of the adjusted transformer design are different from the original assumptions. The characteristic parameters can be obtained by standard simulation tools based on the adjusted geometric and electrical parameters. For example, the new characteristic parameters can be:

[0142] Δθ hr : 18K

[0143] Δθ or : 60K

[0144] R: 10

[0145] x: 0.8

[0146] y:1.6

[0147] Accordingly, the characteristic parameter y changes from 1.3 to 1.6 in the new design.

[0148] Although the new design meets the temperature limits according to the formulated expressions at rated conditions, evaluation of equation (14) using actual transformer characteristics at full load (K = 266.67 / 210 = 1.27) shows that the hot spot temperature exceeds 140° C. Therefore, the method will be iterated using new characteristic parameters until all criteria are met.

[0149] The size of the transformer can be further reduced by using high temperature materials with a higher temperature limit. Suitable materials may include, for example, thermally modified paper or aramid paper. In addition, lower humidity levels in the transformer can be ensured. Furthermore, impregnation liquids with a higher temperature limit can be used in the transformer, such as synthetic esters or natural esters. In this case, the restrictions of the aging criteria can be reduced, so that the transformer can be more compact with the same rating as when using materials with a lower temperature limit. In addition, the ratings obtained in the process can be further reduced by using different materials, such as ester-filled transformers instead of mineral oil-filled transformers.

[0150] Reference numerals

[0151] s The apparent power output of the power plant

[0152] S Maximum apparent power output of the power plant

[0153] κ Load level of the power plant

[0154] K Transformer load level

[0155] N is the number of transformers in the grid.

[0156] r Transformer rating

[0157] Lambda Factor

[0158] H Hours

[0159] θ a Ambient temperature

[0160] n K Load discretization steps

[0161] n θa Ambient temperature discretization steps

[0162] D Transformer Design

[0163] Δθ hr Gradient from hot spot to top liquid at rated current

[0164] Δθ or Top liquid temperature rise at rated loss

[0165] R Ratio of load loss to no-load loss under rated conditions

[0166] x Oil index depending on cooling mode

[0167] y Winding index depending on cooling mode

[0168] DP start Initial degree of polymerization

[0169] DP end Degree of polymerization at end of life

[0170] L Specified life of transformer

[0171] C The specified life subset of the transformer operating at the emergency level

[0172] E A activation energy

[0173] R g Gas constant

[0174] θ h Hot spot temperature

[0175] θ o Top liquid temperature

[0176] θ hw normal Weighted hot spot temperature at normal load

[0177] θ hw contingency Weighted hot spot temperature of emergency load

[0178] θ h normal Hot spot temperature under normal load

[0179] θ h contingency Hot spot temperature of emergency load

[0180] θ o,max Limit of top liquid temperature

[0181] θ h,max Hot spot temperature limit

[0182] θ hw,max normal Weighted hot spot temperature limit under normal load

[0183] θ hw,maxEmergency limit of weighted hot spot temperature under emergency load

[0184] λ h Critical value of the factor of the winding hot spot temperature

[0185] λ o Critical value of the factor for the top liquid temperature

[0186] λ n Critical value of the factor for normal operation

[0187] λ c Critical values ​​of emergency operation factors

Claims

1. A method for an improved transformer design for a power plant, comprising the following steps: i) defining at least two critical temperatures when the transformer is in operation, the critical temperatures being selected from at least two of a weighted hot spot temperature at normal load, a weighted hot spot temperature at emergency load, a top liquid temperature at maximum load and maximum ambient temperature, and a hot spot temperature at maximum load and maximum ambient temperature, and determining limits for the critical temperatures, ii) providing a data set of transformer load levels for one or more transformers or power plants, wherein the data set includes the amount of time during operation of the one or more transformers or power plants that the one or more transformers or power plants are at a specific load level at a specific ambient temperature, iii) providing an expression for said critical temperature in dependence on said transformer load level and design parameters related to the geometrical and / or electrical design of the transformer, iv) adjusting the geometrical and electrical design of said transformer and obtaining adjusted values ​​of said design parameters such that said critical temperature calculated according to at least one of said expressions is equal to said determined limit, v) after determining the adjusted design, checking whether all other limits in the limits of the critical temperature are observed, and if not, iterating the method starting from step iii) and setting the design parameters to their adjusted values, comprising the steps of producing a transformer having the adjusted values ​​of said design parameters and / or using a transformer having the adjusted values ​​of said design parameters in said power plant, Wherein, the expression of the critical temperature includes: The expression for the weighted hot spot temperature for normal load is: in, The expression for the weighted hot spot temperature of the emergency load is: in, Expression for top liquid temperature at maximum load and maximum ambient temperature: as well as Expression for hot spot temperature at maximum load and maximum ambient temperature: The parameters are defined as follows: E A :activation energy; R g : gas constant; H(κ,θ a ): The specified ambient temperature θ from the input data set a and the number of hours at a specified load level κ; λ: factor; Δθ hr : Gradient from hot spot to top liquid at rated current; Δθ or : Top liquid temperature rise under rated loss; R: Ratio of load loss to no-load loss under rated conditions; x: liquid index depending on the cooling mode; y: winding index depending on the cooling mode; nκ: number of load discretization steps; nθ a : number of steps for discretization of ambient temperature; N: number of transformers in the power plant; θ a,max : Maximum ambient temperature; κ max : Maximum load level of the transformer.

2. The method according to claim 1, in, When all other limits of the critical temperature are observed by the adjusted design, the design is accepted.

3. The method according to claim 1 or 2, in, In step iv), the design is adjusted so that the expression is equal to the determined limit or differs from the determined limit by at most 10°C.

4. The method according to claim 1 or 2, in, The expressions in step iii) are further dependent on factors related to the rating of the transformer design, wherein, in step iv), one or more critical values ​​of the factors are determined at which the one or more expressions are equal to the limits determined thereby, Therein, the geometrical and electrical design of the transformer is adjusted so that the expression for the smallest of the critical values ​​is at least closer to the determined limit than a previous design of the transformer.

5. The method according to claim 1 or 2, in, A limit of at least one of the critical temperatures is determined by an aging model depending on aging parameters based on at least one of the solid insulation material type, humidity level and oxygen level of the transformer, degree of polymerization at an initial time and at the end of life, a specified total life of the transformer, and a life subset of the transformer operating at an emergency level.

6. The method according to claim 1 or 2, in, The dataset of the transformer load is provided as the amount of time of ambient temperature discretization steps and load discretization steps, wherein the sum of the amount of time is equal to the number of hours in a year.

7. The method according to claim 1 or 2, in, The obtained transformer design is configured for use in a wind power plant.

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