Influence of Oil-Immersed Transformer Insulation Paper on Winding Temperature

JIANG Zhangnan，LIU Chang，ZHAO Zhengang，ZHANG Dacheng，ZHANG Changsheng

（Faculty of Information Engineering and Automation，Kunming University of Science and Technology，Kunming 650500，China）

Abstract：The winding insulation paper in the oil-immersed transformer is a vital component of the insulation system.It evenly wrapped insulation paper outside winding and immersed in the transformer oil has a significant barrier effect on the heat transfer between the winding and the transformer oil，resulting in the winding being unable to dissipate heat effectively.In this paper，through the Finite Element Method（FEM），a coupled heat transfer model of the natural oil circulation transformer winding area is established，which couples the temperature field and flow field.This model is used to analyze the temperature distribution of the winding when the winding is wrapped with insulation paper.Moreover，the winding temperature wrapped with different thickness insulation paper is studied.The simulation results are verified by the experiments，in which the winding temperature is directly measured by optical fiber temperature sensors.By comparing the experimental results with the simulation results，the maximum error between the simulation result and the experiment result is within 1 K when considering the insulating paper，while the minimum error is1.2 K and the maximum error is2 K when the insulation paper is neglected in the simulation.From the simulation results of winding insulation paper of different thicknesses，it can be seen that the winding hot spot temperature increases with the increase of the thickness of the insulation paper.

Key words：oil-immersed transformer；coupled heat transfer；temperature distribution；insulation paper；fiber optic temperature sensor

蒋张楠，刘 畅，赵振刚，张大骋，张长胜

（昆明理工大学信息工程与自动化学院，云南 昆明650500）

摘要：油浸式变压器中的绕组绝缘纸是绝缘系统重要的组成部分，是一种热的不良导体，均匀的外包住绕组并浸泡于变压器油中，对绕组与变压器油之间的热量传递有很大的阻隔效果，导致绕组无法有效散热.本文运用有限元法，建立自然油循环式变压器绕组区域温度场与流场的耦合传热模型，分析绕组在绝缘纸外包时的温度分布，研究不同厚度绝缘纸对绕组温度的影响，并通过光纤温度传感器直接测量绕组温度的实验验证仿真结果.通过对比实验结果与仿真结果，以实际厚度绕组绝缘纸进行仿真计算得到的结果与温升实验结果最大误差在1 K以内，而忽略绕组绝缘纸的仿真结果与温升实验结果存在最小1.2 K最大2 K的误差.由其他不同厚度绕组绝缘纸的仿真结果可知，绕组热点温度随着绝缘纸厚度增加而升高.

关键词：油浸式变压器；耦合传热；温度分布；绝缘纸；光纤温度传感器

0 Introduction

The power transformer is a core power equipment in power systems.The stability of power transformers operating status directly affects the safety of the whole power system.The power system failures caused by the transformer winding issues counts for more than 60%，attributed to the frequent failures［1］.It is due to the heat caused during its operation，which leads to a temperature increase inside the transformer.The iron core，the winding and the insulation parts have been in a high temperature for a long time，which leads to the loss of insulation capacity of the insulation system［2］.According to Chinese national standards，the service life of class A insulation materials commonly used in oil-immersed transformers is shortened by half when the winding temperature increased by 6°C.Therefore，it is important to ensure that the system is operating normally［3-4］.

In 2011，Chen et al.［5］used Fluent software to calculate the winding temperature under different load conditions and compared it with the measured temperature data.The simulation value is about 2 K lower than the experiment value.In 2015，Liu et al.［6］used Comsol to establish a simulation model of the transformer，obtained the distribution law of the internal temperature field，and compared it with the temperature rise test of the transformer.In 2016，Ma et al.［7］used Fluent software to analyze the oil flow and the internal heat transfer process，and compared the obtained hot spot temperature with experiment results.According to their work，due to the thermal resistance effect of the insulation paper is not considered，the winding temperature simulation result is lower than the experiment result.As an essential part of the insulation system，insulation paper is often neglected in the simulation.In current studies of transformer winding temperature distribution，most researchers considered that the heat is transferred directly from the windings to the transformer oil，without considering the winding insulation paper’s thermal resistance，which affects the heat transfer and causes the winding temperature rise.

The winding wrapped the insulation paper is not conducive to the heat dissipation of the winding，and the winding temperature is higher than in the case without insulation paper.To analyze its influence on the temperature rise of the winding，in this work the10 000kVA and 35kV oil-immersed transformers produced by a company in Yunnan is taken as the case study.The fluid-solid coupling simulation of the transformer is carried out by using the fluid mechanics simulation software ANSYSCFX.The fiber grating temperature sensor is used to detect the winding temperature of the transformer directly，and the experimental results are compared with the simulation results to verify the effectiveness of the simulation model.

1 Oil-immersed transformer structure and heat source analysis

Oil-immersed transformers are mainly composed of the iron core，windings，insulation components，and oil tanks.The iron core and winding act as a heat source to transfer heat to the cooling medium transformer oil.The loss in the transformer consists of the no-load loss generated by the iron core and the load loss generated by the high and low voltage windings［8］，which is expressed by

where，PTis the total loss，PLis the load loss andPCno-load loss.

During the operation，the iron core and the winding generate heat and transfer heat to the transformer oil，heat transfer is mainly performed by heat conduction and heat convection，with supplementary heat radiation，the temperature of the iron core and windings rises rapidly，transferring heat to the surface，creating a temperature difference with the transformer oil，and transferring heat to the transformer oil in the form of thermal convection.A small amount of heat is transmitted directly to the air through the heat radiation of the tank wall.The wall and air transfer most of the heat to the air by convection.When the oil flow rate is stable，the transformer reaches an internal thermal equilibrium state［9］.

The insulation paper is a poor conductor of heat.It has different body thermal resistance according to the thickness and material.The contact thermal resistance between the winding and the insulation paper is also very large.Therefore，the impacts of the insulation paper on the heat generated by the winding transferred to the oil can not be ignored.

As shown in Figure 1，the relationship between heat flux density and unit thermal resistance can be described as

whereqis the heat flux density，ΔTis the temperature difference between the winding surface and outside insulation surfaces，andRAis the thermal resistance per unit area.

Yuan et al.［10］measured the total heat flux of the insulation paper with different thicknesses in the oil-immersed state by the steady-state double heat flow meter method.Install a heating plate that outputs constant heat on the top heat flow meter，and set a cooling system under the bottom heat flow meter.When the heat is transferred from top to bottom，the temperature of each temperature collection point can be measured by a thermocouple.The temperature of the upper and lower surfaces of the insulation paper，and the average heat flux density through the insulation paper can be inferred.

Experiment results with the insulation papers of different thickness at the same temperature are shown in Figure 2，as the thickness of the insulation paper increases the thermal resistance increases linearly.

Fig.1 Heat transfer diagram

Fig.2 Thermal resistance fitting curve of insulation paper

2 Analysis of three-dimensional temperature field of transformer

According to the actual size of the transformer core and winding structure as show in Table 1 and the transformer material properties as show in Table 2.the finite element analysis software ANSYS is used to build the three-dimensional coupling calculation model of the core and winding.The three-phase model of the transformer is built by simplifying the core into a cylinder.The low-voltage winding is close to the core，and the high-voltage winding is outside the low-voltage winding.The simplified three-dimensional physical model of the transformer is based on the core and winding actual parameters.The three-dimensional model is illustrated in Figure 3.According to the insulating paper of different thicknesses on the winding surface，an equivalent barrier effect is set in the boundary conditions to simulate the barrier effect of heat transfer between the winding and the transformer oil.

Tab.1 Structural parameters of various components of the transformer

Tab.2 Transformer material properties

Fig.3 Three-dimensional model of the iron core and winding

The calculation of the temperature rise of the winding involves the coupling calculation of the transformer oil flow field and the temperature field，and some simplified assumptions need to be made for the solution environment.Under rated load，the thermal power of the winding is 76 753W/m3，and the thermal power of iron core is 1 062W/m3：

1）The external environment of the transformer is constant at 297K.

2）The heat source is a uniform heating body，and the insulation paper evenly covers the winding.

3）The effects of other adjacent windings are ignored.

Unlike forced oil circulation transformers，the oil flow force of natural oil circulation transformers is mainly derived from the thermal buoyancy lift of the transformer oil.The oil density changes to force the oil flow to circulate，so the flow state of the transformer oil determines the internal heat dissipation effect.The material properties of the fluid need to be set before the model is numerically solved.The physical properties of the transformer oil vary with temperature is shown in Table 3.

Tab.3 Transformer oil physical parameters

The fluid mechanics software ANSYSCFX uses the finite volume method to divide the calculation area into a series of non-repeating control volumes.Each control volume is integrated through the differential equation to be solved.Use the conservation control equation to establish a discrete equation.The general form of the conservation control equation is expressed as

The finite volume method is used to solve the integral of the governing equation in the control volume.

In the formula，ρrepresents the fluid density，Γrepresents the generalized diffusion coefficient，∈represents the universal variable，Vrepresents the velocity vector，andSis the generalized source term.Discrete the differential equations in the control volume by the finite volume method to obtain a discrete equation system.After the governing equations are discretized，the solution of fluid-solid coupled heat transfer can be obtained.

The fluid-solid coupling between the iron core，windings，and transformer oil is carried out through convective heat transfer at the interface between the solid and fluid domains of the transformer.The heat exchange process occurs under the combined action of heat transfer and heat convection.The premise of heat exchange is the temperature difference between the fluid and the solid surface.The software uses the fluid-solid coupling model calculation formula to determine the convective heat transfer coefficient.The Newtonian cooling formula describes the heat of convective heat transfer，efficient of the surface，ΔTis the temperature difference between

whereαis the convective heat transfer co the fluid and the solid surface.

Since the simplified model does not include the actual modeling of the insulation paper，there is no actual modeling in the simulation calculation，and the contact surface between the winding and the oil exists in the form of thermal resistance.The temperature of the outer surface of the insulation paper is calculated by：

Whereqis the heat flux density，Ris the thermal resistance of the oil-impregnated insulation paper at the temperature to the unit area of the winding，δis the thickness of the insulation paper，tpis the outer surface temperature of the insulation paper，tcis the winding surface temperature，andλis the thermal conductivity of the insulation paper at this temperature.

The convective heat transfer coefficient of the outer part of the tank wall in contact with the air is manually calculated because in the three-dimensional modeling，the fuel tank is simplified to a wall without thickness.In the actual heat transfer，the oil first convectively exchanges heat with the inner tank wall，then transfers heat to the outside of the tank through heat conduction.Therefore，the influence of the heat conduction process on the wall temperature needs to be considered when calculating the convective heat transfer coefficient.

3 Winding temperature rise and experimental results

By solving the three-dimensional fluid-solid coupling heat transfer model of the transformer，the temperature distribution of the winding and the oil flow distribution can be obtained.The oil flow velocity distribution is shown in Figure 4，and the temperature distribution of winding is shown in Figure 5.

Fig.4 Oil flow distribution map

Fig.5 Temperature distribution of winding without insulation paper（left）vs.Temperature distribution of winding with 0.15mm thickness insulation paper（right）

It can be seen from the oil flow velocity distribution that the oil flow velocity at the uppermost part of the winding is faster than the other parts，and the oil path is vast，which is favorable for the top winding to sufficiently dissipate heat.Hence，the hot spot temperature is at the bottom of the top.When the insulation paper is outsourced，the windings reach the maximum temperature at the 45th pie from bottom to top.

In order to verify the effectiveness of the simulation model，the temperature rise experiments are conducted on the oil-immersed transformer，and the fiber Bragg grating temperature sensor is installed in the pad of the winding oil passage to accurately measure the temperature rise of the winding accurately.The sensor tail stays no the left，and the pad is placed in the low voltage winding temperature measuring point.A total of 5 temperature sensors are placed at 1cm，13cm，25cm，37cm and 49cm from the bottom of the winding，as shown in Figure 6.

Fig.6 Sensor installation and layout

The experiment performed temperature detection on the windings of different heights and compared with the simulation results，as shown in Table 4.

The temperature rise test results are compared with the winding temperature rise curve diagram with or without insulation paper wrapping winding，as shown in Figure 7.There is a particular gap between the parameter setting of the simulation calculation and the actual transformer environment.The simulation result of the actual winding insulation paper thickness model has an error of up to 1K within the experimental result.The model simulation results without the paper outer winding are slightly lower than the experimental results，and the errors ranged from1.2K to 2K.It implies that the simulation model considering the thermal resistance effect of the insulation paper is more accurate than the one without consideration，and the overall distribution trend of the winding temperature obtained by the simulation is nearly consistent with the measured results，which proves the feasibility of the model.

4 Analysis of the influence of insulation paper on the temperature rise of winding

To further investigate the influence of insulation paper on the winding temperature，a three-dimensional model of the outer winding of different thickness insulation paper is shown in Table 5，keeping the rest settings unchanged.

Tab.4 Transformer temperature rise experimental data and simulation values

Tab.5 Insulation paper thickness and unit area thermal resistance

Fig.7 Comparison of experimental and simulation results

The winding temperature distribution of the insulation paper with different thicknesses is shown in Figure 8，and the fitting curve of the insulation paper thickness and the hot spot temperature is shown in Figure 9.

Fig.8 Winding temperature distribution of outer windings of different thickness insulation paper

Fig.9 Relationship between hot spot temperature and insulation paper thickness

By comparing the winding temperature distribution when the insulation paper of different thicknesses wraps the winding，it can be seen that the hot spot location and distribution trend of the winding have not changed significantly.The winding hot spot temperatures in the six cases are 341.4K，342.3K，343.1K，343.4K，344.3K，and 345.1K，respectively.Compared with the case without insulation paper，the hot spot temperature difference is 0.9K，1.7K，2K，2.9K，and 3.7K.Through the fitting curve of insulation paper thickness and thermal resistance combined with the simulation results，it is known that the winding hot spot temperature increases with the increase of insulation paper thickness.In engineering applications，it can know the hot spot temperature when winding insulation paper with different thicknesses through the fitting curve of insulation paper thickness and hot spot temperature.

5 Conclusions

In this paper，the finite volume method is used to study the influence of different thicknesses of insulation paper on the temperature distribution of the windings，and it is also verified by transformer temperature measurement experiments.

The simulation results obtained without considering the insulation paper and the temperature rise experimental results have a minimum error of 1.2 K and a maximum of 2 K，and the maximum error of the result obtained in consideration of the actual thickness of the insulation paper is within 1 K.By comparing the temperature distribution of the outer windings of different thickness insulation papers，it can be known that the insulation paper has a significant influence on the hot spot temperature of the windings.When the insulation paper is not considered，the hot spot temperature of the winding is 341.4K，and the temperature difference between the hot spots of 0.05mm，0.15mm，0.18mm，0.27mm，and 0.34mm outer insulation paper is 0.9K，1.7K，2K，2.9K，and 3.7K.In summary，the simulation model is more accurate when considering insulation paper，and the temperature of the winding hot spot increases as the thickness of the insulation paper increases.