Improving the cooling performance of electrical distribution transformer using transformer oil – Based MEPCM suspension

Engineering Science and Technology, an International Journal xxx (2016) xxx–xxx

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Improving the cooling performance of electrical distribution transformer using transformer oil – Based MEPCM suspension Mushtaq Ismael Hasan Mechanical Engineering Department, College of Engineering, Thi-Qar University, Thi-Qar, Iraq

a r t i c l e

i n f o

Article history: Received 17 September 2016 Revised 4 November 2016 Accepted 5 December 2016 Available online xxxx Keywords: Electrical transformer Transformer oil MEPCM Phase change material Cooling performance Distribution transformer

a b s t r a c t In this paper the electrical distribution transformer has been studied numerically and the effect of outside temperature on its cooling performance has been investigated. The temperature range studied covers the hot climate regions. 250 KVA distribution transformer is chosen as a study model. A novel cooling fluid is proposed to improve the cooling performance of this transformer, transformer oil-based microencapsulated phase change materials suspension is used with volume concentration (5–25)% as a cooling fluid instead of pure transformer oil. Paraffin wax is used as a phase change material to make the suspension, in addition to the ability of heat absorption due to melting, the paraffin wax considered as a good electrical insulator. Results obtained show that, using of MEPCM suspension instead of pure transformer oil lead to improve the cooling performance of transformer by reducing its temperature and as a consequence increasing its protection against the breakdown. The melting fraction increased with increasing outside temperature up to certain temperature after which the melting fraction reach maximum constant value (MF = 1) which indicate that, the choosing of PCM depend on the environment in which the transformer is used. Ó 2016 The Author. Production and hosting by Elsevier B.V. on behalf of Karabuk University This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction A transformer is a device that transfers electrical energy from one circuit to another. It is used to step up or down the voltage of power system. The transformer has vital rule and considered the main component of the electrical network due to many reasons such as, its high cost, direct effect on network operation, location and its contents of oil and toxic materials. Therefore it is very important to protect it from breakdown which will cause many risks on neighboring people life and electrical network. The main cause for transformer breakdown is the oil temperature rise. So it is important to pay more attention to the research works trying to improve the cooling performance of transformers. When a transformer fails, problems occur in the operation of distribution networks cause an increase in the power system operation cost and reduce the reliability of the electricity delivery. Transformers utilize transformer oil as a cooling medium and insulation material, together with cellulose. Breakdown voltage (dielectric strength) is one of the most important parameters of transformer oil. Winding temperature limit the transformer loading, therefore, for full load and normal surrounding temperature, the transformer oil temperature must remain in a certain limit set by industry E-mail address: [email protected]

standards. The temperature of the transformer winding is not uniform and the limiting value is actually the higher temperature section of the winding which called winding hot spot temperature. The insulation temperature represents the main factor of transformer aging. The paper insulation depolymerized with temperature and time which represent the end of life of the insulation materials, which defined as the transformer end of life. The temperature of the transformer oil is increased by increasing transformer load, so loading above a certain value include some risk potential. The rise in the maximum temperature of transformer oil over the ambient temperature in modern transformers is about 65 °C [1]. A phase change material characterized with high latent heat of fusion since it is able to absorb and release extra amounts of heat through melting and solidification. Microencapsulated phase change materials suspensions are received more interest and importance due to their potentials of enhancing heat transfer and thermal storage performances. The heat transfer improvement is caused by the latent heat absorption through melting of the phase change materials in the dispersed MEPCM particles. There are many researches in literature deals with basic principles of transformers and transformer oil specifications. Dejan S. [2] (2005) presented new and accurate methods for temperature calculation in transformer oil, he used heat transfer theorem, using the method of lumped capacitance, and the analogy

http://dx.doi.org/10.1016/j.jestch.2016.12.003 2215-0986/Ó 2016 The Author. Production and hosting by Elsevier B.V. on behalf of Karabuk University This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: M.I. Hasan, Improving the cooling performance of electrical distribution transformer using transformer oil – Based MEPCM suspension, Eng. Sci. Tech., Int. J. (2016), http://dx.doi.org/10.1016/j.jestch.2016.12.003

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Nomenclature Symbol A c Cp H he k T u v DH w F

Description SI Unit area m2 MEPCM volumetric concentration % specific heat J/(kg K) enthalpy of suspension (W) sensible heat (W) thermal conductivity W/m K temperature K fluid x-component velocity m/s fluid y-component velocity m/s latent heat (W) fluid z-component velocity m/s natural convection Source term

between thermal and electrical resistance at many locations within an electrical transformer. He takes into consideration the changes in oil viscosity and variation with temperature loss. The change in time constant due to viscosity changes is also considered in the models. He found that the hotspot temperature rise over top oil temperature due to a load change can be represented as exponential function with a time constant equal to the winding time constant. Mohammad R. et al. [3] (2008) Studied experimentally the oil models taken from many power transformers with different ages, to correlate the relation between the real age of the oil and its different specifications and to know the appropriate characteristics which can indicate its aging. They accomplished experiments for accelerated aging on oil elements at different values of temperatures. Also they used Arrhenius law to predict the remaining lifetime of the oil. They found that, Arrhenius law can be applied accurately to the thermal degradation phenomena of the oil, and to find out the left lifetime of the oil. Diaconu et al. [4,5] (2010) investigated experimentally the natural convection heat transfer of a PCM in a vertical helically coiled tube. New microencapsulated PCM suspension with concentration up to (45%) was employed. They first tested pure water to formulate natural convection correlations and then a comparison was made with the results obtained from MEPCM. Their results indicated that during the phase change period the heat transfer coefficient for the PCM was higher than that for pure water. D.J. Smith et al. [6] (2010) used COMSOL software to study numerically the dielectric changes and the resultant capacitance and dissipation parameters variation from moisture and temperature ingress. Their results showed to give an improved understanding about modeling and analyzing faults within OIP bushings. Arslan A. et al. [7] (2011) covered the research performed in a series of tests done on the temperature rise in distribution transformers. They studied the ambient effects represented by relative humidity and natural air flow that a transformer surface generates. They also discussed the experimental results with the theoretical values to match the theoretical study. They found that, the relative humidity affects temperature rise considerably. Their results may help in minimizing the predicted temperature rise ranges in contrast to empirical models. M. Srinivasan [8] (2012) Proposed a new semi-physical model including of the environmental parameters to estimate the hot spot temperature in addition to the insulation life loss in the transformers. The winding hot spot temperature was calculated as a function of the top oil temperature that can be estimated using the transformer loading data, top oil temperature value, atmospheric temperature, air velocity in addition to the effect of solar heat radiation. They validated their proposed model with real data gathered from a 100 MVA power transformer. Saleem A.H. [9] (2012) Studied the effect of many parameters on the properties of transformer oil such as, the gap

x y z MMF

l

/

q d

axial coordinate m vertical coordinate m horizontal coordinate m melted mass fraction dynamic Viscosity m2/s mass fraction density kg/m3 diameter lm

Abbreviations PCM phase change material MEPCM microencapsulated phase change material MF melting fraction

distance, water content and temperature on the transformer oil breakdown voltage. He observed that the dielectric of transformer oil increased as increasing of the gap distance. Also he showed that, there is a marked reduction in the breakdown voltage with increasing of the amount of water content. Also he found that, there is a strong dependence of the DC breakdown voltage on the temperature, where breakdown voltage decreases with increasing temperature up to 100 °C. Murtaza H. [10] (2013) Performed a steadystate calculations using IEC guidelines to determine the hot spot temperatures of power and distribution transformers in the harsh weather as a results of long summer season. Also he considered the effect of increase in winding resistance due to increase in ambient temperatures. He found that, the power and distribution transformers should be progressively de-rated under certain conditions for their safe operations, to improve also the reliability of the electricity supply in the challenging future smart grid environment. In this paper a transformer oil-based MEPCM suspension will be used as a cooling medium in distribution transformer instead of pure transformer oil. As the author best knowledge and According to the reviewed literature this type of suspension is never used in transformer cooling, and there is no research include modeling of complete transformer, therefore the results of this paper represent a pioneer step in this field. 2. Problem description The transformer selected as a case study in this paper is 250 KVA distribution transformer which is widely used in Iraqi electricity network. Fig. 1.a. shows a picture for this transformer while Fig. 1.b represents a schematic drawing illustrating the outer view of this transformer. The transformer consists of (coils and core assembly) which consists of three copper coils and a steel core linking them, all these items are immersed in transformer oil contained in the transformer body which equipped with fins to increase the heat transfer area. The transformer oil play two important roles, as a cooling medium transmit the heat generated in coils and core into outer walls to dissipate it and as an electric insulator. According to the available data the heat generated in transformer in full load situation is 1000 W from each coil and 500 W from core. This heat generated must be dissipated to maintain the temperature of oil at certain accepted level. 3. Mathematical formulation Heat is generated in coils and core due to electrical resistance, this heat is absorbed by oil to transfer it to the outer walls by natural convection and then dissipated from outer walls to the surrounding air by convection and radiation. The governing

Please cite this article in press as: M.I. Hasan, Improving the cooling performance of electrical distribution transformer using transformer oil – Based MEPCM suspension, Eng. Sci. Tech., Int. J. (2016), http://dx.doi.org/10.1016/j.jestch.2016.12.003

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The energy equation for oil-based MEPCM suspension is [13].

r½~ Vðqs HÞ ¼ rðks ÞrT s Þ

ð4Þ

where s refers to the oil based MEPCM suspension. Total enthalpy of the suspension (H) is indicated in Eq. (5) which calculated as the sum of the sensible heat (he) and the latent heat (DH) of the phase change material.

H ¼ h e þ DH

ð5Þ

Sensible heat is given in Eq. (6), based on reference enthalpy at reference temperature [14].

Z

T

he ¼ href þ

ð6Þ

Cpf dT T ref

The latent heat of suspension shown in Eq. (7) is a function of the PCM latent heat L, suspension mass fraction / and the melted mass fraction MMF. The melted mass fraction MMF is the mass ratio of melted PCM to the total mass of PCM in the suspension. The phase change material starts in melting at Tsolidus and melts completely at Tlquidus where the liquid fraction can vary from zero at Tsolidus to one at Tlquidus. Eq. (8), describes the melted mass fraction MMF.

DH ¼ MMF / L

ð7Þ

where:

MMF ¼ 0 if

Tf < Tsolidus

MMF ¼ 1 if

Tf > Tlquidus

MMF ¼

Ts  Tsolidus Tlquidus  Tsolidus

ð8Þ if

Tsolidus < Tf < Tlquidus

The boundary conditions used to solve the above set of equations are: No slip velocity on all solid walls (coils, cores and all transformer walls). Coils and cores are subjected to constant heat generation source term calculated from the actual heat losses generated in real transformer and its values are calculated from the available data for electrical losses in coils and core which are converted to heat generation. All the outer walls of transformer including the fins surfaces are subjected to combined natural convection and radiation, the value of convection heat transfer coefficient is assumed 1000 (W/m 2 .K) and the external emissivity is 1. The above model is numerically solved to calculate the distributions of temperature, then the oil average and maximum temperatures and heat transfer rate and heat transfer coefficient can be calculated. 4. properties of microcapsules Fig. 1. a Picture for studied 250 KVA transformer. b. Schematic figure for studied 250 KVA transformer.

equations for 3D, steady and incompressible oil are continuity, momentum and energy equations respectively as below [11] and [12]:

r: ~ V ¼0

qðV:r: ~ VÞ ¼ rP þ r:ðlr: ~ VÞ þ qcp ðV:rTÞ ¼ kr2 T

ð1Þ

q  q1 g q1

ð2Þ ð3Þ

Single microencapsulated phase change materials capsule consist of wall made from polymers which envelope a core of phase change material. The polymer wall must be flexible to interact with changes in volume caused by solid - liquid phase change. The average diameter of MEPCM particles studied is 5 lm. The PCM used is paraffin wax which has a melting temperature range of 43.8–50.6 °C, which is suitable for hot climate regions with air temperature may exceed 50 °C in summer. The wall material is polymethylmethacrylat (PMMA), the PCM in a single MEPCM capsule is about 70% by volume. The properties of transformer oil and suspension components are listed in Table1 below [15–18]. In addition to its heat absorption abilities the paraffin wax is an excellent electrical insulator, with a resistivity of between 1013–1017 O meters which is nearly better than all other materials

Please cite this article in press as: M.I. Hasan, Improving the cooling performance of electrical distribution transformer using transformer oil – Based MEPCM suspension, Eng. Sci. Tech., Int. J. (2016), http://dx.doi.org/10.1016/j.jestch.2016.12.003

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Table 1 Thermo physical properties of oil and suspension components. Density (kg/m3)

Cp (J/kg.K)

k(W/m.K)

l (kg/m.s)

Transformer oil Paraffin wax

870 900 1190

0.109 Solid = 0.21 Liquid = 0.12 0.21

0.0124 –

PMMA (MEPCM wall)

2000 Solid = 2380 Liquid = 2446 1470

except some types of plastics [19] therefore using paraffin wax as PCM in suspension modify both the thermal performance and electrical insulation. Due to difference in properties of the paraffin wax and the wall material, microencapsules properties must be computed based on the properties of the individual components. The mass and energy balance respectively where used to compute the density and specific heat of the microencapsules. The density of paraffin wax was considered as the mean of its solid and liquid densities [20,21].

qcapsule ¼

 3 10 dwax qwax 7 dcapsule

Cpcapsule ¼

ð7Cpwax þ 3Cpwall Þqwax qwall ð3qw ax þ 7qwll Þqcapsule

ð9Þ

ð10Þ

The composite sphere approach is used to calculate the thermal conductivity of the microcapsules. Wall thickness gives the heat transfer resistance of the wall material; while the heat transfers resistance of the core material was calculated using the solid sphere in an infinite medium model.

dcapsule  dwax 1 1 ¼ þ kcapsule dcapsule kwax dwax kwall dcapsule dwax

ð11Þ

5. Properties of suspension The average properties of suspension are a combination of the properties of the transformer oil and the microcapsules. The density and specific heat are calculated from the following relations [13,20,21].

qs cqcapsule þ ð1  cÞqoil

ð12Þ

Cps ¼ /Cpcapsule þ ð1  /ÞCpoil

ð13Þ

The viscosity of the suspension is calculated from the Vand’s correlation as following [22]:

ls ¼ loil ð1  c  1:16c2 Þ

2:5

ð14Þ

The following relation is used to calculate the bulk thermal conductivity of suspension:

Ks

2koil þ kcapsule þ 2cðkcapsule  koil Þ   k k 2 þ capsule  c capsule 1 k k oil

ð15Þ

oil

The relation between the mass and volume fractions is:

/¼

cqcapsule ðqw þ cðqcapsule  qoil ÞÞ

ð16Þ

The volume fractions selected in the present model were 5%, 10%, 15%, 20% and 25%. 6. Numerical model Simulate numerically the complete transformer shown in Fig. 1. a. and 1.b. is difficult and time and memory consuming, therefore to simplify the numerical solution and due to the geometrical

–

symmetry a quarter of transformer can be used as a computational model to represent the complete transformer as shown in Fig. 2.a. which represent the meshed computational model (quarter of transformer) and Fig. 2.b which represent the meshed computational model for coils and core assembly where the coils are assumed as a solid cylinders. The governing equations are solved numerically by finite volume method (FVM); the ‘‘first order upwind” scheme is used to convert the governing equations to algebraic form which are solved by using the segregated solver. To enforce the continuity equation and solve the velocity- pressure coupling problem the SIMPLE algorithm is used. By solving above mentioned equations by a CFD code the distribution of temperature is obtained in the oil and suspension, then the heat transfer coefficient and fluid maximum temperature are calculated. The computational domain (quarter of transformer including coils, core and oil) was descritzed by selected size grids (regular mesh). Mesh independence was studied by using seven mesh sizes and the results for oil average temperature for different meshes used are listed in Table 2 below for outer walls temperature To = 303 K. From Table 2 it can be observed that, after sixth mesh further increase in the grids will not have a significant effect on the solution so the sixth mesh is used for all numerical computations. The mesh used is indicated in Fig. 2.a. and b. The solution residuals used as a convergence criteria for both momentum and energy equations were selected to be less than 106 which gives high accuracy for numerical solution. 7. Results and discussion Solution was conducted first with pure transformer oil, and then repeated with transformer oil based MEPCM suspension with volume percentages of 5%, 10%, 15%, 20%, and 25%. The used numerical model has been previously validated in my previous work [20] by solving the numerical model presented in [16] and the results were compared. The numerical model presented in [16] is a microchannel heat sink has a width of 5.1 mm, height of 1.5 mm and length of 10 mm. It consists of 25 equally spaced rectangular microchannels each one with 166.6 lm hydraulic diameter and the channels are separated by a 100 lm wall thickness of Aluminum. Due to computational difficulties of modeling the whole heat sink with 25 channels and due to symmetry in geometry of channels, the numerical model used in [16] includes only a half of individual channel with its surrounding of the heat sink metal. Thermal boundary condition is a constant heat flux of 100 W/cm2 acting at the bottom wall of the heat sink. The inlet velocity is 1 m/s and inlet temperature is 300 K, the PCM used with melting range of 300–305 K. Fig. 3 represents a distribution of bulk temperature of MEPCMwater suspension along microchannel for results of used numerical model and results of [16]. From this figure it can be seen that, the agreement between results of used numerical model and results of [16] is accepted since the mean error for all points is 2.1%. This model has been also used to study the MEPCM suspension as in [20] therefore the present numerical model is reliable and can be used to study the distribution transformer with oil-based MEPCM suspension.

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Present Presentmodel model 308

Reference [16] Refrence [18]

Bulk (K) Bolktemperature temperature (K)

307

306

305

304

303

Symmetry 302

q 301

Fluid inflow 300 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

x/L Fig. 3. Distribution of bulk temperature along channel as a comparison between present model and results of reference [16].

Fig. 2. a computational mesh (outer view). b. Computational mesh for coils and core surfaces.

Table 2 Mesh refinement results. Mesh Mesh1 Mesh2 Mesh3 Mesh4 Mesh5 Mesh6 Mesh7

Average oil temperature (K) (Number (Number (Number (Number (Number (Number (Number

of of of of of of of

nodes = 43681) nodes = 57789) nodes = 73680) nodes = 89226) nodes = 104877) nodes = 109945) nodes = 112013)

444.59 398.79 369.67 350.14 336.16 331.52 330.01

Figs. 4 and 5 shows the temperature contours at (x-z) and (y-z) planes at middle height and width of transformer respectively for pure oil at outside temperature of 30 °C. It is clear form these figures that, the temperature is distributed from its maximum values near the surfaces of coils and core to its minimum values at the outer surfaces of transformer corresponding to the surrounding air temperature due to transferring of the generated heat from the coils and core to the outside air through the transformer oil.

Fig. 6 indicates the variation of both of the oil maximum and average temperature with outside air temperature for pure oil case. The average temperature calculated as a volume average for whole oil. From this figure it can be noted that, both of average and maximum oil temperatures are increased with increasing the outside air temperature due to decreasing of the heat dissipation to the outside as a results of decreasing of the temperature difference which lead to accumulate the heat in the oil. The outside air temperature is selected up to expected higher values in hot climate regions 51 °C. It is observed from the results of this figure that, the maximum oil temperature reaches high and dangerous values at higher values of air temperature which is the main problem in transformer in hot climate regions especially at full load conditions, since the outside air temperature reach a higher level decreasing the dissipation of transformer generated heat. Increasing of transformer oil temperature to a certain level lead to decrease its dielectric and cause a transformer breakdown, also increasing the oil temperature cause decreasing of the insulation of paper used to cover and insulate the coils. Therefore it is important to use new and efficient technologies to improve of the cooling performance of transformers. Fig. 7 shows the variation of average temperature with outside air temperature for both of pure oil and oil based MEPCM suspension with different values of volume concentration. From this figure it can be observed that, the average temperature increased with increasing the surrounding temperature as discussed before in Fig. 6 due to reduction in heat transfer process as a results of decreasing in temperature difference. Also it can be noted form this figure that the average temperature reduced in case of using MEPCM suspension instead of pure oil due to increasing the heat absorption process as a results of paraffin wax melting which is absorbs extra heat as a latent heat. The reduction in average temperature increased with increasing the PCM volume concentration as a result of increasing the melted amount of PCM with volume concentration which causes extra heat absorption. Fig. 8 represents the Variation of fluid maximum temperature with outside air temperature for pure oil and MEPCM suspension with different values of volume concentration. From this figure one can observe that, the maximum temperature increased with

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Fig. 4. Temperature contour on the x-z plan at middle height of transformer.

120 115 110

(oC) Oil temperature (oC)

105 100

o Oil average temperature (oC) C o Oil maximum temperature e (oC) C

95 90 85 80 75 70 65 60 30

33

36

39

42

45

48

51

54

o

( C) Outside temperature e (oC) Fig. 6. Variation of oil temperature (maximum and average) with outside temperature for pure transformer oil.

Fig. 5. Temperature contour on the y-z plan at middle width of transformer.

increasing air temperature for all cases as discussed in Fig. 7 above. Also the maximum temperature in case of using MEPCM suspension as a cooling medium is smaller than that of pure oil in same conditions due to extra latent heat absorbed as a result of melting of the paraffin wax (PCM) in the suspension. For all range of air temperature the maximum temperature of suspension decreased with increasing the volume concentration of PCM in the suspension due to increasing the latent heat absorbed with increasing the amount of melted PCM. The value of temperature reduction with using MEPCM suspension is depends on the phase change material used and its melting temperature and latent heat of fusion, therefore extra reduction in oil temperature can be

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77 Pure oil

76

Oil average temperature (oC)

MEPCM suspension (c=5%)

75

MEPCM suspension (c=15%)

74

MEPCM suspension (c=25%)

73 72 71 70 69 68 67 66 65 64 63 62 61 60 27

30

33

36

39

42

45

48

51

54

Outside temperature (oC) Fig. 7. Variation of oil average temperature with outside temperature for pure oil and MEPCM suspension with different values of concentration.

118

118

Outside temperature = 30 oC Outside temperature = 39 oC Outside temperature = 45 oC

117

Oil maximum temperature (oC)

Oil maximum temperature (oC)

117

116

115

114 Pure oil

Outside temperature = 51 oC

116

115

114

MEPCM suspension (c=5%) MEPCM suspension (c=10%)

113

113

MEPCM suspension (c=15%) MEPCM suspension (c=20%) MEPCM suspension (c=25%)

112

112 27

30

33

36

39

42

45

48

51

54

o Outside temperature ( C)

0

5

10

15

20

25

30

Suspension concentration (%)

Fig. 8. Variation of oil maximum temperature with outside temperature for pure oil and MEPCM suspension with different values of concentration.

Fig. 9. Variation of oil maximum temperature with suspension concentration for different values of outside temperature.

obtained by using other PCM with suitable melting range and higher melting latent heat. The variation of fluid maximum temperature with suspension volume concentration for different values of outside temperature is shown in Fig. 9. The case of zero con centration represents the pure oil case. From this figure it can be concluded that, the maximum temperature is decreased with increasing the volume concentration of MEPCM suspension for all selected air temperatures due to increasing the amount of PCM in the suspension with increasing the concentration which lead to increase the melted amount of PCM and consequently increasing the amount of heat absorbed. Also it can be seen that, for all values of selected volume

concentration the maximum temperature increased with increasing the air temperature as discussed before. Fig. 10 shows the variation of melting fraction (MF) of MEPCM suspension with outside temperature for case of volume concentration (c = 25%). The plotted melted fraction is the average value calculated as a volume average in whole suspension, since at each point in the suspension domain there is a value of (MF) corresponding to its temperature. As discussed before the suspension temperature increased with increasing outside temperature and as mentioned in section of PCM properties the melting of PCM occur in range of temperature started at solidus temperature at which all PCM exist as solid phase and end at liquidus temperature

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the temperature difference which lead to reduce the amount of heat that can be dissipated from the transformer which lead as a consequence to increase the transformer temperature. Also it can be seen from this figure that, the heat transfer coefficient in case of using MEPCM suspension is larger than that for pure oil case which indicate the enhancement of heat transfer process due to absorption extra amount of heat as a latent heat when phase change material melted and the heat transfer coefficient increased with increasing the PCM volume concentration as a results of increasing heat absorption as discussed before.

1.05 1.00 0.95

Melting fraction

0.90 0.85 0.80 0.75

8. Conclusions

0.70

The transformer oil based MEPCM suspension was used as a cooling fluid in distribution transformer instead of pure transformer oil to modify its cooling performance. From the results obtained the following conclusions can be drawn:

0.65 0.60 0.55 27

30

33

36

39

42

45

48

51

54

57

Outside temperature (oC) Fig. 10. Variation of melting fraction of suspension with outside temperature.

1.6 Pure oil

1.5

MEPCM Suspension (c=10%) MEPCM Suspension (c=20%)

1.4 1.3

1- Transformer oil temperature increased with increasing the outside air temperature and it may reach a dangerous values cause a transformer breakdown. 2- Using of transformer oil based MEPCM suspension leads to modify the cooling performance of an electrical distribution transformer by decreasing its inside temperature. 3- Transformer temperature reduction increased with increasing the volume concentration of PCM in the suspension. 4- The type of PCM in the suspension must be selected according to its melting temperature to be suitable for temperature of region at which the transformer will be used.

References

h (W/m2 .K)

1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 30

32

34

36

38

40

42

44

46

48

50

52

o

Outside temperature ( C) Fig. 11. Variation of heat transfer coefficient with outside temperature for pure oil and MEPCM suspension.

at which all PCM changed to liquid. From this figure it can be seen that, the amount of PCM melted (MF) increased with increasing the outside air temperature up to certain temperature at which the (MF) reach its maximum value, MF = 1 and remain constant regardless of outside temperature, which reveal the melting of all PCM in the suspension. After this temperature all PCM is melted to liquid phase therefore there is no extra latent heat absorption and there is no considerable improvement in cooling performance of transformer after this temperature for this type of PCM. The variation of heat transfer coefficient at the transformer outer walls with outside air temperature for pure oil and MEPCM suspension is presented in Fig. 11. This figure declare that, the heat transfer coefficient decreased with increasing surrounding temperature for both the pure oil and suspension due to decreasing

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Please cite this article in press as: M.I. Hasan, Improving the cooling performance of electrical distribution transformer using transformer oil – Based MEPCM suspension, Eng. Sci. Tech., Int. J. (2016), http://dx.doi.org/10.1016/j.jestch.2016.12.003

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Please cite this article in press as: M.I. Hasan, Improving the cooling performance of electrical distribution transformer using transformer oil – Based MEPCM suspension, Eng. Sci. Tech., Int. J. (2016), http://dx.doi.org/10.1016/j.jestch.2016.12.003