Comparasion between Oil Immersed and SF6 Gas Power Transformers Ratings

Power transformers present the largest portion of the capital investment in addition the power transformer remains oil immersed which presents a fire hazard that is particularly objectionable in urban and metropolitan environment. After careful studies of various alternatives to conventional transformer technology to produce a gas insulated power transformer with increased mega volt-ampere ratings, gas insulated power transformer has been developed with a use with non flammable SF6 gas. For changing the insulating material from oil to SF6 gas a comparative study between these types of transformers should be made. In this paper two mathematical models are suggested and simulated by computer programs to calculate the temperature of the cooling mediums and transformer windings. The obtained results are in agreement wit the measured values in the field.


Introduction
The energy losses in transformer are classified as no load losses and load losses which divided into I 2 R and stray losses.The insulating medium (oil or SF6) is used for insulation and to remove heat from the winding and core assembly to surrounding [1].The heat generated inside transformer must be transfers to the insulating medium and further to surroundings via tank and heat exchanger.Although the winding copper holds its mechanical strength up to several hundreds degree Celsius and transformer oil dose not significantly degrade below about 140 o C.The paper insulation deteriorates very rabidly if it s temperature exceeds 90 o C [2].It has been reported that from 90 to 110 o C the tensile strength aging rate is doubled for approximately each 8 o C increase in temperature [2].Other authors have observed that the life of different transformer insulation materials is halved by an increase in temperature ranging from 5 to 10 degrees [3].The IEC 60354 loading guide for oil immersed power transformers [4], and the IEEE guide for loading mineral oil immersed transformer [5], indicate how oil immersed transformers can be operated in different ambient conditions and load levels without exceeding the acceptable deterioration limit of insulation due to thermal effects.
To increase transformer operational efficiency and minimize the probability of an unexpected outage, several on-line and off-line monitoring systems have been developed [6], [7][8], [9][10], and [11][12].Direct measurement of a has been increasing since the mid

Thermal Models of Power Transformers 1. Thermal Model of Oil Power Transformer
The final thermal over all model for oil based on the thermal-electrical analogy and heat transfer theory [17], [18], [19].Figure 1.Thermal over all circuit models where: q tot is the total losses; q heat generated in the winding; R ambient temperature; θ hs is the hot spot temperature; θ R th-hs-oil is the non linear winding to oil thermal resistance; C capacitance and Cth -oil is the oil load and load transformer losses are represented by two ideal heat sources [19], [20].
The ambient temperature is represented as ideal temperature source [19], [20].The nonlinearities i.e., oil viscosity and other transformer oil parameter changes and loss variation with temperature are taken into account by employing non The differential equations of the thermal circuits that given in Figure 1 for modeling both the top oil temperature and the hot spot temperature respectively are as follows [17]: Where: R is the ratio of rated load losses and no load losses [21] K is the load factor [21]; µpu is the oil viscosity per unit value [17]; Coilpu is the specific heat capacity of oil in per unit value; βpu is the coefficient of thermal cubic expansion in per unit value; kpu is the thermal conductivity of oil in per unit value; ρpu is the oil density in per unit value; 1, March 2012 : 43 -54 Direct measurement of actual transformer winding temperatures using fiber optic probes has been increasing since the mid-1980s [13], [14], [15], [16].

Thermal Models of Power Transformers Thermal Model of Oil Power Transformer
The final thermal over all model for oil immersed power transformer is given in Figure 1 electrical analogy and heat transfer theory [17], [18], [19].
Figure 1.Thermal over all circuit models is the total losses; q fe is the heat generated by no load losses; q heat generated in the winding; R th-oil is non linear oil thermal resistance; is the hot spot temperature; θ oil is the top ar winding to oil thermal resistance; C th-wdn the oil thermal capacitance.The heat generated by both no load and load transformer losses are represented by two ideal heat sources [19], [20].
The ambient temperature is represented as ideal temperature source [19], [20].The nonlinearities i.e., oil viscosity and other transformer oil parameter changes and loss variation with temperature are taken into account by employing non-linear thermal resistances [17].
The differential equations of the thermal circuits that given in Figure 1 for modeling both the top oil temperature and the hot spot temperature respectively are as follows [17]: R is the ratio of rated load losses and no load losses [21]; µpu is the oil viscosity per unit value [17]; is the specific heat capacity of oil in per unit value; βpu is the coefficient of thermal cubic expansion in per unit value; kpu is the thermal conductivity of oil in per unit value; ρpu is the oil density in per unit value; ISSN: 1693-6930 ctual transformer winding temperatures using fiber optic probes immersed power transformer is given in Figure 1 electrical analogy and heat transfer theory [17], [18], [19].
is the heat generated by no load losses; q wdn is the is non linear oil thermal resistance; θ amb is the is the top-oil temperature; winding thermal thermal capacitance.The heat generated by both noload and load transformer losses are represented by two ideal heat sources [19], [20].
The ambient temperature is represented as ideal temperature source [19], [20].The nonlinearities i.e., oil viscosity and other transformer oil parameter changes and loss variation hermal resistances [17].The differential equations of the thermal circuits that given in Figure 1 for modeling both the top oil temperature and the hot spot temperature respectively are as follows [17]: (1) (2) θamb is the ambient temperature ; θoil is the top oil temperature; ∆θoil is the rated top oil temperature rise over ambient temperature; ∆θhs,is the rated hot spot temperature rise over top oil; Pl, pu (θe) is the temperature dependence on the load losses in per unit value; Pwdnpu (θhs) is the winding losses dependence on temperature losses in per unit value; τoil,rated is the rated oil time constant [22]; Τwdn, rated is the rated winding time constant and n is constant equal to 0.25 [17].
The winding loss's dependence on temperature, Pwdn,pu(θhs), is as follows: Pdc,pu(θhs) and Peddy,pu(θhs) describe the behaviour of the DC and eddy losses as a function of temperature.The DC losses vary directly with temperature, whereas the eddy losses vary inversely with temperature.θk is the temperature factor for the loss correction θk = 235 for copper.
The temperature dependence of the load losses, Pl,pu( e), is also taken into account as follows: Pdc,pu is the DC loss per unit value; Pa,pu is the additional loss (i.e., equal to the sum of eddy and stray losses) per unit value; θe is the temperature at which the losses are estimated ºC; θk is the temperature factor for the loss correction, θk = 235 for copper.

Thermal Model of SF6 Power Transformer
The theoretical thermal model consists of three basic energy balance equations.A single equation results from an energy balance on each of the three major transformer components.Considering the first component of the gas insulated transformer under the transient condition, the energy generated within the core and coil assembly is equal to the energy stored in it plus the heat loss through convection to the insulated gas.The energy balance equation is: Wgen is the total energy generated within the core and coil assembly of the transformer.Wconv,cg is the convection heat transfer rate between the core and coil assembly and the insulating SF 6 gas= hcg Ac (Tc -Tg).
The natural convective heat transfer coefficient between the core and coil assembly and surrounding gas (hcg) is given by classic Nusselt number correlation's as [23], [24].
where the Nusselt number for the laminar flow is : For transformer loading in excess of half of its rating, the mode of heat transfer along the core and coil assembly become forced convection.The convective heat transfer coefficient in this case takes the form [25]: Where the Nusselt number for the turbulent flow is as follows: .
For the SF 6 insulating gas, the energy transferred by convection from the core and coil assembly is equal to the energy stored in the SF 6 insulating gas plus the energy transferred through convection to the tank inner wall and to the cooling radiators system.Thus, the energy conservation equation under transient conditions is: Wconv,gt is the convective heat transfer rate between the tank inside surface and the insulating SF 6 gas = hgt At (Tg-Tt) Wcon,gr is the convective heat transfer rate between the radiators inside surface and the insulating SF 6 gas = hgr Ari (Tg-Tt).
The convective heat transfer coefficient between the SF 6 insulating gas and the inside of the transformer tank, hgt, can be evaluated using similar procedure equations ( 12) to (20) where Ht is the height of the transformer tank and the convective heat transfer coefficients are given by: The following correlation has been proposed for conditions which result in combined free and forced convection between the tank inside surface and the SF 6 insulating gas [23]: Where Dr is the cooling tube diameter The natural or free convection velocity Vgas,free was measured using laser velocimeter [24] and found to be about 0.3 m/sec.The following relation for evaluation of the Nusselt number in flow through along tube is recommended [26]: The heat transfer coefficient in this case can be determined using the expression: The following correlation has been proposed for conditions which result in combined free and forced convection between the inside of the cooling tubes and the SF 6 insulating gas [23]: At the out side surface of the tank and the cooling radiators, the energy transferred through convection to the tank and cooling radiators from the insulating SF 6 gas, are balanced by the energy stored in the tank plus the convective and radiative energy losses to the atmosphere.Therefore, the energy conservation equation is: where Wconv,ta is the rate of heat flow by convection between the transformer tank outside surface and the ambient air = ) ( Wconv,ra is the rate of heat flow by convection between the outside surface of the radiators and the ambient air = ) ]( [ is the rate of heat flow by radiation from the transformer tank outside surface to the ambient air is the rate of heat flow by radiation from the outside surface of the radiators cooling system to the ambient

× ε σ
The free convection heat transfer Nusselt number can be approximated by the expression [25]: The convective heat transfer coefficient for free convection between the outside of the tank and the air is given by: Where the Renold's number, Re is defined as: The convective heat transfer coefficient for forced convection between the tank outside surface and the air is: The following correlation has been proposed for conditions which result in combined free and forced convection between the outside enclosure of a tank and outside air [23]: Heat transfer for the outer fins is evaluated by [27]: The convective heat transfer coefficient for free convection between the outside of the radiators and the air is given by: Heat transfer from the interior fin passages is evaluated by [27]: The amount of heat transferred by a radiation depends upon a number of factors including surface temperature and emissivity.The radiation exchange factor for rectangular U-channel radiator Fu may be calculated following the same procedure described in [27].thefactor Fu takes the form: Cnet is the net radiation conductance.It is a function of the U-channel can be found is [27].

Oil immersed transformer
The suggested thermal model is applied on 66/11 kV transformer.The applied load as a function of time is given in Figure 2. The obtained results are compared with the measured values and agreement between them is noticed as shown in Figure 3.

SF6 Transformer
The SF6 gas transformer thermal model is applied on transformer 66/11.The measured temperature was in agreement with the calculated temperature as shown in Figure 5 .The load applied to this transformer as shown in Figure 4.The transformer component temperatures are affected by circulation speed of the gas, the higher gas speed the lower transformer component temperature.For comparison between gas insulated gas cooled power transformer and oil immersed power transformer, 66/11 kV 25 MVA, 1200 Amp transformer insulated by oil and another transformer insulated by SF 6 gas.The effect of changing gas pressure and gas circulation velocity on the transformer components temperature as shown in Figures 6 and 7 by applying the load cycle given in figure 8 on the SF6 and oil type transformer the temperature distribution is given in figure 9.The reason of higher temperatures of SF 6 transformer is due to the heat transfer coefficient of SF 6 gas is lower than the heat transfer in oil at stated gas pressure and forced gas velocity, also the thermal capacitance of oil is higher than that of SF 6 gas.It can be noticed that the transformer component temperature decreases with the increase of SF 6 gas pressure.This is because the increase in gas pressure causes an improvement in thermal capacity of the SF6 gas.
As given in Conti Elektro-Berichte [28] the heat transfer in SF 6 gas is equal to the heat transfer in oil at SF 6 gas pressure = 2 MPa and gas velocity = 4.5 m/s, it is noticed that the relationship between the SF 6 gas component temperatures and gas pressure and also with forced gas velocity, when SF 6 gas pressure or velocity increased, the SF 6 gas temperature also increased and transformer windings temparatures are decreased, and vise versa.For increasing the heat transfer and lowering the SF 6 gas transformer component temperature the gas pressure should be increased from 0.24 to 2 MPa and forced gas velocity from 1.5 to 4.5 m/s.has the advantage that having lower temperature, on the other side.The change in the gas temperature with the variation in the load with time is very small compared with the change of oil temperature with the change in the load cycle, also decrease the winding temperature.The two transformers have the same losses "iron losses and copper losses" and at the same conditions "ambient temperature and load".The suggested load distribution applied on both transformers is as shown in the Figure 9. From Figure 10 it is noticed that the temperature of SF 6 gas transformer components is higher than the temperature of oil immersed power transformer components.This difference is between 30 o C and 42 o C depending on the load cycle at SF 6 gas pressure is .24MPa and its velocity is 1.5 m/s

Verification of the obtained results by using QUICK FIELD program
By using quick field program we verify the obtained results by using MATLAB program in case of oil immersed transformer and SF6 gas transformer.This program gives the temperature distribution of the transformer components in contour lines; it means that the temperature at any point in the transformer can be calculated.
Figure 11 (a) and (b) give samples of the obtained results It is noticed that the calculated temperature using quick field program is closed to the actual temperature of the transformer component, moreover quick field programs shows temperature distribution inside transformer.

Conclusions
Two thermal mathematical models to simulate the heat flow and to calculate the cooling medium temperature and the winding temperature of oil and SF 6 transformers are presented and simulated by computer programs.The obtained results from the used models are in agreement with that measured in the field.Due to the higher thermal capacitance of the oil than that of SF 6 gas, the heat transfer coefficient in oil is higher than that of SF 6 gas, so the temperature of oil immersed power transformer component is lower than the temperature of SF 6 gas insulated gas cooled power transformer components having the same ratings.In SF6 transformer the winding temperaturs are affected by gas pressure and temperature Comparasion between Oil Immersed and SF6 Gas Power …. (Osama Elsayed Gouda) case of forced convection the following expression can be used to evaluate the average Nusselt number for turbulent flow over the external surface of the tank[26]: Comparasion between Oil Immersed and SF6 Gas Power …. (Osama Elsayed Gouda) 49

Figure 6 .
Figure10shows the temperatures of oil and SF 6 gas temperature after increasing gas pressure from 0.25 to 2 MPa and gas velocity from 1.5 to 4.5 m/s.From this figure it is noticed that the SF 6 temperature is reduced to be in the range between 47 o C to 55 o C but still the oil

Figure 10 .
Figure 10.Temperature in two transformers after changing gas pressure and circulation

Temperatures in two transformers after increasing gas pressure to 0. 5
MPa and gas circlation velocity to 4 still apply without modification, however the Rayleigh number are determined from the expression: