On the correlation between winding temperature

and I 2R losses of generator transformer at

Siguragura Power Station
Bagus Brahmantya
Power Station Maintenance Section
PT Indonesia Asahan Aluminium (Persero)
bagusbpg@inalum.co.id

Abstract—Recent efforts have been done to decrease the II. T HE NATURE OF I 2 R L OSSES
temperature at both oil and winding of generator transformer
at Siguragura Power Station, such as by periodic flushing of Each conductor at both primary and secondary winding of
oil cooler tubes and operating standby oil circulating pump. transformer has resistance R which depends upon the length
However, the purpose was mainly to maintain a safe operating L, cross section A, and resistivity ρ of the conductor [1], as
temperature while transformer is being loaded close to its full- follow
capacity continuously, moreover after long service time (near 35 ρL
years). This paper aims to show that decreasing the temperature R= (1)
A
of oil, which in turns will decrease the temperature of winding,
may also decrease I 2 R losses. However, further study should be Current flows through the windings, thus, will inevitably
performed to determine whether decreasing oil temperature will pass through their resistances and generate I 2 R losses as given
directly imply transformer’s efficiency improvement or not. by
2 2
Wcopper = Ipri. Rpri. + Isec. Rsec. (2)
I. I NTRODUCTION
with Rpri. and Rsec. are primary and secondary winding
A practical power transformer can never transform all of resistance, respectively. As shown in Eq. 2 above, I 2 R losses
its input power into output. According to [1], losses of power depends on current the transformer carries: Ipri. and Isec. .
transformer are composed of the following: Measurement of losses and efficiency of generator trans-
1) I 2 R losses in the winding former has been done during commissioning test [2], as
2) Hysteresis and eddy-current losses in the core, and summarized in Table I1 . As Eq. 2 dictates, I 2 R losses2 varies
3) Stray losses due to currents induced in the tank and only by magnitude of load (i.e. current), independent of power
metal supports by the leakage fluxes factor.
Those losses, expressed in watts or kilowatts, appear in the
form of heat and will increase the temperature of transformer. TABLE I
M EASUREMENT OF LOSSES
Without proper handling, the temperature will continue to rise
and may deteriorate transformer’s insulation, thus shortening L. F. P. F. Wcopper WH Wtotal Pinput η
its life. Tests made on many insulating materials have shown +Wstray +WE
(%) (%) (kW) (kW) (kW) (kW) (%)
that the service life of electrical apparatus diminishes approx- 100 100 366.9 81.0 447.9 79848 99.44
imately by half for every 10◦ C increase in temperature [1]. 80 100 234.8 81.0 315.8 63836 99.51
Generator transformer at Siguragura Power Station specifies 60 100 132.1 81.0 213.1 47853 99.55
40 100 58.7 81.0 139.7 31900 99.56
its temperature rise of winding at 60◦ C [2]. This means 100 90 366.9 81.0 447.9 71908 99.38
that transformer would probably last for 2 to 5 years if 80 90 234.8 81.0 315.8 57484 99.45
operated continuously at 105◦ C of winding temperature. Con- 60 90 132.1 81.0 213.1 43089 99.51
sequently, longer service life may be achieved by operating the 40 90 58.7 81.0 139.7 28724 99.51
transformer at lower temperature. To maintain safe operating
temperature, generator transformer at Siguragura Power station
III. E VALUATION OF I 2 R L OSSES
is equipped with forced-oil forced-water cooling system.
AGAINST W INDING T EMPERATURE
However, with high demand for availability and reliability
of generator transformer at Siguragura Power Station to serve Imagine the transformer load is changed from Pout,1 and
increasing load at aluminum smelter in Kuala Tanjung, ques- kept to certain higher value Pout,2 with same power factor.
tions like how to reduce transformer losses? how to extend This will result in increased Joule heating due to higher I 2 R
service live? is it possible to connect more loads than power 1 Test was done at rated voltage of 10.5kV/281.25kV, corresponding to tap
rating? are frequently asked. This paper focuses on the study position no. 2. ”L. F.” stands for load factor, ”P. F.” stands for power factor.
about the first question, specifically addressing I 2 R losses. 2 In Table I, values of I 2 R losses have been corrected to 75◦ C.
losses, causing the winding temperature rise from T1 until transformer uses tap 1 and is loaded at 90% of its rated
reaching new thermal equilibrium at T2 with oil circulating capacity, for example, then I 2 R losses will be reduced as much
around the winding. However, any change in I 2 R losses as
does not solely depend on change in load current, since both 2  3
× 4.52 Ω

Rpri. and Rsec. in Eq. 2 may also change in value when 79400 kVA
90% × √ × 2 +
the temperature change. Such changes may occur because 3 (287.5 kV)2
9
× 0.0044 Ω

resistivity ρ of conductor in Eq. 1 varies with the temperature
[3], as follow √2 ×0.00321/◦ C
( 3 × 10.5 kV)2
ρ2 = ρ1 [1 + α1 (T2 − T1 )] (3) or 775 watts for every 1◦ C drop in winding temperature.
with ρ2 and ρ1 are winding resistivity at temperature T2 and
TABLE II
T1 , respectively, and α1 is numerical coefficient corresponding M EASUREMENT OF WINDING RESISTANCE
to the fractional change in the resistivity per unit temperature
at T1 as reference. Winding Tap R at 17◦ C (Ω) R at 75◦ C
Pos. U-V V-W W-U Average Average
The direct consequence of Eq. 3 is that I 2 R losses will H. V. 1 3.67 3.67 3.69 3.68 4.52
be slightly higher than that of caused by current increase H. V. 2 3.59 3.60 3.60 3.60 4.42
alone. To account for the effect of temperature, Eq. 3 may H. V. 3 3.52 3.53 3.53 3.53 4.34
u-v v-w w-u
be extended using Eq. 2 to calculate the corrected value of L. V. - 3.52m 3.59m 3.62m 3.58m 4.40m
I 2 R losses as follow
Wcopper,2 = Wcopper,1 [1 + α1 (T2 − T1 )] (4)
IV. M ETHODS OF D ECREASING W INDING T EMPERATURE
2
where Wcopper,1 and Wcopper,2 are I R losses for the same In this section, three methods are briefly presented and
load current but at different temperature, i.e. T1 and T2 , compared each other. Natural temperature variations of trans-
respectively. former oil during shutdown are also presented as reference
As Eq. 4 predicts, lower I 2 R losses may be achieved by case and to help identifying the effectiveness of each of
decreasing winding temperature. Consider again, somehow, the those methods. Indirect comparison based on oil, rather than
cooling system can decrease the winding temperature from T2 winding, temperature will be applied5 .
to T20 , while Pout,2 is maintained, then the I 2 R losses will be
reduced3 to A. Oil Temperature Variations During Shutdown
Wcopper,20 = Wcopper,1 [1 + α1 (T20 − T1 )] Fig. 1 shows a typical example of oil temperature variations
naturally occurred in a day during which 7-hours shutdown is
Thus, the decrease in I 2 R losses due to temperature change performed. In such case, oil temperature drops about 1.7 to
∆T = T2 − T20 is 1.8◦ C from its initial value.
∆Wcopper = Wcopper,1 α1 ∆T (5)
75
Generalized interpretation of Eq. 5 suggests that for the
same load current, the decrease in I 2 R losses is proportional shutdown interval
to the decrease in winding temperature, and vice versa. 60
The value of α will be calculated at 75◦ C as reference
temperature since data of resistances and I 2 R losses at that
45
temperature are made available in test report [2], which in turn
will be very helpful in determining ∆Wcopper in Eq. 5.
Referring to Table II4 , α at 75◦ C, or simply denoted as α75 , 30
can be calculated as follow
R75 − R17
α75 = (6) 15
58R75
Oil Temp. (◦ C)
where R17 and R75 are winding resistance at 17◦ C and Load (MVA)
75◦ C, respectively, which yields the value of α75 to be 0
0.003221±0.000008/◦ C. Applying this to Eq. 5 and assuming 0 2 4 6 8 10 12 14 16 18 20 22 24
Time (hour)
3 Remember that Eq. 4 can also be used to estimate I 2 R losses at any
temperature between T1 and T2 .
4 ”H. V.” in Table II stands for high voltage side, ”L. V.” stands for low Fig. 1. Temperature variations during shutdown
voltage. Tap #1 corresponds to 287.5kV, tap #2 281.25kV, and tap #3 275kV.
High voltage windings are star connected with neutral, while low voltage
windings are delta connected. U-V-W and u-v-w corresponds to terminals at 5 Collected data of previous years show that drop in winding temperature
H. V. and L. V. windings, respectively. can be fairly approximated by drop in oil temperature
B. Method I: Major Flushing on Oil Cooler Tube 75
The water used for generator transformer cooling system is
taken from the draft tube and discharged to the tail-race surge
60
chamber [4]. Although this cooling water passes through a
strainer for debris removal before being used in the cooling
system, a small amount of mud contained within the cooling 45 oil temperature before minor flushing
water can still get pass through the strainer. The mud may
accumulate inside oil cooler tubes of generator transformer
after some period of time and decrease their overall cooling 30
capacity. A regular 7-hours shutdown is required, typically for
every 2 years, in which oil cooler tubes are cleaned thoroughly.
15
Oil Temp. (◦ C)
75 Load (MVA)
0
0 2 4 6 8 10 12 14 16 18 20 22
60 Time (days)
oil temperature before major flushing
Fig. 3. Temperature variations after minor flushing
45

Applying Eq. 7 for the case of minor flushing, the energy

30 saving attained is around 1MWh, again assuming 90% loading
of transformer. If minor flushing is done every two months,
the resulted energy saving will be equivalent to 6MWh per
15
year.
Oil Temp. (◦ C)
Load (MVA) D. Method III: Operating Stand-by Oil Pump
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Recently, so much interest is put to increase oil flow rate,
Time (months) thus increasing cooler performance, by operating a stand-by
oil pump. Experience shows that this action could decrease oil
Fig. 2. Temperature variations after major flushing temperature by an average from 2.5 to 3.5◦ C. However, oil
pump consumes power about 1.5kW while operating. Using
Fig. 2 shows a typical example of oil temperature variations this fact together with Eq. 7, net energy saving attained while
after major flushing, in which an initial drop as much as 15◦ C operating stand-by oil pump thus becomes 3.5 to 10.5MWh
is usually observed, and then the temperature gradually rises per year. Surprisingly, this value is comparable to that of major
until reaching its former value after 4 to 5 months later. flushing discussed earlier6 .
The energy saving due to temperature drop after major V. D ISCUSSION
flushing can be approximated by
A. Energy Saving In Terms of Total Energy Output in A Year
fX
inal
Assuming constant 90% capacity loading and 95% power
∆E = Wcopper,75 α75 ∆Ti ∆ti (7) factor, energy saved by method I, II, and III are all much less
i=initial
than 0.1% of total energy output of the transformer in a year.
where ∆Ti is average temperature drop during time inter-
B. Connection Between Temperature and Service Life of
val ∆ti . ∆E is evaluated between the moment after major
Transformer
flushing (tinitial ) and after reaching back the initial tempera-
ture (tf inal ). Using this Eq. 7, every major ushing can save According to [5], relative ageing rate V for thermally
energy up to 18MWh, assuming 90% loading of transformer. upgraded insulation paper is given by
 15000 15000 
C. Method II:Minor Flushing on Oil Cooler Tube V = exp − (8)
110 + 273 θh + 273
Simplified method of fushing has been put into trials, in from which the loss of life L of transformer over a certain
which high pressure water and air were circulated inside oil period of time can be calculated as follow
cooler tubes during 7-hours shutdown. A typical example of Z t2
oil temperature variations after minor flushing is shown in Fig. L= V dt (9)
3, in which an initial drop as much as 7◦ C is usually observed, t1

and then the temperature gradually rises until reaching its 6 Because major flushing is done every two years, the energy saving
former value after around 20 days later. calculated in subsection IV-B is then equivalent to 9MWh per year.
where θh is hot-spot temperature7 , while t1 and t2 are bound- (Persero) for their hard work in keeping generator transformers
ary of time period under consideration. in prime condition, also members of power plant operation
Eq. 8 and 9 can be applied for each method discussed in section for the idea of operating stand-by oil pump and engi-
Section IV. Generally, the decrease of hot-spot temperature neering section for their cooperation during data collection.
will result in smaller ageing rate V , and finally reducing
transformer loss of life L. R EFERENCES
[1] T. Wildi, Electrical machines, drives, and power system, 5th ed. Prentice-
VI. C ONCLUSION Hall, 2002.
Three methods of decreasing transformer operating tem- [2] H. Tanaka et al., “Test record of underground main transformers and
halon 1301 fire extinguishing system,” 1981.
perature are presented and examined. I 2 R losses are shown [3] S. O. Kasap, Principles of electronic materials and devices, 3rd ed.
reduced. Although energy saving in a year from such reduction McGraw Hill, 2006.
is not significant compared to the total energy output of the [4] “Project completion report for asahan hydroelectric and aluminum
project: hydroelectric part,” 1984.
transformer, other advantage of extending transformer service [5] “Power transformers — Part 7: Loading guide for oil-immersed power
life is gained. By selecting the most optimum combination transformers,” IEC 60076-7:2005, 2007.
of those three methods, more energy saving and extension of
transformer service life are expected.
ACKNOWLEDGMENT 7 in this case can be approximated by the one recored via dial thermometer

The author wishes to thank all members of power plant for winding temperature
maintenance section of PT Indonesia Asahan Alumunium