2014 IEEE International Conference on Liquid Dielectrics, Bled, Slovenia, June 30 - July 3, 2014

High Thermal Conductivity Transformer Oil Filled

with BN Nanoparticles

B. X. Du X. L. Li
,

School of Electrical Engineering and Automation

Tianjin University
Tianjin 300072, China

Abstract-Liquid insulation plays a significant role in the volume resistivity, relative permittivity and dissipation factor
design and operation of power system. There is good evidence in with the addition of magnetic nanoparticles. In the thennal
the literature that the addition of nanoparticles to conventional properties, millimeter and micrometer size particles were
transformer oils can lead to significant increases both in earlier used in liquids to increase their effective thennal
electrical and thermal characteristics. In this paper, spherical BN conductivity. But unfortunately these suffer from stability
nanoparticles with high thermal conductivity were added into
problems and cause severe clogging. The advancement in
transformer oil to form nanofluids, with the aim of enhancing
Science and Technology has led to the development of nano
electrical and thermal properties. Electrical properties and
sized « 100 nm) particles which form stable suspensions when
thermal characteristics of oil samples before and after
introduced into heattransfer liquids [3]. Y. F. Du et al have
modification were measured. It was found that the nanofluids
used TiOz and Zr02 nanoparticles and pointed enhancement in
have higher AC breakdown voltage and relative permittivity,
dielectric and thermal properties. B. X. Du et al [4] have
lower dissipation factor which indicates better electrical
reported that the epoxy mixed with BN particle have
properties compared with pure oil. In order to investigate the
effect of BN nanoparticles on thermal properties, temperature
significant improvement in thermal conductivity compared to
changes and heat distribution of the nanofluids with different the pure epoxy.
concentrations of BN particles in thermal conductivity process
Many efforts have mainly been focused on the nano-metal
were measured by an infrared thermal imager and a temperature
oxide particles or magnetic particles. However, the side effect
sensor. Obtained results showed that, under the same
of utilizing magnetic fluids is their heating under the influence
experimental condition, the heat dissipation of nanofluids with
of an alternating magnetic field, especially at high frequencies
higher concentration of BN particles was faster. It is concluded
[9]. However, so far most studies on nano-modified
that the added BN fillers have a very significant effect on
electrical and thermal properties in transformer oil-based fluids.
transformer oil have been limited to the electrical properties or
thennal properties only. Very few studies have considered
Keywords-BN nanoparticles; thermal conductivity; electrical developing a type of nanofuilds with improved properties of
property; ballistic phonon transport; interfacial region; nanofluids both electrical strength and thennal conductivity, combined
with the relation between the improved thermal conductivity
and the electrical property.
I. INTRODUCTION

Transfonner oil is a highly refined mineral oil which This paper presents a new type of nano-modified
performs the dual functions of insulation and cooling. The oil transformer oil by adding different amounts of BN
used in transfonners also has an effect on the dielectric nanoparticles into pure transformer oil. Breakdown strength
properties of paper and pressboard insulations [ I]. The oil fills and dielectric properties of transfonner oils with different
up the pores in the fibrous insulation thus increasing its weight ratios of BN nanoparticles were presented combined
breakdown strength. From the viewpoint of thermal properties, with thennal conductivity in heating and cooling process.
the oil removes the heat generated during the operation of the
transformer by convective heat transfer. Thus any work related II. EXPERIMENTAL DETAILS
to transformer oil must take into account both its dielectric
properties and thermal characteristics. A. Preparation ofNano Fluids

Recently, much effort has been focused on preparing The experimental specimens were made by dispersing BN
magnetic nanofluids by suspending magnetite nanoparticles nanoparticles with grain diameter of 50 nm into pure
into the transformer oil and testing their electrical property. It transfonner oil with different weight ratios. The mineral oil
has been reported that the AC breakdown voltage, the lightning (25# Karamay) in the experiment was filtered through a
impulse breakdown voltage and the negative DC breakdown membrane filter in order to remove the impurity particles and
voltage achieves increase of 20%, 53% and 28% respectively meet the demand of clean oil defined by ClORE working group
with the presence of nanoparticles compared to that of pure oil 12.17 as the particle content with a diameter larger than 5 [.tm is
[2]. For dielectric properties, there is significant increase in down to 300 per 100 ml in oil sample by the supplier [5].

Thanks to Chinese National Natural Science Foundation and State Key

Lab. Electrical Insulation and Power Equipment Foundation.

978-1-4799-2063-1 $31.00 ©2014 IEEE

 2014 IEEE International Conference on Liquid Dielectrics, Bled, Slovenia, June 30 - July 3, 2014

The transfonner oil was prepared in five copies, one copy heating coil in each oil sample was measured by a temperature
for base oil as contrast test, the other four types were mixed sensor.
with BN nanoparticles with different weight ratios of 0. 01,
To study the influence of BN nanoparticles in heat
0. 03, O.OS, 0.1 wt%, respectively. All the nanofluids were
dissipation process inside the oil, overall heat transfer test was
prepared under water bath and ultrasonic vibration at 2S°C for
taken. Coil in oil samples with different weight ratios of BN
40 minutes, and then they were stirred for 1 hour by
nanoparticles was heated under the same heat power for 20
mechanical stirring rod at 40°C to ensure the nanoparticles
min, and then the power was off, the coil was cooled in the
dispersed in the transformer oil uniformly. After mixing nanofluids for another 20 min. The temperature change of the
process, all the samples were dried in the vacuum box for 2 oil nearby the heating coil was measured by thennocouple
hours in order to extrude moisture and air. After these which was fixed at a distance of 3mm next to the heating coil.
treatments, the samples were kept in a sealed cabin. A time sequence of temperature measurements of the
nanofluids was conducted at intervals of 1 min for each oil
B. AC Breakdown Voltage Test sample. The setup of thermal conductivity experiment is shown
The AC breakdown test is generally used as acceptance test in Fig. l.
before filling the oil inside the transformers. AC breakdown

i� i
test was performed using copper electrodes which were set 2 Data acquisition
system
mm apart. AC voltage is generated by using a 220VIl00kV
step up transformer. Considering the dispersion of the
experimental results, for each type of transformer oil sample,
Infrared thermal
five sets of breakdown test were carried out. The experiments Om
Temperature
' "�
were performed at room temperature. The waveforms were
recorded using the OWON SDS8202(V) Digital Storage
L
1�
Dc
Oscilloscope (DSO) at the sampling rate of IGS/s and source l 11111111""""

bandwidth of 300MHz. Temperature

-= measurement device

C. Relative Permittivity and Dissipation Factor Test Heating coil Oil sample

Relative permittivity is a physical quantity of dielectric

polarization characteristics. Dissipation factor is a good tool to Fig. 1. Experimental setup for thermal conductivity tests
indicate the perfonnance of insulation. A high value of
dissipation factor is an indication of the presence of
III. EXPERIMENTAL RESULTS
contaminations or deteriorations products such as water,
oxidation products, and etc. Relative permittivity and
A. AC Breakdown Strength
dissipation factor of the oil samples were measured according
to IEC 60247-2004, using a high voltage automatic dissipation The AC breakdown strength measured for the four different
factor measurement kit. types of the nano-modified transformer oil and the non­
modified transformer oil are showed in Fig. 2. The breakdown
D. Measurement of Thermal Characteristics strength of nano-modified transformer oil samples was all
greater than that of non-modified transformer oil sample. From
The oil used in transformer perfonns the dual functions of
Fig. 2, it is observed that there is a significant increase in
insulation and cooling. In order to investigate the effect of BN
breakdown strength value of BN nanofluids if the
nanoparticles on thermal conductivity in transfonner oil-based
concentration increases. The results demonstrate that the
fluids, oil samples with different concentrations of BN
presence of the BN nanoparticles in the oil effectively inhibits
nanoparticles were heated and cooled under the same
the processes of electrical breakdown.
environment temperature. To simulate the temperature rise of
146 ,...-----,
transfonner windings, heating coil was heated inside the BN
nanofluids for SO min at the same power by a DC source. The
initial heating temperature was ISoC, and the power was SW.
The changes of the surface temperature of the oil samples were E 144

detected by the infrared thennal imager (HY-G90 by SAT Ltd. �E

temperature resolution: < I °C, frame time: 0.2 s). A time c;,
c
� 142
sequence of thermal conductivity measurements of the BN 1;)

nanofluids was conducted at intervals of S min.

c
;:
o
�
'"
From the viewpoint of thennal properties, the oil removes � 140
(!l
the heat generated during the operation of the transformer and
cools the transformer windings. For cooling process, the
heating coil was heated up to 7SoC outside the oil samples and 138
0.01 0.03 0.05 0.1
then was cooled in the transformer oils for S min with the
Concentration(wt%)
initial oil temperature of ISoC. The surface temperature of the
Fig. 2. Relation between the breakdown strength and the concentration

978-1-4799-2063-1 $31.00 ©2014 IEEE

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 2014 IEEE International Conference on Liquid Dielectrics, Bled, Slovenia, June 30 - July 3, 2014

Discharge magnitude is an important characterization in C. Dissipation Factor

breakdown strength, and a large discharge magnitude indicates The dissipation factor of the nano-modified transformer oils
severe breakdown process and serious deterioration of and non-modified transformer oil was also measured. From
breakdown on insulation material. Fig. 3 shows the discharge Fig. 5, it is shown that the dissipation factor of the nano­
magnitude in the first 1ms in the AC breakdown process for the modified transformer oil samples is smaller than that of the
oil samples. As shown in Fig. 3, the discharge magnitude non-modified transformer oil sample. As the nanoparticle
shows a significant decrease in the nano-modified oil compared concentration is varied, the dissipation factor decreases from
with pure oil, especially with the concentration of the 0.315% to 0.226%. This indicates that with the presence of BN
nanoparticles increasing up, this decrement becomes obvious, nanoparticles, there is a significant improvement in dielectric
which demonstrates the enhancement in breakdown strength. properties which results in restricting the insulation aging
The small discharge magnitude indicates the difficult in the process in transformer oil.
break of the chemical bond, which shows that the inhibition of 0.32 r------,
deterioration in breakdown process with the addition of BN
nanoparticles.
0.30
42 r-
---,
�
o
U 0.28
40 J11
c
13 o

l'"
::l

Q)
'0 O.26
.a 38
'1" is
'"
'"
E
Q) 0.24
e> 36
'"
"
.c

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is
34 0.22
0.01 0.03 0.05 0.1
Concentration(wt%)
32
0.01 0.03 0.05 0.1
Fig. 5. Relation between the dissipation factor and the concentration
Concentration(wt%)

Fig. 3. Relation between the discharge capacity and the concentration D. Thermal Accumulation

The thermal accumulation process of the nano-modified

B. Relative Permittivity transformer oils and non-modified transformer oil was given in
Fig. 6. The temperature of the samples was measured by an
The relative permittivity of the nano-modified transformer
infrared thermal imager every five minutes for 50 minutes. [t is
oils and non-modified transformer oil was measured. From Fig.
observed that the surface temperature of the nano-modified
4, it is observed that the relative permittivity of the nano­
transformer oils rises up fast as the nanoparticle concentration
modified transformer oils is always greater than that of the
increases. There is an obvious increase in the heating up
non-modified transformer oil sample. There is an obvious process in the nano-modified oils compared with pure oil. The
increase in relative permittivity as the nanoparticle results demonstrate that, heat dissipation rate of the BN
concentration is increased, which is beneficial in causing a nanotluids has a distinct increase due to the addition of BN
more uniform electric field in oil-paper insulation. Over the nanoparticles.
concentration range considered the relative permittivity is
80 r-------,
increasing from 2.21 to 2.34. ---- O%wt
2.36 r-------, ---.- 0.01 %wt
--4- 0.03%wt
70
� 0.05%wt

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p
2.32
.i='
·5
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::l
60

::: 1§OJ
E "- Surface
E temperature .....-;;::: Infrared
� 2 50

flt2f1
Q) 2.28
>
� 6
W
0::
40
2.24
Heating coil

30
10 15 20 25 30 35 40 45 50

2.20 Heating time(min)

0.01 0.03 0.05 0.1
Concentration(wt%)
Fig. 6. Relation between the surface temperature and the heating time with
Fig. 4. Relation between the relative permittivity and the concentration different concentrations

978-1-4799-2063-1 $31.00 ©2014 IEEE

3
 2014 IEEE International Conference on Liquid Dielectrics, Bled, Slovenia, June 30 - July 3, 2014

E. Cooling Process However, in the down part of the curve, the difference in

The cooling process of the nano-modified transformer oils temperature drop rate of the five oil samples is not obvious
and non-modified transfonner oil was given in Fig. 7. The compared with the heating process. It is well known that
surface temperature of heating coil was measured by a temperature difference has a significant influence on heat
temperature sensor every 10 seconds for the first 30 seconds as conduction process. Therefore, we can speculate that for the
the temperature drops very fast, then it was measured every 30 non-modified oil, a relative higher start temperature contributes
seconds. It is observed that the cooling process of the nano­ to the heat dissipation process, although it has a lower thennal
modified transformer oils accelerates as the nanoparticle conduction compared with nano-modified oils as shown in Fig.
concentration is increased. 6. Hence, it is concluded from Fig. 8 that with the addition of
BN particles, there is a distinct improvement in thermal
75
Temperature ---- O%wt conductivity in the overall heat transfer.
Heating coil I- sensor ----- 0.01%wl
----A---- 0.03%wl

�
65 ---T- 0.05%wl
---+- 0.1%wl IV. CONCLUSIONS
0'
� 55 This work focuses on electrical properties and thermal
.3
!" temperature characteristics of nano-modified transformer oiI. Differences of
Ql
"- of coil
E electrical and thermal properties between nano-modified and
.$ 45
non-modified insulating oils are also presented. The
'0
()
conclusions can be summarized as follows:
35
• Compared with non-modified transfonner oil, the
relative permittivity of BN nano-modified transformer
25
oil samples is higher and the dissipation factor is lower,
30 60 90 120 150 180 210 240 270 300
which demonstrates an increment in dielectric
Cooling lime(sec)
properties.

Fig. 7. Relation between the coil temperature and the decay time with • There is significant improvement in breakdown strength
different concentrations
in non-modified transformer oil. Furthermore, the
breakdown voltage value of nano-modified transformer
F. Overall Heat Transfer oil increases with the concentration increasing up

Overall heat transfer process of transfonner oils was given • The thermal conductivity tests results demonstrate that
in Fig. 8. The power was off after the coil was heated for 20 there is a significant improvement in thermal properties
min, then the coil was cooled in the oil for another 20 min. For due to the addition of BN particles. Furthennore, with
the heating process, it is observed that the temperature rise in concentration of BN particles increasing up, the
pure oil is faster compared with nano-modified transfonner increase in thermal properties becomes evident
oils. Moreover, the temperature rise slows down if the
concentration is increased up in the nanofluids. It can be
ACKNOWLEDGMENT
confirmed from Fig. 6 and Fig. 8 that the heat generated by the
heat source was conducted to the surrounding oil and then This work is supported by the Chinese National Natural
dispersed to the entire oil in a fast way due to the addition of Science Foundation under the Grant 51277131,State Key Lab.
the BN nanoparticle. Electrical Insulation and Power Equipment under the Grant

42 .-
EIPEI0204 and State Key Lab. Power System under the Grant
----�--__,
Cooling SKLDllKZ06.
40 ---- 0% wt process
----- 0.01%wt
38 ----A---- 0.03%wt REFERENCES
---T- 0.05%wt
0' 36 [I] C. P. Sugumaran, "Experimental Evaluation on Dielectric and Thermal
Characteristics of Nano Filler Added Transformer Oil", IEEE Int. Conf
�
.a 34 High Voltage Eng. AppL (ICHVE), pp. 207-210,2012.
!"
Ql [2] 1. C. Lee, W. H. Lee, S. H. Lee and S. Y. Lee, "Positive and Negative
@" 32 Heating ceil
.$ Effects of Dielectric Breakdown in Transformer Oil Based Magnetic
30 Fluids", Mater. Res. Bull.,Vol. 47, No. 10,pp. 2984-2987,2012.
o
[3] H. Jin, T. Andritsch, P. H. F. Morshuis and 1. 1. Smit, "AC Breakdown
28
Voltage and Viscosity of Mineral Oil Based Si02 Nanofluids", IEEE
Conf Electr. InsuL Dielectr. Phenomena (CEIDP), pp. 902-905,2012.
26
[4] B. X. Du, M. Xiao and 1. W Zhang, "Eflect of Thermal Conductivity on
--�----�--�
24 U- Tracking Failure of Epoxy/BN Composite under Pulse Strength", IEEE
10 15 20 25 30 35 40
Trans. Dielectr. Electr. InsuL,VoL20,No. 1, pp. 296-302,2013.
Lapse lime(min)
[5] CIGRE, "Effect of Particles on Transformer Dielectric Strength",
Working Group 17 of Study Committee 12,June 2000.
Fig. 8. Relation between the oil temperature and the lapse time with different
concentrations

978-1-4799-2063-1 $31.00 ©2014 IEEE