2019 IEEE 2nd International Conference on Power and Energy Applications

The Effects of Vacuum and Oil Impregnation Duration on Partial Discharge of 167
MVA 500/150 kV Single Phase Power Transformer

Agus Indarto1, Rudy Setiabudy1, Radin Rahmatullah1, Iwa Garniwa1,2, Chairul Hudaya1,2*
1
Department of Electrical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus Baru UI Depok 16424,
Republic of Indonesia
2
Energy System Engineering, Faculty of Engineering, Universitas Indonesia, Kampus Baru UI Depok 16424, Republic of
Indonesia
Email: c.hudaya@eng.ui.ac.id

Abstract—Manufacturing process of oil immersed Power will incredibly affect the overall performance of electrical
Transformer (PT) has significant effect to the value of its power system [3]. Thus, failure to maintain the PT
partial discharge (PD). PD is one of the most important reliability will result in significant lost and degrade the
parameters of PT to ensures the reliability and quality of the reliability of electrical power system [4].
PT's operation PT. As PT is one of the most important parts In the recent years, many electric utilities demand the
in electrical power system, PD becomes one of the key PT specifications to have lower partial discharge, and this
performance indicators for PT. The quality of manufacturing parameter becomes crucial as it becomes the important
process of both liquid and solid insulations will reflect to the indicator of transformer health [5]. For this reason, the low
PD value of PT. Both solid and liquid insulation will form the
partial discharge value of PT becomes an important factor
overall insulation of integration of PT. One of the important
manufacturing processes is vacuum and oil impregnation
to be considered to meet the high quality of product for the
process. These processes ensure the removal of the moisture buyer or owner. Partial discharges (PD) are electrical
from PT insulation and keep maintaining PT from moisture discharge (arcs) which form the surges or breakdown
ingress. In this study, the experiment is conducted to between electrode of any partial area of the insulating
investigate the relation between vacuum and impregnation media of the transformer and the conductors [6]. These are
process duration to partial discharge value of 167 MVA recognized as the main cause of insulation deterioration
500/150 kV on single phase power transformer. Twelve (12) process in power transformers [7]. Therefore, the optimum
samples are investigated during manufacturing process and insulation structure is one of the challenges of transformer
final acceptance testing by measuring vacuum and design and manufacturing process.
impregnation process duration as well as the value of partial The power transformer life is mainly affected by the
discharge. The result shows that vacuum process duration has quality of the insulation system [7]. While the function of
significant effect to PD values. Combined with oil insulation is to separate electrically each conducting part of
impregnation process of the insulation, the results show that equipment and also from earthed components [8]. This
to achieve PD below 70pC (<70pC), the optimum vacuum function might be endangered if partial discharge occurs
duration is 57 hours and the oil impregnation duration is inside the insulation. It makes low partial discharge value
ranged 75-120 hours. This practical study is beneficial for become crucial parameter for high voltage power
both the owner and the manufacturer of PT, so it gives the transformer. Insulation should also be able to deal with
optimum duration for manufacturing process to get lower
various dielectric stresses, oil gap stresses, and creep
partial discharge at optimum duration and cost.
stresses.
Keywords - partial discharge, power transformer, Partial discharges sources are coming from electric field
manufacturing process, oil impregnation, vacuum process in the area of non-homogeneous materials (gaseous, liquid,
or solid media), the deterioration of the insulation and the
I. INTRODUCTION formation of gas that may accumulate at a critical stress
Power transformer (PT) is one of the most important area [8]. Most of the transformer insulation failures are due
parts in electrical power system with the functions of to partial discharges which damage the insulation over the
stepping-up-and-down the voltage and delivering electricity transformer lifespan by gradual degradation of organic
from the power plant to the consumer [1, 2]. The material, such as oil and cellulose-based materials i.e.,
transformer is a fundamental part in electrical power paper and pressboard [9]. In the end, low PD leads to
systems. This equipment represents high cost and requires increase transformer lifespan and to avoid failures.
greater attention during operation as one of critical Based on literature study, the mapping researches
equipment in electrical power system. On the other hand, related to partial discharge of power transformer is shown
the manufacturing process of PT takes considerable time on Fig. 1. The study of PD sources are found from material
and complicated procedures to meet the high standard insulation due to cavity [10], insulation void [11, 12], and
design and requirement. The reliability and quality of PT insulation material impurities [13]. Moreover, for liquid
insulation (transformer oil), the sources of PD come from

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moisture inside transformer oil [14], bubbles inside oil [15], 1. To identify and to analyze the effect of vacuum and
and particles inside oil [16]. Other researches study the impregnation time of active parts on transformer partial
effect of PD on power transformer ageing [16], insulation discharge.
deterioration [17], and insulation breakdown due to voltage 2. To determine the optimum duration of vacuum and oil
impulse [18]. In other study also found the effect of solid impregnation duration to achieve low partial discharge.
insulation drying process to PD value [19]
II. METHODOLOGY
PD's sources: PD's effect on In this research, twelve (12) new and identical samples
Insulation Materials Insulation Materials of 500/150 kV 167 MVA on single power transformers are
investigated, observed, and tested at the factory in
Indonesia. The study focuses on vacuum process and
Solid Insulation: Liquid Insulation: impregnation duration during manufacturing process.
- Cavity [10] - Moisture [14] Vacuum process and impregnation duration are measured
- Void [11,12] - Bubble [15] for each power transformer, and then partial discharge
- Impurities [13] - Particle [16] testing is to measure its value. Within the study, the active
- Drying Effect [19] parts are processed started from drying to partial discharge
- Ageing [16]
testing. The flow chart is shown in Fig. 2.
- Deterioration [17]
- Breakdown [18] Drying
Clamping/Tightening
PD on Power Transformer Exposure Time
Tanking
Fig.1.Mapping of Partial Discharge in Power Transformer
Vacuum
Based on those studies, the PD sources mostly might Oil Filling
come from the insulation. Furthermore, the effect of PD Impregnation Oil Filtration
causes faster insulation damage and ageing. Moreover, Duration and
material insulation condition is also affected by the Resting Time
streamlining
manufacturing process. Therefore, this process is very Partial Discharge Testing
important and critical for partial discharge issues at high
voltage power transformer. Fig.2. Step-by-step of manufacturing process for active parts of PT
This research focuses on vacuum and impregnation A. Exposure Time and Vacuum Duration
process of active parts of insulation as a part of overall
manufacturing process of power transformer. The Exposure time starts when active part is taken out from
investigation covers the phenomenon of how the the drying oven and ends when the active part is put in the
manufacturing processes might cause partial discharge tank ready for vacuum application. For certain time the
source and how effective those processes in eliminating the active part was applied vacuum process. The process
partial discharge sources. The study is to investigate the representation of vacuum of insulation is shown in Figure
relation of vacuum and total impregnation time to the PD 3A.
values of transformer. B. Impregnation Duration
The failure to remove the moisture from the insulation Oil impregnation is a process in which the oil is used to
system can affect the partial discharge value. Liquid fill the pores existing in an insulation material. For
insulation process includes filtering and impregnation cellulosic material (paper, pressboard), the oil impregnation
process which remove particles’ contents, bubbles, and can happen in the cavities between the fibers or in the voids
moisture from the liquid insulation. within the fiber itself [20]. This is used to improve the
Manufacturing process is started by winding, including dielectric strength and to protect the material from
insulation and the core. The next stage is core coil degradation.
assembly by putting winding, core, and insulation together,
then clamping it. After that, the complete active parts are
built, including connection, lead, cover, and all the
insulations. These active parts are then processed for drying,
tightening, tanking, vacuuming, oil filing, and impregnation
before the final testing.
This research will address the following question of
research problem: What are the effects of vacuum and
impregnation time on the value of power transformer
partial discharge source and what is the optimum value?
The purposes of this research are as follow: A B
Fig. 3. Vacuum and impregnation process

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 Impregnation duration includes time of completing up to partial discharge testing. The measurement of vacuum
initial oil filling (active part and all insulation materials process and total oil impregnation duration are done.
under oil), complete oil filling, oil circulation/streamlining,
rest time, and time for other than high voltage tests. The A. Experiment Process and Results
process of oil impregnation of insulation is shown in Figure The purpose of the vacuum process is to remove
3B. Pressboard is assumed as a porous body that moisture absorbed during clamping and tanking as well as
behaves as an assemblage of capillaries with very small to avoid any air pockets in the transformer insulation
radius, which obeys the same law as the rise of the liquid structures. Further ingress of moisture during vacuum
in capillary tubes of circular cross section and in porous application due to leak is also avoided at any time. The
media like wood with different liquid flows that might active parts vacuum process is shown on Fig. 5.
occur [21].
C. Partial Discharge Testing
The equivalent circuit diagram and testing picture for
the measurement of partial discharge is shown on Fig. 4.

Fig. 5. Vacuum application process

After vacuum application, the active part must be

impregnated by oil. This is to avoid remaining air bubbles
and air pockets in the insulation material, which could lead
to partial discharges. The oil has to be degassed, dried, and
filtered before being fed into the tank. The impregnation
duration of transformer board or wood depends on
dimensions, weight and oil temperature. A higher oil
temperature leads to lower viscosity which increases the
speed of impregnation.
In high-voltage oil-filled transformers, both liquid and
solid insulation are used. The insulation is composed of
mineral oil and cellulosic material such as paper,
pressboard, and wood. The cellulosic material is dried
under the vacuum at high temperature and then saturated
with an impregnating medium. This impregnation is used to
Fig. 4.Electrical equivalent circuit and testing facilities for PDs replace the free spaces inside the cellulosic material by
measurement at [22] oil, and hence to avoid partial discharge activity [23]. The
cellulosic materials are used to provide electrical insulation
It consists of high voltage source (Vs), Low Pass Filter as well as mechanical structure for the windings in the
(Z), measuring capacitor (Cm), Coupling Capacitor (Cc), transformer.
Solid insulation model (Ct), measuring instrument (MI), The processes after vacuum are oil filing, filtration, and
and high pass filter PD detector circuit parallel combination streamlining as shown on figure 6, which is a starting point
of resistor, and capacitor and inductor. In this model, the of oil impregnation process. Oil filling is putting oil inside
testing object is shown in the form of small capacitances in the transformer tank. Oil streamllining is process of
which C3 represents the void capacitance, C2 represents the directing or flowing the oil through out active part of power
remaining series (upper and lower) capacitance of the solid transformer to ensure all parts is oil impregnated and
insulator, and C1 represents the remaining discharge free cleaned from the dirt or particles. Oil is oil cleaning
capacitance of the material. processto remove moisture, dirts and particles .
III. RESULTS AND DISCUSSION B. The Effect of Vacuum Duration on Partial Discharge
Values
Twelve (12) active parts of 500/150kV 167MVA on
single phase transformer processing is started from drying Based on the experiment results, the relationship
between vacuum duration and partial discharge value is
shown on graph on Figure 6. Twelve (12) transformer

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samples are investigated by varying the vacuum duration in C. Effect of Oil Impregnation Duration to Partial
two (2) groups of oil impregnation duration (75h and 120h). Discharge Values
In this experiment, two (2) times of impregnation
duration is set to see the effect and the optimum duration of
oil impregnation. Based on the experiment, the relationship
between vacuum duration and oil impregnation and partial
discharge value are shown in graph on Figure 7.
The impregnation process is affected by capillary action
mechanism, in which the oil can flow in the narrow spaces
within pressboard without supported by any external forces
and affected by force applied by the oil on the surface of
insulation or the walls of the pressboard [24]

dV  r 4
Q  . P (1)
dt 8 L
where:
Fig. 6. Oil filling, streamlining and filtration process
Q : the conductivity (m3/s) or the volumetric rate of
liquid
V : the volume of oil inside the capillary, and this
equation is πr2L (m3)
r : the average equivalent capillary radius (m)
η : dynamic viscosity of the liquid (N s/m2)
ΔP : is the pressure differential giving rise to flow (N/m2)
The capillary forces values can be estimated by Jurin’s
law, where the capillary rises as a function of the radius of
the capillary, surface tension, and dynamic viscosity of the
liquid [24]

rt
h2  (2)
2
where:
h : capillary rise (m)
σ : surface tension at interface (pa)
Fig. 7. Relationship between vacuum duration and partial t : time (s)
discharge value η : is the dynamic viscosity of the liquid (N s/m2)
As the sample of active parts (insulation) type,
As vacuum application is to remove the surface dimension and quantity are the same, so it is only the time
moisture absorbed during resting time or moisture which is that should give effect on PD values. From graphic on Fig.
still left during drying time as well as to avoid any air 7, it is clearly shown that the longer impregnation duration
pockets in the transformer insulation structures, the vacuum will result lower PD values as the oil impregnation into
pressure takes important role in the process. Since the cellulose takes most to reduce the moisture of bubble inside.
vacuum pressure applied is the same as for all samples, the Based on the experiment value, to achieve PD values 70 pC
duration gives significant effect on removing the moisture. or lower, the optimum value would be 75 to 120 h.
With the PD values target is below 70pC, the vacuum time
average of 12 experiment of 57 hours is choosed to give IV. CONCLUSION
expected result PD value average of 55 pC. From the The effects of vacuum and oil impregnation duration
graphic, it is clear that a longer vacuum duration will have during active parts (insulation) processing on partial
lower PD values, but at the same time, the cost of vacuum discharge of 167 MVA 500/150 kV power transformers
process will increase. The optimum time of vacuum have been investigated. It is found that the two process
process is 57 hours. parameters affect the PD values. A longer vacuum and
impregnation duration will result in lower PD values. As
the process is related to cost, the optimum value to achieve
the PD target below 70 pC is 57 h for vacuum duration and
75-120 h for oil impregnation duration. As the oil

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impregnation duration consists of oil filling, filtering, and [11] M. G. Niasar, Partial discharge signatures of defects in insulation
streamlining, and resting time, in which oil filling, filtering, systems consisting of oil and oil-impregnated papers: Electrical
Engineering, KTH Royal Institute of Technology Stockholm, 2012.
and streamlining depend on oil quantity, therefore the
[12] W. Zhang, Z. Li, G. Ji, J. Guo, and Y. Yang, "Partial Discharges
optimum oil impregnation duration can be adjusted in the Behaviors of Internal Void in the Oil-Paper Insulation and its Effect
resting time. on the Material," 2015.
[13] M. G. Niasar, R. Kiiza, X. Wang, R. Nikjoo, and H. Edin, "Aging of
ACKNOWLEDGMENTS oil impregnated paper due to PD activity," in 18th Int‗ l. sympos.
The authors would like to thank to Universitas High Voltage Eng.(ISH), Paper No. OF, 2013, pp. 4-04.
Indonesia for the financial support through the 2019 PIT9 [14] M. G. Niasar, H. Edin, X. Wang, and R. Clemence, "Partial
funding scheme managed by Directorate Research and discharge characteristics due to air and water vapor bubbles in oil,"
in XVII Intern. Symp. on High Voltage Eng., Hannover, Germany,
Community Engagement (DRPM) of Universitas Indonesia. 2011, 2011.
[15] Z. Zheng, L. Chen, J. Diao, and R. Wang, "Evolution of bubble in
REFERENCE oil-paper insulation and its influence on partial discharge," in
[1] E. I. Amoiralis, M. A. Tsili, and A. G. Kladas, "Transformer design Electrical Insulation and Dielectric Phenomena (CEIDP), 2014
and optimization: a literature survey," IEEE Transactions on Power IEEE Conference on, 2014, pp. 220-223.
Delivery, vol. 24, pp. 1999-2024, 2009. [16] M. Ghaffarian Niasar, H. Edin, and R. Kiiza, "Oil Aging due to
[2] A. Indarto, I. Garniwa, R. Setiabudy, and C. Hudaya, "Total cost of Partial Discharge Activity," in 23rd NORDIC INSULATION
ownership analysis of 60 MVA 150/120 kV power transformer," in SYMPOSIUM, NORDI-IS13 June 9–12, 2013 Trondheim, Norway,
Quality in Research (QiR): International Symposium on Electrical 2013, pp. 85-88.
and Computer Engineering, 2017 15th International Conference on, [17] R. Khawaja, T. Blackburn, and M. R. Arif, "Ageing and Partial
2017, pp. 291-295. Discharge Patterns in Oil-Impregnated Paper and Pressboard
[3] J. H. Harlow, Electric Power Transformer Engineering, Third Insulation at High Temperature," World Academy of Science,
Edition: Taylor & Francis, 2012. Engineering and Technology, International Journal of Mechanical,
[4] W. H. Bartley, "Analysis of transformer failures," in International Aerospace, Industrial, Mechatronic and Manufacturing Engineering,
Association of Engineering Insurers 36th Annual Conference, 2003, vol. 3, pp. 734-739, 2009.
pp. 1-5. [18] R. Nikjoo, N. Taylor, and H. Edin, "Effect of high voltage impulses
[5] A. Mukhtaruddin, M. Isa, M. R. Adzman, S. I. S. Hasan, M. N. K. H. on partial discharge characteristics of oil-impregnated paper for
Rohani, and C. C. Yii, "Techniques on partial discharge detection online diagnostics," in Power Modulator and High Voltage
and location determination in power transformer," in Electronic Conference (IPMHVC), 2016 IEEE International, 2016, pp. 151-156.
Design (ICED), 2016 3rd International Conference on, 2016, pp. [19] A. Indarto, R. Rahmatullah, and C. Hudaya, "The effects of drying
537-542. time during manufacturing process on partial discharge of 83.3 mva
[6] C57.113-2010 IEEE Recommended Practice for Partial Discharge 275/160 kv power transformer," in MATEC Web of Conferences,
Measurement in Liquid-Filled Power Transformers and Shunt 2018, p. 04008.
Reactors: IEEE / Institute of Electrical and Electronics Engineers [20] A. U. Dufour and G. G. Molinari, "A Study of Pressboard
Incorporated. Monodimensional Impregnation with Transformer Oil and Its
[7] C. Krause, "Power transformer insulation–history, technology and Influence on Dielectric Strength," IEEE Transactions on Electrical
design," IEEE Transactions on Dielectrics and Electrical Insulation, Insulation, pp. 135-144, 1975.
vol. 19, 2012. [21] T. Suzuki and M. Takagi, "Oil Impregnation in Transformer
[8] J. Fuhr and T. Aschwanden, "Identification and localization of PD- Boards," IEEE Transaction in Electrical Insulation, vol. 19, p. 4,
sources in power-transformers and power-generators," IEEE 1984.
Transactions on Dielectrics and Electrical Insulation, vol. 24, pp. [22] E. M. A. Khan, "Measurement Of Partial Discharge (PD) In High
17-30, 2017. Voltage Power Equipment."
[9] S. A. M. Najafi and P. Werle, "Investigation on the possibility of [23] J. Dai, Z. Wang, P. Dyer, A. Darwin, and I. James, "Investigation of
silent partial discharges inside power transformers," in IEEE the impregnation of cellulosic insulations by ester fluids," in
Electrical Insulation Conference (EIC), 2016, 2016, pp. 276-279. Electrical Insulation and Dielectric Phenomena, 2007. CEIDP 2007.
[10] M. G. Niasar, N. Taylor, P. Janus, X. Wang, H. Edin, and R. C. Annual Report-Conference on, 2007, pp. 588-591.
Kiiza, "Partial discharges in a cavity embedded in oil-impregnated [24] N. Irahhauten, "Optimization of the Impregnation Process of
paper: effect of electrical and thermal aging," IEEE Transactions on Cellulose Materials in High Voltage Power transformers-
Dielectrics and Electrical Insulation, vol. 22, pp. 1071-1079, 2015. susceptibility of high-density materials to partial discharge activity,"
2015.

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