1. Performances of distribution
transformer installed in
metallic enclosures - An
Australian experience
Neha kardam
M.Tech (power
system)
11/pps/010

2. Introduction
 Highly restrictive local environmental and urban
planning regulations have resulted in the
development of very compact packaged substation
with extremely arduous service conditions for built
in transformer.
 In this oil immersed, ONAN cooled, and
hermetically sealed distribution transformers rated
750-2000 KVA is used, highlighting their distinct
features as follows:-
1.Unique design
2. loading capability
3. reliability performance
4. safety features

3. Development of kiosk substation
 The packaged substation in Australia
are better known as pad mounted or
kiosk substations.
 Prefabricated substations as type
tested equipment comprising
transformer, low voltage, and high
voltage (HV) switchgear, connections,
and auxiliary equipment in an
enclosure to supply low voltage
energy from high voltage system.

4.  In Europe, distribution power transformers
and MV and LV switchgear are fully
standardized and type-tested ―off-shelf‖
products.
 The first IEC standard for prefabricated
HV/LV substations was published in 1995.

5. Performance of kiosk transformer
A. Factors affecting life of distribution
transformer
 Australian electrical utilities expect that an
average design life for a modern oil-
immersed distribution transformer should
be in excess of 20–30 years.
 Distribution transformers are relatively
inexpensive, very reliable, and easy to
replace.

6.  First, there is a very emaciated possibility of
using a non optimal rating of those
transformers.
 Second, degradation of insulating materials
caused by increased ambient temperatures
due to restricted air flow around the
transformer is considered to be much more
critical for its lifetime than the external
influences.

7. B. Loading of distribution
transformers
 The maximum intermittent loading of
distribution transformers for normal cycling,
long-term, and short-term loading as 1.5, 1.8,
and 2.0 p.u. of the rated current, respectively.
 The ―distribution transformers‖ should be
further classified into the following three
categories:
• small distribution transformers: below 500 KVA;
• medium distribution transformers: 500–1500
KVA;
• large distribution transformers: above 1500
KVA.

8. Design features
 Distribution transformers in Australia
have bushings mounted on the side tank
walls.
 The kiosk transformers are extremely
compact and usually very narrow and tall.
 Kiosk transformers have very low
electrical losses.
 It employ very efficient cooling systems.

9. Ambient temperature
 Function of the hot-spot temperature and
the top-oil temperature.
 varies as a function of the climate, the
season, the time of the day, etc.

10. Normal service condition for
transformer in free air
Maximum ambient temperature 40 c
Average daily ambient temperature 30 c
Average yearly ambient temperature 20 c
• Australiankiosks employ both–the
hermetically sealed and the free-breathing
distribution transformers.

11. Impact of enclosure on transformer
temperature rises
 the enclosure is defined by its rated
maximum power.
 It is clearly stated that the maximum power,
expected to be delivered from the kiosk, is
lower than the free-air rating of the
transformer.
 The Australian standard recommends two
methods in assessing the impact of the
enclosure on the transformer hot-spot
temperature and the top-oil temperature

12. Correction for an increase in an
ambient temperature due to kiosk
enclosure with 1 transformer
Temperature Transformer size (KVA)
increase in 250 500 700 1000
ambient due to
10 15 20 -
enclosure (c)
The authors have thoroughly investigated both
variations of the second method and it appears
that Table III , which provides recommendations
for correction for increase in ambient
temperature due to the enclosure, should be
extended by considering the following :-

13.  constructional features of the enclosure,
including equipment arrangement, ventilation
systems, and protection (IP) level.
 losses in transformer and switchgear; with a
large number of transformer-switchgear
arrangements, the range of losses released in
the kiosk-substation could be very wide.
 external conditions (solar radiation, wind,
slope sites)
 Larger distribution transformers (1000–2500
kVA)
 Provision for enclosures manufactured from
alternative materials.

14. Enclosure
 The kiosk consists of a metallic enclosure
with transformer and switchgear
compartments and base.
 The enclosure and compartments are made
of 2.5-mm-thick galvanized mild steel sheets.
 The kiosk base is made of a reinforced
concrete or hot-dip-galvanized steel
channels.
 The transformer compartment is in the
middle, completely segregated from the LV
and the MV switchgear compartments.

15.  The standard required degree of
protection for switchgear and
transformer compartments is IP24D.
 Ventilation openings are arranged to
prevent any undesired condensation on
electrical equipment and inner wall
surfaces.
 The optimum airflow is achieved

16. Metallic enclosure with ventilation
opening on side walls

17. Air-Temperature diagram along
sidewall for 1000kVA kiosk
substation

18. The authors adopted temperatures at two
heights as relevant for transformer
loading assessment as follows:
 Top height
 mid height

20. Conclusion
 The reliability of the entire LV network and, thus,
most activities in residential, industrial, and
commercial areas depends on the reliability of
kiosk substations and their most important
part—the distribution transformer.
 A design investigation was formulated to show
the performance of optimized distribution
transformer designs when installed in kiosk
substations. Simple methodology was developed
to forecast temperature rises in transformer
compartments at two different levels: - mid height
and top height of the transformer compartment.
Heat-run tests confirmed calculated temperature
rises under different overload conditions

21. Thank you