This document discusses the performance of distribution transformers installed in metallic enclosures based on Australian experience. It addresses factors that affect the life of distribution transformers such as loading and ambient temperature. The development of compact substations known as kiosk or pad-mounted substations is described. The document also examines the design features of kiosk transformers and their cooling systems, as well as methods to assess the impact of enclosures on transformer temperature rises. Testing confirmed that an optimized design approach can provide distribution transformers with reliable performance when installed in kiosk substations.

1. Performances of distribution
transformer installed in metallic
enclosures - An Australian
experience
UNDER THE BY :-
GUIDANCE OF :- NEHA KARDAM
Dr. ASHU VERMA M.TECH
(POWERSYSTEM)
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

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.

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.
 Australian kiosks employ both–the
hermetically sealed and the free-breathing
distribution transformers.

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
Temperature limits for oil
immersed distribution transformer
Insulation system (top oil 105 c
temperature )
Rated hot spot winding temperature 98 c
Maximum permissible hot spot 140 c
temperature

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