Performances of distribution transformer installed in metallic enclosures - An Australian experience

Neha kardamM.Tech (power system)11/pps/010

IntroductionHighly 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

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.

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.

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.

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.

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.

Design featuresDistribution 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.

Ambient temperatureFunction 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.

Normal service condition for transformer in free air

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

Maximum ambient temperature

40 c

Average daily ambient temperature

30 c

Average yearly ambient temperature

20 c

Impact of enclosure on transformer temperature risesthe 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

Correction for an increase in an ambient temperature due to kiosk enclosure with 1 transformer

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 :-

Temperature increase in ambient due to enclosure (c)

Transformer size (KVA)250 500 700 1000

10 15 20 -

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.

EnclosureThe 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.

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

Metallic enclosure with ventilation opening on side walls

Air-Temperature diagram along sidewall for 1000kVA kiosk substation

The authors adopted temperatures at two heights as relevant for transformer loading assessment as follows:

Top height mid height

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

Thank you