Broadly, transformer can be divided into two types-
‘Step up’Transformer- which converts low voltage to high voltage
‘Step down’Transformer-which converts high voltage to low voltage.
Application wise Transformers can be categorized as below-
Power Transformers- Typically Used in transmission network, usually high rating (kVA capacity) 3000 kVA & above. Distribution Transformers- Step down transformers; mostly used in the distribution system of electricity boards for auxiliary supply in industrial & domestic purposes having LV voltage 433V. Usually low to medium rating & lower than that of Power transformers.

Special purpose transformers- Step down or Step up transformers used for specific application in various fields/industries with specific LV voltage other than 433V e.g. 575V or 460V or 690V etc. Examples- Windmill Transformer, Solar transformer, Furnace transformer, Booster transformer etc.

Construction wise Transformers can be categorized as below-
Non-Sealed Transformers- Oil immersed transformers design, where the dielectric insulating oil in the transformer tank is in contact with the external air through a conservator tank & a device known as silica gel dehydrating breather attached to its conservator tank. As external air is constantly moving in & out of transformer with expansion & contraction of oil, it is also called as breathing transformer.

Sealed Transformers- Oil immersed transformers design, where the dielectric insulating Oil in the transformer tank is in zero contact with the external air & in that sense is completely sealed. Since the oil within does not come in contact with external air there is hardly any oxidation of oil which increases the operating lifetime of the transformer. As there is no movement of air in & out of the transformer; it is also called a non-breathing transformer. These are mostly mounted in compact substations where it is to be accommodated in compact dimensions due to space constraints. Scheduled yearly filtration of oil is not required for this type therefore it is also called maintenance free transformers.

IS 1180 : 2014- For Distribution Transformer (used for auxiliary purpose i.e. output voltage of 433V)

IS 2026 : 2011- For rest of the special purpose Transformer types (Windmill, Solar, Furnace etc.)

Losses: In any electrical machine, 'loss' can be defined as the difference between input power and output power. Transformer consists of No load loss (iron losses)& Load loss (Conductor losses). Losses play a significant role in total ownership cost by way of electricity consumption as well as operating life of the transformer.
Inshort: Greater losses = Less purchase cost = Higher recurring cost in the form of electricity bill = Sad Owner Less losses = Higher purchase cost = Less recurring cost in the form of electricity bill = Happy Owner Impedance: It is the supply voltage, expressed as a % of rated voltage that is required to circulate rated current through the transformer. It is also an indicator of how the transformer will behave, in terms of its fault level and voltage regulation aspects. The impedance of a transformer has a major effect on system fault levels. It determines the maximum value of current that will flow under fault conditions. Hence, It is used for determining the interrupting capacity of switchgear employed to protect the primary of a transformer. In Short - Lower the impedance higher the fault level & Vice Versa.

BIL or Basic Impulse Level Is a method of expressing the voltage surge that a transformer will tolerate without breakdown. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground.

For Distribution Transformers with IS 1180 : 2014
i) Total loss at 50% loading
ii) Total loss at 100% loading
iii) % Impedance

For Special Purpose Transformers with IS 2026 : 2011
i) No load loss
ii) Load loss at 750C
iii) % Impedance

Other than above parameters customer should also ask & ensure for all legal & statutory requirements as well as Quality aspects, which is always over looked

LEGAL & STATUTORY REQUIREMENTS:
For Distribution transformer mandatory requirements from manufacturer are as listed below- Manufacturer must have valid (Renewed) BIS license for manufacturing of a particular rating. Manufacturer must have valid Type Test Certificate & its inclusion endorsement by BIS in License of a particular rating to sale in a market.
NOTE- For any standard rating of distribution transformer (with IS 1180:2014) if manufacturer manufactures & sales a particular rating WITHOUT valid Type Test Certificate & its inclusion endorsement by BIS, then it amounts for breach of BIS guidelines of License & criminal offence as per Gazette.

QUALITY ASPECTS:

For Distribution Transformers with IS 1180 : 2014
Total loss at 50% & 100% loading- No tolerance & quoted as maximum w.r.t standard ratings % Impedance- Tolerance is applicable as per IS 2026 : 2011

For Special Purpose Transformers with IS 2026 : 2011
No Load loss, Load loss & % impedance - Tolerance is applicable as per IS 2026 : 2011

Taps are provided on transformers (Normally 200kVA & above as per IS) on the high voltage winding.  It is an arrangement to deliver full rated Secondary / outgoing Low voltage in case of fluctuating Primary / Incoming voltage conditions.

As Mentioned above taps are provided on high voltage winding to deliver full rated Secondary/outgoing Low voltage. The purpose of a tap changer is to regulate the output voltage of a transformer. It does this by altering the number of turns in one winding and there by changing the turns ratio of the transformer.

Off Circuit Tap Changer (OCTC) or De-energised Tap Changer (DETC) :
As the name suggests, it is a tap changer that cannot be moved while the transformer is energized. It often has 5/7/9 positions with step voltage of 2.5% above & below of Normal/Principal tap.

OnLoad Tap Changer (OLTC) :
As the name suggests, it is a tap changer that can be moved while the transformer is energized & carrying load. It comes with an Automatic Voltage Regulator (AVR) & Remote Tap Changing Controller (RTCC) Panel. It often has 9/12/17 positions with step voltage of 1.25% above & below of Normal/Principal tap.
Although OLTC is a convenience it can create nuisance if not maintained as per OEM’s guideline. While changing taps, there is frequent sparking in oil of OLTC chambers which deteriorates its insulating value. If oil is not replaced with a new one after recommended operation it can lead to failure of OLTC mechanism or transformer itself.    

Indoor type transformer are those that are mounted inside commercial complexes or in the basement of a building. Typically, an indoor transformer is a "dry" transformer (it uses air rather than oil to "cool" the transformer) designed for safety purposes.

Outdoor type transformers are those that are mounted outside industrial sheds or commercial complex. They are either Pole mounted or plinth mounted. Typically, an outdoor transformers are Oil cooled transformers. Sometimes it is provided with Cable boxes on High voltage & Low voltage terminal side so that no trace passer is harmed due to the open terminal.