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Transformer Riddle No.103 - Difference between power & distribution transformers
What is the difference between power & distribution transformers, I have heard a lot about it that power transformer designed for max efficiency at  full load and Distribution transformers designed for half load. But does this reflect in any Standard?
Or smaller rating transformers are distribution transformers & large rate transformers are Power?
Author : Deepak
Wed, April 9th, 2014 - 13:09
The U.S. DOE (Department Of Energy) settle efficiency of distribution transformer. As part of the settlement, a negotiated rulemaking working group (WG) of interested stakeholders was formed consisting of transformer manufacturers, steel manufacturers, utilities, environmental interest groups, and consultants.
As a result of this work, the DOE published a Notice of Proposed Rulemaking (NOPR) on February 10, 2012.
The NOPR puts forward a standard of Efficiency Level 1 (EL1) for most liquid-filled distribution transformers. The current DOE efficiency standards implemented on January 1, 2010, were considered EL0. The WG essentially divided into three groups with distinct proposals:
- Group 1 proposed efficiency levels ranging from EL0 to EL0.5 for the various product classes under consideration. This group’s message to the DOE was that the standard currently in effect is a good standard and it should not be changed
- Group 2 agreed to support EL1 in an attempt to gain consensus within the WG
- Group 3 proposed EL3 for all liquid-filled product types under consideration. They claimed that this efficiency level could be economically justified based on the efficiency gains regardless of the long payback period for the investment in the new transformers

Allocated Indian Standared for distribution and power transformer illustrated as below:

-Distribution Transformers:
Reference Standard: IS 1180 --1989
-Power Transformers:
Reference Standards
IS 2026 Part I    1977– General
IS 2026 Part II   1977– Temperature Rise
IS 2026 Part III  1981– Insulation Levels and Dielectric Tests
IS 2026 Part IV   1977– Terminal markings, Tappings and Connections
IS 2026 Part V    1994– Transformer/ Reactor Bushings- Minimum external clearance in air
Author : Hamid - From: Iran
Mon, January 5th, 2015 - 10:21
Those transformers installed at the ending or receiving end of long high voltage transmission lines are the power transformers. The distribution transformers (generally pole mounted) are those installed in the location of the city to provide utilization voltage at the consumer terminals.

     Power transformers are used in transmission network of higher voltages for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.
    Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V) and are generally rated less than 200 MVA.
    A power transformer usually has one primary and one secondary, and one input and output. A distribution transformer may have one primary and one divided or “Tapped” secondary, or two or more secondaries.
    Power transformers generally operate at nearly full – load. However, a distribution transformer operates at light loads during major parts of the day.
    The performance of the power transformers is generally judged from commercial efficiency whereas the performance of a distribution transformer is judged from all – day – efficiency.
    The rating of a high transformer is many times greater than that of distribution transformer.
    In Power Transformer the flux density is higher than the distribution transformer.
    Power transformer’s primary winding always connected in star and secondary winding in delta while in distribution transformer primary winding connected in delta and secondary in star.
    In The Sub station end of the transmission line, The Power Transformer Connection is Star-Delta.( For the purpose of Step down the Voltage Level)
    In the star up of the  Transmission line (H-T), The Connection of the power Transformer is Delta – Star (For the purpose of Step Up the Voltage Level) But in case of Distribution Transformer, But Generally it is used in there-phase Step down distribution transformer( Delta – Star).
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