THREE-PHASE UNIT VERSUS SINGLE-PHASE UNITS

-->

Three-phase unit versus single-phase units:

Since the transmission system is 3-phase, transformers may be built as 3-phase single units or as three single-phase units into delta and star combinations or groups.

Advantages of 3 phase units

• They occupy less space
• No extra support equipment is required to form a 3-phase Delta or Star

connection.

• They are cheaper
• They can be transported from factory as a compact unit, erected and

commissioned at site quickly

• Compact on-load tap changing (OLTC) gear can be provided as a built in unit.

Disadvantages of 3 phase units

·         Problem of transportation in case of large capacity units weighing more than 100 tons.

·         Takes time in assembling, erecting and commissioning if parts are dismantled and sent to site.

·         The cost of one spare 3-phase transformer is more.

·         Change of connections from star to delta or vice-versa cannot be done.

·         If reconditioning is undertaken then the complete unit has to be taken out of service and this becomes a problem if no spare capacity is available.

          Advantages of Single-Phase Units

• The cost of a spare transformer is the cost of a single-phase unit, which is comparatively very much less than the cost of a complete spare 3-phase unit.
• They can be transported to site as completely assembled units and commissioned quickly.
• Reconditioning can be undertaken on individual units with a minimum outage time.
• It is possible to obtain different possible pairs of connections between the primary and secondary.

          Disadvantages of Single-Phase Units

• They occupy more space
• They require additional support structure to form 3-phase connections.
• Expenditure on civil engineering works is more
• The problem of providing on-load tap changing gear and even if provided the cost of providing tap changing gear on each unit works out costlier by at least 50% when compared to a compact unit in a 3-phase transformer.

          Considering all the above, there is little argument in favour of the adoption of single-phase units as compared to 3-phase units.  Single-phase units are the only choice where 3-phase units cannot be transported because of their weight and dimensions and also if there are no facilities at site for the assembly, preparation and commissioning of the 3-phase unit

          Power Transformers

These are transformers of high rating of generally not less than 5MVA and 33KV and the rating also increases with the voltage rating. They may be of the step-up type installed at generating stations or of the step-down type installed at substations. They have a high utilisation factor, which means that they are arranged to work at a constant load equal to their rating.  Hence their maximum efficiency is designed to be at or near full load.  Such power transformers installed in substations are provided with OLTC gear to regulate the voltage to be within permissible limits during peak load and off peak load hours.

However, generator step-up power transformers are provided with only off circuit taps.

          Distribution Transformers

These are transformers installed in H.V. distribution feeders to meet consumer voltage requirements.  These are generally rated at 11KV and have a rating not exceeding 1000KVA.  These transformers are characterised by an intermittent variable load, which is usually considerably less than the full load rating.  They are therefore designed to have their maximum efficiency at between half and three quarter of full load.  These transformers are not provided with any OLTC gear but with only off circuit taps.

        Auto Transformers

An Auto Transformer is a transformer with a common winding for both primary and secondary. They are used in place of two winding power transformers where the ratio of transformation does not exceed 2 as they are cheaper than two winding transformers such as in a 132KV/66KV system or 66KV/33KV system.