Distribution transformers are used both in electrical power distribution and transmission systems. The power rating of a transformer is normally determined by the cooling method and the coolant used. Oil or some such other heat conducting material is commonly used as coolant. Ampere rating is increased in a distribution transformer by increasing the size of the primary and secondary windings; voltage ratings are increased by increasing the voltage rating of the insulation used in making the transformer.
The criteria for selection of a transformer and its technological details depend on its intended use or purpose, working conditions and operating requirements. For example, a power transformer is selected in the sub-transmission level because it is consistently loaded up to full rating in order to obtain maximum efficiency at full load. On the other hand, distribution transformers (in the power distribution system) are under-loaded most of the time in order to ensure maximum all day efficiency at the expense of lower efficiency during peak hours. In order to understand these criteria, you need to know about transformer classification.
Classification of Transformers
Apart from classification on the basis of purpose (as power transformers and distribution transformers) transformers are also classified on the basis of:
Type of Core Used; and
Type of Cooling Used.
Type of Core Used
In laminated-steel-core transformers, two main types of cores are used:core type and shell type.
•Core type transformers have cores with a hollow square through the centre (Fig. ). Note that the core is made up of many laminations of steel.Distribution
The criteria for selection of a transformer and its technological details depend on its intended use or purpose, working conditions and operating requirements. For example, a power transformer is selected in the sub-transmission level because it is consistently loaded up to full rating in order to obtain maximum efficiency at full load. On the other hand, distribution transformers (in the power distribution system) are under-loaded most of the time in order to ensure maximum all day efficiency at the expense of lower efficiency during peak hours. In order to understand these criteria, you need to know about transformer classification.
Classification of Transformers
Apart from classification on the basis of purpose (as power transformers and distribution transformers) transformers are also classified on the basis of:
Type of Core Used; and
Type of Cooling Used.
Type of Core Used
In laminated-steel-core transformers, two main types of cores are used:core type and shell type.
•Core type transformers have cores with a hollow square through the centre (Fig. ). Note that the core is made up of many laminations of steel.Distribution
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| Core Type Transformer |
•Shell type transformers are the most popular and efficient transformers and they have a shell core (Fig.6.2). Note that each layer of the core consists of E- and I-shaped metallic sections, which are butted together to form the laminations.The laminations are insulated from each other before being pressed together to form the core.
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| Shell Type Transformer |
Type of Cooling Used
There are two types of transformers in this category: Dry type and oil-filled.
• Dry type transformers use natural air cooling and are usually of very small ratings. They are rugged and simple in construction and are not plagued with the failures related with oil cooling.
• Oil filled transformers are of two types: One type uses self-cooling and the other type uses forced cooling.
Self-cooling oil filled transformers have natural circulation of insulating oil within which the entire transformer is immersed. These
are of moderate ratings and are suitable for outdoor duty as these
require no housing other than their own and thereby save on cost. For higher ratings, either the smooth surface of tank is corrugated or is provided with radiators/pipes to get greater heat radiation area.Large transformers require forced oil/water cooling. The coolant
is circulated by a pump to radiate high quantity of heat generated and also to minimise the size of the transformer.
A.Criteria for Transformer Selection
The criteria used for selection of proper rating/size of DTRs are described below:
While the secondary side voltage rating of the transformer is fixed as 400 V, 3 phase with neutral available along with the three phase wires in star configuration, the primary side voltage is decided by the voltage of incoming feeder(s). If there are more than one incoming feeders, the feeder for fixing the criterion for transformer selection can be decided on the basis of its proximity with the substation, load delivering capability and availability of suitable voltage transformer. The number, steps and type of tap changer, i.e.,on load or off load, is decided by operating requirements of voltage and current.
B.Size/Capacity/kVA Rating kVA
There are two types of transformers in this category: Dry type and oil-filled.
• Dry type transformers use natural air cooling and are usually of very small ratings. They are rugged and simple in construction and are not plagued with the failures related with oil cooling.
• Oil filled transformers are of two types: One type uses self-cooling and the other type uses forced cooling.
Self-cooling oil filled transformers have natural circulation of insulating oil within which the entire transformer is immersed. These
are of moderate ratings and are suitable for outdoor duty as these
require no housing other than their own and thereby save on cost. For higher ratings, either the smooth surface of tank is corrugated or is provided with radiators/pipes to get greater heat radiation area.Large transformers require forced oil/water cooling. The coolant
is circulated by a pump to radiate high quantity of heat generated and also to minimise the size of the transformer.
A.Criteria for Transformer Selection
The criteria used for selection of proper rating/size of DTRs are described below:
While the secondary side voltage rating of the transformer is fixed as 400 V, 3 phase with neutral available along with the three phase wires in star configuration, the primary side voltage is decided by the voltage of incoming feeder(s). If there are more than one incoming feeders, the feeder for fixing the criterion for transformer selection can be decided on the basis of its proximity with the substation, load delivering capability and availability of suitable voltage transformer. The number, steps and type of tap changer, i.e.,on load or off load, is decided by operating requirements of voltage and current.
B.Size/Capacity/kVA Rating kVA
rating of transformer(s) is decided on the basis of the following factors:
• existing load to be catered;
• future load growth to be absorbed;
• diversity factor (DF) of load for different categories of consumers.
Diversity factor is greater than or equal to one and can be used to get the required rating by dividing the sum of maximum demand of individual consumer categories by DF;
• margin for future load growth;
• safety factor for avoiding overloading;
• level of all day efficiency to be achieved; and
• selection of an optimum loading of transformer(s), usually around 60%,used to calculate the required capacity of the transformer(s).
C.Number of Transformers
Distribution substations are seldom provided with a single transformer of required rating (except pole mounted transformers/substations). However,reliability of supply can be severely affected if only one transformer is used especially when it fails or when it is under maintenance. Reliability of supply increases with increase in the number of transformers.
It is important to note that capacity requirement and hence cost per transformer reduces with increase in the number of transformers, as the load gets divided, but the total cost increases as the relative benefit of reduction inthe cost of each transformer is lower than the reduction in capacity.Moreover, there is additional cost for associated equipment for each additional unit of transformer. Thus, in order to optimise the reliability and the cost involved, the number of transformers is usually kept between two to
four. Transformers are required to comply with the latest edition of IS 2026.The size specifications for this purpose are given in Table.
• existing load to be catered;
• future load growth to be absorbed;
• diversity factor (DF) of load for different categories of consumers.
Diversity factor is greater than or equal to one and can be used to get the required rating by dividing the sum of maximum demand of individual consumer categories by DF;
• margin for future load growth;
• safety factor for avoiding overloading;
• level of all day efficiency to be achieved; and
• selection of an optimum loading of transformer(s), usually around 60%,used to calculate the required capacity of the transformer(s).
C.Number of Transformers
Distribution substations are seldom provided with a single transformer of required rating (except pole mounted transformers/substations). However,reliability of supply can be severely affected if only one transformer is used especially when it fails or when it is under maintenance. Reliability of supply increases with increase in the number of transformers.
It is important to note that capacity requirement and hence cost per transformer reduces with increase in the number of transformers, as the load gets divided, but the total cost increases as the relative benefit of reduction inthe cost of each transformer is lower than the reduction in capacity.Moreover, there is additional cost for associated equipment for each additional unit of transformer. Thus, in order to optimise the reliability and the cost involved, the number of transformers is usually kept between two to
four. Transformers are required to comply with the latest edition of IS 2026.The size specifications for this purpose are given in Table.
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| Standard Sizes of Transformers |
Placement of Transformers
You have learnt in Unit 4 that in the High Voltage Distribution System (HVDS),transformers are usually of very small rating (5 to 25 kVA) and provide supply to 10 − 25 consumers. Such transformers, being large in number and very small in size are mounted on a pole at an optimum location and are provided with metering and isolating equipment. Similarly, medium sized self-oil cooled transformers are also placed in open space and may be pole mounted. Large sized transformers are fixed on the ground with proper foundation and are enclosed within premises housing the associated equipment of transformers.
You have learnt in Unit 4 that in the High Voltage Distribution System (HVDS),transformers are usually of very small rating (5 to 25 kVA) and provide supply to 10 − 25 consumers. Such transformers, being large in number and very small in size are mounted on a pole at an optimum location and are provided with metering and isolating equipment. Similarly, medium sized self-oil cooled transformers are also placed in open space and may be pole mounted. Large sized transformers are fixed on the ground with proper foundation and are enclosed within premises housing the associated equipment of transformers.
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