Distribution Transformer-types and comparison with Power transformer

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Distribution Transformer

Distribution Transformer is one of the important electrical isolation transformers which provide the final output voltage to the end users. As the name specifies it distributes the power to the costumers depends on their need. The basic function of distribution transformer is to step down the line voltage to the consumer voltage for supplying it to them. Because normally the transmission line voltages will be high to increase the power transfer without losses but the electrical devices which the consumer uses will operate for low voltage. So the line voltage will be stepped down to consumer level and distribute it them efficiently. The application of distribution transformers can be residential, industrial and commercial sectors.

Distribution Transformer

Distribution Transformer

Important features

  1. They only have one KVA ratings. Because they are small and the customer peak loadings are relatively short duration.
  2. These can be single phase or three phase transformers. In some cases the three phases are loaded on each phase balanced.
  3. Distribution transformers are output rated, that they can deliver their rated KVA without exceeding temperatures rise limits.

Properties of distribution transformer:

Impedance of distribution transformer

The impedance of distribution transformer is low compared to power transformer. This is to provide better voltage regulation and less voltage flickering for major motor loads and other functional loads at the consumer side. The major disadvantage of this low impedance is that it increases the fault currents on the secondary side. This increase in fault current in secondary impacts the voltage sure at the primary side and hence the primary fault current increases tremendously.

Corrosion in Distribution Transformer

Corrosion is one of the main concerns in distribution transformer, especially for anything on the ground or in the ground. Overhead and Pad-mounted distribution transformer tanks are made up of carbon steel which tend to corrode at the transformer base. Submersible units, being highly susceptible corrosion, are often made up of stainless steel.

Cooling of Distribution transformer:

Distribution transformers are self cooled, they do not require extra cooling arrangement like as power transformer because they are mostly placed at outdoor. Rarely in submersible transformers are equipped with oil cooling.

Types of distribution transformers:

Type of insulation:

  1. Liquid immersed transformer
  2. Dry type transformer

Number of phases

  1. Single phase
  2. Three phase

Voltage Level

  1. Low Voltage
  2. Medium Voltage

Dry type transformers:

dry type transformer

dry type transformer

Dry type or air cooled transformers are electrical transformers, which doesn’t use any liquid (oil) for heating and cooling purpose. These are also acts as distribution transformers which reduces the high line voltages to low end user voltages. Dry type, or air-cooled, transformers accomplish this function so safely and efficiently that they are commonly used for indoor applications where other transformer types are considered too risky. Normally these dry type transformers have F and H class insulation.

Advantages:
  • They require minimum service and provide reliable service for many years.
  • These are very compact in size and suitable for placement inside high complex buildings.
  • These offers high efficiency

Liquid Immersed Transformers:

Liquid Type Transformer

Liquid Type Transformer

Liquid Immersed transformers are one of type electrical transformers which use liquid (oil) as a coolant. The term liquid immersed indicates the type of insulation medium. These transformers are generally installed in indoors unlike the dry type transformers.

Advantages:
  • Efficient cooling of the transformers.
  • These can be used at indoor where the dry type transformer cannot to be used because of heating losses.

Difference/comparison between distribution transformer and power transformer:

1) Voltage level

  • Power transformers are used at higher power level with higher voltages and step-up and step down. Normally these are rated above 200 MVA
  •  Distribution transformers are used at lower consumer power levels with low voltages step down and step up.

2) Transformer size:

  • Power transformer size is large because it will be operated at higher powers the size of auxiliaries are large.
  • In distribution transformers the auxiliaries are less sized so the total size is less compared to power transformer.

3) Loading of transformer:

  • Power transformer are connected near generating plants in power network, so the load fluctuations are less thus they are loaded fully.
  • The distribution transformers are connected near the consumer, so load fluctuations are high thus these are partially loaded to meet the peak load demand.

4) Efficiency

  • The efficiency of power transformer is Maximum at 100% of load because it always operates at full load.
  • The efficiency of the distribution transformer is Maximum at 60-70% and deviates from the maximum when the load is reduced or increased.