Archive for the ‘Transformers – Dry’ Category

Substations and transformers – The basic considerations

Saturday, June 13th, 2009

The location of a site sub-station needs to be carefully considered, the key factors being:

Proximity to the heaviest loads
Equipment with heavy loads is generally grouped together into what is termed a load centre, and ideally the sub-station should be located as close as possible to the load centre. This avoids the need for lengthy runs of low voltage cable which is relatively expensive and can cause a degree of power loss under heavy loads. Large installations may have several load centres located at different points around the building and may consequently require multiple sub-stations.

Supplier access:
The supplier must have easy access to their high voltage switchgear. This can either be located in the same area as consumer’s switchgear or alternatively in a separate switch room.

Transformer type:
There are a variety of different types of transformers that can be used, all of which fall into two basic categories: liquid-filled and dry type.

Liquid filled transformers:

Key points:

  • Mineral oil transformers are relatively cheap but present a slight fire risk
  • Liquid-filled transformers are generally more energy efficient than dry transformers


  • Mineral oil transformers generally need to be located outside due to the fire risk
  • Internally located mineral oil transformers may require automatic fire extinguishers and a purpose built soak-away to deal with any oil spillages
  • Synthetic ester filled transformers provide a non-flammable alternative but cost approximately 30% more than the mineral oil option

Dry transformers:

Key points:

  • Available with high grade insulation that renders them fire resistant enabling them to be used in locations where an oil-filled transformer may not be suitable
  • Transformer performance can be boosted by using mechanical ventilation to enhance cooling
  • Limitations:

    • More expensive than oil-filled transformers, especially the cast resin type
    • Dry transformers tend to be significantly heavier and larger than oil-filled transformers, which can be a problem where space is restricted
    • At low to medium loads, dry type transformers tend to be less efficient, which results in more electrical energy being converted to heat and lost to the atmosphere. This wasted energy must be paid for by the consumer and represents a significant operating cost

    High voltage distribution:
    In many buildings such as offices, a single sub-station is often all that is required to supply the low voltage distribution system. In such cases, the incoming high voltage supply and substation can be located in a single area and that will be the extent of the high voltage system.

    Larger commercial buildings and industrial applications may have several load centres and will require a high voltage distribution system to serve multiple sub-stations. There are two systems which can be used, each of which has advantages and disadvantages. These systems are:

    • Ring main

    The high voltage distribution circuit is arranged in the form of a ring which starts and finishes at the high voltage intake. The load centres are connected at convenient points around the ring main. The benefit of this system is that each load centre effectively has a high voltage supply from either side of the ring main, so if a fault occurs with one of the supplies, the load centre can still operate.

    • Radial feeder

    In a radial feeder system each load centre is fed separately from the consumer’s main high voltage switchboard. The main benefit of this system is that a fault in the supply to one of the load centres should not affect the others.