Periodical inspections started on OL3 transformers
The majority of the electricity produced at a nuclear power plant is transferred to the national grid, but part of it is used to operate the plant's own equipment and systems. In order to guarantee the reliability of power supplies, the various components need to be inspected at regular intervals. At OL3, annual periodical inspections are at present carried out on the high-voltage transformers.
Electrical power has already for a few years been supplied to OL3 from the external grid by 110 kilovolt (kV) and 400 kV lines. These power supplies have been used in e.g. the testing of process systems in the nuclear island and the turbine island. 
Transformers, which transform the voltage as needed for the various equipment, play an essential role in power distribution. The periodical inspections of the OL3 high-voltage transformers started at the beginning of September will continue for a few weeks. Maintenance engineer Antti Alitalo is a member of the maintenance team.
– Although the actual inspections are conducted by the manufacturer, we monitor their execution closely. This way we can jointly ensure the operability of the transformers also in the future. There are seven high-voltage transformers at OL3. Some of them are main transformers, some are auxiliary transformers. There are also spare transformers for these transformers. – The inspections are carried out according to a long maintenance check list. This guarantees that the operability of all the parts is tested. Like all devices and components at nuclear power plants, inspections are carried out also on transformers on an annual basis. Maintenance and replacement intervals have been specified for all components. The objective of good advance planning is to prevent through planned replacement the failures of equipment and components, which are critical to safety and production. In order to guarantee the safe operation of the plant unit, the availability of critical equipment has been secured with four parallel and redundant power supply systems. The design of the systems ensures an adequate capacity in terms of nuclear safety even in case one subsystem is lost while another is undergoing maintenance. Moreover, an emergency diesel generator serves as a backup power supply to critical equipment in each subsystem. Electricity can also be supplied to the busbars of the emergency diesels from the Olkiluoto gas turbine plant. Two smaller diesel generators are also available for use as emergency power supplies. – In addition to normal power distribution and diesel generator backup, we also have battery sets that ensure uninterrupted power supplies. In my opinion, the supply of electricity is secured extremely well and with versatile sources at the Olkiluoto nuclear power plant, Mr. Alitalo sums up. Auxiliary transformer at OL3
Inspections based on detailed plans
The entire operation of the transformers is verified in the periodical inspection by means of visual and functional tests as well as oil samples taken to check the cleanliness of the insulating oil. According to Mr. Alitalo, careful preparations precede every maintenance activity.
Secured power supply systems
At OL3, electrical systems comprise e.g. the main generator, the 400 kV switchgear and the power lines, in addition to the transformers. Power distribution inside the plant unit consists of medium-voltage supplies, emergency diesel generators and low-voltage supplies. 
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