Life management of primary distribution transformers

This presentation comprises of two parts. Part 1 deals with primary distribution transformers. Part 2 deals with their associated tapchanger. Primary transformers are defined as those with a primary voltage of 132 kV, 66 kV, 33 kV or 22 kV. Secondary system transformers by comparison are low cost and can be replaced quickly. London Electricity´s (LE) current practice on the management of transformers and tapchangers is presented. London has 120 primary substations many of which vary in size, complexity and equipment type. The old standard was 4 transformers per substation with double busbar 11 kV switchgear and a minimum rating of 15 MVA per transformer. All transformers are free-breathing with conservators and silica gels. Since the early 1980s the standard has been 3x60 MVA 132/11/11 kV double wound secondaries with a double busbar 11 kV switchboard consisting typically of 50 or more panels. These transformers are also free breathing, but refrigerated breathers are used instead of silica gel. They also have Hydrans fitted to both the main tank and selector compartments. There are several nonstandard primary substations in LE the largest of which consists of 16 transformers with a combined capacity of 290 MVA, three 11 kV switchboards, two 22 kV switchboards and one 66 kV switchboard all being of a metalclad double busbar design. The largest capacity transformers in the company are 90 MVA 132/33 kV units. Load densities are among the highest in the world. This is reflected in high transformer utilisation, which often leads to difficulties accessing the system and short outage windows