High-power autotransformers up to 1500 MVA, up to 800 kV

Description

Autotransformers are installed where large power has to be transferred between voltage classes: at system substations that interconnect extra-high and high voltage grids. Compared with a two-winding transformer of the same throughput rating they have lower mass, dimensions and losses. According to the manufacturer's data the rating reaches 1000 MVA, and the production programme table states up to 1500 MVA. The voltage class is up to 800 kV and the frequency 50 or 60 Hz.

Each autotransformer is designed individually to the technical specification of the grid company, like the other power transformers of the manufacturer. The calculations cover the voltage distribution under lightning and switching impulses, electrodynamic withstand during short circuits, zones of increased field strength, additional losses and thermal state. The design is optimised for the total cost of ownership (TCO), which includes the price, no-load and short-circuit losses, commissioning costs, reliability and maintenance.

Manufacture takes place at the plant in Tapiószele in Hungary, where the crane lifts up to 280 t and the annual output is 12,000 MVA. Testing is carried out in the company's own accredited laboratory to IEC and IEEE. The list covers measurement of the transformation ratio and vector group, resistances and insulation, capacitance and dissipation factor, losses and short-circuit voltage, zero-sequence impedance and temperature rise. Separately, impulse tests, applied and induced voltage tests with partial discharge measurement, and sound level measurements are performed. The laboratory is accredited to ISO/IEC 17025. If required, the autotransformer is supplied with a monitoring system.

An autotransformer differs from a two-winding transformer in that part of the winding is common to the higher and lower voltage sides. At interconnection substations this gives an advantage in mass, dimensions and losses at high throughput rating. The two rating figures, 1000 and 1500 MVA, are taken from different sections of the manufacturer's documentation, so the final rating and voltage class are confirmed against the technical specification of the project. The grid parameters, winding connection scheme and cooling method are also agreed at the design stage so that the loss and heating calculations match the actual operating mode of the substation.

Technical specifications

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