The inverter transformers are used to step up the voltage of the AC supply network. By using a star-delta connection, the inverter can operate with 12 pulses in each cycle of the AC supply, which eliminates numerous harmonic current components. The insulation of the transformer windings must be specially designed to withstand a large DC potential to earth. Inverter transformers can be built as large as 300 MVA as a single unit. It is impractical to transport larger transformers so when larger ratings are required, several individual transformers are connected together. Either two three-phase units or three single-phase units can be used. With the latter variant only one type of transformer is used, making the supply of a spare transformer more economical. Inverter transformers operate with high flux densities, and so produce more acoustic noise than normal three-phase power transformers. This effect should be considered in the siting of an HVDC static inverter plant. Noise-reducing enclosures may be applied.