the converter transformers step up the voltage of the ac supply network. using a star-to-delta or "wye-delta" connection of the transformer windings, the converter can operate with 12 pulses for each cycle in 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. converter transformers can be built as large as 300 megavolt-amperes (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.