be used only as short-time rating is required, provided the defective load will be automatically disconnected in a fault condition. The transformer's impedance should not be too low for desired maximum fault current. Impedance can be added after the secondaries are summed to limit maximum fault currents (the 3Io path). A combination of Y (wye or star), delta, and zigzag windings may be used to achieve a vector phase shift. For example, an electrical network may have a transmission network of 110 kV/33 kV star/star transformers, with 33 kV/11 kV delta/star for the high voltage distribution network. If a transformation is required directly between the 110 kV/11 kV network an option is to use a 110 kV/11 kV star/delta transformer. The problem is that the 11 kV delta no longer has an earth reference point. Installing a zigzag transformer near the secondary side of the 110 kV/11 kV transformer provides the required earth reference point. Applications Zigzag transformers are often required by utilities when connecting three-phase inverters (usually for renewable energy) to the grid to provide a stable neutral voltage and prevent excessive phase-to-ground voltages. This also protects the switching devices inside the inverters, which are usually insulated-gate bipolar transistors (IGBTs). References Electric transformers