paraformer

A paraformer is a type of power transformer designed to increase voltage in electrical distribution systems, typically found in utility substations and industrial plants. Unlike distribution transformers that step voltage down for end-use, the paraformer works in the opposite direction, raising voltage from medium levels (often 6 to 35 kV) to higher transmission levels (69 kV and above). This voltage boost allows power to travel longer distances with reduced line losses, since power loss in transmission is inversely proportional to voltage squared.

The name "paraformer" derives from the step-up or "para" configuration of its winding arrangement, where the secondary coil has more turns than the primary. Physically, the transformer consists of two wire coils wound around an iron core; alternating current flowing through the primary induces a changing magnetic field that transfers energy to the secondary without direct electrical connection. Core materials vary from laminated silicon steel in smaller units to amorphous metal alloys in high-efficiency designs, chosen to minimize core losses during continuous operation.

Construction and ratings

Paraformers are oil-cooled units in larger ratings (above 10 MVA), with mineral or synthetic oils circulating through internal ducts and external radiators to dissipate heat. Smaller dry-type paraformers rated below 5 MVA use air cooling and epoxy insulation, common in indoor substations where oil containment is problematic. Typical three-phase units range from 5 MVA to over 500 MVA in utility applications, with impedance (the resistance to current flow) specified between 5 and 15 percent depending on system requirements.

Common failure modes include insulation breakdown from moisture ingress, bushing failures at high voltage terminals, and core overheating during sustained overload. Winding hot-spot temperatures are monitored via embedded thermometers; units are usually rated for operation up to 65 degrees Celsius rise above ambient. Tap changers, either off-load or on-load designs, allow field adjustment of output voltage by connecting different points on the primary winding, accommodating seasonal load variations and voltage regulation needs.

Paraformers occupy a distinct place between substation power transformers (which handle bulk transmission) and conventional distribution transformers. In utility grids, they typically feed intermediate voltage networks that supply industrial customers and smaller distribution zones. Their efficiency ratings, usually 98 to 99 percent for modern units, make them critical components in minimizing systemic power losses, particularly in networks with long transmission distances or significant load growth.