The HFA outdoor rated station-post current transformer is a window-type suitable for use on line voltages up to 36kV / 200kV BIL. It may be used with insulated or un-insulated conductors, and mounts to sub-station structures or pedestals in any orientation. It is molded in a rugged cast resin ideal for harsh environments. Its size makes it an economical solution when upgrading sub-station protection and adding additional metering points. It can also be used in wind farm and hydro-generation applications, as two examples. Its simple construction provides a low leakage product with virtually unlimited short circuit capabilities, far surpassing its stand alone wound-type counterpart.


The toroidal core is continuously tape wound using cold rolled grain oriented electrical grade silicon steel, which receives a full stress relief anneal after it is wound to its specified dimensions. The secondary winding is then wound of insulated copper magnet wire over the insulated core with the turns equally spaced around the core periphery. When taps are pulled, they are wound in a manner that assures a fully distributed winding between any connections.

The high voltage insulator is produced by Electro Composites Industries, a subsidiary of Hubbell Power Systems Products, using their proprietary cycloaliphatic epoxy resin formulation. This material has been used for outdoor bushing applications since 1994. It has high arc track resistance and high mechanical strength. The interior wall of the insulator is lined with a copper shield to provide uniform voltage distribution throughout, thus eliminating corona between the un-insulated conductor and air inside the sleeve. A pigtail lead is provided for making this connection to un-insulated conductors (not required with fully insulated conductor).

The coil and HV insulator are then fully encapsulated in a ANSI 70 (sky grey) resin system that has been developed and tested for outdoor use. The resin compounds are U.V. stabilized, have a working temperature from -50°C to +75°C ambient, good chemical resistance, high dielectric strength, rugged physical properties to endure mechanical shock, and overall excellent weather resistance.


The primary polarity H1/P1 is identified on its surface by an embossed colored dot. The secondary terminals are ¼-20 silicone bronze studs provided with hex nuts and cupped washers. Terminal identification is permanently embossed with dual designations “X” and “S” for compatibility in the global market. The X1/S1 terminal, also identified by a colored dot, represents the instantaneous polarity reference with respect to H1/P1. A cast aluminum conduit box with (2) 1” NPT hubs and blanking plugs is provided with a removable cover.

Outdoor Mounted Slip-Over Bushing Current Transformer


An aluminum nameplate with all required information and ratings, along with serial number and connection diagram, is affixed to each unit adjacent to the secondary conduit box.


The HFA station post unit may be mounted in any orientation (upright, under hung, cantilever) directly to the sub-station structure or on its own pedestal.

Handling and Storage:

The HFA station-post is usually shipped upright in a protective crate. Ideally they should be lifted using endless slings on each side of and adjacent to the main body around the HV sleeve, lifting by fork lift or overhead hoist, one unit at a time. Caution should be employed while moving to avoid damaging or chipping the insulation and any sudden impacts to the unit or the conduit box. The units are outdoor rated so they may be left outside as originally packaged but preferably should be stored indoors until ready for use.