TYPES OF CURRENT TRANSFORMERS - BASIC INFORMATION

Current transformers are greatly used in protective relaying since the magnitude of fault current are far too big for the relays to handle. Design of these devices is discussed only to the extent that it relates to the application and use of instrument transformers in protection applications.

Current transformers (CTs)

A CT transforms line current into values suitable for standard protective relays and isolates the relays from line voltages. A CT has two windings, designated as primary and secondary, which are insulated from each other. The various types of primary windings are covered below.

The secondary is wound on an iron core. The primary winding is connected in series with the circuit carrying the line current to be measured; and the secondary winding is connected to protective devices, instruments, meters, or control devices. The secondary winding supplies a current in direct proportion and at a fixed relationship to the primary current.

Types of CTs

The four common types of CTs are as follows:

a) A wound CT has a primary winding consisting of one or more turns mechanically encircling the core or cores. The primary and secondary windings are insulated from each other and from the core(s) and are assembled as an integral structure (see Figure 1).

b) A bar CT has a fixed, insulated, straight conductor in the form of a bar, rod, or tube that is a single primary turn passing through the magnetic circuit and is assembled to the secondary, core and winding (see Figure 2).

c) A window CT has a secondary winding insulated from and permanently assembled on the core, but has no primary winding as an integral part of the structure. Primary insulation is provided in the window through which one or more turns of the line conductor can be passed to provide the primary winding (see Figure 3).

d) A bushing CT has an annular core and a secondary winding insulated from and permanently assembled on the core, but has no primary winding or insulation for a primary winding.

This type of CT is used with a fully insulated conductor as the primary winding and used typically in equipment where the primary conductor is a component part of other apparatus, for example, on bushings of a transformer or circuit breaker.

The secondary windings of bushing CTs are usually fully distributed around the core. Typically they are multiratio with each winding tap also being fully distributed.