Current Transformer

an instrument transformer for the measurement and monitoring of large currents by means of standard measuring instruments and automatic control and monitoring devices. It also serves to insulate apparatus from the potential of the circuit that is being measured or monitored. A distinction is made between AC transformers (usually known simply as current transformers) and DC transformers.

The primary winding of an AC current transformer (seeINSTRUMENT TRANSFORMER, Figure 2) consists of one or several turns of wire w₁ of relatively large cross section; it is connected in series with the circuit whose current is to be measured or monitored. The secondary winding consists of a large number of turns of wire w₂ of comparatively small cross section; instruments having negligible internal resistance—for example, ammeters, meters, and relays—are connected to it.

The distinguishing feature of a current transformer is that the current I₁ in the primary winding is independent of the operating conditions of the secondary (in practical terms, the secondary winding is short-circuited). The primary magnetomotive force I₁w₁ is balanced by the magnetomotive force I₀w₁ (which gives rise to the main magnetic flux in the core) and by the magnetomotive force I₂w₂, which determines the demagnetizing effect of the current I₂. Under normal operating conditions, I₀w₁ is usually 1–3 percent of I₁w₁—that is, I₁w₁ ≈ I₂w₂. If the transformation ratio is known, this relationship makes it possible to determine the large current I₀ by measuring the relatively small current I₂. Since, nevertheless, I₀w₁ is nonzero, the measured value of I₁ has a current error determined by the relative value of I₀w₁ and an angular error determined by the phase shift between the currents I₀ and I₂. In some transformers, called compensated current transformers, the measurement errors are canceled out. The rated value of the current I₂ in most current transformers is 5 amperes. Since current transformers are used in circuits where short-circuit currents may occur, the windings of such transformers are also required to withstand short-duration currents that substantially exceed the rated values.

Current transformers are made for various purposes; types distinguished on this basis include instrument, protective, line, and laboratory transformers. Transformers may be of the outdoor or indoor type, or they may be built into electrical apparatus and machines or attached in various ways, such as by means of a bushing; they may also be portable. Other bases for distinguishing various types of transformers include the number of stages (single-stage or multistage), the method of attachment (through-type, including hook-on meters and pole transformers), the number of turns in the primary winding (single-turn, or rod, and multiturn), the operating voltage (low-voltage or high-voltage), and the type of insulation on the windings (dry, resin-paper laminate, and poured insulation). (For DC current transformers, see.)