Instrument Transformers

Instrument Transformers

Instrument transformers insulate meters or other instruments from high voltages and currents and transform them (i.e., step-down) to values where they can be measured by standard meters.

All instrument transformers are classified as either current transformers or voltage transformers. They are used for revenue metering and also with relays and indicating instruments.

A number of considerations are involved in selecting proper instrument transformers including the system voltage, load current, rating factor and accuracy class at specified loads (burdens).  Physical requirements such as indoor or outdoor installation; window (solid core or split core) or bar type (for 600 volt CT’s); and window diameter are also important.  Each transformer has its own unique combination of features that best meet the application for which it was designed.

R. L. Laster & Associates strives to provide its customers with a total package solution. Instrument transformers are often needed to complete a monitoring system, and we are proud to provide W.I.C.C. instrument transformers where needed. For specific information on W.I.C.C products.

Current Transformers

The primary windings of current transformers are connected in series with the line and their secondaries are connected to the current coils or elements of meters, relays or instruments. Normally, all standard current transformers have a 5 ampere rated secondary regardless of their primary current. However, 1 ampere rated secondaries are becoming more common.

Each current transformer has specifications based upon continuous current, short time current, and mechanical stress. There are two accuracy classifications, one for metering and one for relaying.  The metering accuracy classification tells what error in transformation can be expected at normal currents. The relaying accuracy classification tells what percent accuracy can be expected when the secondary is called upon to induce a certain voltage.

The secondary of the current transformer should always be connected to a load (burden) or short-circuited.  If the secondary is open circuited, the transformer may develop a dangerously high secondary voltage.

Voltage Transformers

The primary windings of voltage transformers are connected across the line. They step-down the line voltage to a value suitable for standard meters, normally 115 volts or 120 volts.

In basic construction, voltage transformers are much like power transformers except that the current and the kVA capacity are very low. They are designed to give a very accurate voltage transformation.