C57.13: IEEE STANDARD REQUIREMENTS FOR INSTRUMENT TRANSFORMERS GUIDE

Guide for instrument transformer standard referring to IEEE standards.

Scope of the Standard
This standard is intended for use as a basis for performance, interchangability, and safety of equipment covered, and to assist in the proper selection of such equipment. This standard covers certain electrical, dimensional, and mechanical characteristics, and takes into consideration certain safety features of current and inductively coupled voltage transformers of types generally used in the measurement of electricity and the control of equipment associated with the generation, transmission, and distribution of alternating current.

Terms Used as Defined in the Standard
All definitions, except as specifically covered in this standard, shall be in accordance with IEEE Std 100-1992.

bar-type current transformer: One that has a fixed and straight single primary winding turn passing through the magnetic circuit. The primary winding and secondary winding(s) are insulated from each other and from the core(s) and are assembled as an integral structure.

basic lightning impulse insulation level (BIL): A specific insulation level expressed in kilovolts of the crest value of a standard lightning impulse.

burden of an instrument transformer: That property of the circuit connected to the secondary winding that determines the active and reactive power at the secondary terminals.

NOTE — The burden is expressed either as total ohms impedance with the effective resistance and reactance components, or as the total voltamperes and power factor at the specified value of current or voltage, and frequency.

 bushing-type current transformer: One that has an annular core and a secondary winding insulated from and permanently assembled on the core but has no primary winding and no insulation for a primary winding. This type of current transformer is for use with a fully insulated conductor as the primary winding. A bushing-type current transformer usually is used in equipment where the primary conductor is a component pan of other apparatus.

cascade-type voltage transformer: A voltage transformer that has an insulated-neutral or grounded-neutral terminal and that has the primary winding subdivided into two or more (usually equal) series connected sections, mounted on one or more magnetic cores, and that has the secondary winding located about the core at the neutral end of the primary winding. The sections of the primary winding are coupled by “coupling windings.” The cores, if more than one, are insulated from each other and connected to definite voltage levels along the primary winding.

continuous thermal current rating factor (RF): The number by which the rated primary current of a current transformer is multiplied to obtain the maximum primary current that can be carried continuously without exceeding the limiting temperature rise from 30 °C average ambient air temperature. The RF of tapped-secondary or multi-ratio transformers applies to the highest ratio, unless otherwise stated. (When current transformers are incorporated internally as parts of larger transformers or power circuit breakers, they shall meet allowable average winding and hot spot temperature limits under the specific conditions and requirements of the larger apparatus.)

current transformer (CT): An instrument transformer intended to have its primary winding connected in series with the conductor carrying the current to be measured or controlled.

double-secondary current transformer: One that has two secondary windings each on a separate magnetic circuit with both magnetic circuits excited by the same primary winding.

double-secondary voltage transformer: One that has two secondary windings on the same magnetic circuit with the secondary winding insulated from each other.

excitation losses for an instrument transformer: The power (usually expressed in watts) required to excite the transformer at its primary terminals.

NOTE — Excitation losses include core, dielectric, and winding losses due to the excitation current. 

fused-type voltage transformer: One that is provided with means for mounting one or more fuses as integral parts of the transformer in series with the primary winding.

grounded-neutral terminal type voltage transformer: One that has the neutral end of the primary winding connected to the case or mounting base in a manner not intended to facilitate disconnection.
3.13 indoor current (or voltage) transformer: One that, because of its construction, must be protected from the weather.

instrument transformer: One that is intended to reproduce in its secondary circuit, in a definite and known proportion, the current or voltage of its primary circuit with the phase relations substantially preserved.

voltage transformer (VT): An instrument transformer intended to have its primary winding connected in shunt with the voltage to be measured or controlled.

window-type current transformer: One that 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 turn of the line conductor can be passed to provide the primary winding.

wound-type current transformer: One that has a primary winding consisting of one or more turns mechanically encircling the core or cores. The primary winding(s) and secondary winding(s) are insulated from each other and from the core(s) and are assembled as an integral structure.

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