IEEE C37.110-1996 pdf download IEEE Guide for the Application of Current Transformers Used for Protective Relaying Purposes
1. Overview
1.1 Scope
This standard describes the characteristics and classification of current transformers (cts) used for protective relayingIt also describes the conditions that cause the ct output to be distorted and the effects on relaying systems of thisdistortion. The selection and application of cts for the more common protection schemes are also addressed.
1.2 Purpose
The purpose of this document is to present a comprehensive treatment of the theory and application of cts to assist therelay application engineer in the correct selection and application of cts for protective relaying purposes.
2. References
This standard shall be used in conjunction with the following publications. When the following standards aresuperseded by an approved revision, the revision shall apply.
IEC 44-6 (1992), Instrument transformers–Part 6: Requirements for protective current transformers for transientperformance.
IEC 185 (1987),Current transformers.
IEEE Std C37.103-1990, IEEE Guide for Differential and Polarizing Relay Circuit Testing (ANSID.
3. Definitions The following deÞnitions are taken from IEEE Std 100-1992 except as noted. The symbol ( a ) indicates that at the time the standard was approved there was no corresponding deÞnition in IEEE Std 100-1992. Symbol ( b ) indicates the deÞnition was taken from British Standard B.S.3938, SpeciÞcation for Current Transformers.
3.1 accuracy: The extent to which the current in the secondary circuit reproduces the current in the primary circuit in the proportion stated by the marked ratio, and represents the phase relationship of the primary current.
3.2 accuracy classes for relaying (instrument transformer): Limits in terms of percent ratio error that have been established. 3.3 accuracy ratings for relaying: The relay accuracy class is described by a letter denoting whether the accuracy can be obtained by calculation or must be obtained by test, followed by the minimum secondary terminal voltage that the transformer will produce at 20 times rated secondary current with one of the standard burdens without exceeding the relay accuracy class limit. (This is usually taken as 10%.)
3.4 burden (of a relay): Load impedance imposed by a relay on an input circuit, expressed in ohms and phase angle at speciÞed conditions.
3.5 burden on an instrument transformer: That property of the circuit connected to the secondary winding that determines the active and reactive power at the secondary terminals. The burden is expressed either as total ohms impedance, together with the effective resistance and reactance components, or as the total voltamperes and power factor of the secondary devices and leads at the speciÞed values of frequency and current.
3.6 bushing type current transformer: A current transformer that has an annular core with 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 for use with a fully insulated conductor as a primary winding. A bushing type ct is usually used in equipment where the primary conductor is a component part of other apparatus.
3.7 continuous thermal current rating factor (RF): The speciÞed factor by which the rated primary current of a ct can be multiplied to obtain the maximum primary current that can be carried continuously without exceeding the limiting temperature rise from 30 ° C ambient air temperature. When current transformers are incorporated internally as parts of larger transformers or power circuit breakers, they shall meet allowable average winding and hot-spot temperatures under the speciÞc conditions and requirements of the larger apparatus.
3.8 current transformer (ct): An instrument transformer that is intended to have its primary winding connected in series with the conductor carrying the current to be measured or controlled. In window-type cts, the primary winding is provided by the line conductor and is not an integral part of the transformer.
3.9 instrument transformer: A transformer that is intended to reproduce in its secondary circuit, in a deÞnite and known proportion, the current or voltage of its primary circuit with the phase relations substantially preserved.
3.10 knee-point voltage (Class C transformers): The point on the excitation curve where the tangent is at 45 ° to the abscissa. The excitation curve shall be plotted on log-log paper with square decades. This deÞnition is for nongapped cts. When the ct has a gapped core, the knee-point voltage is the point where the tangent to the curve makes an angle of 30 ° with the abscissa.
IEEE C37.110-1996 pdf download
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