IEEE 3004.1-2013 pdf download IEEE Recommended Practice for the Application of Instrument Transformers in Industrial and Commercial Power Systems
3 Definitions
For the purposes of this document, the following terms and definitions apply. The IEEE StandardsDicfionary On/ine should be consulted for terms not defined in this clause.
burden: The load connected to the secondary terminals, which may be expressed as voltamperes andpower factor at a specified value of current, total ohms impedance and power factor, or ohms of theresistance and reactive components.
composite error: The root-mean-square (rms) of the instantaneous difference between the actual primarycurrent and the actual secondary current multiplied by the rated current transformer transformation error.
current transformer (CT): Transforms line current into values suitable for use with standard protectiverelays and meters while isolating these instruments from line voltages.
dynamic current rating (lam): The crest value of the asymmetrical primary current which a currentransformer must withstand without being damaged electrically or mechanically by the resultingelectromagnetic forces with the secondary winding short-circuited.
knee-point voltage: (A) The voltage at which a line tangent to the secondary excitation characteristic.when drawn on log-log coordinates, is at an angle of 45°to the horizontal. (B) The rated-frequencysecondary voltage above which a 10% increase in voltage results in an increase of 50% or more in excitingcurrent.(adapted from IEC)
polarity: The instantaneous phase relationship between the currents flowing in the primary and secondaryof a current transformer.In simple applications. polarity is not important, but it is a critical considerationwhenever multiple current transformers are used in combination, or when the output of a currenttransformer is used in conjunction with the output of a voltage transformer.
rating: The rating of a current transformer consists of a primary curent rating and an associated secondarcurrent rating. These ratings are related by the nominal transformation ratio of the current transformerwhich is usually also the physical turns ratio of the transformer.
ratio correction factor (RCF): The ratio of the true, or measured, ratio of the current transformer to themarked.or nominalratio.
ratio error: The degree to which the ratio correction factor deviates from the ideal. or textbook case, and istypically expressed in percent.
short-time thermal current rating: The maximum current that the current transformer can carry for aspecified period of time.
transformer correction factor (TCF): Takes into account both the magnitude of error and any associatederror in phase angle. TCF tends to be more of a concer in metering applications and is the factor by whichthe reading of a wattmeter may be adjusted to compensate for inaccuracies.
voltage transformer (VT): Transforms line voltage into values suitable for standard protective relavs andmeters while isolating these instruments from the stresses associated with the primary power system.
4. Current transformers
A current transformer (CT) transforms line current into values suitable for use with standard protectiverelays and meters while isolating these instruments from line voltages. A typical CT has two windings.designated as primary and secondary, which are insulated from each other. Most CTs are conventional inthe sense that they are transforers consisting of winding on iron cores. However, air core CTs have beenused in power system applications, and CTs utilizing optical technology are becoming available. Theprimary winding is connected in series with the circuit carrying the line current to be measured: and thesecondary winding is connected to protective devices, instruments, meters, or control devices.
Ideally, CTs change the magnitude of the current being measured without changing the phase angle orwave shape of the current.Practically, however, the output of CTs does contain some error and distortion.and dealing with these errors and distortion is one of the primary challenges in applying CTs.
4.1 Equivalent circuit for current transformers
To understand the performance and application of CTs, it is necessary to start with an equivalent circuitThe circuit shown in Figure 1 is representative, although variations on this circuit may be found in varioustexts.