Editing Current transformer (section)

== Accuracy ==
The accuracy of a CT is affected by a number of factors including:
* Burden
* Burden class/saturation class
* Rating factor
* Load
* External [[electromagnetic field]]s
* [[Temperature]]
* Physical configuration
* The selected tap, for multi-ratio CTs
* Phase change
* Capacitive coupling between primary and secondary
* Resistance of primary and secondary
* Core magnetizing current

Accuracy classes for various types of measurement and at standard loads in the secondary circuit (burdens) are defined in IEC 61869-1 as classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3. The class designation is an approximate measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class&nbsp;1 CT is 1% at rated current; the ratio error of a Class&nbsp;0.5 CT is 0.5% or less. Errors in phase are also important, especially in power measuring circuits. Each class has an allowable maximum phase error for a specified load impedance.<ref name=SHEE11/>

Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults. A CT with a rating of 2.5L400 specifies with an output from its secondary winding of twenty times its rated secondary current (usually {{math|5&nbsp;A × 20 {{=}} 100&nbsp;A}}) and 400&nbsp;V (IZ drop) its output accuracy will be within 2.5 percent.

=== Burden ===
The secondary load of a current transformer is termed the "burden" to distinguish it from the primary load.

The burden in a CT metering [[electrical network]] is largely [[electrical resistance|resistive]] [[electrical impedance|impedance]] presented to its secondary winding. Typical burden ratings for IEC CTs are 1.5&nbsp;[[volt-ampere|VA]], 3&nbsp;VA, 5&nbsp;VA, 10&nbsp;VA, 15&nbsp;VA, 20&nbsp;VA, 30&nbsp;VA, 45&nbsp;VA and 60&nbsp;VA. ANSI/IEEE burden ratings are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. This means a CT with a burden rating of B-0.2 will maintain its stated accuracy with up to 0.2&nbsp;[[Ohm|Ω]] on the secondary circuit. These specification diagrams show accuracy parallelograms on a grid incorporating magnitude and phase angle error scales at the CT's rated burden. Items that contribute to the burden of a current measurement circuit are switch-blocks, meters and intermediate [[electrical conductor|conductors]]. The most common cause of excess burden impedance is the conductor between the [[Electricity meter|meter]] and the CT. When substation meters are located far from the meter cabinets, the excessive length of cable creates a large resistance. This problem can be reduced by using thicker cables and CTs with lower secondary currents (1&nbsp;A), both of which will produce less voltage drop between the CT and its metering devices.<ref name=SHEE11/>

===Knee-point core-saturation voltage===
The '''knee-point voltage''' of a current transformer is the magnitude of the secondary voltage above which the output current ceases to linearly follow the input current within declared accuracy. In testing, if a voltage is applied across the secondary terminals the [[magnetizing current]] will increase in proportion to the applied voltage, until the knee point is reached.  The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%.<ref>{{Cite web|url=http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=321-02-34|title=IEC 60050 - International Electrotechnical Vocabulary - Details for IEV number 321-02-34: "knee point voltage"|website=www.electropedia.org|access-date=2018-07-12}}</ref> For voltages greater than the knee point, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals. The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much higher but constrained within a very small range of the current transformer rating, typically 1.2 to 1.5 times rated current. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to fault currents of 20 to 30 times rated current.<ref name=GEC75>Anon, ''Protective Relays Application Guide Second Edition''The General Electric Company Limited of England, 1975 Section 5.3</ref>

===Phase shift===
Ideally, the primary and secondary currents of a current transformer should be in phase. In practice, this is impossible, but, at normal power frequencies, [[phase shift]]s of a few tenths of a degree are achievable, while simpler CTs may have larger phase shifts. For current measurement, phase shift is immaterial as [[ammeter]]s only display the magnitude of the current. However, in [[wattmeter]]s, [[electricity meter|energy meter]]s, and [[power factor#Measurement techniques|power factor]], phase shift produces errors. For power and energy measurement, the errors are considered to be negligible at unity power factor but become more significant as the power factor approaches zero. The introduction of electronic power and energy meters has allowed current phase error to be calibrated out.<ref>{{Cite web|url=https://ctlsys.com/support/ct_phase_angle_correction/|title=CT Phase Angle Correction – Continental Control Systems|website=ctlsys.com|language=en-US|access-date=2017-09-21}}</ref>