Current transformers are used to measure high alternating currents and provide safety isolation. They work by inducing a current in the secondary winding that is proportional to the primary current passing through the transformer core. Current transformers scale down large primary currents to safer secondary currents used for instrumentation and protection devices. They are used extensively in power generation, transmission and distribution systems to monitor operations and protect equipment.

2. A current transformer (CT) is a type of transformer that is used to measure alternating
current (AC). It produces a current in its secondary which is proportional to the current
in its primary.
Current transformers, along with voltage or potential transformers are instrument
transformers. Instrument transformers scale the large values of voltage or current to
small, standardized values that are easy to handle for instruments and protective
relays. The instrument transformers isolate measurement or protection circuits from
the high voltage of the primary system. A current transformer provides a secondary
current that is accurately proportional to the current flowing in its primary. The current
transformer presents a negligible load to the primary circuit.
Current transformers are the current-sensing units of the power system and are used
at generating stations, electrical substations, and in industrial and commercial electric
power distribution.

3.  Like any transformer, a current transformer has a primary winding, a
core and a secondary winding, although some transformers, including
current transformers, use an air core. In principle, the only difference
between a current transformer and a voltage transformer (normal type)
is that the former is fed with a 'constant' current while the latter is fed
with a 'constant' voltage, where 'constant' has the strict circuit theory
meaning. The alternating current in the primary produces an
alternating magnetic field in the core, which then induces an
alternating current in the secondary. The primary circuit is largely
unaffected by the insertion of the CT. Accurate current transformers
need close coupling between the primary and secondary to ensure that
the secondary current is proportional to the primary current over a wide
current range. The current in the secondary is the current in the
primary (assuming a single turn primary) divided by the number of turns
of the secondary. In the illustration on the right, 'I' is the current in the
primary, 'B' is the magnetic field, 'N' is the number of turns on the
secondary, and 'A' is an AC ammeter.

4. Current transformers typically consist of a silicon steel ring core wound with many
turns of copper wire as shown in the illustration to the right. The conductor carrying
the primary current is passed through the ring. The CT's primary, therefore, consists
of a single 'turn'. The primary 'winding' may be a permanent part of the current
transformer, i.e. a heavy copper bar to carry current through the core. Window-type
current transformers are also common, which can have circuit cables run through the
middle of an opening in the core to provide a single-turn primary winding. To assist
accuracy, the primary conductor should be centered in the aperture.
CTs are specified by their current ratio from primary to secondary. The rated
secondary current is normally standardized at 1 or 5 amperes. For example, a 4000:5
CT secondary winding will supply an output current of 5 amperes when the primary
winding current is 4000 amperes. This ratio can also be used to find the impedance
or voltage on one side of the transformer, given the appropriate value at the other
side. For the 4000:5 CT, the secondary impedance can be found as ZS = NZP = 800ZP,
and the secondary voltage can be found as VS = NVP = 800VP. In some cases, the
secondary impedance is referred to the primary side, and is found as ZS′ = N2ZP.
Referring the impedance is done simply by multiplying initial secondary impedance
value by the current ratio. The secondary winding of a CT can have taps to provide a
range of ratios, five taps being common.

5. Current transformer shapes and sizes vary depending on the end user or switch
gear manufacturer. Low-voltage single ratio metering current transformers are
either a ring type or plastic molded case.
Split-core current transformers either have a two-part core or a core with a
removable section. This allows the transformer to be placed around a conductor
without having to disconnect it first. Split-core current transformers are typically
used in low current measuring instruments, often portable, battery-operated, and
hand-held (see illustration lower right).

6.  Current transformers are used extensively for measuring current and
monitoring the operation of the power grid. Along with voltage leads,
revenue-grade CTs drive the electrical utility's watt-hour meter on
virtually every building with three-phase service and single-phase
services greater than 200 amperes.
 High-voltage current transformers are mounted on porcelain or polymer
insulators to isolate them from ground. Some CT configurations slip
around the bushing of a high-voltage transformer or circuit breaker,
which automatically centers the conductor inside the CT window.
 Current transformers can be mounted on the low voltage or high
voltage leads of a power transformer. Sometimes a section of a bus bar
can be removed to replace a current transformer.
 Often, multiple CTs are installed as a "stack" for various uses. For
example, protection devices and revenue metering may use separate
CTs to provide isolation between metering and protection circuits and
allows current transformers with different characteristics (accuracy,
overload performance) to be used for the devices.

7. The burden (load) impedance should not exceed the specified maximum value
to avoid the secondary voltage exceeding the limits for the current transformer.
The primary current rating of a current transformer should not be exceeded or
the core may enter its non linear region and ultimately saturate. This would
occur near the end of the first half of each half (positive and negative) of the AC
sine wave in the primary and would compromise the accuracy.

8.  Current transformers are often used to monitor high currents or
currents at high voltages. Technical standards and design practices
are used to ensure the safety of installations using current
transformers.
 The secondary of a current transformer should not be disconnected
from its burden while current is in the primary, as the secondary will
attempt to continue driving current into an effective
infinite impedance up to its insulation break-down voltage and thus
compromise operator safety. For certain current transformers, this
voltage may reach several kilovolts and may cause arcing. Exceeding
the secondary voltage may also degrade the accuracy of the
transformer or destroy it. Energizing a current transformer with an
open circuit secondary is equivalent to energizing a voltage
transformer (normal type) with a short circuit secondary. In the first
case the secondary tries to produce an infinite voltage and in the
second case the secondary tries to produce an infinite current. Both
scenarios can be dangerous and damage the transformer.

9. The accuracy of a CT is affected by a number of factors including:
1. Burden
2. Burden class/saturation class
3. Rating factor
4. Load
5. External electromagnetic fields
6. Temperature
7. Physical configuration
8. The selected tap, for multi-ratio CTs
9. Phase change
10. Capacitive coupling between primary and secondary
11. Resistance of primary and secondary
12. Core magnetizing current

10. 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 1 CT is 1% at rated current; the ratio
error of a Class 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.
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 5 A × 20 = 100 A) and
400 V (IZ drop) its output accuracy will be within 2.5 percent.

11. 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 circuit is
largely resistive impedance presented to its secondary winding. Typical
burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA,
20 VA, 30 VA, 45 VA and 60 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 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 conductors.
The most common cause of excess burden impedance is the conductor
between the 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 A), both of which will produce
less voltage drop between the CT and its metering devices.

12. 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%.[citation needed] 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.

13. 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 shifts of a few tenths of a degree are
achievable, while simpler CTs may have phase shifts up to six
degrees.[2] For current measurement, phase shift is immaterial
as ammeters only display the magnitude of the current. However,
in wattmeters, energy meters, and power factor meters, 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. At zero power-factor,
any indicated power is entirely due to the current transformer's phase
error.[2] The introduction of electronic power and energy meters has
allowed current phase error to be calibrated out.

14. Construction
Bar-type current transformers have terminals for source and load connections of
the primary circuit, and the body of the current transformer provides insulation
between the primary circuit and ground. By use of oil insulation and porcelain
bushings, such transformers can be applied at the highest transmission
voltages.
Ring-type current transformers are installed over a bus bar or an insulated cable
and have only a low level of insulation on the secondary coil. To obtain non-
standard ratios or for other special purposes, more than one turn of the primary
cable may be passed through the ring. Where a metal shield is present in the
cable jacket, it must be terminated so no net sheath current passes through the
ring, to ensure accuracy. Current transformers used to sense ground fault (zero
sequence) currents, such as in a three-phase installation, may have three
primary conductors passed through the ring. Only the net unbalanced current
produces a secondary current - this can be used to detect a fault from an
energized conductor to ground. Ring-type transformers usually use dry insulation
systems, with a hard rubber or plastic case over the secondary windings.

15. For temporary connections, a split ring-type current transformer
can be slipped over a cable without disconnecting it. This type
has a laminated iron core, with a hinged section that allows it to
be installed over the cable; the core links the magnetic flux
produced by the single turn primary winding to a wound
secondary with many turns. Because the gaps in the hinged
segment introduce inaccuracy, such devices are not normally
used for revenue metering.
Current transformers, especially those intended for high voltage
substation service, may have multiple taps on their secondary
windings, providing several ratios in the same device. This can
be done to allow for reduced inventory of spare units, or to
allow for load growth in an installation. A high-voltage current
transformer may have several secondary windings with the same
primary, to allow for separate metering and protection circuits,
or for connection to different types of protective devices. For
example, one secondary may be used for branch overcurrent
protection, while a second winding may be used in a bus
differential protective scheme, and a third winding used for
power and current measurement.

16. Special types
Specially constructed wideband current transformers are also used (usually with
an oscilloscope) to measure waveforms of high frequency or pulsed currents
within pulsed powersystems. Unlike CTs used for power circuitry, wideband CTs are
rated in output volts per ampere of primary current.
If the burden resistance is much less than inductive impedance of the secondary
winding at the measurement frequency then the current in the secondary tracks
the primary current and the transformer provides a current output that is
proportional to the measured current. On the other hand, if that condition is not
true, then the transformer is inductive and gives a differential output.
The Rogowski coil uses this effect and requires an external integrator in order to
provide a voltage output that is proportional to the measured current.
Chemtrols Solar Pvt Limited, Amar Hill Saki Vihar Road, Powai Mumbai – 400 072
, Jay Ranvir- 9594998390
Email jay.ranvir@chemtrolssolar.com