current transformer (CT) is used for measurement of alternating electric currents.

Current transformers,
together with voltage transformers (VT) (potential transformers (PT)), are known as instrument
transformers. When current in a circuit is too high to directly apply to measuring instruments, a current
transformer produces a reduced current accurately proportional to the current in the circuit, which can be
conveniently connected to measuring and recording instruments. A current transformer also isolates the
measuring instruments from what may be very high voltage in the monitored circuit. Current transformers
are commonly used in metering and protective relays in the electrical power industry.
Contents
[show]
Design [edit]




SF6 110 kV current transformer TGFM series, Russia


Current transformers used in metering equipment forthree-phase 400 ampere electricity supply
Like any other transformer, a current transformer has a primary winding, a magnetic core, and a
secondary winding. The alternating current flowing in the primary produces an alternating magnetic field
in the core, which then induces an alternating current in the secondary winding circuit. An essential
objective of current transformer design is to ensure that the primary and secondary circuits are efficiently
coupled, so that the secondary current bears an accurate relationship to the primary current.
The most common design of CT consists of a length of wire wrapped many times around a silicon steel
ring passed 'around' the circuit being measured. The CT's primary circuit therefore consists of a single
'turn' of conductor, with a secondary of many tens or hundreds of turns. The primary winding may be a
permanent part of the current transformer, with a heavy copper bar to carry current through the magnetic
core. Window-type current transformers (aka zero sequence current transformers, or ZSCT) 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. When conductors passing through a CT are not centered in the circular (or
oval) opening, slight inaccuracies may occur.
Shapes and sizes can vary depending on the end user or switchgear manufacturer. Typical examples of
low voltage single ratio metering current transformers are either ring type or plastic moulded case. High-
voltage current transformers are mounted on porcelain bushings to insulate 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.
The primary circuit is largely unaffected by the insertion of the CT. The rated secondary current is
commonly standardized at 1 or 5 amperes. For example, a 4000:5 CT would provide an output current of
5 amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or
multi ratio, with five taps being common for multi ratio CTs. The load, or burden, of the CT should be of
low resistance. If the voltage time integral area is higher than the core's design rating, the core goes
into saturation towards the end of each cycle, distorting the waveform and affecting accuracy.
Usage [edit]


Many digital clamp meters utilize a current transformer for measuring AC current
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 amps.
The CT is typically described by its current ratio from primary to secondary. 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.
Safety precautions [edit]
Care must be taken that the secondary of a current transformer is not disconnected from its load while
current is flowing in the primary, as the transformer secondary will attempt to continue driving current
across the effectively infinite impedance up to its core saturation voltage. This may produce a high
voltage across the open secondary into the range of several kilovolts, causing arcing, compromising
operator and equipment safety, or permanently affect the accuracy of the transformer.
Accuracy [edit]
The accuracy of a CT is directly related to a number of factors including:
Burden
Burden class/saturation class
Rating factor
Load
External electromagnetic fields
Temperature and
Physical configuration.
The selected tap, for multi-ratio CTs
For the IEC standard, accuracy classes for various types of measurement are set out in IEC 60044-1,
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, and 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 20 times its rated secondary
current (usually 5 A x 20 = 100 A) and 400 V (IZ drop) its output accuracy will be within 2.5 percent.
Burden [edit]
The secondary load of a current transformer is usually called the "burden" to distinguish it from the load of
the circuit whose current is being measured.
The burden, in a CT metering circuit is the (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
& 60 VA. As for 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 can tolerate up to 0.2 of impedance in the metering circuit before its
secondary accuracy falls outside of an accuracy specification. 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 source of excess burden is the conductor
between the meter and the CT. When substation meters are located far from the meter cabinets, the
excessive length of wire creates a large resistance. This problem can be reduced by using CTs with 1
ampere secondaries, which will produce less voltage drop between a CT and its metering devices.
Knee-point core-saturation voltage [edit]
The knee-point voltage of a current transformer is the magnitude of the secondary voltage after 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, up until the knee point. The knee point is defined as the voltage at which a 10%
increase in applied voltage increases the magnetizing current by 50%. From the knee point upwards, the
magnetizing current increases abruptly even with 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 tighter but constrained within a very small bandwidth 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 currents of 20 or 30 times rated
current during faults.
[1]

Rating factor [edit]
Rating factor is a factor by which the nominal full load current of a CT can be multiplied to determine its
absolute maximum measurable primary current. Conversely, the minimum primary current a CT can
accurately measure is "light load," or 10% of the nominal current (there are, however, special CTs
designed to measure accurately currents as small as 2% of the nominal current). The rating factor of a CT
is largely dependent upon ambient temperature. Most CTs have rating factors for 35 degrees Celsius and
55 degrees Celsius. It is important to be mindful of ambient temperatures and resultant rating factors
when CTs are installed inside padmount transformers or poorly ventilated mechanical rooms. Recently,
manufacturers have been moving towards lower nominal primary currents with greater rating factors. This
is made possible by the development of more efficient ferrites and their corresponding hysteresis curves.
Special designs [edit]
Specially constructed wideband current transformers are also used (usually with an oscilloscope) to
measure waveforms of high frequency or pulsed currents within pulsed power systems. One type of
specially constructed wideband transformer provides a voltage output that is proportional to the measured
current. Another type (called a Rogowski coil) requires an external integrator in order to provide a voltage
output that is proportional to the measured current. Unlike CTs used for power circuitry, wideband CTs
are rated in output volts per ampere of primary current. CT RATIO
Standards [edit]
Depending on the ultimate clients requirement, there are two main standards to which current
transformers are designed. IEC 60044-1 (BSEN 60044-1) & IEEE C57.13 (ANSI), although the Canadian
& Australian standards are also recognised.
High voltage types [edit]
Current transformers are used for protection, measurement and control in high voltage
electrical substations and the electrical grid. Current transformers may be installed inside switchgear or in
apparatus bushings, but very often free-standing outdoor current transformers are used. In a
switchyard, live tank current transformers have a substantial part of their enclosure energized at the line
voltage and must be mounted on insulators. Dead tank current transformers isolate the measured circuit
from the enclosure. Live tank CTs are useful because the primary conductor is short, which gives better
stability and a higher short-circuit current withstand rating. The primary of the winding can be evenly
distributed around the magnetic core, which gives better performance for overloads and transients. Since
the major insulation of a live-tank current transformer is not exposed to the heat of the primary conductors,
insulation life and thermal stability is improved.
A high-voltage current transformer may contain several cores, each with a secondary winding, for
different purposes (such as metering circuits, control, or protection).
[2]