A current transformer is used for measurement of alternating electric currents. Current
transformers, together with), are known as instrument transformers. When current in a
circuit is too high to apply directly 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 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.
Design
Like any other transformer, a current transformer has a primary winding, a magnetic core and
a secondary winding. The alternating current 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 the primary and secondary
circuits are efficiently coupled, so the secondary current is linearly proportional 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 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 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 molded case. High-voltage current
transformers are mounted on porcelain 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 bus bar
is arranged to be easily removed for exchange of current transformers.
Usage
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. 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 to be used for the devices.
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 secondary winding
will supply an output current of 5 amperes when the primary winding current is 4000 amperes.
The secondary winding can be single or multi-ratio, with five taps being common for multi-ratio
CTs. The load, or burden, of the CT should be a
low resistance. If the voltage time integral area is higher than the core's design rating,
the core goes into saturation toward the end of each cycle, distorting the waveform and
affecting accuracy. Safety precautions
Care must be taken that the secondary of a current transformer is not disconnected from
its load while current is 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
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
Phase change For the IEC standard, accuracy classes for
various types of measurement are set out in IEC 61869-1, 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 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 and 400 V its output accuracy will be within
2.5 percent. Burden
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 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. 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 Ω 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 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%. 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. Phase shift
Ideally the secondary current of a current transformer should be perfectly in phase with
the primary current. In practice, this is impossible to achieve, but phase shifts as
low as a few tenths of a degree for well constructed transformers up to as much as six degrees
for simpler designs may be encountered. For the purposes of current measurement, any phase
shift is immaterial as the indicating ammeter, only displays the magnitude of the current.
However, if the current transformer is used in conjunction with the current circuit of
a wattmeter, energy meter or power factor meter, any phase shift in the measured current
can affect the accuracy of the target measurement. For power and energy measurement, this error
is generally considered to be negligible at unity power factor but increases in significance
as the power factor approaches zero. At true zero power factor, all the measured power
is entirely due to the current transformer's phase error. In recent years the introduction
of electronic based power and energy meters has allowed the phase error to be calibrated
out. Special designs
Specially constructed wideband current transformers are also used 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 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.
Standards Ultimately, depending on client requirements,
there are two main standards to which current transformers are designed. IEC 60044-1 & IEEE
C57.13, although the Canadian and Australian standards are also recognised.
High voltage types 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 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. A neutral current transformer is used as earth fault protection
to measured any fault current flowing through the neutral line from the wye neutral point
of a transformer. See also
Rogowski coil Transformer
Instrumentation Transformer types
References Guile, A.; Paterson, W.. Electrical Power
Systems, Volume One. Pergamon. p. 331. ISBN 0-08-021729-X. ^ Anon, Protective Relays Application Guide
Second Edition, The General Electric Company Limited of England, 1975 Section 5.3
^ a b http:www.ccontrolsys.comMeasurement_Errors_Due_to_CT_Phase_Shift ^ http:www.ccontrolsys.comCT_Phase_Angle_Correction
^ Protective Relays Application Guide, pages 78-87
External links High Voltage Transformer / Custom Transformers
Introduction to Current Transformers Introduction to Metering / Measurement Current
Transformers Introduction to Protection Current Transformers
Transformer Terminology Testing Current Transformers