NOTES ON CURRENT TRANSFORMERS

A current transformer is a transformer, which produces in its secondary
winding a current, which is proportional to the current flowing in its primary
winding. The secondary current is usually smaller in magnitude
than the primary current.
The principal function of a CT is to produce a proportional current at a
level of magnitude, which is suitable for the operation of measuring or
protective devices such as indicating or recording instruments and relays.
The rated secondary current is commonly 5A or 1A, though lower currents
such as 0.5A are not uncommon. It flows in the rated secondary load,
usually called the burden, when the rated primary current flows in the
primary winding.
The primary winding can consist merely of the primary current conductor
passing once through an aperture in the current transformer core or it may
consist of two or more turns wound on the core together with the secondary
winding.
These are two basic CT types. The first is commonly called a “ring” type
CT as the core is usually annular, but in some cases it may be square or
rectangular in shape. The second is usually known as a “wound primary”
type CT.
The primary and secondary currents are expressed as a ratio such as
100/5. With a 100/5 ratio CT, 100A flowing in the primary winding will
result in 5A flowing in the secondary winding, provided the correct rated
burden is connected to the secondary winding. Similarly, for lesser primary
currents, the secondary currents are proportionately lower.
It should be noted that a 100/5 CT would not fulfil the function of a 20/1 or
a 10/0.5 CT as the ratio expresses the current rating of the CT, not merely
the ratio of the primary to the secondary currents.
The extent to which the secondary current magnitude differs from the calculated
value expected by virtue of the CT ratio is defined by the [accuracy]
“Class” of the CT. The greater the number used to define the class,
the greater the permissible “current error” [the deviation in the secondary
current from the calculated value].
Except for the least accurate classes, the accuracy class also defines the
permissible phase angle displacement between primary and secondary
currents. This latter point is important with measuring instruments influenced
both by magnitude of current and by the phase angle difference
between the supply voltage and the load current, such as kWh meters,
wattmeter’s, var meters and power factor meters.
Common burden ratings are 2.5, 5, 10, 15 and 30VA.
Current transformers are usually either “measuring” or “protective” types,
these descriptions being indicative of their functions. The principal requirements
of a measuring CT are that, for primary currents up to 120% or
125% of the rated current, its secondary current is proportional to its primary
current to a degree of accuracy as defined by its “Class” and, in the
case of the more accurate types, that a specified maximum phase angle
displacement is not exceeded.
A desirable characteristic of a measuring CT is that it should “saturate”
when the primary current exceeds the percentage of rated current specified
as the upper limit to which the accuracy provisions apply. This means
that at these higher levels of primary current the secondary current is less
than proportionate. The effect of this is to reduce the extent to which any
measuring device connected to the CT secondary is subjected to current
overload.

On the other hand the reverse is required of the protective type CT, the
principal purpose of which is to provide a secondary current proportional
to the primary current when it is several, or many, times the rated primary
current. The measure of this characteristic is known as the “Accuracy
Limit Factor” (A.L.F.). A protection type CT with an A.L.F. of 10 will produce
a proportional current in the secondary winding [subject to the allowable
current error] with primary currents up to a maximum of 10 times the
rated current.
Preferred primary and secondary current ratings [and therefore ratios],
classes, burdens and accuracy limit factors are defined in BS3938 and
other comparable national standards, together with other minimum performance
requirements, physical construction requirements, etc.
It should be remembered when using a CT that where there are two or
more devices to be operated by the secondary winding, they must be
connected in series across the winding. This is exactly the opposite of
the method used to connect two or more loads to be supplied by a voltage
or power transformer where the devices are paralleled across the secondary
winding.
With a CT, an increase in the burden will result in an increase in the CT
secondary output voltage. This is automatic and necessary to maintain
the current to the correct magnitude. Conversely, a reduction in the burden
will result in a reduction in the CT secondary output voltage.
This rise in secondary voltage output with an increase in burden means
that, theoretically, with infinite burden as is the case with the secondary
load open circuit, an infinitely high voltage appears across the secondary
terminals. For practical reasons this voltage is not infinitely high, but can
be high enough to cause a breakdown in the insulation between primary
and secondary windings or between either or both windings and the core.
For this reason, primary current should never be allowed to flow with no
load or with a high resistance load connected across the secondary winding.
When considering the application of a CT it should be remembered that
the total burden imposed on the secondary winding is not only the sum of
the burden(s) of the individual device(s) connected to the winding but that
it also includes the burden imposed by the connecting cable and the resistance
of the connections.
If, for example, the resistance of the connecting cable and the connections
is 0.1 ohm and the secondary rating of the CT is 5A, the burden of
the cable and connections (RI2) is 0.1 x 5 x 5 = 2.5VA. This must be
added to the burden(s) of the connected device(s) when determining
whether the CT has an adequately large burden rating to supply the required
device(s) and the burden imposed by the connections.
Should the burden imposed on the CT secondary winding by the connected
device(s) and the connections exceed the rated burden of the CT
the CT may partly or fully saturate and therefore not have a secondary
current adequately linear with the primary current.
The burden imposed by a given resistance in ohms [such as the resistance
of a connecting cable] is proportional to the square of the rated secondary
current. Therefore, where long runs of cable between CT and the
connected device(s) are involved, the use of a 1A secondary CT and a 1A
device rather than 5A will result in a 25-fold reduction in the burden of the
connecting cables and connections.
All burden ratings and calculations are at rated secondary current.

Because of the foregoing, when a relatively long [more than a very few
metres] cable run is required to connect a CT to its burden [such as a
remote ammeter] a calculation should be made to determine the cable
burden. This is proportional to the “round trip” resistance, i.e. twice the
resistance of the length of twin cable used. Cable tables provide information
on the resistance values of different sizes of conductors at 20o C per
unit length. The calculated resistance is then multiplied by the square of
the CT secondary current rating [25 for 5A, 1 for 1A]. If the VA burden as
calculated by this method and added to the rated burden(s) of the device(s)
to be driven by the CT exceeds the CT burden rating, the cable size must
be increased [to reduce the resistance and thus the burden] or a CT with
a higher VA burden rating must be used, or a lower CT secondary current
rating [with matching change in the current rating of the device(s) to be
driven] should be substituted.

General


CTs should be specified as follows:

  • RATIO: input / output current ratio
  • VA: total burden including pilot wires.
  • CLASS: Accuracy required for operation
  • DIMENSIONS: maximum & minimum limits

Metering CTs
In general, the following applies:
CLASS

  • 0.1 or 0.2 for precision measurements
  • 0.5 for high grade kilowatt hour meters for commercial grade kilowatt hour meters
  • 3 for general industrial measurements
  • 3 or 5 for approximate measurements

BURDEN (depending on pilot lead length)
• Moving iron ammeter 1-2VA
• Moving coil rectifier ammeter 1-2.5VA
• Electrodynamic instrument 2.5-5VA
• Maximum demand ammeter 3-6VA
• Recording ammeter or transducer 1-2.5VA

Protection CTs
In addition to the general specification required for CT design, protection
CT’s require an Accuracy Limit Factor (ALF). This is the multiple of rated
current up to which the CT will operate while complying with the accuracy
class requirements.
In general the following applies:

  • Instantaneous overcurrent relays & trip coils - 2.5VA Class 10P5
  • Thermal inverse time relays - 7.5VA Class 10P10
  • Low consumption Relay - 2.5VA Class 10P10
  • Inverse definite min. time relays (IDMT) overcurrent - 15VA Class 10P10/15
  • IDMT Earth fault relays with approximate time grading - 15VA Class 10P10
  • IDMT Earth fault relays with phase fault stability or accurate time grading required - 15VA Class 5P10

Class X CTs
Class X CTs are special CTs used mainly in balanced protection systems
(including restricted earth fault) where the system is sensitively dependent
on CT accuracy. Further to the general CT specifications, the manufacturer
needs to know:

  • Vkp - Voltage knee point
  • Io - Maximum magnetising current at Vkp
  • Rs - Maximum resistance of the secondary winding

 

Burden (VA) of copper wires between instrument
and current transformer for 1A and 5A secondaries

Table for 1A Secondaries

Burden in VA (twin wires) Distance in metres
Cross section
mm2
10m 20m 40m 60m 80m 100m
1 0.36 0.71 1.43 2.14 2.85 3.57
1.5 0.23 0.46 0.92 1.39 1.85 2.31
2.5 0.14 0.29 0.57 0.86 1.14 1.43
4 0.09 0.18 0.36 0.54 0.71 0.89
6 0.06 0.12 0.24 0.36 0.48

0.6

 

Table for 5A Secondaries

Burden in VA (twin wires) Distance in metres
Cross section
mm2
1m 2m 4m 6m 8m 10m
1.5 0.58 1.15 2.31 3.46 4.62 5.77
2.5 0.36 0.71 1.43 2.14 2.86 3.57
4 0.22 0.45 0.89 1.34 1.79 2.24
6 0.15 0.3 0.6 0.89 1.19 1.49
10 0.09 0.18 0.36 0.54 0.71 0.89