Subject : Switch Gear & Protection

Factors To Be considered while

selecting CT

- Shivam Patel
 Current Transformer (CT)

 Definition
 A current transformer is an instrument transformer, used along with measuring or
protective devices, in which the secondary current is proportional to the primary
current (under normal conditions of operation) and differs from it by an angle that is
approximately zero.
 Functions
 Current transformers perform the following functions:
 Current transformers supply the protective relays with currents of magnitude
proportional to those of power circuit but sufficiently reduced in magnitude.
 The measuring devices cannot be directly connected to the high magnitude supplies.
Hence current transformers are used to supply those devices with currents of
magnitude proportional to those of power.
 A current transformer also isolates the measuring instruments from high voltage
circuits.
 Current Transformer (CT)

 Principle
 The basic principle of the current transformer is the same as that of the power transformer. Like the power
transformer, the current transformer also contains a primary and a secondary winding. Whenever an alternating
current flows through the primary winding, alternating magnetic flux is produced, which then induces alternating
current in the secondary winding. I
 n the case of current transformers, the load impedance or “burden” is very small. Therefore the current
operates under short circuit conditions. Also the current in the secondary winding does not depend on load
impedance but instead depends on the current flowing in the primary winding.
 The current transformer basically consists of an iron core upon which primary and secondary windings are
The primary winding of the transformer is connected in series with the load and carries the actual current flowing
the load, while the secondary winding is connected to a measuring device or a relay. The number of secondary
turns is proportional to the current flowing through the primary; i.e., the larger the magnitude of current flowing
through the primary, more the number of secondary turns.
 Current Transformer (CT) (Principle…..)

 Principle :
 The ratio of primary current to the secondary current is known as the current transformation ratio of the CT. Usually
current transformation ratio of the CT is high. Normally the secondary ratings are of the order 5 A, 1 A, 0.1 A, whereas
primary ratings vary from 10 A to 3000 A or more.
 The CT handles much less power. Rated burden can be defined as the product of current and voltage at the secondary
side of the CT. It is measured in volt ampere (VA).
 The secondary of a current transformer should not be disconnected from its rated burden while current is flowing in
primary. As the primary current is independent of the secondary current, the entire primary current acts as a
current when secondary is opened. This results in deep saturation of the core, which cannot return to normal state and
the CT is no longer usable.
 Current Transformer (CT) : Types

Based on the function performed by the current

Based on the function construction
transformer

• Bar Type. This type consists of a bar of suitable size

and material forming an integral part of the
• Measuring current transformers. These current transformer.
transformers are used along with the measuring
devices for the measurement of current, energy, • Wound Type. This type has a primary winding of
ore than one full turn wound over the core.
power.
• Window Type. This type has no primary winding.
The secondary wind of the CT is placed around the
• Protective current transformers. These current current flowing conductor. The magnetic electric
transformers are used along with the protection
field created by current flowing through the
equipments such as trip coils, relays, etc.
conductor induces current in the secondary
which is used for measurement.
 Current Transformer (CT) : Types

Based on the function construction

Bar Type Wound Type Window Type.

 Protective Current Transformer

 Purpose :
 Protection Current Transformers are designed to
measure the actual currents in power systems and to
produce proportional currents in their secondary
windings which are isolated from the main power
circuit.
 Satisfactory operation of protective relays can
on accurate representation of currents ranging from
small leakage currents to very high overcurrent's,
requiring the protective current transformer to be
linear, and therefore below magnetic saturation at
values up to perhaps 30 times full load current .
 Protective Current Transformer

 This wide operating range means that protective current transformers require to be constructed
with larger cross-sections resulting in heavier cores than equivalent current transformers used for
measuring duties.
 For space and economy reasons, equipment designers should however avoid over-specifying
protective current transformers ITL technical staff are always prepared to assist in specifying
protective CT's but require some or all of the following information;
• Protected equipment and type of protection.
• Maximum fault level for stability.
• Sensitivity required.
• Type of relay and likely setting.
• Pilot wire resistance, or length of run and pilot wire used.
• Primary conductor diameter or busbar dimensions.
• System voltage level.
 Factors To Be considered while selecting CT

 Accuracy Class :

 CT Accuracy is defined in terms of departure from the true ratio . It is expressed as

 The accuracy varies with the secondary biurden and also dependent upon the satureation flus density of the
core .
 For many relays ,an accuracy of +_ 10% TO +_ 15% is acceptable. For example incase o fIDMT overcurrent
relays , the accuracy is less omportanr , particularly at high current as the time of operation od relay is
practically constant. On the other hand , with the distance and differential relay a CT accuracy of +_3% to +_ 5%
is derirable.
 CT errors can be also be expressed in terms of composite error.
 Factors To Be considered while selecting CT

 Knee-point 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. T
 he 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
 Factors To Be considered while selecting CT

 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 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 and 60 VA. 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 (1A),
both of which will produce less voltage drop between the CT and its metering devices.
 Factors To Be considered while selecting CT

 Short- Time current rating of the Transformer:

 Short time current is maximum fault current that a CT can withstand for a short duration of time. This
current can be decided by the following factors.
1. The rms value of the fault current for a short period
2. The duration for which fault current may perisist.
3. Peak asymmetric for which fault current may persist.
4. Peak asymmetric value of fault current
5. Transient Voltage at the instant of fault occurance and the instant of extension of arc by a circuit beaker.

o The First two factor constitue the thermal limit. The more th fault current and greater is the time period
secified ; larger will be the primary conductor cross-section.
o The peak Value o fthe symmetric falult current imposes the mechanical limit. The electrodynamic forces
produced depends the peak value the number of primary turns ans the configuration of the coil.
 Factors To Be considered while selecting CT

 Accuracy Limit Factor :

 Protective CT are reqired to faithfully transform maximum possible fault currents . This value of the fault
current can be calculated by assuming a three-phased bolted shor – circuit immediately following a CT
location .
 Accuracy Limit factor (ALF) is given by :

ALF = Maximum fault current / Rated Primary current

 A CT is so often sopecifid as 15VA, 5P10 where 15Va denotes burden , 5 denotes composite error and 10
is the ALF
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