- Current Transformer Definition: A current transformer (CT) is defined as an instrument transformer where the secondary current is proportional to the primary current and ideally has zero phase difference.
- CT Accuracy Class: The accuracy class of a current transformer measures how accurately the CT replicates the primary current in its secondary, crucial for precise metering.
- Working Principle: Current transformers operate on the principle of a power transformer, where the primary current is the system current, and the secondary current depends on the primary current.
- Ratio Error in Current Transformer: Ratio error in current transformer occurs when the primary current isn’t perfectly mirrored in the secondary current due to core excitation.
- Reducing CT Errors: Errors in current transformers can be reduced by using high-quality core materials, matching the rated burden with the actual burden, and minimizing the core length.
Definition of Instrument Transformer
Instrument transformers means a current transformer and a voltage transformer used in an electrical power system to step down currents and voltages for metering and protection. The relays and meters used for those jobs are not designed for high currents and voltages.
High currents or voltages in an electrical power system cannot be fed directly to relays and meters. A current transformer steps down the system current to 1 Amp or 5 Amp, and a voltage transformer steps down system voltage to 110 V. Relays and meters are generally designed for 1 Amp, 5 Amp and 110 V.
What is a Current Transformer?
A current transformer (CT) is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree.
CT Accuracy Class or Current Transformer Class
A current transformer is similar to an electrical power transformer to some extent, but construction and operating principle differ. For metering and indication, the ratio of primary to secondary current must stay accurate in the normal working range. IEC 61869-2 asks for accuracy of a current transformer up to 120% of rated current, because system current in normal service should stay within that band.
After that limit it is desirable for a metering core to saturate, so a system overcurrent does not stress the meter on the secondary. Secondary current then stays within a set band even if primary current rises far above rating. Accuracy in the working range is the main criterion for a metering current transformer. That degree of accuracy is expressed as current transformer accuracy class or simply current transformer class or CT class.
For protection, the current transformer need not match the accuracy of a metering current transformer, but its core should not saturate while high fault current passes through the primary.
A protection current transformer core is therefore designed so it does not saturate across a wide current range. If the core saturates at a lower primary current, the secondary no longer tracks the primary, relays on the secondary may not operate, and the protection system loses reliability.
Take a current transformer with a ratio of 400/1 A whose protection core saturates at 500 A. If primary current reaches 1000 A, secondary current stays at 1.25 A because of saturation. If the relay on the secondary needs 1.5 A to pick up, it will not operate even though primary current is 1000 A.
Accuracy of a protection current transformer is coarser than a metering current transformer, but it is still stated as current transformer accuracy class or simply current transformer class or CT class as for a metering current transformer, though the class letters and numbers are assigned in a different way.
Theory of Current Transformer or CT
A CT uses the same basic principle as a power transformer, with differences in how current is set. In a power transformer, primary current varies with the load or secondary current. In a CT, primary current is the system current. That current is transformed to the CT secondary, so secondary current depends on primary current.
If the load on a power transformer is disconnected, only magnetizing current flows in the primary. Primary current of a power transformer is proportional to the load on the secondary. A CT primary is connected in series with the power line, so primary current is the same as line current.
Primary current of the CT therefore does not depend on whether a burden is connected to the secondary, or on the impedance of that burden. A CT generally has few primary turns and many secondary turns. If Np is the number of turns on the CT primary and Ip is primary current, primary ampere-turns equal NpIp AT.
If the number of secondary turns and secondary current of that current transformer are Ns and Is respectively, secondary ampere-turns equal NsIs AT.
In an ideal CT, primary ampere-turns equal secondary ampere-turns in magnitude.
From that statement, a CT with one primary turn and 400 secondary turns that carries 400 A in the primary will have 1 A in the secondary burden.
The turns ratio of that CT is therefore 400/1 A.
Current Transformer Error
Errors in an actual current transformer are easiest to follow from a CT phasor diagram,
Is – Secondary current.
Es – Secondary induced emf.
Ip – Primary current.
Ep – Primary induced emf.
KT – Turns ratio = Numbers of secondary turns/number of primary turns.
I0 – Excitation current.
Im – Magnetizing component of I0.
Iw – Core loss component of I0.
Φm – Main flux.
Take flux as the reference. EMF Es and Ep lag the flux by 90°. The magnitude of the phasors Es and Ep are proportional to secondary and primary turns. The excitation current Io is made up of two components Im and Iw.
The secondary current Is lags the secondary induced emf Es by an angle Φ s. The secondary current is then transferred to the primary side by reversing Is and multiplying by the turns ratio KT. Total primary current Ip is then the vector sum of KT Is and I0.
The Current Error or Ratio Error in Current Transformer or CT
From the phasor diagram it is clear that primary current Ip is not exactly equal to the secondary current multiplied by turns ratio, i.e. KTIs. That difference exists because part of the primary current is the core excitation current. The error in current transformer from that difference is called current error of CT or sometimes ratio error in current transformer.
Phase Error or Phase Angle Error in Current Transformer
For an ideal CT, the angle between the primary and reversed secondary current vector is zero. In a real CT there is always a phase difference because the primary current also supplies the excitation current. That phase difference is the phase angle error in a current transformer.
On the phasor diagram it is β. Phase angle error is usually expressed in minutes.
Cause of Error in Current Transformer
The total primary current is not all transformed in a CT. One part of the primary current is used for core excitation and the rest is transformed with the turns ratio of the CT, so there is error in current transformer means there are both ratio error in current transformer as well as a phase angle error in current transformer.
How to Reduce Error in Current Transformer
Those errors should be kept small for better performance. For minimum error in a current transformer, apply the following,
- Using a core of high permeability and low hysteresis loss magnetic materials.
- Keeping the rated burden to the nearer value of the actual burden.
- Ensuring minimum length of flux path and increasing cross-sectional area of the core, minimizing joint of the core.
- Lowering the secondary internal impedance.
