Current Transformers occupy a vital part in the measurement and protection scheme of any electric installation.

Hence, it is imperative that the choice of CT is made with full knowledge of the application and the number of relays and meters which are to be connected to it. A current transformer with a wrong burden rating or a wrong accuracy class will seriously compromise the effectiveness of the measuring or protection system.

Calculating the burden of a Current Transformer.

The burden of a current transformer is indicated in its nameplate. The burden is rated in VA. such as 15VA or 25 VA. The rated VA indicates the load the transformer can take.

The current transformer is connected to a measuring instrument or a protective relay by means of wires. The burden on the current transformer is imposed by the connected device and the impedance of the connecting wires which connect it. The VA load of the device can be obtained from the datasheet provided by the manufacturer. The total burden is the sum of the burden of the connected devices and the resistance of the wires. The inductive component of the wire impedance is usually neglected as it is minimal.

The burden of a current transformer can increase over time as the resistance of the connecting wires may slightly increase due to age, change in temperature and loosening of connections. Hence, the current transformer should never be loaded to 100% of its capacity.

Classification of Current Transformers

Depending on their application, current transformers can be classified into measuring and protection current transformers.

Measuring transformers have high accuracy. However, they have a low saturation point. They are deliberately designed this way, so that the measuring instruments are not damaged by the high currents during a fault. During a fault, the measuring transformers get saturated and the output stays within the range of the measuring instruments.

Measuring transformers are classified into 0.1,0.3, 0.5, 1. The values indicate the percentage error at the rated primary current. Thus a 100/5 transformer with 0.3 accuracy will have a maximum error of 0.3 when a current of 100 A passes through the primary.

The current transformers used for protection have lesser accuracy as compared to measuring current transformers. They have a very high saturation limit. This is necessary as they need to continue sensing the current even at high fault values.

Protection Transformers are classified as 5P10,10P10, etc. The first letter in the notation indicates the maximum percentage error. The last number indicates the number of times the rated current. Thus a 5P10 transformer would indicate a maximum error of 5 % at 10 times the rated current.

Peterson coils are used to in ungrounded 3-phase grounding systems to limit the arcing currents during ground faults. The coil was first developed by W. Petersen in 1916.


When a phase to earth fault occurs in ungrounded 3 phase systems, the phase voltage of the faulty phase is reduced to the ground potential. This causes the phase voltage in the other two phases to rise by √3 times. This increase in voltage causes a charging current, Ic between the phase-to-earth capacitances. The current Ic, which increases to three times the normal capacitive charging current, needs to complete its circuit. This causes a series of restrikes at the fault locations known as arcing grounds. This can also lead to overvoltages in the system.

A Petersen coil consists of an iron-cored reactor connected at the star point of a three phase system. In the event of a fault, the capacitive charging current is neutralized by the current across the reactor which is equal in magnitude but 180 degrees out of phase. This compensates for the leading current drawn by the line capacitances. The power factor of the fault moves closer to unity. This facilitates the easy extinguishing of the arc as both the voltage and current have a similar zero-crossing.

IC=3I=3Vp/(1/ωC) =3VpωC

Where IC is the resultant charging current that is three times the charging current of each phase to ground.

Consider a Petersen coil connected between the star-point and the ground with inductive reactance ωL, then

The current flowing through it is given by

IL =Vp/ωL

To obtain an effective cancellation of the capacitive charging currents, IL to be equal to IC.



From which we get,

L=1/ (3ω2C)

The value of the inductance in the Petersen coil needs to match the value of the line capacitance which may vary as and when modifications in the transmission lines are carried out. Hence, the Petersen coil comes with a provision to vary the inductance.