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.