The burden is the maximum load that we can connect to the secondary CT and PT. The burden of CT and PT are expressed in apparent power ( VA – Volt-Ampere).
Before getting the exact answer of why PT burden high compared to CT burden, first, let us understand how the secondary current flows in the secondary of CT & PT.
PT receives AC voltage at the primary and steps down the voltage according to the turns ratio of the transformer. The secondary voltage depends on the primary voltage, and it does not depend on the load connected at its secondary(Burden). The secondary voltage remains fairly constant with the load.
The current through the PT depends on the load connected to its secondary. The load on PT secondary remains almost constant for certain loads like meter, protection relay & monitoring relay after its installation.
PT secondary has to supply the voltage to the electrical equipment connected in parallel to the PT secondary.
|PT Burden( VA) = Vs * ( I1 + I2 + I3 +……. In )|
Therefore, more PT burden needs to connect the meter and relay.
Contrarily, the CT secondary current depends on the burden of the individual meters and protection relay. The CT has to supply the current to the electrical meters and protection relays connected in the series to the CT secondary. Thus, the meters connected to CT secondary must have the least burden.
|CT Burden (VA) = Vs * Is|
The higher CT burden cause error in the measurement and in a long run may fail the CT.
That is why PT burden is high Compared to CT burden.