Why PT burden is high Compare with CT burden?
First of all we must know, what is burden? Burden is nothing but an output apparent power of the instrument transformer. The burden of the transformer is normally denoted in VA. VA means Volt Amp.
In instruments transformer, the potential transformer burden is high as compare with current transformer because Potential transformers are parallel loaded device, here we can connect N number of loads with the potential transformer, so that the PT has to ready to fulfill all the existing loads and featured loads. But Current transformers are series connected device, hence all the load current or the Current transformer always gives constant current to the load circuits, even if you connect N number of loads in series with the current transformer. For this, less burden is enough to carry all the loads.
Refer the diagram:
Also Potential transformers are higher in cost as compare with the current transformer. The cost will be reduced as the burden increases, instead of that if we go for New potential transformer for feature loads we have to install additional PTs. Also PTs are very sensitive equipment, to protect the PT we need to install additional surge protection, i.e LAVT panels.
Due to above reason the potential transformer will be installed with the higher capacity one time installation.