According to IEC 186 a voltage transformer is required to fulfill its accuracy class for burdens between 25 and 100% of rated burden.
A turn's correction is mostly selected to give a positive error + max at a burden of 25% of rated burden and - max at a burden Of 100% of rated burden.
The values are rated values per phase for a three-phase set. The Standard values on burden is given at cos Ø=0,8.
The accuracy class is fulfilled for 25-100% of rated burden.
Burden of today protection and metering equipment is very low(in range 5-10 VA for an object) and considering that accuracy class is fulfilled down to 25% only a low selected rated burden
should be used.
But addition thermal burden of each VT can be used by ferro-resonance damping resistor. Both the CVT and a magnetic VT can be involved in Ferro-resonance phenomenon.
The ferro resonance for a magnetic VT is an oscillation between the inductance of the VT and the capacitance of the network. Ferro resonance can only occur at ungrounded networks, but note the risk that some part becomes ungrounded under certain circumstances.
The damping of ferro-resonance is normally done with a 27-60 ohm ,200 W resistor connected across the open delta winding. The resistor value should give a current as high as possible but a current below the thermal rating of the voltage transformer.