Joined: 04 March 2014
I have voltage instrument transformer (VT) rated as followed: (110/sqrt(3))/(0.1/sqrt(3)) kV
secondary winding of my interest (4 secondary windings in total):
rated burden: 25 VA class 3P (protection type class).
VT is connected to distance protection relay (Alstom P433). Relay gives 0.5 VA burden per phase.
According to IEC 60044-2 and some other standards, the accuracy class shall be fullfilled from 25% to 100% of the rated burden.
It is seen that 25% of 25VA is 6.25 VA which is far above 0.5 VA.
Hence VT works under 25% of rated burden which results in increased measurement error.
Should the load resistors be used to obtain winding load between 25-100 % to get rated accuracy class?
Load resistors are commonly used for metering windings ( ie. 0.5 class) connected to energy meters for revenue metering.
However I've never seen (so far) resistors connected to protection class windings and I am not convinced if it is correct to let protection class winding be loaded belov minimum 25% of rated load.
Please give me any suggestions.