Question

A 20MVA, 11kV/132 kV, star-delta transformer is being protected by a biased differential protection
system. The transformer is supplied from 132 kV grids and on full load operation. Determine the
suitable CTs ratio at both side of the transformer if current at restraints coil is rated at 5 A. (CT ratios
available: 100/5A, 300/5A, 600/5A, 800/5A, 900/5A, 1000/5A, 1500/5A, 2000/5A, 3000/5A) If
lightning strikes at the secondary of the transformer which injecting IL=10kA to the system;

(i) Determine the primary current, Ip and secondary current, Is for the 20MVA transformer at the CTs’
output. Ignore the impedance of the cables.
(ii) Determine the average restraint current and operating current in the relay.
(iii) The bias characteristic of the relay is y = mx + c where y is the operating current, x is the
average restraint current and m is slope. Draw a neat sketch of the characteristic curve of this
relay if relay setting IS = 0.2A and percentage bias setting is set to 25%. Clearly mark the scales
used for both the x-axis and y-axis neatly on the sketch.
(iv) Mark the operating point of the relay due to this fault on the sketch. Will the relay operate? If
yes, give your explanation.

Answer 1