Question $4$

$3-$phase Transformer Differential protection

pper taps:

$,4\mathrm{.}0,4\mathrm{.}5$ & $5$ A

wer taps:

$7$&$8\mathrm{.}5$ A

a$)$ Calculate the full$-$load $($or normal$-$load$)$ power transformer line currents.

b$)$ Select CT connections and ratios so that the primary CT rating is greater than the $3-$ph transformer full$-$load line current.

c$)$ Calculate the CT secondary currents at full load.

d$)$ Keeping in mind the rule for CT connections, calculate the current flowing from the CTs to the relays.

e$)$ Select relay taps so the ratio of tap settings is as nearly as possible equal to the ratio of currents that flow to the relay. Also, the relay taps should be as nearly as possible equal to the respective currents that flow under full$-$load $($or normal load$)$ conditions.

f$)$ Calculate the resulting $\%$ unbalance

g$)$ Calculate the $\%$ unbalance for a $300$:$5$A HV CT and $5$A relay tap paired with a $1600$:$5$A LV CT and a $8\mathrm{.}5$ A relay tap.