Question $3$

$3-$phase Transformer Differential protection

All que

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 $($or normal$-$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.

$6$

Select $4\mathrm{.}6$A tap for HV side

Since there is only one tap available on the LV side $(8\mathrm{.}7$A$)$ a quicker way to find the corresponding tap on the HV side is shown below:

Select $4\mathrm{.}6$A tap for HV side

Since there is only one tap available on the LV side $(8\mathrm{.}7$A$)$ a quicker way to find the corresponding tap on the HV side is shown below:

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

g$)$ Determine the minimum relay pickup that will avoid nuisance tripping on no$-$load of $10\%$ FL$.$ Settings in increments of $0\mathrm{.}1$ A $,$ with a minimum of $0\mathrm{.}3$ A $,$ are available.