Question $2$

Shown in Figure Q$2$ is a main LV switchboard fed from a $2000$kVA$11k\frac{V}{400}V$ distribution transformer and protected by a $3000$A MICB $($main incoming circuit breaker$).$ One $100$m$-$long feeder circuit is wired with two $\frac{3}{c}300m{m}^2$ PVC$/$SWA$/$PVC cables on tray, and is protected by a $630$ A ACB $($air circuit breaker$).$ The earth fault loop impedance at the main LV switchboard is $50$ mohm. Both $3000$A and $630$A circuit breakers are controlled with IDMT relays for earth fault protection. Electronic relays with SI and EI curves are available for selection.

$($a$)$ Determine the breaking capacity of $630$A ACB.

$(4$ marks$)$

$($b$)$ Find out the min. fault current in the feeder circuit for the chiller plant.

$(4$ marks$)$

$($c$)$ Select the type, PS and TM of the earth$-$fault relay for MICB so that the relevant requirement in CoP can be achieved. Explain the meaning of $5$P$15$ using the CTs for MICB.

$(9$ marks$)$

$($d$)$ Select the type, PS and TM of the earth$-$fault relay for ACB if the discrimination between MICB and ACB is achieved for the possible range of earth fault current in this feeder circuit. The grading time is assumed to be $0\mathrm{.}1$ sec $.$

$(8$ marks$)$

The time$-$current characteristic of relays is described by

$t=\frac{0\mathrm{.}14}{PS{M}^{0\mathrm{.}02}-1}TM$

$t=\frac{80}{PS{M}^2-1}TM$

The plug setting $($PS$)$ is in the range of $10\%-40\%$ with a step size of $10\%,$ and the time multiplier $($TM$)$ is in the range of $0\mathrm{.}1-1\mathrm{.}0$ with a step size of $0\mathrm{.}05.$ Additional information is given in Appendix.