Question $2$

Shown in Fig. Q$2($a$)$ is a main LV switchboard fed from a $1050$kVA$11k\frac{V}{380}V$ distribution transformer and protected by a $1600$A MICB $($main incoming circuit breaker$).$ One outgoing circuit with the max. loading of $600$A is protected by a $630$A ACB. Both MICB and ACB are controlled with IDMT relays for overcurrent protection. Electronic relays with SI and EI curves are available for selection. SWB reactance is assumed to $1\mathrm{.}0$ mohm $.$

$($a$)$ Find the maximum fault current after the transformer. Select CT rating, and the type, PS and TM of the O$/$C relay for MICB so that the power company's requirement in Fig. Q$2($b$)$ can be achieved.

$(10$ marks$)$

$($b$)$ Find the maximum fault current in this feeder circuit. Select CT rating, and the type, PS and TM of the overcurrent relay for ACB if the discrimination between MICB and ACB is achieved for the maximum fault current in this feeder circuit. The grading time is assumed to be $0\mathrm{.}05$ sec $.$

$(10$ marks$)$

$($c$)$ Different current parameters are introduced for discussing the protection of electrical circuits, such as Ib$,$ In$,$ Inf, I$2,$ Iz$,$ Ir$,$ Im$,$ Icu. Illustrate briefly, with equations and$/$or line diagrams, the relationship among these parameters for the circuit protection.

$(5$ 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 $50\%-200\%$ 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 $0\mathrm{.}05.$ The possible CT rating includes $630$ A $,1000A,1600A,2000A35$nA