Can I get an idea as to why for delta-star (secy) transformer, (normally 11kV/0.433kV level), if a barrel fuse blown on primary side results one phase healthy, second phase dim, third phase no supply?
Responding to Krishna's 13-Sep (22:54) question... what happens when a fuse, located in the primary supply D-Y transformer, operates?
A) Qualitative Mathematical Analysis.
The simplest approach to this problem is to (mathematically) substitute the single 3-phase transformer with three 1-phase transformers, whose primaries are delta-connected and secondaries wye-connected. In this way a more rigorous solution is avoided because the effects of mutual-inductances, and mutual magnetic flux-fields can be ignored.
B) Pre-Fault and Post-fault Primary Voltages.
The pre-fault connections for the three transformers are: Tr1 is connected to phases A-B; Tr2 to B-C; and Tr3 to C-A. Now, assume that the C-phase fuse operates. Then, post-fault voltages are: Tr1 is supplied with full voltage; Tr2 and Tr3 are placed in series so their primary terminal voltages are 50% each.
C) Post-fault Secondary Voltages.
Calling the secondary phases a, b, and c, then the corresponding phase-to-neutral voltages are: 100% for Tr1; and 50% each for Tr2 and Tr3. But, because all secondaries are "in-phase", the corresponding secondary phase-to-phase voltages are: 87% between a-b; 87% between a-c; and 0% between b-c!
D) Post fault Secondary Currents.
What about currents? The corresponding secondary bus-currents are: 2-units in bus a; and 1-unit each in bus b and c! Recall that Kevin Cooper described a similar situation in his 19-Jan-07 Thread #1026230518, "False Clearing of Transformer Primary Fuse"! He observed that the fault was cleared by operation of the secondary incoming breaker... not by operation of a second fuse in the primary supply!
Krishna, please note that this qualitative analysis also ignored the effects of residual voltages produced by the motors as they decelerate! If a quantitative analysis is required, let me know!