If the current of one side either primary or secondary is
known than the current of other side will be in same
proportion as the volotage ratio of primary and secondary
side.
For example -for a 1000 KVA 11kv/0.4 kv transformer if there
is 1 amp. current in HT side than current in LT side will be
1x11x1000/400 i.e almost 26 amps.

Let suppose u have taken 1500 kva transformer of 11kv to 415 v u wanna be the current calculation for the both HT side and LT side. But u also think that u have connect load of the transformer 1000 kw. In this case both HT & LT SIDE power drawn is same. But this is three phase so we take formula
P equal to SQ ROOT 3 *V* I COS phai
Remember cos phai is necessary becuse of we know KVA :::: KW/ COS phai
U have connect 1000kw and its drawn full lo
ad then it wil be calculate in HT SIDE
1000*1000:::1.732*11000*0.8* I(u take power factor as0.8)
I::::65.6A
WHILE calculating in LT SIDE IT wil be
1000*1000::: 1.732*440*0.8* I
I:::1739.05A
because power drawn HT & LT SIDE depend on load connected to the transformer

In a distribution system,having 3phases & 1 nuetral.ecah
phase to neutral is 230V,line-line voltage is 440V,but what
is the voltage of each line.but it should obey the KVL law.

When in a large power system if the demand of power is
larger than the generated power the frequency of the power
system drops.(assume the rated frequency is 50Hz and now it
is 49Hz)
So does this mean the demand is totally supplied with a
lower value of generated power in a lower frequency?