why use CT in series & PT in parallel

I think maybe you are misinterpreting what you see.

"CT in series"? A CT is a pass-through device, the current conductor passes through the core of the CT and the rise and fall of the magnetic field is what creates the proportional current output on the secondary. it's not really in series with the conductor. In large CT's they include a piece of bus bar for convenience.

In a PT, you have to have two different phase relationships for them to work, as in any voltage transformer, but would one consider that to be "parallel"? I don't. That's just "connected".

hi Qartas

The "CTs" , "PTs" represent "a high voltage based Ammeter" , "a high voltage based voltmeter" respectively ,…. So as you know when measuring out the current, we use Ammeter in series because its internal resistance is very small, which ensures no effect on the circuit that containing that Ammeter. On the other hand, we connect the Voltmeters as parallel owing to its high internal resistance…………….. the same thing can approximately be said……

Hope this helps,,,,,,,,,,,,,

how do you measure current??..or system voltage???..this question must have been anticipated before....we go back to elementary circuits: series,parallel, and the series-parallel combined...as rule of thumb: "in a series circuit the current is equal that is It = I1=I2=..=In" in "in a parallel circuit voltages at nodes is equal"...furthermore, CT's and PT's are not Measuring devices..but sensing...so your main intention is to sense not to measure drop in the cicuit itself...