Regarding the parallel and series connection of CT's:
Secondary of the CT can be considered as CURRENT SOURCE!
Connecting two CT'S in parallel means two current sources outputs connected to the load, which means the result is the algebraic summation of the two secondaries of the CT's i.e:
If CT1 = 300/5 A
CT2 = 300/5 A
Then output result of parallel connection of the two secondaries of the CT's:
CT3 = 300/10 A, which is equivalent to 150/5 A.
NOTE: any two CT's can be connected in parallel even when they are not similar (they have different ratio)
Connecting two CT's in series is possible just in case if the two CT's are similar to avoid any current circulation between the two CT's!
This connection has an advantage to increase the burden of the output of the CT.
If CT1= 300/5, burden 15 VA
CT2= 300/5, burden 15 VA
Then:
CT3= 300/5, burden 30 VA
I hope this can simplified the idea of parallel and series connection of the CT's