Question

Why do ybus used in load flow study instead of z bus?

Answer 1

Load flow studies are done to find mainly the current. We know that I= V/Z and Z= 1/Y. Hence I=Y*V.  All these are matrices and hence division by matrices is very hard we use multiplication using Y  instead of dividing with Z.
2. Y values are small compared to Z. Hence calculation becomes simpler.
3. Easy to incorporate effect of mutual coupling.
4. Y matrix is symmetrical and there are many zeros in it. Hence it requires less memory as load  flow analysis are done in computers.