Typically, an efficient design of transformer would correspond to choosing the core cross section such that  should be near the knee point of B - H curve. One obvious way of avoiding CT saturation on dc flux is to oversize the core so that for flux  , corresponding B is below the knee-point. Hence, the factor  is called core-oversizing factor.
Core-oversizing factor =
Note that X/R in above equation is the transmission line X/R ratio. For a 220KV line with X/R  10, this would imply that transformer core should be oversized by a factor of 11. For a EHV line, with X/R  20, this would imply an oversizing requirement of about 21 times the usual design. Clearly, this high amount of oversizing is not practical. Thus, an important conclusion is that, protection engineers have to live with the saturation problem. Under the situation one should try to quickly reach the decision, before CT saturates. However, this brings in the picture, the well discussed 'speed vs accuracy conflict'. We will have more to say on the accuracy aspect of relaying in later lectures. |
