It is required that power plant units can run in island operation in the contract with a foreign element, which is a strict requirement for the safety of units. During the commissioning process of India Sagardighi 300MW power plant unit, there were many problems due to the existing logic limitations, such as reheater overpressure in island operation, frequent action of spray water adjusting valve in low pressure (LP) bypass, poor regulating quality, spray water isolating valve in high pressure (HP) bypass opening without command, superheating in turbine with reheater overpressure, and turbine trip with high HP exhaust temperature. In order to solve these problems, a new logic for bypass system is put forward. The practice shows that the application of the bypass systems logic can significantly reduce the probability of problems in island operation.