Security constrained optimal power flow(SCOPF)for large-scale power systems is a complex and difficult problem with its highly demanding computational requirements. A practical SCOPF model is presented based on power flow transfer relations and related constraint reduction methods are proposed. According to the power flow transfer relations established by contingency analysis

the active power flows on branches in post-contingency state can be described as linear functions of those in pre-contingency state. Therefore

the active power flow constraints of the branches in post-contingency state are described by the reformed linear inequality constraints of the active power flows of the branches in pre-contingency state. Furthermore

by grouping the parallel lines and parallel transformers

the number of monitored branches is reduced by using the distribution relations of active power flows in grouped branches. Finally

since the short-term capacity of power equipment is usually much greater than the long-term capacity

the power flow constraints of the branches in post-contingency state are accordingly screened to reduce the scale of SCOPF problem as much as possible. Simulations on the IEEE 14-bus test system and the East China power grid verify the correctness and effectiveness of the proposed SCOPF model and the constraint reduction methods.