In most provincial power systems in China， the generation company signs a long-term generation contract， and the dispatching objective of a dispatching department is to keep generation scheduling synchronous， which brings a new challenge to the security constrained economical dispatch（SCED）. A SCED model is developed to achieve the minimum deviation objective under the equilibrium power generation dispatching mode. Based on the unit power deviation measuring function and optimized objective evaluating function， three typical optimized objective models are given， namely， the linear sum of bias model， linear sum of bias penalty function model and minimax bias model. Results show that the linear sum of bias penalty function model could lead to minimal gross deviation and avoid the randomness of deviation distribution. Actual provincial system test results show the validity and practicability of the SCED model proposed.