Power system security and reliability are more complex issues in a deregulated environment. Various criteria have been considered for power system reliability. In the day-ahead market, a successful trade schedule should be able to accept various disturbances with sufficient flexibility to be adjusted during the re-dispatch process. This paper describes the dispatch liquidity theory and some liquidity indices. The liquidity indices evaluate the effective operating reserves with the network constraints taken into consideration. A model is presented to calculate the liquidity index. An extended trade scheduling model with minimum liquidity index constraints is presented that considers the distribution requirements of the operating reserves. The liquidity indices could also be used to coordinate the security and reliability between multistage markets and for contingency selection. The algorithms were tested with real power system data. The results show that the dispatch liquidity theory is reasonable and the algorithms are effective.