Power systems in China are being interconnected into a unified one which is running according to its physical rules (KCL, KVL and Ohm’s Law) but being managed and controlled by own regional control centres separately and independently. Each sub-system in the unified power system gathers SCADA data real time from its own regional network and makes dispatching decision itself independently. How the Energy Management System (EMS) in each regional control centre follows the variation of whole power system correctly and timely is a very concerned and real problem especially when some transmission line outage occurs in its external network. To solve this problem, some interaction and coordination between control centres in different hierarchies are needed to ensure that a consistent result is achieved by each of the separate control centres. Based on the principle of network piecewise, decomposition and coordination calculation modes with three different interaction levels for real time network analysis applications are proposed in this paper. Through interaction and coordination between master and slaver control centres, a consistent network analysis result with whole network can be achieved at the sub-system level. A practical and efficient real time tracking equivalence system (TES) for EMS applications is developed, implemented and put into on-line operation in this paper. Correctness and exactness of the EMS simulation in sub-systems by using TES have been greatly improved.