Load growth, high penetration of distributed generators (DG) and energy storage systems (ESSs) have caused different challenges for the distribution networks (DNs) such as congestion in their lines. This paper assesses the impact of congestion on the operational cost of the multi microgrid (MMG) based DN firstly. Then, a security constrained energy management system (SC-EMS) for the MMG system is proposed by considering the effect of the network's load flow constraints on the scheduling of that. Obviously, adding extra constraints to the optimization problem of the MMG system leads to increment in the operational cost. In order to resolve this problem, tie lines between MGs are modelled. In fact, in the proposed model, MGs can trade energy not only with the DN, but also with other MGs. Moreover, it is evaluated that considering the MGs as an integrated unit decreases the overall operational costs. Simulations on a modified IEEE 33-bus test system with multiple MGs are performed in GAMS environment. As expected, the gained results demonstrate the increment in the operational costs of the DN and the MGs when the contingencies occur in the network's lines. On the other hand, taking the tie lines into account as a recommended solution leads to a decrement in the operational cost of the MMG system.