Power system security is an important issue in power system operation and security assessments should be performed regularly both in online and off-line modes. Evaluation of protection coordination performance of an interconnected power system. When it transits from one operating state to another operating state is very crucial. However, such analyses are not being performed on Ethiopian existing power network. As a result, the power system operators do not have thoroughly analyzed remedial action plans for each credible contingency and this will impact the day to day power system operation. The objective of a protection schemes is mainly to keep the power system stable by isolating only the affected components or the section of the electricity network in which the fault has developed while allowing the rest of the network to continue operating. It is important to note that the protection equipment does not prevent faults from occurring, but it limits the damaging effect of the fault and protects other healthy equipment. This is only achieved if the protection system of the electrical network involved complies with the requirements and purpose of the electrical protection standards. These requirements include the Operational speed, Reliability, Security and Sensitivity. This thesis will assess the protection co-ordination security of the Ethiopian existing power network in off-line and online mode considering the occurrence of credible contingencies on critical 132kv Substation in Addis Ababa network and outage of large demand. The performance of transmission line protection relays when a fault occurs in the system is Important for improvements in the stability of the system and reduction of their effect on sensitive loads. Reducing the fault clearing time for more possible fault conditions is one of the main goals in the development application and time dial setting of such relays. using that the over current relay have been investigated using dig silent, Inverse Definite Minimum Time (IDMT) type of IEC61850 characteristic have been used to determine the proper settings of the over current relays as a backup local and remote protection down to full protection coordination for the thesis. In the case of 132kV Addis Ababa Substation investigated in details only over current relay protection system focus. Assessment on protection coordination/2018 Page xx The line protection schemes are composed of directional over current relays where the setting of the relays must be computed considering relay. Separate relay computation would lead to loss of selectivity. The standard IEC 60255 combined with the considerations of tripping time have been used in order to overcome the coordination problems with the Backup protection (directional over current ) as a local protection and to provide remote backup protection to the other over current relays in the grid. This approach may be used to improve the reliability of a transmission grid and reduce its vulnerability to cascading outages. It may be added to the existing contingency analysis tools to assess the impact of cascading events in both on-line and off-line environments. Moreover, the project poses a challenge in that reliable protection of power system is highly required as the electrical networks expand on daily basis. This is necessary for the safe and economical operation of power systems. (i.e. Investigation into alternative protection solutions for a Substation network using the IEC61850 standard) the performance requirement for setting protection application with time setting coordination.