International Journal of Integrated Engineering – Special Issue on Electrical Electronic Engineering Vol. 9 No. 4 (2017) p. 13-20 *Corresponding author:hhgoh@uthm.edu.my 2017 UTHM Publisher. All right reserved. penerbit.uthm.edu.my/ojs/index.php/ijie 13 An Overview of Out-Of-Step Protection in Power Systems Hui Hwang Goh 1,* , Mohamed Dahir Osman 1 , Omer Mohamed Basaleem 1 , Qing Shi Chua 1 , Chin Wan Ling 1 , Goh Kai Chen 2 1 Department of Electrical Engineering, Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia. 2 Department of Construction Management, Faculty of Technology Management and Business, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia. 1. Introduction Power systems are persistently vulnerable to numerous disturbances either in small or large type during steady state operation [1]. Therefore, design engineers and planners are working firmly to initiate most reliable power system structure which is capable to deal with most of the contingencies. However, one of the most unpredictable power system instability situation is the angular instability or known as out-of-step (OOS). Whenever insufficient transmission line capacities or any faults occurred in the systems will originate the generation and load demands is not equilibrium. Hence, the generation side of the power system will begin to work asynchronously with the other parts of the system. The power system generation is unable to tolerate the prolonged time of OOS due to a negative effect of the system materials. Consecration of OOS protection must respond to asynchronous operation acting on power network separation to determine the place with the objective to preserve the generation load equilibrium with each of the system. All sorts of OOS power protection can be categorized as follows [11]:  Protection of local type, that usually operated locally and gives obtainable measurement of OOS detection.  The measurement of the whole system based on OOS protection which applies to measurements of numerous places in the power system. 2. Background study 2.1 Power Swing Power swing refers to the oscillation in active and reactive power flows on a transmission line following by large disturbances to power system transmission lines which consequently cause wrong process to the distance relay if not blocked, Moreover, it may generate undesired tripping on the power system transmission line circuit breaker. Therefore, if that undesired tripping did not isolate with the generator, then it may cause some damages to the machine [2]. The authors in [3] defined the power swing as the variation in three phase power flow in the power system which occurs when the rotor angels of the generator advancing each other because it response to the changing of the load factors such as the change in direction and the magnitude of the load, line switching, faults and system disturbances that happened to the power system and the loss in generation. The power swing has a negative impact on the power system such as protective relays malfunction which cause an electrical equipment damage. Power system operates on a frequency of 50Hz or 60Hz during steady state conditions [4]. The entire synchronism of nominal frequency and voltage at the sending and receiving ends causing entirely equilibrium of active and reactive power between generated and consumed active and reactive powers [2]. The sudden adjustments of electrical power are resulted by the power system faults, line switching, generator interruption, and the loss or implementation of large groups of load [5]. These losing will appear in unpredicted results alteration towards the electrical power Abstract: Power system is subjected to an extensive variety of little or bigger disturbance to the system during the operation. The power system that designed as one of the main requirement is to survive from the larger type of disturbances like faults. The power swing in certain system is the variation in three phase power flow in the power system. This paper mainly discussed the power swing and distance relay and the effect of the power swing on the distance relay and demonstrate about the basic power system stability and power swing phenomena. Moreover, out of step protection and detection applications are revised as well. At the end, the paper also demonstrated the past study of out of step application of TNB 275 KV network. Keywords: Out-of-Step Blocking (OSB), Out-of-Step Trip (OST), Out-of-Step (OOS), Power swing