Enhancing the security and reliability of power systems is of great significance to maintaining social stability. The power electronic devices of power systems can be utilized by attackers to launch indirect attacks, which could cause devastating impacts on the power grids. In this paper, load side attacks (LSAs) on relay protection devices is proposed as the first step to analyze the features of the attack. To achieve detection, a relation-based detection network is introduced to distinguish the LSAs from the normal faults.