Abstract : To reduce the dead zone of directional relays, the phase voltage of the non-faulty phase is introduced to determine the phase of the fault current (i.e., 90° connection). However, when a two-phase short circuit occurs on the delta side of a Y/Δ connected transformer, the directional relay of this connection method is very likely to operate incorrectly on the star side of the transformer. This paper analyzes the malfunction mechanism and proposes countermeasures to prevent directional relay maloperation, thus improving the reliability of directional relays. Keywords: Directional relay; 90° connection; Maloperation; Reliability 1 Introduction Power directional relays can clearly determine the direction of the fault and have always been favored by relay protection R&D personnel. In dual-power supply networks, in order to clearly distinguish which side the fault occurs on at the protection installation point, directional elements are generally required to prevent the protection from operating indiscriminately. For directional elements used in line protection, it is usually specified that they operate when the short-circuit power direction is from the bus to the line, and do not operate when the short-circuit power direction is from the line to the bus. The directional element is connected at 90°, which has no dead zone in the event of an asymmetrical short circuit and can correctly respond to phase-to-phase faults occurring on the line. When a two-phase short circuit occurs on the delta side of the YO/A-11 connected transformer, the directional element is highly likely to malfunction. Many documents only provide conclusive descriptions without specific demonstrations. This paper analyzes the causes of directional element malfunction and proposes a power directional element based on the positive sequence component principle, which can effectively improve the reliability of the directional element. To reduce the dead zone of the directional element, a 90° connection is used in practical applications to introduce the voltage of the non-faulty phase to determine the phase of the current, as shown in Figure 1. θ represents the internal angle of the relay. The paper describes the operation of the directional element when various phase-to-phase faults occur on the line. [b][align=center]For more details, please click: Analysis and Improvement of 90° Connected Power Directional Relay[/align][/b]