Abstract : Relay protection auxiliary devices play a crucial role in power system relay protection, and therefore their operational reliability is of great concern. The main reasons affecting the operational reliability of relay protection auxiliary devices include open circuits in intermediate relay coils, poor contact, or burnt-out contacts. Based on the design requirements of the relevant circuits of the auxiliary devices, some measures to improve their reliability are proposed. Keywords: Auxiliary device; Reliability; Measures 1. Problem Statement Relay protection auxiliary devices include AC voltage switching boxes, three-phase operating relay boxes, and phase-by-phase operating relay boxes, etc. They are mainly used in secondary relay protection and automatic control circuits of power systems for switching secondary circuits and as auxiliary control for circuit breaker operation, to meet the control operation requirements of circuit breakers. These auxiliary devices play a crucial role in power system relay protection, and therefore their operational reliability has always been a concern. The main reasons affecting their operational reliability include open circuits in intermediate relay coils, poor contact, or burnt-out contacts. Over the years, research, manufacturing, design, and operation departments have done a great deal of effective work to improve their operational reliability. This article, in conjunction with the situation of new generation auxiliary devices (such as ZSZ-11S three-phase operating box, ZFZ-11S, ZFZ-12S, ZFZ-22S series phase-separated operating boxes and ZYQ-11S, ZYQ-12S voltage switching boxes, etc.), explains the design requirements of the relevant circuits of the auxiliary devices and proposes some effective measures to improve the reliability of such products. 2 Effective measures to improve the reliability of auxiliary devices (1) Adopt a new chassis structure to completely change the traditional structure of relay protection auxiliary devices, which is a combination of electromagnetic relays with protruding or embedded structures, or the structure of the entire screen consisting of relays installed in a decentralized manner. For example, adopt a 4u or 6u structure, a standard 19-inch chassis, and adopt a plug-in structure with a split plug-in printed circuit board, modular plug-in functions, and wiring with a winding method, etc. (2) Pay attention to the use of new devices and new processes, develop the use of new devices, keep up with the trend of world technology development, continuously improve product processes, and improve assembly quality. (3) Since the main key component in the relay protection auxiliary device is the relay, a high-reliability intermediate relay is selected to improve the technical parameters of the relay, such as the power consumption and thermal stability of the relay. Moreover, the selection of relay model, manufacturer and parameters are optimized in the design so that the relay and related circuit components work under the best parameters. For example, the intermediate relays in the new generation of auxiliary devices of XJ Electric Co., Ltd. are all imported high-power intermediate relays. The main technical indicators of the relay are shown in Table 1. (4) When using a low rated voltage specification relay (such as 110V or 48V or 24V relay for 220V power supply) in series with a resistor, one end of the series resistor should be connected to the negative power supply, and the model and parameters of the series resistor should be optimally selected. Special design is made for the heating resistor that is constantly energized in the device (such as placing the heating resistor that is constantly energized in the device on the back panel of the device chassis) to further reduce the working temperature inside the device chassis. For example, the resistor element in the new generation of auxiliary devices produced by XJ Electric Co., Ltd. is a military-grade power resistor with good temperature characteristics and high working reliability. (5) Select a high-voltage diode, connect it in series with a resistor of appropriate value and connect it to the voltage coil of the intermediate relay to form an arc suppression circuit. Note that the resistance value of the series resistor should not be too large so that the "diode series resistor" circuit has a good arc suppression effect; but it should not be too small either, so as to prevent the fuse from blowing when the intermediate relay operates due to a short circuit of the diode, causing the protection to fail to operate. After calculation, it is generally more appropriate to select a resistance value of 250 to 300Ω when the DC power supply is 110 to 220V. (6) Ensure the technical data of the relay operation in each circuit, such as the intermediate relay operating voltage of 50% to 70% of the rated voltage, the holding current of no more than 80% of the rated current, and the holding voltage of no more than 70% of the rated voltage; it is necessary to ensure that the starting voltage of the trip output relay is not lower than 50% of the rated voltage to prevent false operation when the positive power supply of the relay coil is grounded; but it should not be too high either, so as to prevent the reliable operation of the relay and the fast operation under normal conditions when the DC power supply decreases. (7) Fully consider and ensure the insulation resistance and withstand voltage level of each circuit of the auxiliary device. (8) For some important circuits, a parallel operation mode with two sets of contacts is adopted, and operation monitoring measures are added, such as voltage switching and pressure interlocking circuits. (9) For the output contact control circuit of the circuit breaker tripping and closing coil, in order to ensure reliable tripping and closing of the circuit breaker, a series self-holding relay circuit must be provided, and the tripping and closing output relay contacts must be kept free of arcing and reliably tripped and closed. For circuits with only a single output relay, a current self-holding coil can be connected in series in the tripping and closing contact circuit of the output relay. The current self-holding coil should be connected between the output contact and the circuit breaker control circuit, and the relay self-holding current should not be greater than about half of the rated tripping and closing current, and the coil voltage drop should be less than 5% of the rated value. When multiple output relays may trip at the same time, it is advisable to use an anti-tripping relay to achieve the above tasks. The anti-tripping relay should be a fast-acting relay, with an operating current less than half of the tripping current and a coil voltage drop less than 10% of the rated value. For example, the new generation of auxiliary devices produced by Xuji Electric Co., Ltd. uses specially designed dedicated multi-coil intermediate relays to achieve the above tasks, such as the current-starting voltage holding anti-trip relay TBJ in the three-phase operation box and the phase-separated operation box, and the voltage-starting current holding reclosing and manual closing relays ZHJ, SHJ, and the trip pressure lockout relay 1YJJ, etc. (10) The position relays in the circuit breaker trip and close circuits should ensure that the relay coil can operate reliably after the additional resistor is connected in series, and when the relay coil or the additional resistor is short-circuited, the voltage on the circuit breaker trip and close coils should not be insufficient to make the circuit breaker operate, and the margin should not be less than 1.3; the current Icq flowing through the trip coil and close coil in the control circuit for a long time should not cause the coil to overheat, that is, Icq should not be greater than 0.15 of the rated current. Therefore, the rated voltage of the position relay can be taken to be equal to half of the rated voltage of the operating power supply, and the parameters of the additional resistor should be selected according to this condition. The resistance value of the additional resistor is determined according to the condition that the voltage on the relay is equal to its rated voltage. (11) Dual measures are adopted for certain tripping circuits, such as the three-phase tripping circuit in the phase-separated operation relay box, as shown in Figure 1. 3 Other auxiliary measures (1) Set up monitoring circuits for intermediate relay coils, such as the tripping position and closing position relay coil monitoring circuits in the three-phase operation box and the phase-separated operation box, as shown in Figure 2. (2) Set up relay contact monitoring circuits, such as setting up monitoring circuits for relay output contacts, disconnecting the output circuit, allowing it to conduct instantaneously, and monitoring the integrity of the contacts. (3) Strengthen the inspection and operation and maintenance of auxiliary devices. (4) Manufacturers should test and demonstrate the quality of products, and pay special attention to the research on component and product testing methods to improve the quality of the components used and improve the quality of the products leaving the factory. 4 Conclusion As a necessary and important auxiliary supporting product for relay protection devices, the reliability of auxiliary devices plays an important role in ensuring the correct operation of relay protection and the safe and stable operation of the power system. Therefore, taking some effective measures to improve the reliability of auxiliary devices is of great significance. This article, in conjunction with the design process of auxiliary devices, explains the circuits and requirements of the relevant auxiliary devices, providing a useful reference for the design, operation, and maintenance of such products.