With the improvement of power plant safety management, people are paying more and more attention to single-phase grounding protection of low-voltage plant power systems. Most low-voltage plant power systems in China are directly grounded neutral systems. When a single-phase grounding occurs, the symmetry of the line voltage is disrupted, and continued operation is not permitted. However, for systems with non-directly grounded neutral systems, when a single-phase grounding occurs, the symmetry of the line voltage is not disrupted, allowing short-term operation for 1-2 hours. In this case, the cause should be identified and the fault eliminated. How to correctly select and configure single-phase grounding protection devices to improve the operational reliability of plant power systems is discussed in the following three aspects: 1. Single-phase grounding protection of low-voltage plant transformers a. For transformers with directly grounded neutral points on the low-voltage side, when a single-phase grounding short-circuit fault occurs on the low-voltage side, zero-sequence overcurrent protection on the neutral line can be formed by an inverse-time current relay. Alternatively, a two-phase three-relay connection can be adopted, allowing the overcurrent protection on the high-voltage side to also serve as the single-phase short-circuit protection on the low-voltage side; b. For low-voltage plant auxiliary power systems grounded through a large resistance, single-phase grounding protection should utilize the zero-sequence voltage generated on the neutral point grounding device. After the protection operates, a grounding signal should be sent to the duty station. The feeder circuit on the central busbar of the low-voltage plant auxiliary power system should be equipped with a grounding fault detection device. The detection device should preferably consist of elements that reflect the zero-sequence current and operate on a local signal. Single-phase grounding protection for low-voltage plant auxiliary transformers has mature configurations and many years of operating experience, so it will not be elaborated here. 2 Single-phase grounding protection for low-voltage plant auxiliary motors a. When the neutral point of the low-voltage plant auxiliary power system is directly grounded, single-phase grounding short-circuit protection should be installed for motors with a capacity of 100kW or more; while for motors of 55kW and above, if the phase-to-phase short-circuit protection can meet the sensitivity of single-phase grounding short circuit, the phase-to-phase short-circuit protection can also serve as grounding short-circuit protection. If it cannot meet the sensitivity, a separate grounding short-circuit protection should be installed. The protection device consists of a current relay connected to a zero-sequence current transformer. This relay trips the circuit breaker instantaneously. b. When the neutral point of a low-voltage plant auxiliary power system is grounded through a large resistor, a ground fault detector should be installed on the low-voltage motor circuit to facilitate locating the ground fault. To meet the sensitivity requirements of the ground fault detector, the grounding current should ideally be around 3A when selecting the large resistor value. There have been disagreements among professionals regarding single-phase grounding protection for low-voltage plant auxiliary motors, primarily concerning the setting values. Since the "Technical Regulations for the Design of Auxiliary Power Systems in Thermal Power Plants" does not address setting values, the following discussion seeks to reach a consensus on this issue. The 1981 edition of the "Electric Power Engineering Design Manual," Volume 2, states that "the protection device consists of a DL-11/0.2 type relay connected to an LJ or LJF type zero-sequence current transformer." The setting value of the DL-11/0.2 type relay is 0.05~0.2A. Since the manual does not specify whether this protection is applicable to low-voltage plant power systems with a directly grounded or non-directly grounded neutral point, the design, installation, commissioning, and operation at that time were all based on the maximum setting value of the DL-11/0.2 type relay. Due to the high sensitivity of the operating value, frequent tripping occurred. The book "Protection of Plant Auxiliary Power Systems" published by the Ministry of Water Resources and Electric Power in 1986 states that: "For low-voltage plant auxiliary power systems with a directly grounded neutral point, the setting value of the zero-sequence operating current of low-voltage motors should meet the following two requirements: 1) It should be able to operate correctly in the event of a single-phase ground fault, and have sufficient sensitivity. The sensitivity coefficient should not be less than 1.5; 2) It should avoid the unbalanced current during motor starting and self-starting. Based on operating experience, the value of the primary operating current is 10% to 20% of the rated current of the motor (generally less than 2A on the secondary side). In cases where the unbalanced current is large, the secondary operating current can be set to 2A." The second volume of the 1991 edition of "Power Engineering Design Manual" states: "In 380V plant auxiliary power systems with a directly grounded neutral point, the DL-11/2 type relay should be selected. In 380V and 3-10kV plant auxiliary power systems with an ungrounded neutral point, and where the single-phase ground fault current is not greater than 10A, the DL-11/0.2 type relay should be selected." In 380V plant auxiliary neutral point ungrounded or high-resistance grounded systems, the GRD high-resistance grounding indicator is selected. The setting value of the DL-11/2 type relay is 0.5～2A, while the LJ type zero-sequence current transformer is used in conjunction with the DL-11/0.2 type relay. When a 20A current is applied to the primary side of the LJ type zero-sequence current transformer, a current of no more than 0.2A can be measured on the secondary side, ensuring reliable operation of the DL-11/0.2 type relay. When a 50A current is applied to the primary side of the LJ type zero-sequence current transformer, the current on the secondary side does not reach the minimum setting value of 0.5A for the DL-11/2 type relay. Therefore, the LJ type zero-sequence current transformer used with the DL-11/0.2 type is only suitable for 380V and 3～10kV plant auxiliary neutral point ungrounded systems. The LJZ type zero-sequence current transformer manufactured by Yangzhou Switchgear Factory, when used with the DL-11/2 type relay, is suitable for 380V plant power systems with directly grounded neutral points. When the relay coils are connected in parallel and the setting value is 1A, the primary operating current does not exceed 60A. This satisfies the compatibility issue between the zero-sequence current transformer and the DL-11/2 type relay. In summary, the above analysis can be summarized as follows: In a 380V plant auxiliary neutral point directly grounded system, an LJZ type zero-sequence current transformer and a DL-11/2 type relay are selected. The primary setting value is 10% to 20% of the motor's rated current, and the secondary setting is 1 to 2A. In cases of large unbalanced current, a setting of 2A is used; in cases of small unbalanced current, a setting of 1A is used, triggering the circuit breaker tripping. In 380V and 3-10kV plant auxiliary neutral point ungrounded systems, assuming a single-phase grounding current (i.e., single-phase ground capacitance current) of no more than 10A, a DL-11/0.2 type relay is selected, with a secondary setting value of 0.15A. The current relay operates, and the signal device is activated. In a 380V plant auxiliary neutral point ungrounded or high-resistance grounded system, a GRD high-resistance grounding indicator is selected. Single-phase grounding protection for low-voltage plant auxiliary feeders If a single-phase grounding occurs in the feeder circuit of the 380VMCC workshop panel, and the upstream protection can quickly eliminate the fault without affecting production, it is not necessary to install protection. Otherwise, single-phase grounding protection for the feeder circuit should be installed. In a 380V plant auxiliary neutral point directly grounded system, if the 380VMCC workshop panel does not have a motor with separate single-phase grounding protection, the protection device should be coordinated with the protection device of the largest motor that also serves as phase-to-phase protection and grounding protection. The protection device can consist of an LJZ type zero-sequence current transformer and a GL-21/5 type inverse time relay. If the 380VMCC workshop panel has a motor with separate single-phase grounding protection, the protection device consists of a LJZ type zero-sequence current transformer and a GL-21/5 type time-limit relay. The protection device can be configured to work in conjunction with the largest motor for phase-to-phase protection and grounding protection. The setting of the device determines which one the feeder's single-phase grounding protection should be paired with. The protection setting must be greater than the single-phase grounding protection setting of the next-level low-voltage motor and less than the single-phase grounding protection setting of the previous-level low-voltage transformer. The operating time of the protection device should be coordinated with the next-level protection; the melting time of the next-level fuse is generally 0.2–0.3 seconds, and the operating time of the protection device should be 0.7 seconds. The operating time of the protection device should also be coordinated with the previous-level protection; the operating time of the previous-level protection is generally 1.2 seconds. The above discusses three aspects. Because the "Technical Regulations for the Design of Auxiliary Power Systems in Thermal Power Plants" does not provide sufficient detail for the second and third aspects, further practical experience from professionals is needed to make the single-phase grounding protection of the low-voltage auxiliary power system in power plants safer and more reliable.