State Grid 18 Anti-Accident Measures: Full Guide to Transformer Neutral Point Overvoltage Prevention

1. Overview of Anti-Accident Management Specifications

   To eliminate frequent neutral point overvoltage faults of high-voltage transformers and ensure the safe and stable operation of 110kV and 220kV power transmission systems, State Grid Corporation has formulated standardized control requirements in the 18 Key Anti-Accident Measures for Power Grid Operation. All power grid operation, maintenance and engineering management departments must strictly implement national and industry standards including Grounding for AC Electrical Installations (DL/T621-1997), Measurement Guidelines for Power Frequency Characteristic Parameters of Grounding Devices (DL/T475-1992), and Overvoltage Protection and Insulation Coordination of AC Electrical Installations (DL/T620-1997) to standardize the whole-process management of transformer neutral point overvoltage prevention.

 

    Transformer neutral point overvoltage accidents are one of the common hidden dangers in high-voltage power grids. Abnormal overvoltage caused by switching operations, isolated ungrounded system faults and power frequency voltage surges will damage transformer neutral point insulation, lead to equipment breakdown, and even trigger large-scale power grid tripping accidents. Standardized implementation of State Grid anti-accident measures is the core means to block such operational risks.

 

2. Key Technical Requirements for Preventing Transformer Neutral Point Overvoltage Accidents

  The 12th item of State Grid 18 anti-accident measures focuses on grounding grid and overvoltage accident prevention, and puts forward mandatory operational and configuration specifications for neutral point protection of 110kV~220kV high-voltage transformers, covering switching operation standards and protection device configuration requirements.

 

2.1 Temporary Grounding Specification for No-Load Transformer Switching

  In the 110kV and above effective grounding power system, when operators switch no-load transformers with ungrounded neutral points, temporary neutral point grounding operation is mandatory before switching. This key operation specification is designed to restrain instantaneous power frequency overvoltage and switching overvoltage generated during circuit breaker closing and opening.

   Without temporary grounding measures, the ungrounded neutral point of the no-load transformer will form a floating potential during switching, which is extremely vulnerable to induced overvoltage from line capacitance and system electromagnetic transients, resulting in insulation aging or flashover breakdown of the transformer neutral point. This standardized operation is the primary preventive measure for neutral point overvoltage faults in daily substation operation.

 

2.2 Protection Configuration Standard for 110kV~220kV Ungrounded Transformer Neutral Point

  To avoid abnormal operating conditions of isolated ungrounded systems and excessive power frequency overvoltage in effective grounding systems, all 110kV to 220kV transformers adopting ungrounded neutral point operation must be equipped with stick gap protection as the basic overvoltage protection scheme. The stick gap can effectively limit persistent power frequency overvoltage and low-amplitude operating overvoltage, and isolate the transformer neutral point from excessive potential surge.

 

2.3 Matching Configuration of Stick Gap and Metal Oxide Arrester (MOA)

    For conventional 110kV transformers with neutral point insulation impulse withstand voltage ≤185kV, single stick gap protection cannot fully resist lightning impulse overvoltage and severe operating overvoltage. Therefore, it is required to connect a metal oxide arrester (MOA) in parallel with the stick gap to form a combined protection system.

   The combined protection mode realizes complementary advantages: the stick gap undertakes long-term power frequency overvoltage protection, while the MOA suppresses instantaneous high-amplitude lightning and switching overvoltage. Power grid operation units must complete parameter matching calibration of gap distance and arrester performance before equipment commissioning to ensure coordinated action of the two protection devices and avoid protection failure or misoperation.

 

2.4 Post-Fault Inspection and Parameter Recalibration Requirements

     Once the neutral point stick gap acts to discharge overvoltage, operation and maintenance personnel must conduct an on-site inspection immediately. The key inspection items include checking the burning and ablation degree of the gap electrode, measuring the actual gap distance, and recalibrating the protection threshold parameters. Severe gap ablation will change the discharge characteristics, and uncalibrated gap distance will lead to insufficient overvoltage suppression ability, which needs timely adjustment and maintenance to ensure long-term reliable operation of the protection device.

 

3. Core Value of Standardized Implementation

   Strictly implementing the transformer neutral point overvoltage prevention requirements of State Grid 18 anti-accident measures can effectively solve the pain points of high-voltage grid operation such as floating neutral potential, isolated system overvoltage and insulation damage. Standardized operation specifications and protection configuration schemes form a full-chain risk prevention and control mechanism from switching operation to equipment protection and post-fault maintenance, which greatly improves the operational safety and reliability of 110kV and 220kV main transformers, and provides solid technical support for stable power grid operation.

 

4. Summary

  The prevention of transformer neutral point overvoltage accidents relies on the dual guarantee of standardized operation and professional protection configuration. The core requirements of State Grid anti-accident measures are summarized as follows: mandatory temporary grounding for no-load transformer switching, unified stick gap protection for 110kV~220kV ungrounded transformers, matched MOA configuration for low-insulation 110kV transformers, and standardized post-action inspection and parameter calibration. All specifications are formulated based on national industry standards, which are the essential guidelines for power grid operation, maintenance and safety management.