Southern Grid Records Multiple 400 kV Forced Transmission Outages: Technical Summary

Incident Overview

On April 21–22, the Southern Regional Grid experienced a significant disturbance characterized by severe frequency instability and forced outages. Data from GRID-India confirms the all-India grid frequency dropped to a critical low of 49.42 Hz on April 21. The frequency remained below the Indian Electricity Grid Code (IEGC) safe operating band of 49.90–50.05 Hz for nearly six hours. Specific 400 kV elements under operational stress during this disturbance included the Gadag PSS–Kudgi PG-1 line and the Doni–Guttur corridor, coinciding with power overdraws that approached the threshold for Automatic Under Frequency Load Shedding (AUFLS) activation.

Technical Breakdown: 400 kV Transmission Outages

Technical data from the Southern Regional Power Committee (SRPC) 128th Meeting of the Protection Coordination Sub-Committee (PCSC) and the 223rd Meeting of the Operation Coordination Sub-Committee (OCC) identify multiple forced outages involving the Kudgi complex.

• Gadag PSS–Kudgi PG-1 Incident: This 400 kV line experienced forced trippings that required a review of the protection scheme by the Protection Setting Sub-Group (PSSG) and consultation with the Original Equipment Manufacturer (OEM).
• Relay Mal-operation: Analysis recorded in PCSC-128 (Item 127/IV/1) revealed that the D60 relay (Main-2) consistently initiated a lock-out function for near-end faults during high fault current events.
• Categorization: These events are officially recorded as “tripping incidents” or “forced outages” within the PCSC and OCC grid occurrence logs, occurring during periods of high regional demand and multiple network overload alerts.

Renewable Energy Infrastructure Impact

Recent grid occurrences have significantly impacted high-capacity renewable energy (RE) evacuation corridors in Karnataka and Andhra Pradesh, leading to isolated island formations and technical failures.

• Island Formation and Frequency Excursion: Forced outages at the 400 kV Greenko Central Pooling Substation (CPSS) and the 400 kV AMGEPL Wind/Solar plants resulted in the formation of an electrical island. During this incident, the frequency within the isolated subsystem dropped to approximately 42 Hz.
• AMGEPL Solar ICT-2 Tripping: A differential relay operation caused an unwarranted trip at the AMGEPL Solar ICT-2. Technical analysis (Item 127/III/4) determined the root cause was unbalanced currents during a through-fault, specifically a dead short circuit between phases in a 33 kV feeder.
• Doni–Guttur Line: The 400 kV Doni–Guttur line 1, managed by KPTCL, reported trippings that led to recommendations for deactivating Current Transformer Detection (CTD) for line breakers and stricter adherence to Standard Operating Procedures (SOP) during maintenance.

Technical Failure Summary:

• Island Formation: Documented at Greenko CPSS, AMGEPL Wind, and Pinnapuram PSP.
• Differential Relay Operation: Triggered by 33 kV feeder short circuits causing unbalanced currents.
• Protection Logic Errors: Lock-out relay activations for near-end faults observed in D60 relay units.

Regional Grid Context and Load Imbalance

The frequency dip on April 21 was exacerbated by substantial power overdraws by Southern Regional constituents during non-solar hours. These overdraws created a stark imbalance between regional demand and available generation.

Peak Overdraw by State (April 21)

Substation Maintenance and Regional Stability:

• Guttur Substation: The 400/220 kV Guttur substation remains a critical node with frequent trippings due to persistent DC earth faults and control cable failures.
• Regulatory Compliance: Maintenance at Guttur has shown violations of CEA (Technical Standards for Construction of Electrical Plants and Electric Lines) Regulations, 2022, specifically Regulation 48(2b) regarding the grouping of DC supply.
• Cascading Impacts: Regional stability is further challenged by the frequent tripping of the 110 kV Konaje–Manjeshwaram line, which has caused repeated supply failures to critical loads in Kerala, including the Uppala traction loads.

System Restoration and Current Status

The SRPC has provided updated restoration timelines for critical transmission elements following diversion works and maintenance:

• 400 kV Puducherry-NNTPS Line: Following diversion works for the NHAI, this line revived on February 5, 2025.
• 400 kV Somanahalli-Mylasandra Line: This line has been under shutdown since December 2024 for Dommasandra substation works. KPTCL has been requested to provide an updated revival schedule due to ongoing delays.
• 230 kV Puducherry-Villianur Line: Approved for NHAI works in early February 2025, the expected completion and revival date is set for mid-February 2025.
• Operational Directives: To mitigate disturbances during the high-demand season, the SRPC recommends meticulous demand forecasting, proactive scheduling by Variable Renewable Energy (VRE) generators, and the rigorous monitoring of battery bank health and communication systems. The committee emphasizes that immediate decisive action is required to address the rise in grid disturbances.