Short Answer:

Under-Frequency Load Shedding (UFLS) is a protective control mechanism used in power systems to automatically disconnect certain loads when the system frequency falls below a safe limit. This helps prevent a complete system blackout caused by an imbalance between electricity generation and consumption.

When there is a sudden increase in demand or loss of generation, frequency drops rapidly. UFLS detects this drop and disconnects pre-selected loads in stages, allowing frequency to recover and protecting the grid from collapse.

Detailed Explanation:

Under-frequency load shedding (UFLS)

In an alternating current (AC) power system, frequency stability is crucial for safe and reliable operation. The nominal frequency (like 50 Hz or 60 Hz) is maintained by keeping power generation and demand in balance. However, sudden events like generator failures or line trips can cause a large mismatch between supply and load, leading to a drop in system frequency. If the frequency continues to fall, it may result in equipment damage, generator instability, or a complete blackout.

Under-Frequency Load Shedding (UFLS) is a method used to protect the power system by automatically reducing the load when the frequency drops below certain thresholds. This action helps restore the balance between generation and demand, allowing the frequency to return to its normal range.

How UFLS Works

1. Frequency Monitoring:
   Special relays continuously monitor system frequency at key points in the power grid. These relays are programmed with set frequency thresholds.
2. Detection of Frequency Drop:
   When frequency falls below a preset limit (e.g., 49.5 Hz), it indicates that demand is greater than generation.
3. Automatic Load Disconnection:
   The UFLS relays send signals to circuit breakers to disconnect specific non-critical loads. This reduces the total demand and allows the remaining generators to stabilize the frequency.
4. Staged Shedding:
   Load shedding is usually done in multiple stages:
   • Stage 1: Shed small percentage of load at slight frequency drop (e.g., 49.5 Hz)
   • Stage 2: Shed more load at a lower frequency (e.g., 49.2 Hz)
   • Stage 3: Further load is shed if frequency continues to fall (e.g., 48.8 Hz)
5. System Recovery:
   Once the frequency recovers to a safe value, loads may be gradually reconnected either manually or through an automatic load restoration scheme.

Importance of UFLS

• Prevents Blackouts:
  Stops cascading failures that could lead to widespread power outages.
• Maintains System Stability:
  Helps in quickly restoring balance between load and generation.
• Protects Generators and Equipment:
  Low frequency can damage generators, transformers, and motors. UFLS prevents operation under such dangerous conditions.
• Supports Renewable Integration:
  With fluctuating sources like wind or solar, UFLS helps in managing sudden generation losses.
• Complies with Grid Standards:
  Most regional and national grid codes mandate UFLS schemes for frequency control.

Design Considerations in UFLS

• Selection of Loads:
  Typically, non-critical or interruptible loads like industrial processes or street lighting are chosen for shedding.
• Relay Coordination:
  Proper timing and staging must be set so that UFLS operates only when necessary.
• System-Wide Communication:
  SCADA systems and frequency measuring devices are used for real-time coordination and monitoring.

Conclusion

Under-Frequency Load Shedding (UFLS) is a crucial defense mechanism in power systems that protects the grid from instability and blackouts. By disconnecting pre-determined loads during frequency drops, UFLS ensures that the remaining system stays operational and recovers quickly. It is a vital part of grid protection, especially in today’s power networks that include variable renewable energy sources and complex load patterns