How do you design and coordinate under-frequency load shedding (UFLS) schemes to prevent frequency collapse?

1 UFLS objectives and criteria

When designing and coordinating UFLS, the first step is to define objectives and criteria. These objectives and criteria may vary depending on system characteristics, regulations, and policies, but they typically include preventing frequency collapse and maintaining system stability, minimizing load shedding amount and duration, distributing load shedding equitably among different regions and customers, coordinating UFLS actions with other protection and control devices, and ensuring personnel and equipment safety.

2 UFLS settings and zones

The next step is to determine the UFLS settings and zones. This includes frequency thresholds, time delays, and the amount of load to be shed at each stage of the scheme. The settings and zones should be chosen to meet the UFLS objectives and criteria, taking into account system dynamics, uncertainties, and contingencies. Factors to consider are the frequency nadir (lowest frequency reached after a disturbance), the rate of change of frequency (indicating severity and speed of imbalance), generator inertia and droop (affecting frequency response and regulation), load characteristics (elasticity, diversity, priority, sensitivity), and network topology/constraints (transmission lines, transformers, interconnections).

3 UFLS simulation and validation

The third step in implementing a UFLS scheme is to simulate and validate it. This involves testing the performance and robustness of the scheme with software tools and models under various scenarios and conditions. These should include the normal operating state, as well as potential disturbances, worst-case scenarios, extreme events, and sensitivity analysis. Additionally, the results must be compared and verified with the UFLS objectives and criteria, as well as historical data and measurements.

4 UFLS implementation and coordination

The final step is to implement and coordinate the UFLS scheme. This involves installing, commissioning, and maintaining the UFLS devices and communication systems, as well as ensuring their compatibility and interoperability with other protection and control devices. Additionally, it is important to document and communicate the UFLS scheme and settings to all stakeholders. Personnel involved in the UFLS operation should be trained and educated accordingly. Furthermore, it is essential to monitor and evaluate the UFLS performance, providing feedback and making improvements based on experience and lessons learned.