One of the critical functions in synchronous generator protection is the Under Excitation Limiter (UEL), which prevents excessive under-excitation that could result in field overheating or even loss of synchronism. The UEL ensures that the generator's excitation remains above a threshold level, thereby avoiding the risk of generating negative reactive power (or "field weakening"). When simulating these systems, it is essential to configure machine initial conditions accurately to ensure steady-state operation unless the simulation of transient behaviour, such as under-excitation events, is required.
In PSCAD, UELs are modeled as dynamic control systems that monitor parameters such as generator field voltage or reactive power. When certain limits are breached, the UEL adjusts the excitation system to prevent under-excitation, thus safeguarding the generator.
PSCAD offers pre-defined UEL models that comply with IEEE Standard 421.5-2016. For specific simulation requirements, users can also develop custom UEL logic using building blocks from the CSMF (Continuous System Modelling Functions) section of the PSCAD Master Library.
The following UEL models are available:
UEL1: This model represents a simple under-excitation limiter based on a feedback mechanism. It limits the generator's excitation by adjusting the field voltage to avoid excessive under-excitation, using a basic time constant and threshold.
UEL2C: This model offers a more advanced under-excitation limiting function, incorporating a lead/lag filter structure for more refined control. The UEL2C model uses multiple inputs such as generator field voltage, current, and reactive power, and provides a time-dependent limiting response, which is often applied in systems where a smoother and more precise excitation adjustment is required.
For further details on background theory for these UEL models, refer to IEEE Standard 421.5-2016 (PSCAD References [39]).