One of the key considerations in modelling excitation control systems is the inclusion of appropriate limiting functions to ensure equipment protection and realistic system response. Over Excitation Limiters (OELs) are designed to protect synchronous machines from sustained over-excitation that can lead to overheating of the generator field winding or other associated components. When modelling these limiters, care should be taken to ensure that machine initial conditions are properly set to represent a steady-state operating point, unless the simulation of abnormal or transient events is intended.
In PSCAD, OELs are implemented as dynamic control models that monitor signals such as generator field current, field voltage, or reactive power. These models act to reduce excitation when defined thresholds are exceeded, typically by introducing limiting signals to the automatic voltage regulator (AVR).
PSCAD includes several pre-defined OEL models that conform to IEEE Standard 421.5-2016. If none of the built-in models are sufficient for a particular study, custom OEL logic may be constructed using control elements from the CSMF (Continuous System Modelling Functions) section of the PSCAD Master Library.
The OEL models covered in this library include the following types:
OEL2C: A signal-level takeover limiter that responds to generator field current or a proportional signal (e.g., from a brush-less exciter) to regulate field output based on a time delay and threshold limit.
OEL3C: A summing-type limiter that monitors field current and introduces a soft limiting effect by gradually modifying the AVR input, allowing smoother transitions under over-excitation conditions.
OEL4C: Similar to OEL3C but includes additional logic to respond to reactive power input, enabling more comprehensive system behaviour under prolonged over-excitation conditions.
OEL5C: A takeover-type limiter with configurable logic for more abrupt intervention, typically used in systems where decisive limiting action is required to protect generator field equipment.
For further details on background theory for these OEL models, refer to IEEE Standard 421.5-2016 (PSCAD References [39]).