Excitation Limiters

An exciter model is central to representing a generator's excitation system. The field voltage output from an exciter is used to excite the field winding of an electric machine, and the level of excitation determines the generator's reactive power output. At each level of active power (MW) output, the generator has minimum and maximum limits on the amount of the reactive power (Mvar) that it is safely capable of providing. These reactive power bounds might not be adequately represented by just the static limits found in exciter models. To allow for more detailed modeling in this aspect, there are additional excitation limiter models that can be configured with an exciter model. These limiter models would then work in synergy to limit the field voltage, and hence limit the generator's reactive power output during a dynamic simulation.

There are 3 types of excitation limiters:

• Under excitation limiter (UEL), which prevents the generator from exceeding its core-end heating limit and/or its stability limit, when it is operating at a leading power factor, i.e., absorbing Mvar.

• Over excitation limiter (OEL), which prevents the generator from exceeding its field current heating limit, when it is operating at a lagging power factor, i.e., supplying Mvar.

• Stator current limiter (SCL), which prevents the generator from exceeding its stator current heating limit, when it is operating at a high active power output, but is not limited by an UEL or OEL constraint.

The bounds enforced by these excitation limiters would actually be similar to the limits defined by a generator capability curve. This is also known as a D-curve, which is often part of a generator's input data in power flow studies. In contrast, UEL, OEL and SCL dynamic models also have a dynamic response in addition to enforcing these bounds, and some of these models could also enforce time-varying limits based on an activation logic.

All UEL/OEL models have a transient stability result field called Activation Status that succinctly captures their current operation. In all SCL models, there are two fields called UEL Activation Status and OEL Activation Status reflecting under or over excitation limiting action.

• Idle: Activation Status = 0, or UEL Activation Status = 0, or OEL Activation Status = 0

  Activation logic is not triggered.
  For UEL/SCL models: Output signal is V_UEL-MIN
  For OEL/SCL models: Output signal is V_OEL-MAX
  

• Engaged: Activation Status = 1, or UEL Activation Status = 1, or OEL Activation Status = 1

  The limiter model's activation logic is triggered, and the output is varying between its lower and upper limits
  For UEL/SCL models: Output signal is in the range (V_UEL-MIN, V_UEL-MAX)
  For OEL/SCL models: Output signal is in the range (V_OEL-MIN, V_OEL-MAX)

• Saturated: Activation Status = 2, or UEL Activation Status = 2, or OEL Activation Status = 2

  The limiter model's activation logic is triggered, but the output is already capped
  For UEL/SCL models: Output signal is V_UEL-MAX
  For OEL/SCL models: Output signal is V_OEL-MIN