Auto-Correction at Excitation Limiter Models

UEL/OEL/SCL models have model-specific auto-correction that are listed in their respective documentation pages. Additionally, a broader set of checks are applied regardless of their specific type, depending on the Output Used (OutputUsed) field of an UEL/OEL/SCL.

1. UEL/OEL/SCL‑exciter compatibility check

• Error is shown if Output Used ∈ [Not Suitable, Not Configured]

  Manual correction could be to change either the type of limiter to match the exciter configuration, or the type of exciter, or the exciter configuration (if possible) to match the limiter that is present.

  Both cases would cause exciters to have atypical signal clamping (if a summation point signal were to be fed into a takeover gate), or abnormally large error signals (if a takeover gate signal were to be fed into a summation point). Therefore, these must be corrected, or otherwise they would result in weird and unintended consequences during the dynamic simulation.

  If not manually corrected, auto-correction will make the limiter model not active, and not change anything at the exciter models.

• Warning is shown if Output Used ∈ [No Input, Not Used, No Exciter]
  

  Manual correction could be to change either the type of exciter, or the exciter configuration (if possible) to match the limiter that is present, or add a compatible exciter.

  All three cases would simply not use the limiter model, so the simulation can continue without any correction

  If not manually corrected, no auto-correction is done
  

2. UEL/SCL_UEL signal lower limit (V_UEL-MIN) check
   (only if Output Used = Summation Point)

   • Error shown if (V_UEL-MIN < 0).
     (only if model does not have a fixed zero lower limit)
     

     Manual correction could be to set V_UEL-MIN = 0.
     A negative UEL signal at a summation point would curb over-excitation, instead of its intended goal of curbing under-excitation.

     If not manually corrected, auto-correction will set V_UEL-MIN = 0.

   • Error shown if (V_UEL-MAX < V_UEL-MIN).
     

     Manual correction could be to set V_UEL-MAX > V_UEL-MIN.
     

     If not manually corrected, auto-correction will set V_UEL-MAX = V_UEL-MIN + 0.05.

3. OEL/SCL_OEL signal upper limit (V_OEL-MAX) check
   (only if Output Used = Summation Point)

   • Error shown if (V_OEL-MAX > 0).
     (only if model does not have a fixed zero upper limit).
     

     Manual correction could be to set V_OEL-MAX = 0.
     A positive OEL signal at a summation point would curb under-excitation, instead of its intended goal of curbing over-excitation.

     If not corrected manually , auto-correction will set V_OEL-MAX = 0.

   • Error shown if (V_OEL-MIN > V_OEL-MAX).
     

     Manual correction could be to set V_OEL-MIN < V_OEL-MAX.
     

     If not corrected manually, auto-correction will set V_OEL-MIN = V_OEL-MAX - 0.05.

For UEL/OEL/SCL models that meet the criteria (Output Used = Takeover Gate), upper and lower limits checks are not generalizable like those in B and C, which aim to ensure zero initial derivatives at the exciter when limiter models are not activated. Nevertheless, limits of a takeover gate limiter still needs to be consistent with the internal limits of the associated exciter model, and they need to be tuned relative to each other.

Therefore, the lower and upper limit checks for takeover gate limiters can not be done during auto-correction (i.e., before initialization), because they would have to be specific to each exciter's internal excitation signal. If the "Modify Limits and Run" option is chosen in Transient Stability options, then these checks would be performed during the subsequent step of initialization.