A: Model aggregation is a useful technique for simplifying the network model and improving solution convergence. However, in some instances, aggregation may result in an unacceptable level of model error.
Consider an IBR plant with four identical CIR models that are connected in parallel with each other, as shown below. In this case, all four converters will see the same voltage at their terminal bus and should produce the same current output during the fault. Therefore, aggregation of these models to a single CIR model should reduce the computations required to solve the network without impacting the fault current that would flow into the POI bus from the IBR plant.
Next, consider an IBR plant with four identical CIR models that are daisy-chained together, as shown below. In this case, each of the four converters will see a different voltage at their terminal bus (affected by the impedance of the daisy-chain links). Therefore, each CIR model will produce a different current output during the fault. In cases where the actual voltage difference between the converter terminal buses is expected to be small, it may be acceptable to aggregate the units. This modeling decision will require some judgment.
