Adapted from CIM data models. The class represents IEEE Std 421.5-2005 type ST4B model. This model is a variation of the Type ST3A model, with a proportional plus integral (PI) regulator block replacing the lag-lead regulator characteristic that is in the ST3A model. Both potential and compound source rectifier excitation systems are modeled. The PI regulator blocks have non-windup limits that are represented. The voltage regulator of this model is typically implemented digitally. Reference: IEEE Standard 421.5-2005 Section 7.4.