Secondary Low-Voltage Circuit Models — How Good is Good Enough?

Utility power distribution system analysis has traditionally been performed considering only the primary, or medium-voltage (MV), system. Distributed energy resources are increasingly being located on the secondary, or low-voltage, side of the distribution transformer, requiring the low-voltage system to be included in the analysis. However, most distribution system models do not include secondary circuits at all or they are modeled with limited detail. In the absence of the appropriate model data, secondary circuits models commonly rely on simplifications such as single-phase equivalent models, constant power factor, etc. This paper highlights the implications of some of these assumptions and provides guidance in terms of how more accurate secondary models are needed. This paper shows that split-phase secondary circuits can be modeled accurately with single-phase equivalents under perfectly balanced conditions. It also shows that assuming constant power factor or using smart meter measurements with practical 15-min, or larger, measurement time-granularity can lead to considerable errors in simulated secondary circuit losses or voltages.