## 2 Load Profile Development
### Harmonic Distortion Evaluation  
Derating distribution, industrial, or commercial transformers due to heating induced by harmonics distortion is a well-documented application [35; 36; 37; 38; 39]. As defined in [PERSON], [PERSON], and [PERSON], the four methods applicable to derating transformers include IEEE recommendations, analytical methods, experimental approaches, and finite elements [14]. Depending upon the application, load type, and available transformer and system information, each approach possesses disadvantages and advantages. Within these classifications, researchers have developed numerous approaches for derating transformers serving nonlinear loads [40; 41; 42; 43]. However, as defined by the research in [PERSON] and [PERSON], not all transformers serving nonlinear loads must be derated. Derating is dependent upon the transformer's top oil temperature rise (TOR), which is a function of ambient temperature, loading, solid and liquid dielectric properties, and transformer construction [9]. Historically, most system problems caused by harmonic distortion originate at a single source or customer, which prompts mitigation efforts to be directed exclusively at one location, bus, circuit, or subdistribution circuit. In fact, most research endeavors focus on derating transformers rated less than 2 MVA that serve mostly dedicated nonlinear loads [9; 35; 36; 37; 38; 39; 40; 41; 42; 43]. For substation transformers that serve a combination of residential, industrial, and commercial loads, accounting for harmonic is typically ignored due to the low levels of distortion and isolation of problems [1]. As the penetration of nonlinear loads increase on the grid in the form of DER power electronics, computer supported equipment, and electronic devices, should harmonic distortion be accounted for in determining substation transformer capacity? The heat generated by harmonic distortion--both current and voltage--can accelerate the aging of transformers depending upon it effects on TOR and the surrounding ambient temperatures [9; 13; 17]. The research performed previously by the authors identified additional heating as an effect of serving a flattened load profile due to high penetrations of PV, ES applications, and EV charging [6]. Therefore, combining the heat produced by nonlinear devices and the heat produced by a flattened load profile can accelerate insulation aging when compared to that induced by a historical demand. DER applications, residential and utility owned, rely upon inverter based power electronics, which introduce harmonics onto the system they are connected with. The increased implementation of DER applications contribute to the existing levels of harmonic distortion, which is a result of consumer electronic devices, fluorescent lighting, high efficient appliances, and computers. Although several industry standards limit the amount of harmonic distortion produced by individual nonlinear devices (inverters, CFLs, etc. ), the overall accumulation of distortion may still rise above acceptable limits listed in IEEE Std. 519-2014 [15; 16; 17; 44; 45]. Before one can determine how severe harmonic distortion will become due to high penetrations of DER devices, one must first answer the question: \"What are the typical existing levels of harmonic distortion?\"  
To identify an existing harmonic distortion baseline for the transformer capacity analysis, the existing harmonic distortion on a 15 kV distribution system located in an urban area (>750,000 end-users) was evaluated by the authors. The results of the analysis are to be utilized to develop the appropriate harmonic spectrum for the transformer capacity analysis. The assessment included analyzing voltage and current harmonic distortion measurements acquired during 401 quality service investigations performed by the utility company at end-user service meters between 2009 and 2015.