# Efficiency Comparison of AC and DC Distribution Networks for Modern Residential Localities
## 5 Overall System Model and Analysis  
An overall picture of the system model is depicted in Figure 2. The left side represents a DC distribution system. The 11-kV primary distribution lines are depicted in red and black. DC-DC converter transformers are employed for the primary DC-DC conversion, which steps down the voltage level from 11-kV to 325-V. From each DC-DC Converter transformer, 325-V power lines are drawn to energize the buildings. The in-building \"A, D, I\" loads are powered through converters where required. 'A' category loads are driven through inverters that convert 325-V DC to 230-V AC. Since 'D' category loads are small devices, they require power at a lower voltage level. Therefore, they are driven through DC-DC converters that convert 325-V DC to 24-V DC. 'I' category loads being independent are driven directly through the service mains.  
The right side of Figure 2 is the AC analogous to the left side. Three-phase lines run from the 11-kV grid. The transformers perform their duty of voltage transformation to 230-V per phase. Power at this voltage level is brought to the building, where it is converted to DC via AC-DC converter to drive 'D' category loads. In contrast, category 'A' and 'I' category loads directly run on 230-V available at the service mains. For analysis, the system voltage at the primary distribution voltage level is chosen to be 11-kV for both AC and DC systems. Secondary distribution is performed at 230-V per phase for the AC system and 325-V for DC system. The purpose of choosing these voltage levels draws support from [30]. We assumed a system of 63 buildings. For distribution, 21 buildings are assumed; hence, in total3 DC-DC converter transformers are employed. For the AC system, since there are three conductors per phase, each conductor is assumed to supply 7 buildings. The figure  
Figure 2: Overall system model.  
presents a conceptual diagram of the two systems. We have chosen \"Hare\" aluminum conductor steel reinforced (ACSR) for both (AC and DC) distribution networks and spacing of 300 m between distribution transformers is assumed. The physical systems of AC and DC distribution draw their support from [8]. Conductor losses are almost comparable at the secondary distribution level, therefore neglected. At the primary distribution level, DC resistance of a conductor is accounted in the DC system analysis, whereas, AC, as well as DC resistances, are accounted in the AC system analysis. Moreover, the power factor of AC loads and the loss associated with the power factor is also considered in the analysis of the AC system.  
The input data are acquired through a bottom-up approach using the mathematical model discussed in the previous section. The 'loading vs. efficiency' curves of converters along with their equations (acquired from MATLAB curve-fitting tool) are presented in Figure 3 and (17), respectively. The curve-fitting tool provides a function that fits the curve data, and then approximate results can be obtained by increasing the degree of the function.  
\[\eta\ (l)=\sum_{i=0}^{j}k_{i}\times(l-10i)\times u(l-10i) \tag{17}\]  
where \"\(\eta\)\" is the efficiency of converter as a function of percentage loading '  
The mathematical model and data from Table 2 are combined for converter efficiencies against loading using the above expressions resulting in the efficiency comparison of AC and DC distribution systems presented in Figure 4. The comparison of AC and DC home is also presented in Figure A2.