SIMULATION TOOL FOR TECHNO-ECONOMIC ANALYSIS OF HYBRID AC/DC LOW VOLTAGE DISTRIBUTION GRIDS

Partial operation of the distribution grid in DC instead of AC has been identified as a possible strategy for cost-effective management of future grid scenarios driven by international decarbonisation goals. By applying a simulation tool for techno-economic analysis on synthetic test grid models, it has been shown that the conversion of AC low-voltage grid feeders to DC is a suitable solution to mitigate overloading and decrease voltage fluctuations caused by, inter alia, integration of electric vehicles (EVs), photovoltaic systems (PV) or increased energy demand. Cost models were applied to the simulation results. The economical findings indicate that the implementation of DC in low-voltage grids can be financially beneficial, especially when future developments and learning curves of DC technologies are considered.


Introduction
Direct current (DC) transformation was only made possible by the introduction of semiconductors, therefore alternating current (AC) had won the War of Currents in the late 19th century. Nowadays, DC can be efficiently transformed using converters; the advantages of DC stay, while the biggest disadvantage disappeared.
The benefits of DC distribution have been investigated in many industries. Following the development of high voltage direct current (HVDC) for transmission networks, there are efforts to use DC at lower voltage levels. Significant developments can be observed in the field of hybrid electric propulsion [1], marine systems [2], data centres [3] and DCbased medium-voltage distribution networks [4]. Furthermore, DC enables a more efficient way of integrating the loads and renewable energy sources (RES) of the future in lowvoltage distribution grids due to the reduction of conversion stages [5]- [8]. In [9], hybrid AC/DC low voltage (LV) grid simulations focusing on system efficiency using Modelica Electric Power System Library (EPSL) are presented. The simulation tool proposed in this paper is designed for technoeconomic analysis of hybrid AC/DC LV networks from a grid planning perspective by the means of quasi-stationary simulations in Power Factory.

Methodology
The presented simulation tool enables automated conversion of low voltage alternating current (LVAC) lines and feeders to DC and the subsequent simulation of the hybridised grid using Power Factory, controlled through its integrated Python API. A web interface provides an input mask to parameterize and start the simulations. For direct comparison, both pure AC and hybrid AC/DC systems are simulated under identical conditions. Moreover, different EV and PV penetrations can be added to the grid. If this is the case, two additional simulations are performed including the supplemented loads. An economic analysis can be applied on the simulation results through the same interface, whereas parameters are set by the user. The implemented DC configurations and scenarios as well as the grid and economic models are explained in detail in the following sections.

LVDC configuration and voltage levels
DC distribution lines and feeder must be configured as either unipolar or bipolar systems. Based on 4-conductor LVAC lines, the configurations shown in Figure 1 are applied in the simulation tool. For parallel AC lines, three instead of one conductor are used for both plus and minus conductor in bipolar DC operation.

Synthetic grid model
The utilized synthetic grid model (presented in [11], available at [12]) covers all elements between the transmission level (220 kV), distribution level (110 kV and 20 kV, urban and rural) and the LV connection points. Ten LV feeders (Table 2) with a total of 190 loads are modelled in detail, representing typical rural and urban LV feeders in the European distribution grid. Beside a radial LV grid scenario, the model is available with meshed LV topology. Furthermore, equivalent loads (operated in AC) at all network levels are available, modelling the part of the grid which is not directly under investigation. The yearly load and PV profiles in 15-minutes resolution are based on measured European data [12]. EV profiles are modelled with realistic static charging profiles [13].

Line types:
The simulation tool offers automatized evaluation of maximum power transmission capacity vs line length for AC and DC operation of any given line type, taking simulated line losses into account. Within the applied synthetic grid model, the rural LV grid mainly consists of typical European overhead line types, whereas the urban grid is modelled using typical LV cables, such as evaluated in Figure  3. By applying recommended DC voltage levels (

AC/DC hybrid LV grid scenarios
Defined scenarios are considered within the simulation tool, listed in Table 3, to convert LV lines and feeders from AC to DC, resulting in hybrid LV distribution grids.

Simulation Parameters
In Table 4   The maximum feeder input apparent power Sin_max only occurs for short time periods within all DC feeders, therefore the voltage source converter (VSC) rated power is selected to be 0.9 * Sin_max considering dynamic overload capability of AC/DC converters. An in-depth analysis of the load profiles per feeder would be necessary to determine most economic rated power factors for each converter. Optimal sizing of the converter allows operation at higher efficiency, since weak utilization can be limited. Although not considered in this analysis, the possibilities of peak shaving or storage within LV feeders would most likely allow the VSCs to be dimensioned smaller, which would have a positive impact on the costs for conversion to DC due to reduced losses and capital expenditure (CAPEX).
EV and PV penetration as well as load scaling were chosen to represent possible future scenarios in which the existing grid reaches its capacity limits.

Economic analysis
Changing LV feeders from AC to DC requires considerable efforts and investments. The driving force of such significant structural changes must be the potential of economic benefit. For the purposes of this techno-economic analysis, the costs for AC reinforcement versus conversion to DC for the presented LV feeders are modelled considering the parameters listed in Table 5.

Results
In the following, the simulation results implementing the previously explained variables in the synthetic grid model are presented. Table 4). Figure 3 shows an overall reduction of maximum loading for all simulated feeder using 1400 VDC instead of 400 VAC. Additional PV and EV loads affect the loading of LV rural 1 severely, whereas max. loading could even be reduced in other feeders by adding future loads. Evaluating the max. loading throughout the simulated year, feeder LV rural 1 and LV urban 3 experience overloading violations.  Similarly, the voltage profile for feeder LV rural 1 on 23 June ( Figure 6) shows that the allowed voltage band cannot be adhered using 400 VAC in the investigated scenario. Voltage fluctuations are reduced remarkably in the 1400 VDC system. The cost model was applied to the yearly simulation results. Figure 7 shows that the conversion of any LV feeder to DC would be economically beneficial in comparison to reinforcing the AC lines under the given circumstances. For feeder LV Rural 1, which is one of the feeders where the AC network reached its limits, it is clearly economically beneficial to  Table 5 and yearly simulation results.

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implement DC, even if scenarios 1a or 1b are considered. As already demonstrated in [14], it was confirmed that long rural feeders such as LV rural 1 are important use-cases for LVDC. Moreover, it was shown that even for urban LV feeder such as LV urban 3, if the current AC grid is reaching its capacity limitations, the conversion to DC is an option worth considering from a techno-economic perspective.  Table 4) Overall yearly feeder losses in the given scenario could only be reduced for feeder LV rural 1 and 4 in case EVs and PVs are included in the test grid (comparison of LOSS costs in Figure 7). In general, DC system losses resulted to be higher mainly due to the high VSC no-load losses and frequent low load operation of the converter. Nevertheless, beside potential cost reduction of DC equipment, VSC losses and CAPEX costs can be further reduced by minimizing the converter size. Moreover, Figure 8 shows that the efficiency gains applied to loads, EVs and PVs if connected to DC resulted in a notable reduction of maximum feeder input apparent power Sin_max.

Conclusion
The presented simulation tool enables flexible and fast hybridisation of complex grid models in Power Factory. An economic model helps identifying cost sensitivities and facilitates the high-level evaluation of the implementation of LVDC from a grid planning perspective.
Selected simulation results underline the potential benefits of DC in LV grids. In case capacity limits of an LVAC system are reached, it is shown that a conversion of the LV feeder to DC instead of reinforcing the existing lines can be economically beneficial, for both rural and urban LV feeders, especially considering future technological development of DC equipment.
Further investigations are planned in the optimal sizing of the converters as well as the inclusion of more complex converter control strategies. This will allow the investigation of possible effects of the hybridization on the overlaying AC grid, as well as the identification of synergies to tackle future challenges such as low inertia grids or management of decentralized energy communities.