Feasibility study of MVDC Hybrid Grids for large scale research facilities
Creators: name: Hetzenecker, Katharina
Publisher: name: The HYPERRIDE Consortium
Year: 2022
Subjects: MVDC
DC grid
grid planning
feasibility
European Union (EU)
H2020
Project
HYPERRIDE
GA 957788

description: At the RWTH-Aachen University a new Campus West will be built. This Campus area includes a thermal network as well as an IT-center, PV power plants and charging stations for electric vehicles. It is considered to connect these components using a customer owned medium voltage grid, in order to maximise self consumption. Within this deliverable, a study is implemented based on the plans for Campus West, which identifies the advantages of using a Medium Voltage Direct Current (MVDC) grid configuration compared to a 10 kV Alternating Current (AC) reference grid for that purpose. Parameters considered in this analysis are different voltage levels, transformer efficiency, converter efficiency and cable transmission losses. The aim of this study is to identify the key influencing factors along with the conditions that result in the MVDC grid configuration being more feasible than the AC reference scenario.

In order to carry out the comparison between AC and Direct Current (DC) systems, grid systems are first designed accordingly for the use of the respective technology. Within the scope of the comparison, both monopolar and bipolar DC system configurations are compared with the AC reference scenario.

For the three-level neutral-point-clamped converter (3LNPC) and the modular three-phase dualactive bridge (DAB3) the efficiency curves are calculated by means of  simulation.

In order to compare the efficiency of the concepts, the power flows for a whole year are calculated in hourly resolution based on the load profiles, including the resulting losses in the converters, transformers and cables. The results show the following:



DC power transmission is advantageous for voltage levels UDC >= 5 kV compared to the 10 kV AC reference scenario.

Considering material consumption as well as efficiency allows to identify monopolar grid structures as more advantageous, since they only need two conductors instead of three.

While designing the DC-DC converter, it is financially rewarding to oversize the converters instead of sizing them only for the maximum power needed, if the load profile is constant. Hence, a more efficient operating point can be achieved.

If the efficiency of the MVDC-LVDC converter is 1.7 % higher than the efficiency of the active rectifier used in the AC reference scenario, the implementation of a MVDC grid is potentially more feasible.

The losses in the MVDC-MVAC substations exceed the losses in the MVAC-MVAC substation by far, because of the additional converter needed for rectification in the DC scenarios.

Distributing the power supply evenly among the active substations using power-flow control does not make the substations more efficient, even though it allows to design the components for smaller nominal power values. However, distributing the power supply leads in this example to higher cable transmission losses.

Increasing the cable transmission distances by a factor of ten leads to the DC systems being more efficient than the AC-reference system. Especially for monopolar configurations with grid voltages UDC >= 12 kV.


Limitations and critical review of this analysis:



As the case study includes a consumer that withdraws constantly a high amount of power, it could be that the application of the results to other use cases needs to be critically evaluated, especially concerning the derivation of more generalised conclusions.

The comparison of the cable transmission losses is based on the assumption that the current densities that can be realised without exceeding the thermal limits of the cable are the same for AC and DC transmission.


In the context of this case study different parameters have been identified, that lead to better feasibility of a DC-grid, compared to the AC reference grid scenario. While the impact of the cable transmission losses is comparably low and only becomes relevant when the transmission length is increased, the key influencing factor is the efficiency of the converters used in the grid systems. DC grid scenarios become potentially feasible after nine years or more, if the DC-DC converters connecting the MVDC grid to the LVDC grids have a maximum efficiency that exceeds the efficiency of the active rectifiers in the AC reference scenario by 1.7 %. Also, for the use case investigated in this case study, the efficiency of the MV Active Front End (AFE) was identified to have significant impact on the overall system efficiency.

Future work should further investigate the modelling of converter efficiency, in order to obtain results that can further differentiate between different types of semiconductor switches or types of converter control. In addition, further consideration should be given to the aspect of material consumption by determining tolerable current densities in DC cable configurations and comparing the material consumption of the medium frequency transformers and 50 Hz transformers implemented in different DC and AC grid configurations.