Linear PV power plant based on MVDC collection network

Abstract

The European Green Deal targets the climate-neutral continent by 2050. This very ambitious target requires much higher shares of renewable energy. Just in France, based on the existing 15 GW photovoltaic installed power, the objective is to reach up to 44 GW in 2028. This will require the development of large-scale PV power plants. The PV requires large areas (approximately 1 ha/MWp) and the available lands are limited. A solution is to reuse the lands which are already occupied by agriculture, buildings etc. An interesting solution is to install PV power plants along motorways, railroads, rivers etc. These linear PV power plants would be only few meters large but could reach the length of many kilometres and the power up to a few tens of megawatts (roughly 1 MWp/km). The maximum potential for linear PV power plants in France represents roughly 60 GWp.

The distance between the PV power plant and the available AC grid substation may be up to a few tens of kilometres. The length of the linear PV power plant and the distance to the AC grid substation present some challenges for the electric power collection networks. The medium voltage AC collection network has high power losses and high voltage drop. A novel network architecture based on medium voltage DC (MVDC) is analysed in this article. It is an interesting candidate to reduce power loss and voltage drop of linear PV power plants.

The case study linear PV power plant is 20 MW spreading along 20 km. The AC grid substation is 20 km away from the PV plant centre. A radial MVDC network at ±20 kV is selected and discussed. There are eighty 250 kW isolated DC-DC converters for PV string voltage step-up and one DC-AC converter for interfacing MVDC collection network with the AC grid. There are DC circuit breakers and DC fuses for network protection. A special MVDC cable is discussed.

The steady state analysis of the MVDC network is presented. It shows the power losses and voltage drop across the MVDC network compared to 20 kV AC network. The voltage drop is at least 2 times lower in DC. The power losses of a ±20 kV DC high efficiency solution are 470 kW lower at 20 MW than 20 kV AC.
The total yearly energy loss is calculated considering the energy price of 100 €/MWh. Over 10 years, the MVDC solution allows to save between 150 k€ and 560 k€, excluding CAPEX. This saving must compensate the CAPEX difference between AC and DC. The DC solution is expected more expensive, at least due to the 20 MW DC-AC converter which is present only in DC. Considering that the cost of all equipment in DC is the same as in AC, then the budget for a 20 MW DC-AC converter is <150 k€ or <560 k€, depending on the hypotheses.

Piotr Dworakowski, Silvain MARACHE, Eric LAMARD, Caroline RAMONDOU

Presented at CIGRE 2024