Photovoltaics to boost low-voltage grids?
The 'Security of supply photovoltaic inverter: combined UPS, power quality and grid support function in a photovoltaic inverter for weak low voltage grids' (SOS-PVI) project aimed to develop an inverter capable of injecting photovoltaic energy into low voltage grids. SOS-PVI envisioned a number of advantages to employing such an approach. These include minimising the impact of PV systems on grid operation and planning, ensuring security and quality of electricity supply to buildings with PV installations and increased penetration of PV in the networks. To meet project objectives, partners performed a market study of data on weak grids in Europe and estimated market potential for small-scale distributed generation and grid stabilisation systems. Activities then turned to practical implementation and work focused on prototype production. The first prototype constructed, controlled and submitted for preliminary testing was based on a lithium-ion based storage system. A supercapacitor bank was designed and sent for integration in the hybrid storage system and testing, and lead-acid battery cells were designed and subjected to preliminary capacity tests. Following cabinet development, two valve-regulated lead-acid battery banks were sent for integration in the hybrid storage system and testing. Other project efforts led to the development of parallel inverters, algorithms and several electric devices. The latter were realised on the strength of demand side management (DSM) studies. A demand signal generator device capable of providing the system with grid status information was developed. Two prototypes were constructed thanks to the development of an automated demand management device and a user demand management device. After work on prototype constructions, team members conducted final tests on field system installations, validated operations and performed a life-cycle analysis to assess whether cost and environmental impact objectives were achieved. Results indicate that the impact of the hybrid system is higher thanks to larger material amounts, and predominantly, its shorter lifetime. Comparing operational costs of lithium and hybrid systems, project outcomes reveal that over a 20-year lifetime, the lithium system is actually less expensive.
