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Security of Supply PhotoVoltaic Inverter: combined UPS, power quality and grid support function in a photovoltaic inverter for weak low voltage grids

Final Report Summary - SOS-PVI (Security of Supply PhotoVoltaic Inverter: combined UPS, power quality and grid support function in a photovoltaic inverter for weak low voltage grids)

The 'Security of supply photo voltaic inverter' (SOS-PV) project intended to:
- minimise the impact of PV systems on the grid operation and planning;
- insure security and quality of electricity supply to houses and buildings with PV installations;
- increase performance ratio of PV systems;
- increase penetration of PV in the networks.

In order to meet these global objectives, the target of the project was the production of a PV inverter with UPS functionalities basing on two different technologies for the storage system:
- a system based on a lithium-ion battery bank;
- a hybrid storage system, including a lead-acid battery bank, a super capacitor and the electronic device to manage them.

A market study was performed with collection of data on weak grids in Europe (load profiles, grid quality), estimation of market potential for small scale distributed generation and grid stabilisation systems in Europe, especially considering power and storage capacity provided.

During the first working period, the general information and energy flow in the system was defined in order to implement DSM strategies in the SOS-PV system. After this first step of definition of the system and components, the project entered an active phase of making of the prototypes:
- lithium-ion based storage system: the first prototype was constructed, controlled and submitted to preliminary testing by SAFT;
- supercapacitor bank: the supercapacitor bank was designed to fulfil the application requirements defined earlier. The development of a 165 F module for 48 V was performed by Maxwell, and the use of two such modules in series was fitting SOS-PVi requirements. Maxwell sent one supercapacitor bank to CEA and another one to TTA for integration in the hybrid storage system and testing.
- lead-acid batteries: Hawker and Enersys designed a new cell, then did some first capacity tests, and developed a cabinet. Two valve-regulated lead-acid battery banks were then sent to INES and TTA for integration in the hybrid storage system and testing.

In parallel inverters and algorithms were developed. Studies concerning the demand side management (DSM) led to the development of several electric devices:
- development of a demand signal generator device, which is able to inform the system about the grid status - normal, overcharge and construction of the prototype;
- development of an automated demand management device (to connect or disconnect individually the loads according to the grid status and their level of priority) and construction of the prototype;
- development of a user demand management device (information about the system energy status, definition of energy status modes) and construction of the prototype.

After the second step dedicated to the making of the prototypes, the third and last step of the project aimed at the final tests of the prototypes, the installation of the whole systems in the field (France and Spain) and the validation of operation. Both complete storage systems have been tested according to charge / discharge profiles (including power peak pulses) that had been designed from real Spanish consumption patterns. In the meanwhile inverters were updated and tuned up (lithium based one and hybrid system one); they are in fact real systems consisting each of a synchronisation and fast cut-off switch device, a PV-input converter with MPPT control unit, a multifunctional inverter unit, components for the load management and components for the system monitoring. Systems were then ready for field testing during the last months of the project. During the last period of the project a life cycle analysis was done to check the fulfilment of two objectives concerning cost and environmental impact.

During the whole duration of the project, networking activities were undertaken to promote the issue of grid-connected storage among the community of European project researchers, and in particular, assess the possibility of integration of SOS-PV systems in virtual power plants (VPP), which are a centrally controlled aggregation of decentralised energy resources.

The impact of the hybrid system appears to be higher due to a bigger amount of material and mainly a shorter life time. Costs regarding the lithium and hybrid systems are in the same range considering a 10 year life time today; in the long term (year 2020) and considering a 20 year life time, the lithium system is less expensive.

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