Siemens is helping the French Polynesian island of Tahiti reduce its reliance on diesel fuel as part of a wider plan to help decarbonise the island.
Currently, around 36 per cent of Tahiti’s electricity supply comes from renewables – 30 per cent from hydro-electric stations, and 6 per cent from solar PV.
The rest remains heavily dependent on diesel, which is not only carbon-intensive, but can be highly vulnerable to price volatility and supply disruptions. Tahiti’s aim is to have 75 per cent of its energy provided by renewable energy by 2030.
The project was commissioned by French company Engie subsidiary Electricite de Tahiti (EDT), which operates Tahiti’s public energy service for the main island as well as 19 other islands in French Polynesia.
The project will see a 15MW/10.4MWh lithium-ion battery energy storage system (BESS) offered by Kokam, a global provider of innovative lithium-ion battery solutions and a subsidiary of SolarEdge Technologies, integrate with Siemens’ SINAMICS S120 virtual synchronous generator (VSG) liquid cooled inverter with marine grade protection. It is specifically designed for the island and its tropical environment.
Importantly, the grid forming VSG functionality together with specifically designed Static VAR Compensation (STATCOM) 16 Mvar of the SINAMICS S120 enables the inverter to provide grid support functionality. This is equivalent to the synchronous services and reactive power capabilities traditionally provided by a synchronous generator.
The SINAMICS S120 inverter system provides grid support functionality not only for active power and frequency stability, but also provides dynamic power support to the Tahiti network for voltage stability.
The Siemens inverter system is critical to the success of the project, as putting more renewable energy into the grid means it becomes more unstable. The project will offset EDT’s existing spinning reserve diesel generators and is expected to reduce diesel usage by around $2.2 million annually.
Additionally, the introduction of VSG will strengthen the stability of the local grid and increase energy savings through reduced generator maintenance costs and an increase in the generator’s lifespan.
The importance of inverters is that they are able to convert the direct current (DC) or variable frequency electricity produced by renewable energy and store it in batteries into the nominally 60Hz alternating current (AC) form of electricity used by the power grid.
A key challenge is that whilst the grid’s frequency is nominally 60Hz, in reality, it constantly varies due to the mismatch between demand and supply balance in the power system.
It is therefore necessary to have mechanisms in place to stay synchronised with the grid. This is particularly challenging during system disturbances when the local grid’s voltage waveform can become significantly distorted.
By injecting reactive current during a grid fault condition, the STATCOM functionality of the SINAMICS S120 will help the generators ride through the system fault events as required for grid stability.
As per the requirement of EDTs specifications, the inverter needs to provide reactive power as a function of AC voltage. In voltage regulation mode, the SINAMICS S120 unit is fed with an external set point of the measured voltage at the point where voltage regulation is required.
Voltage regulation mode only requires measurement of voltage by either VTs or by external source such as a power meter. The Inverter provides the required reactive power to maintain the supply voltage within limits.
Manufacturing and testing of the system are currently underway, with the project expected to go live in late 2022.
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