Abstract
The variations in irradiance produced by changes in cloud cover can cause rapid fluctuations in the power generated by large photovoltaic (PV) plants. As the PV power share in the grid increases, such fluctuations may adversely affect power quality and reliability. Thus, energy storage systems (ESS) are necessary in order to smooth power fluctuations below the maximum allowable. This article first proposes a new control strategy (step-control), to improve the results in relation to two state-of-the-art strategies, ramp-rate control and moving average. It also presents a method to quantify the storage capacity requirements according to the three different smoothing strategies and for different PV plant sizes. Finally, simulations shows that, although the moving-average (MA) strategy requires the smallest capacity, it presents more losses (2–3 times more) and produces a much higher number of cycles over the ESS (around 10 times more), making it unsuitable with storage technologies as lithium-ion. The step-control shown as a better option in scenery with exigent ramp restrictions (around 2%/min) and distributed generation against the ramp-rate control in all ESS key aspects: 20% less of capacity, up to 30% less of losses and a 40% less of ageing. All the simulations were based on real PV production data, taken every 5 s in the course of one year (2012) from a number of systems with power outputs ranging from 550 kW to 40 MW.
Highlights
The concern shown by a number of transmission system operators (TSO) over the increasing presence of large photovoltaic (PV) plants is leading to constant changes in grid codes, directed at facilitating PV penetration yet without detracting from the safety and quality of supply
The results shown can be used as a guide to select the best control strategy, and the optimum design and selection of the energy storage system (ESS), making it possible to assess the impact of the ESS on the PV plant energy/economic balance
This paper has quantified the storage requirements needed to smooth short-term PV power fluctuations based on different control strategies and for a wide range of PV system power outputs
Summary
The concern shown by a number of transmission system operators (TSO) over the increasing presence of large photovoltaic (PV) plants is leading to constant changes in grid codes, directed at facilitating PV penetration yet without detracting from the safety and quality of supply. The researchers [5,13,14] have demonstrated that the larger the PV system is, the lower the PV fluctuations are: typically, for this 1 min time window, a 1 MWp PV plant records fluctuations of up to 90%, a 9.5 MWp plant exceeds 70% [14], whilst a 45.6 MWp fluctuates by up to 33% [15] As these new regulations gradually come into force, it will be necessary to equip PV plants with an energy storage system (ESS) in order to smooth out any fluctuations to within the maximum limits. We describes the method in order to calculate, for any PV plant size and maximum allowable ramp-rate, the energy storage requirements for both MA and step-control strategies. The results shown can be used as a guide to select the best control strategy, and the optimum design and selection of the ESS, making it possible to assess the impact of the ESS on the PV plant energy/economic balance
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