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ENERGIX-Stort program energi

Design and development of modular multilevel converters for large scale grid connected Photovoltaic and Battery Energy storage systems

Alternative title: Et "Modular multilevel converters (MMC)"-basert grensesnitt for fotovoltaiske (PV)-systemer som innebygger et batteri energilagringssystemer

Awarded: NOK 2.5 mill.

By 2022, a dramatic increase in solar energy production is expected in India. An ambitious target of 100 GW installed power is established. (Although COVID situation has slow down the numbers, there are 44 GW till July 2021). Despite the low energy prices, the Norwegian solar market is growing as well, there is around 160 MW installed where 40 MW were installed only in 2021. A collaboration between Norwegian and Indian institutions can bring new opportunities in the near future in the field of solar energy. PVBESS is funded by The Department of Science and Technology (DST) of the Government of India and the Research Council of Norway (RCN) in a program for joint funding of Indo-Norwegian joint research projects called INDNOR. The large integration of PVs represents a great opportunity but a great challenge as well. Here some of the greatest challenges: 1) Non-dispatchable. Even with the most advanced forecasting technologies to predict the generation, it is not possible to guarantee that a PV will be available when is needed. 2) Intermittent generation. Even if the generation is predicted, the output power in a PV will be intermittent. The balance between generation and load must be maintained all the time to avoid synchronization issues. 3) Uncertainty generation. Besides the daily fluctuations caused by the sunrise and sunset, the output power in a PV can suddenly change because of a cloudy condition. For the grid operator is a great challenge to calculate the output power in the rest of the conventional generators to accommodate these fluctuations. The intermittent and uncertain nature of PV power could be managed with energy storage systems (ESS). In specific, batteries are getting more attention due lower cost, the scalability, reliability, and efficiency. The batteries can be charged during high PV production, and it can be discharged during low PV generation. How to connect batteries to a PV? Is there only one way to do it? The core of the integration of PV generation with batteries into the grid is the power-electronic interfaces. A new, promising grid interface is the modular, multilevel converter (MMC). This converter can connect PV arrays in each of the submodules which enables individualized MPPTs for each PV array. Moreover, the MMC can without transformers create medium-high voltages and has an outstanding ability to control internal power flows. The control features are important to handle any power mismatches that can occur between the submodules. For example, the PV arrays can receive different amounts of irradiances which makes the submodule produce unequal amounts of power. The main objective of the PVBESS project is to produce both theoretical and experimental evidence to assess the performance of the conceptual configuration. The work in PVBESS can be summarised as: (i) A suitable interface for PV and batteries with MMC is under test in IIT Delhi (The module was designed and developed) (ii) It has investigated how the MMC can internally handle the power mismatches using simulation models Some scenarios with different power mismatches were simulated to evaluate the MMC performance using key performance indicators and to identify converter deficiencies. (This was done in cooperation with NTNU in a master thesis project). (iii) A first effort towards a general energy management system (EMS) for DERs such as PV arrays and BESSs is made. An advance optimization is under development. The designed interface for PV and batteries will be finished next year in India. The final demo is scheduled for 2022.

Due to the uncertain nature of solar radiation, a steady power flow cannot always be guaranteed in a PV installation. Thus, various energy storage elements, such as batteries, can be integrated for supplying energy when the demand is needed. Centralized grid-connected converters are a matured technology for large PV installations because of its simplicity and relatively low price. However, centralized topologies have high harmonic content, a non-flexible design that can jeopardize the reliability, and low energy harvesting. Distributed grid-connected converter topologies can increase the reliability since multiple inverters are connected to PV cells and increase energy harvesting since every inverter can operate with an independent maximum power point tracking (MPPT). However, distributed topologies are normally installed in small PV installations because of the complexity of the circuit design. Modular multilevel converters (MMCs), which have recently become an established technology for HVDC applications, can facilitate the installation of distributed grid-connected converter topologies in large PV installations. MMC can offer modularity, scalability, low harmonic distortion, and high reliability also to PV applications. MMC has particular advantages for PV applications as each PV string can operate with an independent MPPT to maximize efficiency, and the PV system can easily reach medium voltage level. Several individual lower rated modules with inherent bypass feature in MMC improve the fault-tolerant capacity of the converter. The MMC topology is also very well suited for integrating batteries to the system, where batteries are used to form the dc link of each converter module. The proposed topology in this project is consists of an MMC-based interface for PV systems embedding a BESS. With the batteries in the system, another degree of flexibility is obtained. The SOC of batteries can be controlled by the PV system and the MMC converter.

Funding scheme:

ENERGIX-Stort program energi