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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.

The COVID-19 pandemic and the Ukraine conflict have underscored the need for affordable, reliable, and flexible energy systems. These events have made it more challenging to achieve the United Nations' sustainable development goal of ensuring access to affordable, sustainable, and modern energy for all. However, some positive outcomes have been, such as the increased share of renewables in the electricity generation mix during the pandemic. This presents an opportunity to accelerate the transition towards renewable energy. Solar photovoltaic (PV) energy, in particular, has a significant role in society's electrification. PV solar has become increasingly affordable, with a 70% cost decline over the past decade. It is now competitive with fossil fuel power generation. PV solar offers flexibility and can be used for various applications and scales, from small residential installations to large-scale solar farms. It is a clean energy option that helps mitigate energy-related carbon emissions. Alongside wind power, PV solar has the potential to lead the decarbonization of the global energy system. However, scaling up PV solar presents challenges that require technological innovations, supportive policies, and grid infrastructure upgrades. As PV solar becomes more prevalent, challenges arise regarding its intermittency, uncertain generation, and non-dispatchable nature. Energy storage technologies can address these challenges by storing excess energy produced by solar panels and releasing it when needed, balancing supply and demand effectively. This helps make solar power more reliable and useful for the grid, reducing dependence on traditional energy sources and contributing to a more sustainable and efficient energy system. Commercial solar power plants are connected to the grid using power electronic interfaces, which convert the DC electricity generated by solar panels into AC electricity. Different configurations, such as centralised, string, multi-string, and micro-module, exist for connecting PV with the grid. Each configuration has its advantages and disadvantages in terms of scalability, energy harvesting, reliability, and cost. The integration of PV generation with batteries into the grid relies on power-electronic interfaces. The PVBESS project focused on the integration of PV generation with batteries into the grid using a promising interface which is the modular, multilevel converter (MMC). This converter allows for the connection of PV arrays in individual submodules, enabling customized maximum power point tracking (MPPT) for each array. The MMC has the remarkable capability to generate medium-high voltages without the need for transformers and offers excellent control over internal power flows. These control features are crucial for managing power mismatches that can arise between the submodules, such as when varying irradiance levels result in unequal power production. Moreover, the MMC is well-suited for integrating the batteries. The PVBESS project aims to provide theoretical and experimental evidence to evaluate the performance of this conceptual configuration. The project encompasses the following key activities: (i) Design and test a suitable interface for PV and batteries with MMC at IIT Delhi, where the modules were designed and developed. (ii) Investigating the MMC's ability to handle power mismatches internally through simulation models. Various scenarios with different power mismatches were simulated to evaluate the performance of the MMC, utilizing key performance indicators and identifying any converter deficiencies. This work was carried out in collaboration with NTNU as part of a master's thesis project. (iii) An energy management system (EMS) for distributed energy resources (DERs), including PV arrays and BESSs. A holistic algorithm was used in the experiments. (iv) Partial integration of the interfaces with a multilevel converter that was done in IIT. The PVBESS project was giving a step forward in the integration of new converter technologies, in specific the multilevel converters for connecting Photovoltaic (PV) cells embedding with Batteries. We obtained improvements both in the hardware and the software, but this concept needs further R&D with industrial partners to step forward in the technology readiness level (TRL). More specifically, it is necessary to finish the integration of all the pieces that were made in the project and gather more evidence that this concept can assist in the scaling up of solar energy.

WP1: Selection of reference configurations and test cases. The objective was to select a suitable MMC power converter topology and interfaces for grid integration of PV systems embedded with a BESS. Outcome: A report with the selection of the interface for connecting PV and batteries was done in WP1. Several configurations were explored, but the dual active bridge (DAB) was selected for simplicity, efficiency, and controllability. All the specifications were selected in the step. Moreover, some test cases were also formulated and included in a master thesis that was part of the project. WP2: Control design of proposed configuration. Here the objective was to develop control strategies for power sharing and voltage balancing in PV cells and batteries. Outcome: Two levels of controllers were proposed. One at the cell level (Submodule control) and the other at the converter level (Energy management control). Submodule control has been tested in the laboratory with satisfactory results in India. Energy management control at the converter level controls using holistic/optimization algorithms were tested in Norway using simulations with the test cases proposed in WP1. The control details are included in a conference paper. WP3: Demonstration of proposed configuration. Here the objective was the experimental validation of the control algorithm in laboratory conditions. Outcome: The integration was made into two parts: In India, the experiments were done using an H-bridge converter since MMC was not available. Some results can be found in the publications. In Norway, the integration was made using lab-validated MMC models since DAB bridges were not available. The experimental part is included in the journal paper. WP4: Technical and economic feasibility assessment. The objective was to check the operation boundary analysis of the power converter at the system level, and the potential of the power converter for providing ancillary services. Outcome: The converter can provide a variety of ancillary services including controlled active and reactive power injection. The focus was the unbalanced generation of PV since this is probably the most challenging aspect of the concept. The results can be found in the journal paper that was part of PVBESS. Outcome: The dissemination for a wide audience was done in a final workshop and a blog.

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