Innovative Approaches to Modular Multilevel Converters with Embedded Energy Storage: A Comprehensive Review

### Topology, Control, and Applications of MMC with Embedded Energy Storage: A Brief Review

Abstract

In recent years, the rising energy demand and the extensive deployment of renewable energy resources have significantly increased the need for high-capacity power transmission and energy storage systems in power systems. This has led to the emergence of modular multilevel converters (MMCs) integrated with energy storage systems, referred to as ES-MMCs. These systems leverage the advantages of both MMCs and ES systems. This paper reviews the ES-MMC technology, focusing on electrical topology, steady-state control strategies, common applications, and associated challenges. The analysis includes a comparison of various energy storage interfaces and the techno-economic feasibility of different submodules. Control strategies for distributed ES-MMCs are explored from multiple perspectives, and the paper concludes by discussing the advantages of ES-MMCs over traditional solutions in various application contexts and potential future research directions.

1. Introduction

To combat global warming and reduce reliance on fossil fuels, the development and utilization of renewable energy have become paramount. According to the International Energy Agency’s Electricity Market Report 2023, the increase in renewable energy generation will outpace that of all other energy sources, with an annual growth rate exceeding 9%. By 2025, renewable energy is expected to account for over one-third of global electricity generation. Modular multilevel converters (MMCs) have shown exceptional adaptability to renewable energy sources, such as offshore wind and large-scale photovoltaic generation, due to their flexible control modes, high voltage tolerance, and strong grid stability.

However, high penetration of renewable energy poses challenges to power system stability, including difficulties in power balance and reduced power quality. To address these issues, the ES-MMC has been proposed, significantly enhancing the flexibility of renewable energy transmission systems. ES-MMCs function as flexible AC–DC interface converters with energy storage capabilities, reducing the need for additional investments in energy storage and dedicated power conversion systems at grid connection points.

Recent research has focused on various technical issues related to ES-MMCs, including topology construction and control design. This paper provides a structured overview of the current status of ES-MMC research, with a focus on topology, control strategies, application scenarios, and future research prospects.

2. Electrical Topology of ES-MMC

The ES-MMC topology facilitates efficient power exchange between the AC and DC buses and energy storage units (ESUs). Depending on how the ESU connects to the converter, ES-MMCs can be categorized into centralized and distributed forms. Each form’s connection methods have distinct impacts on the converter’s performance.

2.1. Centralized ES
– 2.1.1. ES Integrated on the AC-Side of Converter: In this configuration, ESUs connect to the AC side of the MMC. The internal parallel connection scheme allows ESUs to connect to the MMC inductors via modular AC–DC conversion arms, providing additional energy storage power without altering the original constraints of the MMC. External parallel connections involve a separate ES system linked to the AC side, which can enhance voltage transformation ratios but may lead to higher construction costs.

• 2.1.2. ES Integrated on the DC-Side of Converter: This approach connects multiple independent ESUs in series to the DC side of the MMC. While this method is straightforward, it can affect overall power quality if one ESU fails. Adding a DC–DC converter can mitigate some issues associated with direct series connections, improving energy storage management and fault tolerance.

2.2. Distributed ES
In distributed ES-MMCs, ESUs are integrated into each submodule, leading to several topology variations based on the submodule types, such as half-bridge and full-bridge configurations. Each design has implications for efficiency, control complexity, and fault tolerance, with several studies highlighting various configurations and their performance characteristics.

3. Control Strategy for ES-MMC

Control strategies for ES-MMCs vary based on the topology. Centralized ES-MMCs can often use independent control for the ES and MMC systems. In contrast, distributed ES-MMCs require coordinated control between AC, DC, and ES power. Key aspects include:

3.1. Control for Single-Stage SMs-Based ES-MMCs: Energy balance-based control methods can effectively maintain SM voltage stability but may sacrifice control accuracy for active power and DC voltage.

3.2. Control for Two-Stage SMs-Based ES-MMCs: The introduction of DC–DC converters allows for more nuanced power control, requiring careful coordination to manage power flow between AC, DC, and ESU ports.

3.3. AC Current Controller: The AC current controller can operate in grid-following or grid-forming modes, with recent research indicating that a hybrid approach combining both may enhance system stability under various operating conditions.

3.4. Modulation Methods: Various modulation methods, including CPS-SPWM and NLM, are utilized to ensure quality output voltage and current, with NLM often deemed optimal for high-voltage DC applications.

4. Applications of ES-MMC

4.1. Active Power Margin for VSC-HVDC Transmission: ES-MMCs are well-suited for high-voltage DC transmission systems, particularly for renewable energy sources located in remote areas. They offer significant advantages over traditional HVAC and LCC-HVDC systems, such as enhanced voltage stability and active power support.

4.2. STATCOM with Energy Storage: ES-MMCs can also function as energy storage-based STATCOMs, providing both active and reactive power support, which is crucial for grid stability, especially in renewable energy-dominated grids.

4.3. Grid-Forming Equipment for Weak Grids: In areas with weak grids, ES-MMCs can stabilize voltage and frequency, allowing for reliable integration of renewable energy sources.

5. Research Prospects

Future research directions include:
– Topology Optimization: Balancing technical feasibility and economic viability in designing ES-MMCs.
– Efficient Control: Developing control strategies that minimize sensing requirements to enhance reliability and reduce component complexity.
– Mathematical Modeling: Establishing comprehensive models for steady-state and transient analyses to understand the interactions of ES-MMCs in power systems.
– Asymmetrical Operation Mechanisms: Investigating the ES-MMC’s performance under asymmetric conditions and developing control algorithms to manage these scenarios.

6. Conclusions

The integration of energy storage into MMC technology represents a significant advancement in providing power support and ancillary services for grids that incorporate large-scale renewable energy. This paper reviews existing topologies, control strategies, and practical applications of ES-MMCs, highlighting their advantages over traditional systems and identifying future research challenges. The findings suggest that ES-MMCs hold considerable potential for enhancing grid stability and power quality in renewable energy integration efforts.