Advanced Control Techniques for T-Type Three-Level Converters in Hybrid Power-to-X Applications

Enhanced Control Strategy for Three-Level T-Type Converters in Hybrid Power-to-X Systems

This paper introduces a dual-loop control system specifically designed for three-level three-phase T-type converters, optimizing their performance within the hybrid operations of Power-to-X systems. With the rise of distributed power generation from renewable sources, Power-to-X systems are essential for converting surplus renewable energy into other energy forms, including hydrogen, synthetic fuels, and chemical storage. These can be stored for later conversion back into electricity or utilized in various applications.

Bidirectional converters are integral to the operation of hybrid systems, necessitating efficient and reliable power conversion to ensure stability and performance. The proposed dual-loop control system features an inner current loop for rapid current regulation and an outer voltage loop that maintains stable voltage levels. This design guarantees precise output control of the converter while enhancing responsiveness to dynamic load and generation changes. Additionally, the control system employs a technique to balance the voltages of split DC-link capacitors, addressing a significant challenge in three-level converters. Simulation and experimental results validate the effectiveness of the proposed control system in sustaining high power quality and supporting the hybrid operation of Power-to-X systems.

Keywords: Power-to-X; three-level converter; hybrid system; T-type inverter; resonant control

1. Introduction

The shift towards sustainable energy systems is accelerating the integration of renewable energy sources and advanced energy storage technologies into the power grid. Power-to-X (P2X) systems, which convert surplus electrical energy into various storable forms, including hydrogen, synthetic fuels, or valuable chemicals, are pivotal in this transformation. By enabling the storage and utilization of renewable energy across multiple sectors, these systems contribute significantly to a more sustainable energy landscape.

In a Power-to-Hydrogen (P2H) system, excess renewable generation is utilized to produce hydrogen, which is stored and can later be converted back into electricity using fuel cells to meet demand. This process, termed the hydrogen-to-power (H2P) process, plays a crucial role in efficiently managing energy resources.

Bidirectional DC-to-AC or AC-to-DC converters are fundamental in controlling power flows within hybrid systems, ensuring compatibility between various energy forms and maintaining the stability and reliability of the power grid. Various bidirectional topologies, including T-type, neutral point clamped (NPC), and active neutral point clamped (ANPC), have rapidly evolved in high power and high voltage hybrid systems. Among these, T-type converters are preferred in this study due to their lower conduction losses, reduced complexity, and improved efficiency in comparison to NPC and ANPC topologies.

DC-to-AC converters, or inverters, facilitate the integration of batteries and fuel cells into P2X systems, enhancing energy management flexibility and resilience. Batteries provide immediate response capabilities and high efficiency for short-term energy storage, while fuel cells are well-suited for long-term storage, offering high energy density. Additionally, inverters are key components in the integration of renewable energy into hybrid systems, such as photovoltaic systems connected to the grid.

This research addresses the growing challenge of maintaining power quality and system stability in hybrid P2X energy conversion systems. Current methods often experience steady-state errors, limited adaptability to grid disturbances, and inefficient power flow management. To overcome these limitations, this study proposes a novel dual-loop control strategy that enhances system robustness, improves power quality, and ensures accurate tracking of reference signals, which is vital for the effective integration of renewable energy into the power grid.

2. Literature Survey

For hybrid systems to receive instantaneous power, sinusoidal current and voltage must be injected, necessitating that the controller tracks or compensates for these sinusoidal signals. Traditionally, proportional-integral (PI) control within the synchronous reference frame has been employed to achieve zero steady-state tracking error. However, this approach often results in steady-state errors concerning both amplitude and phase when managing AC signals. These issues have been mitigated through the use of stationary frame resonant compensators, which provide exceptionally high gain at specific non-zero frequencies, effectively minimizing steady-state errors to near-zero levels.

This paper proposes a new dual-loop control configuration for three-level three-phase T-type converters. Existing dual-loop control frameworks typically include an inner current loop and an outer voltage loop, but their efficiency in handling disturbances can vary significantly. Conventional implementations often rely heavily on PI-based controllers, which struggle to maintain robust performance under dynamic grid conditions. Our proposed control strategy enhances these frameworks by incorporating a novel feed-forward (FF) action that improves disturbance rejection, transient response, and voltage regulation stability.

The proposed dual-loop control configuration consists of an inner loop with a two-degrees-of-freedom control structure, incorporating an FF action to cancel disturbances and an outer loop that tracks the inverter output current injected into the grid or an AC load. The outer loop is also responsible for controlling the capacitor voltage. With disturbances cancelled by the FF action, the control challenge for both outer loops reduces to ensuring tracking behavior, thus allowing for the design of tracking controllers based on the time-domain approach of single-resonant controllers.

Another critical aspect addressed in this study is the balancing of DC voltage capacitors, which becomes increasingly complex in three-level T-type converters due to their inherent topology. Unlike two-level converters that utilize a single capacitor for the DC link, three-level converters employ split DC links, necessitating control structures that manage two DC link voltages. This paper introduces a control strategy to address the imbalance problem and implement the dual-loop control setup.

The structure of this paper is as follows: Section 2 discusses three-phase three-level bidirectional AC-to-DC and DC-to-AC converter structures and their roles in P2X systems. Section 3 presents the proposed control structure in detail, including the analysis of DC link capacitors’ voltage balancing. Finally, Section 4 showcases simulation and experimental results validating the proposed methodology.

3. Three-Phase T-Type Bidirectional Converter

Bidirectional multilevel converters consist of DC voltage sources or loads and switching power electronic components (IGBT, SiC MOSFET, etc.) connected to AC loads or sources through filters. This paper focuses on a three-phase T-type filtered converter powered by a DC voltage source, representing energy sources like photovoltaic systems, battery storage, or fuel cells connected via a DC/DC converter. The converter operates as a DC/AC converter connected to an AC load or the grid.

The proposed control maintains stable voltage levels across the capacitor voltages and regulates the current injected from the inverter to the grid or an AC load, enhancing the dynamic response to load and generation changes.

4. Proposed Control Design

The block diagram of the proposed dual-loop control system is outlined, with controllers designed to minimize steady-state errors for the inverter-side inductor current and capacitor voltage reference tracking.

4.1. Feed-Forward Capacitor Voltage Loop

The dynamics of the inverter-side inductor are described using a switching-period-averaged equation. The capacitor voltage is employed as an FF signal for the controller to decouple system dynamics from external disturbances.

4.2. Inverter Side Inductor Inner Control Loop

The objective of the controller for the inverter-side inductor current is to track a reference signal, with a control strategy based on time-domain transient responses proposed to achieve this.

4.3. Output Voltage Outer Control Loop

The plant transfer function relating to capacitor voltage and inverter current is described, with a voltage controller defined to achieve the desired response.

4.4. DC-Link Capacitors Voltage Balancing Control

Effective balancing control is critical to maintaining operational stability, and a strategy to ensure voltage balance between DC-link capacitors is introduced.

5. Verification

5.1. Simulation Validation

Simulations conducted using Powersim software validate the feasibility of the proposed control strategy. The results demonstrate satisfactory tracking performance, confirming the validity and robustness of the controller.

5.2. Experimental Results

An experimental setup for a three-level, three-phase T-type converter was designed and tested. The results show that the proposed control effectively maintains balanced voltages across the DC-link capacitors and achieves precise tracking of reference signals.

6. Conclusions

This paper presents an effective dual-loop control system for three-level three-phase T-type converters in hybrid Power-to-X systems. The proposed control strategy, which includes an inner current loop and an outer voltage loop, has proven successful in managing the complexities of P2X applications. The dual-loop control approach ensures precise regulation of converter output, enhancing the efficiency, stability, and overall performance of hybrid Power-to-X systems.

Future research should explore integrating machine learning-based controllers for improved adaptability and robustness, particularly under dynamic grid conditions. Additionally, further optimizations in hardware and circuit design will be essential for enhancing system efficiency and reducing costs.