Virtual Synchronous Machine Control with Adaptive Inertia Applicable to an MMC Terminal

Renewable energy sources (RES) penetration levels are increasing in the power grid. However, it does not have inertia as a traditional synchronous generator, causing a reduction in the inertia and damping in the power grid, impacting the stability during power changes in the grid, causing large frequency deviations. The virtual synchronous machine (VSM) concept has become an attractive solution to emulate the synchronous machine characteristics and supply the inertia and damping property in the system. It consists of emulating the synchronous machine’s static and dynamic properties by power electronic converters and energy storage systems. Nevertheless, the implementation and design of the VSM is a challenge since it must be flexible in the presence of load fluctuations, preventing the oscillations and frequency overshoot from increasing during system disturbances. Hence, the VSM with adaptive inertia has become a potential solution because it provides the inertia and damping factor to the grid according to the load variations and different RES penetration levels in the system. Therefore, the inertia estimation is necessary to use the special techniques that guarantee the balance between the power and frequency response..

Virtual Synchronous Machine Control with Adaptive Inertia Applicable to an MMC Terminal

Renewable energy sources (RES) penetration levels are increasing in the power grid. However, it does not have inertia as a traditional synchronous generator, causing a reduction in the inertia and damping in the power grid, impacting the stability during power changes in the grid, causing large frequency deviations. The virtual synchronous machine (VSM) concept has become an attractive solution to emulate the synchronous machine characteristics and supply the inertia and damping property in the system. It consists of emulating the synchronous machine’s static and dynamic properties by power electronic converters and energy storage systems. Nevertheless, the implementation and design of the VSM is a challenge since it must be flexible in the presence of load fluctuations, preventing the oscillations and frequency overshoot from increasing during system disturbances. Hence, the VSM with adaptive inertia has become a potential solution because it provides the inertia and damping factor to the grid according to the load variations and different RES penetration levels in the system. Therefore, the inertia estimation is necessary to use the special techniques that guarantee the balance between the power and frequency response..

Virtual inertia for suppressing voltage oscillations and stability mechanisms in DC microgrids

Renewable energy sources (RES) are gradually penetrating power systems through power electronic converters (PECs), which greatly change the structure and operation characteristics of traditional power systems. The maturation of PECs has also laid a technical foundation for the development of DC microgrids (DC-MGs). The advantages of DC-MGs over AC systems make them an important access target for RES. Due to the multi-timescale characteristics and fast response of power electronics, the dynamic coupling of PEC control systems and the transient interaction between the PEC and the passive network are inevitable, which threatens the stable operation of DC-MGs. Therefore, this dissertation focuses on the study of stabilization control methods, the low-frequency oscillation (LFO) mechanism analysis of DC-MGs and the state-of-charge (SoC) imbalance problem of multi-parallel energy storage systems (ESS). Firstly, a virtual inertia and damping control (VIDC) strategy is proposed to enable bidirectional DC converters (BiCs) to damp voltage oscillations by using the energy stored in ESS to emulate inertia without modifications to system hardware. Both the inertia part and the damping part are modeled in the VIDC controller by analogy with DC machines. Simulation results verify that the proposed VIDC can improve the dynamic characteristics and stability in islanded DC-MG. Then, inertia droop control (IDC) strategies are proposed for BiC of ESS based on the comparison between conventional droop control and VIDC. A feedback analytical method is presented to comprehend stability mechanisms from multi-viewpoints and observe the interaction between variables intuitively. A hardware in the loop (HIL) experiment verifies that IDC can simplify the control structure of VIDC in the promise of ensuring similar control performances. Subsequently, a multi-timescale impedance model is established to clarify the control principle of VIDC and the LFO mechanisms of VIDC-controlled DC-MG. Control loops of different timescales are visualized as independent loop virtual impedances (LVIs) to form an impedance circuit. The instability factors are revealed and a dynamic stability enhancement method is proposed to compensate for the negative damping caused by VIDC and CPL. Experimental results have validated the LFO mechanism analysis and stability enhancement method. Finally, an inertia-emulation-based cooperative control strategy for multi-parallel ESS is proposed to address the SoC imbalance and voltage deviation problem in steady-state operation and the voltage stability problem. The contradiction between SoC balancing speed and maintaining system stability is solved by a redefined SoC-based droop resistance function. HIL experiments prove that the proposed control performs better dynamics and static characteristics without modifying the hardware and can balance the SoC in both charge and discharge modes

Issues and Challenges of Grid-Following Converters Interfacing Renewable Energy Sources in Low Inertia Systems : A Review

The integration of renewable energy sources (RESs) is a key objective for energy sector decision-makers worldwide, aiming to establish renewable-rich future power grids. However, transitioning from conventional systems based on synchronous generators (SGs) or systems with a low RESs share presents challenges, particularly when accompanied by decommissioning large central generation units. This is because the reduction in inertia and system strength, traditionally provided by SGs, can lead to a loss of essential system support functions like voltage and frequency. While current converter technologies attempt to compensate for the grid support provided by SGs by enhancing converter capabilities, they still heavily rely on the presence of SGs to function effectively. These converters, known as grid-following (GFL) converters, depend on the grid to operate in a stable and secure manner. As the penetration of RESs increases, the efficacy of GFL converters diminishes, posing stability challenges in low inertia systems and limiting the integration of RESs. Therefore, it is crucial to reassess the existing GFL converter technologies, control mechanisms, and grid codes to understand their status and future requirements. This will shed light on the advancements and limitations of GFL converters, enabling greater RESs integration and grid support independent of SGs. This paper aims to provide an up-to-date reference for researchers and system operators, addressing the issues and challenges related to GFL converter technologies, control systems, and applications in low inertia systems. It serves as a valuable resource for facilitating the transition towards future systems with 100% RESs penetration scenarios.© 2024 The Authors. This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/fi=vertaisarvioitu|en=peerReviewed

DC & Hybrid Micro-Grids

This book is a printed version of the papers published in the Special Issue “DC & Hybrid Microgrids” of Applied Sciences. This Special Issue, co-organized by the University of Pisa, Italy and Østfold University College in Norway, has collected nine papers and the editorial, from 28 submitted, with authors from Asia, North America and Europe. The published articles provide an overview of the most recent research advances in direct current (DC) and hybrid microgrids, exploiting the opportunities offered by the use of renewable energy sources, battery energy storage systems, power converters, innovative control and energy management strategies

Adaptive active power sharing techniques for DC and AC voltage control in a hybrid DC/AC microgrid

IEEE Energy Conversion Congress and Exposition (9th. 2017. Cincinnati