Sustainable Wind Power Systems: Recent Advancements in AC/DC Collector Grids, and High-Voltage DC (HVDC) and Low-Frequency AC (LFAC) Transmission Systems

Dear Colleagues,

The integration of large-scale wind generation systems into the existing power grids continues to grow, as an increasing number of onshore and offshore wind farms are being installed to meet the demands of clean energy. For transmission grids in wind power systems, traditionally high-voltage AC systems have been used; however, with the flexibility and controllability that power electronic systems offer and their competitive costs, high-voltage DC (HVDC) are becoming more popular, especially as the wind farms become larger. For collector grids, which collect the power from wind turbines before sending it to an offshore/onshore substation, AC collector grids have been the primary choice of technology. However, as DC systems and their power electronic-based components gain more popularity while their cost continues to decrease, there is an interest in the use of DC systems either in medium-voltage DC (MVDC) or HVDC for collector grids. A DC collector and transmission grid system is sometimes referred to as an all-DC grid.

The purpose of this Special Issue is to provide a platform for authors and the scientific community to share their recent findings on the topics of interest, which include, but are not limited to, the following:

• All technical, societal, and environmental aspects of offshore and onshore wind power systems.
• AC and DC wind power systems design, control, analyses, and operations.
• MVDC and HVDC collector and transmission grids.
• High-voltage power electronics systems with applications in wind power systems, such as modular multilevel converters (MMC), solid-state power substations (SSPS), and high-voltage high step-up DC-DC converters.
• DC circuit breakers, and protection of DC systems.
• Addressing grid integration issues, controls, and stability.
• Application of artificial intelligence (AI) and machine learning (ML) in wind power.
• Open dataset and data analytics for wind power operation.
• Automation aspects of wind generation systems.

We are seeking both original research as well as review papers and look forward to receiving your contributions.

Dr. Omid Beik
Dr. Qiuhua Huang
Guest Editors

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• AI and ML in wind power
• control and stability
• DC circuit breaker
• grid integration
• high-voltage power electronics
• HVDC systems
• MMC converters
• SSPTs
• wind generation systems

Title: High-Voltage Wind Energy Conversion System Using Hybrid Generator and Modular Multilevel Converters: State-of-the-Art
Authors: Muhammad Owais Manzoor; Omid Beik
Affiliation: Dept. of Electrical and Computer Engineering, North Dakota State University, Fargo, ND, USA
Abstract: The wind energy conversion systems (WECS) have experienced substantial growth, with current wind turbines approaching close to 20 MW capacity. This upward trend has led to research and development in transformation of the WECSs where low-voltage back-to-back converter configurations will be replaced by high-voltage converters. This evolution represents a crucial development in WECS technologies enabling more efficient and powerful wind generation systems. In this paper a high-voltage WECS is introduced where the conventional generator is replaced with a high-voltage multiphase hybrid generator (HG) integrated with a passive rectifier, and the back-to-back converters are replaced with a modular multilevel converter (MMC) as the grid-side converter. In this configuration the HG controls the dc-link voltage in the wind turbine, while the MMC controls the active and reactive power flow in a grid forming mode (GFM). The paper provides, (i) a detailed modelling of the proposed WECS, (ii) proposes a control system for the HG that controls the dc-link and the maximum power point tracking (MPPT) for the wind turbine, and (iii) proposes a control strategy for the MMCs that maintains a balanced voltage across the capacitors at all wind turbine operating point, i.e., from cut-in to cut-out speed.

Title: Comparison of Grid-Forming and Grid-Following Modes for High-Voltage Wind Turbines and MMC-based HVDC Wind Energy Systems
Authors: Mahnoor Fatima; Omid Beik
Affiliation: North Dakota State University
Abstract: The high-voltage DC (HVDC) systems, particularly for offshore wind farms, have gained much attention. Compared to AC wind farms, the HVDC systems may be less exposed to conventional stabilities issues, however, they may still be susceptible to small-signal instabilities due to negative interactions that may occur during integration with weak grids. One of the solutions to handle these interactions at the wind turbine level is the grid forming mode (GFM) of the grid-side converters (GSCs) that interface the wind turbines to the wind farm 3-phase collector grid. The existing wind turbines, however, use the reference values from collector grids, and hence the GSC operates in grid-following mode (GFL). To enable GFM a fixed DC-link voltage between the back-to-back converters in wind turbines is required, which may be implemented by using batteries in wind turbines. In this paper a high-voltage wind turbine conversion system based on a hybrid generator is introduced that enables regulating the DC-link voltage and facilitates implementation of GFM control for the GSC without use of batteries. Similarly, the existing practice in HVDC modular multilevel converters (MMCs) is to use GFL mode. Given the proposed wind turbine conversion system this paper proposes a GFM control for the rectified-based as well as for the inverter-based MMCs. The MMC-inverter controls the voltage and frequency while MMC-rectifier is controlled to maintain a constant HVDC voltage. Comparison of GFM and GFL modes show that HVDC-MMCs in GFM mode show tolerance to weak grids and small signal instability.

Title: Stability Analysis via Impedance Modelling of a Real-World Wind Generation System with AC Collector Grid, and LLC-Based HVDC Transmission System
Authors: Muhammad Arshad; Omid Beik; Muhammad Owais Manzoor
Affiliation: Dept. of Electrical and Computer Engineering, North Dakota State University, Fargo, ND, USA
Abstract: This paper studies stability of a real-world wind farm, Bison Wind Generation System (BWGS) in the state of North Dakota in the United States. The BWGS uses an AC collector grid rated at 34 kV, and a symmetrical bipolar high-voltage DC (HVDC) transmission grid rated at 250 kV. The HVDC line transfers a total power of 0.5 GW, while both the rectifier and inverter substations use line-commuted converters (LCCs). The LCC-based rectifier adopts constant DC current control to regulate DC current, while the inverter operates in constant extinction angle control mode to fix DC-link voltage. The paper develops, (i) a detailed model of wind turbines (WTs) in the BWGS that use permanent magnet synchronous generator (PMSG) in both time-domain and direct-quadrature (d-q) domain, (ii) develops the impedance-model of the AC collector and LCC-based HVDC grids (rectifier and inverter), (iii) develops models to study the influence of commutation overlap and the control parameters of phase lock loop (PLL) on the overall system stability, (iv) study the possible instability within the collector system and investigate resonance between wind farm and the LCC-based rectifier, and (iv) explores the impact of varying delays and filtering on impedance, providing valuable insights for the design and verification of an AC collector and LCC-based HVDC transmission control systems. The stability of the overall system is assessed through the utilization of Bode plot analysis and the Nyquist stability criterion. The developed analytical impedance model of BWGS is verified by performing a frequency scan simulation in PSCAD/EMTDC.

Title: Multilevel Middle-Point Clamped (MMPC) Converter for Wind Turbines in an All-DC Wind Generation System
Authors: Awais Karni, Omid Beik, Mahdi Homaeinezhad
Affiliation: North Dakota State University
Abstract: This paper proposes a multilevel middle-point clamped (MMPC) DC-DC converter for wind turbines (WTs) in all-DC wind generation systems. The proposed WT topology includes, (i) a generator with two sets of 3-phase output windings, (ii) two sets of passive rectifiers that each converts one set of the generator 3-phase output, and (iii) two proposed MMPC converters connected in series that take the two rectified output of passive rectifiers, and step them up before connection of WT to a medium voltage DC (MVDC) collector grid. The MMPC converter consists of two legs, each leg having five series switches, and two series clamping diodes that form a 3-level topology. The middle point of the clamping diodes is tied to a middle point of two series capacitors on the DC link. Along with reduced voltage stress across switches and reduced output ripple, the proposed converter offers the capability to form modular structure which facilitates the high voltage applications of DC-DC converters in wind power systems. In the proposed WT conversion system as the wind velocity varies from cut-in to cut-out the rectified output of the generator varies from 6.5 kV – 18.5 kV. The MMPC converter incorporates a series inductor per leg to enable the step-up functionality for the converter. Further, the paper proposes a DC switching scheme for the MMPC converter that is based on a space modulation diagram (SMD) in a space vector modulation domain. The effectiveness of the proposed converter is verified and validated with mathematical derivations and simulation results.