Comprehensive overview of maximum power point tracking algorithms of PV systems under partial shading condition

Artificial intelligence techniques for stability analysis and control in smart grids: Methodologies, applications, challenges and future directions

PV arrays reconfiguration for partial shading mitigation: Recent advances, challenges and perspectives

Abstract Offshore wind farms (OWFs) have received widespread attention for their abundant unexploited wind energy potential and convenient locations conditions. They are rapidly developing towards having large capacity and being located further away from shore. It is thus necessary to explore effective power transmission technologies to connect large OWFs to onshore grids. At present, three types of power transmission technologies have been proposed for large OWF integration. They are: high voltage alternating current (HVAC) transmission, high voltage direct current (HVDC) transmission, and low-frequency alternating current (LFAC) or fractional frequency alternating current transmission. This work undertakes a comprehensive review of grid connection technologies for large OWF integration. Compared with previous reviews, a more exhaustive summary is provided to elaborate HVAC, LFAC, and five HVDC topologies, consisting of line-commutated converter HVDC, voltage source converter HVDC, hybrid-HVDC, diode rectifier-based HVDC, and all DC transmission systems. The fault ride-through technologies of the grid connection schemes are also presented in detail to provide research references and guidelines for researchers. In addition, a comprehensive evaluation of the seven grid connection technologies for large OWFs is proposed based on eight specific indicators. Finally, eight conclusions and six perspectives are outlined for future research in integrating large OWFs. 

https://pcmp.springeropen.com/track/pdf/10.1186/s41601-022-00239-w

A critical survey of technologies of large offshore wind farm integration: summary, advances, and perspectives

This paper proposes a two-level combined control (TLCC) scheme of voltage source converter-based multi-terminal high-voltage direct current (VSC-MTDC) integrated offshore wind farms to provide frequency support for onshore system. The proposed TLCC scheme consists of two levels, which are the step start-up and adaptive inertial droop control of the offshore wind turbine level, and the communication-free allocation control of the onshore VSC station level. On the first level, each wind turbine adopts the inertial and droop control with adaptive coefficients, and all wind turbines (WTs) work at the maximum power point tracking (MPPT) mode without energy reserve. To reduce the second frequency drop (SFD), the WTs are divided into different clusters according to their rotor speed, and a step start-up control scheme is adopted for the WT clusters to provide frequency support sequentially. On the system level, the communication-free allocation control strategy is proposed using local frequency signal of onshore VSC stations to share the active power among onshore VSC stations reasonably. The proposed TLCC scheme can provide onshore system with frequency support and reduce the SFD simultaneously, while all WTs work at MPPT mode. Case studies are carried out on a 3-area 4-terminal VSC-MTDC based offshore wind farms (OWFs). Simulation results demonstrate the effectiveness and universality of the proposed TLCC scheme under different scenarios.

Two-Level Combined Control Scheme of VSC-MTDC Integrated Offshore Wind Farms for Onshore System Frequency Support

A new type of subsynchronous oscillation (SSO) has been observed recently in double-fed induction generator (DFIG)-based wind farm integrated via voltage source converter-based HVdc (VSC-HVdc) system. However, the mechanism of this emerging oscillation is not entirely understood. In this paper, the impedance models of DFIG with and without considering the phase-locked loop (PLL) dynamics are both derived. Then, the impedance-based simplified equivalent circuit of the multiple DFIGs interfaced with VSC-HVdc system is established. This model can be further represented as the RLC series resonance circuit to quantify the start-oscillating condition intuitively. The theoretical analysis results show that DFIGs behave as an inductance in series with a negative resistance at the resonance point, whose interaction with wind farm side VSC (WFVSC) (regard as a resistance–capacitance) constitutes an equivalent RLC resonance circuit with negative resistance. Therefore, the oscillation tends to occur due to the negative damping. In addition, the impact of various factors including number of grid-connected DFIG-wind turbines (WTs), wind speed, and parameters of PI controllers and PLL on the SSO characteristics is analyzed based on the proposed simplified model. Finally, the correctness of the theoretical analysis is validated by both the time-domain simulation and hardware-in-loop experiments.

/pdf/impedance-modeling-and-stability-analysis-of-grid-connected-a14enrktis.pdf

Impedance Modeling and Stability Analysis of Grid-Connected DFIG-Based Wind Farm With a VSC-HVDC

In Northwest of China, a strategy to transmit the wind-thermal-bundled power from the local grid and doubly fed induction generator (DFIG)-based wind farms through a voltage source converter-based HVDC (VSC-HVDC) can be widely applied. However, since the local grid is usually weak, a new type of electrical oscillation in the medium-frequency region may occur in the sending-end converter (SEC) of VSC-HVDC with PQ-control. The mechanism of this oscillation caused by the interaction among DFIG, local grid and SEC is not entirely understood. In this paper, the sequence impedance model of the sending end converter (SEC) of VSC-HVDC with the PQ-control outer loop and PLL is derived with the explicit analytic expression, and then the oscillation mechanism is explored based on the intuitive analysis of the system impedance frequency characteristics. Compared with the subsynchronous oscillation (SSO) caused by DFIG or power inverter, the theoretical analysis shows this medium-frequency oscillation (MFO) mainly origins from SEC due to its negative damping effect between about 100 to 200 Hz. In addition, the impact of the system controller parameters and operating condition of DFIG, local grid and SEC on the oscillation characteristics is analyzed in detail. Finally, the correctness of the theoretical analysis is validated by time-domain simulation.

/pdf/impedance-modeling-and-analysis-of-medium-frequency-2j16x5wi6j.pdf

Impedance Modeling and Analysis of Medium-Frequency Oscillation Caused by VSC-HVDC Connected to Local Weak Grid and DFIG-Based Wind Farms

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Impedance Modeling and Stability Analysis of Grid-Connected DFIG-Based Wind Farm With a VSC-HVDC

Impedance Modeling and Analysis of Medium-Frequency Oscillation Caused by VSC-HVDC Connected to Local Weak Grid and DFIG-Based Wind Farms