Abstract
DC subgrids consisting of modern active loads (ALs) and local dc distributed generation (DG) units are normally interfaced with the main ac grid by utilizing bidirectional voltage source converters (VSCs). Under the very weak grid (VWG) conditions, the integration of voltage-oriented controlled (VOC) VSCs in the inversion mode becomes very challenging and therefore undamped oscillations in the power and angle responses are yielded. Most of the existing works address this issue for VSCs in the inversion mode of operation. However, integration of VSCs in the rectification mode with the consideration of the outer loop controllers into the VWGs has not been reported. To fill up this gap, a state-space model of the bidirectional VSC-to-weak grid (VSC-WG) system is developed in this work with an emphasis on the rectification mode of operation. A modal-sensitivity analysis is then utilized to evaluate small-signal stability of the system, identify the dominant modes, and investigate the system states that have a major influence on these modes. The results reveal two pairs of unstable complex modes that are correlated with the dynamic interaction between the VOC-based VSCs and the VWG impedance. It is also shown that the stability margin of VSCs in the rectification mode is less than that of the inversion mode under the same VWG conditions. To enhance the integration of the VSCs in the rectification mode, a dual-active compensation (DAC) scheme is proposed to mitigate the instabilities under VWG conditions. Several time-domain simulation results are presented to verify the validity of the small-signal model and demonstrate the effectiveness of the DAC scheme under the VWG conditions. Finally, hardware-in-the-loop (HIL) real-time experimental results are presented to validate the simulation results.
Highlights
Power electronic converters (PECs) are used to interface many types of clean and renewable resources of energy such as photovoltaic panels and fuel-cells to the ac power grid registering them as distributed generations (DGs) [1]
A dual-active compensation (DAC) scheme is proposed by which, two linear functions of the voltage source converters (VSCs) output voltage are added to the ac voltage control (AVC) and dc voltage control (DVC) loops to change the distribution of the system eigenvalues in the complex plane and stabilize the system at the nominal power without compromising the system dynamics
Simulation Results A series of time-domain simulations are carried out on the nonlinear model of VSC-to-weak grid (VSC-weak grid (WG)) system to verify the results that are obtained based on the small-signal model that is developed in the previous sections
Summary
Power electronic converters (PECs) are used to interface many types of clean and renewable resources of energy such as photovoltaic panels and fuel-cells to the ac power grid registering them as distributed generations (DGs) [1]. Λ4-5 relocation is limited to the LHP, while λ2-3 cross the jω axis and the system becomes unstable at Pac = 0.87 pu This shows that first, the stability of the VSCWG system at the nominal ac power is mostly influenced by the dominant low-frequency (range) eigenvalues and highfrequency (range) eigenvalues in the rectification mode and the inversion mode, respectively; second, the stability of the system in the inversion mode is slightly better than the TABLE I PARTICIPATION FACTOR OF THE STATES IN THE DOMINANT MODES. The instability can be avoided only under the condition that the DVC dynamics are extremely compromised which is not desired To mitigate this issue, a DAC scheme is proposed by which, two linear functions of the VSC output voltage are added to the AVC and DVC loops to change the distribution of the system eigenvalues in the complex plane and stabilize the system at the nominal power without compromising the system dynamics. Since vod is constant and voq is zero in the steady state, the compensators have zero effects on the steady-state operation of the VSC according to (9) and (10)
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