Power converter-based resources are susceptible to control interactions, particularly under weak grid conditions. Increasing the DC-link capacitance can help mitigate unstable oscillations. However, in engineering practices, there is a preference for reducing DC-link capacitance to decrease the size and cost of power converters. This study proposes the use of a low-pass filter (LPF)-based virtual impedance in the outer DC-link voltage control loop of a grid-tied voltage source converter (VSC) to enhance its stability in weak grids without using a large DC-link capacitor. Moreover, theoretical calculation results indicate that the virtual impedance can reduce the required DC-link capacitance, and the reduction in weak grid condition is more significant.

The Impedance-based stability assessment (IBSA) methods are widely used to identify control interaction issues between inverter-based resources (IBRs) and transmission grid. The IBR impedance models can be obtained and analyzed in different reference frames and forms, such as positive sequence single-input single output (SISO) and dq-frame multi-input multi-output (MIMO) forms. Accordingly, different IBSA methods should be applied. In this paper, six IBSA methods are reviewed, compared, and their applications are demonstrated. Merits and shortcomings of each method are discussed, and recommendations are presented for engineers and researchers. The IBSA results are compared with electromagnetic transient (EMT) simulations in two test systems involving doubly-fed induction generator (DFIG) and full-size converter (FSC)-based wind parks (WPs).

Impedance-based stability assessment (IBSA) is an effective method to study sub-synchronous control interaction (SSCI) with inverter-based resources (IBRs) in modern power systems. An IBR's required frequency-dependent impedance model can be obtained via the positive sequence scanning method. In this paper, single-sinusoidal signal and multi-sinusoidal signal-based methods for frequency scanning are investigated and evaluated. To benefit from the advantages of both schemes, hybrid scanning is proposed including perturbation signal, amplitude, and frequency segmentation settings. Additionally, a modified quadratic phase shift scheme is proposed for reducing the high magnitudes of multi-sinusoidal signals. All methods are validated and compared using the average value model of a doubly fed induction generator. The guidelines and recommendations are provided for the proper usage of the proposed hybrid scanning in the sub-synchronous frequency range.