No abstract

Voltage source converters (VSCs) are nowadays widely integrated in the power grid, nevertheless they can induce low frequency stability problems under weak grid conditions. The interaction of PLL, dc-link voltage control, and ac voltage control generates a positive feedback which threatens the power system stability. The existing researches mainly focus on modeling strategies and stability analyses tools, however still few studies dealt with active damping in the low frequency range. In this paper, a nonlinear state space model of a VSC is presented and linearized around the operating point. From the model, a linear state feedback control law is designed and incorporated in the dc-link and ac voltage control in order to increase the system damping. Eigenvalue analysis is used to investigate the performance of the proposed controller. The simulation results based on a 2 MW grid connected wind generation unit, clearly show the effectiveness of the proposed solution. Experimental results with a 4 kW scaled-down setup validate the analytic and simulation results.

Low frequency instability phenomena in power electronic based power systems can originate both at converter and at power system level. At the converter level, the interaction between PLL, dc-link and ac voltage control in voltage source converters (VSCs) connected to a weak grid can lead to instability. At the power system level, the interactions among different parallel VSCs can produce oscillatory phenomena, and even result in instability. However, a model to study the instability phenomena at both levels is still under development. In this paper, a scalable VSC state space model, which captures the interactions among PLL, dc-link and ac voltage control is proposed. The proposed model is suitable for interconnection, and an example of power system modeling is shown. The model is then validated through simulations and experimental tests. Eigenvalue analysis is carried out to investigate the influence of the control parameters on the stability.

The increasing penetration of Distributed Generators (DG) in the modern electric distribution network poses high priority on the problem of the stability. In this article the Harmonic Stability of a Smart Transformer-fed microgrid is investigated under different control strategies. The considered microgrid is composed by a Smart Transformer and three Distributed Generators, considering the bandwidth of the DGs unknown. The robustness is evaluated analysing the eigenvalues as a consequence of a variation of the DGs bandwidth. The system is modelled as a Multi Input Multi Output System (MIMO); the eigenvalue based analysis is carried out to assess the stability and compare the robustness of the traditional double-loop PI and a state-feedback (SF) integral controller. The results show that the SF controller ensures a higher robustness than the traditional PI controller with respect to increasing bandwidths of the DGs.

Admittance reshaping is a widely used strategy to address the converters low-frequency stability issues in weak grid, caused by the PLL and its interaction with the dc and ac voltage control. However, the asymmetric control of the d-and q-axis current references and the coupling between the converter ac and dc side restricts the damping capability of Single-Input-Single-Output feedbacks. This phenomena gets even worse in presence of nearby converters. This paper extends the concept of admittance reshaping to Multi-Input-Multi-Output (MIMO) control. A full state-feedback is added to the current reference of the converter to increase the damping of the conventional multi-loop control. A systematic offline algorithm is delegated to design the feedback, and a scalar coefficient is employed to activate/deactivate online the reshaping feedback, making the proposed solution user-friendly. The proposed control is analyzed both in time-and frequencydomain and tested in parallel-operation with other converters, and shows higher damping capability than conventional solutions and good robustness with respect to grid impedance and operating point variations. Experimental tests under ac and dc disturbances are conducted both in lab setup and in Hardware-In-the-Loop.