This paper presents a control method for modular multilevel converters (MMCs) as an interface between renewable energy sources and the grid. With growing penetration of renewable energy sources in the power grid, the developments in converter technologies and controller designs become more prominent. In this regard, dynamic and steady state analysis of the proposed model for an MMC use in a renewable energy based power system are provided through dc, 1st, and 2nd harmonic models of the converter in dq reference frame. This detailed configuration is then used to accomplish converter modulation and controller design. The first novel contribution of this control method is to provide an accurate pulse width modulation (PWM) strategy based on network and converter parameters, in order to achieve a stable operation for the interfaced MMC during connection of renewable energy sources into the power grid. In addition, the proposed method is able to mitigate the converter circulating current by inserting a second harmonic reference in the modulation process of the MMC, which is the second contribution this paper provides over other control techniques. A capacitor voltage balancing algorithm is also included in this control method to adjust each sub-module (SM) voltage within an acceptable range. Finally, converter's maximum stable operation range is determined based on the dynamic equations of the proposed model. The functionality of the proposed control method is demonstrated by detailed mathematical analysis and comprehensive simulations with MATLAB/Simulink.

This paper presents a dynamic model of modular multilevel converters (MMCs), which are considered as an effective interface between energy sources and the power grid. By improving the converter performance, appropriate reactive power compensation is guaranteed. Modulation indices are calculated based on detailed harmonic evaluations of both dynamic and steady-state operation modes, which is considered as the main contribution of this paper in comparison with other methods. As another novelty of this paper, circulating current control is accomplished by embedding an additional second harmonic component in the modulation process. The proposed control method leads to an effective reduction in capacitor voltage fluctuation and losses. Finally, converter’s maximum stable operation range is modified, which provides efficiency enhancements and also stability assurance. The proficiency and functionality of the proposed controller are demonstrated through detailed theoretical analysis and simulations with MATLAB/Simulink.

This paper presents an analytical dynamic model of a modular multilevel converter (MMC) in grid-connected operating mode. The proposed model is then efficiently used to design a circulating current controller. The proposed controller can accurately study the dynamic and steady-state performance of the converter through harmonic evaluations. Based on this configuration, the modulation is accomplished so that the stable performance of the converter in the network is obtained, which is considered as the foremost novelty of the proposed control method over existing control methods. In order to mitigate the circulating currents, the proposed controller inserts an estimated second harmonic component into modulation indices, which is considered as the second novelty of the proposed control method. The functionality and capability of the proposed control method are validated using detailed theoretical analysis and simulations with MATLAB/Simulink.

This article presents an enhanced control strategy for voltage source converters (VSCs) based on virtual inertia concept. Considering the significant share of renewable-based generations, frequency stability is noted as one of the most significant challenges of modern low-inertia power systems. Therefore, as the first contribution of this article, dynamic equations of the converter are utilized in order to study its proficient active and reactive power compensation capabilities. Then, this effective power factor correction of the proposed VSC is improved by adding inertia emulation characteristics in order to provide a more synchronous generator (SG) like behavior, which is beneficial from the frequency stability point of view. In addition to that, the impacts of changes in the main control parameters such as inertia and damping on dynamic performance are analyzed using the root locus method. This applied eigenvalue analysis has then been used to support a proper selection of parameter values, which is considered as another novelty of this article. In this regard, the designed algorithm providing auto adjustment capability of controller parameters based on a desired frequency response characteristic can improve the dynamic frequency stability of the system. The functionality of the proposed controller is validated through state-space analysis and simulations with MATLAB/Simulink. The superior performance of the proposed algorithm demonstrated through simulation results confirms that the proposed control method can be considered as a simple yet effective solution for the challenges of a reduced inertia power system.

This paper presents an automatic parameter adjustment technique for synchronous power controllers (SPC) in order to improve the dynamic frequency stability of the low inertia power systems. The proposed control method is based on a novel transfer function in which various grid and converter controller parameters involved in the stability studies have been thoroughly included. As the main contribution of this paper, the eigenvalue trajectory of the proposed transfer function has been determined, considering the variations of both virtual inertia and damping control parameters simultaneously. Furthermore, any corresponding operating point on this eigenvalue trajectory can be specified based on the desired frequency response characteristics of the system. Therefore, the inertia and damping coefficients of the converter controller can be simultaneously adjusted through the proposed controller algorithm as the second contribution of this paper. Also, as another novelty of this paper, it is demonstrated through analytical and theoretical studies that both damping ratio and natural frequency characteristics of a dynamic frequency response have profound effects on the controller parameter value adjustments. Simulation results have been employed in MATLAB/Simulink to confirm the performance of the proposed controller regarding the appropriate parameter value adjustments, development of the desired dynamic frequency responses, and the prominent interactions between the frequency response characteristics and the converter controller parameters.