The premise that the virtual synchronous generator (VSG) can actively support the weak grid is that the VSG can operate stably. In this paper, the small-signal sequence impedance models of the voltage-controlled VSG and the current-controlled VSG are built and the sequence impedance characteristics of these two types of VSGs are compared and analyzed. The sequence impedance of the voltage-controlled VSG, being consistent with the grid impedance characteristics, is generally inductive. By contrast, the sequence impedance of the current-controlled VSG is mainly negative impedance in the middle- and high-frequency areas and the impedance amplitude is quite low. Based on the built sequence impedance models and the impedance stability criterion, the influence of the weakness of the grid on the stability of these two types of VSG grid-connected system is analyzed. The results of stability analysis show that the current-controlled VSG is prone to instability while the voltage-controlled VSG remains stable. Therefore, the voltage-controlled VSG is more suitable than the current-controlled VSG for grid-connected renewable energy generation in an ultra-weak grid from the point of view of system stability. Finally, experiment results validate the stability analysis.

Analogizing the sub-synchronous resonance (SSR) phenomenon of the series-compensated grid-connected system based on synchronous generator, the virtual synchronous generator (VSG) also has the low-frequency oscillation problem owing to the impedance interaction between VSG and series-compensated network. Stability of the series-compensated grid-connected system controlled by VSG is analyzed by the impedance-based Nyquist stability criterion. Due to the inductive impedance of VSG, VSG is easy to couple with the series-compensated network in the low-frequency areas, which can lead to the SSR. With the increase of the series compensation level, the stability of VSG becomes worse and the induced oscillation frequency becomes higher. Therefore, a mod-ulation signal feedforward active damping control strategy with time delay compensation is proposed to suppress this SSR, which directly feeds the active-damping control signal to the PWM modulator for reshaping the output impedance of VSG from the inductance to the resistance-inductance. Moreover, a value range of virtual resistance is given with considering the influence on the regular operation of VSG. Finally, simulation and experimental results verify the correctness of the SSR analysis and the feasibility of the proposed active-damping control strategy.

For the LCL-type grid-connected distributed generation system, the grid-current-feedback active damping (GCFAD) methods have a conflict between the resonance-suppression ability and harmonic-currents amplification. For this, an inverter-current-feedback reso-nance-suppression (ICFRS) method without additional sensors is proposed to reduce resonance-frequency offset and grid-inductance effect due to its unattenuated damping characteristic under high-frequency bandwidth. By analyzing two types of equivalent impedance models of ICFRS and GCFAD with a high-pass filter (HPF), GCFAD can suppress the resonance peak well only with a large resonance-frequency offset and might result in a narrow control bandwidth. However, ICFRS can suppress the resonance peak well with smaller resonance-frequency offset and maintain a high power quality compared with GCFAD. Considering the influence of control delay and grid inductance, the positive/negative critical point of an equivalent virtual resistance of ICFRS derived in the interval of [1/6, 1/3) is obtained, and a robust ICFRS is further proposed to improve system stability by maintaining a positive resistance regardless of grid-inductance variation. The stable-state and dynamic properties of the overall system are analyzed in detail in this paper, and its proper control parameters are selected without a complicated trial. Finally, simulation and experimental results verify the validity of the proposed method.

In this paper, a railway power conditioner (RPC) based on a modular multilevel converter (MMC) with a split supercapacitor energy storage system (SCESS) is studied. In this case, the MMC-SCESS-based RPC could not only provide normal negative-sequence current compensation, but also reduce the impact of power fluctuations caused by the locomotive braking or startup on the electric grid. First, this paper analyzes the power flow patterns and deduces the reference circulating current under different operation modes of the MMC-SCESS-based RPC. Then, the control objectives of the MMC-SCESS-based RPC are divided into two categories—the balance control and the current tracking control. The balance control methods are developed for the submodule capacitor voltages and the state of charge of the supercapacitor, which are associated with the operation modes. To ensure the current tracking performance, a model-predictive direct current control method is presented for the MMC and the bidirectional energy converter. Finally, the effectiveness of the proposed control methods is verified by the experimental results of a downscaled prototype.

Modular multilevel converter (MMC) has become a promising topology applied in medium/high-voltage applications due to its advantages. However, there are several challenges in the MMC, such as the appearance of harmonic circulating current. In this paper, a novel two-stage model predictive control (TSMPC) is proposed to suppress the harmonic circulating current. First, the mathematical model of the MMC is discretized to describe steady-state objectives including output currents, circulating currents, summation, and imbalance of capacitor voltages. Then, an optimal control options set consisting of inserted numbers is built by the front stage of TSMPC, while the inserted number applied to the MMC ultimately is selected from the above-mentioned set by the back stage of TSMPC. The proposed TSMPC can not only improve the control performance of harmonic circulating current but also ensure the overall control performance of the MMC system. In addition, a simplified rolling optimization method is proposed to reduce computation from the aspect of simplifying each control option prediction. An optimized search space method is also proposed to design the control options set to be evaluated from the aspect of ensuring the output current tracking and harmonic circulating current suppression. Finally, the effectiveness of the proposed method is validated by experiment results.

LLC resonant converters are widely used in high power density and high efficiency applications. Practically, the parasitic capacitors of mosfet s and diodes, the stray capacitor of the high frequency transformer, and the dead time will have a great impact not only on the steady state resonant waveforms but also on the zero voltage switching- on (ZVS- on ) of the mosfet s. This article presents a piecewise-approximated time domain analysis of LLC both considering the parasitic/stray capacitors and the dead time. On this basis, not only the accurate steady state resonant waveforms, but also the ZVS- on boundary can be obtained, which will guide the parameter design, and switching frequency/dead time configuration instead of slow and cumbersome simulations. The major applications of the presented analysis, including the resonant/switching waveform illustration, the critical dead time calculation, and the optimal converter design are verified by a 500 W asymmetric half bridge LLC experimental prototype.