Most existing theoretical studies of momentum transport focus on calculating the Reynolds stress based on quasilinear theory, without considering the nonlinear momentum flux-hṽ rñũk i. However, a recent experiment on TORPEX found that the nonlinear toroidal momentum flux induced by blobs makes a significant contribution as compared to the Reynolds stress [Labit et al., Phys. Plasmas 18, 032308 (2011)]. In this work, the nonlinear parallel momentum flux in strong electrostatic turbulence is calculated by using a three dimensional Hasegawa-Mima equation, which is relevant for tokamak edge turbulence. It is shown that the nonlinear diffusivity is smaller than the quasilinear diffusivity from Reynolds stress. However, the leading order nonlinear residual stress can be comparable to the quasilinear residual stress, and so may be important to intrinsic rotation in tokamak edge plasmas. A key difference from the quasilinear residual stress is that parallel fluctuation spectrum asymmetry is not required for nonlinear residual stress. V C 2015 AIP Publishing LLC. [http://dx.

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Virtually, all existing theoretical works on turbulent poloidal momentum transport are based on quasilinear theory. Nonlinear poloidal momentum flux -ṽ rñṽθ is universally neglected. However, in the strong turbulence regime where relative fluctuation amplitude is no longer small, quasilinear theory is invalid. This is true at the all-important plasma edge. In this work, nonlinear poloidal momentum flux ṽ rñṽθ in strong electrostatic turbulence is calculated using Hasegawa-Mima equation, and is compared with quasilinear poloidal Reynolds stress. A novel property is that symmetry breaking in fluctuation spectrum is not necessary for a nonlinear poloidal momentum flux. This is fundamentally different from the quasilinear Reynold stress. Furthermore, the comparison implies that the poloidal rotation drive from the radial gradient of nonlinear momentum flux is comparable to that from the quasilinear Reynolds force. Nonlinear poloidal momentum transport in strong electrostatic turbulence is thus not negligible for poloidal rotation drive, and so may be significant to transport barrier formation.

The virtual inductor is widely used in the virtual synchronous generator (VSG) control to attain predominantly inductive impedance and reduce the power coupling. In fact, 'predominantly inductive impedance' is a misleading indicator of decoupling performance. In this study, a comprehensive perspective on virtual inductor is given for the improved decoupling performance of VSG control. A small-signal model is proposed to make a thorough inquiry into the effects of virtual inductor on power decoupling. With the proposed model, the optimal value of virtual inductor is obtained, the physical interpretation of the optimal decoupling is revealed, and the necessity of using virtual inductor is discussed. First, with optimisation of the value of virtual inductor, some negative effects of excessively large virtual inductor on power decoupling can be avoided. The excessively large value of virtual inductor will deteriorate the performance of power decoupling, aggravate the reactive power shortage of the grid and increase the capacity demand of voltage-source converter. Additionally, when the grid inductance is larger than the threshold value, virtual inductor is not needed. After that, the external physical characteristics of VSG when the grid inductance is under that critical condition are analysed. Finally, the theoretical analysis is validated by simulation and experiments.

Virtual synchronous generator (VSG) control is an attractive method to solve the stability issues of future renewables (e.g. photovoltaic [PV])‐dominated power system. Unfortunately, a VSG is prone to transient instability under grid faults, and the existing work ignores the instability issues caused by current limitation and reactive power loop (RPL). To tackle with the problem, this article analyzes the transient stability of VSG considering the impact of current limitation and RPL. It indicates that the roots of transient instability can be attributed to two aspects, that is, (1) the positive feedback and continuous phase angle integration in active power loop (APL) which are caused by the P‐f control structure, (2) the decline of active power transfer limit which is caused by the Qvsg‐vcd‐iod‐Pvsg control structure. Based on it, an optimized VSG control method is proposed to improve the transient stability, by optimizing the design of power loops for VSG. Compared with other stability control methods, the proposed method avoids the use of frequency detection, and it is simple in design and implementation. Finally, the proposed method is validated by the experiments.