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Under the low-voltage ride-through (LVRT) of the doubly fed induction generator, extra efforts are required for the model predictive control (MPC)-based direct stator/rotor current controls (DSCC/DRCC) against the stator flux oscillation. Constant stator/rotor currents are maintained at the cost of rotor/stator current oscillations, respectively, due to stator flux oscillation. Necessity to incorporate the stator flux transient in the MPC-based DSCC/DRCC under the LVRT is analysed based on tracking effects of the current references. To quantify LVRT effect with the MPC-based DSCC/DRCC, analytical expressions of the stator current, rotor current and rotor voltage are proposed, whose accuracy is validated based on comparison with the time-domain simulations. On the basis of analytical study, advantages and disadvantages of the DSCC/DRCC are analysed considering impacts of the current oscillations on the security of the rotor-side converter. With the DRCC, rotor voltage increase is suppressed with damping to the stator flux oscillation and rotor current constraint is satisfied without decreasing stator output power; thus, the LVRT effect is desirable compared with the DSCC. The MPC-based DSCC/DRCC is extended to LVRT under the asymmetrical fault to realise effective control to both the positive and negative sequence stator/rotor currents.

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A novel fully hydraulic variable valve system is described in this paper, which achieves continuous variations in maximum valve lift, valve opening duration, and the timing of valve closing. The load of the unthrottled spark ignition engine with fully hydraulic variable valve system is controlled by using an early intake valve closing rather than the conventional throttle valve. The experiments were carried out on BJ486EQ spark ignition engine with fully hydraulic variable valve system. Pumping losses of the throttled and unthrottled spark ignition engines at low-to-medium loads are compared and the reason of it decreasing significantly in the unthrottled spark igntion engine is analyzed. The combustion characteristic parameters, such as cyclic variation, CA50, and heat release rate, were analyzed. The primary reasons for the lower combustion rate in the unthrottled spark ignition engines are discussed. In order to improve the evaporation of fuel and mix with air in an unthrottled spark ignition engine, the in-cylinder swirl is organized with a helical intake valve, which can generate a strong intake swirl at low intake valve lifts. The effects of the intake swirl on combustion performance are investigated. Compared with the throttled spark ignition engine, the brake specific fuel consumption of the improved unthrottled spark ignition engine is reduced by 4.1% to 11.2%.

The terminal-connected series dynamic braking resistor (SDBR) is applied to assist the lowvoltage ride-through (LVRT) of the doubly-fed induction generator (DFIG). With the fault current and switch-in of the SDBR, the stator voltage oscillates, thus the constant stator voltage drop assumption is invalid and the effect of the converter current control is weakened with the changing voltage-oriented reference frame. In this paper, the xy frame of the point of common coupling is applied to the converter control to avoid oscillation of the reference frame. The analytical expression of fault current with the SDBR and constant converter current control is derived. To evaluate the LVRT effect, the analytical analysis of the LVRT transient is carried out. The resistance of the SDBR is optimized based on an index combining the capabilities of the DFIG to provide the active power support and damp the electromagnetic torque oscillation. The uncertainties of the fault scenario are considered in the optimization algorithm by applying the probabilistic method. Simulation results show that the improved LVRT effect of the DFIG is realized with optimization to the SDBR resistance and its switch-in criterion.INDEX TERMS Doubly-fed induction generator, low-voltage ride-through, series dynamic braking resistor, constant current control, analytical fault current expression, fault uncertainties, probabilistic evaluation.