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Increase in doubly fed induction generator (DFIG)-based wind farms degrades the short-term frequency regulation of power systems. However, such wind farms may have large amount of kinetic energy which can be rapidly injected into the power system to support system frequency by using an appropriate supplementary control loop. This study first analyses the impacts of DFIGs and their supplementary loop on power system short-term frequency regulation. Then, the average power system frequency model is modified to include the participation of wind farms in frequency control. Moreover, a new method is proposed to derive an analytical expression for minimum frequency of a power system, as an important index of frequency regulation, after a power imbalance occurrence. This analytical expression provides a tool for better insight into frequency behaviour of power systems with high levels of wind generation. The results of the analysis are verified by simulation of the nine-bus test system, using MATLAB/SIMULINK.

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In this paper, coupling between active and reactive powers in conventional direct power control (DPC) strategies is analyzed and a new direct DPC method for doubly fed induction machine without rotor position sensors is presented. Coupling analysis is done on an improved DPC strategy with rotor flux controllers in the stator reference frame. The presented control strategy is done by controlling the rotor flux in the grid flux reference frame. The rotor flux command is calculated using a predicted stator flux, the stator current command, and the stator resistance. Moreover, the rotor position is estimated by comparing measured and estimated values of the rotor current. Furthermore, to reduce the method's sensitivity to the parameter inaccuracies, the mutual inductance of the machine is updated during the machine operation by the error between the magnitudes of the measured and estimated values of the rotor current. Index Terms-Component, direct decoupled power control (DDPC), direct power control (DPC), doubly fed induction generator (DFIG), doubly fed induction machine (DFIM), model reference adaptive system (MRAS), sensorless.NOMENCLATURE Ψ s , Ψ r Stator and rotor flux linkage, vectors. V s , V r Stator and rotor voltage vectors. I s , I r Stator and rotor current vectors. ω 1 , ω r , ω slip Synchronous, mechanical, and slip speeds. R s , R r Stator and rotor resistance. L ls , L lr Stator and rotor leakage inductance. L m Machine mutual inductance.

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A new non-isolated high-voltage gain three-port converter for standalone photovoltaic systems is proposed. The magnetic element of this converter is only one coupled inductor. The primary winding of the coupled inductor is shared between battery charger circuit and main converter. Leakage inductance energy of the coupled inductor is either transferred to battery or is regenerated via the passive-clamp circuit. Using switched capacitor and voltage lift techniques, the voltage gain is significantly increased for both low-voltage ports. Single magnetic element, high-voltage gain with reasonable duty cycle, lowvoltage stress on the switches, low winding turn ratio of coupled inductor and high efficiency are the merits of this converter. The operation principles and the steady-state analysis are described for the three modes of: single-input single-output, single-input dual-output and dual-input single-output. To verify the theoretical analysis, a laboratory prototype with 28 V input, 48 V battery voltage and 380 V output voltage is implemented and tested.