Impact of Grid Connection of Large-Scale Wind Farms on Power System Small-Signal Angular Stability

The grid connection of a large-scale wind farm could change the load flow/configuration of a power system and introduce dynamic interactions with the synchronous generators(SGs),thus affecting system small-signal angular stability.This paper proposes an approach for the separate examination of the impact of those affecting factors,i.e.,the change of load flow/configuration and dynamic interactions brought about by the grid connection of the wind farm,on power system smallsignal angular stability.Both cases of grid connection of the wind farm,either displacing synchronous generators or being directly added into the power system,are considered.By using the proposed approach,how much the effect of the change of load flow/configuration brought about by the wind farm can be examined,while the degree of impact of the dynamic interaction of the wind farm with the SGs can be investigated separately.Thus,a clearer picture and better understanding of the power system small-signal angular stability as affected by grid connection of the large-scale wind farm can be achieved.An example of the power system with grid connection of a wind farm is presented to demonstrate the proposed approach.

Small-signal Angular Stability of Power System as Affected by Grid-connected Variable Speed Wind Generators—A Survey of Recent Representative Works

This paper reviews the status and progress of the investigation on power system small-signal angular stability as affected by grid-connected variable speed wind generators(VSWGs).The review is carried out on the basis of a survey of recently published representative papers.Strategies of the investigation made in those selected papers are classified into two groups:1)VSWGs displacing synchronous generators(SGs);2)VSWGs simply being added in a power system.The diversification of the results of the investigation is highlighted in the survey.Careful analysis on two strategies of the investigation is conducted in this paper.It is revealed that in the strategy of VSWGs displacing SGs,there are two factors which could affect power system small-signal angular stability differently:1)Withdrawing the SGs’dynamics;2)adding the VSWGs’dynamics.In the strategy of adding VSWGs in the power system,there are also two affecting factors:1)Change of load flow brought about by the VSWGs;2)dynamic interactions with the SGs introduced by the VSWGs.Hence diversified results of the investigation obtained so far are not only due to the dependence of the investigation on sample power systems used,but also caused by the mixture of different affecting factors.This paper is concluded with a summary of key issues of the investigation for future work.

Single-layer angularly-stable bandpass frequency-selective surface with interdigital resonator

A miniaturized periodic element for constructing bandpass frequency selective surface(FSS)independent of incident angles and polarizations is presented.An interdigital resonator(IR)with one extending finger to connect the two separate parts of the interdigital capacitor is explored to achieve parallel resonance.The equivalent circuit model(ECM)and electric field distributions are introduced to explain frequency performance of FSS.The whole structure has only one layer and possesses a low profile(a thickness of 0.0015λ,where λ represents the resonant wavelength in free space)as well as a small size(0.03λ×0.03λ).This FSS performs as a spatial bandpass filter which exhibits a great angular stability with incident angles ranging from 0° to 80° for both transverse electric(TE)and transverse magnetic(TM)polarizations.As an example,a prototype of one proposed FSS is fabricated and tested.The measured results show a good angular stability.

Underactuated spacecraft angular velocity stabilization and three-axis attitude stabilization using two single gimbal control moment gyros

Angular velocity stabilization control and attitude stabilization control for an underactuated spacecraft using only two single gimbal control moment gyros （SGCMGs） as actuators is investigated. First of all, the dynamic model of the underactuated spacecraft is established and the singularity of different configurations with the two SGCMGs is analyzed. Under the assumption that the gimbal axes of the two SGCMGs are installed in any direction, and that the total system angular momentum is not zero, a state feedback control law via Lyapunov method is designed to globally asymptotically stabilize the angular velocity of spacecraft. Under the assumption that the gimbal axes of the two SGCMGs are coaxially installed along anyone of the three principal axes of spacecraft inertia, and that the total system angular momentum is zero, a discontinuous state feedback control law is designed to stabilize three-axis attitude of spacecraft with respect to the inertial frame. Furthermore, the singularity escape of SGCMGs for the above two control problems is also studied. Simulation results demonstrate the validity of the control laws.

Switchable Electromagnetic Absorber/Reflectors Using Back-Loaded Active Radio Frequency Circuits

In this paper,three switchable electromagnetic absorbers/reflectors using back-loaded active radio frequency circuits are presented.In the first design,the incident waves are received by an antenna array,transmitted to the back-loaded active circuits and then reflected into the free space when the PIN diode in series in the circuit is off.When the diode is on,the incident waves can be absorbed by a matching load.In the second design,the phase of the reflected waves in the unit cell can be manipulated by adjusting the length of the microstrip lines,allowing the reflection polarization to be tuned.In the third design,by integrating RF switching chips into the back-loaded circuits,a switchable absorber/reflector with a switching polarization state is presented.The first design is fabricated and measured.The profile of the switchable absorber/reflector is approximately 0.14.A total of 2 PIN diodes are used per unit cell for both TE and TM polarizations.By using active circuits,flat in-band reflection performance and stable absorption performance can be achieved in the frequency band of 7 to 18.5 GHz(FBW of 90%)for normal incidence,and an angular stability of 45°for TE and TM polarizations is realized.