THE APPLICATION OF WEINSTEIN-CHIEN′S METHOD——THE UPPER AND LOWER LIMITS OF FUNDAMENTAL FREQUENCY OF RECTANGULAR PLATES WITH EDGES ARE THE MIXTURE OF SIMPLY SUPPORTED PORTIONS AND CLAMPED PORTIONS

In this paper, the method of relaxed boundary conditions is applied to rectangular plates with edges which are a sort of the mixture of simply supported portions and clamped portions, so that the lower limit of fundamental frequency of such plates is evaluated. A kind of polynomial satisfying the displacement boundary conditions is designed, os that it is enabled to evaluate the upper limit of fundamental frequency by Ritz' method. The practical calculation examples solved by these methods have given satisfactory results. At the end of this paper, it is pointed out that the socalled exact solution of such plates usually evaluated by the force superposition method is essentially a kind of lower limit of solution, if the truncated error of series which occurs in actual calculation is considered.

Study of a Single-Phase Series Active Power Filter with Hysteresis Control

The paper presents results from a study of a series-connected single-phase APF （active power filter） with a control system based on hysteresis control, both with and without limitation of the maximum switching frequency. The general purpose of the series APF is to eliminate the low order harmonics of the source （grid） voltage. The filter operation at limited and unlimited maximum switching frequency is explained and it is described using building blocks and time diagrams illustrating the tracking down of the reference curve. Waveforms from the computer simulation and waveforms from the experimental tests of the filter are presented also for the two situations. Operations at limited and unlimited maximum switching frequency are compared regarding the quality of the voltage across the load, the complexity of the implementation of the control system, and the electromagnetic compatibility. The investigation proved capability of the series APF to achieve its general purpose using the hysteresis control methods studied （limited and unlimited switching frequency）.

Frequency Control at the Power Sending Side for HVDC Asynchronous Interconnections Between Yunnan Power Grid and the Rest of CSG

For a large-scale high voltage direct current (HVDC)asynchronous interconnected power grid, the frequency issue atthe power sending side under DC faults is a crucial problem.To solve this problem, based on rotor motion equations, theeffect of unbalanced power on the system frequency under DCfaults is analyzed. The characteristics and dynamic developmentprocess of frequencies after the injection of disturbances areanalyzed. In addition, the actions and coordinated strategies ofvarious frequency control measures are also investigated. Basedon the testing projects of an asynchronous interconnection in theChina Southern Power Grid (CSG), the frequency features arestudied according to the measured PMU data. The outcome showsthat the frequency problem of the Yunnan Power Grid after anasynchronous interconnection can be solved and controlled. Italso shows that the frequency limit control (FLC) is importantfor the frequency regulation of large scale HVDC asynchronousinterconnected DC power grids. As demonstrated, DC FLC caneffectively suppress the deviation of the transient frequency.However, reasonable frequency regulation parameters shouldbe set and other area frequency control measures should becoordinated to maintain the frequency stability of the system.

Resolution beyond classical limits with spatial frequency heterodyning

A technique for coherent imaging based on spatial frequency heterodyning is described. Three images corresponding to three physical measurements are recorded. For the first measurement, a scene is simply illuminated with a coherent beam and for measurements 2 and 3, the scene is projected with cosine and sine fringes, respectively. Due to spatial frequency heterodyning, upper and lower side hand information falls in the pass band of the imager. These bands are separated and correct phases and positions are assigned to these bands in the spatial frequency domain. An extension of bandwidth is achieved in the frequency domain and the inverse frequency domain data then give a high resolution coherent image.

Impact of Frequency Anti-windup Limiter on Synchronization Stability of Grid Feeding Converter

Loss of synchronization is one of the main issues for a grid-feeding converter in a weak grid after being subjected to a large disturbance.The synchronous transient is highly nonlinear due to phase movement and frequency limiters.However,none of the previous research has considered the anti-windup PI in the phase-locked loop,which is commonly implemented in reality and introduced as an additional nonlinear transient.This work provides a taxonomy to evaluate and compare the effect of different anti-windup PI limiters on synchronization stability,including clamping,back-calculation and combined method.Different anti-windup PI limiters allocate zeros and poles differently and have different impacts on damping and stability enhancement.A case study implemented in Matlab/Simulink serves to compare the trajectory of the converter phase and frequency using different anti-windup PI in the scenario of both with and without equilibrium points during the fault.Simulation results show that anti-windup PI limiters increase damping during the fault and thus improve the synchronization stability margin.