Small-xignal stability analysis methods based on an impendance criterion originat from the minor lop gain mehtod and are gradually utilized om twp-stage dc distributed power systems. In this paper, we conclude that the impedance criterion directly dependent on output impedance Z。(s) of the source subsystem and input impedance Zi(s) of the load subsystem is possible but gives and invorrect stability analysis for systems with a regulated source subsystem. Through introducing a mapped pure impedance of the load subsystems and the preliminary system, we develop a general three-step impedance criterion, with which a correct small-signal stability analysis can be guaranteed. regardless of the type of sorce subsystem. Furthermore, we introduce the application of the three-step impedance criterion in two small-signalstability analysis cases and utilize it in an example system to predict the sability shift process arising from the variation on the load resistance and input voltage value.

In order to reduce the vibration of the stator core in the large hydro-generators, the recent tendency is to stack the stator core at site for the hydro-generators with the diameter larger than ten meterws. Owing to the temperature difference between the core and frame of the stator, the frame will restrain the expansion of the core, which will lead to buckling of the stator core in case of high-stiffened frame. But in order to prevent forom the eletrmagnetic vibration the stator frame will be required to have an appropriate supporting stiffness for the stator core. The investigation of the thermal deformation in the generator must be based on comprebensicve analysis of the temperature distrbution within the begnerator under different operation conditions. Inthis paper a method by FEM for calculation of the 3D thermal field within the benerator is applied. Based on the results of the thermal field calculation, the thermal deformation of the stator core and the frame can be calculated. As an example, the thermal field and deformation of a 400MW evaporating cooled generator have been calculated. The comparison of the calculated results with those measured at site proves that the applied method in this paper has apprpiate precision.

After more than 40 years theory research and industry paractice, evaporation-cooling technology has miade great breakthrough at the end of 20th centruy. On December Ist 1999. a 400MW evaporation-cooling hydro-generator was put to use in Chinese power system at lijiaxia power sation of Qinghai province. Industry experience shows that evaporation-cooling technology keeps the advantages of the old ones and overcome the main disadvantages of them. So it has many prospective applications in 21st century.