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.

Impedance character as well as relevant impedance criterion is important in thesmall-signal stability analysis of multi-sgage dc distributed power systems, where subsystems are integrated. At the same time, we usually approximately simplify a tightly controlled load-converter as constant power load. In this paper, we focus on the impedance character and impedance criterion in two-stage dc distributed system, which is in fact criterion in two-stage dc distributed system, which is in fact inter-dependent dynamical system with both interaction loop and many individual regulation loops. We verify that minor loop gain method can be directly extended to inter-dependent subsystems, while original impedance criterion cannot. Furthermore, we develop a substitutional stability analysis. With this substitutional impedance criterion, we discuss the influence of regulation loop gain, load resistance, input voltage and various duty-cycle ration on system small-signal stability. In additon, we analyze the impedance shift process of the imput impedance of load-converter and build the relationship between it and rgeulation loop gain. Negative rgsistor approximation of load-converter is only regarded reasonable when high regulation-loop gain is assumed and low-frequeney character of load-converter is considered. Simulation results illustrate the validity of the substitutional impedance criterion.

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.

21世纪随着国民经济的快速发展，中国水力发电业将会有巨大的发展，将要安装许多巨型机组。大工率、大尺寸、大重量是这些机组的物点，而为制造出这些机组，生产厂将面临许多技术难题。本文简述了国内水电设备制造业的现状，存在的主要问题，生产这些巨型机组将遇到的技术难题等，文末提出了若干条建议。