The paper reviews the role of impedance relaying in modern transmission line protection systems The speed and reach of the first zone of an impedance relay are identified as being the most important attributes of a relay The paper develops a mathematical model of the relaying process in the presence of transient phenomena which accompany faults It is shown that there is an uncertainty associated with the impedance estimated by a relay and this uncertainty depends upon the speed with which these estimates are made A quantitative relationship between the relay speed and its reach is obtained

Limits to Impedance Relaying

The first-swing stability of synchronous generators (SGs) has a close relationship with synchronizing ability of the grid and inertia characteristic of all the generators. This paper studies inertia characteristic of doubly fed induction generator (DFIG) based wind turbine (WT) under transient control to analyze its impact on the first-swing stability of SGs. First, an analytic model of DFIG-based WT following the concept of motion equation is developed to describe and explain its inertia characteristic. The foundation is the definition of internal voltage of DFIG. Based on the model, an important conclusion is drawn that DFIG-based WT under transient control during grid fault has adjustable inertia characteristic that has a clear link with control parameters. Then, the impacts of transient control parameter of DFIG-based WT on the first-swing stability of SGs are studied through numerical simulation. Finally, the impact mechanism of transient control of DFIG-based WT on the first-swing stability of SGs is clearly explained by physical concept of inertia characteristic and synchronizing force.

Inertia Characteristic of DFIG-Based WT Under Transient Control and its Impact on the First-Swing Stability of SGs

Results of studies exploring the effects of early valve actuation on the stability of a nuclear station and a fossil-fired station are presented. The studies were performed on multimachine systems using a detailed representation of steam turbines. The sensitivity effects of the valve actuation times were investigated and the importance of such an investigation is demonstrated.

A study of early valve actuation using detailed prime mover and power system simulation

It is important to develop new laboratories for teaching and research in order to keep up with the advancement of modern power engineering1,2. This paper presents salient features of a "micro-machine" dynamic power system developed for teaching Power System Dynamics. It also has been used to develop new stabilizers to improve both dynamic and transient stabilities of power systems.

A dynamic power system model for teaching and research

Fast valving of steam turbine-generators is a method of rapidly reducing the mechanical power output to assist in preventing instability (loss of synchronism) of the steam turbine-generator with the rest of the power system. Although rarely used, fast valving can be a very effective stability aid [l]. Many operating turbines made by the authors' company have fast valving capabilities. This paper describes the current fast valving capabilities of these turbines, both reheat types and straight condensing types, and illustrates some of the benefits for system stability that can be obtained with these capabilities. Planning considerations for fast valving are also discussed, including some advanced fast valving concepts.

Fast Valving with Reheat and Straight Condensing Steam Turbines

The trend in turbine-generator characteristics as unit sizes become larger is in an adverse direction from the standpoint of power system stability. Results of specific studies as well as a general rule of thumb for evaluating this effect are given. The effectiveness of independent pole switching and other control measures available to the system planner are shown.

C. Concordia

Papers

Effects of Trends in Large Steam Turbine Driven Generator Parameters on Power System Stability