Power System Stability in Single Machine System

Abstract

An interconnected power system consists of generating units (including turbine-governor and excitation systems), transmission lines, loads, transformers (including off-nominal ratio and phase shifting transformers), static var compensators and HVDC lines. The size of the interconnection varies but the technical problems are the same. It is not uncommon for some utilities to study systems which have 10,000 buses. As a rule of thumb about one fourth of these buses will have generators connected. At the planning level, after examining the adequacy of transmission capacity, economy of generation etc., the planner would invariably study the stability of the system for a set of disturbances ranging from the worst, such as three phase to ground fault (whose probability of occurrence is rare) to single phase faults which constitute about 70% of the disturbances. There are good production grade programs which simulate the time behavior of the system for such disturbances. There are literally thousands of variables that can be monitored even on a moderate sized system such as a 500 bus system. The planner wants to see if a fault is properly cleared without the system losing synchronism. A system is said to be synchronously stable (i.e., retain synchronism) for a given fault if the system variables settle down to some steady state values as t → ∞. These simulation studies are called transient stability studies. A typical transient stability study with detailed modeling for a 500 bus 100 machine system might take up to an hour.