Comprehensive preferential load shedding technique

Abstract

Load shedding is an emergency control action designed to ensure system stability by curtailing system load to match generation supply. Load-shedding for preventing frequency degradation is an established practice all over the world. Typically load shedding protects against excessive frequency or voltage decline by attempting to balance real and reactive power supply and demand in the system. If a considerable amount of generation is lost, the only effective way of correcting the imbalance would be to quickly shed loads before frequency falls so low that the power system collapses. Utilities would only resort to load shedding as a final measure and this action has the advantage of disconnecting selected loads for a relatively short period, rather than interrupting all consumers for extended periods. In a power system network, the response of a generator closer to the point of disturbance that creates the imbalance between the load and generation is faster than that of a generator located at a far distant location with respect to the disturbance. At the moment of any disturbance that creates the imbalance between the load and generation, imbalance between the system load and generation is distributed among the generators according to their electrical distance with the load change location. Study will be made to analyze and qualitatively determine the time response of system frequency for load changes at different locations. As the load imbalance distribution changes with time, the imbalance is distributed according to the inertias of generators. Study will be made to analyze and qualitatively determine the effect of generator H constant on the system frequency with sudden change in load. It is expected that initially the impact of load change will be shared immediately by the generators according to their synchronizing power coefficients with respect to the bus at which the load change occurs. Thus, the machines electrically close to the point of impact will pick up the greater share of the load change regardless of their size. After a while it is expected that the imbalance will be shared according to the generator H constant. Furthermore, the size of the load to be shed in an emergency situation is also important to stabilize the power system network quickly. Considering the importance of load size in this respect, loads are also ranked based on their magnitude. Based on the above analysis, a shedding-index will be determined for each load buses depending on the electrical proximity of the loads to generators, the generator inertia constant (H) and the size of the load. The shedding-index shall help to prepare a priority list for shedding loads in a power system to achieve faster frequency stability under a fault condition. The performance of the proposed technique will be verified on a test network.