The Research Of Under-frequency And Under-voltage Load Shedding And Their Coordinated Control

Mostly as a consequence of the deregulated electricity markets and the growth of electrical energy consumption, power systems are now often operated close to their stability limits. In case of large disturbances, such as generators tripping and faults on transmission lines, the resulting active and reactive power imbalance can eventually lead to simultaneous large frequency and voltage deviation, as well as instability. Thus, there is a higher risk that the overall system may collapse.Conventionally, load shedding schemes—under frequency load shedding(UFLS) and under voltage load shedding(UVLS), are designed independently and they constitute the last line of defense against frequency and voltage instability. However, the traditional load shedding schemes are not capable of dealing with combined instabilities. Since the action of UFLS usually only considers the frequency information, it may have unanticipated, adverse consequences on the voltage. A similar issue affects the conventional UVLS which uses only local voltage magnitude information at the individual buses. Another disadvantage of the conventional methods is that they simply follow a preset rule, such as shedding a fixed percentage of loads when frequency is out of the normal range. This type of rule lacks the flexibility to execute load shedding actions fit for the different type of instabilities.For this purpose, the frequency characteristics of the power system are analyzed in detail in this paper, and it points out that the variation of the power system frequency is closely related to the system inertia constant and the load frequency coefficient. The concept of the power system voltage stability is discussed deeply. The influence of different type of load model on voltage stability is analyzed thoroughly. And effects of UFLS on voltage stability are researched in the principle. Considering the regulating effect of voltage on the load, an improved method for the calculation of power imbalance is proposed based on wide area measurement information from PMU. It calculates the total amount of load shedding for different disturbance online. Taking advantage of the sparsity of adjacency matrix, a kind of new power flow tracing algorithm is proposed combining adjacency matrix with the principle of proportional distribution. It improves the computational efficient and is applied in UFLS to determine the location and the corresponding quantity of the load shedding online. Taking Gansu Longnan power grid as an example, the original load shedding schemes are optimized and the coordination of UFLS and UVLS is realized. And then through the coordination between the load shedding and load intertripping devices, a new load shedding scheme is put forward.