Loadflow

The word loadflow or power flow computation is self explanatory: the flows of power in the network are computed. Later on we will see that the name is rather ‘misleading’ as first the node voltages in the network are computed before the power flows can be calculated. The loadflow is the most important network computation, as it brings insight in the steady-state behavior of the power system. Hereunder, some of the applications of the loadflow are described.

When a transmission line is taken out of service temporarily, for planned maintenance for example, the power originally flowing through the transmission line will find itself a new path to the loads. The operators want to be certain, in advance, that other transmission lines and/or cables in the vicinity are not overloaded after taking the particular line out of service. A load- flow computation of the network configuration in which the transmission line is taken out of service, gives insight into the new power flows and indicates possibly overloaded connections or components.

After a blackout has paralyzed a part of the network, the utilities put a lot of effort in finding the cause and want a reconstruction of the phenomena that eventually caused the blackout. A loadflow calculation gives the state of the network for a certain steady-state situation. A black- out is a longer process - of multiple more or less steady-state situations - in the course of time, so a consecutive series of loadflow computations gives information about the system behavior preceding to the blackout.

The node voltages in the network should be kept within close limits, during normal operation between 1.1 and 0.9 pu. A loadflow computes the voltages in the network and visualizes the effect of tap changing transformers, capacitor banks and load shedding on the voltage profile in the system.

The system operation should be robust and therefore the power system is operated n-1 secure. This means that a system component may fail without overloading other components or with- out violating the voltage limits. A list with transformers, transmission lines, cables, generators, and so on, is available from which the components are taken out of service in a simulation one by one, and each time a loadflow is computed. When a loadflow calculation shows an over- loaded connection or transformer, preventive actions can be taken in the real network to pre- vent that particular situation. This analysis, based on a large number of loadflow computations, is called the contingency analysis. The contingency analysis is performed in the control centre of the utilities on a regular time basis.