A flexible software environment for steady-state power flow optimization with series FACTS devices

The current major restructuring of the electricity market and the advances of new technology add new complexities in the power system application software, particularly optimization. Minimization of the impact of a transaction on unrelated thirdparty facilities, or maximization of the power transfer between utilities must now be formulated as new constraints or objective functions. The dispatch and control of generating units, the taps of regulating transformers, mechanically switched capacitors and reactors have been the primary means available for controlling power flows over the years. However, certain applications require frequent operations that result in a rapid wearing out of the switching equipment. Other applications require smooth and continuous real-time control that is impossible with the conventional equipment. The diversity and the volume of the new market based transactions push the network elements closer to their limits. The inadequate line flow control may result in overloaded parts of the networks, while other parts are loaded far below their power carrying capacity. Therefore, the demands for flexibility in power flow control are growing continuously. The FACTS (Flexible Alternating Current Transmission Systems), relying on thyristors, have high speed switching capability that allows for rapid readjustment of line power flows in response to various contingencies. Installation of FACTS devices on key locations in a meshed network changes the effective reactances of the parallel paths and consequently the way that power divides among them. The motivation for this thesis is the need for flexible and expandable power system application software in the continuously changing electricity market. The focus of the thesis is the design and implementation of a prototype solver for power system optimization problems that considers the concepts mentioned above. The primary design goal was to use robust off-the-shelf research and commercial software tools that minimize the required hand coding and increase the code flexibility and maintainability. An automatic differentiation (AD) package is used to generate code for the first-order derivatives and take care of the sparsity pattern. Code generation relieves the program developer of the tedium and inevitable “bugs” normally associated with hand-coding. A general-purpose nonlinear optimization package requiring only first-order derivatives from the user is used for the optimization studies. The thesis concentrates mainly on the description of the optimization software package and the steady-state modeling of the series FACTS devices TCSC (Thyristor Controlled Series Capacitor) and UPFC (Unified Power Flow Controller). The FACTS modeling equations illustrate how the flexibility of the software environment in use allows the easy combination of different objective functions, different sets of variables and different formulations of functions. Case studies demonstrate the operating regions of the series FACTS devices and their effectiveness in increasing the MW power transferability of a particular network.