The effects of local and optimized power flow control logic in the reliability analysis of bulk systems

A conceptual algorithm to the reliability evaluation comprises three basic steps: system state selection, analysis of each selected state and updating of reliability indices. Usually, in composite reliability studies, the state analysis is performed by an AC power flow coupled with a remedial actions algorithm, which alleviates possible operational constraints by using rescheduling of system generators, adjustments in voltage profile and taps of LTC transformers, and, as a last resort, load shedding. The best option to provide a fair comparison of different operational violations caused by distinct contingencies is to use an AC optimal power flow (OPF). One example of a very robust nonlinear AC OPF is the interior point (IF) OPF, which can even deal with power flow solvability and voltage collapse problems. In traditional Newton-Raphson power flow algorithms, the terminal and remote bus voltages are obtained by using a local control logic. On the other hand, most of the OPF's utilized in available reliability programs adopts an optimized logic for voltage control, i.e., the voltage level is optimized inside the pre-specified bounds, meaning that new settings can be calculated in order to avoid or minimize load shedding. In this paper, local voltage control logic was introduced into the nonlinear IP OPF of the NH2 reliability evaluation program. The impacts of these two voltage control logic's on the reliability indices are then analyzed using the new IEEE Reliability Test System (RTS-96) and a 89-bus network derived from the North Northeastern Brazilian System.