Investigation of technical uncertainties in the power network with the aim of improving voltage stability and reducing system losses using the SPEA-II optimization algorithm

Optimal reactive power dispatch (ORPD), is one of the most challenging problems of power system operation. This problem deals with both system security and the economy. The problem’s objective functions are usually decreasing the voltage profile deviation of the system, improving the voltage stability margin, and decreasing system loss. ORPD is modeled as an optimization problem with nonlinear functions and constraint and mixed continuous/discrete decision variables. The decision variables are the reactive power of the generators and compensators and the tap number of the transformers. To consider the security of the system, the effect of unscheduled events must be modeled and investigated in the ORPD problem. This type of ORPD is called security constraint ORPD (SC-ORPD). However, the uncertain nature of power systems makes it inevitable to consider them in planning issues. These uncertainties are the source of risk in power grids and must be addressed in planning issues, including ORPD. Risk is the possibility of something bad happening. In other words, risk involves uncertainty about the effects of an activity. Therefore, risk functions include a combination of the probability of an event and its impact. Considering deterministic objective functions (not risk-based ones) in ORPD problems may decrease system economy because most of the events are rare in power system operation. In addition, due to low event probability, the calculated risk may not reflect the true risk that operators and networks are facing. These uncertainties are classified into two categories: technical and economic. Technical uncertainties, which affect the system security, include network topology and operation (like demand and generation) whereas economic uncertainties include micro and macro economies. This paper presents, security-constrained optimal reactive power dispatch by considering the technical uncertainties of power systems. Maximizing the voltage stability margin based on the new voltage instability risk index and minimizing losses are the technical and economic objective functions, respectively. The proposed index is more compatible with practical criteria than existing indicators because it excludes the effect of the low events outage probability of power system components on the risk index. The SPEA-II optimization algorithm is then used to find non-dominant solutions, and finally, the fuzzy decision-maker is used to select the dominant solution. The proposed method is applied to standard 30 IEEE-bus system. The results show, that the proposed algorithm is superior to existing methods and increases the voltage stability margin for postulated contingencies and at the same time reduces operating costs.