A coordinated strategy for multi-machine power system stability

Owing to the fast growth of the energy demand, the current power system may reach the marginal operating design resulting in unfavourable frequency/voltages conditions, the timeframe of stability and unexpected faults. Therefore, it is necessary to use all capacity without any additional development costs and having a secure and safe system. Hereby, this study addresses a novel coordination strategy for improving low-frequency oscillations in multi-machine models under various operating conditions. To achieve this goal, the proposed solution is divided into three main parts, the first part presents eigenvalues and time-domain analyzes to extract the unstable modes for a nonlinear model of multi-machine to capture a real-world problem. The second part proposes two controllers based on fuzzy theory and thyristor controlled series compensator (TCSC) with power oscillation damper (POD) structure to reach considerable damping of low-frequency oscillations. Since an uncoordinated design between two controllers in the power system aggravated instability, therefore, the last part proposes a modified virus colony search (VCS) to optimally adjust the decision variables with two conflicting objectives based on time and frequency domains. To demonstrate the proposed coordinated strategy, the well-known two-area and four-machine test system have been selected and the obtained results are compared in several loading conditions such as ± 20 load changing, with other available controllers through several analysing indices. According to numerical results, the proposed design can improve the statical indices such as integral of time multiplied absolute value of the error (ITAE) and figure of demerit (FD) about 12 and 11%, respectively. Also, the eigenvalues can be collected between the damping ratio 0.2 and real part -1.0.