Novos algoritmos para proteção de linhas de transmissão durante oscilações de potência

Abstract

The current technical literature, concerning power system protection, has given emphasis to the study of power swings, especially symmetrical power swings. In such cases, equal or approximately equal effects are observed in the three-phase voltage and current signals. However, asymmetrical power swings can also occur. Under these conditions, the existing unbalance in the three-phase signals is noticeable. During any power swing condition, symmetrical or asymmetrical, the apparent impedance shifts in the RX plane and may enter the operating zones of the distance protection. This is a problem for the distance protection, which can detect a power swing condition as a fault. To avoid this malfunction, the power swing blocking function must block the distance protection during power swings. However, as the distance protection is blocked, fast unlocking is required if a short circuit occurs during power swing. In this case, the unblocking function must detect the existence of the fault and unblock the distance protection, which will clear the fault. It is in this context of complementary functions, blocking and unblocking, that this study is developed, and two novel algorithms are proposed. The first proposed algorithm (ALGA) is developed considering mathematical formulation, widely found in the literature, used to model symmetrical power swings. Therefore, the ALG-A proposed algorithm is limited to this context. The second proposed algorithm (ALG-B) is developed from a novel mathematical formulation, originally proposed by this study, capable of modeling symmetrical and asymmetrical power swings conditions. Therefore, the ALG-B proposed algorithm applies to a broad context, which considers symmetrical and asymmetrical power swings conditions, characterizing it as a multifunction protection method. Common to both proposed algorithms (ALG-A and ALG-B) is the employment of analytical solution, also originally developed in the scope of this study. It is shown that the Fourier filter and Fortescue’s theorem estimates negative sequence phasors during power swings. Then, these phasors can be used to detect power swings and faults during power swings. Among the advantages, it is mentioned that the proposed algorithms do not require complex systemic studies to define parameterization adjustments. In addition, they work with low sampling frequency and use mathematical tools commonly found in protective relays, such as Butterworth filters, Fourier filter and Fortescue’s theorem. Both algorithms are initially validated against test scenarios obtained from the software Matlab/Simulink, where an IEEE-9 Bus electrical system was modeled. The results show that the blocking function worked correctly in all tests performed. The unblocking function also performed well, outperforming two other algorithms available in the literature. When analyzing real cases of power swings, among the proposed algorithms, only the ALG-B proposed algorithm performed fully adequately, corroborating the reliability and safety of this novel method.