Power Flow Algorithm Of AC/DC System And Its Interconnected Characteristics

As the technology of power electronics becomes mature, the long-distance and large-capability HVDC transmission system has developed rapidly, making China a mixed power grid with AC system and DC system. On the one hand, with the implement of global power energy internet and continuous attachment of new energy sources, the proportion of DC transmission lines grows a lot, causing the power calculation of AC/DC system and the control strategies change in nature. In the other hand, many scholars pay more attention to the voltage stability of the receiving system since the load has increased a lot. Compared to load bus, the DC transmission lines is unique, causing the analysis of voltage stability of the receiving system reconfirm. In the paper, a power flow algorithm based on minimum Jacobin matrix for AC/DC hybrid systems is proposed, a dominant index is proposed on the basis of Thevenin equivalent parameters and the relationship of the behavior of the receiving system on the HVDC transmission is analyzed. The main research content is as follows.Firstly, on the basis of existing AC/DC power flow algorithms, a power flow algorithm based on minimum Jacobin matrix for AC/DC hybrid systems is proposed. According to the basic equations and the connectivity of various control modes, the current is regarded as a state variable, making the DC transmission unit combine with AC power system organically. The study aims to minimize the dimension of the Jacobin matrix and realize the direct observation of the operation states of the DC transmission unit, providing a basic tool to evaluate or control the AC/DC system. At the same time, compared to the algorithm with all DC variables and the one with none DC variable, the suggested method is more efficient and takes less time.Secondly, considering the previous voltage stability of the node bus, the mechanism of the voltage stability of the receiving system is stated on the basis of reactive power and voltage trait. In order to analyze the interconnected AC/DC system, the inverter and the compensation capacitor are summed as inverter station. With the use of the trait that the power of DC transmission system is the function of the receiving line voltage, new forms of the characteristics of the reactive power supplied by the receiving system and the reactive power absorbed by the inverter station are raised, pointing out that the mismatch of the reactive power is the cause of voltage instability. Considering the control strategy of the DC transmission line, a dominance index is proposed on the basis of Thevenin equivalent parameters, which can be used to compare the stability level of various strategies.At last, the paper concludes the influence on the voltage stability of the AC/DC system, which is caused by the behaviors of the receiving system. The operation modes includes the load change, the adjust of the excitation voltage control system and the compensation capacitor. The paper uses the extended power flow calculation to show the variations of ESCR, the commutation angle of the inverter, the voltage amplitude and the reactive power of the system while the receiving system changes its operation mode. In the end, the factors threatening the security of the inverter are summed and relative control strategies are proposed according to the example.