When studying the harm of geomagnetic storms to the power system, the focus is often placed on the influence of transformers reactive power loss (GIC-Q) caused by geomagnetic induced current (GIC) on voltage fluctuation. GIC-Q, as a reactive load, can cause power flow changes, and the small disturbance stability is affected by the power flow distribution before disturbance, so GIC-Q connects geomagnetic storms with small disturbance stability. In this study, the probability distribution function of induced geoelectric fields is established, and the sample data is obtained by sampling. Combined with the GIC benchmark network model, multiple sets of GIC-Q in each substation are calculated and added to the system one by one to study the influence of geomagnetic storms on the oscillation characteristic parameters and establish the risk index of small disturbance instability. The result shows that the risk index value of oscillation instability in the occurrence of geomagnetic storms increases both in the AC system and the AC/DC hybrid system, but the increased range of risk value of local oscillation instability decreases with the addition of DC systems. The research results can provide a reference for evaluating the influence of geomagnetic storms on the stable operation of power systems.

The reactive power loss of transformers (hereinafter referred to as GIC-Q) caused by geomagnetically induced current (GIC) has the characteristics of large total amount and strong fluctuation. As a kind of reactive load added to the system, GIC-Q can cause the change of operating state. The influence of geomagnetic disturbance (GMD) on system stability focuses on the static stability of conventional systems, and its influence on the transient stability of hybrid systems has not been studied. This study establishes the random fuzzy model of induced geoelectric field components and calculates the expected values of critical clearing angle and acceleration/deceleration area. And the transient stability margin is quantitatively analyzed considering the influence of GMD. The result shows that GMD deteriorates the transient stability of the system, and the transient stability margin is the smallest when the wind power access ratio is about 50%. The research results provide a basis for disaster prevention and control of GMD.

The inaccuracy of the geoelectric structure model is the main reason for the low accuracy of the evaluation of the impact of the ground-return current (GRC) of ultra-high voltage direct current (UHVDC) on the AC power grid. Given insufficient raw data of soil resistivity, the Kriging method combined with the original data of magnetotelluric sounding (MT) is proposed to predict the soil resistivity in the unknown area and form the Kriging soil electrical structure model. In view of the high cost and slow speed of large-scale earth surface potential (ESP) calculation, it is proposed to separate the small-scale resistivity mutation region in the Kriging soil electrical structure model from the large-scale model, and use the Dirichlet Neumann (DN) iterative algorithm to calculate the ESP in parallel for multiple sub-regions. Based on the measured data of the bias current of each substation around the Zhalute grounding electrode of ± 800 kV Zhalute-Qingzhou UHVDC transmission project, it is proved that the accuracy of using the data obtained by Kriging method to establish the soil electrical structure model. INDEX TERMS Bias current, Dirichlet-Neumann (DN) iterative algorithm, grounding electrode, Kriging method.

When geomagnetic disturbances (GMDs) occur, the reactive power loss (GIC-Q) caused by geomagnetically induced currents (GICs) flowing through transformers can lead to system instability. Previous studies have been focused on the impacts of GMD on the static stability of conventional systems, while transient stability of hybrid systems left unexplored. In this paper a hybrid system based on wind and conventional energy sources are proposed as a research object. The wind farm output power and GIC-Q are equivalent to the grounding impedance. Considering the uncertainty of GMD, based on establishing the random fuzzy model of induced geomagnetic fields, the expected value of critical clearing angle and acceleration/deceleration area is calculated by using credibility theory. Then the influence of GMD on the transient stability under the different proportions of wind farm output power is analyzed quantitatively by using transient stability margin.Taking the Mengdong power grid under four operation modes as an example, the results show that no matter which operation mode, the transient stability of the system is reduced when GMDs occur. Under heavy loading conditions in summer and when the proportion of wind farm output power reaches 50%, the transient stability of the system is the lowest. The research results provide a basis for disaster prevention and control of GMDs.

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