Ancheng Xue

Under slow power-flow dynamics, a voltage instability phenomenon induced by low-voltage ride-through (LVRT) control switching rather than by saddle-node bifurcation (SNB) has been observed at a renewable energy station (RES) in China. The phenomenon leads to a large-scale disconnection of RESs. However, the understanding of the patterns and mechanisms of this LVRT-control-switching-induced (LCSI) voltage instability is still insufficient, resulting in potential operating risks for grid-connected RESs. This paper investigates the potential LCSI voltage instability patterns and develops a graphical discrimination method. First, three scenarios involving the triggering of LVRT control switching are identified. Second, a mathematical model of the grid-connected RES system is established. Then, based on the accessibility of equilibrium points, the LCSI voltage instability pattern and the corresponding bifurcation properties in each scenario are revealed. Furthermore, a graphical discrimination method that accounts for both SNB and LCSI voltage instability is proposed. Finally, the existence of three LCSI voltage instability patterns is verified, and the effectiveness and robustness of the proposed graphical discrimination method are validated.

The voltage instability induced by low‐voltage ride‐through (LVRT) control switching may occur in renewable energy stations (RESs) after faults, affecting the safe and stable operation of the power system. However, the post‐fault voltage patterns and practical discrimination methods associated with LVRT‐control‐switching‐induced (LCSI) voltage instability remain insufficiently understood, leading to potential operational risks for grid‐connected RESs. This paper investigates the potential post‐fault LCSI voltage patterns and proposes a graphical discrimination method. First, based on the physical model of LVRT control, five potential post‐fault LVRT control switching scenarios of the RES are identified. Second, an analysis method for the LVRT process based on equal voltage curves (EVCs) is proposed. The mechanisms of the LVRT control switching and voltage stability in each scenario are investigated. Two voltage recovery patterns and three voltage instability patterns are identified, including two additional patterns not reported in the previous studies. Then, a graphical method for distinguishing the LCSI voltage patterns is proposed, and the corresponding identification process is provided. Finally, EMT simulations verify the occurrence of the two additional voltage patterns. The computational efficiency, effectiveness, and robustness of the proposed method are further validated using the modified IEEE 39‐bus system and a simplified provincial‐level power system.

The mechanism of oscillations that occur in the system connected to renewable energy sources is complex, generally including negative damping oscillation, limit induced oscillation etc. Combining with the dynamic characteristics of the low‐dimensional system after limit working, this paper analyses the mechanism of a type of limit induced oscillation that occurs when the output of the DC voltage outer loop continuously touches the limit for single‐machine infinite‐bus system of permanent magnet synchronous generator with positive damping under a large disturbance. Specific study as follows: firstly, a non‐smooth state space model of single‐machine infinite‐bus system of permanent magnet synchronous generator with nonlinear state limit (clarified as a Filippov system) is established. Secondly, the small disturbance characteristics of the system are analysed when the limit does not work. Thirdly, the piecewise dynamic description of the system is analysed with the limit working, and the reason that the final contraction of the system to an eight‐dimensional manifold is explained based on mathematical derivation and physical characteristics. Finally, the characteristics of equilibrium point of the system in a low‐dimensional manifold are analysed when the limit continuously working, revealing that the existence of a pair of conjugate unstable eigenvalues in the low‐dimensional system is the reason of system oscillation.

The phasor measurement units (PMUs) are the eyes for the dynamic of the power system. However, the measured phase angle data from PMUs for a transmission line (TL) may coexist with different deviations, i.e., the deviations in voltage/current phase angle difference (DVPAD/DCPAD) between different ends of the TL, and the deviations in power factor angle (DPFA, i.e., the deviations in phase angle difference between voltage and current at the same end), which threaten the advanced application with PMUs. This article proposes a correction method for the phase angle data when the DVPAD/DCPAD and DPFA coexist for the TL with unknown line parameters. Specifically, first, the abnormal voltage phase angle differences and power factor angle anomalies are discussed, and the corresponding illustrative examples are presented. Second, the mathematical model for estimating phase angle deviations in a general case is presented. Third, a “two-step” method to correct the phase angle data of PMU for the TLs with unknown parameters is proposed, in which the DVPAD/DCPAD between two ends is estimated in advance, and then, the DPFA is estimated. Finally, simulation and examples with field data verify the feasibility and engineering applicability of the proposed method.

Errors in voltage and current transformer and compensation algorithms may cause errors in local voltage and current phase angle measured by phasor measurement units (PMU), whose difference is defined as the power factor angle (PFA). Consequently, the error of local PMU voltage and current phase angle difference, i.e., power factor angle deviation (PFAD), can cause reactive power measurement deviation, which leads to reactive power measurement inconsistency of symmetrical double-circuit lines, deteriorating the application of dynamic monitoring. In this paper, a three-step correction method for PFAD of double-circuit lines is proposed based on the operation characteristics and reactive power of double-circuit lines:1) estimate the difference between PFADs of different lines at the same end; 2) estimate the difference of PFADs at both ends of one line of the double-circuit lines; 3) estimate the local PFAD. The method does not require line parameters and can realize the correction of PFAD at each end of the double circuit line. Finally, the effectiveness and robustness of the proposed method are demonstrated by the simulation results using both simulated and field data.

The voltage stability issue of renewable energy stations (RES) cannot be ignored. In this paper, for the Voltage Oscillation Instability phenomenon caused by the Low voltage ride through during High active power output of the RES with No faults (VOILHN), works on both theory and criterion are conducted. In theory, it is explained that the VOI-LHN corresponds to a High active power output and Low voltage ride through control induced Non-smooth Bifurcation (HLINB). Furthermore, based on theoretical derivation and actual power grid parameters, the boundary between HLINB and Saddle Node Bifurcation (SNB) is obtained. The boundary suggests that in actual power grids, wind farms are more prone to HLINB than SNB. In criterion, a criterion for the occurrence of HLINB is developed, and a threshold calculation method for the criterion is proposed.