Static Var Generators (SVG) have been widely adopted in renewable power systems to improve voltage stability. The capacity and location of SVG installation in a renewable power system both impact the improvement effect. In this paper, we investigate the optimal SVG configuration that cost-effectively improves the voltage stability of power systems with high renewable energy penetration. First, recognizing the short-circuit ratio as a key and simple indicator of system voltage stability levels, a multi-objective optimization model is established, with objectives of improving the short-circuit ratio of renewable generators and minimizing the overall SVG installation cost. Then, we propose a modified NSGA-II algorithm to solve this model, thus obtaining the number and location for installing the SVGs in a power system. Experiments conducted on the SG118 system, which integrates renewable energy, validate the effectiveness of our proposed method. Our work presents power system operators with an effective measure to maintain the voltage stability of power systems with high penetrations of renewable energy.

Unbalanced faults could inject negative sequence currents and voltages into the power system, which in turn affects the operational quality of the whole system. This paper mainly studies the harmonic characteristics of traditional HVDC (Line Commuted Converter High Voltage Direct Current, LCC-HVDC) transmission system under unbalanced conditions. Analysis results show that when there is a negative sequence voltage on the bus of converter, harmonic voltages of 2ndand 2nthorder harmonics will be generated on the DC side. This paper gives the calculation formula of harmonics in DC voltage, which is verified by simulation. The interesting phenomenon is that amplitude of 2ndand 4thharmonic voltage only depend on the voltage amplitude of negative sequence when the commutation overlapping angle is not taken into account.

The resilience of a power system refers to its ability to resist and recover from multiple physical failures under extreme operation conditions. In this paper, we study the power grid resilience considering different time scales of recovery strategies and propose a method for identifying critical elements whose physical damage can significantly degrade the resilience of a power system. The maximum power supply (MPS) of the remaining network containing the elements that are not physically damaged is obtained and used to indicate the resilience performance of the system. We identify the critical elements in the grid by iteratively selecting and removing the link with the lowest MPS. We do simulations in the IEEE 118 Bus case to evaluate the grid resilience under various extremes and test the proposed strategy. Simulation results validate the efficacy of our proposed method in the identification of critical elements.

Voltage source converters (VSCs) have been increasingly applied in power systems for the past decade, whose interactions with the grid give rise to a variety of complex behaviors. In this paper, we study the low-frequency bifurcation phenomena of weak-grid-tied VSCs considering the phase-locked loop (PLL). The linearized state-space representation of the weak-grid-tied VSC system is firstly obtained, on which the eigenvalue-based analysis is performed. The loci of the eigenvalues show that the weak-grid-tied VSC system loses stability through a Hopf bifurcation and that improper grid parameters-the active power and the equivalent impedance of the AC system seen from the point of common coupling (PCC), can contribute to the occurrence of the bifurcation behavior of the system. Time-domain simulation results validate the correctness of the analysis and conclusions.