Mechanism and Mitigation Strategy of Sub-synchronous Oscillation Caused by Grid-connected DFIG with Series Compensation

Power systems are at risk of sub-synchronous oscillation (SSO) when connecting doubly-fed induction generator (DFIG)-based wind turbines to transmission lines with series compensation. This paper focuses on the mechanism and mitigation strategy of SSO. Firstly, the mechanism of SSO is studied from two aspects: induction generator effect (IGE) and sub-synchronous control interaction (SSCI). An equivalent circuit is developed to illustrate the negative resistance effect. Based on the result of the analysis, the SSO mitigation strategy using notch filter and virtual resistance control is proposed. In order to verify the result of analysis and the effectiveness of mitigation strategy, time-domain simulations are carried out based on PowerFactory DIgSILENT.

A Novel Approach for Mitigating Power Quality Issues in a PV Integrated Microgrid System Using an Improved Jelly Fish Algorithm

A two-step methodology was used to address the power quality concerns for the PV integrated microgrid system. In the first step, partial shading was included to deal with the real-time issues. The Improved Jelly Fish Algorithm integrated Perturb and Obserb (IJFA-PO) has been proposed to track the Global Maximum Power Point (GMPP). In the second step, the main unit powered by a DC-AC converter is synchronised with the grid. To cope with the wide voltage variation, an auxiliary unit is connected to the main unit in the opposite phase. This study evaluates various switching approaches to determine the optimal solution for achieving the stated goals. It was found that the IJFA-based SHE in 120 օ conduction gives improved results. A novel series compensation technique has been employed to further eliminate harmonics before grid integration. The proposed IJFA has been used to determine the switching angles for the SHEPWM converter. The objective function and novel series compensation work together to determine the harmonics that should be decreased and kept to a minimum. Three switching angles' performance was equivalent to that of nine switching angles. As a result, higher-order harmonics can be minimised with fewer switches, and switching losses can be lowered without compromising efficiency. The THD of the proposed system was 1.32 percent, which is very much within the tolerable limit of IEEE 1547, IEC, and CIGRE WG 36-05 standards. In terms of efficiency, metaheuristics, and convergence, the proposed system outperformed the existing ones. The model was developed in MATLAB/Simulink 2016b. To verify the simulation results, an experimental prototype of grid synchronised PV capacity of 260W was tested under various loading conditions. The present model is reliable and features a simple controller that provides more convenient and adequate performance for practical reasons.

Stability Domain Analysis and Enhancement of Squirrel Cage Induction Generator Wind Turbines in Weak Grids

There are significant concerns regarding the stability of increased wind power generation in weak power grids. This paper investigates and improves the stability of Wind Turbine Squirrel Cage Induction Generators (WT-SCIGs) with series compensation and weak interconnections to the power grid. Detailed time-domain and state-space modeling have revealed new bifurcations and oscillatory modes for a WT-SCIG connected radially to a weak grid through a series compensated line. The stability domain analyses are carried out by computing bifurcations in the system by analyzing eigenvalues of the linearized system. The analyses demonstrate for the first time how the degree of compensation at which the Hopf bifurcation occurs depends on the X/R ratio of the line, operating slip of the induction generator, and voltage regulator parameters as well as the time delays in measurements. A new damping controller is proposed, which greatly improves the dynamic stability of the WT-SCIG and eliminates destructive Hopf bifurcations in weak grids for a wide range of series compensation. This allows for a much larger percentage of series compensation than what is usually possible, while avoiding instabilities, thereby maximizing the power transfer capability.

Augmented Power Dispatch for Resilient Operation through Controllable Series Compensation and N-1-1 Contingency Assessment

Research on enhancing power system resilience against extreme events is attracting significant attention and becoming a top global agenda. In this paper, a preventive augmented power dispatch model is proposed to provide a resilient operation. In the proposed model, a new N-1-1 security criterion is proposed to select disruptive N-1-1 contingency cases that might trigger cascading blackouts, and an iterative contingency assessment process based on the line outage distribution factor is proposed to deal with security constraints. In terms of optimization objectives, two objectives related to power flow on the transmission line are considered to reduce the possibility of overload outages. Controllable series compensation devices are also considered in the model to improve the power flow distribution. Case studies conducted on the modified IEEE 30-bus, 118-bus and Polish 2382-bus systems show that the power flow solution of the proposed power dispatch model can avoid some branches from undertaking excessively heavy loads, especially lines forecasted to be affected by extreme events. The results of blackout simulations through a hidden failure cascading outage simulation model show that the average power losses of the proposed model are reduced by around 40% in some cases as compared to the classical economic dispatch model.