On the stability of power systems containing doubly fed induction generator-based generation

This article is concerned with the impact of large-scale wind farms utilising doubly fed induction generators on the stability of a general power system. Inspection of the eigenstructure of the power system provides a foundation for assessing the impact, which is then quantified by means of detailed numerical simulations.     

Improvement of power system stability by using a VSC-HVdc

The capabilities of a VSC-HVdc to improve the stability in power systems are analyzed in this paper. The analysis considers a power system which has the need for increasing the transmission capacity. Two options are analyzed: connection of a new ac transmission line or connection of a VSC-HVdc link. Different disturbances are applied in the system in order to analyze the dynamic response of the system. Supplementary control is included in the control of the VSC-HVdc. The control strategies in the supplementary control are based on nonlinear and linear theory. Furthermore, remote and local information are used as input signals in the control strategies. Simulation results clearly showed the benefits of VSC-HVdc in the improvement of power system stability.     

Multi-terminal VSC HVDC for the European supergrid: Obstacles

For many, the supergrid is seen as the solution that allows the massive integration of renewable energy sources in the European power system. It connects different remote energy sources to the existing grid while offering additional control. It offers balancing through geographic spread and allows a more diversified energy portfolio. In the meanwhile it increases the security of supply.However, technical limitations exist, and it is not yet possible to construct such a supergrid. Several outstanding issues need to be solved. This paper first describes the potential and need for a supergrid. The paper focuses on a meshed, multi-terminal VSC HVDC, and it is explained why this relatively new technology is believed to be the best suitable one for such a grid. The different difficulties or challenges that still exist are addressed. Not only the remaining technical limitations are addressed, but also the techno-economic, control and operational issues are discussed, as well as some regulatory obstacles.     

Active and reactive power control of a VSC-HVdc

Voltage source converter-based HVdc (VSC-HVdc) systems have the ability to rapidly control the transmitted active power, and also to independently exchange reactive power with transmissions systems. Due to these characteristics, VSC-HVdcs with a suitable control scheme can offer an alternative means to enhance transient stability, to improve power oscillations damping, and to provide voltage support. In this paper, a VSC-HVdc is represented by a simple model, referred to as the injection model. Based on this model, an energy function is developed for a multi-machine power system including VSC-HVdcs. Furthermore, based on Lyapunov theory (control Lyapunov function) and small signal analysis (modal analysis), various control strategies for transient stability and damping of low-frequency power oscillations are derived.     

Control Lyapunov functions for controllable series devices

Controllable series devices (CSD), i.e., series-connected flexible AC transmission systems (FACTS) devices, such as unified power flow controller (UPFC), controllable series capacitor (CSC) and quadrature boosting transformer (QBT) with a suitable control scheme can improve transient stability and help to damp electromechanical oscillations. For these devices, a general model, which is referred to as an injection model, is used. This model is valid for load flow and angle stability analysis and is helpful for understanding the impact of the CSD on power system stability. Also, based on Lyapunov theory a control strategy for damping of electromechanical power oscillations in a multi-machine power system is derived. Lyapunov theory deals with dynamical systems without inputs. For this reason, it has traditionally been applied only to closed-loop control systems, that is, systems for which the input has been eliminated through the substitution of a predetermined feedback control. However, in this paper, we use Lyapunov function candidates in feedback design itself by making the Lyapunov derivative negative when choosing the control. This control strategy is called control Lyapunov function (CLF) for systems with control inputs