Investigation into voltage and angle stability of a hybrid HVAC-HVDC power network

This study investigates the power stability problems of HVDC and VSC-HVDC interactions on their hybrid networks with HV AC link, with the intention of bringing out their weaknesses and strengths. The knowledge of this will assist network planners to be informed on ways of improving the efficiency and quality of power systems network. The simulations for this study was done using DIgSILENT Powerfactory software version 14.0.515. This study encapsulates the three major stability problems affecting power systems network, namely, the voltage stability, transient stability and small signal stability. The voltage stability study was conducted using series of load flows at various levels to plot the VP and QV curves, and the results were used to analyze the systems proximity and sensitivity to voltage collapse, as well as the maximum loading point (MPL) of the network. Furthermore, the voltage angle, and terminal voltage responses during a three-phase short circuit disturbance was also used to analyze the voltage stability of the networks. For the transient stability study, several case studies were investigated and their dynamic performances during three-phase short circuit perturbations were analyzed. The small signal stability investigation was done using modal analysis to determine the small signal stability of the three transmission schemes mentioned above. The transient and small signal stability, which are both subsets of rotor angle stability, were further investigated to show the effect of power systems stabilizer (PSS) and automatic voltage regulator (A VR) on rotor angle stability. The results of the analyses show that the HVDC transmission scheme provides the best alternative for bulk power transmission over a long distance. The VSC-HVDC transmission network is suitable for interconnections where the tie with HV AC networks have a low short circuit ratio (SCR). Other conclusions reached with the investigations are explained in chapter ten

Control of fluctuating renewable energy sources: energy quality & energy filters

This doctoral study discusses how to control fluctuating renewable energy sources at converter, unit, and system layers to deliver smoothed power output to the grid. This is particularly relevant to renewable power generation since the output power of many kinds of renewable energy sources have huge fluctuations (e.g. solar, wind and wave) that needs to be properly treated for grid integration. In this research, the energy quality is developed to describe the friendliness and compatibility of power flows/waveforms to the grid, by contrast with the well-known concept of power quality which is used to assess the voltage and current waveforms. In Chapter 1 & 2, a background introduction and a literature review of studied subjects are presented, respectively. In Chapter 3, the problem of determining the PI parameters in dq decoupling control of voltage source converter (VSC) is studied based on a state-space model. The problems of the conventional method when there is insufficient interface resistance are addressed. New methods are proposed to overcome these drawbacks. In Chapter 4 & 5, energy quality and the energy filters (EFs) are proposed as tools to assess and manage power fluctuations of renewable energy sources. The proposed EFs are energy storage control systems that could be implemented on a variety of energy storage hardware. EFs behave like low-pass filters to the power flows. Finally, in Chapter 6, as an application example of renewable power plant with energy filter control and smoothed power output, a master-slave wave farm system is proposed. The wave farm system uses enlarged rotor inertia of electric machines as self-energy storage devices