Investigation of Voltage Stability of Wind Rich Electric Power Systems

With the increasing penetration levels of wind power generation in power systems around the world, it is imperative to understand the impact of wind generators on power system dynamics and stability. Wind generators have distinct characteristics compared to synchronous generators used in conventional power systems, such as the intermittency and limited predictability of wind resources, and utilisation of asynchronous power generators and power electronic converters, which partially or fully decouple the mechanical and electromagnetic dynamics. Due to these distinct characteristics, wind generators can affect stability of existing networks in relation to voltage stability, frequency stability and rotor angle stability. Through a critical review of existing literature related to these areas, it was identified that there are significant research gaps related to voltage stability in wind rich networks, which is the focus of the work presented in this thesis. The main consequence of voltage instability is the inability to meet the reactive power demand of power systems. Voltage stability is affected by the reactive power support available in a network, where generators are the main reactive power sources. Therefore, with the increased penetration levels of wind power generation in power systems, it is important to investigate the capabilities of wind generators to support the voltage stability. Voltage stability of a power system is usually assessed using steady-state methods such as PV curves, VQ curves and QV modal analysis. These steady-state methods which inherently require less computational capability can indicate the level of stability of a power system. Therefore, VQ curves and QV modal analysis tools are used in the preliminary investigations of voltage stability in wind rich networks presented in this thesis. The importance of accurate representation of the reactive power capabilities of wind generators in voltage stability studies was emphasised in the analysis. Test systems integrated with doubly-fed induction generator (DFIG) based variablespeed wind generators (VSWGs), modelled in DIgSILENT Power Factory, are used to investigate the impacts of voltage control strategy, reactive power capabilities and iii iv loading levels of the VSWGs on steady-state voltage stability. Moreover, the impact of synchronous generators and, DFIG and full-converter wind generator (FCWG) based VSWGs on steady-state voltage stability are compared. Typically, placement of VSWGs in power systems depends on the availability of the wind resources. However, there can be a number of possible connection options available for integration of a wind farm to an interconnected power system. In the next phase, a method is developed which can aid the decision on identifying the optimal location of VSWGs, considering steady-state voltage stability of a network. QV modal analysis tool is used to identify strong and weak areas of a power system in terms of voltage stability. Also, VQ curves are used in the analysis. Voltage stability is a dynamic phenomenon which is affected by the generator and load dynamics, time dependent control actions of power system components such as, excitation system controller, over-excitation limiters (OXLs), under-excitation limiters (UELs), transformers with on-load tap changers (OLTCs), and the response of reactive power compensation devices. Steady-state analysis methods are not capable of incorporating the dynamics of these components and hence, the steadystate analytical methods are not sufficient to accurately evaluate the voltage stability of a network. Therefore, dynamic simulations with accurate dynamic models of power system components have to be conducted to obtain more conclusive results. This led to the examination of dynamic voltage stability of wind rich networks, which is detailed in the thesis. Dynamic voltage instability phenomenon is usually categorised into two types: short-term voltage instability and long-term voltage instability. This classification is based on the different time frames over which the voltage instability is observed, depending on various power system components responding at different time frames. It is important to consider both short-term and long-term voltage stabilities in voltage stability studies. Therefore, this thesis investigates the impact of VSWGs integrated to an electricity network on both short-term and long-term voltage stabilities. Long-term voltage stability is considered in the first phase of the dynamic voltage v stability analysis. Using a dynamic model of a test system developed in DIgSILENT Power Factory, the impact of DFIG and FCWG based VSWGs on long-term voltage stability is investigated. With increasing penetration levels of the renewable power generators in power systems, some of the existing synchronous generators will normally be retired. Therefore, it is vital to note that VSWGs will be replacing some of the existing synchronous generators in power systems. This factor has been extensively considered in the long-term voltage stability analysis presented in this thesis, which investigate the behaviour and the impact of VSWGs to support a network under such conditions. Two common mechanisms which can drive a network to long-term voltages instability are considered, and impact of VSWGs on those types of long-term voltage stability is evaluated. Furthermore, the impact of connection point of VSWG based wind farms and the loading level on long-term voltage stability are examined. In addition, the impact of DFIG and FCWG based wind generators on long-term voltage stability are compared. Conducting complete dynamic simulations on large networks requires considerable time, effort and computational capability. Already well developed direct methods exists, which are alternative methodologies to evaluate system stability without explicitly solving differential equations which represent system dynamics. These methods are generally developed using energy function based techniques. The practical application of most of these methods are limited mainly due to the limitations in application of the methodologies for large interconnected networks which are represented using detailed models. A novel energy function based method is developed in this thesis to examine long-term dynamic voltage stability, which overcomes the aforementioned limitations, using the conventional energy function approach. The application of this method to a large interconnected network is illustrated using Reliability and Voltage Stability (RVS) test system. The application of the proposed method to evaluate the long-term voltage stability of a wind rich network is also demonstrated. One of the main applications of this method could be in online voltage security assessment. vi The last part of this thesis focuses on the impact of VSWGs on short-term voltage stability, which is the second category of dynamic voltage stability. The main driving force of short-term voltage instability is the behaviour of dynamic loads, such as induction motors. Therefore, a test system model which includes VSWGs and significant induction motor loads is used in the studies. Two common mechanisms which can drive a power system to short-term voltage instability are considered, and the impact of VSWGs on short-term voltage stability is examined. Impact of different loading level of VSWGs on short-term voltage stability is also examined. Moreover, the impact of DFIG and FCWG based VSWGs on short-term voltage stability are compared.