Power system dynamic security assessment with high penetration of wind generation in presence of a line commutated converter DC link

Traditionally energy has been generated with large synchronous generators. These large plants have characteristics that are well understood and are the basis for the operation of the electricity grid. Most grid codes are based on the assumption that new plant will be composed of synchronous generators. Most of these plants are powered by non-renewable fuels that come with significant carbon emissions. The realisation that there is not an infinite supply of these fuels and their emissions are harming the world’s environment has resulted in policies being implemented aiming at reducing these sources of emissions. This energy is to be replaced with energy from renewable sources. There are many renewable generator types available but wind generation has the highest focus in most countries. As of 2013 there is approximately 318 GW of wind energy installed worldwide. Integrating all of this wind generation into the synchronous power system presents many challenges to grid companies. Wind generation usually does not have the same characteristics as synchronous plant as it is asynchronous. Many of the services that are assumed to be provided by synchronous plant such as inertia or fault contribution are unavailable or come with additional cost. Compounding this wind generation will displace synchronous plant, reducing the system strength further. It is important for grid companies to gain an understanding of the impact of wind generation on the electrical system before the wind integration becomes an issue. Usually when issues begin to arise it is too late to alter existing plant. This means any mitigation of system issues will be expensive or result in an inefficient market. This means that new generators would be required to meet much higher connection standards as there is little system strength left to allocate to the new generators. Ireland has tacked this integration issue by adopting a simple wind integration metric System Non Synchronous Penetration (SNSP) to flag when the system is approaching critical non-synchronous generation levels. This thesis aims to investigate wind generation integration issues in small power systems, in particular ones that are not connected or only weakly connected to other larger grids. It will: • Develop a wind integration metric similar to that used in Ireland or determine application guidelines for the Irish SNSP; • Determine what regulatory approach may reduce the impact of new wind generation minimising the requirement for the integration metric; and • Determine what effect wind generation may have on other plant, particularly those that will not be mitigated by the first two points. For this study the power system of Tasmania is used as the case study. Tasmania is a relatively small (~1700 MW peak load, ~900 MW minimum load) power system connected weakly to the much larger mainland Australia power system via a single HVDC interconnector. This interconnector has a transfer capability of 500 MW into Tasmania and 630 MW out of Tasmania. Additionally this connector is monopolar and can lose all transfer capability in a single fault. This means that during low load approximately half of Tasmania’s generation needs to be able to be tripped at any moment. This is before any response from wind farms is taken into account. Tasmanian generation is predominantly hydro. This type of plant is very flexible. It can be started and shut down very quickly and has no real minimum operating level. This means that when wind generation is high it will tend to shut down rather than operate at a minimum level. This thesis is presented in five sections: Chapter 1. Introduction: This chapter introduces this thesis and its objectives. It also summarises the experiences of other jurisdictions and how they may be similar to the study case. Chapter 2. Mathematical description of a wind plant: This chapter describes a wind plant in mathematical terms, and then it shows how a wind plant responds differently to grid disturbances. Chapter 3. Impact of wind generation on a small power system: This chapter studies the impact of wind generation on the case study power system and investigates how this impact may be mitigated. Chapter 4. Conclusion: This chapter summarises this thesis and explains its outcomes