Steady state analysis of HVDC grid with Wind Power Plants

Doctoral thesis, 2017

The idea of building a multi-terminal, even a meshed HVDC grid in the North Sea, that can inter-connect surrounding nations, is being discussed widely. Such a system is expected to be realised in steps through the interconnection of wind power plants and electricity markets. Hence, the aim of this work is to accomplish steps to realise the idea through modelling, investigating and quantifying economical and the technical aspects of building the grid. Particular attention is placed on quantifying economically optimum sizing of system components and establishing economic connection requirements of the markets. In order to fulfil part of the aim, wind speed modelling procedures, that can be used to simulate temporally and specially correlated time series wind speed, are proposed. A special contribution in one of the modelling procedures is the introduction of frequency decomposition in the procedure. The procedures are later used as inputs to simulate time series wind power which is used in the economical analysis of building the grid in steps. In addition, a steady state model of a control strategy, which is based on local primary controllers and a central secondary controller, is also presented. Moreover, based on the investigated cases, it is verified that the optimum size of a VSC HVDC transmission system, connecting two electricity markets having a pre-defined exchange power data, is approximately equal to the absolute mean of the exchange. The result is determined for a cable length of 300 km and the optimum cable size decreases by about 5% for every 300 km increases in cable length. Furthermore, it is quantified that the investment cost of a VSC HVDC transmission system is 3 €/MWh for a cable length of 300 km. For every 300 km increase in cable length, the investment cost increases by about 2 €/MWh. Furthermore, a minimum slope difference requirement, between the marginal costs of markets, for an economical inter-connection, is established. The established requirement focuses on the cases where the net exchange power between the markets, during some time interval, is close to zero. Based on the studied cases, it is determined that, for a given distance between the markets, the minimum slope difference, that could make a feasible investment, decays exponentially as the size of the system increases. For example, for two markets which are 300 km apart and have a maximum demand of 20 GW each, the minimum angle between the markets should be 0.006. For 100% increase in cable length, the minimum angle also increases by the same percentage.