Steady state analysis of HVDC grid in the North Sea with offshore wind power plants

Licentiatavhandling, 2014

In this thesis, some of economical and technical aspects of expanding the HVDC (High Voltage Direct Current) transmission systems in the North Sea are studied at steady state. In the thesis, it is assumed that the expansion of the system is made by connecting WPPs (Wind Power Plants) to the system which is built for power exchange. From the economical point view, it is observed that the system operator can gain from the expansion of the system. The gain can vary depending upon the total capacity of the WPPs, the transmission tariff paid by the WPP (Wind Power Plant) operators and the cost of altering the scheduled exchange powers due to the added WPPs. It is also observed that, in most of the studied cases, significant economic gains can be achieved by reinforcing the systems when the total WPP capacities are higher than 60 % of the total possible WPP capacities from a control point of view. Furthermore, in order to simulate the power from the WPPs in the system, which is a function of wind speed, a standard ARIMA (Auto Regressive Integrated Moving Average) based modelling procedure is used. Due to a limitation in the standard modelling procedure to model a time series wind speed data, a modified modelling procedure is proposed which is based on decomposing the data into HF (High Frequency) and LF (Low Frequency) components. In addition, for a 10 minute average wind speed data, a six order AR (Auto Regressive) model structure can be used to model the HF component of the data and a six orderMA(Moving Average) model structure can be used to model the LF component of the data. In order to split the 10 minute average wind speed data into HF and LF components, a cut off period of Tcutoff = 4day was found to be sufficient. Moreover, proposed model structures and the corresponding model parameters that can be used to simulate 10 minute average wind speed data in the Baltic Sea areas, where there are no measured wind speed data, are also presented in the thesis. In addition, a control strategy which is based on local primary controllers followed by a central secondary controller is proposed in the thesis. The proposed control strategy can be used to enable the integration of WPPs into a system designed for power exchange.