Design, analysis and control of DC/DC converter based DC wind farms

Abstract

This thesis discusses the design, operation and control of DC wind farms that use high power DC/DC converters, DC cables and DC collection networks. DC wind farms are proposed as alternatives to traditional AC wind farms due to the potential to reduce the system size, improve the speed of dynamic response and improve the system efficiency.

DC wind farms involve different types of high-power DC/DC converters in different stages of power conversion. Isolated DC/DC converters are chosen as the wind turbine converters in which the intermediate transformer design is of great importance. A general and comprehensive medium frequency transformer modelling and design methodology is presented in this thesis, which considers the efficiency, leakage inductance and thermal management. The proposed methodology is applied to transformers for single phase and three phase DC/DC converters.

Isolated Single Active Bridge DC/DC converters are appealing topologies for medium voltage applications. The operation of these DC/DC converters is complex and important for the converter control design. The comprehensive operational principles of three-phase single active bridge converters under changing duty cycle are investigated. Eight operating modes are identified with detailed derivation of power flow and current dynamics. The converter performances are evaluated and compared theoretically and experimentally.

Then, the control of wind turbine converters in DC wind farms is designed considering both DC-link and network dynamics. To deal with the oscillations caused by smoothing reactors, a power system stabilizer based control design is developed and implemented. Furthermore, a multi-variable feedback control design using pole-placement technique is proposed. This method is able to achieve the minimum oscillatory time without compromising the dynamic performance of the DC-link voltage.

Finally, taking into account the low capacitance issue in wind farms, the voltage stability of DC wind farms is investigated and different stabilizing methods are designed and analyzed. The impedance models of aggregated wind turbine converters, DC cables and the station DC/DC converter with control action are derived, in order to study the interactions between the station converter and the DC wind farm. A new equivalent capacitor control strategy to enhance the system capacitance is proposed and analyzed through various case studies.