Combined stability and electro-magnetic transients simulation of offshore wind power connected through multi-terminal VSC-HVDC

Offshore wind energy has a high potential especially in Northern-Europe. Future wind power plants may be situated further from the shore and therefore a high-voltage direct current connection based on voltage sourced converters is most suitable for grid integration. Connection utilization can be improved by interconnecting several wind power plants leading to multiterminal schemes. This paper describes a modeling approach that facilitates the incorporation of such (offshore) dc-systems into transient stability simulations. It enables the possibility to use a different simulation approach for each side of the converters, i.e. to represent the acside by complex phasors and the dc-side by electro-magnetic transients. Coupling between the ac and dc-sides is obtained by the active power balance. To study the interaction between the multi-terminal scheme and the onshore network an illustrative test-network has been taken. A chopper-controlled braking resistor that protects the dc-circuit against overvoltages has been included and is expressed as a variable resistance. Methods to distribute the wind power among the onshore converters are explored and operation without a supervisory dispatch controller has been studied