Offshore wind farm cluster based DC collection network — Operation and design considerations

Models of a DC cluster based wind farm collection network with permanent magnet generators (PMG) and passive rectification are described and implemented to investigate the operability of groups of up to 5 paralleled wind turbines. The presence of a degree of slip between the rotating speeds of the machines and its relationship with the PMG stator inductances is discussed. The results of modelling the system with a single machine, two machines and five machines are given, highlighting the presence of 6th harmonic torque components and interactions between the harmonics produced by adjacent machines, introducing lower frequency harmonic components. The presence of circulating currents between the machines in the cluster when a wind speed change is imposed on one of the machines is discussed, highlighting transient and steady state responses. Finally the impact of increasing the machine branch inductances is studied, which affects the rates of acceleration of the machines in response to a change of the wind speed on one machine.