Large offshore wind farms are usually composed of
several hundred individual wind turbines, each turbine having
its own complex set of dynamics. The analysis of the dynamic
interaction between wind turbine generators (WTG), interconnecting
ac cables, and voltage source converter (VSC) based High
Voltage DC (HVDC) system is difficult because of the complexity
and the scale of the entire system. The detailed modelling and
modal analysis of a representative wind farm system reveal the
presence of several critical resonant modes within the system.
Several of these modes have frequencies close to harmonics
of the power system frequency with poor damping. From a
computational perspective the aggregation of the physical model
is necessary in order to reduce the degree of complexity to a
practical level. This paper focuses on the present practices of
the aggregation of the WTGs and the collection system, and
their influence on the damping and frequency characteristics of
the critical oscillatory modes. The effect of aggregation on the
critical modes are discussed using modal analysis and dynamic
simulation. The adequacy of aggregation method is discussed.