Wind power penetration is rapidly increasing in today's energy generation industry. In
particular the doubly-fed induction generator (DFIG) has become a very popular option in wind
farms due to its cost advantage compared with fully rated converter-based systems. Wind farms
are frequently located in remote areas far from the bulk of electric power users and require
long transmission lines to connect to the grid. Series capacitive compensation of DFIG-based
wind farm is an economical way to increase the power transfer capability of the transmission
line connecting the wind farm to the grid. For example a study performed by ABB reveals that
increasing the power transfer capability of an existing transmission line from 1300 MW to 2000
MW using series compensation is 90% less expensive than building a new transmission line.
However a factor hindering the extensive use of series capacitive compensation is the
potential risk of subsynchronous resonance (SSR). The SSR is a condition where the wind farm
exchanges energy with the electric network to which it is connected at one or more natural
frequencies of the electric or mechanical part of the combined system comprising the wind farm
and the network and the frequency of the exchanged energy is below the fundamental frequency
of the system. This oscillatory phenomenon may cause severe damage in the wind farm if not
prevented. Therefore this book studies the SSR phenomenon in a capacitive series compensated
wind farm. A DFIG-based wind farm which is connected to a series compensated transmission line
is considered as a case study. The book consists of two main parts: Small-signal modeling of
DFIG for SSR analysis: This part presents a step-by-step tutorial on modal analysis of a
DFIG-based series compensated wind farm using Matlab Simulink. The model of the system includes
wind turbine aerodynamics a 6th order induction generator a 2nd order two-mass shaft system
a 4th order series compensated transmission line a 4th order rotor-side converter (RSC)
controller and a 4th order grid-side converter (GSC) controller and a 1st order DC-link model.
The relevant modes are identified using participation factor analysis. Definition of the SSR in
DFIG-based wind farms: This part mainly focuses on the identification and definition of the
main types of SSR that occur in DFIG wind farms namely: (1) induction generator effect (SSIGE)
(2) torsional interactions (SSTI) and (3) control interactions (SSCI).
