Figure 3 . DFIG based WECS Schematic.
2.3. Synchronous Condenser
An 8 MVA synchronous condenser (SC) has been used to provide a voltage
reference for stable operation of the windfarm. It also has an added
advantage of increasing system inertia, short circuit current, short
term overload and low voltage ride through (LVRT) capabilities. In terms
of modelling, the SC model is simply a synchronous machine with no
mechanical input and a static excitation system [23]. Excitation
limiters have also been included in the excitation system to obtain a
near accurate response. Excitation limiters included and modelled are
overexcitation limiter (OEL) [24], volt/hertz limiter (V/Hz)
[24] and underexcitation limiter (UEL) [25].
2.4. Dump Load
A dump load has been used to absorb the excess energy produced by the
windfarm. A model similar to [26] has been used in this study. 8
three phase resistors, in series and controlled by ideal GTO switches,
are varied in steps of 17.5 KW from 0 to 4.4625 MW in steps of 17.5 KW.
They are connected in series and controlled by ideal GTO switches.
System frequency has been used as the input to control the switches that
in turn control the resistor banks.
2.5. Transformer
Different sizes of the 33/132 kV collector point step-up transformer
have been considered in the study to investigate the transformer
energization capabilities of the hybrid windfarm. Fig. 4 shows the high
frequency model of up to 100kHz proposed by [27] suitable for
transformer energization studies. Description and derivation of the
parameters have been discussed in detail in [28]. The model includes
saturation effect with hysteresis, short circuit impedance incorporating
skin effect, eddy current losses and coupling capacitances.