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.