ISSN 1905-7873 Â© 2012 - Maejo International Journal of Science ...

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14 Maejo Int. J. Sci. Technol. 2012, 6(01), 12-27 Figure 1. SEIG-STATCOM system supplying the load Figure 2. Control scheme for STATCOM The control scheme is based on direct control of the source current, which consists of in-phase I sd and quadrature I sq components. I sd is the current drawn from SEIG to maintain the DC bus voltage by charging (or discharging) the DC bus capacitor. I sq is the reactive current required to maintain the SEIG terminal voltage. To regulate the DC bus voltage, the error signal for the PI controller is obtained from the sensed DC bus voltage V dc and its reference value V dc . The output of this PI controller is taken as I sd . The 3- reference in-phase current i sdabc is obtained by multiplying I sd with the sinusoidal in-phase unit voltage template u sdabc . To regulate the SEIG terminal voltage, the error signal for PI controller is obtained from the amplitude of SEIG voltage V sm and its reference value V sm . The output of this PI controller is taken as I sq . The 3- reference quadrature current i sqabc is obtained by multiplying I sq with the sinusoidal quadrature unit voltage template u sqabc . The total reference source current i sabc is taken as the sum of i sdabc and i sqabc . The sensed and reference source currents are processed in a rule based carrier less hysteresis band current controller to generate gating signals (S a , S b , S c ) for IGBT of CC-VSI of STATCOM.
Maejo Int. J. Sci. Technol. 2012, 6(01), 12-27 15 DESIGN METHODOLOGY The STATCOM is designed considering that the VAR rating of STATCOM is fixed, the source voltage is completely sinusoidal and the pulse width modulation (PWM) inverter operates in linear mode. The VAR rating of STATCOM, (VAR) STATCOM , is estimated using the capacitance needed for providing the rated voltage at no load (C NL ) and that needed for a stable operation of the induction motor while being loaded to its rated capacity (C FL ). These capacitances are computed using sequential unconstrained minimisation technique (SUMT) in conjunction with Rosenbroack’s direct search method [23]. The problem formulation is briefed in Appendix I. The (VAR) STATCOM is computed as 2 1 1 (VAR) STATCOM = 3V ab (1) XC X FL CNL and STATCOM line current I st is calculated as ( VAR) STATCOM Ist (2) 3V ab The reference DC bus voltage Vdcr of the voltage source converter of STATCOM depends upon the AC voltage. The STATCOM does not provide adequate compensation during the transient condition of low V dcr , whereas high V dcr may stress the devices. The V dcr is calculated using the peak supply line voltage V m [15] as V dcr = (1.2-2.0)V m , Vdcr Vm (3) The DC bus capacitor C dc stores energy and maintains the DC bus voltage with small ripple. The C dc is calculated using an energy balanced equation characterised by appropriate response time t and actual DC bus voltage V dca : C dc V 3V ab I st t 2 2 dcr Vdca The AC filter inductance L f is decided by the allowable ripple in the compensation current [20]. L f is calculated by assuming a linear mode operation (modulation index m=1) and a switching frequency f s of 10 kHz: 3 Vdc 2 Lf (5) 6af K s rp where the range of factor a (transient current) =1.2-2.0 and K rp (peak-to-peak ripple) =0.05-0.1. The ratings of IGBT suitable for medium rating and high frequency operation are decided as follows: Vdev (1 KL Kt ) Vm (6) I 2 K ( K 1) I dev s rp st where the ranges of K L (factor for filter inductor drop), K t (factor for transient voltage) and K s (factor of safety) are taken as 0.05-0.1, 0.1-0.2 and 1.25-1.50 respectively. (4)
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