In Chapter 3 a control scheme for a three-phase PV inverter has been designed and analyzed. A linear model of the LCL filter is presented. An active damping scheme for the inverter is proposed and described. This method uses feedback of both the inverter-side current and the
filter capacitor current to damp the resonance of the LCL filter. Because two feedback variables are used, the damping ratio can be directly specified and no iteration is required in calculating the feedback gains. Using the inner inductor current in addition to the capacitor current increases the phase margin and reduces the low frequency gain. Using the transfer function of the damped LCL filter, a method of calculating the gains for a PR controller based on phase and gain margin specifications has been shown. The current control loop had a ½ cycle step response time. A modification is proposed for a PR controller to prevent resonator windup. The proposed anti-windup scheme compares the output of the active damping loop to a maximum and minimum value based on the DC link voltage. If the limits are exceeded, the difference is subtracted from the input of the resonator. Compared to previously proposed anti-windup methods for PR controllers, the scheme presented in this chapter has been shown to more accurately track the phase of the current reference. The DC link voltage control scheme discussed in [1] is considered. An FLL was designed to estimate the grid frequency. Finally, the well-known P&O technique for maximum power point tracking has been adopted.
3.9 References
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Chapter 4
4 Calculation of the Negative Sequence Current Component
In this chapter the method used for calculating the negative sequence of the load current is shown and compared to existing techniques for calculating negative sequence components. For control applications the calculation of sequence components must be accomplished in real time using no more than the available processing power. Several techniques for the calculation of symmetrical components were presented in Chapter 2. In this chapter, the work presented in [1] is extended for the calculation of the negative sequence current component using a compensated Half-Cycle Discrete Fourier Transform (HCDFT). Section 4.1 provides a brief introduction to the Discrete Fourier Transform (DFT). The method for calculating the accurate negative sequence components is then derived. This method is evaluated and compared with existing algorithms in section 4.2. Section 4.3 describes the integration of the HCDFT into the inverter control structure.