SENSORLESS DIGITAL CONTROL FOR SOLAR DC OPTIMIZERS USED IN DC MICROGRIDS
This thesis presents novel control techniques to improve the performance of DC optimizers for harvesting solar energy in DC microgrids. The focus of this thesis is to implement a reliable and efficient structure to harvest the maximum power from solar panels. To increase the efficiency of the system, a control scheme is presented that provides the converter with zero-voltage-switching (ZVS) capability. ZVS can significantly eliminate switching losses and provide highly efficient and reliable operation for the DC optimizer. This task is done through a variable frequency peak current mode (PCM) control technique. Implementing this ZVS control scheme increases the input inductor current ripple. Therefore, an interleaved structure is used, which is able to reduce the input current ripple through the phase shifting of the inductor currents in the system. To implement the phase shifting technique, the first interleaved converter is selected as the base converter. The switching period of the base converter is measured by a switching frequency indicator unit. The one-third point and two-thirds point of the base converter’s switching period is calculated and imposed as delays to the second and third interleaved converters’ gate signals, respectively. Finally, an observer-based control technique is proposed that can estimate the control parameters, including the precise inductor current, both in continuous conduction mode (CCM) and discontinuous conduction mode (DCM). Thus, instead of an expensive and complex current sensor-based analog variable frequency PCM design, the inductor values in the system are estimated by a digital observer-based estimation block and used in the closed loop control system. Therefore, the whole closed-loop control system is implemented by a fully digital controller. This control scheme eliminates the need for expensive and inaccurate current sensors in the system. The proposed techniques are verified for performance through simulation and experimental results.
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