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Issues on Dynamic Modeling and Design of GridConnected ThreePhase VSIs in Photovoltaic Applications
Tiivistelmä:
This thesis analyzes the dynamic properties of gridconnected voltage source inverters (VSI) and shows that the inverter dynamics are determined not only by the power stage but also by the application where it is to be used. In power production, under gridparallel mode, a VSI controls its input voltage and has to be analyzed as a currentfed topology having corresponding dynamic properties. Important information e.g. about the control dynamics is lost if the same powerstage is analyzed as a voltagefed topology.
In addition, a general method to model the dynamic effect of source/load nonidealities in gridconnected currentfed and voltagefed inverters regardless of the topology is presented. The source/load nonidealities can include either the internal impedance of the source/load subsystem and/or the dynamic effect of a passive input/output filter.
It is also demonstrated that a gridconnected inputvoltagecontrolled VSI incorporates an operatingpointdependent pole in the inputvoltagecontrol loop caused by the cascaded inputvoltageoutputcurrent control scheme. The location of the pole on the complex plane can be given explicitly according to the input capacitance, operating point voltage and current, and the dynamic resistance of the PV generator. The pole shifts between the left and right halves of the complex plane according to the PVG operating point. Naturally, the pole causes controlsystemdesign constraints when it is located on the right half of the complex plane (RHP). Furthermore, the RHP pole frequency is inversely proportional to the input capacitance, which implies that minimizing the input capacitance can lead to an unstable input voltage loop because the control loop has to be designed so that the loop crossover frequency is higher than the RHP pole frequency.
Therefore, a design rule between the inputcapacitor sizing and inputvoltagecontrol design is proposed. Typically, the input capacitor design is based on energybased design criteria, e.g. inputvoltage ripple or transient behavior. The energybased criteria are important, although subjective, and do not necessarily guarantee the inverter stability. Therefore, in addition to the energybased criteria, the controlbased rule proposed in this thesis has to always be considered because it can be used to determine the inverter stability, which results in more reliable and robust PV inverter design.