Dynamics of Grid-Forming Inverter
Berg, Matias Uolevi (2017)
2017
Sähkötekniikka
Tieto- ja sähkötekniikan tiedekunta - Faculty of Computing and Electrical Engineering
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Hyväksymispäivämäärä
2017-08-16
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201708241697
https://urn.fi/URN:NBN:fi:tty-201708241697
Tiivistelmä
Islanded micro-grids are increasingly becoming a vital factor in improving the reliability of a power system. In the islanded micro-grid power is produced from a local energy source that can be a renewable energy plant or an energy storage system.
If for example, the energy produced from solar panels and energy storage systems is fed to the grid an inverter is required to change the direct current produced by the solar panels to the alternating current. In the islanded micro-grid the inverter is used to form the phase voltages.
Dynamic models of power electronic converters have been previously developed in the literature. A useful feature of a dynamic model is that it can be used to tune controllers for the inverter and analyze stability. However, a dynamic model of the grid-forming inverter with respect to its true dynamics has not been reported and analyzed at the same time properly.
This thesis investigates, how to develop a dynamic model of the grid-forming inverter. A circuit diagram of the inverter, where the load is modeled as an ideal current sink is used as a basis for the modeling. Equations from the circuit diagram are written for a state space model. The state space model consists of linearized equations in a synchronous reference frame. The synchronous reference frame is used so that traditional control theory can be employed. Transfer functions that describe the dynamics from the system inputs to the system outputs are solved from the state-space presentation.
Analysis of the solved open-loop transfer functions shows that the dynamics of the grid-feeding and grid-forming inverter differ significantly. In orderer to tune reasonable controller the size of the capacitor in a commonly used LCL-filter has to be increased compared to grid-feeding inverter. The dynamic model could be used to develop a cascaded controller. It is analyzed that capacitor current feedback provides good output current disturbance rejection.
If for example, the energy produced from solar panels and energy storage systems is fed to the grid an inverter is required to change the direct current produced by the solar panels to the alternating current. In the islanded micro-grid the inverter is used to form the phase voltages.
Dynamic models of power electronic converters have been previously developed in the literature. A useful feature of a dynamic model is that it can be used to tune controllers for the inverter and analyze stability. However, a dynamic model of the grid-forming inverter with respect to its true dynamics has not been reported and analyzed at the same time properly.
This thesis investigates, how to develop a dynamic model of the grid-forming inverter. A circuit diagram of the inverter, where the load is modeled as an ideal current sink is used as a basis for the modeling. Equations from the circuit diagram are written for a state space model. The state space model consists of linearized equations in a synchronous reference frame. The synchronous reference frame is used so that traditional control theory can be employed. Transfer functions that describe the dynamics from the system inputs to the system outputs are solved from the state-space presentation.
Analysis of the solved open-loop transfer functions shows that the dynamics of the grid-feeding and grid-forming inverter differ significantly. In orderer to tune reasonable controller the size of the capacitor in a commonly used LCL-filter has to be increased compared to grid-feeding inverter. The dynamic model could be used to develop a cascaded controller. It is analyzed that capacitor current feedback provides good output current disturbance rejection.