Modeling and Electrical Emulation of Grid Impedance for Stability Studies of Grid-Connected Converters
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Verkkoimpedanssin mallinnus ja sähköinen emulointi vaihtosuuntaajien stabiiliustarkasteluun
The access to reliable and aﬀordable energy is vital for a modern society. The climate change and increased consciousness of the environment has shifted the global energy production towards new, renewable alternatives. The rapid growth of renewable energy production increases the amount of grid-connected converters in the power system. However, the dynamics and grid requirements for converters are very diﬀerent than for conventional rotating generators. The interface between the converter and grid is prone to stability issues. The stability can be assessed based on the ratio of the inverter output impedance and the grid impedance. However, the grid impedance is often an unknown parameter and modeled based on simpliﬁed assumptions. The most common model for the grid impedance is a series connected inductor and resistor. Grid impedance measurements have shown the grid impedance to have more complex, resonant and time-variant characteristics, which are neglected in the conventional modeling approach. This thesis presents grid impedance models, in which the complex nature of the grid impedance is accurately considered. The enhanced models are based on aggregation of grid elements into sub-models, where the resonant and time-variant behavior is clearly shown. In addition, this work introduces a power hardware-in-the-loop setup for testing a grid-connected inverter in various grid conditions. The derived grid models are applied in a real-time grid simulator, which in turn provides references for a linear ampliﬁer operating as a grid emulator. Thus, a real inverter can be connected to a simulated grid conditions. The versatility and desired behavior of the setup are veriﬁed with grid impedance measurements from the grid emulator.