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Stability Impact of Photovoltaic Inverter Generation on Medium-voltage Distribution Systems

System diagram
Fig.1. One line diagram of the distribution system
Renewable energy generators in distribution systems and at the transmission level have recently increased in proliferation. Due to their high impedances, distribution systems are weaker than transmission systems. In addition, the impact of photovoltaic (PV) generators on the voltage profile impels the requirement of PV reactive power regulation from grid code. In the near future, PV generators will have more complicated reactive-power control strategies instead of working under unity power factor. This raises the question whether PV generators in the distribution sys-tem will cause any stability problems. To assess small-signal stability, this paper employs the Generalized Nyquist Criteria (GNC) method based on measured impedances in DQ frames at connection interfaces. The GNC meth-od has the advantage that interconnection stability can be judged without knowing details about the grid and PV generator model. Based on a distribution system that has 56 buses, a compari-son is done among PV inverters at different physical locations and with different reactive-power control modes and operation states to determine their impact on the system’s operation and stability. The PV farms connected to different branches of a complicated radial distribution system may have interactions with each other when they are under droop-mode control. So the control strategy and parameters of the PV generator should be designed with consideration given to the impact of other PV generators. If models of other generators are not accessible, the GNC meth-od based on impedance measurement should be used for stability assessment. Higher capacity of PV penetration makes this stability problem worse. Moving any of the PV farm closer to the substation reduces the possibility of instability. Droop mode is more flexible and beneficial for grid voltage regulation compared to unity power factor or constant Q control modes, but raise the possibility of unstable PV connection, especially when PV is working in inductive zone in droop curve. There is a tradeoff between grid voltage regulation and stability.
Nyquist plots
Fig. 2. Nyquist plots of eigenvalues of return ratio matrix L when the second PV farm is at a different location
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