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Small-signal Stability Impact of Utility PV with Reactive Power Control on the Medium Voltage Distributed Systems

PV impedances
Fig. 1. PV impedances under different Q control modes.
  With an increasing number of photovoltaic (PV) inverters in the distribution system, their impact is no longer negligible, especially in the aspect of dynamic interaction. Accordingly, a comparison is done among PV inverters of different reactive power control modes, to determine their impact on the system voltage profile, power loss and small-signal stability. Generalized Nyquist Criteria (GNC) based on impedances in DQ frames is used for stability assessment, which is validated by time domain simulation results and also system eigenvalues calculation results from MATLAB. From these, guidelines are formulated to manage PV inverter reactive power control strategies. Reactive power control mode of volt-var Q=f(V) is preferred to other reactive power modes to avoid voltage profile problem and reduce power loss, but will induce small-signal instability and cause PV terminal voltage oscillations. There's tradeoff between static influence and dynamic impact in choosing the local reactive power control strategies.
  A known broad statement is that active power injected by PV inverters increases the system voltage. In what regards the reactive power compensation capability of PV inverters, this paper showed that the reactive power control mode Volt-var droop mode of Q = f (V) is preferred over other reactive power control modes after the static analysis of voltage regulation effect and the impact on grid power loss.
  The terminal impedances in DQ frame are derived of utility-scale PV farm based on small signal model of PV inverters as shown in Fig.1. A comparison is done among impedances of PV inverters under 5 different reactive power control modes, based on which GNC is used to assess the grid-PV connection stability as shown in Fig. 2. The volt-var control mode changes PV terminal impedance signs and magnitudes significantly and may cause unstable connection to the grid. The stability assessment is proved by time domain simulation and also eigenvalues acquisition from the state space model of the whole system with PV generators.
Characteristic loci of PV
Fig. 2. Characteristic loci of PV connection to the grid.

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