Accurate Impedance Model of a Grid-Connected Inverter for Small-Signal Stability Assessment in High-Impedance Grids

Tuomas Messo, Roni Luhtala, Aapo Aapro, Tomi Roinila
2019 IEEJ Journal of Industry Applications  
Power quality problems caused by grid-connected renewable energy inverters have been reported increasingly in recent literature. Excessive harmonics and interharmonics arise when the inverter starts to interact with the grid impedance. Small-signal impedance models have been proven to be useful tools to analyze the stability margins. However, in presented dq-domain models the grid voltage feedforward loop employed by the inverter is not included. To fill this gap, this paper presents a new
more » ... presents a new impedance model, which includes the effect of feedforward, to analyze impedance-based stability in the presence of large grid impedance. The model was verified by impedance measurements from a laboratory prototype. The model gave accurate predictions of small-signal stability when the Nyquist stability-criterion was applied. Thus, the model can be used to re-shape the inverter impedance to avoid stability problems. The developed impedance model also provides a useful tool to monitor stability margins online, which necessitates adaptive impedance-shaping of grid-connected inverters. List of Symbols Z grid Grid impedance matrix in dq-domain Y inv Inverter admittance matrix in dq-domain Z in Inverter input impedance T oid AC-to-DC transmittance (d-component) T oiq AC-to-DC transmittance (q-component) G cid Control-to-DC transfer function (d-component) G ciq Control-to-DC transfer function (q-component) G iod Forward transfer function (d-component) G ioq Forward transfer function (q-component) G cod Control-to-AC transfer function (d-component) G coq Control-to-AC transfer function (q-component) G coqd Control-to-AC cross-coupling (q to d) G codq Control-to-AC cross-coupling (d to q) −o open-loop transfer function −c closed-loop transfer function D d Duty ratio d-component (steady-state) D q Duty ratio q-component (steady-state) V od Grid voltage amplitude (d-component) I od Grid current amplitude (d-component) ω s Grid fundamental angular frequency G ffd Proportional feedforward gain (d-component) G ffd Proportional feedforward gain (q-component) G del Delay transfer function
doi:10.1541/ieejjia.8.488 fatcat:mi7luzl3rffv5bplmgvjjvvkwu