14/06/2004NORPIE Paralleling of 3-phase inverters a part of my PhD study: "Control and monitoring for distributed power supply“ which is a part of the project "Technologies for Reliable Distributed Generation of Electrical Power from Renewable Energy Sources” Erik Hoff, Tore Skjellnes, prof. Lars Norum

14/06/2004NORPIE Overview of lab setup to be built Capacitor Battery Sun Wind : Renewable energy source =/~ : 3-phase inverters DC Energy storage Grid Consumers Internal communication system External communication system Focus today

14/06/2004NORPIE Outline of Presentation Simulation of two 3-phase inverters using a virtual resistor »LC filter dampening »Load sharing »Simulation results

14/06/2004NORPIE Problem: LC filter resonance Inverter A =/~ Inverter B 1.3mH0.3mH 1.3mH1mH 65µF Diode rectifier load Solution: Negative current feedback → Virtual resistor Load

14/06/2004NORPIE Where to place the virtual resistor?  Two possible locations: –R1 is most common, but expensive –R2 is used here, because the inductor current I L,αβ already is available due to overcurrent protection I L,αβ V C,αβ

14/06/2004NORPIE High load problem Series resistor will give voltage drop! Solution: Band-pass filtering

14/06/2004NORPIE Band pass filtering of inductor current Still a resistor at 50Hz?

14/06/2004NORPIE Load sharing between two voltage sources  An inductor L is necessary to decrease the disturbance (load current) frequency  A virtual (series) resistor R can be added for improved load sharing VEVE VSVS L R

14/06/2004NORPIE Phasor diagram – Inductance and resistance contributions to load sharing V s I s,q  R IsIs IsRIsR I s,p  R I s,p VEVE I s,q VsVs I s,p  j  X IsIs IsjXIsjX I s,q  j  X I s,q VEVE I s,p Pure inductance Pure resistance

14/06/2004NORPIE Simulations results for traditional (increased gain by 4x) and virtual resistor based load sharing vector Control assuming an inductive transmission line. P: 4% frequency droop Q: 24% voltage droop + Good load sharing - Poor dampening Control assuming a resistive transmission line P: (24%) voltage droop Q: 4% frequency increase + Good dampening - 20% load sharing difference

14/06/2004NORPIE Conclusion  A band pass filter makes it possible to use a virtual resistor in series with the load for LC-filter dampening. –Cheaper and simpler than a virtual resistor in series with the filter capacitor.  The use of a virtual resistor requires rotation of the load sharing vector. –Good oscillation dampening, but load sharing may be uneven  The traditional control method assuming inductive transmission line has poor active power dampening. –Some special ”tricks” may be used to decrease response time: Adding a phase angle in addition to the frequency (Patent US6,693,809 by Alfred Engler)

14/06/2004NORPIE Thank you for your attention