A robust equal-load-sharing control scheme for parallel UPS units with time delay consideration

This paper proposes a robust control scheme for equal load sharing between n parallel-connected UPS units with LCL output filters. Dual-loop control is used with capacitor voltage feedback in the outer loop, and capacitor current feedback in the inner loop. Time delay in exchanging information between UPS units is taken into consideration in the stability analysis. It is shown that large time delay, if not compensated, affects the dynamic performance of the system and tends to destabilize the system. Simulation results are provided to prove the stability and robustness of the system against load variation and hot-swap operation.

[1]  J.M. Guerrero,et al.  Hierarchical control of droop-controlled DC and AC microgrids — a general approach towards standardization , 2009, 2009 35th Annual Conference of IEEE Industrial Electronics.

[2]  Xinbo Ruan,et al.  Step-by-Step Controller Design for LCL-Type Grid-Connected Inverter with Capacitor–Current-Feedback Active-Damping , 2014 .

[3]  Jiyan Zou,et al.  Delay-Dependent Stability of Single-Loop Controlled Grid-Connected Inverters with LCL Filters , 2016, IEEE Transactions on Power Electronics.

[4]  Jih-Sheng Lai,et al.  Design and Control for LCL-Based Inverters with Both Grid-Tie and Standalone Parallel Operations , 2008, 2008 IEEE Industry Applications Society Annual Meeting.

[5]  Josep M. Guerrero,et al.  An Improved Droop Control Strategy for Reactive Power Sharing in Islanded Microgrid , 2015, IEEE Transactions on Power Electronics.

[6]  K H Ahmed,et al.  Improved Instantaneous Average Current-Sharing Control Scheme for Parallel-Connected Inverter Considering Line Impedance Impact in Microgrid Networks , 2011, IEEE Transactions on Power Electronics.

[7]  Yan Zhou,et al.  Parameter Design of a Two-Current-Loop Controller Used in a Grid-Connected Inverter System With LCL Filter , 2009, IEEE Transactions on Industrial Electronics.

[8]  P.W. Lehn,et al.  Autonomous load sharing of voltage source converters , 2005, IEEE Transactions on Power Delivery.

[9]  Bangyin Liu,et al.  Stationary Frame Equivalent Model of Proportional-Integral Controller in dq Synchronous Frame , 2014, IEEE Transactions on Power Electronics.

[10]  Josep M. Guerrero,et al.  Virtual Flux Droop Method—A New Control Strategy of Inverters in Microgrids , 2014, IEEE Transactions on Power Electronics.

[11]  Y. Li,et al.  Analysis and Digital Implementation of Cascaded Delayed-Signal-Cancellation PLL , 2011, IEEE Transactions on Power Electronics.

[12]  Frede Blaabjerg,et al.  Grid-Current-Feedback Active Damping for LCL Resonance in Grid-Connected Voltage-Source Converters , 2016, IEEE Transactions on Power Electronics.

[13]  Ali Mehrizi-Sani,et al.  Distributed Control Techniques in Microgrids , 2014, IEEE Transactions on Smart Grid.

[14]  P. C. Loh,et al.  Improved transient and steady state voltage regulation for single and three phase uninterruptible power supplies , 2001, 2001 IEEE 32nd Annual Power Electronics Specialists Conference (IEEE Cat. No.01CH37230).

[15]  Frede Blaabjerg,et al.  An Efficient and Robust Hybrid Damper for $LCL$- or $LLCL$-Based Grid-Tied Inverter With Strong Grid-Side Harmonic Voltage Effect Rejection , 2016, IEEE Transactions on Industrial Electronics.

[16]  Frank L. Lewis,et al.  Droop-Free Distributed Control for AC Microgrids , 2016, IEEE Transactions on Power Electronics.

[17]  Xinbo Ruan,et al.  Capacitor-Current-Feedback Active Damping With Reduced Computation Delay for Improving Robustness of LCL-Type Grid-Connected Inverter , 2014, IEEE Transactions on Power Electronics.

[18]  Tsai-Fu Wu,et al.  Current Weighting Distribution Control Strategy for Multi-Inverter Systems to Achieve Current Sharing , 2007, IEEE Transactions on Power Electronics.

[19]  L. Corradini,et al.  Analysis of Parallel Operation of Uninterruptible Power Supplies Loaded through Long Wiring Cables , 2009, 2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition.

[20]  Yun Wei Li,et al.  Generalized Closed-Loop Control Schemes with Embedded Virtual Impedances for Voltage Source Converters with LC or LCL Filters , 2012, IEEE Transactions on Power Electronics.

[21]  Fang Zheng Peng,et al.  Control for Grid-Connected and Intentional Islanding Operations of Distributed Power Generation , 2011, IEEE Transactions on Industrial Electronics.