Design of an Autonomous Control for Parallel-Connected Uninterruptible Power Supplies

This paper proposes an autonomous control for parallel-connected uninterruptible power supply (UPS) systems based on electrical power system interrelations. The controllable variables are the output voltage phase angle and magnitude. Relation between active current and output voltage phase angle is derived from a synchronous machine model. Electrical power systems suggest that voltage regulation and reactive current can be proportionally related to yield another control equation. The resulted control scheme is very simple and only requires an output current measurement to provide a voltage reference signal locally in synchronism with other UPSs without communication signal exchange. A small-signal model represents the entire system and constitutes proof of its stability. Feasibility is confirmed by an experimental setup with three UPSs connected in parallel with different output power ratings. In the experiments, voltage limiters (1%) are employed and three cases are verified. Under all conditions, the results show excellent performance in terms of stability, and active and reactive load sharing.

[1]  Atsuo Kawamura,et al.  Autonomously Decentralized Control of Multiple UPS's for Higher Reliability , 1998 .

[2]  Atsuo Kawamura,et al.  Theoretical and experimental verification of independent control for parallel-connected multi UPS : Powering the broadband network , 2004 .

[3]  Atsuo Kawamura,et al.  A Compensator for Lateral Current Reduction Applied to Autonomously Controlled UPSs Connected in Parallel , 2004 .

[4]  Atsuo Kawamura,et al.  Theoretical and Experimental Verification of Independent Control for Parallel-Connected Multi UPS , 2004 .

[5]  Ching-Lung Chu,et al.  Combination voltage-controlled and current-controlled PWM inverters for UPS parallel operation , 1995 .

[6]  Ernane Antônio Alves Coelho,et al.  Small signal stability for parallel connected inverters in stand-alone AC supply systems , 2000 .

[7]  C.A. Canizares,et al.  Effects of limits in small signal stability analysis of power systems , 2001, 2001 Power Engineering Society Summer Meeting. Conference Proceedings (Cat. No.01CH37262).

[8]  P. Fairley,et al.  The unruly power grid , 2004, IEEE Spectrum.

[9]  Atsuo Kawamura,et al.  Theoretical and experimental verification of independent control for parallel-connected multi-UPS's , 2003, The 25th International Telecommunications Energy Conference, 2003. INTELEC '03..

[10]  T. Kawabata,et al.  Parallel Operation of Voltage Source Inverters , 1986, 1986 Annual Meeting Industry Applications Society.

[11]  Deepak Divan,et al.  Control of distributed UPS systems , 1994, Proceedings of 1994 Power Electronics Specialist Conference - PESC'94.

[12]  Jen Lin-Liu.  Top U.S. officials successfully defend IEEE's Wi-Fi standard , 2004 .