Download Instantaneous Current-Sharing Control Strategy for Parallel

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Instantaneous Current-Sharing Control Strategy
for Parallel Operation of UPS Modules Using
Virtual Impedance
Abstract:
By regulating the inverter output current every switching cycle, instantaneous currentsharing control strategies are usually employed in paralleled modular uninterruptible
power supplies (UPSs). In this paper, virtual impedance, which is usually utilized in the
droop method, is firstly introduced to the instantaneous current-sharing control strategy to
achieve good load current-sharing performance. The instantaneous circulating current
model of the paralleled system is developed and the circulating impedance is derived as
well to clearly identify the intrinsic nature of unequal sharing of load current. By inserting
the virtual impedance to the circulating impedance with the help of proper control strategy,
good current-sharing capability can be obtained. The parallel inductor, e.g., coupled
inductor, which is generally added at the output of each inverter to reduce the circulating
current among modules, can be, therefore, eliminated, contributing to reduced weight,
volume, and cost. In the meantime, the system output-voltage regulation performance, e.g.,
amplitude and waveform quality, is not affected by the introduction of the virtual
impedance. The designation of the virtual impedance is provided and its implementation is
simple but rather effective. Experimental results are also provided to verify the feasibility of
the proposed method.
INTRODUCTION:
I N RECENT years, uninterruptible power supplies (UPSs) have been widely employed to
provide continuous and reliable electrical power to critical loads. With the ever-increasing
power and reliability demand, several UPS units are usually connected in parallel. These
units often have similar characteristics and power ratings, contributing to the system
features of N+X redundancy operation and modularity [1], [2]. Due to the voltage source
nature, proper current sharing is of extreme
Manuscript received October 25, 2011; revised January 10, 2012 and March 24, 2012;
accepted May 5, 2012. Date of current version September 11, 2012 Recommended for
publication by Associate Editor J. M. Guerrero.
Y Zhang and Y Kang are with the Huazhong University of Science and Technology, Wuhan
430074, China (e-mail: [email protected]; [email protected]).
M. Yu is with the Dongguan Leke Electronic Company, Ltd., Dongguan 523718, China (email: [email protected]).
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[email protected]
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2
F. Liu is with the Huazhong University of Science and Technology, Wuhan 430074, China,
and also with the Department of Electrical and Computer Engineering, Ryerson University,
Toronto, ON M5B 2K3 Canada (e-mail: [email protected]).
Architecture Diagram:
CONCLUSION:
The virtual impedance, which is usually employed in the droop method, is firstly proposed
for the instantaneous current-sharing strategy in this paper to achieve good load current-
sharing performance. By developing the circulating current model of the paralleled inverter
system, the circulating impedance is derived and the intrinsic nature of unequal load
current sharing is clearly demonstrated. With the help of the virtual impedance, the
stability of circulating current is guaranteed. Another significant feature is that the virtual
impedance does not cause additional voltage drop and distortion in the system output,
which are common in the traditional virtual impedance- based strategies. Moreover, the
circulating current caused by the difference between parameters of each inverter power
stage can also be well suppressed, contributing to good load currentsharing performance. A
simple and valid designation of the virtual impedance is provided. The feasibility and
validity of the proposed method are validated by the experimental results.
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[email protected]
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[email protected]
+91 7200247247