ABSTRACT:
In this paper,
a new control method for the parallel operation of inverters operating in an
island grid or connected to an infinite bus is described. Frequency and voltage
control, including mitigation of voltage harmonics, are achieved without the need
for any common control circuitry or communication between inverters. Each
inverter supplies a current that is the result of the voltage difference
between a reference ac voltage source and the grid voltage across a virtual
complex impedance. The reference ac voltage source is synchronized with the
grid, with a phase shift, depending on the difference between rated and actual
grid frequency. A detailed analysis shows that this approach has a superior behavior
compared to existing methods, regarding the mitigation of voltage harmonics,
short-circuit behavior and the effectiveness of the frequency and voltage
control, as it takes the to line impedance ratio into account. Experiments show
the behavior of the method for an inverter feeding a highly nonlinear load and during
the connection of two parallel inverters in operation.
KEYWORDS:
1. Autonomous power systems
2. Converter control
3. Dispersed generation
4. Finite output-impedance ac voltage source emulation
5. Frequency and voltage droops
6. Harmonics
7. Parallel
connection
8. Power quality
9. Microgrids
10. Stand-alone systems
11. Uninterruptible power supplies (UPS)
12. Virtual impedance
13. Voltage source inverter
14. Mixed
voltage-current control
SOFTWARE: MATLAB/SIMULINK
CONCLUSION:
A time-domain method for controlling
voltage and frequency using parallel inverters connected to the mains or in an
island grid is developed. By imitating a voltage source with a complex finite-output
impedance, voltage droop control is obtained. Frequency droop control results
from synchronizing the power source with the grid, with a phase angle
difference that depends on the difference between rated and actual grid
frequency. Compared to existing techniques, the described method exhibits superior
behavior, considering the mitigation of voltage harmonics, the behavior during
short-circuit and, in the case of a non-negligible line resistance, the “efficient”
control of frequency and voltage. Two experiments are included to show the described
behavior.
REFERENCES:
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[2] E. A. A. Coelho, P. C. Cortizo, and
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pp. 533–542, Mar./Apr. 2002.
[3] M. C. Chandorkar, D. M. Divan, and
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1993.
[4] A. Engler, “Regelung von
Batteriestromrichtern in modularen und erweiterbaren Inselnetzen,” Ph.D.
dissertation, Dept. Elect. Eng., Univ. Gesamthochschule Kassel, Kassel,
Germany, 2001.
[5] M. Hauck and H. Späth, “Control of
three phase inverter feeding an unbalanced load and operating in parallel with
other power sources,” in Proc. EPE-PEMC’02 Conf., Sep. 9–11, 2002.
