Renewable energy sources are quickly becoming r -lnE operation of two or more inverters in parallel is incorporated into increasingly distributed electrical utility possible in situations where two or more DC sources grids. An ever present issue when interfacing these sources is that of harmonic distortion reduction at the inter-tie point.are available (e.g. fuel cells, solar cells, some wind based One very interesting approach to reduce harmonic distortion sources, microturbine or other sources with a DC link). is to synchronize power transistor operation for inverters Parallel operation of multiple inverters has been identified operating in parallel.as means to reduce both relative and net harmonic In this paper, a rarely employed technique for harmonic distortion of an inverter system [1,2]. In more recent years, reduction using synchronized phase shifted parallel PWM this idea has received renewed interest as alternative energy inverters with current-sharing reactors is presented and . .analyzed. Using the technique presented inverter output sources are being connected to electric utility grid systems.distortion can be reduced significantly, for example total Many approaches employ an output reactor so that a harmonic reduction can be reduced typically in half using just voltage source inverter can be connected to the grid and at two synchronized phase-shifted parallel inverters (our the same time provide current waveform control. simulation and experimental studies indicate that even smaller Techniques have been suggested on how to minimize such distortion reduction factors can be achieved),
reactors [3]. Another set of researchers have suggestedWe present analyses for two and three phase-shifted * -inverters operated in parallel employing current sharing randong th opeain of etep hranistortga reactors. Our analyses consider the impact of a wide range of functions so as to obtain uncorrelated harmonic spectra phase-shift delays on net output total harmonic distortion, amongthe parallel inverters [4].with the phase-shift delay from zero to one entire cycle of the With the increasing interest in renewable energy fundamental component. Some results are expected, such as a sources, we expect increased attention on parallel operation dramatic increase in distortion when the fundamental of multiple inverters. Indeed paralleling inverters is useful components between multiple inverters tend to cancel out, on in itself for a number of reasons; where modularity and the other hand it is interesting to observe a leveling-off, and r gradual, smooth increase in the distortion reduction factor redundancy are some of the key points, paralleling may over a significant portion of the phase-shift delay variable.lead to other benefits such as ease of maintenance through operation of identical units, scalable designs, increased Index Terms--multiple inverters, pulse width modulation, reliability through redundancy, etc, without over sizing the distortion reduction.single inverter. One shortcoming is that many of these paralleli...
The elimination of low order harmonics in grid-connected renewable-energy inverter systems is desirable both from the point of view of grid health as well as efficient inverter operation. In this paper, one inverter (called reference inverter) employs pulse width modulation (PWM) with conventional selective harmonic elimination (SHE) to remove the 5 th and 7 th harmonics. By adjusting a delay in the gating signals to four inverters for example, it is possible also eliminate other higher significant harmonics, namely the 11 th and 13 th harmonics, without any increase in the switching frequency. This approach may be useful for high power grid-connected inverter systems supplied by alternative energy sources where switching losses may become prohibitive if the switching frequency is too high. Two and four parallel inverters systems are investigated and simulated in PSPICE and MATLAB.
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