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Thread: VFD motor drive harmonics vs. grid-tied inverter

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    VFD motor drive harmonics vs. grid-tied inverter

    I'm designing a system for a pumping station. There will be 3 x 7.5 HP pumps driven from variable frequency drives (VFDs). The customer would like to install a PV system to offset electricity use. The motor power will be a very large proportion of the total load on the main service with very low additional linear loads (just a few lights, etc.). The VFDs are a large rectifier load, and I read that VFDs in a scenario like this can create significant harmonics on the line side even though they will be equipped with DC line reactors. I'm concerned that the extra harmonic distortion will cause the grid-tied inverter to drop off line due to poor power quality when the pump motors run. Does anyone have any experience along these lines? Is this a realistic concern?

    Thanks,

    Tom

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    Quote Originally Posted by tfrsolar View Post
    I'm designing a system for a pumping station. There will be 3 x 7.5 HP pumps driven from variable frequency drives (VFDs). The customer would like to install a PV system to offset electricity use. The motor power will be a very large proportion of the total load on the main service with very low additional linear loads (just a few lights, etc.). The VFDs are a large rectifier load, and I read that VFDs in a scenario like this can create significant harmonics on the line side even though they will be equipped with DC line reactors. I'm concerned that the extra harmonic distortion will cause the grid-tied inverter to drop off line due to poor power quality when the pump motors run. Does anyone have any experience along these lines? Is this a realistic concern?

    Thanks,

    Tom
    The VFDs will draw harmonic currents which will cause some distortion on your grid. How much depends on how stiff your grid is.

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    Quote Originally Posted by Besoeker View Post
    The VFDs will draw harmonic currents which will cause some distortion on your grid. How much depends on how stiff your grid is.
    And the extent to which the resulting harmonic voltages will affect the GTI will depend entirely on the design of its anti-islanding circuits. There are so many different ways to perform that function that you cannot really generalize from one type GTI to another.

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    There is another possibility, feeding DC into the VFD.

    Years ago we did several PV assisted heat pumps in conjunction with Trane and EPRI. At that date (about 1990) Trane had a variable speed heat pump that used a PF corrected input on a single phase 240V input to produce a nominal 415 volts DC for the inversion part of a 3-phase variable frequency/variable voltage output that operated a 3-phase compressor. Trane made a slightly different version of the input to boost the PV input to a slightly higher voltage so that if PV power was available, it would be used. The systems worked well and a check of power quality by the participating utility showed no harmonics on the QC service. The only problem we had at that date was convincing the AHJ that an ungrounded/floating PV system was safe.

    It may be that your VFD has low harmonics if it has an input with PF correction.

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    Thanks for the replies so far. The DC option is intriguing but not a possibility at this point due to the pump skid being already fully defined with a 240VAC single phase input. I sent out queries to the inverter manufacturers I'm considering - one has replied so far with the expected conservative response that "Yes, this will be a problem." The reference material I read about VFDs and an EE I consulted also tend toward suggesting trouble. On the other hand, I just located a similar system that I believe I can visit to observe the inverter when the pumps run. Stay tuned.

    - Tom

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