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VSD and harmonic mitigation requirement

Paul Screen

We are providing a 900kw VSD to a customer in the UK. A 6 pulse drive has been sold with no additional harmonic mitigation apart from the the drive's in-built d.c. link choke.

Could the THD of around 35% be a problem?

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    Where/ how  is the 35% measured ?  If it is the waveform of the mains current I'd expect the DNO to be unimpressed unless that is a small fraction of the total connected load  ! Figures like <5% target, 8% upper bound under worst case are commonly specified at point of connection.


    If it is the voltage or the current on the side of the customer's wiring and loads, well its a matter for them, but problems are entirely possible, unless the load is very robust - a large motor that is mostly under-run may be OK or anything producing heat - but then you dont need the VSD.
    Depending on its exact composition, 35% THD is pretty badly non-sinusoidal and may be enough to overheat magnetic cores as well as increase losses in the copper.

    Mike

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  • 0

    Where/ how  is the 35% measured ?  If it is the waveform of the mains current I'd expect the DNO to be unimpressed unless that is a small fraction of the total connected load  ! Figures like <5% target, 8% upper bound under worst case are commonly specified at point of connection.


    If it is the voltage or the current on the side of the customer's wiring and loads, well its a matter for them, but problems are entirely possible, unless the load is very robust - a large motor that is mostly under-run may be OK or anything producing heat - but then you dont need the VSD.
    Depending on its exact composition, 35% THD is pretty badly non-sinusoidal and may be enough to overheat magnetic cores as well as increase losses in the copper.

    Mike

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