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how to measure dirty power

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how to measure dirty power

Postby Wolle2022 » Sun Jan 07, 2018 10:42 am

We need to check whether in our house the Total Harmonic Distortion (THD) is well below 8% in sum and - if possible - from which equipment the biggest distortion comes from. Can we use the PicoScope 2208B for that purpose? Which probes would we have to purchase in addition, means would we need both voltage and current probes? Would we have to conduct the measurement at the point from where a distortion may come from (only single line equipments like LED-drivers of relevance!!) and/or also in the central electricity board where all 3 lines (L1, L2, L3) are present? In the latter case: would we have to use 3 or more sets of probes in such circumstances?

We suspect certain LED-drivers (with active PF and with up to 90W / 24V DC or 90W / 12V ) to generate too much THD.

Thanks for your advise before we can decide what to purchase!
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Re: how to measure dirty power

Postby Gerry » Tue Jan 09, 2018 2:56 pm

Hi Wolle2022,

The answer to your questions will be influenced by your reason for you asking them. So, I have to ask "why do you need to know if you have 'dirty' power?"

If you are trying to find the contribution made to the national grid by the harmonics generated from the items in your house (seen as loads to the system) then you really need to be monitoring the system voltage (at the closest point where it joins the supply lines) and doing this (a) in isolation, and (b) with each load switched in indepenedently. As you need to check the signal integrity for the system you would need to monitor the individual voltage phases, so you would need to capture 3 channels of data simultaneously with 3 probes.

As we are talking about potentially hazardous voltages, you would need to use High Voltage (High consumer voltage, Low grid voltage) Differential probes, which would most likely need to be rated at CAT III or CAT IV according to the latest recommendations for the use of measurement equipment at Low Grid voltages IEC 61010-1 Edition 3 (for Europe) or UL 61010B-1 (for the USA). We have a range of Active differential Probes here: (https://www.picotech.com/accessories/ac ... ope-probes) that may be applicable.

Your actual choice of PicoScope will depend upon the answer to my question, but to answer yours, a PicoScope 2208B only has 2 channels, but a 4-channel PicoScope 2408B would be able to capture data at the rate and resolution required to detect THD at 8%. The PicoScope 2408B , also, has inherent harmonic distortion at <-50dB typically at 100kHz for a fullscale input which is 0.3%, and low enough to not adversely affect the measurement of <8% THD. Our PicoScope 6 software would be able to calculate and Plot the FFT, and perform the measurement for that level of distortion.

However, bearing in mind the cost of 3 Active Differential Probes, a PicoScope 4444 (see here: https://www.picotech.com/oscilloscope/4 ... 4-overview) with 3 of our PicoConnect 442 Passive Differential Probes (see here: https://www.picotech.com/accessories/d9 ... tial-probe) would be more cost-effective, if applicable.


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