Poor man's options to measure THD of grid tied solar inverters - Page 1
Forum section: electronics / general

author: nikbry
author_rank: Regular Contributor
timestamp: September 17, 2022, 02:05:10 pm
content: Hi ,As the title of the thread gives away, I'm interested in measuring the Total Harmonic Distortion of grid tied solar inverters.Since my background in the electrical field is quite basic, I'd appreciate any help in understanding how to conduct these measurements and the test equipment that is required.I understand that dedicated power analyzers are used professionally, but as a hobbyist I'm on a poor man's budget mission to see if there are affordable alternatives.Should this topic have a better fit in another subforum, please let me know.Cheers.

author: Marco
author_rank: Super Contributor
timestamp: September 17, 2022, 02:40:20 pm
content: You could use a clamp meter with THD function, clamp on the phase or neutral to the inverter.I doubt it's a relevant metric for any modern grid tier solar inverter, they are all going to be too good for it to make a difference.

author: nikbry
author_rank: Regular Contributor
timestamp: September 17, 2022, 05:38:52 pm
content: Thanks for pointing me in the right direction. Those clamp meters seem way over my budget.From what I gather there seem to be quite a few microinverters from reputable manufacturers whose THDi does not comply with the VDE-AR-N 4105 standard. So apparently it is a problem.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 17, 2022, 07:39:20 pm
content: Isolated current sensor connected to sound card would be very cheap. Use an audio amplifier and dummy load to characterize the frequency response of the sensor.

author: nikbry
author_rank: Regular Contributor
timestamp: September 18, 2022, 09:25:06 am
content: OK. I happen to have a clamp from a Shelly EM and an OWON VDS1022i USB Oscilloscope.I connected the scope leads directly to the clamp's wires and did some measurements of a mains 230V phase cable.I can't seem to make any sense of what I am seeing on the scope. Besides being a very noisy signal, I'm measuring frequencies of around 800Hz. I'm guessing an amplitude of 1V per 50mA.Would a basic capacitor/resistor filter help with the noise?Shouldn't I be expecting a 50Hz (mains) frequency?

author: Marco
author_rank: Super Contributor
timestamp: September 18, 2022, 12:36:20 pm
content: Only when there's a load or a source (such as a solar inverter feeding back on the mains).You're measuring current, not voltage. This is important because you want to ignore the THD in the voltage signal, which you don't control.

author: nikbry
author_rank: Regular Contributor
timestamp: September 18, 2022, 01:41:46 pm
content: Maybe I'm missing something, please excuse my ignorance.I put a 50W light bulb as a load connected to mains and put the clamp on the phase cable.I thought the ac current flowing through the clamp's toroid would induce an oscillating voltage on its leads, proportional in amplitude to the current flowing through the clamp and having the same frequency, 50 Hz.My plan was to capture that voltage wave and do an FFT analysis on it, to then calculate the THD from the data.Where's the fault in my reasoning?

author: Marco
author_rank: Super Contributor
timestamp: September 18, 2022, 02:07:56 pm
content: Does the Shelly current transformer have a built in shunt? Otherwise you need to use a shunt ...

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 18, 2022, 02:09:26 pm
content: Check the peak current rating and max pulse width rating of the probe. Passive probes are often only rated milliamps for 100s of microsec.For mains freq you might need a gapless cored C/T with a burden resistor, or better, a Rogowski.Anyway you should see a nice pure sine wave of lamp current before doing a FFT.

author: jonpaul
author_rank: Super Contributor
timestamp: September 22, 2022, 06:26:14 am
content: Bonjour, Many Standards exist for Solar Power invertersANSI, NIST ASTM, IEEE, UL, NIST, IEEE, UL, SEMI, SAC, CENELEC and IEC, bOne paper on IEC std review.http://proceedings.ises.org/paper/swc2011/swc2011-0401-Salas.pdfCurrent transformers are available in many ratings, for power monitoring.Most are rated for mains freq but not rated for harmonics.CT Ratio typically 10, 20, 50, 100, 1000:1100 turn CT will generate 10 mA in the specified burden (load) resistor for each 1 A in the conductor under test.https://www.electronics-tutorials.ws/transformer/current-transformer.htmlClamp on probes are less accurate and the airgap varies each clamp. Most are designed for a VOM /DVM.Their freq response is worse, so harmonics are not accurate.https://en.wikipedia.org/wiki/Current_clampA good quality solar inverter spec provides the harmonic content and regulatory/safety compliance.Cheap (clones) may not comply or be unreliable/unsafe.Jon