Electronics > Power/Renewable Energy/EV's

Unusual method of measuring PV system EMI

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I have a long telephone line to the exchange (53dB attenuation) so my broadband (ADSL) is always hard to maintain. Having checked out my PV system with the radio searching for excessive EMI I thought it was OK. But today I happened to run a broadband SNR plot (enclosed) and low and behold it is being modulated ~1db according to whether the sun is bright or diffused, this in turn determines if the converter runs in buck or boost mode.

So it seems the buck mode of the converter has an EMI problem and needs attention!

I am not entirely surprised, I call it buck as it reduces the PV input voltage to a lower voltage across the fixed heater load to allow the panels to maintain MPPT in low light, but it's actually a PWM, hence the long (maybe 10 Mtrs) heater connecting cable carries the switching current  :o but I thought the common mode choke and filtering components would be sufficient (slow down the edges).

Either I have to build a real buck converter (big job, extra inductor etc) or another idea is to reduce the PWM frequency as that will reduce the density of the fast edges.

Anyway if anybody is interested in a different kind of quantitative EMI sensor there it is :)

Are you also using this fixed heater load to heat your water in the house? If not, is there another reason not to build the buck?

Yes it is the water heater. The only reasons I don't want to build the buck are economy and space. According to simulation I should be able to reduce EMI substantially by reducing frequency and remain within the ripple current capability of the other components so I am going to try that first especially as it is a simple software change.

Well I tried different PWM frequencies and it had no effect whatsoever (maybe unsurprising) from 0741 onward in the plot several different frequencies were tried ranging from 16Khz to 64Khz. The only reason for a change in level was the increasing power (as it was early morning) over the duration of the test.

Another strategy I had thought of was to remote the PWM stage itself to immediately next to the heater thus dramatically shortening the loop length of the modulated signal however this has quite a few implementation difficulties.

In simulation a modest LC filter added to the PWM output has a dramatic effect (see fft but ignore title) so I will pursue this path with the contents of the junkbox and see what transpires with a real field trial :)

Frequencies of interest as far as my broadband goes are approx 138-860Khz.

Started building the LC filter today, junk box gifted four 2.2uF/250V poly capacitors (brown jobs in picture) and an ETD34 gapped at 2mm. Wound this with Litz made up of 3*1mm, got 13 turns on and put it on my inductor tester.
2nd pic is scope shot at 8A/cm vertical showing saturation at ~28A & inductance as 18uH (V is ~14.5).

This will do the job perfectly. The design equations gave a somewhat higher inductance and that's why I prefer to measure it :)


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