AC voltage rising at night?

Using OEM has allowed me so see things I could not before...


Wikipedia ( says 230v +10 / -6  which would be to 225v to 253v. We can be far off the top end most nights. 

I can see that the voltage coming in to our house rises during the night. Does anyone know why this is or why its so high?

Robert Wall's picture

Re: AC voltage rising at night?

Like any other power source, the mains supply has a finite source impedance (referred to by network engineers as the "fault level"). When the system is under high load - normally during the daytime - this internal impedance causes a large voltage drop. Under light load, the voltage drop is smaller and you see the voltage rise as a result.

If you look very carefully at the voltage graph, you should see steps in it as the controllers take generating sets off-line (during the evening when they have surplus generating capacity) or put them back on-line during the early morning to meet the rising demand.

If you run MartinR's PLL sketch or look at the published data, you'll see the mains frequency follows a similar pattern.

The first question to ask about it being "so high" is: Have you accurately calibrated the voltage input against a known good voltmeter? Unless you have, the absolute values you are recording are largely meaningless, as they depend on the actual values of components within your emonTx, and those actual values can lie anywhere within a tolerance band, the size of which varies from component to component. There's a page in Building Blocks about this, and another about multimeter accuracy.

JetJackson's picture

Re: AC voltage rising at night?

Yes, I have checked. Its the first thing I did. I used my 'trusty' ol' voltmeter, this agrees within 0.01v with the OEM figure. I have never calibrated it, but the values it produces agrees to 2nd DP with 2 different LiPo chargers I have.

I said 'so high' as I had never read the spec to be honest. I was aware it was nominally 220 to 240v, now I know that its 225v to 253v, so its not so high, just at the top end of spec.


The graph is interesting. The rise following 23:00 caught my eye, as we all go to bed and the load decreases the voltage rises until almost exactly 00:00 when it sharply steps down, probably when some generating switches off at midnight!

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