The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Wednesday, 3 August 2016

Live power feed

I find it useful having a display on my computer of how much power my Powerspout is producing.  Many times a day, when doing other stuff on my desktop, I'll check to see what the output is. It saves a walk downhill to the turbine or uphill to the header tank, - though I do those as well !

The data stream I view is the same as anyone can view by clicking on this "Live power" link.

Viewing it often as I do has thrown up observations which make me curious as to what I'm seeing. In this diary entry I want to write about these.  I am, after 3 years, still reaching for explanations for some of them.  

The output should in theory be absolutely constant. And mostly it is, as this record shows, captured just now as I started writing.  The variation of 1 watt either side of 216 W simply represents the limit of the system's ability to resolve and display the true reading; the base line is dead flat for 7.5 hours:

But the record is not always a flat line like this. Earlier in the year, I captured this record showing how a change in temperature in the SmartDrive housing, created artificially by blocking or allowing ventilation, made the power output anything but a flat line. As can be seen the baseline dropped 20 watts with the compartment warming up and rose again when it cooled down:

Discussion about this 'temperature effect' can be found on these blog pages.

Recently, I think I've come to understand what's going on in another type of record I see almost every day.  Unlike the fluctuation caused by temperature, which is a genuine change in the power output of the Powerspout, this one is only an apparent change.  It arises from the way in which the monitoring system works and not from a real variation in the power being put out.

An example can be seen in this trace from last week:

What is noticeable here is that output is dead level between midnight and 06.00 hrs, and dead level again between 20.00 hrs and the following midnight. In between these times it appears to jump around all over the place.  This in-between time is day-time, the time when activity in the house is causing most electricity to be used. It is also the time when my solar panels are working, putting their output into the grid via the house consumer unit.  Could it be that the level of energy consumption in the house and the output from my solar panels have an effect on the display of hydro generation?

I think the answer is yes and the explanation is twofold. It lies in understanding the way the hydro monitoring gets its data and what happens to mains voltage when household consumption is high or the solar panels are generating.

The way the live power feed gets its data is from the frequency of a pulse taken from the installation's kWh meter.  The meter is calibrated to give 1000 pulses per kWh.  The parameter which the meter actually measures to give these pulses is current.  It doesn't measure voltage, which is the other parameter necessary to give an energy reading.  It just assumes that voltage will be constant, and for my Elster meter it assumes 230v.

Now in reality, house voltage is never quite constant at 230v. It varies slightly up and down from this value as a result of fluctuations in voltage in the main grid network; in addition to these fluctuations there are further fluctuations contributed by voltage drop over the supply cable from grid to house, and these will be dependent on how much power is being taken moment by moment.  So the voltage seen in the house actually varies significantly through the day, going down as power consumption goes up, but going up if there is much solar input.

What this variability in voltage means for the display of live power is that what is, in reality, a constant power output now appears to be not constant; and it mostly appears to be not constant in daylight hours because that is the time when the factors causing disturbance to voltage are most prevalent. It appears not constant because when the prevailing voltage is less than the meter's nominal 230v, the current the meter will measure will be higher by as much as the voltage is lower; since the meter measures only current, and this current measurement alone is the source of the frequency of the pulses on which the power display relies, the result will be an apparent upward blip on the power trace.

Conversely, transient or sustained voltages above 230v will cause downward blips and troughs.

It is only because the sensitivity of the system which is measuring live power is so good that such fluctuations in voltage can be seen to have any effect.  For all practical purposes, they matter not at all !

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