Consumption meters and the 'sweet zone'

My consumption meter is an Ampy 5196A. This is believed to have a 3600 Joule sweet zone which can be used to ‘buffer’ energy by rocking power back and forth through the meter without generating charge pulses.
At high levels of house consumption and using 1800 Joules passing back and forth the meter LED will stay on for as long as you are prepared to stare at it. This is a good demonstration that there is no bias towards import as no charge pulses are seen.
At lower power levels when several seconds are required to fill the 1800 Joule energy bucket the meter LED stays on most of the time but occasionally the meter LED will extinguish briefly indicating that a meter pulse has occurred.
The reason for this difference in performance is believed to be the poor power factor that applies when the house power requirement is very low. When high power loads are on the load is predominantly resistive and the current waveform will be more sinusoidal.
In the quest for perfection and to eliminate those charge pulses I have experimented with an adaptive export bias mechanism to prevent these unnecessary charge pulses occurring.
So, the test conditions were house load about 100W – 200W and roughly 100W surplus PV available with an overcast sky.
The energy being rocked back and forth was 1500 Joules and the experiment involved increasing the export bias until the meter LED remained permanently on.
Bias (Joules)       Triac pulses before LED extinguished
00                      10
50                      20
100                     27
150                     Stays on
The result for what is quite a severe test is that a 10% bias towards export was required to ensure the meter LED remained on. This does not necessarily mean that 10% is being wasted. It probably means that the diverter’s measurement accuracy is not so good when the current waveform is distorted and phase shifted.
Some more experiments suggested that at around 500W the meter LED stays on without any bias towards export. Clearly what is therefore required is a bias that is applied when say the surplus PV falls below 600W.
A bias mechanism as described above has now been added to my adapted version of Martin’s eMC sketch and this appears to work exactly as required.
What surprises me is that to my knowledge no one has mentioned this before. Is this because we are once again talking about micro pence or has it simply passed unnoticed?

Robert Wall's picture

Re: Consumption meters and the 'sweet zone'

Robin is away this week. I'm sure he will have a comment concerning synchronisation between the controller and the meter, which I know is something he's working on.

calypso_rae's picture

Re: Consumption meters and the 'sweet zone'

While any of my Mk2 router rigs have been actively diverting surplus power, I have never noticed the meter's LED going from 'on' to 'off'.  As you correctly say, any transition of this type means that a charge has been applied.  While surplus power continues to be available, my router design ensures that import and export remain in balance thus no chargeable events should be registered by the meter.   Providing that the measurement system is linear, the degree of balance that is achieved should be nigh-on perfect, well below the anti-creep threshold of the meter.

Did you notice this same behaviour while you were running my Mk2 code?  By calling the checkLedStatus() function every mains cycle, each transition of the meter's LED can be analysed.  If a chargeable event ever occurs while a dump load is on, then that situation would be well worth noting. 

Switching between multiple dump-loads would be the most likely occasion for charge pulses to sneak in.  Particularly so if the primary dump load is unable to consume all of the surplus power.  When a cloud passes by, more than one load may need to be turned off which is asking a lot of the logic if A/F precautions are in place.  Rev5 of my Mk2i code uses a different type of A/F algorithm which I believe to be particularly well suited to this challenge.

During periods of consumption, it would undoubtedly be helpful to note and respond to pulses from the meter.  This would allow power diversion during each period of surplus power to start up cleanly without any loss of energy to the grid.  The meter's LED would then be 'off' whenever surplus power is being diverted, rather than 'on'.  Once actively diverting power, the only event that should happen at the meter is that its LED may go from 'off' to 'on' after some minutes as the absence of any significant net flow causes the meter to enter its anti-creep mode.

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