Digivation Hack Central

How Smart Meters can be Fooled

by on Sep.20, 2010, under Solar

As we all know, smart meters are being rolled out everywhere so that electricity companies and charge by time of use. This means that when you want the power most and so does everyone¬† else, they will increase the tariff for that period. I prefer my flat rate but apparently this is not a choice if you want a premium rate feed in tariff for your solar system. It will cost us at least $150 more per year for the power we buy with AGL’s 3 rate tariff.

The old disc analog power meters record power/current usage with a motor which spins a disc, and turning the counter in a calibrated way.

The new smart meters are digital. This means they sample the current and voltage waveform for a period of time and then do some maths to calculate the resultant power used in that period. This is then recorded.

There are several designs for smart meters, a common one using an MSP430 micro, but this could as easily be a PIC or an AVR. These devices generally have an A/D converter with multiple inputs. These A/D converters can only sample so fast. They also can only sample 1 input at any one time.

The sample rate would typically be in the order of 500us to 1ms due to limitations in the hardware. This leave gaps between the samples where power could be acquired leaving no trace on the meter. Then there is also the time between sample bursts, where the calculations are done or another phase/meter input is sampled. There is no record of anything used in this period either. If a second input is sampled, the first input is not sampled. No record of this either.

Then there is also outlier detection and removal. This means any sample that is not near excepted value compared to other adjacent samples is discarded, though not all meters would necessarily implement this algorithm, but it does guard against noise creating high false readings which customers may object to.

I have direct evidence that at least 1 product does this. The Fronius inverter which a colleague has had installed for his PV system draws 70W at night, measured with a plug in  power meter from Jaycar and also measured at 314 mArms. Looking at the current and voltage waveform on an oscilloscope, it shows several pulses, the main one just after peak voltage, and has a duration of between 100 and 200us. The EM1200 meter installed does not register this current draw at all!

The crux of this is that you could design a power supply along these principals, and get free power. This power could then be fed immediately back through a suitable grid connect inverter, the power which would be registered by the meter.

Conversely, if you get the timing wrong, the meter may record a higher than average value by several times.

I wonder how long it will be before this is fixed. ;-)

There is also the legal question. Is Fronius breaking the law.


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