After installing solar panels on my roof I got my electric meter replaced with a bidirectional one. This meter can measure both energy that I draw from the network and the energy that I send, depending whether my consumption is higher or lower than my production.
It got me thinking though. If the network uses AC which has no constant direction, then how can a meter by measuring the current and voltage know if the power is consumed or given back?
In: 7
Same as DC really. DC also needs to tell direction of current flow. Just with AC you also need to check the polarity of the voltage at the same time, which this needs a third terminal to neutral in addition to the current in and current out probes. When the voltage is negative you just flip the sign on the measured current direction.
As for direction on a regular DC current meter. With a shunt resistor, when current flows through the shunt resistor you measure the difference in voltage across the resistor. If the voltage on leg 1 of the resistor is higher than leg 2 current is flowing from leg 1 to 2. And vice versa. Hall effect sensors current transformers and such also output readings the t let you know direction of current flow.
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