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?
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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.
The power meter will measure both current and voltage. When you are using power the current goes into your house when the voltage is positive and out of your house when the voltage is negative. But when you are making power the current goes out of your house when the voltage is positive and into your house when the voltage is negative. The meter will either measure the voltage and current very regularly and calculate this for each sample, or there are analog circuits which can multiply current and voltage and then do analog integration for the meter to read every once in a while.
It’s easier to describe in terms of DC but it wouldn’t really answer your question.
With AC, as you say, the voltage is changing ‘direction’ all the time, but so is the current.
So power = volts x amps. When a device is USING power the voltage is positive the current is also positive, and when the voltage goes negative, so does the current.
Plus x plus = plus.
Minus x minus = plus.
So power is still flowing the same ‘positive’ direction towards the device that’s using it even when the voltage and current are negative.
So when a device PRODUCES power, from its perspective, the current and voltage are opposite.
Think about a water pump and a hose nozzle. The water flows towards the hose nozzle and away from the pump, but the water pressure is positive in both cases.
Plus x minus = minus.
Minus x plus = minus.
So back to your original question. The way your meter can tell which way the power is flowing is to measure which direction the current is flowing when the voltage is positive.
I hope that helps but if youre interested and want to go further then try to read about ‘passive sign convention’ and ‘power factor’
The meter looks at instantaneous current and voltage.
In an AC system, the voltage is positive for half the time and negative the other half. If you are importing energy, then the current will be into your house when voltage is positive and out when voltage is negative.
The meter takes the voltage and current readings, multiplies them and sums the result. This way if you are importing, the calculation will give a positive power, or negative if exporting.
In fact, all electricity meters do this.
Regular electricity meters have a block (either mechanical or software) which stops the counter going down when export power is measured.
A bidirectional meter has two separate meter counters. One only goes up during import and one only goes up during export. They are both blocked from going down.
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