Solid State Meter - Net Metering

[saladhawks]

I understand how an older electromechnical meter can be used in net metering applications by reversing the direction of the meter disk when exporting power back to a utility.

When using a solid-state meter for a net metering application, can someone explain the internals of how the direction of power flow is determined?

 

[davidbeach]

The same way. In both cases, the phase angle between the voltage and the current determines the direction of the power.

 

[wbd]

In the newer microprocessor based relaying, it will display or record what quadrant the power is in. That will tell you wif it is power received or delivered and can be totalized.

 

[waross]

I understand that the electronic meters can give independent reports of power recieved and power delivered. When the buying and selling rates are different, as they often are, two meters with ratchet attatchments were often needed with the electro mechanical meters. The ratchets were also sometimes used on KVARHr. meters. In the old days when permanently connected power factor capacitors were common, you could increase your power factor to unit and bank the difference between 90% and 100%. Going a little leading was OK with many utilities as long as there were no problems, but the ratchet on the meter denied you credit for any KVARHrs in excess of those needed to achieve unity power factor.

 

[JBinCA]

The power at a particular point in the system is the product of voltage and current and the cosine of the phase angle between them. Electromechanical meters set up a magnetic circuit that physically measured this by consuming a fixed fraction of the measured power. Microprocessor based or solid-state meters measure the instantaneous values of voltage and current inputs and do math to come up with power.

But it is important to note that the meter itself doesn't know[//b] what "direction" positive watts and vars are in. You have to make that determination and apply the meter appropriately.

In most cases there is only one acceptable definition of positive or "delivered" watts and VARs, in others it's an arbitrary determination.

Once that determination is made, you have to apply a specific meter based on that determination. A lot of modern Microprocessor based or solid-state meters will allow you to define that direction in their software (since they're just doing math, this mean's multiplying the calculated current phasor by -1 to add the effect a 180 degree phase shift). If a meter does not give that option, you would have to reverse the current inputs to the meter to get it to change which direction of power it records as negative, and which it records as positive.

Regards,

JBinCA

 

[saladhawks]

If microprocessor based relays calculate power based on the instantaneous values of voltage and current internally via software algorithms and do not inherently know the specific "direction" of power flow - how difficult would it be to add the functionality to determine the specific "direction?"

Would this have anything to do with setting up a "dummy" load within the meter that determines the power angle between the voltage inputs?

 

[davidbeach]

A microprocessor based relay (and I would assume a meter too) knows the direction as defined by the angle between a voltage and the corresponding current. Anyway it knows both angles. The metering command for a relay with both voltage and current inputs will provide information including watts and vars in or out (forward is toward the wye point of the CTs, away from the polarity of the CTs). Piece o' cake.

 

[stevenal]

JBinCA was referring to how the thing is wired to the circuit. All the meter "knows" is forward or reverse, with no knowledge of whether forward is east, west, up, down, toward the cogen, away from the cogen, etc. This choice is made by the circuit designer or meter programmer.

 

[dpc]

There is a standard IEEE definition of power flow relative to CT and PT polarity. The micro-processor meter will be configured to register power according to this standard as a default. If you hook up the CTs with reversed polarity, it will not know this - just like the electromechanical meter. It will certainly be able to distinguish the direction of power flow. How you configure it to register this is another matter entirely.

The digital meter "knows" at least as much as the old electro-mechanical Whm and generally "knows" a lot more.