One CT - two different sources

[Bodoelectric]

A customer of mine asked me if he could use one set of CT's to measure both street power AND generator power. I wasn't sure how to answer because at some point - whether the utility power came back and the house was on generator or the utility power was normal and the generator was being exercised - it's conceivable that both sources would be live and likely way out of sync with each other. Will it drastically affect the accuracy of the CT to have both sources live even if only one source is actually supporting the load?

 

[GoldDigger]

A customer of mine asked me if he could use one set of CT's to measure both street power AND generator power. I wasn't sure how to answer because at some point - whether the utility power came back and the house was on generator or the utility power was normal and the generator was being exercised - it's conceivable that both sources would be live and likely way out of sync with each other. Will it drastically affect the accuracy of the CT to have both sources live even if only one source is actually supporting the load?

I suspect that it will not cause any problems if and only if the two sources are never active at the same time. But a real concern is that the potential terminals of the meter must be reading the voltage of the connected source. If the reason that the CTs cannot be mounted downstream of the transfer switch while the voltage connection can be made there, then I suppose it would be tolerable.

This would, of course, have to be private metering function, not the one used by POCO.

 

[Bodoelectric]

I suspect that it will not cause any problems if and only if the two sources are never active at the same time. But a real concern is that the potential terminals of the meter must be reading the voltage of the connected source. If the reason that the CTs cannot be mounted downstream of the transfer switch while the voltage connection can be made there, then I suppose it would be tolerable.

This would, of course, have to be private metering function, not the one used by POCO.

They cannot be made downstream of the transfer switch, because there IS no transfer switch...it's a main breaker interlock. The CT's would have both the generator feeders and the POCO feeders in them. 99% of the time there would be no issue, but in the event that the generator was being used due to a power outage and power was restored there would be two sets of feeders from two different systems in the same CT - albeit only ONE would actually be serving the load. It's my understanding that a CT is measuring the strength of the magnetic field around the wire it's surrounding, so by simply having voltage applied is there enough of a field generated to create a problem?

 

[GoldDigger]

They cannot be made downstream of the transfer switch, because there IS no transfer switch...it's a main breaker interlock. The CT's would have both the generator feeders and the POCO feeders in them. 99% of the time there would be no issue, but in the event that the generator was being used due to a power outage and power was restored there would be two sets of feeders from two different systems in the same CT - albeit only ONE would actually be serving the load. It's my understanding that a CT is measuring the strength of the magnetic field around the wire it's surrounding, so by simply having voltage applied is there enough of a field generated to create a problem?

A current transformer, like any transformer, operates on the basis of a magnetic field supplying the coupling between windings, not the electric field.
In a power/voltage transformer the primary is in parallel with the power source and the secondary is lightly loaded, making it appear that all that is happening is that the output voltage gets scaled by the turns ratio.
A current transformer's primary (in the simplest case, just the current carrying wire itself) is in series with the load and the secondary is very heavily loaded with the result that the secondary current is scaled from the primary current by the turn ratio.
A properly designed current transformer will not deliver any output when a voltage is applied to the sensed conductor without any current flowing.

If you connect the voltage sensing leads of the power meter to the panel bus (possibly through a branch breaker or a fuse) then what you propose will work. Make sure you get the polarity right when you pair the two source wires in each CT.

 

[Bodoelectric]

A current transformer, like any transformer, operates on the basis of a magnetic field supplying the coupling between windings, not the electric field.
In a power/voltage transformer the primary is in parallel with the power source and the secondary is lightly loaded, making it appear that all that is happening is that the output voltage gets scaled by the turns ratio.
A current transformer's primary (in the simplest case, just the current carrying wire itself) is in series with the load and the secondary is very heavily loaded with the result that the secondary current is scaled from the primary current by the turn ratio.
A properly designed current transformer will not deliver any output when a voltage is applied to the sensed conductor without any current flowing.

If you connect the voltage sensing leads of the power meter to the panel bus (possibly through a branch breaker or a fuse) then what you propose will work. Make sure you get the polarity right when you pair the two source wires in each CT.

So, the voltage is certainly measured from a dedicated branch circuit that would only be supplied power from one source at a time...generator or utility - whichever was selected. Much like a typical transfer switch, an interlock will do this...which I'm sure you already know...

So assuming now that it CAN be done without affecting the measurement the CT making or creating a hazard, are there any code issues that would affect an installation like that? Just to be clear, it would be a typical 200A residential panel with a main breaker interlock, and the CT's would be installed on both sets of feeders in the main panel...

 

[gar]

180223-0711 EST

Bodoelectric:

A single CT with many wires passing thru the CT will produce an instantaneous output current that is the instantaneous sum of all the currents passing thru. None of these currents have to be related to each other. If only one wire of the many wires is carrying current at an instant of time, then the output current is only proportional to that one current carrying wire.

The only way the voltage on a conductor with no current flow thru the wire could affect the CT output would be by capacitive coupling, insignificant because of the impedances involved, or by leakage current, very unlikely.

What is important is the direction the wires go thru the CT and is there space to get the wires thru.

If this is a TED system, then the CT center hole is not very large, and it is important that the clamp-on CT can fully close over all wires thru it. With the TED system there is substantial phase shift thru the CTs and as PF changes from unity to lower values a power reading error develops from its actual value where power calibration was done with a resistive load. This error can be great enough to make someone think that a PF correction capacitor will save a residential customer money on their bill.

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[gar]

180223-0741 EST

Changing the wire direction thru the CT changes the output phase by 180 degrees, and in a power measurement from positive power to negative or vice versa.

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