CT's and PT's connected to a power analyzer

[rockman7892]

I am using a power analyzer to monitor a 480V wye system. I have both CT's and Voltage inputs that come with the monitor which I will connect to the equipment. Because of the access to the service I am going to hook the CT's directly around the secondary of CT's and hook the voltage leads to the secondary of PT's.

My question is, the PT's are a 480:120V ration arranged in a corner grounded open delta arrangement. I was concerned that if I hooked up the PT's and CT's as described above then I might get false power readings because of a possible phase shift or wrond phase angle resulting from using the open delta PT arrangment.

Because the CT's are direclty on the wye system, I would think that thier phase angles would be referenced L-N. The PT's would have thier phase anlges referenced at L-L angles therefore the voltage phasors would be:

V a-b = 480V @ -330deg
V b-c = 480V @ -90deg
V c-a = 480V @ -210deg

Would this 30deg phase shift from L-N to L-L phase angles for the voltage give false current readings since the currents would be referenced at L-N angles?

To use the meter to monitor PT's you have to set it up for a delta system configuration and I was concerned about false readings as described above.

 

[rbulsara]

That is a very common set up. The PQ analyzer only cares for L-L voltages and line currents, which you have. If there is a setting, you need to tell the analyzer that it is a delta connection. And the get the ratio right.

Analyzer/meter is (should be) designed to calculate phase angles on its own for metering, as long as polarity (CTs) and phasing (A to a , B to b and C to c) are correct.

If in doubt measure a known load and see it matches the reading.

As for the reference, it for the display for user's convenience. There will be only one reference for I and V quantities in all phases. It should also be selectable, normally as Van or Vab.

 

[scottf]

What are you trying to measure?

Harmonics? Up to what order?

 

[rockman7892]

I am actually just trying to take power readings and was thinking about the various connections.

If I connect the meter to a three phase 4-wire wye system and I put the CT's and PT's on each phase + neutral then I'm guessing that the power meter will read both L-L phase angles and L-N phase angles. I guess it will always use the L-L phase angles when calculating power?

Is all line current measured in regards to the phase angle difference between current and L-N voltages for determining power factor. In other words for a power factor of .76 the phase angle between current and voltage is 40deg. This 40 deg is referenced between current phasor and L-N phasor not L-L phasor. Therefore when refering to current phase angle it may be represented as 70deg if the L-L phase angle is given as 0deg.

So i guess my other question is how line curren phase angles are typically represented. Are they represented by the difference between L-N or L-L voltage phase angles when displayed?

When watts are calculated then I'm guessint that the current has to be referenced to the L-N voltage to get correct power in watts? Is this correct?

 

[rbulsara]

Do not confuse with referece selected for representation/ display on the meter for current and voltages with how the meter will calculate kw and pf.

The meter will coorectly calculate the power. It is true when one says pf is .76, the 40 degree phase angle is between each phase (L-N) and its current as you mention.

Just by selecting a different reference, the relationship with betwen any two vectors on the same system do not change.