capacitor problem..help appreciated

[meddy]

we have been experiencing some weird issues concerning exported energy into the distribution grid ..the meters show readings for exported active energy at customers having capcitors installed for power factor correction
when the capacitors are disconnected the meters show 0 KW exported
i m struggling finding a logical explanation..i checked the polarities of the CT's connected to the meters and they seem to be connected as the should

many thanks in advance

 

[dpc]

Improperly connected metering seems the most likely cause. You need to check voltage and current polarities and phasing and verify phase sequence assumptions.

 

[meddy]

Already have done that ..and that still wouldnt explain the presence of an export only when a capacitor is connected

 

[davidbeach]

Sure it would. If the meters are miswired such that they are metering kVArHr rather than kWHr, there would be export only when the capacitors are connected.

 

[dpc]

I would verify that the phase sequence matches what you think it is. I've seen a lot of screwy metering results when phases got swapped somewhere between here and there.

You probably need to go up to the primary CT and PT connections and work your way back, assuming, as you go, that everything is hooked up wrong. A donut CT installed "backwards" will flip the polarity, even when the secondary wiring looks fine.

 

[meddy]

already did that before..but ill try that again tomorrow i ll get back to you tomorrow
thanks for the suggestions

 

[Skogsgurra]

Is it an electromechanical or an electronic meter?

Gunnar Englund

http://www.gke.org

 

[meddy]

it s a digital meter(electronic)

 

[dpc]

Can you display or record the voltage and current phasors (magnitude and angle)?

 

[meddy]

sure... and the all have the desired amplitudes and angles

 

[dpc]

What is "desired"? You need to verify angles between all voltage and current phasors preferably with and without the cap banks and make sure the current is changing in the manner it should be.

Can the meter be configured for either positive or negative phase sequence?

 

[amps21]

What kind of meter is this??

 

[meddy]

there are altaris(french) meters
and they only display the magnitudes or the phase votages and currents...they also show a warning when the phase sequences are not respected.. so my guess is that it s not a sequence problem ..unfortunately
that was my first guess also but doesnt seem to be a wiring problem and we dont have meters available to see the angles of each phase

 

[dpc]

Do you have three PTs and three CTs?

 

[meddy]

just 3 CT's 75/5A in the box with the meter

 

[meddy]

all polarities were checked on the CT's
i m new on the job ..yet none of the senior engineers seem to find the solution to this issue either

 

[davidbeach]

Whoa there. You can't talk about energy if you only have CTs. You have to have voltage also. You need to verify how the meter is getting voltage from the three phases, or you need to ignore everything out of the meter except current.

 

[buzzp]

A possibility outside of wiring/polarity issues:

Donut style CT's are known to be 'flipped over' in their packaging. There would be no way to tell unless you opened up the packaging. Without a piece of equipment to tell you this, your kind of out of luck. If this is the case, the only thing to do is start flipping them over (one at a time) or pull out three new ones and install again. I had this happen to me twice in the last ten years.

 

[dpc]

I think the meter must be directly connected to the system - no voltage transformers?

 

[waross]

buzzp
Good suggestion. Flipped packeging or incorrect labelling is something we all have seen at one time or another.
As for checking. It doesn't require much equipment.
a 75:5 CT will have 15 turns on the secondary. If you hand wind 5 more turns in the proper direction you should have a 100:5 CT.
If the CT is reversed internally, the new turns will subtract and the effective ratio will be 50:5. You can check the ratio with a clamp-on ammeter.
yours