three element metering on a delta

[stevenal]

Any problem with using full three element metering on an ungrounded delta service? Normally this would require two elements and an 8 jaw meter, but it is already in place with three elements and 13 jaws. The potential transformers are connected wye, and are rated for line to line voltage. I was originally concerned that the wye point potential could move about based on PT impedance and burden. But after drawing a few phasor diagrams, I cannot find a wye point position that will cause an error, even if it should move outside the delta. Other than the weird no-load voltages Davibeach is seeing, is there any other problem with this arrangement? This is for revenue, and the meter in question is an electronic auto ranging meter. Thanks.

 

[waross]

Hi stevenal;
I am concerned with the electronic meter that may have a very low burden. I would be more comfortable connecting the VTs line to line and removing or shorting one CT.
I am concerned that if the wye point migrates from the geometric neutral, there may be an error with power factor and VARHr measurements.
Alternatively, how about using capacitors to establish a wye point as David Scott mentions in thread #238-173479

DaveScott (Electrical)
18 Dec 06 0:10
Ah, yes. I think you are right. I just checked what we do with electric arc furnaces (floating delta), and there is a set of 0.1uF capacitors, with fuse protection, which are in star connection to provide the metering phase-gnd measurement.

respectfully

 

[ScottyUK]

Stevenal,

I have seen a couple of instruments which exhibited some strange behaviour on a VT group with a grounded 'Y' phase ('B' phase in the US?) instead of a grounded star point. In a previous thread I thought I could remember what they were: I'll confirm that a GE Multilin PQM II and a Yokogawa WT 2030 both had a problem with this connection even though the literature suggested otherwise. Later PQMs appear to have no problem; I've never used a WT 2030 since to comment further.

I don't know what you are planning to do with the secondary VT earth, but it might be worth running by the meter manufacturer and verifying that they warrant its operation with whatever connection you use.

----------------------------------

Sometimes I only open my mouth to swap feet...

 

[stevenal]

Waross,

On my phasor diagram, wye position doesn't appear to matter to either to total W, VArs, or pf. Anything subtracted on one phase shows up in the other.

Also trying to contain costs while not getting too far out of spec. Capacitors in an instrument transformer can would be unexpected.

Scotty,

I avoided using the words earth or ground. The wye point is established by the balance (or lack thereo of) of the PTs and the burden of the meter coils. It is brought into the meter's potential neutral position.

 

[apowerengr]

stevenal - take a look at davidbeach's thread on the voltage measurements, seems a similar condition. Form 5 meters are still widely used.........

 

[stevenal]

I read David's thread with great interest.

Update: There was a center tap available on one of the cans, which we tied to ground and the PT wye point. Now it is a 240/480 4 wire delta being metered just like the common 120/240 4 wire delta. After this change, the meter was observed reporting an error condition. Further inspection showed a toasted PT. Not sure just when it smoked. Did the wye point move so far that the 600V PT went over-voltage?

 

[waross]

On a wye delta bank with a floating primary neutral, you may experience switching surges. This may have been the cause of the PT toasting. Some utilities connect the primary wye point to neutral temporarily when closing single phase fuse cutouts on a wye delta bank. This avoids the switching surges. Once the bank is online, the neutral is floated to avoid circulating currents and backfeeds.
Hopefully someone who works with in the field with wye delta banks can give us some information on the magnitude of the switching surges that may be expected.
respectfully

 

[ePOWEReng]

Here is a link to a paper regarding single phase switching causing an over-voltage on a floating wye-delta bank. The over-voltage on the primary system can be 2.5 PU phase-neutral on the last phase closed...enough to cause failure of MOVs.

http://hubbellpowersystems.com/powertest/literature_library/pdfs4lib/OB/EU1159-HR.pdf

 

[ePOWEReng]

Here is a link to a paper regarding the over-voltage present during single phase switching on a ungrounded wye-delta transformer. The phase-neutral voltage on the last phase to be closed can be as high as 2.5 PU.

http://hubbellpowersystems.com/powertest/literature_library/pdfs4lib/OB/EU1159-HR.pdf

 

[waross]

Nice link, hold6448;
An additional issue is backfeeds when a feeder supplies mixed residential and wye-delta loads.
If these feeders are single phase fused or switched you may expect a lot of issues with residential loads.

With a floating neutral you may experience overvoltage backfeeds from the transformer bank when two phases are missing. These backfeeds may aproach 1.73 times normal voltage BEFORE switching surges are reflected onto the primary.
This may cause the failure of voltage sensitive loads.

With the wye point connected to the primary neutral, there are other issues with residential loads.
On a single phase condition, the secondary voltages on the two unpowered phases will sum to line voltage. With two phases energised the voltage on the third phase will approach normal voltage, but the wye delta bank will be supplying the power to the third phase. The transformer bank may be overloaded and the voltage drop may be excessive.
If a feeder has been de-energised and is re-energised with single phase switching, the common sequence is:
1> First phase is energised. Second and third phases back-fed with about half voltage. Many freezers and refrigerators are demanding cooling and try to start. The voltage is insufficient to supply sufficient starting torque and the compressors are stalled on about half voltage and start heating up.
The transformer bank may be overloaded.
2> Second phase is energised. It is now supplying normal line voltage. Many of the compressors remain stalled by head pressure and heating continues until the internal thermal overload device disconnects them.
The third phase is back-fed from the transformer bank. The transformer bank is more likely to be overloaded now and the voltage drop may be noticable. Refrigeration compressors continue to heat.
3> Third phase is energised. The voltages are normal. It is not unusual for one or two residential refrigerators or freezers to fail. The internal thermal cutouts are supposed to protect them, and do protect most of them, but the combination of an unusual heating pattern and repetition often overtakes one or two units.
respectfully

 

[stevenal]

The service is fed floating wye/delta. For the reasons mentioned above, this wye point is temporarily grounded when switching the transformers in and out of service. The metering PTs are wye/wye.

 

[apowerengr]

stevenal - if I read your post correctly you are using 3 PT 3 CT connection and a form 9S meter so I'm assuming you have 277/120 volt PT's. You stated the PT's are rated for full line-to-line voltage however I wonder what their nominal voltage is as you later indicated that the PT's are rated 600 volts. Connecting the neutral point of the assumed 277/120 volt PT's to the center-tap grounded leg on a 4 wire 480/240 volt service would connect one PT on the "wild-leg" with a nominal voltage to ground of 416 volts. I'd guess you're smoking the PT and/or the meter with this high of a continuous input voltage. Three-phase 4 wire delta is correctly metered with a form 8 meter. If your PT's have a different ratio you're meter is most likely erroring as it is seeing a significant imbalanced voltage with the wye connection (input voltages of 240, 240 and 416). I'm also curious, when you grounded the center-tap did you take the grounded conductor to the service so you have a ground-fault path?

 

[stevenal]

The PTs have a ratio of 2.4 and good for continuous duty at 600V. Both the primary and secondary voltages are within the ratings of the PTs and the meter. The meter is an electronic 9/8S which can correctly meter 4 wire wye or delta. Just the three wire delta that I was unsure of. The grounded center tapped conductor extends to the service.

 

[waross]

Conventional 13 jaw DELTA measuring may be done with a FORM 8 meter.
Conventional 13 jaw WYE measuring may be done with a FORM 9 meter.
I believe that a FORM 8 meter will be a drop-in replacement to measure 4 wire center tapped delta.
Two current coils on a FORM 8 may be connected in series to equal a FORM 5 meter. A FORM 8 meter is derived from a FORM 5 by splitting one current coil. This enables it to measure unbalanced currents on the center tapped phase.
Some of the new electronic meters are self configuring and will conform to either wye (FORM 9) or delta (FORM 8) depending on the voltages applied.
Re The fried PT. I suspect a wrong connection that was corrected after the PT died. If connection instructioNs and diagrams were given for an 8 jaw meter, an electrician may have tried to use the diagram for a 13 jaw meter by ignoring the unused jaws. This may result in a short circuit on one PT.
Please check up to date data sheets on the meters you have available. You may find that a FORM 8 or an auto-configuring FORM 9 may be a plug-in fit.
Use the center-tap on one phase as the system ground and the metering neutral/ground.
I hope i have this right. I've just come a long way to investigate a metering problem.
good luck.
Respectfully

 

[stevenal]

Well I was misinformed on the PT. The insulation class was 600, but the primary voltage rating was 288. Now we know why it smoked.

 

[waross]

Thanks for the feed back, stevenal.
respectfully

 

[stevenal]

I think I found my answer to the original question here:

http://www.themeterguy.com/cgi-bin/yabb/YaBB.pl?board=Ask;action=display;num=1167752113

and here:

http://www.themeterguy.com/cgi-bin/yabb/YaBB.pl?board=Info;action=display;num=1166665132

Works fine with ideal instruments, but real ones like to have a neutral present.