Open Y Connection 1

[vtpower]

Howdy,
I have recently been working on a voltage complaint for one of our customers. It is an elementary school at the end of one of our 12KV feeders. The three phase ends about 5 miles before the school and two primary phases continue on through the town and end up at the school. The original service was set up in the 60's and is connected as an open wye service to get 120/208 off of two phase. I believe this responds similar to an open delta connection, but I am not sure. The issue is I am getting poor voltage on the phantom leg, but not during the school peak, it actually occurs during the town peak, 5PM to 8PM. It has been burning up a motor starting coil for a booster bump. Besides extending the three phase for 5 miles, are there any solutions that would help stiffen this voltage issue? Thank you in advance for any info.

 

[wareagle]

Suggest you set a recorder to monitor the voltage on each primary phase. You probably have an unbalanced system. If this is correect go back an balance the loads between the 2 phases. I would check the loading on the transformers also. Also go basck to the end of the 3 phase line and check the primary voltafes there. Us single phase transformers to make the check. Your entire feeder may need balancing. If the voltage is still low check the regulator setting.

 

[vtpower]

After the original complaint, we set a recorder for a week to monitor voltage, also, there is a set of two regulators approximately 3 miles upstream of the school with data recording control. I did check on the balance and it appears to be fairly reasonable. One idea, as you mentioned is to bump up our regulator setting a volt or two, but that would mean having one odd setting out of hundreds of regulators and I was hoping to avoid that.

 

[waross]

Cheapest quickest, safest. If the only problem is the coil in the control circuit burning up, move the control circuit connection to the solid phase(s)
I am not familiar with the open wye connection except to provide what would normally be single phase power from a 120/208 volt wye system. Is this a three phase transformer with one primary phase missing? If the school doesn't need three phase, go to 120/240Volts.
If it does need three phase go open delta. That seems to be the common way to produce three phase from two primaries.
I have that sinking feeling that I may be going to learn something new about transformers before this thread is over.
Do I understand that the town is on two phases? The neutral will be carrying about the same current as the two phases. You may have a neutral voltage drop issue, particularly if the problem coincides with the town peak loading. If the neutral current is causing a voltage drop on the neutral that can cause a phase displacement even back on the full three phase section of the system. With the neutral current you may also have ground currents depending on ground resistance. Maybe not. Without the regulators you would probably have unbalanced voltages and phase displacement. The regulators are probably masking the unbalance voltages but not really affecting the phase displacement. You should be looking at current baance, as well as voltage balance.
Consider extending the third phase to the town and balancing the load there. You can probably find enough current unbalance to justify a third line. Check for ground currents on your multiple ground connections. There may be none, but if there are it is further justification to run the third phase. A third phase to the town will probably cure the problem at the school, if moving the coil connection doesn't help.
yours

 

[cuky2000]

An open wye in the secondary is not an operative connection. I am guessing that you may have a wye grounded primary with open delta mid-tap grounded in the secondary. This connection provides in the low side 120V, 208V and 240V.

Overvoltage is likely to appear in the secondary if the primary neutral conductor has a poor grounding connection or loss ground by switching some where in the system

[blue]“Static discharge is potentially present on a non-grounded primary wye connection. A high, excessive voltage results on a 3-phase Y-Delta connection on the secondary line to ground when one leg of the primary is open. The voltage present is static with no power and bleeds off when taken to ground. This static can damage a volt-ohm meter. The static is greater when the secondary feeder is short and lesser when the secondary feeder is long. The static problem is resolved by grounding one phase or the center tap of one transformer on the secondary side, but this usually requires special KWH metering. This static condition is present only when a primary line is open, not the secondary. This static condition can occur on an open (2-transformers) or closed (3-transformers) bank. This static condition can occur with any primary voltage.” [/blue]

 

[waross]

Hello cuky2000,
I ran into a similar system but different problems a few years ago. The two wire primary causes a neutral current which elevates the neutral above ground or causes ground currents through the multiple protective earthing. Or both.
We were having severe problems back on the full three phase system because of the neutral voltage drops.
A couple of questions about the static your post mentioned. Is this a capacitive coupling between the windings in the transformer?
It was pointed out to me recently that RUS standards dictate floating neutrals on Star delta banks to avoid burning out transformers on missing phase or voltage unbalance conditions, but the secondaries are grounded.
Re the special metering. My old metering hand book shows a lot of connections on several meter forms that are suitable for grounded Delta systems. The form 5S and form 8S may be used for either Delta or Wye, but the connections are definitely different and may need multiple conductors through the the CT's. There is also a Wye metering scheme for form 5 that requires delta connected CTs.
However.
The new electronic meters I am seeing auto detect and work on either center tapped Delta or Wye systems with no change in connection. The last Delta to Wye changeover that I did I paid the customary visit to the utility metering department to co-ordinate the metering. The engineer told me, just leave all the metering connections the same and don't worry about it. We'll check it sometime when we have time.
I think the meters are based on the form 9S or 9A. but I'm not sure.
Yours

 

[vtpower]

Waross and cuky,
Thanks for the posts. The current connection uses three single phase transformers, two 25KVA and 1 50KVA, all 120/240 transformers. The two end transformers (25KVA's) have their windings parelled, as you would in a normal 120/208 connection, but the 50KVA has its windings in the normal 120/240 configuration. One of the 25KVA is connected Primary B to Ground and Secondary B to ground as a normal 120/208. The second 25KVA is connected off the same primary phase as the other 25KVA but has one of its secondary bushings connected to C on the secondary bus and its other bushing is connected to the X2 bushing of the 50KVA. The 50KVA then has X1 conncted to A on the secondary and X0 connected to ground. I wish I could post the drawing but I am not sure how.
The school has some 208 volt air handling equipment, otherwise I was looking to convert them too 240 open delta. I have had them move their motor starter to the stiffer legs, but I was hoping to solve the issue an avoid other complaints.
Extending the three phase is certainly an option, but to get it far enough to make a difference would be around $1 Million.
It is certainly not a transformer connection used today, but unfortunately, when dealing with antiquated system, it is something we have to deal with.

 

[waross]

I just sketched out the transformer connection you described. I had to give you a star for showing me a new connection. I didn't think it was possible to get 120/208 from 2 primary phases but I was wrong.
Re your problem. Most of the airconditioning compressors I run across have a wide enough voltage tolerance to work on either 208 or 240. The standard motor for 480 volts has been for years rated at 460 volts.
I would certainly consider an Open Delta connection but one of the factors against may be the loading of the existing panels. When you change to open delta on an existing panel, every third breaker becomes "Wild" and is unusable for 120 volt circuits. You may have to change out panels, or add a single phase panel for the displaced single phase loads. At the least you would have to do a lot of breaker shuffling.

Most older motors running on 208 volts are actually rated at 230 volts. A new trend seems to be 200 volt rated motors for 208 volt operation but your motors are probably not that new.
To state my earlier post in a different way the problem is probably being caused by the town load on the neutral.
If the town is experiencing normal growth rates, you can expect the problem to get worse. Extending the third phase to the next major load will be a big help, but that's probably the $1,000,000 job in any event.
Please let us know what develops in any event.
respectfully

 

[vtpower]

Thank you greatly for the information. I will have a look at the air handling motor ratings, and the panel. If I used a dry type stepdown from 240 to 120, would I be able to use the existing panel, while having a 240 open delta service?

 

[waross]

Hi vtpower
One thing that we haven't mentioned yet is to examine the neutral from the town to the school to detect any damage or bad connections. A high impedance connection on the neutral may cause a large part of the neutral current to be returning through ground with a resulting neutral voltage displacement.
In regards to changing to an open delta system.
You would use the existing 50 KVA transformer for the single phase 120V and 240V loads. One of the 25 KVa transformers would be used for the open delta connection. This could be the third phase in existing panels or a new three phase panel.
The voltages to ground will be 120V, 120V, and 208V "Wild" leg.
You can use the existing panel, and the connection changes are all done at the transformer bank. You will have to rearrange the breakers in the existing panel, but if it's a three phase panel you may run out of available spaces, as the wild leg should only be used for three phase loads.
A change from 4 wire delta to star is usually straight forward. A change from star to 4 wire delta can get messy.
One development of the open delta has been to address
situations with a large single phase load requiring a small amount of three phase. A second transformer may be added to supply three phase. The primary service may be a normal commercial or residential single phase installation with one extra transformer and one extra wire added to supply the three phase loads. Typically there would be all single phase panels with a three phase panel added at the location where the three phase is needed.
Where more three phase is required there are some reasons to go open delta, There may be a saving in installation costs with two larger transformers rather than three smaller transformers. As in your case there may only be two primary phases available.
It's a case by case situation. You have to investigate the panels in the school to determine if you can rearrange the breakers or if you have to change panels and/or add panels.

However your root cause is probably the voltage drop on the primary neutral.
As I mentioned this may also be caused by a bad connection as well as by umeven loading. As a first response try checking for current on your grounding conductors along the line. If the town is using any open wye connections without problems then a bad neutral connection on the neutral is a real possibility.
Are you supplying three phase in town and if so are you having any problems?
If the problem is being caused by neutral currents the voltage drop will depend on the neutral current and will probably extend some distance into the section of your system with full three phase power. You may be able to mitigate the problem by shifting loads on the full three phase section of the line so as to load up the short phase and create a neutral drop that is opposite to the drop at the school and will partially cancel it. I don't recomend this. It is not solving the problem but rather spreading it out. As in a bunch of small problems rather than one big problem.
respectfully

 

[cuky2000]

Hello Vtpower,

Here is a draft sketch with my interpretation of the connections as you described in your post. I took the liberty to assume transformers with additive polarity and standard terminal markings nonmenclature per ANSI C57.12.20.
I also Include in the sketch a few questions for your verification and clarification as shown in the enclose link.

http://cuky2000.250free.com/Wye_Open_Delta_Open_Draft_1.jpg

For comparison with your system, a similar open wye-Open Delta grounded configuration using only two transformers is included below for information.

http://cuky2000.250free.com/Wye_Open_Delta_Open_CASE_1.jpg

 

[waross]

Hello cuky2000
I always like your drawings.
I interpreted the connections differently. May I make a suggestion?
Assuming;
Transformer #1 120/240 as shown.
Transformer #2 and #3 Reconnected for 120 volts, X1 to x3, no connection to X2
In your diagram;
1> Transformer #1, X1 to "A"
X3 to neutral, & ground.
2> Transformer #2, X1 to "B"
X3 to neutral, & ground
3> Transformer #3, X2 to "C"
X1 to X3 on transformer #1.
In this connection, the combination of the X2-X3 winding on the 50KVA transformer and the winding on the #3 transformer form a 120 volt open delta. The resultant voltage and phase angle are correct for the third leg of the Wye system.
Can you redo the drawing and sketch the vectors? Thanks.
respectfully

 

[cuky2000]

Hello Waross,

Enclose is the revise sketch with your comments incorporated.

http://cuky2000.250free.com/Wye_Open_Delta_Open_Rev_a.jpg

Vtpower- Did you concur with Waross’ comments?

Here is extra information extracted from JEA Engineering Manual:

Third harmonic component of exciting current must be supplied from the supply. Unbalanced primary voltages cause negative sequence voltages and currents on the secondary, which can lead to overheating of motors on the system.
The maximum and minimum voltage variation should not exceed 5 to 6% and the bank should not be used with motors in excess of 25 HP.

 

[vtpower]

Cuky2000,
The first change, which really doesn't matter but it is how it is connected in the field, is the two 25KVA xf are on either end with the 50 in the middle.

The X2 bushings on the two 25KVA are not used. The windings are parelled with one end connected to X1 and the other too X3. X2 has no connection.

To follow my above transformer swap, I will say Transformer 1 is a 25 KVA and that #2 is the 50.

Transformer #1 (your #2) has the primary connected exactly as you show it. X1 is connected to A, X2 is not connected to anything, and X3 is connected to X1 of the 50KVA.

For Transformer 2 (your #1, 50KVA)- The primary is connected as you show it. X2 is connected to neutral and X3 is connected to phase C.

Transformer #3 has X1 connected to Neutral, X2 not connected and X3 connected to B.

Primary neutral and secondary neutral are connected and the transformer tanks are solid grounded. I apologize if this is clear as mud, but let me know if something is unclear and I will try to re-word it. Is there a way to attach a PDF to these posts?

 

[waross]

Hi Cuky2000
We're getting closer.
As per vtpowers comments;
Let me try to describe a vector diagram that will explain the voltage relationships, and you can rotate it or reverse it to
apply to actual field conditions.
The first Vector is the 50 KVA transformer.
I visualize it as a horizontal line, 1X1, 1X2, 1X3.
1X1 is "A" phase. 1X2 is the neutral and 1X3 is just hanging out there for now.
The second vector. This is one of the 25 KVA transformers.
2X1 is "B" phase and 2X3 is the neutral. I wisualize this vector as originating at the neutral (1X2) and extending above the line at a 120 degree angle to 1x2, 1X1.
Now the third transformer has a vector of the same magnitude and phase as the second transformer. If this is connected 1x2, 1X3-3X1, 3X3, the resulting vector should originate at the previously neglected end of the 50 KVA transformer winding, 1X3. It will extend down at an included angle of 60 degrees. Now half the winding of transformer #1, (1X2, 1X3) and the winding of transformer #3 will form an open delta. The voltage and phase angle across the open delta will be correct to supply the third phase of the 120/208 volt system.
I have never seen this connection in a text book, and I am impressed with the ingenuity of the originator.
respectfully
Ps. and thanks Cuky2000 for your help an patience.

 

[cuky2000]

Here is the revised connections.

 

[vtpower]

The only issue is it appears the two 25KVA are connected 240, and they are actually connected 120 with the windings parelleled. I assume you can't make that change in the drawing, so if we imagine the windings parelled it is perfect.

 

[cuky2000]

Hello Vtpower,

The correction was done as suggested.

Regarding your question if is there a way to attach a PDF to these posts? The answer is yes. This is a very effective way to communicate ideas in this forum.

The first step is to obtain an account in a server. There are several free services such as the enclose link.

http://250free.com/

I hope other members of this forum will be interested to use this feature. I will be happy to share what I know.

Best regards.

 

[waross]

Hello cuky2000
It looks good to me as far as generating the right voltages in the right phase order..
Thanks for staying with it.
respectfully

 

[mc5w]

One of the kookier service configuration that I have seen are two 400 amp 120/240 volt services metered by 1 CT rated meter. The meter nameplate says that the voltage is 120/240 5 wire X and is a form 6s rewired to act like 2 form 4s meters. This is in an area where single phase services were at one time restricted to 400 amps maximum. The 2 supply transformers are on a 4,400 volt delta primary system with 1 transformer connected across 1 side of the primary triangle and the other connected across the second side of the primary triangle. Sounds a lot like this is what you might have.

One disadvantage of open wye open delta transformer banks is that balanced 3 phase loads produce a primary neutral current that is 1.73 times the current in the hot leads. In other words, the people who need all 3 primary phases the most are the ones who are farthest from the substation. This was one of the arguments back in the old days for using 3-wire 3-phase ungrounded ( e.g. 4,400 volts, 4,800 volts, or 7,200 volts ) for rural distribution is that 3 wires gives you full 3-phase and not open wye.

In actuality, a 7,200Y12,470 volt system should be considered to be a 7,200 volt system that can only go about 7 miles from a substation. Step change voltage regulators do not do anything for the voltage change that occurs when motors start.

An alternative for 7.2Y12.47 KV systems is to connect some single phase transformer phase to phase rather than phase to neutral. There is such a thing as a 2 primary bushing 14,400 volt transformer that has taps for 13.8 KV, 13.2 KV, and 12.5 KV. I also believe that 12 KV transformers for use on 6,928Y12,000 volt unigrounded systems have an internal taps for approximately 12.3 and 12.6 KV so you could do things that way too.

Running that long of a line as a 7,200/12,470 3-wire network line is a bad economy in the long run and you might as well upgrade to a 3rd hot wire. If this line really is 40 years old you probably might even need to replace/upsize the 3 existing wires as well.