PROS AND CONS OF OPEN WYE/OPEN DELTA TRANSFORMER BANKS

[jing09]

I VISIT A SITE RECENTLY AND INSTALLED A POWER QUALITY ANALYZER. TO MY SURPRISED THE RECORDED DATA IN THE INSTRUMENT SHOWED A VOLTAGE SAG. IT OCCURS 5 TIMES, ALL HAPPENED AT DAWN. NOW MY QUESTION IS HOW DOES THIS VOLTAGE SAG HAPPENED WHEN IT IS DAWN AND NO BUSINESS ESTABLISHMENT WAS OPEN? IS THIS CONNECTED TO HOW THE TRANSFORMER BEEN CONNECTED INTO A OPEN WYE OPEN DELTA?

THE UTILITY COMPANY USES TWO 100 KVA TRANSFORMER CONNECTED IN A OPEN WYE/OPEN DELTA. IS IT MUCH BETTER IF A PROPOSE TO INSTALL A THREE 75 KVA TRANSFORMER CONNECTED IN CLOSE SYSTEM?.

 

[davidbeach]

Do you think that three transformers would fix the failed CapLock key on your computer? ;-)

 

[Mbrooke]

How much is the sag and how many cycles does it last?

 

[waross]

As the street lights are turning off the voltage is rising.
You may be seeing the on-load tap changer dropping the voltage back to normal.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jing09]

230 voltage is the normal voltage

phase 12 108.26 volts .267 secs
phase 12 202.06 volts .008 secs
phase 31 108.63 volts .267 secs
phase 31 108.07 volts .267 secs
phase 31 196.62 volts .008 secs

 

[jing09]

can you elaborate? @ waross

 

[itsmoked]

jing; Power companies have voltage regulators that they can adjust neighborhood voltages with. Unfortunately they have to be fixed changes. For instance, one might drop or raise the voltage 8 volts in one step. They are usually controlled by simple time clocks that switch the voltage based only on the time of day. However, with changing seasons and length of day, they can get to a point every year where they switch at poor times.

waross is saying that as lots of street light loads start going away at sunrise the neighborhood voltage regulator could be shifting the voltage down because of the loss of street lighting loads.

It could be at a bad time or mis-adjusted. This possibility should be examined before you go to any large changes while trying to mitigate the issue.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jing09]

@Itsmoked; the establishment I put the PQA suffered losses, lots of their air conditioning unit broke specially the ic chips of those

can you suggest any possible solution?

 

[waross]

Thanks Keith. Well said.
Those dips don't look like voltage regulator action.

jing09
Can you give us the voltage dips in the following format please?
Normal Voltage A to B xxxVolts dropping to yyyVolts for zzzseconds at 00:00 o-clock
Normal Voltage B to C xxxVolts dropping to yyyVolts for zzzseconds at 00:00 o-clock.
Normal Voltage C to A xxxVolts dropping to yyyVolts for zzzseconds at 00:00 o-clock.
Normal Voltage A to Neutral xxxVolts dropping to yyyVolts for zzzseconds at 00:00 o-clock.
Normal Voltage B to Neutral xxxVolts dropping to yyyVolts for zzzseconds at 00:00 o-clock.
Normal Voltage C to Neutral xxxVolts dropping to yyyVolts for zzzseconds at 00:00 o-clock.

Do all three phases drop together or individually.
You may have phase angle error issues as well as voltage issues.
You don't want a third transformer closing the delta.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jing09]

Normal Voltage A to B 230 Volts dropping to 108.26 Volts for .267 seconds at 3:22:10.477 o-clock AM JULY 03,2018
Normal Voltage A to B 230 Volts dropping to 202.06 Volts for .008 seconds at 3:55:29.083 o-clock AM JULY 06,2018
Normal Voltage C to A 230 Volts dropping to 108.63 Volts for .267 seconds at 2:45:06.919 o-clock AM JULY 03,2018
Normal Voltage C to A 230 Volts dropping to 108.07 Volts for .267 seconds at 3:22:10.477 o-clock AM JULY 03,2018
Normal Voltage C to A 230 Volts dropping to 196.62 Volts for .008 seconds at 3:55:29.083 o-clock AM JULY 06,2018

I ONLY GOT PHASE TO PHASE VOLTAGES
THE TRANSMISSION LINE THERE HAS A TWO 100 KVA TRANSFORMER CONNECTED IN OPEN WYE/OPEN DELTA. IS IT BETTER IF THEY JUST MAKE A THREE 75 KVA TRANSFORMER INSTEAD OF TWO 100 KVA?

 

[waross]

It looks like the voltage is often dropping on two phases simultaneously.
An open delta on a firm supply has good regulation, when the reduced capacity relative to a similar full delta bank is considered.
The voltage regulation and voltage drop issues are generally a result of voltage drop on the primary neutral.
I don't agree with the oft repeated statement that an open delta has poor regulation.
The open delta is subject to issues caused by feeder impedance, not transformer regulation.
It looks as if there are heavy single phase or unbalanced loads on the primary circuit.
This may be made worse by a break in the primary neutral conductor. That is; the primary neutral current is going to ground and then from ground back to the neutral conductor via multiple grounding.

To better understand draw a sketch of the open delta with transformers at A-B and A-C.
Now imagine a voltage drop in the neutral conductor. This voltage drop will move the common point, A relative to B and C.
It can easily be seen that any movement of the point A will change the angles of the open delta and will change the voltages of one or often both phases.
I would look for an open line or corroded connection on the primary circuit neutral conductor back to the sub station.
Ask the engineering department of the utility what large loads are switched at dawn every day.
Ask the engineering department for help resolving this issue.
You may have to go through the customer service department.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

You have asked several times about a full delta service with three transformers.
A grounded wye:delta only works in a perfect world.
In transmission, with balanced loads and fairly high impedance transformers it may have applications and the balance it provides may be helpful.
In distribution systems it is a disaster. This connection has the ability to transfer power between phases, at the cost of circulating currents in the delta.
When the voltage drops on one phase, the delta will take power from the other two phases and backfeed it into the low phase to try to maintain a balance.
While this may sound like a good idea, in practice it is a disaster.
A grounded primary line anywhere on the system may result in blown fuses on the wye:delta primary.
Any unbalance in supply phase voltages will result in transformer heating and possibly overheating.
Phase angle errors will result in transformer heating and possibly overheating.
Are you on a long line with remote voltage regulators? Remote voltage regulators are very good at maintaining phase to neutral voltages, but on rural circuits often introduce phase angle errors and phase to phase unbalances. Both these conditions lead to transformer heating and possibly overheating.
Please let us know how you make out with the utility folks.
As fuse protection of distribution feeders is replaced with breaker protection, the issues of blown refrigerators and freezers, circuit wide, are a thing of the past.
This was common when distribution circuits were energized by closing fused cutouts, phase by phase.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Generally, closed wye delta distribution banks run with the primary wye point ungrounded to avoid circulating current. Grounded only during single phase switching, then ungrounded again.

 

[waross]

Yes, I agree. I saw such a bank years ago, with the fourth empty cutout.
I have since looked for it on Google Mapview and driving when I was in the area. Alas, I can no longer locate it.
Please forgive my rant. I spent over 15 years in a grounded wye:delta culture. Your method works well for a single bank.
Unfortunately it was common to kill entire circuits by pulling fused cutouts on Sunday to perform dead-line maintenance.
You will agree that switching feeders with an open neutral and a mix of single phase residential loads and some three phase wye:delta banks is not feasible.
Have I shared recently the anecdote about the time that a customer demanded a grounded wye:four wire delta service?
The customer demanded a 150 KVA transformer bank to support a 17 KVA three phase load and about a 5 KVA single phase load.
It had taken me several years to have the customer owned wye:delta transformer banks on our small system reconnected to wye:wye.
All the issues with failed refrigeration compressors ceased.
We did not want to return to the wye:delta problems but the customer was the government owned telephone company and the utility manager was not willing to buck them.
The manager did agree to let me implement a red-neck, off the books, solution.
We took a fuse holder from a cut-out and, without a fuse link installed, tack welded the toggle in position.
When the dummy was installed, it looked like a full closed delta bank, but in fact it was an open delta bank with the third transformer just along for the ride.
It worked that way for years and the customer never knew the difference.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

PS Steven. It sounds as if you have some experience with wye:delta banks. You will recognize the issues that will arise if the bank in question is changed to a full delta with those voltage drop-outs.
I agree that a full delta may smooth out the voltage dips, but the problems in the surrounding area may not be worth it.

Bill
--------------------
"Why not the best?"
Jimmy Carter