Open Delta Regulator Bank in Substation 2

[TomScott]

I'm designing a small 25kV to 4.14kV distribution substation for a customer who needs a delta service. We don't typically supply a delta service so I do not have much experience with it. The station transformer will be a 5/7.5 MVA wye - delta and will be too small for an LTC on the tank. For regulation in our wye - wye stations of this size, we typically use 3 single phase regulators in a grounded wye configuration. Connecting 3 regulators in a delta configuration is giving us some headaches concerning the design of the metering / regulator bay structure and it's associated bus work.

One solution we have come up with is to use 2 regulators wired open delta instead of our traditional 3. This greatly simplifies the bus work and allows us to use a standard structure we have already deigned. I'd like to know if any of you operate an open delta regulator bank in any similar situations? If so, what are the downsides? Do you need to use larger regulators to reach the same capacity? Any info would be greatly appreciated.

Tom

 

[dpc]

What does the customer mean when they say "delta" service? Is he just referring to the delta secondary on the transformer? Why do you think you need regulation on the transformer?

 

[TomScott]

We are replacing our existing substation. It's a high value account so we decided to maintain an actual substation for service instead of just setting an auto transformer. We always build substations with regulation to ensure proper voltage.

Tom

 

[waross]

Are you aware of the serious circulating current and back feeding with a grounded wye:delta transformer or bank?
You may float the wye point of the wye and the problems go away to be replaced by possible over voltage switching transients.
The grounded wye:delta transformer will back feed into a line to ground fault anywhere on the supply circuit.
The available fault current will be increased by the available fault current of one phase of the wye:delta bank, limited by three times the % impedance of the bank.

The old 4160 kV wye systems were frequently derived by reconnecting 2400 kV delta systems.
I suggest a meeting with the customers electrical people.
Suggest a high resistance grounded wye to replace the delta service.
Their equipment should never know the difference.

But if you must use a delta system, the open delta should work fine.
If you have a 173 Amp load on a delta bank, each transformer will carry 100 Amps.
With open delta the two regulators must carry the entire 173 Amps.
Oversize by root 3. Or, size the regulators for the actual line current.

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

 

[prc]

waross, "The grounded wye: delta transformer will back feed into a line to ground fault anywhere on the supply circuit.The available fault current will be increased by the available fault current of one phase of the wye: delta bank, limited by three times the % impedance of the bank. " Can you explain a bit more how it happens or any reference or link to understand more.

I have seen many stations in India with grounded star /delta (HV/MV) with a grounding transformer on secondary side. Will the problems that you mentioned will be avoided in this case?
Tom, Today special LTCs are available to suit even 1 MVA transformers -cost effective and reliable. This need came because of the transformers required for renewable sector. Distribution transformers are provided with LTC to feed power to MV grid.

 

[TomScott]

Changing the customers facilities is not an option. Their system was designed to be fed by a delta when it was built in the 60's. The substation replacement is a project we've initiated to improve reliability and serviceability. For better or worse, we're stuck with the setup as is.

Tom

 

[magoo2]

TomScott-
Let me ask some stupid questions so I don't waste your time.

What I understand from what you've said is the existing substation site has 25 kV on the high side. I assume this is a 4-wire multi-grounded neutral circuit or supply. Correct?

You need to supply an existing customer load with a 4.16 kV (or so) 3-wire delta service. How bad is the voltage regulation on the 25 kV at this point? I'm asking this because I wonder if you need additional regulation. If you do, are you proposing to install it on the 4 kV side (which I suspect) rather than on the 25 kV side?

I'm assuming what you call a wye delta substation transformer is actually a 25 kV ungrounded to 4.16 kV delta. So this should take care of Bill's concerns about a grounding bank (i.e., a grounded-wye delta connection). But I'd like you to confirm this.

On a delta (4 kV) circuit, 2 voltage regulators can be used and have been used over the years to regulate the 3 phases. On a 3-phase 4-wire circuit, I'd be more inclined to use 3 regulators since the loads could be different.

As a retired utility guy, my only concern on the downsides is that you avoid single phase switching on the 25 kV side. We have used floating-wye delta connections to supply a number of 240 V delta customers from 34.5 kV and 13.2 kV. These are typically 500 kVA or less - smaller than the 5 or 7.5 MVA that you are dealing with. We typically install a cutout between the high side neutral and ground so that we avoid the possibility of ferroresonance when we energize the transformer bank. Once the bank is energized, the neutral cutout is opened. In your case, you probably have a circuit breaker so it should not be an issue.

prc- I think the typical distribution substation transformers in India, whether supplied by transmission voltages of 220 kV or 400 kV, are typically ungrounded on the distribution side. So, if there's a delta connection on the low side of the substation transformer, a grounding transformer (grounded-wye delta or zigzag) is often added to provide a ground source and assist with the protective relaying associated with the distribution circuits involved. Also, the distribution system in India is quite different than what is used in North America. I have a 100 kVA transformer serving my house and about 6 neighbors. In India, as I recall, the typical distribution transformer is a three phase transformer serving maybe 200 or more customers.

 

[waross]

Is the customer using a four wire center tapped delta. This is often used to provide 120:240 single phase and 240 Volt delta.
Other than that, can you explain why they cannot use power from a wye connection.
Not doubting you, I'm trying to learn something.
I understand that even if the customer is wrong, A high value account is a valid reason.
prc;
I have not seen much on this in textbooks.
I spent a number of years in an area where grounded wye:delta was the standard and became the system engineer of a tiny utility.
I found out most of this by dealing with issues and going back to first principles.
Historically many distribution systems were originally 2400 Volt delta.
After WW2 power usage increased dramatically.
The distribution transformers had 2400 Volt primaries.
Many utilities dealt with expanded growth by going to a 4160 Volt grounded wye system.
A neutral conductor would be run and the multiple earths installed.
Then there would be an outage and the crews would reconnect the transformers line to neutral/ground from line to line.
This could be done one circuit at a time.
I did come across a note in an old text book to the effect that a ground fault anywhere on a distribution circuit may lead to fuse blowing throughout the circuit.
In those days protection was often by fused cutouts.

What happens.
Consider an open delta, A to B and A to C.
This forms a virtual transformer across B to C.
If you put a load on the virtual transformer, and determine the voltage drop and available fault current you will see that the virtual transformer has similar characteristics to the two real transformes.
If you have two 100 KVA transformer then the virtual transformer formed by the open delta will be a 100 KVA transformer.
Now install a real transformer across the open delta. But the taps are set wrong and the voltage is 5% low. On a 240 volt system that is a deficit of 12 Volts. This 12 Volts will cause a circulating current limited by three times the individual transformer impedance.
What happens if the tap is correct but the voltage on one phase is 5% low or high. Same thing. Heavy circulating currents.

Put a wye:delta bank at the end of a long rural line and what happens?
Rural lines are mostly single phase loads and it is impossible to keep the loads balanced.
Long rural distribution often have one or more voltage regulators. These are typically single phase units running independently.
Many times one line will have a greater voltage drop and the regulator will correct this.
However a voltage drop as a result of unbalanced loads implies a current on the neutral.
The neutral impedance is often higher than the line impedance and even though the neutral current is just the unbalanced current, it is in a direction to add to the deficit that the regulator must make up.
The result is a displaced neutral.
When the neutral is displaced the phase angles are no longer equal.
The delta connection demands equal phase angles.
More circulating currents.
The circulating currents are in a direction and of a magnitude to correct the errors causing them.
This is limited by the ratio of the transformer bank to the impedance of the source.

Now what happens when there is a line to ground fault somewhere on the circuit?
Lets disconnect the B to C phase transformer for a minute and go back to the open delta.
We will leave the B to C transformer connected to the primary but disconnected from the secondary.
10 blocks away we have a solid line to ground fault on B phase.
The voltage on B phase drops to zero.
The voltage on the secondary of the B to C phase transformer drops to zero.
Now with this transformer in the delta bank the secondary is fed full voltage by the open delta. This then back feeds into the faulted line. The circulating current that causes this back feed must pass through all three transformers. Hence the limit on the current of three times the impedance of one transformer.
When this connection was common, there may be a number of grounded wye:delta transformer banks on a circuit. All would be back feeding the fault.
As all these banks would be overloaded, many of them would blow fuses.

When I was working in the area where the grounded wye:delta connection was the standard, the individual circuits were controlled by fused cutouts. Outages on Sunday for maintenance were common.
When the work was done, the circuit would be energized one phase at a time by closing the cutouts.
Wen the first cutout was closed one phase would be energized. In all the grounded wye:delta banks the other two transformers would be back fed in series and each would back feed about 50% of normal voltage.
All over town the refrigerators and freezers would be waiting to start, but two thirds of them would get half voltage. They would stall and hum and get hot.
Then the second cutout would be closed. Have you ever used a 30 foot telescoping hot stick at the end of a hard day to close cutouts?
It is not fast.
Now two thirds of the residential loads have full voltage.
The grounded wye:delta banks would be back feeding nominally full voltage to the last third of the residential transformers but at a lower voltage due to heavy loading on the transformers and resultant voltage drops.
Finally the third cutout would be closed.
Sure the refrigeration compressors have thermal protection but it is not meant to handle this profile of abuse.
Every week several compressors would die.

If you spend some time going over this you should be able to satisfy yourself from first principles.
Wow! That's a lot.
I hope it help prc.
Yours
Bill Ross

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

 

[TomScott]

Magoo see answers below.


Let me ask some stupid questions so I don't waste your time.

What I understand from what you've said is the existing substation site has 25 kV on the high side. I assume this is a 4-wire multi-grounded neutral circuit or supply. Correct?

(That is correct.)

You need to supply an existing customer load with a 4.16 kV (or so) 3-wire delta service. How bad is the voltage regulation on the 25 kV at this point? I'm asking this because I wonder if you need additional regulation. If you do, are you proposing to install it on the 4 kV side (which I suspect) rather than on the 25 kV side?

(Our current station has 3 regulators and we have no voltage issues. We are replacing the station due to age and lack of redundancy. Yes they are on the low side.)

I'm assuming what you call a wye delta substation transformer is actually a 25 kV ungrounded to 4.16 kV delta. So this should take care of Bill's concerns about a grounding bank (i.e., a grounded-wye delta connection). But I'd like you to confirm this.

(The new transformers will be wye (grounded)on the high side and delta on the low side.)

On a delta (4 kV) circuit, 2 voltage regulators can be used and have been used over the years to regulate the 3 phases. On a 3-phase 4-wire circuit, I'd be more inclined to use 3 regulators since the loads could be different.

(On the rest of our system we use 3 regulators. To my knowledge we've never built and open delta regulator bank. I'm only considering it now because I don't want to redesign our standard meter / regulator bay to accommodate the extra bus work to tie the delta together.)

As a retired utility guy, my only concern on the downsides is that you avoid single phase switching on the 25 kV side. We have used floating-wye delta connections to supply a number of 240 V delta customers from 34.5 kV and 13.2 kV. These are typically 500 kVA or less - smaller than the 5 or 7.5 MVA that you are dealing with. We typically install a cutout between the high side neutral and ground so that we avoid the possibility of ferroresonance when we energize the transformer bank. Once the bank is energized, the neutral cutout is opened. In your case, you probably have a circuit breaker so it should not be an issue.

(We have done this on smaller banks just like you describe. In this situation we will have standard 25kV ABB RMAG feeder breakers on the source and load sides of the station. I will be setting the relays to allow only 3 phase operations.)

Tom

 

[TomScott]

Waros,
The reason I fear changing the service to the customer is mainly fault current. My knowledge in this area is limited, but in general, it's my understanding is that delta 3 wire will have low fault current compared to grounded wye. Additionally the facility is a prison that was built in the 60's. I'm not sure why delta 3 wire was chosen and no one can tell me. They have very uneducated staff concerning their electrical system so it's difficult getting information on their equipment and power needs. My fear is that changing their service will negatively effect their system and will cause them problems and cost them money. So since we aren't sure what they have equipment wise, we have chosen to supply them "as is" even though it's not our standard service.

Tom

 

[waross]

With a wye on the 4.14 side rather than delta, you may high resistance ground the system and the ground fault current will be limited by your choice of grounding resistor value.
But you gotta do what you gotta do.

I have seen politics get involved with grounded wye:delta systems as well, sort of.
Driving through a city or area where grounded wye:delta is common it is common to see many transformer banks with one fused cutout open. The bank functions as an open delta and the problems go away.
Then there is an election and a new political appointee takes charge.
The appointee issues orders that all fuses must be replaced.
The fuses start blowing.
The crews replace the fuses with larger sizes.
Eventually the transformers start failing.
Finally the appointee stays in his office and the fuses are left hanging with the banks operating on open delta.
No more problems.

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

 

[prc]

Thank you Bill for your time and patience ! Can I understand that these issues will disappear if you provide a zig- zag grounding transformer after delta?
magoo2 - You are right. In India we have only Dy11 distribution transformers universally. From this forum, I understood some time back that US has many, many variations being the great experimental ground where all the power technology was developed in last century. My observation was only about the sub station transformers where YG/yg is the standard in our country. But seen exceptions with YGd, but with a grounding transformer on secondary. This is seen in some other places also like Oman, Syria(yes Aleppo !)

 

[waross]

Hi prc
The issue is not with grounding.
The issue is with the ability of a grounded wye:delta bank to transfer power from one phase to other phases.
Many years ago when air conditioning was new, The larger compressors could not be started on the existing 120:240 Volt residential services.
The answer was to add one small transformer and supply four wire delta.
The open delta connection provided the ability to start larger A/C units on three phase power and the original center tapped transformer continued to supply the residential loads.
The 240 Volt delta does not provide a source of 120 Volts for lighting and utility loads.
It became common for factories to install the four wire delta system with the famous "Wild" leg.
The center tap of the 120:240 Volt transformer was always and continued to be grounded.
The ground on the delta side made no difference to the issues.
When there is an unbalance on the grounded wye:delta system, there is a neutral/ground current on the primary wye side.
There is no current to ground on the delta side.
The neutral current on the wye side is reflected by a circulating current in the delta.
Interestinglly, just as three phase 240 Volt delta power may be derived from two primary phase conductors and a neutral with an open delta connection, 120:208 Volt pseudo wye power may be derived from two primary phase conductors and a neutral.
I first heard about this connection on this forum. I eventually met one engineer in the field who was familiar with the connection.
He called it a "Winnipeg" connection.

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

 

[TomScott]

So I've broached the idea of replacing the Delta with a high resistance grounded wye and we are going to consider it. I've been reading everything I can find on the subject however I have a few questions about converting the current Delta system to high resistance wye.

The current substation feeds a network of 3 wire overhead distribution. If I change the transformer from a delta to a high resistance grounded wye do I need to add a neutral conductor to the overhead line or can the overhead distribution system remain 3 wire?

Obviously all the current loads on the system are wired delta on the source side but the facilities are a bit of a mystery past our meter so I am unsure of how the load sides of any transformer banks are wired. My gut is telling me that with a resistor between the neutral and the ground that I do not want to distribute the neutral and that it will essentially make no difference to the equipment.

Concerning protection of the circuit, how do you trip for line to ground faults? We use SEL 351S controls on our breakers so my first thought would be to use negative sequence components to trip for a line to ground fault. We could also monitor the voltage across the ground resistor to detect line to ground faults.

The customer has backup generators on site that will carry their full load. Will converting from delta to high resistance grounded wye affect the syncing and operation of their generators in any way?

What else if anything will need to be done to the customers 3 wire delta facilities in order to ensure they receive the same service they get now with the Delta?

Tom

 

[waross]

Hi Tom;
What is the current to ground with a fault on the delta system?
Select a grounding resistor that will limit the current to the same value.
I have seen zero sequence CTs used around the phase conductors, CTs used around the grounding conductor and I have seen the voltage across the grounding resistor monitored to determine the current.
Often I see a "Thumper" circuit added to a grounding resistor bank.
This is a second resistor that may be switched in and out by a contactor and suitable timers.
The resistor may be sized to increase the ground current by 5 Amps, but you are free to pick your own value.
Timing may be of the order of one second on and one second off, but again you are free to choose.
When a ground fault is detected the Thumper may be turned on to help locate the ground with Amprobe type meters.
If you have no way of safely checking the current on your 4.14 kV distribution lines then you probably don't need a Thumper.
I don't see any need to run a neutral conductor with no line to neutral loads.

prc
Transmission systems tend to have better balance, three phase switching and higher impedance transformers. All these factors lessen the magnitude of the issues with the the grounded wye:delta.
Distribution systems with often unbalanced loads, possible single phase fusing and switching, lower impedance transformers and a greater possibility of damage (Think something like an automobile striking a power pole) all contribute to greater circulating currents and the possibility of more issues.
The grounded wye:delta system works well as long as everything is balanced. It is when there are unbalanced voltages, a missing phase, a ground fault and/or unequal phase angles that the problems arise.

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