Y-D transformer Energization 3

[NickParker]

At a Pumping station, the Main transformer is of Y-D, with the primary neutral being connected via disconnector switch to the ground. The neutral also has surge arrestor on them. Normally the disconnector switch of the primary neutral is disconnected, It means the primary is ungrounded. Only when energizing, the operator connects the disconnector switch to earth, after energization again the disconnector switch is disconnected from the ground. Can anybody explain why this practice is been followed?

 

[magoo2]

Any ungrounded primary transformer connection is succeptable to ferroresonance if single phase switching is used to accomplish the energization. This is more a problem of higher voltage distribution systems like 25 kV and 35 kV, but it also can occur on 15 kV systems.

By contrast, the grounded wye primary connection is not one that is succeptable to ferroresonance. However, if you left a grounded wye delta transformer bank connected, it acts as a grounding bank and will often have nuisance fuse blowing due to backfeed to system imbalances.

By temporarily grounding the neutral, you avoid ferroresonance. Then you unground the neutral connection so it doesn't cause problems with backfeed due to unbalances on the circuit.

 

[Mbrooke]

Ferroresonance. Having the primary grounded avoids that during opening and closing of cutouts (single phase switching)


Now, why open the neutral to begin with? To avoid nuisance fuse blowing during faults elsewhere on the system. A wye-delta is basically called a "grounding" transformer, which means that, any imbalance on any of the phases (from a fault) causes current to flow through the primary neutral on the transformer as the secondary delta tries to "balance" this condition. This current of course is also reflected on the phases of the transformer bank, and can be large enough to blow the fuses protecting the bank or unit.

 

[stevenal]

In addition to the above:
Floating the primary neutral helps even out any system voltage unbalance, and avoids circulating current in the delta secondary. It also helps you to detect a fuse open situation. Opening a single fuse on a grounded wye-delta will not result in in an outage until the now open wye-open delta bank fails from overload.

 

[waross]

Nothing to do with ferro-resonance.
The wye delta transformer connection has two issues.
1. With the neutral connected, any unbalance in primary voltages or phase angles will cause heavy circulating currents in the delta secondary. The transformer bank will also back feed into an open primary phase.
The circulating current is limited by three times the impedance of the transformer or bank.
2. With the neutral floating, there is a danger of overvoltage transients being generated in the secondary when the bank is energized.
Energizing with the neutral connected avoids transient overvoltages. Then opening the neutral connection avoids overvoltages.
I remember about 10 or 12 years ago seeing a small wye delta bank installed with three fused cut-outs for that reason.
Coincidentally I was in that area about a week ago and looked for the wye-delta bank to get a picture of the cut-outs.
No luck. Either it has been removed from service or I was on the wrong street.

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

 

[marks1080]

Nothing to do with ferro-resonance.

This thread just got juicy...

I agree with waross... I think a situation can be imagined where ferroresonance might be an issue here, but certainly not enough information is given to say that for sure. I'd be curious to know what the rating of the switch is in relation to the transformer neutral insulation rating.

 

[marks1080]

Also, it's possible they are using a neutral overcurrent element to detect faults for a 'line test' situation. Once operators are satisfied they didn't close the until into a fault, they can remove the neutral connection.

 

[stevenal]

http://www.ece.mtu.edu/faculty/bamork/FR_WG/Panel/Paper%2003GM0829%20-%20Walling-rev1.pdf[/URL]]Ferroresonance most readily occurs for
transformers with ungrounded primary windings (i.e., delta,
floating wye, open delta)

 

[NickParker]

Ferroresonance. Having the primary grounded avoids that during opening and closing of cutouts (single phase switching)

The transformer is energized by three phase Gas Circuit Breaker, not single phase switching. The primary voltage is of 110kV and the secondary is of 6kV

 

[waross]

History, not speculation.
Back one hundred or so years ago, delta:delta transformer banks were the norm.
After WWII when the use of electricity started to increase greatly many utilities gained a cheap and easy 1.73 increase in distribution line capacity by switching the delta:delta systems to wye:delta. The voltage was often increased from 2300 Volts to 4160 Volts.
It was found that the wye:delta bank had some serious issues. Overheating and failing due to circulating crrents, and back-feeding into a fault were two issues. Also a ground fault on one phase of the distribution circuit would be back-fed by the wye:delta bank. This could result in the blowing of one of the fuses on the wye:delta bank.
It was found that these problems could be avoided by floating the primary wye point.
Leaving the wye point floating eliminated the issues with circulating currents, back-feeding and blown fuses.
A new issue arose. Over-voltage transients when the bank was energized.
The solution to this was to connect the wye point to the neutral before energizing the transformer bank. Once the bank was energized, the connection from the wye point to the neutral was opened.
That is why you occasionally see a wye delta bank with an open fused cut-out or a switch to connect the wye point to the neutral.
Closed to energize and then opened and left open.
By the way, I spent over 15 years in a country where the wye:delta connection was popular and I am familiar with most of the issues with wye:delta transformer banks.
Another issue when a residential circuit had one or more wye:delta banks was the frequent failure of residential refrigerators and freezers.
I served as system engineer for a small utility for a number of years. It took about three years to get all of the wye delta banks on our system changed over from wye:delta to wye:wye.
The accepted connection for a wye delta bank is with a floating wye point.
Been there, done that and got the tee shirt.

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

 

[NickParker]

Thank you for your post Waross, that gives a good starting point. My initial thought was if the power source which supplies the power to this transformer also grounded, then grounding this transformer gives some inconveniency for the co-ordination of ground fault for the faults between the source and this transformer. (Do you refer this as "backfeeding"?). Could you elaborate your sentence "Overheating and failing due to circulating crrents, and back-feeding into a fault were two issues"

 

[waross]

The best way I have found to explain this is to start with an open delta. Rather than a three phase transformer consider a three transformer bank for the purpose of explanation.
Remove the transformer from C phase. On the delta secondary there will be 6 kV volts generated by the open delta.
Now connect the low voltage winding only of the third transformer across the open delta. It is easy to see that the open delta will power the third transformer low voltage winding and the transformer will develop 110 kV on the open high voltage winding.
Now consider a fault at another location, on C phase of the distribution circuit.
C phase voltage drops to zero.
But we have shown that the C phase transformer is developing full voltage on the high voltage winding, powered by the healthy A and B phases of the circuit. Connected to C phase a heavy current will flow back into the fault.
The available current will be limited by three times the %impedance of the transformer bank.
The current will be further limited by the system impedance.
The current will still be high enough to take out a fuse.
Once one fuse clears the open delta no longer functions and the current drops to a very low value.

Second issue. Circulating currents.
Consider that the voltage on C phase is 10% low at 99 kV.
The healthy phases are back feeding the C phase transformer and it is trying to develop 110 kV while it is connected to a 99 kV source.
In this example the bank acts as a stabilizing force and may raise the voltage on the low phase.
The issue is that some or all of the transformer capacity may be used to correct the voltage on C phase with little or no capacity left to power the intended load.
Overheated transformers and blown fuses are common.
I hope that this explanation is clear.

Cultural issues.
Yes, cultural issues may have a direct influence on technical issues.
In the country where the wye:delta was common it was also common to see virtually all the wye:delta banks with one fused cut-out hanging open.
Part of the culture was kick-backs from the suppliers to any contractor or consultant bringing in a transformer order.
Transformers tended to be 200% to 300% oversized and most transformer banks could easily handle the load in open delta.
Then there would be an election and a change in regime.
The incoming Energy Minister would replace the head of the national power company with his favorite. The new head would replace a lot of department heads with his favorites. Someone would notice the open cut-outs all over town and issue orders that all fuses must be replaced. The fuses would be replaced with progressively larger fuses until the transformers started failing. This would continue for several months until the permanent workers were able to "train" the incoming political appointees.

Single phase switching.
In this area power outages on Sundays were common. Live line work was avoided and maintenance was regularly done on dead lines on Sundays.
Switching was generally done by closing fused cut-outs, one phase at a time.
On a residential circuit, when the first phase was energized, there was a single phase condition. With A phase energized, the B and C phase low voltage windings would be in series across the healthy A phase low voltage winding. On a balanced circuit each of B phase and C phase transformers would back-feed about 50% voltage into the circuit. After a 4 hour outage all the refrigerators and freezers on those circuits would try to start, but with 50% voltage most would stall and start heating.
When the second phase was energized, the residences on that phase would get about 95% of normal voltage applied to the stalled and heating refrigerator compressors.
Generally the thermal overload protection would save the motors but every weekend it was common for some refrigerators to fail.

There are some other conditions as well.
NickParker asked:

Can anybody explain why this practice is been followed?

With respect to those suggesting fero-resonance issues, and an acknowledgement that at times ferro-resonance may be an important issue, I have seen and have read descriptions of wye point grounding when energizing wye:delta banks to avoid over-voltage transients.
This was at one time and in some areas a common practice.
This was a recommended practice to mitigate known issues.
As I said:
"Been there, done that, got the tee shirt". grin




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

 

[NickParker]

Thank you very much Waross, that was clear. I also heard that they do de-energize the transformer just to change the taps on the transformer even though it has On load tap changer. I wonder why did they purchase the transformer with On Load Tap changer?

 

[prc]

Thank you waross for the explanation, though certain part of it you had given earlier too. I had seen the arrangement explained by Nick in certain countries (Syria ?) but never understood the purpose or sequence of operation. I have a question to waross. How the situation will change if the incoming 110kV is from a D or YN (ie delta or earthed star) source? How it will be with YNd but with a ground=ing transformer connected to 6.9 kV side?

 

[Mbrooke]

Transient over voltages- I don't get that though- I mean wont the delta secondary balance that out?

Another issue when a residential circuit had one or more wye:delta banks was the frequent failure of residential refrigerators and freezers.

Can't a delta wye do the same? 1/3 of the customers will be at 230 volts while the other will be at about 115 (assuming equal load) when one phase is open.

 

[waross]

As I understand the over voltages, it has to do with the energizeation surge. When the second phase is energized with a floating neutral, two windings will be in series. These windings may have different levels of residual magnetism. As a result, one winding may saturate a and cause an unequal division of the voltage across the series windings.
If anyone has an authoritative alternate explanation, please share it.
Different systems:
Most of these effects are due to the wye primary four wire connection. This "locks in" the phase angles and voltages on the secondary.
With the phase angles locked in, the secondary delta must close on itself.
If you draw a "head to tail style" vector diagram of unequal secondary voltages, you will see a gap where the delta does not close. This will drive a circulating current limited by three times the transformer % impedance.
With a delta source the wye point must be floating and the circulating current issue will be avoided.
While energizing with the neutral connected may cause circulating currents this can usually be accepted for the time it takes to energize and then open the neutral connection.
It can get exciting if the last phase is closed with a fault on the circuit.
A 25 Foot hot stick is good protection.
Been there too, done that as well and got that tee shirt. The tee shirt's a litle sweaty. grin

I don't have any experience single phase switching delta:wye transformers.
I have seen a lot of single phase switching of Wye:delta banks.
Historically, the majority of the wye:delta banks resulted from conversions of delta:delta banks to increase capacity and reduce losses. Most of the issues may have been identified and mitigated and forgotten before many of us were born.

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