Grounding connection of High voltage transformer 1

[Al_Pa]

Hi everyone,

i'm approaching a substation in US and i found this scheme for a Main transformer connected to a 230Kv Substation

Do you know the meaning?

the secondary side is grounded to an earthing resistor, but the primary side i don't know how to read the symbol

thanks in advance for supports.


Alex.

 

[wcaseyharman]

Looks like to me it’s a Y-delta-Y.
Grounded wye high side, a corner grounded delta tertiary, and a resistance grounded secondary.

My company commonly orders YDY transformers for distribution substations in certain divisions.

 

[waross]

I spent quite a few years in an area where four wire wye/delta was common for distribution circuits.
In the event of a lost phase on a distribution circuit, any wye/delta banks would supply the missing phase.

How do your grounded wye/delta/wye circuits react to a missing primary phase?
Are such circuits disconnected completely ASAP?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

In my utility the tertiary is not connected to anything. It’s just used to provide a low impedance path for ground faults on the secondary Wye connected side. It’s funny because different areas of the company standardized on different transformer configurations - in certain areas delta-wye transformers were standard, but in others wye-delta-wye banks were standard. I’m not sure anyone still at the utility knows why the system developed that way.

 

[dpc]

It's a three-winding transformer - grounded wye primary, corner-grounded delta tertiary, grounded wye secondary with neutral grounding resistor. Aside from the corner-grounding of the delta, this is a fairly common configuration for transmission substation transformer in the US. In most cases, the delta is "buried" with no leads brought out, but it can be used if the leads are brought out to bushings.

 

[wcaseyharman]

I think the only think that is unusual to me is the grounded primary wye. I think most substation transformers in my company have the primary neutral disconnected so the bank doesn’t supply ground fault current to the transmission system. There are a few exceptions in the company (other than generating stations) but not many.

 

[waross]

I realize that the delta is not connected.
The delta does not have to be connected to anything to backfeed into a primary, grounded wye.
With fused cutout protected distribution circuits, a wye/delta bank will cause a number of serious issues on the whole circuit if a primary phase is lost.
I am wondering how primary phase loss is handled on transmission circuits feeding wye/delta/wye transformers.
Based on 15 years experience with wye/delta, I can see a wye/delta/wye transformer feeding all three phases of the secondary when a primary phase is lost. It does this by way of a circulating current in the delta.
The configuration will supply light loads.
As the secondary load increases, the delta will become overloaded and overheat.
The wye/delta/wye will also back feed into a grounded primary phase.
In the instance of a grounded primary phase, the delta may be damaged by the fault level circulating current.
Similarly a ground fault on the secondary of a wye/delta/wye transformer may damage the delta.
I am aware of all the negative effects of the wye/delta on a distribution circuit.
Most people have never had the misfortune to have experienced the negative effects of a wye/delta transformer bank on a distribution circuit.
Wye/delta circuits were common in the 40s 50s and into the 60s as old delta/delta distribution circuits were upgraded to wye/delta circuits to increase circuit capacity.
It was soon discovered to be a bad idea and the configuration has not been used for years.
I found myself in a technical time warp, where the circuit still was being installed.
The wye/delta configuration was famous for killing refrigerators and household freezers.
One wye/delta bank on an otherwise residential circuit would imperil all of the refrigerators on the same primary circuit.
My question is what protection and what settings are used on transmission circuits feeding a wye/delta/wye transformer.
Is the damage curve of the delta winding considered when designing protection schemes for wye/delta/wye transformers?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

Great question. I’m sorry I didn’t realize you were asking about the transmission circuit protection. The company typically uses phase distance and directional ground for the transmission system. Substation protection has changed with newer technology; Older banks use HV fuse protection and newer banks use circuit switchers with differential protection. when the primary neutral is disconnected I don’t believe there is any additional protection provided. When the primary neutral is grounded I am not sure what special provisions, if any, are provided but I will find out.
I have a vague memory that the old timers said the company had issues with undersized delta windings failing a long time ago, but they since required larger capacity delta windings and that problem went away.

 

[Kiribanda]

Your primary side is 230kV. Therefore utility will not allow you to install a WYE ungrounded transformer.
Also if your facility is having generation synchronized to the 230kV utility, then your 230kV side shall
be always WYE grounded to ensure that 230kV side is effectively grounded during a single line to ground fault at the
230kV system. Also the cost of a YN primary transformer is little lower than a 230kV DELTA
due to insulation issues. Therefore, normally as the voltage increases you have to go to YN from transformer design
point.

 

[prc]

1) The HV side neutral is solidly grounded. LV neutral is grounded through a resistor to reduce fault current and arc flash issues in distribution lines. Such resistors are used when the transformer feeds a dedicated load like in a factory. But in grid substations, LVN is also solidly grounded. The delta winding is the stabilizing tertiary - to stabilize the neutral of Ynyn connected transformers. One corner of such winding shall be kept grounded

2) I don't know why in the US such stabilizing tertiary is provided in Ynyn transformers. In India, we eliminated such stabilizing winding from all grid Ynyn transformers nearly 45 years back.

3) There were many discussions in this forum on Tertiary windings and various aspects of it. You can search and get it. Some years back, I prepared a tutorial on this subject that I put in this forum- thread 238-437739. This paper gives the historical evolution of stabilizing tertiary, why and where it is required, and protection aspects for tertiary winding.