Lost Phase on a Three Phase System

[ProjectDelta]

My company recently had an issue with a temporary installation at a new jobsite where lightning struck a pole feeding the installation and in their investigation, it was determined that the B phase fuse had blown at that pole over a mile away. I've always wanted to see a case study like this to better understand the underlying theory in these situations. I was given the information from the team that troubleshooted the issue and have been analyzing the voltage readings. This is a 13.2kv transformer stepping down to 480v on a delta to wye configuration. The secondary voltages measured are as follows:

A to C: 416V
A to B: 410V
B to C: 99V
A to ground: 273V
B to ground: 149V
C to ground: 143V

These readings where taken under no load condition. Upon review it seems clear now that because the utility B phase was lost, two of the coils are now in series which explains the nearly identical ground undervoltages and if you add the voltage vectors of A coil to B coil and A coil to C coil, the 416V voltages check out. I cant seem to work out the math to explain why B to C is only 99V. Mathematically, the voltage should read 257V. I do have a theory however. Because two of the primary coils are now in series and those both also now in parallel with the A to C coil, perhaps that changes the characteristics of the circuit. Maybe the polarity of the windings impacts the voltage reading. I would appreciate any insight as to how to mathematically prove the B to C voltage. I'm hoping to use this as a case study for my weekly lessons I've started having at our shop for our apprentices. Thank you

 

[Ziyad Alahrbi]

This is a difficult one. You have to obtain the vector voltage at the disconnected corner B. This voltage will be (Va-Vc)/2 with 90 degree lag because winding is an inductor.

In the secondary side, it is better to focus in the phase to ground because you dont have to do polar calculation.
13200/2=6600V
6600V/27.5(transformer ratio)=240 L-L in secondary
For L-N 240/1.732=138.5V (almost same as your reading)



Ziyad Alharbi

 

[Hv&Lv]

My company recently had an issue with a temporary installation at a new jobsite where lightning struck a pole feeding the installation and in their investigation, it was determined that the B phase fuse had blown at that pole over a mile away. I've always wanted to see a case study like this to better understand the underlying theory in these situations. I was given the information from the team that troubleshooted the issue and have been analyzing the voltage readings. This is a 13.2kv transformer stepping down to 480v on a delta to wye configuration. The secondary voltages measured are as follows:

A to C: 416V
A to B: 410V
B to C: 99V
A to ground: 273V
B to ground: 149V
C to ground: 143V

These readings were taken under no load condition. Upon review it seems clear now that because the utility B phase was lost, two of the coils are now in series which explains the nearly identical ground undervoltages and if you add the voltage vectors of A coil to B coil and A coil to C coil, the 416V voltages check out. I cant seem to work out the math to explain why B to C is only 99V. Mathematically, the voltage should read 257V. I do have a theory however. Because two of the primary coils are now in series and those both also now in parallel with the A to C coil, perhaps that changes the characteristics of the circuit. Maybe the polarity of the windings impacts the voltage reading. I would appreciate any insight as to how to mathematically prove the B to C voltage. I'm hoping to use this as a case study for my weekly lessons I've started having at our shop for our apprentices. Thank you

It’s very doubtful you can..
Pole over a mile away..

How many other banks / transformers were on this line?
Were there any loads on this or any other banks on the line across the two good phases?

Also, what kind of meter were you using?
Try it with a low Z meter

 

[jim dungar]

Blown primary side fuses can result in changes to the positive, negative, and zero sequences. It is not uncommon for simple voltage sensing relays to be able to see a loss of voltage due to a primary side fuse opening on Delta days connections.

Most of the protection relay manufacturers, like Schweitzer and Basler, have technical papers written about the benefits of using negative sequence sensing.