VT Fuse Failure-How to calculate? 1

[Kiribanda]

Hello all,

We have got an issue about a set of VTs.

A MV metal clad switchgear is supplied by a 13.8-4.16 kV Dyn1 power transformer. A set of PT (or VT) with the ratio 4.2 kV-120V is installed on the 4.16 kV STAR side to feed secondary line to line bus voltages for the metering and relay (ANSI # 27, 47, 59, 60) panel. The connection of the PT set is OPEN DELTA-OPEN DELTA corner grounded (commonly known as V-V grounded).

What will be the Line to Line voltages seen by the above mentioned relays on the 120 V side, and how to calculate them,

1)When one 13.8 kV phase is open due to a fuse blown condition of the primary side of the Dyn1 power transformer?

2)When one phase of the primary side of the VT is open due to VT primary side fuse blown condition?

3)When one phase of the secondary side of the VT is open due to VT secondary side fuse blown condition?

Most of the relay manuals (SEL, BASLER etc) discuss about the corresponding reflected voltages on the 120 V side with YNyn connected VT sets (e.g. generating stations where ANSI# 51V relays are fed).

Thanks in advance!

 

[PWR]

For the normal situation, the phase to phase voltage will be reduced by the PT ratio of 35/1.

For the unbalanced situations, per a previous post, a good article on this can be found at:

http://basler.com/php/download.php?file=practical.pdf&;id=204&type=res

 

[Kiribanda]

PWR,

Thanks for your input.

I had already read BASLER technical paper brfore.

a)Table 11 of that paper gives the secondary voltages (pu)
for the Dyn1 case.Is it possible to divide those by the
ratio 35 to obtain the reflected secondary voltages on
the 120 V side of the PT?

b)It does not show anything about case 3. My calculation
shows it will be 0,1.0,1.0 pu.Is it correct?

Kiribanda

 

[PWR]

Table 11 will work directly. Thus 0.5 pu is 60V on secondary or 2100V on primary.

Be cautious about the effect of the secondary load/burden.

Two weeks ago I was troubleshooting a situation with the blown VT primary fuse just like you described. Because of the effect of what was connected on the secondary the phase voltages actually read: 120/67/138.

On your last example, if there is no secondary load connected you should see 1.0/0/0 pu. The problem is, there always is something connected or you would not be concerned about what will happen!