Star-Broken Delta PT rating 2

[keyrose]

I have circuit of Primary Star secondary broken delta connection connected to 12kv system.

Each individual PT ratios are 14400/120

As in star, each PTs are sublected to phase voltage only. Can I select the PT of ratios (14400/1.732)/120 for this connection?


Keyrose

 

[scottf]

Keyrose-

Yes. If the star point it grounded, you can use single-bushing PTs. If the star point is not grounded, you need to use 2-bushing PTs.

 

[jghrist]

I don't understand the question. You say that VT ratio is 14400/120 (120:1) then ask if you can select it as(14400/1.732)/120. If the system voltage is 12.47 kV, then you can use 7200/120 (60:1) VTs.

 

[burnt2x]

Me too, can't follow. You already have the ratio 120:1. What's there to "select"?

 

[waross]

Google search "broken delta" on this site. This has been discussed quite a bit.
Almost 400 hits.
One of the first hits.
3V0 (broken delta) Resistor Sizing
thread238-225126


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

 

[davidbeach]

NO, no, no. If you are applying these VTs (PTs) to a system that is not solidly grounded - which is often why the broken delta would be used - you need VTs that are rated for at least line-line on the primary. So your 14400 primary would work but your 8314 primary would be subjected to overvoltages and saturation during a ground fault, just when you need accurate response.

 

[scottf]

David-

Your comments are correct. However, depending on the protection, a 2-bushing PT with a L-G ratio (i.e. 7200:120) can be used if the protection will clear in a time that meets the over-voltage limits set-out in the standard (i.e. IEEEC57.13), i.e. 1.73 - 1 minute.

 

[burnt2x]

scottf,
Yes, if the protection scheme is set to clear under 1 minute on SLG faults. Some ungrounded system protection setup do not trip but just "indicate" the occurrence of a SLG fault, allowing the crew to correct the problem. It may take hours of checking and CTs can saturate.

 

[stevenal]

scottf,

The rule does appear to be universal. Depends on indoor versus outdoor, group and on voltage rating. 140% is the 1 minute rating for many of them.

 

[scottf]

Stevenal-

Yes...the overvoltage factor varies with group no., that's why I listed "i.e.".