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Star-Broken Delta PT rating
2

Star-Broken Delta PT rating

Star-Broken Delta PT rating

(OP)
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
 

RE: Star-Broken Delta PT rating

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.

RE: Star-Broken Delta PT rating

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.  

RE: Star-Broken Delta PT rating

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

RE: Star-Broken Delta PT rating

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: 3V0 (broken delta) Resistor Sizing
 

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

RE: Star-Broken Delta PT rating

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.

RE: Star-Broken Delta PT rating

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.

RE: Star-Broken Delta PT rating

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.

RE: Star-Broken Delta PT rating

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.

RE: Star-Broken Delta PT rating

Stevenal-

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

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