Switchgear Line vs Bus PT's

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philly

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When considering Double-ended MV M-T-M switchgear is it standard practice to have PT's located on line side of main breaker, load side of main breaker (bus PT's), or both?

I guess ideally it would be best to have PT's on both sides but this may not always be feasible due to cost and space limitations? I believe with a tie-breaker you need to have bus PT's so that PT power is available to relays, etc... when bus is fed through the tie. I cant seem to think of a good reason to require PT's on the line side of the main other than to be able to see that there is incoming voltage at the line side of the main breaker.

For my question I'm ignoring any PT requirements for synchronization.
 
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SG-1

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Ware Shoals, South Carolina
philly said:
When considering Double-ended MV M-T-M switchgear is it standard practice to have PT's located on line side of main breaker, load side of main breaker (bus PT's), or both?

I guess ideally it would be best to have PT's on both sides but this may not always be feasible due to cost and space limitations? I believe with a tie-breaker you need to have bus PT's so that PT power is available to relays, etc... when bus is fed through the tie. I cant seem to think of a good reason to require PT's on the line side of the main other than to be able to see that there is incoming voltage at the line side of the main breaker.

For my question I'm ignoring any PT requirements for synchronization.
Click to expand...

If you have under voltage protection on the MAIN breakers, it is difficult to close them without LINE PT's. I have seen this defeated using the "O" contact in the control switch to momentarily interrupt the trip signal from the UV just long enough for the breaker to close & the UV protection contacts to open.

If you have an automatic transfer, it has to know if the voltage on the Incoming Lines is good or bad.

The voltage from the Line PTs can be routed to the protective relays through an 83V & the TIE Breaker MOC. The wiring for this is much more complex than just BUS PT's, but if you do not have the room...
 
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Phil Corso

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Boca Raton, Fl, USA
philly said:
... I guess ideally it would be best to have PT's on both sides but this may not always be feasible due to cost and space limitations? I believe with a tie-breaker you need to have bus PT's so that PT power is available to relays, etc... when bus is fed through the tie. I cant seem to think of a good reason to require PT's on the line side of the main other than to be able to see that there is incoming voltage at the line side of the main breaker...
Click to expand...

Philly...

Typically (in my experience) only 2 PT's are required! One on each source. Preferably, each should be mounted in its own withdrawable cubicle (with fusing), connected in the power circuit between the Xfmr Sec'y terminal and upstream of the Incoming CB's bus stab!

They should only provide power relays associated with transfer-logic, protection, and single-phase metering (if permitted)! They should not be used for voltage-controlled loads associated with system re-acceleration! Normally, bus connected PT's are used!

Regards, Phil Corso
 
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Ingenieur

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Earth
As others said if you have uv coils you need power from the line side to close in
if your relay/control power is sourced from the pt's and required to close in obviously line side

they need a disconnect and labelled so if the main is tripped folks know control power is still energized
 
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Ingenieur

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Phil Corso said:
I could... in facilities in which I was involved with electrical-distribution design!

Phil
Click to expand...

our systems are only 100 mv
5 substations
perhaps 50 x 2 mva load centers
designed by multiple engineers with an avg of 20-40 yrs experience
all use the same line side pt's for protection sensing and to derive control power
!!!
 
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