288V vs. 277V PTs 1

[WoodrowJWeen]

Does anyone know why PTs (VTs for the younger folks) for 480Y/277V service are rated 288:120V instead of 277:120V?

 

[pwrengrds]

288/120 is a ratio of exactly 2.40 while 277/120 produces an uneven number 2.308333 that you can not program digital equipment to.

 

[stevenal]

You can't?

How about this? Integer turns of wire are required. Such a PT would require 397 wraps, making it big heavy and expensive.

 

[busbar]

Note 2.5:1 [300:120] is the ANSI C57.13 standard VT ratio. It typically provides an integer combined CT/PT ratio for revenue metering.

 

[jbartos]

Suggestion: A little calculation reveals:
Assume: 277Volt of 480V/277V system
277V x (120V/300V) = 110.8V on the PT secondary
277V x (120V/288V) = 115.4V on the PT secondary

Considering voltage fluctuations, e.g. 10%,
0.9 x 110.8V = 99.72V by ANSI C57.13
0.9 x 115.4V = 103.86V by the mfr offered PT
Apparently, 99.72V may be viewed as too low voltage with respect to needed 120V.
(99.72/120) x 100% = 83.1%, a little bit too low
(103.86/120) x 100% = 86.55%, still good for ac relays

 

[scottf]

stevenal was correct. It has to do with the turns ratio and how the VT/PT is manufactured.

You'll notice that none of the VT ratios specified in IEEEC57.13 actually use the true sqrt3 rated line-to-ground voltage the corresponds to the rated system voltage.

For example, a 115 kV VT has a rated line to ground voltage of 69 kV, where 66.4 kV is really the line to ground voltage with a 115 kV system voltage. 69 kV LG enables a 600/1000:1 ratio that yeilds 115/69V rated seconday voltage.

Nowadays with electronic relays and meters, it doesn't really matter in terms of the actual voltage developed.

 

[jbartos]

Suggestion: If the relays are old, electromechanical, then the old PT ratings have to be used. New (digital) relays are not so demanding on the voltage input.

 

[twosockets]

jbartos,

You are correct in that old induction meters and relays were generally calibrated at 115 or 120 volts. Therefore, the objective was to use a VT that would result in as close to the calibration voltage as was practical to achieve the best accuracy. For 480/277 volt applications, some utilites even selected a 2.3:1 ratio VT (277/2.3 = 120.4 volts).

Wide voltage range solid state meters and relays preclude the need for all the ratios stocked by a typical utility. In fact, there is a 2:1 VT that is available that can be used on virtually all <= 600 volt wye applications and delta applications <= 480 volts.

 

[busbar]

Two other non-integer PT ratios that exist for convenience in protective relaying with no concern for revenue accuracy are 1.73:1 and 0.577:1

 

[stevenal]

It's not the ratio, but the actual number of windings that must be integers. Maybe 52 and 30.

 

[jbartos]

Suggestion: The industry standards are practical on the average, and in a sense that the standardized equipment is the same from project to project, from interface to interface, etc. However, the industry standards do not guarantee the ultimate standardized product convenience for an arbitrary project or application.