288V vs. 277V PTs
288V vs. 277V PTs
(OP)
Does anyone know why PTs (VTs for the younger folks) for 480Y/277V service are rated 288:120V instead of 277:120V?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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RE: 288V vs. 277V PTs
RE: 288V vs. 277V PTs
How about this? Integer turns of wire are required. Such a PT would require 397 wraps, making it big heavy and expensive.
RE: 288V vs. 277V PTs
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.
RE: 288V vs. 277V PTs
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
RE: 288V vs. 277V PTs
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.
RE: 288V vs. 277V PTs
RE: 288V vs. 277V PTs
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.
RE: 288V vs. 277V PTs
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
RE: 288V vs. 277V PTs
RE: 288V vs. 277V PTs