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Scott conection for Transformer
- Thread starter freddycor
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- Thread starter
- #3
thanks jghirst:
Im looking for vectorial drawings and relationships about this kind of tr conection, im tring to stablish the angular displacement and to make some calculation about voltages and currents. Please advise, and thank you very much.
Im looking for vectorial drawings and relationships about this kind of tr conection, im tring to stablish the angular displacement and to make some calculation about voltages and currents. Please advise, and thank you very much.
A Scott conection is just simply 2 single phase transformers connected in the T configuration on the 3 phase side and in the X configuration on the 2 phase side. Since the center tapped winding on the T connected side will have poor power factor and capacity will be wasted, a third transformer that serves as a balance coil is used to provide the center tap instead.
Thus: ___________
| |
A | C | B
| | | | |
| | | | |
| | | | |
^^^^^^^^^^^^^ ^^^^^^^^^^^^^
============= =============
^^^^^^^^^^^^^ ^^^^^^^^^^^^^
| | | | | |
D | D' E | E'
|_______________|
| |
N Ground
The KVA rating of the left transformer will need to be greater than the right transformer because the left transformer has more primary voltage and current unless a balance coil is used to provide the extra center tap.
For 480 volt 3 phase side transformer 1 is 240/480 volts and transformer 2 is 416 volts. The secondaries would both be 120/240 volts or by using 4 or 5 transformers the secondaries can be 240/480 volts.
At one time General Electric made a 2 phase generator that was internally T connected to make 120/240 volts 4-wire delta. One winding was 120/240 volts center tapped to ground and the other 208 volts end grounded. They called this a monocyclic generator and it avoided circulating current that would occur if a straight 4-wire delta winding was used.
Mike Cole, mc5w@earthlink.net
Thus: ___________
| |
A | C | B
| | | | |
| | | | |
| | | | |
^^^^^^^^^^^^^ ^^^^^^^^^^^^^
============= =============
^^^^^^^^^^^^^ ^^^^^^^^^^^^^
| | | | | |
D | D' E | E'
|_______________|
| |
N Ground
The KVA rating of the left transformer will need to be greater than the right transformer because the left transformer has more primary voltage and current unless a balance coil is used to provide the extra center tap.
For 480 volt 3 phase side transformer 1 is 240/480 volts and transformer 2 is 416 volts. The secondaries would both be 120/240 volts or by using 4 or 5 transformers the secondaries can be 240/480 volts.
At one time General Electric made a 2 phase generator that was internally T connected to make 120/240 volts 4-wire delta. One winding was 120/240 volts center tapped to ground and the other 208 volts end grounded. They called this a monocyclic generator and it avoided circulating current that would occur if a straight 4-wire delta winding was used.
Mike Cole, mc5w@earthlink.net
- Thread starter
- #5
thanks mc5w:
Im sorry for not be enough clear about my question, my doubt is about the 3phase-3phase Scott transformer conection; after a search in the web I found some information but even it is not totally clear to me how locate the neutral point in the secundary winding. Im tryin to help to a friend that rebuilds TR´s but is the first time that make this kind of conection and to me is a new opportunity to learn a little more. Any help will be wellcome and apreciate.
Im sorry for not be enough clear about my question, my doubt is about the 3phase-3phase Scott transformer conection; after a search in the web I found some information but even it is not totally clear to me how locate the neutral point in the secundary winding. Im tryin to help to a friend that rebuilds TR´s but is the first time that make this kind of conection and to me is a new opportunity to learn a little more. Any help will be wellcome and apreciate.
Limited discussion of tee connection in §2.4 of ANSI/IEEE standard C57.105-1978. [For-fee PDF from IEEE] Description of contents standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.105-1978_desc.html
Non-technical historic account in IEEE Industry Applications Magazine, Nov-Dec 2002, pp.6-8
In considering the marked advantage of the two-phase system for distribution and of the three-phase system for transmission, it occurred to me that that a combination of the two systems night secure the advantages of both, and I have worked out a simple and effective method of accomplishing this result.
Some small 480 volts 3 phase primary 120Y208 volt secondary transformers use 2 single phase units in the T primary T secondary configuration. The neutral point on the secondary is displaced up one of the windings to get the 120Y208 volt neutral.
You can also use the T primary T secondary configuration to get 120/240 volt 4 wire delta similar to the monocyclic generator. This will have slightly unbalanced inductive reatance but has simpler overcurrent protection than 3 wire wyre primary genuine 4 wire delta secondary. The latter requires 5 supplemental overload relays, one in each primary lead and 1 in each half of the center tapped secondary.
Mike Cole, mc5w@earthlink.net
You can also use the T primary T secondary configuration to get 120/240 volt 4 wire delta similar to the monocyclic generator. This will have slightly unbalanced inductive reatance but has simpler overcurrent protection than 3 wire wyre primary genuine 4 wire delta secondary. The latter requires 5 supplemental overload relays, one in each primary lead and 1 in each half of the center tapped secondary.
Mike Cole, mc5w@earthlink.net
My copy of GE's "Distribution Transformer Manual" (1991) states that the Scott connection is a three phase to TWO phase connection, and requires extra taps in the primary windings of the two xmfrs at 50% and 86.6% !
Since I am lousy at ASCII diagrams:
Transformer 1 is connected as: H1 to primary phase C, H3 to primary phase B, and an extra primary bushing (H2!) connected from the primary 50% tap to H1 of transformer 2. Transformer 2 is connected as: H1 to H2 of xmfr1, H2 is not connected, and H3 (tapped at 86.6% of primary winding) is connected to primary phase A.
The vector diagram shows two secondary phases at 90° from each other, with two possible secondary connections:
1) a 4-wire 2 phase system with separate neutral for each phase. No attempt is made to combine the neutral returns from the two phases
2) a 3-wire 2-phase system where X1 of xmfr 1 is connected to X2 of xmfr 2, and this is brought out as the neutral, with X2 of xmfr 1 and X1 of xmfr 2 as the two 'hots'.
Note that the neutral current will tend to be LARGER than either of the two phase currents if they are balanced (90° displacement vector sum)
There is a note on the page referring to this as an obsolete connection used historically to supply specially-wound two phase loads.
Since I am lousy at ASCII diagrams:
Transformer 1 is connected as: H1 to primary phase C, H3 to primary phase B, and an extra primary bushing (H2!) connected from the primary 50% tap to H1 of transformer 2. Transformer 2 is connected as: H1 to H2 of xmfr1, H2 is not connected, and H3 (tapped at 86.6% of primary winding) is connected to primary phase A.
The vector diagram shows two secondary phases at 90° from each other, with two possible secondary connections:
1) a 4-wire 2 phase system with separate neutral for each phase. No attempt is made to combine the neutral returns from the two phases
2) a 3-wire 2-phase system where X1 of xmfr 1 is connected to X2 of xmfr 2, and this is brought out as the neutral, with X2 of xmfr 1 and X1 of xmfr 2 as the two 'hots'.
Note that the neutral current will tend to be LARGER than either of the two phase currents if they are balanced (90° displacement vector sum)
There is a note on the page referring to this as an obsolete connection used historically to supply specially-wound two phase loads.
- Thread starter
- #9
thanks tinfoil:
Scott conection have the possibility of make 2 to 3 phase, 3 to 2 phase AND 3 to 3 phase. Now I have some information that makes to me able to make some TTR tests and others to rewind this TR. Thank you very much to all your responses.
Scott conection have the possibility of make 2 to 3 phase, 3 to 2 phase AND 3 to 3 phase. Now I have some information that makes to me able to make some TTR tests and others to rewind this TR. Thank you very much to all your responses.
Aside from legacy information in power-systems texts, there is limited online discussion of various ‘Scott’ configurations on pp 14-15 at [~0.5 MB]
Note that 3ø-3ø-rated types are not Scott connections, but is referred to as Tee-Tee in IEEE C57.105 §2.4 “…two configurations are possible, although the designation of terminals… simulate(s) standard D-Y, Y-D, D-D, and Y-Y…”
For some smaller {3-15kVA} potted-drytype transformers, some manufacturers offer Tee-Tee units in [Δ]—Y voltages [id est, 480—208Y/120V] and [Δ]—[Δ] voltages [id est, 3ø 480—240V]. Using two cores in those sizes is apparently more cost effective than three.
Aside — A historical curiosity in US utilities: ConEd and PECO list 120/240V 2ø 5-wire electric service {not for new work} in their basic tarriffs. Bizarrely, PECO also lists ‘2400V 2ø 3-wire’ service and ConEd lists ‘1950V 2ø.’ Anecdotally, low-voltage 2ø service for motors was once common for sawmills in the western US.
Minor Aside — A company that promotes a range of Tee-configured drytype transformers is in Montreal.
Sorry that my diagram did not come out. Another way for a utility to supply 2 phase power off of 3 phase is to connect 1 transformer primary phase to neutral and the other phase to phase across the other 2 phases. This can even be done with 7,200Y12,470 volts because a 2 primary terminal 14,400 volt transformer has a 12,500 volt tap.
Mike Cole, mc6w@earthlink.net
Mike Cole, mc6w@earthlink.net
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