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A Scott-T transformer (also called a Scott connection) is a type of circuit used to produce
two-phase electric power
Two-phase electrical power was an early 20th-century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by one-quarter of a cycle, 90°. Usually circuits used four wires, two ...
(2 φ, 90 degree phase rotation) from a three-phase (3 φ, 120 degree phase rotation) source, or vice versa. The Scott connection evenly distributes a balanced load between the phases of the source. The Scott three-phase transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
was invented by a Westinghouse engineer Charles F. Scott in the late 1890s to bypass Thomas Edison
Thomas Alva Edison (February 11, 1847October 18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation, mass communication, sound recording, and motion pictures. These inventi ...
's more expensive rotary converter
A rotary converter is a type of electrical machine which acts as a mechanical rectifier, inverter or frequency converter.
Rotary converters were used to convert alternating current (AC) to direct current (DC), or DC to AC power, before the adv ...
and thereby permit two-phase generator plants to drive three-phase motors.
Interconnection
At the time of the invention, two-phase motor loads also existed and the Scott connection allowed connecting them to newer three-phase supplies with the currents equal on the three phases. This was valuable for getting equal voltage drop and thus feasible regulation of the voltage from the electric generator (the phases cannot be varied separately in a three-phase machine).Nikola Tesla
Nikola Tesla ( ; ,"Tesla"
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polyphase power system was based on simple-to-build two-phase four-wire components. However, as transmission distances increased, the more transmission-line efficient three-phase system became more common. (Three phase power can be transmitted with only three wires, where the two-phase power systems required four wires, two per phase.) Both 2 φ and 3 φ components coexisted for a number of years and the Scott-T transformer connection allowed them to be interconnected.
Assuming the desired voltage is the same on the two and three phase sides, the Scott-T transformer connection (shown right) consists of a centre-tapped 1:1 ratio main transformer, T1, and a /2(≈86.6%) ratio teaser transformer, T2. The centre-tapped side of T1 is connected between two of the phases on the three-phase side. Its centre tap then connects to one end of the lower turn count side of T2, the other end connects to the remaining phase. The other side of the transformers then connect directly to the two pairs of a two-phase four-wire system.
The Scott-T transformer connection may also be used in a back-to-back T-to-T arrangement for a three-phase to three-phase connection. This is a cost-saving in the lower-power transformers due to the two-coil T connected to a secondary two-coil T instead of the traditional three-coil primary to three-coil secondary transformer. In this arrangement the X0 neutral tap is part way up on the secondary teaser transformer (see right). The voltage stability of this T-to-T arrangement as compared to the traditional three-coil primary to three-coil secondary transformer is questioned, as the "per unit" impedance of the two windings (primary and secondary, respectively) ''are not'' the same in a T-to-T configuration, whereas the three windings (primary and secondary, respectively) ''are'' the same in a three transformer configuration, if the three transformers are identical.
Three-phase to three-phase (also called "T-connected") distribution transformers are seeing increasing applications. The primary ''must'' be
Technical details
Assuming the desired voltage is the same on the two and three phase sides, the Scott-T transformer connection (shown right) consists of a centre-tapped 1:1 ratio main transformer, T1, and a /2(≈86.6%) ratio teaser transformer, T2. The centre-tapped side of T1 is connected between two of the phases on the three-phase side. Its centre tap then connects to one end of the lower turn count side of T2, the other end connects to the remaining phase. The other side of the transformers then connect directly to the two pairs of a two-phase four-wire system.
Unbalanced loads
Two-phase motors draw constant power, just as three-phase motors do, so a balanced two-phase load is converted to a balanced three-phase load. However if a two-phase load is not balanced (more power drawn from one phase than the other), no arrangement of transformers (including the Scott-T transformers) can restore balance: Unbalanced current on the two-phase side causes unbalanced current on the three-phase side. Since the typical two-phase load was a motor, the current in the two phases was presumed inherently equal during the Scott-T development. In modern times people have tried to revive the Scott connection as a way to powersingle-phase
In electrical engineering, single-phase electric power (abbreviated 1φ) is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when load ...
electric railways
A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply.
Electric railways use either electric locomotives (hauling passengers or freight in separate cars), e ...
from three-phase Utility supplies. This will not result in balanced current on the three-phase of being equal. The instantaneous difference in loading on the two sections will be seen as an imbalance in the three-phase supply; there is no way to smooth it out with transformers. The cited article is a GE paper which points out that railway unbalance, even via Scott-T transformers, affects generators, the motors of other customers and presumably delta connected transformers. Even small unbalances can cause heating. However, because electric systems have grown larger over the 20th century, the paper suggests that the railways are now a tolerable load, provided one has a confirming system analysis. Scott-T transformers may not even be relevant, since direct line-to-line load connections may be sufficient. So this leaves a potential solution, but the single-phase load should then be viewed as being ''tolerated'', not ''balanced''. Allowing it also raises the question: "What if other customers asked for the same toleration?"
Back to back arrangement
The Scott-T transformer connection may also be used in a back-to-back T-to-T arrangement for a three-phase to three-phase connection. This is a cost-saving in the lower-power transformers due to the two-coil T connected to a secondary two-coil T instead of the traditional three-coil primary to three-coil secondary transformer. In this arrangement the X0 neutral tap is part way up on the secondary teaser transformer (see right). The voltage stability of this T-to-T arrangement as compared to the traditional three-coil primary to three-coil secondary transformer is questioned, as the "per unit" impedance of the two windings (primary and secondary, respectively) ''are not'' the same in a T-to-T configuration, whereas the three windings (primary and secondary, respectively) ''are'' the same in a three transformer configuration, if the three transformers are identical.
Three-phase to three-phase (also called "T-connected") distribution transformers are seeing increasing applications. The primary ''must'' be delta
Delta commonly refers to:
* Delta (letter) (Δ or δ), a letter of the Greek alphabet
* River delta, at a river mouth
* D ( NATO phonetic alphabet: "Delta")
* Delta Air Lines, US
* Delta variant of SARS-CoV-2 that causes COVID-19
Delta may also ...
-connected (Δ), but the secondary ''may be'' either delta or "wye"-connected ( Y), at the customer's option, with X0 providing the neutral for the "wye" case. Taps for either case are usually provided. The customary maximum capacity of such a distribution transformer is 333 kVA (a third of a megawatt at unity power factor).
See also
*Alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
* Polyphase coil
Polyphase coils are electromagnetic coils connected together in a polyphase system such as a generator or motor. In modern systems, the number of phases is usually three or a multiple of three. Each phase carries a sinusoidal alternating curren ...
* Symmetrical components
* High-leg delta
High-leg delta (also known as wild-leg, stinger leg, bastard leg, high-leg, orange-leg, red-leg, dog-leg delta) is a type of electrical service connection for three-phase electric power installations. It is used when both single and three-phase po ...
References
{{DEFAULTSORT:Scott-T Transformer Electric transformers AC power fr:Transformateur électrique#Transformateur diphasé-triphasé