29-01-2013, 12:13 PM
Scott-T Connection of Transformer
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Transforming 3 Phase to 2 Phase:
• There are two main reasons for the need to transform from three phases to two phases,
1. To give a supply to an existing two phase system from a three phase supply.
2. To supply two phase furnace transformers from a three phase source.
• Two-phase systems can have 3-wire, 4-wire, or 5-wire circuits. It is needed to be considering that a two-phase system is not 2/3 of a three-phase system. Balanced three-wire, two-phase circuits have two phase wires, both carrying approximately the same amount of current, with a neutral wire carrying 1.414 times the currents in the phase wires. The phase-to-neutral voltages are 90° out of phase with each other.
• Two phase 4-wire circuits are essentially just two ungrounded single-phase circuits that are electrically 90° out of phase with each other. Two phase 5-wire circuits have four phase wires plus a neutral; the four phase wires are 90° out of phase with each other.
The Scott Connection of Transformer:
• A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase power from a three-phase source or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the source.
• Scott T Transformers require a three phase power input and provide two equal single phase outputs called Main and Teaser. The MAIN and Teaser outputs are 90 degrees out of phase. The MAIN and the Teaser outputs must not be connected in parallel or in series as it creates a vector current imbalance on the primary side.
• MAIN and Teaser outputs are on separate cores. An external jumper is also required to connect the primary side of the MAIN and Teaser sections.
• The schematic of a typical Scott T Transformer is shown below:
Key Point:
• If the main transformer has a turn’s ratio of 1: 1, then the teaser transformer requires a turn’s ratio of 0.866: 1 for balanced operation. The principle of operation of the Scott connection can be most easily seen by first applying a current to the teaser secondary windings, and then applying a current to the main secondary winding, calculating the primary currents separately and superimposing the results.
Load connected between phaseY1 and phase Y2 of the secondary:
• Secondary current from the teaser winding into phase X1 =1.0 <90°
• Secondary current from the teaser winding into phase X2 =-1.0< 90°
• Primary current from H3 phase into the teaser winding= 1.1547< 90°
• Primary current from H2 phase into the main winding= 0.5774 <90°
• Primary current from H1 phase into the main winding= -0.5774< 90°
• The reason that the primary current from H3 phase into the teaser winding is 1.1547 due to 0.866: 1 turn’s ratio of the teaser, transforming 1/0.866= 1.1547 times the secondary current. This current must split in half at the center tap of the main primary winding because both halves of the main primary winding are wound on the same core and the total ampere-turns of the main winding must equal zero.
Advantages of the Scott T Connection:
• If desired, a three phase, two phase, or single phase load may be supplied simultaneously
• The neutral points can be available for grounding or loading purposes
Disadvantages when used for 3 Phase Loading
• This type of asymmetrical connection (3 phases, 2 coils), reconstructs three phases from 2 windings. This can cause unequal voltage drops in the windings, resulting in potentially unbalanced voltages to be applied to the load.
• The transformation ratio of the coils and the voltage obtained may be slightly unbalanced due to manufacturing variances of the interconnected coils.
• This design’s neutral has to be solidly grounded. If it is not grounded solidly, the secondary voltages could become unstable.
• Since this design will have a low impedance, special care will have to be taken on the primary protection fault current capacity. This could be an issue if the system was designed for a Delta-Star connection.
Application:
• For Industrial Furnace Transformer.
• For Traction Purpose: The power is obtained from the 220 kV or 132 kV or 110 kV or 66 kV, three-phase, effectively earthed transmission network of the State Electricity Board, through single-phase transformers or Scott connected transformer installed at the Traction Substation. The primary winding of the single-phase transformer is connected to two phases of the transmission network or Where Scott-connected transformer is used, the primary windings are connected to the three phases of the transmission network.
• The single-phase transformers at a Traction Substation are connected to the same two phases of the transmission network (referred as single-phase connection), or alternatively to different pairs of phases- the three single phase transformers forming a delta-connection on the primary side. Out of three single-phase transformers, one transformer feeds the overhead equipment (OHE) on one side of the Traction Substation, another feeds the OHE on the other side of the Traction Substation, and the third remains as standby. Thus the two single-phase transformers which feed the OHE constitute an open-delta connection (alternatively, referred as V-connection) on the three-phase transformers network. The Scott-connected transformer and V-connected single-phase transformers are effective in reducing voltage imbalance on the transmission network. The spacing between adjacent substations is normally between 70 and 100 km.