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Analysis and application of scott connection


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Analysis and application of scott connection

  1. 1. Analysis and Application of Scott Connection November 18, 2013 Nawaraj Kumar Mahato Roll No.120104024 System Id: 2012017620 B.Tech (EEE) 2012 2nd Year
  2. 2. Analysis and Application of Scott Connection Nawaraj Kumar Mahato Department of Electrical and Electronics Engineering School of Engineering and Technology Sharda University, Gr. Noida, UP, INDIA Abstract Scott Connection is mainly used for deriving two-phase current from three-phase supply and vice-versa. Scott Connection is widely used in industrial furnace transformer and for traction purpose. It is used in electric railway system when there are two unbalanced single phase loads as Scott connected transformer can reduce unbalanced currents. This paper investigates the main application of Scott transformer and also analyze weather this connection can reduce unbalanced current. It also includes the complete analysis of balanced and unbalanced condition of the Scott Connected Transformer. 1. Introduction Scott connection (Scott T-transformer) is a type of circuit used to derive two-phase current from a three-phase source or vice-versa. The Scott three-phase transformer was invented by an engineer Charles F. Scott. In 1980s to bypass Thomas Edison‟s rotary converter and thereby permit two-phase generator plants to drive Nikola Tesla‟s three-phase motors. In many situations it becomes necessary to supply a balanced and unbalanced two phase or three phase supply. One way for a two phase load, to connect two phases between two lines, the following is one possibility: Two Phase loads Analysis and Application of Scott Connection Page 1
  3. 3. However, even when these phases are balanced, impedences are balanced; they will draw unbalanced current from supply. Not only that many two phase loads will require a balanced two phase supply as well. Some of these two phase loads are arc furnaces or induction melting furnaces. They will consume power in megawatts. So care should be taken to make sure that the loading of the three phase system is balanced top the extent possible. This can be done by a Scott Connected Transformer. In order to overcome the disadvantage of the T connection, the Scott connection uses two singlephase transformers of a special design to transform three-phase voltages and currents into twophase voltages and currents. The first transformer, called the „„main,‟‟ has a center-tapped primary winding connected to the three-phase circuit with the secondary winding connected to the two-phase circuit. It is vital that the two halves of the center-tapped primary winding are wound around the same core leg so that the ampere-turns of the two halves cancel out each other. The ends of the center-tapped main primary winding are connected to two of the phases of the three-phase circuit. The second transformer, called the „„teaser,‟‟ has one end of its primary winding connected to the third phase of the three-phase circuit and the other end connected to the center tap of the primary winding of the main. The Scott connection requires no primary neutral connection, so zero-sequence currents are blocked. The secondary windings of both the main and teaser transformers are connected to the two-phase circuit. The Scott connection is shown in figure for a two-phase, five-wire circuit, where both secondary windings are center-tapped and the center taps are connected to the neutral of the five wire circuit. Three-wire and four-wire configurations are also possible. If the main transformer has a turns ratio of 1: 1, then the teaser transformer requires a turns 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. Analysis and Application of Scott Connection Page 2
  4. 4. Figure: Transformer connection between a three-phase system and a two phase. 2. Review of Related Works The Scott Connected Transformer can convert a balanced three phase supply to two phase supply if the load on the two phase supply is balanced then the line current drawn from the three phase will be balanced. We will see how it is possible. Analysis and Application of Scott Connection Page 3
  5. 5. In the above diagram we have taken two single phase transformers. Let us say we have N,N1,N2 be the number of turns of the transformers. The second transformer is tapped by 50% so the number of turns of coil becomes N1/2 on both sides of tapping. Then we supplied balanced three phase supply to VA,VB and VC respectively. Let Va and Vb be the induced voltages. Now drawing the phasor VBC, VAB & VCA. Now the voltage phasor VMC will be half of VBC and voltage phasor VAM will be VAC-VMC. It shows that the voltage phasor VAM is right angle to the voltage phasor VBC, hence, induced voltage Va will be in phase with the voltage VAM and its magnitude will be (N2/N)*VAM . Voltage phasor Vb will be in phase with the voltage VBC and its magnitude will be (N2/N1)*VBC . A Va (N2/N)*VAM B M C (N2/N1)*VBC Vb Now we want the voltages VAB and VBC to be balanced and if we want their magnitude also to be same then it is imperative that it should have (N2/N)*VAM=(N2/N1)*VBC or we want N/N1=VAM/VBC From the Phasor diagram VAM/VBC=√3/2=0.867 Hence the number of turns in the primary side of the main transformer should be 0.867N1 So Scott Connected Transformer can be obtained by two single phase transformers with turns N1 and N2 . Analysis and Application of Scott Connection Page 4
  6. 6. 3. Description of the Problem Below there has been discussed the disadvantages of the three phase loading system. To overcome these problems we need a two phase balanced supply indeed we need a Scott Connected Transformer. 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 DeltaWye connection. The inherent single phase construction and characteristics of this connection produces a comparatively bulky and heavier transformer when compared with a normal three phase transformer of the same rating. Analysis and Application of Scott Connection Page 5
  7. 7. In the „‟Auten Stigant & Franklin‟‟ JOP Transformer Book 10th edition, they refer to the Scott T connection for a practical way of connecting a three phase primary to a two phase secondary. However when connecting a three phase primary to a three phase secondary in a Scott T configuration they state that ‘’two single phase transformers can be installed. Their total rated kVA must be 15.5% greater if the transformers are interchangeable or 7.75% greater if non-interchangeable.” Interchangeable refers to separate single phase units that can be replaced independently. In a comparative analysis of the published weights of the one commercially available Scott T transformer (only one manufacturer still constructs a dry-type Scott T) and a number of DeltaWye transformer manufacturers you will find that Scott T‟s are, in most instances, lighter than the similar kVA copper Delta-Wye units. This goes against the mathematical principals and the sizing requirements stated by “Auten Stigant & Franklin”. It suggests that the Scott T transformers do not meet and are not equivalent to their Delta-Wye equivalent kVAs. Example (Note: The following example is taken from an actual case that occurred a number of years ago.) A small industrial electric customer has a plant with an antiquated two phase electrical system. The customer has a Scott-connected transformer bank connecting this system to the local utility company which provides power. This Scott-connected bank catastrophically failed and there are no replacement transformers readily available. The utility company was called in to restore service to the two-phase system, but the utility only has standard single-phase and three-phase distribution transformers in its storerooms. The customer has a five-wire, two-phase system with a phase-to-neutral voltage of 120 V. The utility company‟s distribution system has a phase-tophase voltage of 12,470V and a phase-to-neutral voltage of 7200 V. Customers are routinely supplied with 120/240 V service using single-phase transformers connected either phase-tophase or phase-to-neutral on the primary side. One solution would be to use the T connection with the standard transformers that the utility had on hand, but this would result in unbalanced three phase currents. A bright young engineer working for the utility found a way to construct a Scott-connected bank using standard transformers. The teaser was replaced by a standard distribution transformer with a 12,470 V primary and a 120/240 V center-tapped secondary. The main transformer was replaced by two standard distribution transformers, each having a 7200 V primary and a 120/240 V center-tapped secondary. The primary windings of the replacement Analysis and Application of Scott Connection Page 6
  8. 8. main transformers were connected in series and their secondary windings were connected in parallel. These connections are shown in the figure below. Since the two transformers substituted for the main transformer are not wound on the same core leg, the secondary windings of the main transformers Figure: Scott-connected transformer bank using three standard single-phase distribution transformers, showing the current in the teaser primary current splitting evenly between the two main primaries are interconnected to force the primary current in the primary winding of the teaser to split evenly between the two primary windings of the main in Analysis and Application of Scott Connection Page 7
  9. 9. the same manner that a center-tapped winding forces the teaser current to split evenly in the primary winding of the main in a Scott connection. For a five-wire, two-phase circuit, the center tap of each of the three secondary windings is connected to the common neutral. For a four-wire, two phase circuit, the center taps of the secondary windings are not brought out. Note that the voltage across the 12,470 V primary winding of the teaser is 86.6% of the voltage across the two 7200 V primary windings in series of the main, just like a Scott-connected transformer. This produces balanced operation; however, when the bank is connected to a 12,470 V three-phase source, the phase-to-neutral output voltage is only 86.6% of 120 V, or 104 V. Fortunately in this case, the three-phase distribution voltage at the customer‟s location was 5% higher than the nominal voltage and the transformers also had taps to boost the output voltage by another 5%. The combination of the 5% higher system voltage and the 5% tap boost provided a two-phase secondary voltage of about 115 V. This was still below the 120 V nominal voltage but it was within the operating voltage range of the customer‟s two-phase equipment. 4. Conclusion The Scott T connection in theory would be suitable for supplying a three, two and single phase load simultaneously, but such loads are not found together in modern practice. The neutral points can be available for grounding or loading purposes. Scott transformer causes two unbalanced single-phase loads to be presented less unbalanced to the power system 5. Bibliography i. ii. K Murugesh Kumar, “DC Machines and Transformers”, 2nd Edition, pg no.478-478, Vikash Publishing House Pvt. Ltd. , 2004 iii. Hooman Erfanian MAZIN, Joey GALLANT,” A Probabilistic Analysis on the Harmonic Cancellation Characteristics of the Scott Transformer”, University of Alberta, Canada, Pg no.1-7, 2009 iv. Irving L. Kosow, “Electric Machinery and Transformers”, Second edition, pg no. 559560, Prentice Hall of India, 2004 Analysis and Application of Scott Connection Page 8
  10. 10. X Softw are required, please dow nload from http://w w w Analysis and Application of Scott Connection Page 9