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Prof. Ch. SAI BABU, Professor of Electrical & Electronics Engineering JNTUK Kakinada Lecture VI of Electrical Machines Topic: Transformers

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Prof. Ch. SAI BABU, Professor of Electrical & Electronics Engineering JNTUK Kakinada Lecture VI of Electrical Machines Topic: Transformers ONLINE GATE COACHING CLASSES BY Prof. Ch. SAI BABU, Professor of Electrical & Electronics Engineering JNTUK Kakinada Lecture VI of Electrical Machines Topic: Transformers Jawaharlal Nehru Technological University, Kakinada-533003, Andhra Pradesh 1 Lecture VI Single phase transformer (Topics Covered) Principle of Operation of Three Phase Transformer, Connections, Parallel operation, Conditions Auto Transformer 2 Three phase transformer Normally , when three-phase is required, a single enclosure with three primary and three secondary windings wound on a common core is all that is required. However three single-phase transformers with the same rating can be connected to form a three-phase bank. Since each single-phase transformer has a primary and a secondary winding, then 3 single-phase transformers will have the required 3 primary and 3 secondary windings and can be connected in the field either Delta-Delta or Delta-Wye to achieve the required three-phased transformer bank. 3 Three Phase Transformer Construction The three-limb core-type three-phase transformer is the most common method of three-phase transformer construction allowing the phases to be magnetically linked. Flux of each limb uses the other two limbs for its return path with the three magnetic flux’s in the core generated by the line voltages differing in time-phase by 120 degrees. The shell-type five-limb type three-phase transformer construction is heavier and more expensive to build than the core-type. Five-limb cores are generally used for very large power transformers as they can be made with reduced height. 4 Three Phase Voltages and Currents A transformer can not act as a phase changing device and change single-phase into three-phase or three-phase into single phase 5 Three Phase Transformer Connections 6 Phase Phase Connection Line Voltage Line Current Voltage Current Star VP = VL ÷ √3 VL = √3 × VP IP = IL IL = IP Delta VP = VL VL = VP IP = IL ÷ √3 IL = √3 × IP 7 There are four different ways in which three single-phase transformers may be connected together between their primary and secondary three-phase circuits. These four standard configurations are given as: Delta-Delta (Dd), Star-Star (Yy), Star-Delta (Yd), and Delta-Star (Dy). Connection Primary Winding Secondary Winding Delta D d Star Y y Interconnected Z z 8 Three Phase Transformer Star and Delta Configurations: Transformers for high voltage operation with the star connections has the advantage of reducing the voltage on an individual transformer, reducing the number of turns required and an increase in the size of the conductors, making the coil windings easier and cheaper to insulate than delta transformers. 9 Transformer Delta and Delta Connections: • The delta-delta connection nevertheless has one big advantage over the star-delta configuration, in that if one transformer of a group of three should become faulty or disabled, the two remaining ones will continue to deliver three-phase power with a capacity equal to approximately two thirds of the original output from the transformer unit. 10 Transformer Delta and Delta Connections: • The disadvantage of delta connected three phase transformers is that each transformer must be wound for the full-line voltage, (in our example above 100V) and for 57.7 per cent, line current. The greater number of turns in the winding, together with the insulation between turns, necessitates a larger and more expensive coil than the star connection. 11 Transformer Star and Star Connections In the star-star arrangement ( Yy ), (wye-wye), each transformer has one terminal connected to a common junction, or neutral point with the three remaining ends of the primary windings connected to the three-phase mains supply. The number of turns in a transformer winding for star connection is 57.7 per cent, of that required for delta connection. 12 When the primary and secondary have different types of winding connections, star or delta, the overall turns ratio of the transformer becomes more complicated. If a three-phase transformer is connected as delta-delta ( DD ) or star-star ( YY ) then the transformer could potentially have a 1:1 turns ratio. That is the input and output voltages for the windings are the same. With a 1:1 turns ratio, a star–delta connected transformer will provide a √3:1 step-down line-voltage ratio. 13 The star connection requires the use of three transformers, and if any one transformer becomes fault or disabled, the whole group might become disabled. Nevertheless, the star connected three phase transformer is especially convenient and economical in electrical power distributing systems, in that a fourth wire may be connected as a neutral point, ( n ) of the three star connected secondaries as shown. Primary-Secondary Line Voltage Line Current Configuration Primary or Secondary Primary or Secondary Delta – Delta Delta – Star Star – Delta Star – Star 14 Open-Delta or V-V Connection 15 Consider a bank of three one phase transformers working as Dd or Yd mode (secondary always delta) to run a three phase load. It may happen that due to some reason, one of the transformers is removed; like maintenance, damage, faults etc. In such a case, the secondary delta is broken and the connection resembles a V. Hence this sort of connection is also known as V-V connection or Open- delta connection. Open-delta connection is often used to provide power to new under-construction projects such as a new power station or a substation, to reduce the initial investment. This reduces the project cost. An open-delta connection may be used to provide three phase power up to 57.7 % of the actual capacity of the three phase bank. If the three phase load is not reduced while using open-delta, there is high chance that one transformer gets overloaded; which can be more dangerous. 16 • The disadvantages of this connection are : 1. The average power factor at which the V-bank operates is less than that of the load. This power factor is actually 86.6% of the balanced load power factor. Another significant point to note is that, except for a balanced unity power factor load, the two transformers in the V – V bank operate at different power factors 2. Secondary terminal voltages tend to become unbalanced to a great extent when the load is increased, this happens even when the load is perfectly balanced. 3. for an increase in cost of 50% for the third transformer, the increase in capacity is 73.2% when converting from a V – V system to a D – D system. 17 Scott-T Transformer Connection • 3-phase to 2-phase conversion and vice-versa. • The two transformers are connected electrically but not magnetically. • One of the transformers is called the main transformer, and the other is called the auxiliary or teaser transformer. 18 19 Applications of Scott Connection • The following are the applications of the Scott-T connection. • The Scott-T connection is used in an electric furnace installation where it is desired to operate two single-phase together and draw the balanced load from the three-phase supply. • It is used to supply the single phase loads such as electric train which are so scheduled as to keep the load on the three phase system as nearly as possible. • The Scott-T connection is used to link a 3-phase system with a two–phase system with the flow of power in either direction. 20 Summary on three phase transformers: • Four common methods of connecting three transformers for 3- phase circuits are Δ-Δ, Y-Y, Y-Δ, and Δ-Y connections. • An advantage of Δ-Δ connection is that if one of the transformers fails or is removed from the circuit, the remaining two can operate in the open-Δ or V connection. This way, the bank still delivers 3-phase currents and voltages in their correct phase relationship. However, the capacity of the bank is reduced to 57.7 % of its original value. • In the Y-Y connection, only 57.7% of the line voltage is applied to each winding but full line current flows in each winding. The Y-Y connection is rarely used. • The Δ-Y connection is used for stepping up voltages since the voltage is increased by the transformer ratio multiplied by 3. 21 Parallel operation: The Transformer is said to be in Parallel Operation when their primary windings are connected to a common voltage supply, and the secondary windings are connected to a common load Advantages: it increases the efficiency of the system, makes the system more flexible and reliable Disadvantages: it increases the short-circuit current of the transformers. 22 Necessary Conditions For Parallel Operation For the satisfactory parallel operation of the transformer, the two main conditions are necessary. • Polarities of the transformers must be same. • Turn Ratio of the transformer should be equal. The other two desirable conditions are as follows:- The voltage at full load across the transformer internal impedance should be equal. The ratio of their winding resistances to reactance's should be equal for both the transformers. This condition ensures that both transformers operate at the same power factor, thus sharing their active power and reactive volt-amperes according to their ratings. 23 Reasons For Parallel Operation • It is impractical and uneconomical to have a single large transformer for heavy and large loads. Hence, it will be a wise decision to connect a number of transformers in parallel. • In substations, the total load required may be supplied by an appropriate number of the transformer of standard size. As a result, this reduces the spare capacity of the substation. • If the transformers are connected in parallel, so there will be scope in future, for expansion of a substation to supply a load beyond the capacity of the transformer already installed.
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