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

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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. • If there will be any breakdown of a transformer in a system of transformers connected in parallel, there will be no interruption of power supply, for essential services. • If any of the transformers from the system is taken out of service for its maintenance and inspection, the continuity of the supply will not get disturbed.  Various conditions that must be fulfilled for the successful parallel operation of transformers: • Same voltage Ratio & Turns Ratio • Same Percentage Impedance and X/R ratio. • Identical Position of Tap changer. • Same KVA ratings. • Same Phase angle shift (vector group are same). • Same Frequency rating. • Same Polarity. 24 • Same Phase sequence.

25 Example The primary winding of a delta-star ( Dy ) connected 50VA transformer is supplied with a 100 volt, 50Hz three-phase supply. If the transformer has 500 turns on the primary and 100 turns on the secondary winding, calculate the secondary side voltages and currents. (Gate 2014)

Given Data: transformer rating, 50VA, supply voltage, 100v, primary turns 500, secondary turns, 100.

Then the secondary side of the transformer supplies a line voltage, VL of about 35v giving a phase voltage, VP of 20v at 0.834 amperes.

 If the primary line voltage rating is 3.3 kV (Y side) of a 25 kVA, Y-△△ transformer (the per phase turns ratio is 5:1), then the line current rating of the secondary side (in Ampere) is______. (Gate 2017) Ans: 37 to 39

 Keeping in view the requirement of parallel operation, which of the 3-phase connections given below are possible? (a) delta-delta to delta-star (b) delta-delta to star-delta (c) star- star to delta-star (d) delta-star to star-delta Ans: delta-star to star-delta belongs to -30 phase groups therefore they can be used for parallel operation

 A V-V connected transformer can be connected in parallel to delta-delta connected transformer but not to (Gate 2015) a) delta-star b) star-delta c) star-V d) all of the mentioned Answer: a Explanation: The V-V connected transformer and D-D connected transformers have same phase displacement, so they only can be connected in parallel to each other.

27  The transformer which is more feasible to use in the distribution ends should be a) star-delta b) delta-star c) scott d) delta-delta Answer: a Explanation: Star-delta will have lower voltage at delta end.  The voltage across the terminal ‘r’ and ‘y’ is a) 1.73*V b) V/1.73 c) V d) 3*V

Answer: b Explanation: From star to delta, the line voltage will reduce by 1.73.

 . Open delta transformers can be obtained from a) delta-delta b) star-delta c) delta-star d) any of the mentioned Answer: a Explanation: If one of the transformers is removed from the bank of only delta- delta, then it behaves with 58% power delivery.  The delta-delta connections are used in applications of a) large l.v. transformers b) small h.v. transformers c) large h.v. transformers d) small l.v. transformers Answer: a Explanation: Delta connected winding handle line voltages so it needs more turns in the winding but thin wires.

(Gate 2004)

29 • Figure shows a Δ-Y connected 3 phase distribution transformer used to step down

the voltage from 11000 to 415V line to line. It has 2 switches S1 and S2. Under Normal conditions S1 is closed S2 are open. Under superior condition S1 as open and S2 in closed. In such case the magnitude of voltage across the LV terminal a and c is (Gate 2003)

1. 240 V 2. 480 V 3.415 V 4. 0 V

In Y connection the phase winding is connection is series subtractive polarity

Vac = Van – Vcn , Van = Vcn,, ∴ Vac = 0V

30 The combinations that will operate in parallel: •Following Vector group of Transformer will operate in parallel.

Operative Parallel Operation

Sr.No Transformer-1 Transformer-2 1 ∆∆ ∆∆ or Yy 2 Yy Yy or ∆∆ 3 ∆y ∆y or Y∆ 4 Y∆ Y∆ or ∆y The combinations that will not operate in parallel: •Following Vector group of Transformer will not operate in parallel.

Inoperative Parallel Operation Sr.No Transformer-1 Transformer-2 1 ∆∆ ∆y 2 ∆y ∆∆ 3 Y∆ Yy 4 Yy Y∆ 31 Autotransformer

An autotransformer is a kind of electrical transformer where primary and secondary shares same common single winding. So basically it’s a one winding transformer.

32 33 Applications of Auto Transformers 1.Compensating voltage drops by boosting supply voltage in distribution systems. 2.Auto transformers with a number of tapping are used for starting induction and synchronous motors. 3.Auto transformer is used as variac in laboratory or where continuous variable over broad ranges are required. 4.An auto transformer has higher efficiency than two winding transformer. This is because of less ohmic loss and core loss due to reduction of transformer material. 5.Auto transformer has better voltage regulation as voltage drop in resistance and reactance of the single winding is less. 34

Copper Savings in Auto Transformer weight of copper in winding is directly proportional to product of number of turns and rated current of the winding.

Wa and Wtw are weight of copper in auto transformer and two winding transformer, Saving of copper in auto transformer compared to two winding transformer=

Wtw -Wa = KWtw

constant ′k′ is nothing but turns ratio or voltage ratio of that auto transformer.

35 Power Transformer

 The Power Transformer is installed at various power stations for generation and transmission of power.  It acts as a step-up or a step-down transformer for increasing and decreasing the level of voltages as per the requirement, and it’s also used as an interconnection between two power stations.

36 Distribution Transformer

• The Distribution Transformer is used to bring down or step down the voltage and current level of a transmission line to a predefined level, which is called safety level for the end-user consumer in domestic and industrial purpose.

37 BASIS OF DIFFERENCE POWER TRANSFORMER DISTRIBUTION TRANSFORMER Type of network It is used in transmission network It is used in the distribution of higher voltages network for lower voltages. Availability of ratings 400 kV, 200 kV, 110 kV , 66 kV, 33 11 Kv, 6.6 Kv, 3.3 Kv, 440 V,230 V kV. Maximum rating of usage Power transformers are used for Distribution transformers are used rating above 200 MVA for rating less than 200 MVA Size Larger in size as compared of Smaller in size distribution transformers Designed Efficiency Designed for maximum efficiency Designed for 50-70% efficiency of 100% Application Used in generating stations and Used in distribution stations, also transmission substations for industrial and domestic purposes Losses Copper and iron losses take place Iron losses take place for 24 hours throughout the day and copper losses are based on load cycle Load fluctuation In power transformer the load Load fluctuations are very high fluctuations are very less Operating condition Always operated at full load Operated at load less than full load as load cycle fluctuates Considering time It is independent of time It is time dependent Flux density In power transformer flux density As compared to power transformer is higher the flux density is lower in distribution transformer 38 1. Which type of winding is used in 3-phase shell-type transformer? a) Circular type b) Sandwich type c) Cylindrical type d) Rectangular type Answer: b Explanation: In core type of the transformer, winding is done by normal method, while in the shell type transformer, winding is sandwiched between corresponding shells of core material. Hence, it is sandwich type. 2. 3-phase transformers compare to 1-phase transformers ______a) More economical b) Easy in construction c) Easy to construct d) Easy to handle Answer: a Explanation: 3-phase transformers are also used to power large motors and other heavy loads. A three-wire three-phase circuit is more economical than an equivalent two-wire single-phase circuit at the same line to ground voltage because it uses less conductor material to transmit a given amount of electrical power. 39 3. How 3-phase transformers are constructed? a) A bank of 3 single phase transformers b) A single 3-phase transformer with the primary and secondary of each phase wound on three legs of a common core c) Single 3-phase transformer or a bank of 3 1-phase transformers d) By different method Answer: c Explanation: #-phase transformers can be either constructed by 2 methods. One is a bank of 3 single phase transformers and another method is a single 3-phase transformer with the primary and secondary of each phase wound on three legs of a common core. 4. Three phase transformer compare to a bank of 3 single phase transformers is ____ a) Cheaper b) Costlier c) More in space d) Can’t be determined Answer: a Explanation: 3-phase transformers can be manufactured by 2 methods. The three-phase transformer unit costs about 15% less than that of a bank and furthermore, the single unit occupies less space. There is little difference in reliability. 40 5. In mines we use ______a) a single unit of 3-phase transformer b) a bank of 3 single phase transformers c) use of 3-phase transformer is avoided d) a single unit or a bank Answer: b Explanation: It is cheaper to carry spare stock of a single-phase rather than a three-phase transformer. In underground use (mines) a bank of single-phase units may be preferred as it is easier to transport these units. The bank also offers the advantage of a de-rated open-delta operation when one single- phase comes inoperative. 6. In three phase bank phases are ______a) the phases are electrically not connected and magnetically independent b) the phases are electrically not connected and magnetically dependent c) the phases are electrically connected and magnetically independent d) the phases are electrically connected and magnetically dependent Answer: c Explanation: The 3-phase load bank is constructed by so that all three phases are made to be connected with together electrically. By the same time these banks form three magnetic circuits which are independent. 41 7. Where the tappings are provided in a transformer? a) At the phase end of LV side b) At the phase end of HV side c) At the neutral side end of the HV side d) At the middle of HV side Answer: d Explanation: Tappings are provided in order to maintain the terminal voltage across the loads within the permissible specified limits with respect to load variations. Tappings can be provided at phase end or neutral end of HV side. But to maintain magnetic symmetry tappings are preferred in the middle of the winding only. 8. Tappings are on ______a) LV side of a transformer b) HV side of transformer c) Not on any side d) On both sides Answer: b Explanation: Generally, tappings are preferred on HV side, because the HV side is low current-high voltage side, tap changer involves low current interruption and large variations or small steps are possible due to more no of turns.

42 9. In core type 3-phase transformer flux path chooses how many paths to return? a) 2 b) Single c) 3 d) Many Answer: a Explanation: Flux of each limb in core type 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. Thus, the flux in the core remains nearly sinusoidal, producing a sinusoidal secondary supply voltage. 10. Why shell type 3-phase transformer is used in large power transforming applications? a) Can be made with more height b) Can be made with less height c) More height and less height flexibility d) Due to other reasons Answer: b Explanation: 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.

43 11. A three-phase transformer generally has the three magnetic circuits interlaced. a) True b) False Answer: b Explanation: A three-phase transformer generally has the three magnetic circuits which are interlaced to give a uniform distribution of the dielectric flux between the high and low voltage windings. The exception is a three-phase shell type transformer. In the shell type of construction, even though the three cores are together, they are non-interlaced. 12. When does star/star transformers work satisfactorily? a) Load is unbalanced only b) Load is balanced only c) On balanced as well as unbalanced loads d) Independent of load type Answer: b Explanation: With the unbalanced load connected to the neutral, the neutral point shifts thereby making the three line-to-neutral (i.e. phase) voltages unequal. The effect of unbalanced loads can be illustrated by placing a single load between phase (or coil) a and the neutral on the secondary side. 44 13. When does delta/star transformer work satisfactorily? a) Load is balanced only b) Load is unbalanced only c) On balanced as well as unbalanced loads d) Independent of load type Answer: c Explanation: Large unbalanced/balanced loads can be handled satisfactory. The Y-D connection has no problem with third harmonic components due to circulating currents in D(delta). It is also more stable to unbalanced loads since the D partially redistributes any imbalance that occurs. 14. Scott connections are used in ______a) three-phase to single phase transformation b) three-phase to two-phase transformation c) single phase to three-phase transformation d) all phase transformations Answer: b Explanation: Scott connections are used to convert three-phase to two-phase conversion, to start two phase motors and two phase furnaces. It requires two single phase transformers with adjustable tappings, one transformer is main transformer which is centre tapped through teaser transformer primary with a suitable number of turns to get a balanced two-phase supply. 45 15. In a three-phase star – delta transformer, what is the angle difference between primary and secondary phase voltages? a) Delta side leads by 300 b) Delta side lags by 300 c) Star side leads by 300 d) Star side lags by 300 Answer: a Explanation: This is a vector group and has + 30° displacement. Therefore, delta side leads by + 30°. So, it can be stated that delta side is having lead of 300 over star side because of the connections made. 16. Which can be also called as 00 /1800 connection? a) Star/star b) Direct star c) Delta/star d) Star/delta Answer: a Explanation: Star connection is formed on each side by connecting together phase winding terminals. The voltages of the corresponding phases (and thus of the corresponding lines) are in phase. This is known as the 0°-connection. If the winding terminals on secondary side are reversed, the 180°-connection is obtained.

46 17. What is the ratio of transformation of star/star connection? a) Phase transformation x:1, line transformation x:1 b) Phase transformation x:1, line transformation 2x:1 c) Phase transformation x:1, line transformation x/3:1 d) Can’t say Answer: a Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the star/star connection we will get the same ratios, as at a particular point, voltage reading on primary will be proportional to secondary with x. 18. Delta/delta connection is also called as ______a) 00-connection b) 900-connection c) 1800-connection d) 00/1800-connection Answer: d Explanation: Delta/delta connection is also called as 00-connection as seen from the phasor diagram that primary and secondary line voltages are in phase with each other. By reversing connection, we can get 1800 phase shift.

47 19. What is the ratio of transformation of delta/delta connection? a) Phase transformation x:1, line transformation x:1 b) Phase transformation x:1, line transformation 2x:1 c) Phase transformation x:1, line transformation x/3:1 d) Can’t determine Answer: a Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the delta/delta connection also similar to the star/star, we will get the same ratios, as at a particular point, voltage reading on primary will be proportional to secondary with x. 20. Open delta connection has VA rating of ______a) √3 times delta/delta VA rating b) 1/√3 times delta/delta VA rating c) 3 times delta/delta VA rating d) 1/3 times delta/delta VA rating Answer: b Explanation: When one of the transformer in delta/delta connection is removed we get open delta connection. This connection can handle the power of √3VI. While on the similar line delta/delta connection can handle the power of 3VI.

48 21. Star/delta connection is also called as ______a) 300-connection b) 00-connection c) -300-connection d) 300/-300-connection Answer: d Explanation: Star/delta connection is also called as +/-300-connection as seen from the phasor diagram that primary and secondary line voltages are either ahead or below by 300 phases with each other. By reversing connection, we can get another condition phase shift. 22. What is the ratio of transformation of star/delta connection? a) Phase transformation x:1, line transformation x:1 b) Phase transformation x:1, line transformation √3x:1 c) Phase transformation x:1, line transformation 3x:1 d) Can’t determine with information available Answer: a Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the star/delta connection we will get the √3 factor in ratios, as at a particular point, voltage reading on primary will be proportional to secondary with √3x. 49 23. x/√3:1 ratio is obtained in ______a) Star/delta b) Delta/star c) Delta/delta d) Star/star Answer: b Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the star/delta connection we will get the 1/√3 factor in ratios, as at a particular point, voltage reading on primary will be proportional to secondary with 1/√3x. 24. Which both connections have the same line transformation ratios? a) Star/star and delta/delta b) Star/delta and delta/star c) Star/zig-zag star and delta/zig-zag star d) Star/star, delta/delta and star/delta, delta/star Answer: d Explanation: Star/star and delta/delta both connections have phase transfer ratio of x:1 and line transfer ratio also equal to x:1, while star/zig-zag star and delta/zig- zag star connections have line transformation ratio equal to 2/√3x:1.

50 25. For two transformers connected in parallel, not having unequal percentage impedances, which statement is correct? a) Short-circuiting of the secondaries b) Power factor of one of the transformers is leading while that of the other lagging c) Transformers having higher copper losses will have negligible core losses d) Loading of the transformers not in proportion to their kVA ratings Answer: d Explanation: In parallel operation of a transformer, loading gets divided between the whole set in proportion of their impedances. Thus, if impedances are not same then, loading of transformers will not be in the ratio of their kVA ratings. 26. For the parallel operation of two single phase transformers it is necessary that they should have ______a) same efficiency b) same polarity c) same kVA rating d) same number of turns on the secondary side Answer: b Explanation: Polarity of the two transformers must be same if the are supposed to be operated in parallel mode of operation. It is okay if they have unequal efficiencies, or unequal kVA ratings, as parallel connection is still possible. 51 27. Transformers operating in parallel mode of operation will share the load depending upon their ______a) leakage reactance b) per unit impedance c) Efficiencies d) ratings Answer: b Explanation: In parallel operation of a transformer, loading of the transformers gets divided between the whole set in proportion of their per unit impedances. Thus, if per unit impedances are not same then, loading will not be in the ratio of their kVA ratings. 28. What will happen if the transformers working in parallel are not connected with regard to polarity? a) The power factor of the two trans-formers will be different from the power factor of common load b) Incorrect polarity will result in dead short circuit c) The transformers will not share load in proportion to their kVA ratings d) Cannot be determined Answer: b Explanation: As far polarity is concerned, the transformers are connected in the same sequence when they are operated in parallel combination. An incorrect polarity connection of these transformers will result in dead short circuit. 52 29. If the percentage impedances of the two transformers working in parallel are different, then ______a) transformers will be overheated b) power factors of both the transformers will be same c) parallel operation will be not possible d) parallel operation will still be possible, but the power factors at which the two transformers operate will be different from the power factor of the common load Answer: d Explanation: If the percentage impedances of the two transformers which are working in the parallel are different, then parallel operation is still possible, but load sharing will not be in the ratio of their kVA loads. 30. A delta/star transformer is connected in parallel to a star/delta transformer. The turn ratio former is x times latter, the x is ______a) 3 b) 1/3 c) √3 d) 1/√3 Answer: a Explanation: Turn ratio of star1/ Turn ratio of delta 1= Vph/Vph/√3= √3 Ture ratio of star 2 / Turn ratio of delta 2= Vph/√3/Vph= 1/√3 Thus, turn ratio of star1/delta1 = 3 * turn ratio of star 2/ delta 2. 53 31. While connecting two transformers in parallel voltage around the local loop ______a) positive b) negative c) equals zero d) insufficient information Answer: c Explanation: The transformers must be connected properly as far as their polarities are concerned so that the net voltage around the local loop is zero. A wrong polarity connection results in a dead short circuit. 32. For three phase power transformers relative phase displacement should be ______a) 00 b) 300 c) 900 d) 1800 Answer: a Explanation: Three-phase transformers must have zero relative phase displacement on the secondary sides and must be connected in a proper phase sequence. Only the transformers of the same phase group can be paralleled. 54 33. Y/Y and Y/D transformers can be paralleled. a) True b) False Answer: b Explanation: For example, Y/Y and Y/D transformers cannot be paralleled as their secondary voltages will have a phase difference of 30°. Transformers with +30° and –30° phase shift can, however, be paralleled by reversing the phase- sequence of one of them. 34. Why transformers connected in parallel should have same voltage ratio? a) To avoid full load circulating current b) To avoid no-load circulating current c) To avoid other losses d) To avoid all type of currents Answer: b Explanation: The transformers must have the same voltage-ratio to avoid no-load circulating current when transformers are in parallel on both primary and secondary sides. Since the leakage impedance is low, even a small voltage difference can give rise to considerable no-load circulating current and extra I2R loss.

55 35. Why transformers are paralleled? a) Economical than replacing by one single large unit b) Less maintaining cost c) To handle more load d) Economical factors, more load capacity, less maintenance Answer: d Explanation: When the load outgrows the capacity of an existing transformer, it may be economical to install another one in parallel with it rather than replacing it with a single larger unit. Also, sometimes in a new installation, two units in parallel, though more expensive, may be preferred over a single unit for reasons of reliability—half the load can be supplied with one unit out. Further, the cost of maintaining a spare is less with two units in parallel. 36. Only Scott connection is used for ______a) Converting three-phase to two-phase conversion b) Converting three-phase to single-phase conversion c) Converting single-phase to two-phase conversion d) None of the mentioned Answer: a Explanation: Scott connection is used for obtaining two-phase supply as it is needed for various special purposes like supplying two-phase electric arc furnaces and can easily obtained from three-phase supply by this method. 56 37. In Scott connection, according to the vector diagram two windings are placed at ______a) At 1200 to each other b) Perpendicular with respect to each other c) At 600 to each other d) Can’t say Answer: b Explanation: For a transformer vector diagram, if one winding is kept along the perpendicular axis of another winding then one can obtain two-phase supply from three-phase supply by using Scott connection. Physically this can be obtained by appropriate turns ratio. 38. What is the ratio of voltage/turn of two windings of a transformer in operated in Scott connection? a) ½ b) 1/√2 c) 1 d) 1/√3 Answer: c Explanation: The transformer primaries must have √3 N1/2 (teaser) and N1 turns; this would mean equal voltage/turn in each transformer. A balanced 2-phase supply could then be easily obtained by having both secondaries with equal number of turns, N2. 57 39. The primaries of two transformers in Scott connection are in turns ratio of ______a) √3/2: 1 b) 2/√3: √2 c) 1:1 d) Can’t say Answer: a Explanation: A 2-phase supply could thus be obtained by means of transformers; one connected perpendicular according to the vector diagram is called the teaser transformer and the other is connected across, with turns ratio √3/2: 1 respectively. 40. The secondaries of two transformers are in ratio ____ a) 1:1 b) √3/2: 1 c) √3: 1 d) √2: 1 Answer: a Explanation: Though transformer primaries are in the ratio of √3/2: 1 with respect to each other secondary windings of these transformers are in ratio of 1:1. This simply means that both of these transformers have equal voltage per turns. 58 41. Which of the following is the main advantage of an auto-transformer over a two-winding transformer? a) Hysteresis losses are reduced b) Saving in winding material c) Copper losses are negligible d) Eddy losses are totally eliminated Answer: b Explanation: Auto transformer is a special type of transformer which has primary and secondary winding both located on same winding. Thus, winding material required for a transformer is very less in the case of autotransformer. 42. Auto-transformer makes effective saving on copper and copper losses, when its transformation ratio is a) Approximately equal to one b) Less than one c) Great than one d) Cannot be found Answer: a Explanation: Copper In auto transformer /copper in two-winding transformer = 1- T2/T1. This means that an auto transformer requires the use of lesser quantity of copper given by the ratio of turns. Hence, if the transformation ratio is approximately equal to one, then the copper saving is good and the copper loss is less.

59 43. Total windings present in a autotransformer are ______a) 1 b) 2 c) 3 d) 4 Answer: a Explanation: Autotransformer is the special transformer for which the single winding acts as a primary and secondary both. Thus, by taking the appropriate winding into consideration a variable secondary voltage is obtained. 44. Autotransformers are particularly economical when ______a) Voltage ratio is less than 2 b) Voltage ratio is very high c) Voltage ratio is higher than 2 in smaller range d) Can be used anywhere Answer: a Explanation: Autotransformer is economical where the voltage ratio is less than 2 in which case electrical isolation of the two windings is not essential. The major applications are induction motor starters, interconnection of HV systems at voltage levels with ratio less than 2, and in obtaining variable voltage power supplies (low voltage and current levels). 60 45. Which of the following is not true regarding the autotransformer compare to two-winding transformer? a) Lower reactance b) Lower losses c) Higher exciting current d) Better voltage regulation Answer: c Explanation: Autotransformer is the advance version of normal transformer. It is having better voltage regulation, higher efficiency due to lower losses, lower reactance and thus it also requires very small exciting current. 46. Two-winding transformer of a given VA rating if connected as an autotransformer can handle ______a) Higher VA b) Lower VA c) Same VA d) Cannot be found Answer: a Explanation: A two-winding transformer of a given VA rating when connected as an autotransformer can handle higher VA. This is because in the autotransformer connection part of the VA is transferred conductively. 61 47. When auto-transformation ratio becomes equal to 1, which of the following statement is true? a) VA rating of the autotransformer becomes far greater than VA rating of two winding transformer b) VA rating of the autotransformer becomes far lower than VA rating of two winding transformer c) VA rating of the autotransformer becomes equal to VA rating of two winding transformer d) Can’t comment Answer: a Explanation: VA rating of autotransformer is = [1/1-a] * VA of two-winding transformer, thus, when a i.e. transformation ratio of autotransformer becomes closer to 1 one gets very high value of VA rating of an autotransformer. 48. An autotransformer compared to its two-winding counterpart has a higher operating efficiency. a) True b) False Answer: a Explanation: The losses are less in autotransformer compare to two-winding transformer. Thus, for the given same input to autotransformer as that of two- winding transformer more output will be available to secondary side. 62 49. Ratio of winding material needed for autotransformer to thr two winding transformer is ______a) 1- V2/V1 b) 1- N2/N1 c) 1- V2/V1 and 1- N2/N1 d) 1- V1/V2 Answer: c Explanation: Gauto/GTW= 1- V2/V1, as voltage is directly proportional to the number of turns we can say the value is also equal to the 1- N2/N1. Thus, one can write GTW-Gauto = 1/a’ * GTW = saving of the conductor material using autotransformer. 50. For an auto-transformation ratio tending to the unity value, saving of the conductor material will be ______a) Tend towards 90% or more b) Tend towards 0% c) Can’t say d) Will remain fix Answer: a Explanation: Gtw-Gauto = 1/a’ * Gtw = saving of the conductor material using autotransformer. So, if a’=10, saving is only 10% but for a’=1.1, saving is as high as 90%. Hence it is more economical when the turn-ratio is closer to unity.

63 51. What are the modes in which power can be transferred in an autotransformer? a) Conduction b) Induction c) Conduction and Induction d) Cannot be said Answer: c Explanation: In two winding transformer there is no electrical connection between primary and secondary. So, the power is transferred through induction. But in auto-transformer there is a common electrical path between primary and secondary. So, power is transferred through both conduction and induction processes. 52. A 100/10, 50 VA double winding transformer is converted to 100/110 V auto transformer. tWhat will be the rating of auto transformer? a) 500 VA b) 550 VA c) 100 VA d) 110 VA Answer: b Explanation: Secondary current of the two-winding transformer at rated voltage supply = 50/10= 5 A Thus, autotransformer will also carry the same rated current in secondary giving the power output as 5*110= 550 VA.

64 53. For a 20kVA transformer with turn ratio 0.4 what amount of total power is transferred inductively? a) 8kVA b) 12kVA c) 10kVA d) 50kVA Answer: b Explanation: For an auto transformer power is transferred partially inductively and partially conductively. Thus, out of total power, power transferred inductively is given by (1-k)*total power= 0.6*20= 12kVA. 54. For a 20kVA transformer with turn ratio 0.4 what amount of total power is transferred conductively? a) 8kVA b) 12kVA c) 10kVA d) 50kVA Answer: a Explanation: For an auto transformer power is transferred partially inductively and partially conductively. Thus, out of total power, power transferred conductively is given by (k)*total power= 0.4*20= 8kVA. 65 Previous years questions Transformers operating in parallel mode of operation will share the load depending upon their ______(Gate 1992) a) leakage reactance b) per unit impedance c) efficiencies d) ratings Answer: b Explanation: In parallel operation of a transformer, loading of the transformers gets divided between the whole set in proportion of their per unit impedances. Thus, if per unit impedances are not same then, loading will not be in the ratio of their kVA ratings.

Keeping in view the requirement of parallel operation, which of the 3-phase connections given below are possible? (GATE–96) (a) delta-delta to delta-star (b) delta-delta to star-delta (c) star- star to delta-star (d) delta-star to star-delta Answer: (d)

66 A load of 110kW at 0.8 power factor is to be shared by two single phase transformer with equal turns have impedances equal to (0.4 + j 0.2) and (0.6 + j 0.8) with respect to secondary. How will they share the load if they are operating in parallel? (Gate 2012) a. 80kW, 30kW b. 82.55kW, 28.45kW c. 75.55kW, 24.45kW d. 85.45kW, 24.55kW ANSWER: d If two transformers are operated in parallel with voltage ratio of transformer 1 slightly greater than that of 2. E1 and E2 are the induced emf of transformer 1 and 2, and Z1 and Z2 are the impedances then the circulating current is given by (Gate 2006) a. (E1-E2) / (Z1+Z2) b. (E1+E2) / (Z1+Z2) c. (Z1-Z2) / (E1-E2) d. (Z1-Z2) / (E1+E2) ANSWER: a

67 (Gate 2007)

68 Auto-transformer is used in transmission and distribution (GATE–96) (a) when operator is not available (b) when iron losses are to be reduced (c) when efficiency considerations can be ignored (d) when the transformation ratio is small Answer: a

69 Gate -2018

70 ASSIGNMENT 1. Supply to one terminal of a D-Y connected three- phase core type transformer which is on no-load, fails. Assuming magnetic circuit symmetry, voltages on the secondary side will be :

(a) 230, 230, 115 (b) 230, 115, 115 (c) 345, 115, 115 (d) 345, 0, 345

2. Keeping in view the requirement of parallel operation, which of the 3-phase connections given below are possible? a) delta-delta to delta-star b) delta-delta to star-delta c) star-star to delta-star d) delta-star to star-delta 71 3. A 400V/100V, 10 KVA two winding transformer is reconnected as an auto transformer across a suitable voltage source. The maximum rating of such an arrangement could be : (a) 50 KVA (b) 15 KVA (c) 12.5 KVA (d) 8.75 KVA

4. Which of the following 3-phase connections of transformer create disturbances in communication systems? (a)Star/Delta (b) Delta/Star (c) Star/Star (d) Delta/Delta 5. Two transformers of different KVA rating working in parallel share the load in proportion to their ratings when their 1. per unit leakage impedance on the same KVA base are equal. 2. per unit leakage impedances based on their respective KVA ratings are equal 3. Ohmic values of the leakage impedances are inversely proportional to their ratings 4. Ohmic values of the magnetizing reactance are the same from these the correct answer is

(a) 1, 3, 4 (b) 2, 3 (c) 2, 3, 4 (d) 1, 4 For any queries/clarifications/,suggestions...feel free to contact through... [email protected]

74 ONLINE GATE COACHING CLASSES BY Prof. Ch. SAI BABU, Professor of Electrical & Electronics Engineering JNTUK Kakinada Answers to Assignment in Lecture VI of Electrical Machines Topic: Transformers

Jawaharlal Nehru Technological University, Kakinada-533003, Andhra Pradesh

1 Assignment Questions and Answers 1. Supply to one terminal of a D-Y connected three-phase core type transformer which is on no-load, fails. Assuming magnetic circuit symmetry, voltages on the secondary side will be : (a) 230, 230, 115 (b) 230, 115, 115 (c) 345, 115, 115 (d) 345, 0, 345 Answer: B If the primary applied voltage is 230V then for a core type 3-phase D–Y transformer with one primary supply terminal disconnected, secondary side phase Voltages are of the form 230V, 115V, 115 V. 2. Keeping in view the requirement of parallel operation, which of the 3- phase connections given below are possible? a) delta-delta to delta-star b)delta-delta to star-delta c) star-star to delta-star d) delta-star to star-delta Answer: d D – Y & Y – D transformers both belong to ± 30º phasor group hence they can be used for parallel operation. 2 3. A 400V/100V, 10 KVA two winding transformer is reconnected as an auto transformer across a suitable voltage source. The maximum rating of such an arrangement could be : (a) 50 KVA (b) 15 KVA (c) 12.5 KVA (d) 8.75 KVA Answer : a 4. Which of the following 3-phase connections of transformer create disturbances in communication systems? (a)Star/Delta (b) Delta/Star (c) Star/Star (d) Delta/Delta

Answer : b 5. Two transformers of different KVA rating working in parallel share the load in proportion to their ratings when their 1. per unit leakage impedance on the same KVA base are equal. 2. per unit leakage impedances based on their respective KVA ratings are equal 3. Ohmic values of the leakage impedances are inversely proportional to their ratings 4. Ohmic values of the magnetizing reactance are the same from these the correct answer is

(a) 1, 3, 4 (b) 2, 3 (c) 2, 3, 4 (d) 1, 4 Answer : a and b For any queries/clarifications/,suggestions...feel free to contact through... [email protected]