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Demonstrate knowledge of single and three phase transformers used in the electricity industry US 19323 Training and Assessment Resource Level 3 Credits 4 Electricity Supply Industry Training Organisation Page 1 of 37 www.esito.org.nz Contents Introduction to Training Assessment Resource Glossary Symbols 1.0 3 4 5 Introduction to Transformers........................................................................................................... 7 2. 0 Inductance ...................................................................................................................................... 8 2.1 Self inductance.................................................................................................................................. 8 2.2 Mutual inductance.............................................................................................................................. 9 3.0 Transformer Construction............................................................................................................... 10 4.0 Transformer Load ........................................................................................................................... 13 4.1 Transformer at no load...................................................................................................................... 13 4.2 Loading a transformer....................................................................................................................... 14 5.0 5.1 5.2 5.3 5.4 Transformer Losses and Leakage Flux.......................................................................................... 15 Transformer losses............................................................................................................................ 15 Efficiency ..................................................................................................................................... 16 Transformer rating............................................................................................................................. 16 Leakage Flux..................................................................................................................................... 17 6.0 Voltage regulation........................................................................................................................... 18 6.1Tap-changers.................................................................................................................................... 19 7.0 Auto-transformers.......................................................................................................................... 21 8.0 Three-phase transformers.............................................................................................................. 23 8.1 Three phase transformer connections............................................................................................... 26 8.2 Three phase transformer voltage ratings and turns ratios.................................................................. 30 Answers to activities..................................................................................................................................... 32 Getting started on the ESITO Training & Assessment Resources Activity: A written or spoken exercise or assignment. Keypoint: Important information to remember. Page 2 of 37 TAR 19323 | Rev. 1, 2010 Introduction to Training Assessment Resource This Training Assessment Resource (TAR) contains the information that you require to complete the written assignment in the assessment pack for this unit standard. Purpose People who obtain credit for this unit standard are able to: Demonstrate knowledge of single phase transformers in the electricity supply industry. Demonstrate knowledge of three phase transformer theory. Demonstrate knowledge of transformer outputs and efficiencies. TAR 19323 | Rev. 1, 2010 Page 3 of 37 Glossary When I see this word It means Approximation Not quite exact Catastrophic failure Total failure resulting in a lot of damage and risk of injury Configuration The way the parts of something are arranged and how they fit together Counteract To prevent something having an effect, or to lessen its effect Dissipate To cause something to disappear or reduce Equivalent Being the same, or effectively the same Faraday’s law This is a basic law of relating voltage and flux. It forms the operating principles of transformers, inductors, and many types of electrical motors and generators Fundamental principle An important underlying law or theory Fundamental Underlying or deep-seated Fundamentally Basically Instantaneous Occurring at a given moment in time Load The power drawn or power required Mitigate To lessen the effect, to make something less harsh or severe Proportional There is a constant ratio between two variables. Static Not moving, fixed in position Subscript Printed on a lower level than other characters in a line of type Summation Add together Variable Capable of change Page 4 of 37 TAR 19323 | Rev. 1, 2010 Symbols There are different symbols used in this training manual. Cu means copper Fe means iron ph means phase (phase to neutral) l means line (phase to phase) The following subscripts are used to associate a symbol with a phase or winding. 1 means associated with winding 1 2 means associated with winding 2 P means associated with primary winding S means associated with secondary winding a means associated with phase A b means associated with phase B c means associated with phase C Combining symbols and subscripts Ia means current in A phase Vp is voltage associated with primary winding N2 is the number of turns associated with winding 2 TAR 19323 | Rev. 1, 2010 Page 5 of 37 Table 1 shows the symbols used, their meaning and the units that are used to measure them. Symbol V V1 = N V1 = N Meaning Unit Voltage Volt (V) I Current Ampere (A) P Real power Watt (W) S Apparent power Volt-Ampere (VA) N Number of turns θ Power angle (angle between voltage and current) Degrees Magnetic flux Webers (Wb) Rate of change of flux Webers per second (wb/s) Resistance Ohm ( Ω ) dØ dt dØ dt R = = Output Power Input Power Table 1: Symbols Pout Pout Page 6 of 37 + Efficiency PLosses TAR 19323 | Rev. 1, 2010 1.0 Introduction to Transformers Transformers are static devices that transfer energy from one circuit into another by electromagnetic induction. The energy transferred from the primary circuit to the secondary circuit is equal, excluding losses. The frequency between the circuits is unchanged. However, the voltage and the current of each circuit is either increased or decreased, unless the transformer is being used as a one-to-one isolating transformer. Transformers are used for three reasons: 1. Step-up the voltage. A step-up transformer is used to increase the voltage. An example of a step-up transformer is a generator transformer. In this example the transformer changes the voltage from the generator voltage (eg. 11kV) to the voltage of the transmission system (eg. 220kV). 2. Step-down the voltage. A step-down transformer is used to decrease the voltage. An example of a step-down transformer is a transformer used to supply a rural house. In this example the transformer changes the voltage of the distribution system (eg.11kV) to the mains voltage, 230V. 3. One-to-one (Isolating Transformer). A one-to-one transformer is commonly referred to as an isolating transformer. Isolating transformers are used where it is necessary to have electrical isolation between the primary and secondary circuits. An example of a one-to-one transformer is an isolating transformer used to isolate hand-held power tools on a job site. The construction of all of these transformers is fundamentally the same, with the difference being the relative number of turns on each winding. TAR 19323 | Rev. 1, 2010 Page 7 of 37 2.0Inductance Current passing through a wire will produce a magnetic field around the wire. The creation of any line of magnetic field around a conductor creates an electrical property called “inductance”. 2.1 Self inductance If a magnetic field line is created by the current itself in the wire then this is called “self inductance”. When the wire is wound to form a coil, the magnetic field lines from the individual conductors add together to form larger loops that surround the coil. This summation of the individual self induced magnetic lines is shown in Figure 1. P x Figure 1: Flux formed by a coil Should the current in the coil change, then there would be a corresponding change in the magnetic field. A change in the magnetic field will produce an electromotive force (emf) or voltage that will oppose the change in current. This voltage is often referred to as a back emf. As the back emf is induced by the current in the coil itself, it is referred to as a self induced voltage, and the inductance formed by the coil as self inductance. The back emf can be calculated by Faraday’s Law. V1 = N dØ dt Equation 1: Faraday’s Law Page 8 of 37 TAR 19323 | Rev. 1, 2010 2.2 Mutual inductance If the self induced flux lines of a coil spread out and interact with a second coil then induction will occur in the second coil. This type of induction is given the label “Mutual Inductance”. Any change in the magnetic flux produced by the first coil will induce a voltage into the second coil to oppose the changing magnetic flux. Mutual inductance is a fundamental principal of a transformer. The voltage produced by mutual inductance can also be calculated by Faraday’s Law, Equation 1. With the aid of a diagram explain the terms self inductance and mutual inductance. TAR 19323 | Rev. 1, 2010 Page 9 of 37 3.0 Transformer Construction The fundamental components of a transformer are two or more windings wound around a magnetic steel core. An alternating voltage in one winding will induce an alternating magnetic field in the core. The core is used to transport the magnetic flux to the second winding. The alternating magnetic field will induce an alternating voltage in the second winding, by mutual inductance. A diagram of a transformer is shown in Figure 2. Primary winding Secondary winding Np turns Primary current Ns turns IP Magnetic Flux + Secondary current Is + Primary voltage VP Secondary voltage Vs _ _ Transformer core Figure 2: Diagram of the ideal transformer Page 10 of 37 TAR 19323 | Rev. 1, 2010 In an “ideal” transformer there would be no losses and perfect magnetic flux coupling of both the primary and secondary windings. The relationship between the voltage and the number of turns in each winding in an ideal transformer can be expressed by Equation 2. Vp Vs = Np Ns Equation 2: Voltage / Turns Radio As the transformer is a static device (i.e no rotating parts), and if you ignore losses then “power in” must equal “power out.” Power can be calculated by P=VIcosθ. Using this power equation together with Equation 2, a relationship can be formed between the number of turns and the current in the winding. Vp Vs = Np Ns = Is Ip Equation 3: Voltage / Turns / Current Ratio From Equation 3 it can be seen that the ratio of primary to secondary voltage is equal to the ratio of the number of turns on the primary winding to the number on the secondary winding. It is also inversely proportional to the ratio of the currents in each winding. The ratio of the number of turns on the primary winding to the number of turns on the secondary winding is referred to as the Transformer Turns Ratio. TAR 19323 | Rev. 1, 2010 Page 11 of 37 In a practical transformer design, the windings would not be wound on separate legs (or limbs) of the core as shown in Figure 2. They would be wound one over the other as shown in Figure 3. Typically, the low voltage winding will be wound next to the core with the high voltage winding over the top. The two windings will be separated by solid insulation and insulating fluid. Core - Yoke Core - Limb Note: There is also a limb of the core in the centre of the winding set. Note: Limb covered with insulation, core is laminated steel Winding set - Both low voltage and high voltage winding Figure 3: Photo of a single phase transformer Draw a diagram that shows the fundamental components of a single phase transformer. Page 12 of 37 TAR 19323 | Rev. 1, 2010 4.0 Transformer Load 4.1 Transformer at no load The self induced voltage of the primary winding (V /1 in Figure 4) will be only slightly less than the supply voltage (V1) (due to small internal winding losses) and will oppose any change in the magnetic field and therefore V1. Only a small current will flow; this is required to drive the magnetic flux lines around the core. This small current is known as the magnetising current. The flux will flow through the secondary winding and induce a voltage (V /2) via mutual inductance. The value of this V /2 can be calculated by the formula in Equation 2. Magnetic flux Laminated core Primary winding V1 I Secondary winding I0 / V1 / V2 V2 Figure 4: Transformer diagram TAR 19323 | Rev. 1, 2010 Page 13 of 37 4.2 Loading a transformer When a load is connected to the secondary winding, current will flow within the secondary winding. There is a transformer fundamental principle known as “Lenz’s Law” which states that in mutually coupled coils if current flows in one coil then an opposing current will flow in the other coil. For transformers this means that if current flows out of the loaded side then current will flow into the supply side. Given these opposing electrical currents and opposing self induced magnetic flux lines the mutual flux linking the primary and secondary coils remains constant independent of load current. Page 14 of 37 TAR 19323 | Rev. 1, 2010 5.0 Transformer Losses and Leakage Flux Up to now we have only discussed the ideal transformer which has no losses, and all the flux is coupled between the two windings. In the real world the “ideal” transformer does not exist. All transformers will have losses and leakage flux (i.e. magnetic field lines that do not link both the primary and secondary coils). Both losses and leakage flux will affect the ideal transformer equations. However, these equations are a good approximation of most practical transformers. 5.1 Transformer losses Transformer losses are split into two components; copper losses and iron losses or load and no-load losses respectively. Copper Losses The resistance of each winding is relatively low; however each winding will have resistance. The current flowing through the resistance of the windings will result in heat and therefore losses. These losses are known as copper losses. The copper losses will be equal to: Pcu = Copper losses of primary winding = 2 + Copper losses of secondary winding 2 + Ip R p Is R s Equation 4: Copper Losses As copper losses are proportional to the load current they are often referred to as load losses. Iron Losses There are also losses associated with magnetising the transformer iron core. These losses are not affected by load current. Iron losses are often referred to as no-load losses. Total Losses Total transformer losses are equal to the sum of copper and iron losses, as shown in equation 5. Stray losses and auxiliary losses have been excluded. P Losses = P cu + P fe Equation 5: Transformer Losses TAR 19323 | Rev. 1, 2010 Page 15 of 37 5.2 Efficiency As with all machines, the efficiency of a transformer can be expressed by Equation 6. = = Output Power Input Power Pout Pout + PLosses Equation 6: Transformer Efficiency Transformers have very high efficiency, typically 97-99%. 5.3 Transformer rating The rating of a transformer is expressed in terms of its Volt-Ampere capability. As the power factor of the load is unknown, transformer ratings are expressed in terms of Apparent Power that can be supplied to the load. In general terms the voltage is set by its insulation capability and the current is set by the transformer’s ability to dissipate copper losses. Transformer rating can be expressed in Equation 7. Transformer rating = Ss Vs Output Power rated rated Is rated Equation 7: Transformer Rating Input Power Pout Pout + PLosses Page 16 of 37 TAR 19323 | Rev. 1, 2010 5.4 Leakage flux In the “ideal transformer” equations it is assumed that all of the magnetic flux lines link both windings. However, in a real transformer not all the flux lines produced by one winding will link the other winding and vice-versa. Magnetic flux lines that do not link both windings are known as leakage flux. Leakage flux is shown in Figure 5. As not all of the flux links both windings the voltage ratio of the ideal equations will only be an approximation. Primary winding Secondary winding 12 Main flux 1 2 Leakage flux 21 Figure 5: Transformer diagram showing leakage flux TAR 19323 | Rev. 1, 2010 Page 17 of 37 6.0 Voltage Regulation Voltage regulation is a term that is used to quantify the change in output voltage with load. Voltage regulation is expressed as a percentage, and is shown in Equation 8. Voltage Regulation = Output Voltage No Load - Output Voltage Output Voltage Full Load % No Load Equation 8: Voltage Regulation Voltage regulation is caused by leakage flux lines. As the load increases, the amount of leakage flux produced by each winding also increases. For that reason there will be a reduction in the mutual flux as the load increases. Therefore, by Faraday’s Law (Equation 1) the output voltage will decrease as the load increases. If a transformer has a no-load secondary voltage of 230V and a voltage regulation of 10%, what is the full load voltage? Page 18 of 37 TAR 19323 | Rev. 1, 2010 With the aid of a diagram explain how leakage flux affects voltage regulation. 6.1Tap-changers Transformers are fitted with tap-changers to counteract the effects of voltage regulation, and to allow for system voltage movement. In other words, tap-changers can be used to regulate the output voltage of the transformer, for example to keep it at a required level. Tap-changers are a means of changing the number of turns on either the primary or secondary winding. By changing the number of turns on one of the windings the turns ratio will change. Therefore, by Equation 2, the ratio of primary voltage to secondary voltage can also be changed. TAR 19323 | Rev. 1, 2010 Page 19 of 37 Figure 6 shows a diagrammatic example of a tap-changer winding. Vp Vs Figure 6: Circuit diagram of a tap-changer Tap-changers are referred to as off-load or on-load. An on-load tap-changer is designed to ensure that when the tap-changer is operated the winding is never open-circuited, and therefore the tap-change can occur when the transformer is energised and on load. WARNING: Off-load tap-changers must not be operated when the transformer is energised. Should an off-load tap-changer be operated when the transformer is energised then the winding with the tap-changer would be open circuited, even if the transformer is unloaded. This would result in a large voltage being produced across the tap-changer contacts, and the catastrophic failure of the transformer. Explain the term tap-changer and how it can be used to mitigate the effects of voltage regulation. Page 20 of 37 TAR 19323 | Rev. 1, 2010 7.0Auto-Transformers The transformers discussed up to now are known as double wound transformers. In a double wound transformer the voltage is changed by having two separate windings each with a differing number of turns. Each winding is electrically isolated from the other. Double wound transformers are often referred to as two winding transformers. An auto-transformer is made from a single coil. Change in the voltage between the primary and secondary circuits is achieved by having the primary and secondary windings tapped off at different points, as shown in Figure 7. load Figure 7: Step-down autotransformer diagram In certain applications, autotransformers are used in preference to double wound transformers. The main disadvantage of an autotransformer is the fact that there is no electrical isolation between the primary and secondary winding. In a step-down autotransformer failure of the common winding would result in the primary voltage being applied to the secondary circuit. An autotransformer can be used as either a step-up or step-down transformer. An example of a step-up autotransformer is a line booster. A line booster is used to boost the voltage at the end of a long distribution circuit, where the voltage can start to fall away. Another common use for a step-up autotransformer is as a starter in a fluorescent lamp. An example of a step-down autotransformer is an old starter on an induction motor. In this example the transformer can be used to reduce the voltage to the motor, this is then stepped through different taps until the motor is running at full speed and supplied with full voltage. Another common use of an autotransformer is a variac. A variac is a variable output transformer. These transformers can supply a load with a variable output voltage. The output range of a variac autotransformer is typically 0-110% of the input voltage. TAR 19323 | Rev. 1, 2010 Page 21 of 37 Draw a diagram for a variac with an output range of 0-110%, set at 50%. Page 22 of 37 TAR 19323 | Rev. 1, 2010 8.0 Three Phase Transformers The transformers discussed this far have been single phase transformers. The power grid in New Zealand is a three phase system. A three phase system can be supplied by either: Three single phase transformers, which make a three phase bank of transformers, or One three phase transformer. Three phase transformers have the following advantages over using three single phase transformers: Lower no load losses Lower manufacturing cost Less overall weight Smaller footprint (Less area required) Lower maintenance cost Less insulating oil The disadvantages are: Higher transport weight and size. (The total weight of all three single phase units is more, however, each single phase unit can be transported separately. The single phase transformer used to make a three phase bank of transformers will have a lower weight than a three phase transformer of equivalent rating.) Higher cost of a spare unit. (With a single phase bank only one single phase unit is required to be held as a spare unit.) The theory discussed for single phase transformers also applies to three phase transformers. The construction of a three phase transformer is similar to a single phase unit. Both the primary and secondary windings of each phase are wound onto a common limb. The limbs of the core are attached to each other by the yoke. This forms a magnetic circuit between each phase. Since the flux in each limb is 120o out-of-phase with the other two phases, the instantaneous sum of all three fluxes in the yoke is zero. Therefore, there is no need for a separate return path for each phase. TAR 19323 | Rev. 1, 2010 Page 23 of 37 Figures 8a-c show the construction process of a three phase transformer. Core limbs Note: The core is made up of the vertical limbs and the horizontal yokes. The yokes complete the magnetic circuit between the limbs. Core yoke Note: The upper yoke has not been fitted and the lower yoke is not clearly visible. It is made up of steel laminations, like the limbs, and runs horizontally between the three limbs. It is located under the wooden end rings and behind the white end frame. Figure 8a: Transformer core minus the upper yoke The winding sets are placed over the core limbs Note: One winding set per phase, and a winding set contains both primary and secondary windings. Figure 8b: Winding sets placed over core limbs, upper yoke not fitted Page 24 of 37 TAR 19323 | Rev. 1, 2010 The upper core yoke has been fitted Note: The upper yoke is laminated core steel. It links the three limbs, and is located behind the white end frame Tap-charger Figure 8c: Upper yoke and tap changer fitted A2 B2 C2 a2 b2 c2 N n Figure 9: Graphic representation of a star/star three phase transformer TAR 19323 | Rev. 1, 2010 Page 25 of 37 Explain why the instantaneous sum of flux in the yoke equals zero. 8.1 Three phase transformer connections The three phase electrical system in use today around the world requires the phases of transformers to be connected together. This applies equally to a three phase transformer bank made from three single phase units, or a single three phase transformer. There are two basic connection configurations used; delta and star. Each connection has particular features which affect the design and operation of transformers. The basic configuration of each is shown in Figure10. Delta Star Figure 10: Delta and Star configurations Delta winding connection Delta connected windings are constructed by connecting each end of a winding to a different winding, resulting in a triangular arrangement with three connection points or terminals. There are only three terminals brought through the transformer tank with each connection point connecting to one line terminal of the power grid. This means each winding sees the full phase-to-phase (line) voltage of the power grid. 1 Due to the nature of three phase system, the current in each winding is of the line current. 3 Page 26 of 37 TAR 19323 | Rev. 1, 2010 The relationships between the line (phase to phase) voltage and the voltage across a delta connected winding as well as the current in the line and the current in a delta connected winding are expressed in the following equations and in Figure 11. Voltage across a delta winding = Current in a delta winding = Line Voltage Line Current 3 Iline Ia = Iline 3 Va = Vline Vline Vline Figure 11: Delta winding configurations The apparent power of a delta connected winding can be calculated as shown in Equation 9. S = = S a+ S b+ Sc Va Ia + Vb Ib + Vc I c = 3VI (I I / 3 ) = 3 VI II Equation 9: Apparent power of a delta connected winding TAR 19323 | Rev. 1, 2010 Page 27 of 37 Star or Y connected windings Star connected windings are constructed by connecting one end of each winding together. This connection point is known as the star point or neutral point of the winding. The neutral connection, along with the three phase connections, is usually brought out of the transformer. The neutral connection is usually earthed. 1 The voltage across a star connected winding is only the phase-to-neutral voltage which is of the line voltage. 3 The current through the windings is the same as the line current. The relationships between the line (phase to phase) voltage and the voltage across a star connected winding as well as the current in the line and the current in a star connected winding are expressed in the following equations and in Figure Line 12. Voltage Voltage across a star winding = Line Voltage 3 Voltage across a star winding = 3 Current in a star winding = Line Current Current in a star winding = Iline Line Current Ia = Iline Va = Vline 3 Vline Vline Figure 12: Star winding configurations The apparent power of a star connected winding can be calculated as shown in Equation 10. S = = S a+ S b+ Sc Va Ia + Vb Ib + Vc Ic = 3Vph Iph = 3( VI / 3 )II = 3 VI II Equation 10: Apparent power of a star connected winding From equations 9 and 10 it can be seen that although the voltages and currents in a star or delta connected winding differ, the formula used to calculate the apparent power is the same. This is also true for real and reactive power. Page 28 of 37 P= 3 VL I L Cos θ Q= 3 VL I L Sin θ TAR 19323 | Rev. 1, 2010 Transformer connections It is possible to connect both windings in either star or delta, therefore there are four three phase winding configurations used. These are shown in Figure 13. These configurations are also used when connecting three single phase transformers to form a three phase bank. Primary Secondary (a) Star-star Primary Secondary (b) Delta-delta Primary Secondary (c) Delta-star Primary Secondary (d) Star - delta Figure 13: Three phase transformer connections TAR 19323 | Rev. 1, 2010 Page 29 of 37 Draw a diagram of a delta/star three phase transformer. On the diagram label: Primary winding, secondary winding, core yoke, and core limb. 8.2 Three phase transformer voltage ratings and turns ratios Three phase systems are referred to in terms of line voltages. Likewise, three phase transformers are referred to in terms of the line voltage on either side of the winding. However, if the winding is star connected then the voltage applied to the winding will be 1 the line voltage. 3 Therefore, for a star/delta or a delta/star transformer the actual winding turns ratio will not equal the rated voltage ratio of the transformer. This is demonstrated by the example in Table 2. This example is for a generator transformer that is used to transform the generator voltage of 11kV to the system voltage of 110kV. This transformer has a delta connected primary winding and a star connected secondary winding. Primary System voltage Secondary 11,000 V 110,000 V Winding configuration Delta Star Winding voltage = System voltage = System Voltage / 3 11,000 V 63,500 V 173 999 Number of turns Ratio 1:10 1:5.77 1:5.77 Table 2: Three phase transformer voltage ratio example Page 30 of 37 TAR 19323 | Rev. 1, 2010 Complete the table below Primary Secondary Rated voltage 33,000 V 11,000 V Rated current 141 420 Winding Configuration Delta Star Transformer Power Rating Number of turns on Nominal Tap 5000 Full load voltage on Nominal Tap 33,000 V 9,900 V Tapping range to acheive rated voltage at no load to 5% overvoltage at full load Not tapping winding % to % Copper losses st full load 40kW 35kW Iron Losses at rated voltage 15kW 0 Voltage Regulation on Nominal Tap Number of turn required to increase output voltage at full load to 5% over rated. Efficiency at full load Step up or Step down Example of where this tarnsformer might be used TAR 19323 | Rev. 1, 2010 Page 31 of 37 Answers to Activities Activity (page 9) With the aid of a diagram explain the terms self inductance and mutual inductance. Current flowing in a coil will produce a magnetic field, should that magnetic field change a voltage will be produced in the coil to oppose the change. This effect is called self inductance. Should a second coil be placed in the magnetic field of a primary coil, should the magnetic field of the primary coil change then a voltage will be produced in the secondary coil. This is called mutual inductance. Activity (page 10) Draw a diagram that shows the fundamental components of a single phase transformer. Note: Diagram needs to include a core and two windings. Primary winding Secondary winding Np turns Primary current Ns turns IP Magnetic Flux + Secondary current Is + Primary voltage VP Secondary voltage Vs _ _ Transformer core Page 32 of 37 TAR 19323 | Rev. 1, 2010 Activity (page 18) If a transformer has a no-load secondary voltage of 230V and a voltage regulation of 10%, what is the full load voltage? Voltage Regualtion = Output Voltage Output Voltage No Load Output Voltage 10% = 230 - Output Voltage 0.1x230 = TAR 19323 | Rev. 1, 2010 % No Load Full Load 230 230 - Output Voltage % Full Load Full Load = 230 - 0.1x230 Output Voltage Full Load = 230 - 23 Output Voltage Full Load = 207V Output Voltage Full Load Page 33 of 37 Activity (page 19) With the aid of a diagram explain how leakage flux affects voltage regulation. Primary winding Secondary winding 12 Main flux 1 2 Leakage flux 21 Leakage flux is flux that does not couple both windings. As the current in each winding increases the leakage flux will also increase. As there is a larger percentage of the flux produced by each winding is not coupled then the voltage produced in the secondary winding will also decrease. Activity (page 20) Explain the term tap-changer and how it can be used to mitigate the effects of voltage regulation. A tap-changer is a device that is used to change the number of turn on one of the coils. By changing the turns ratio the voltage ratio will also change. Therefore a tap-changer can be used to increase the output voltage when the load is increased. Activity (page 22) Draw a diagram for a variac with an output range of 0-110%, set at 50%. Note: Winding extends past the primary connect. This is to enable 110% output voltage. Secondary winding connected half way down primary winding. Therefore 50% output voltage. load Page 34 of 37 TAR 19323 | Rev. 1, 2010 Activity (page 26) Explain why the instantaneous sum of flux in the yoke equals zero. Like the current in each coil the flux is 120 degree apart. Therefore the sum of all three phases equals 0. Activity (page 30) Draw a diagram of a delta/star three phase transformer. On the diagram label the: Primary winding, secondary winding, core yoke, and core limbs. Note: Primary winding as the end of one coil connected to the start of the second (Delta). The secondary winding has all the ends connected together (Star) Core Yoke is the horizontal part of the core that links the limbs together Core Limb is the vertical part of the core that has the windings wound over it. Core Yoke A2 B2 C2 Primary Winding Core Limb n Secondary Winding a2 TAR 19323 | Rev. 1, 2010 b2 c2 Page 35 of 37 Activity (page 31) Complete the table below. Primary Secondary Rated voltage 33,000 V 11,000 V Rated current 141 420 Winding Configuration Delta Star Transformer Power Rating = √3 x 11,000 x 420 = 8MVA Number of turns on Nominal Tap 5000 5,000 x (11,000/√3)/33,000 = 962 Full Load Voltage on Nominal Tap 33,000 V 9,900 V Voltage Regulation on Nominal Tap (11,000 – 9,900)/11,000 = 10% Number of turn required to increase output voltage at full load to 5% over rated. Not tapping winding =11,550 – 9,900 = 1650 volts increase Additional turns = 5000 x (1650/√3)/33000= 144 Total number of turns required = 962 + 144 = 1106 Tapping range to achieve rated voltage at no load to 5% overvoltage at full load Not tapping winding 0 % to (144/962)=15% Copper losses at full load 40kW 35kW Iron Losses at rated voltage 15kW 0 Efficiency at full load =8000/(8000 + 40 + 35 + 15) = 98.9% Step up or Step down Step down. Output voltage is lower than input Example of where this transformer might be used. Distribution transformer. Page 36 of 37 TAR 19323 | Rev. 1, 2010 TAR 19323 | Rev. 1, 2010 Page 37 of 37