Unit 8: Transformer I READING AND COMPREHENSION POWER TRANSFORMER – construction of power transformer Brief-Introduction of Transformer The transformer is probably one of the most useful electrical devices ever invented It can raise or lower the voltage or current in an ac circuit, it can isolate circuits from each other, and it can increase or decrease the apparent value of a capacity, an inductor, or a resistor Futhermore, the transformer enables us to transmit electrical energy over great distances and to distribute it safely in factories and homes Construction of Power Transformer Power transformers are usually designed so that their characteristics approach those of an 10 ideal transformer Thus, to attain high permeability, the core is made of iron (Fig.10-2) The resulting magnetizing current Im is at least 5000 times smaller than it would be if an air core were used Furthermore, to keep the iron losses down, the core is laminated, and high resistivity, high-grade silicon steel is used Consequently, the current I f needed to supply the iron losses is usually to times smaller than Im 15 Leakage reactances Xf1 and Xf2 are made as possible by winding the primary and secondary coils on top of each other, and by spacing them as closely together as insulation considerations will permit The coils are carefully insulated from each other and from the core Such tight coupling between the coils means that the secondary voltage at no-load is N2 20 almost exactly equal to N times the primary voltage It also guarantees good voltage regulation when a load is connected to the secondary terminals Winding resistances R1 and R2 are kept low, both to reduce the I2R loss and resulting heat and to ensure high efficiency Fig.10-2 is a simplified version of a power transformer in which 25 the primary and secondary are wound on one leg In practice, the primary and secondary coils are distributed over both core legs in order to reduce amount of copper For the same reason, in larger transformers the cross section of the laminated iron core is not square but is built up so as to be nearly round The number of turns on the primary and secondary windings depends upon their respective voltages A high-voltage winding has far more turns than a low-voltage winding On the 30 other hand, the current in a HV winding is much smaller, enabling us to use a smaller size conductor As a result, the amount of copper in the primary and secondary windings is about the same In practice, the outer coil (coil 2, in Fig.10-2) weighs more because the 35 length per turn is greater Aluminum or copper conductor are used A transformer is reversible in the sense that either winding can be used as the primary winding, where ‘primary’ means the winding connected to the source 61 Task Rephrasing Rewrite the following sentences, replacing the words in italics with expressions from the passage which have similar meanings: The transformers is probably one of the most useful electrical equipment ever invented Futhermore, the transformer allows us to transmit electrical energy over great distances and to distribute it safely in factories and homes As the result, the current If needed to supply the iron losses is usually to times smaller than Im Fig.2-3 shows how the laminations of small transformer are arranged to build up the core Therefore, the amount of copper in the primary and secondary windings is about the same Task Contextual reference What the pronouns in italics in these sentences refer to? It can raise or lower the voltage or current in an ac circuit, it can isolate circuits from each other, and it can increase or decrease the apparent value of a capacity, an inductor, or a resistor (line 3, & 5) (a)Transformer; electrical device; ac circuit (b)Electrical device; electrical device; electrical device (c)Transformer; transformer; transformer (d)Transformer; electrical device; electrical device Futhermore, the transformer enables us to transmit electrical energy over great distances and to distribute it safely in factories and homes (line 7) (a)Transformer (b)Electrical energy (c)Electrical device (d)A resistor Power transformers are usually designed so that their characteristics approach those of an ideal transformer (line 9) (a)Electrical devices’ (b)Factories’ (c)Homes’ (d)Power transformers’ The resulting magnetizing current Im is at least 5000 times smaller than it would be if an air core were used (line 11) (a)Power transformer (b)High permeability (c)The core (d)The resulting magnetizing current 62 It also guarantees good voltage regulation when a load is connected to the secondary terminals (line 19) (a)Tight coupling between the coils (b)The coil (c)The secondary voltage (d)The primary voltage The number of turns on the primary and secondary windings depends upon their respective voltages (line 30) (a)Turns’ (b)The primary windings’ (c)The secondary windings’ (d)The primary and secondary windings’ Task Checking facts and ideas Decide if these statements are true or false Quote from the passage to support your decisions The transformer is the most useful electrical device The apparent value of a capacity, an inductor, or a resistor can be changed by the transformer Transmission of electrical energy and its distribution over great distance are made safely in factories and homes by means of this useful electrical devices Power transformers are only designed so that their characteristics become an ideal transformer The number of turns on the primary and secondary windings varies, corresponding to their respective voltages The The resulting magnetizing current Im is at least 5000 times smaller than it would be if an air core were used A transformer is usually reversible in the sense that either winding can be used as the primary winding The small current in a HV winding allows us to use a small size conductor Arrangement of the primary and secondary coils are made so as to cover both core legs for less amount of copper 10 Winding the primary and secondary coils on top of each other to make Leakage reactances Xf1 and Xf2 must ensure both standard insulation and the interval as close as it is allowed II USE OF LANGUAGE Task Making definitions Make definitions of following concepts: 1) 2) Power Transformer Circuit Breaker 63 3) 4) 5) Protective Relay Transmission Line Load II INFORMATION TRANSFER Task Labeling components Fill up the blanks with the suible word that list below: Transformers, Primary winding, Secondary winding, Windings, Light bulb As we can see in the above experiment, transformers have at least two … (1) or coils One is called the primary, the other the secondary The … (2) is where AC current is fed in The …… (3) is where the current is induced to perform some sort of transfer of energy In this case the current is used to light a … (4) There are many types of ……(5) in existence This is a very simple example The iron bar core helps to transfer more of the magnetic energy from the primary coil to the secondary coil 1) 2) 3) 4) 5) IV …windings………… …primary winding………… …secondary winding… …light bulb………… …transformer………… GUIDED WRITING Task Sentence building Join the following groups of sentences to make the longer sentences Use the words printed in italics at the beginning of each group You may omit words and make whatever changes you think are necessary in the word order and punctuation of the sentences When An AC current is flowing in one of the coils 64 A similar current is induced into the second coil Even though No direct electrical connection exists between the two coils We can induce electrical current in this manner Where The electricity flows to a transmission station In the transmission station, a transformer changes a large current and low voltage into a small current and high voltage Since No current will exist in the secondary coil The secondary coil is open-circuited If More flux were induced in the core It would cause more voltage to be induced voltage in the primary coil V READING AND SUMMARIZING Voltage Transformers Voltage transformers (also called potential transformers) are high-precision transformers in which the ratio of primary voltage to secondary voltage is a known constant, which changes very little with burden Furthermore, the secondary voltage is almost exactly in phase with the primary voltage The nominal secondary voltage is usually 115V, irrespective of what the rated primary voltage may be This permits standard instruments and relays to be used on the secondary side Voltage transformers are used to measure or monitor the voltage on transmission lines and to isolate the metering equipment from these lines The construction of voltage transformers is similar to that of conventional transformers However, the insulation between the primary and secondary windings is always connected to ground to eliminate the danger of a fatal shock when touching one of the secondary leads Although the secondary appears to be isolated from the primary, the distributed capacitance between the two windings makes an invisible connection which can produce a very high voltage between the secondary winding and ground By grounding one of the secondary terminals, the highest voltage between the secondary lines and ground is limited to 115V The nominal rating of voltage transformers is usually less than 500VA As a result, the volume of insulation is often far greater than the volume of copper or steel Voltage transformers installed on HV lines always measure the line-to-neutral voltage This eliminates the need for two HV bushings (sứ xuyên nghĩa thường gặp) because one side of the primary is connected to ground 65 Basic impulse insulation (BIL) of 650kV expresses the transformer’s ability to withstand lightning and switching surges Answer the following questions 1) 2) 3) 4) 5) What is the another name of voltage transformer (VT)? What is usually the nominal secondary voltage of VT? Compare the constrution of VT to that of conventional transformers Compare the volume of insulation to that of copper or steel What is BIL? 66 ... resistor (line 3, & 5) (a )Transformer; electrical device; ac circuit (b)Electrical device; electrical device; electrical device (c )Transformer; transformer; transformer (d )Transformer; electrical... the primary coil V READING AND SUMMARIZING Voltage Transformers Voltage transformers (also called potential transformers) are high-precision transformers in which the ratio of primary voltage... resistor Power transformers are usually designed so that their characteristics approach those of an ideal transformer (line 9) (a)Electrical devices’ (b)Factories’ (c)Homes’ (d)Power transformers’