Tiếng anh chuyên nghành điện chương 8_9

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Unit 8: Transformer

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 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 If needed to supply the iron losses is usually 2 to 4 times smaller than Im

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 almost exactly equal to

2 1

N 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 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 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 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

Task 1 Rephrasing Rewrite the following sentences, replacing the words in italics

with expressions from the passage which have similar meanings:

1 The transformers is probably one of the most useful electrical equipment ever

invented

2 Futhermore, the transformer allows us to transmit electrical energy over great distances

and to distribute it safely in factories and homes

5

15

10

20

25

30

35

3 As the result, the current If needed to supply the iron losses is usually 2 to 4 times smaller than Im

4 Fig.2-3 shows how the laminations of small transformer are arranged to build up the

core

5 Therefore, the amount of copper in the primary and secondary windings is about the

same

Task 2 Contextual reference

What do the pronouns in italics in these sentences refer to?

1 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, 4 & 5)

(a)Transformer; electrical device; ac circuit

(b)Electrical device; electrical device; electrical device

(c)Transformer; transformer; transformer

(d)Transformer; electrical device; electrical device

2 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

3 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’

4 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

5 It also guarantees good voltage regulation when a load is connected to the secondary

terminals (line 19)

(a)Tight coupling between the coils

(c)The secondary voltage

(d)The primary voltage

6 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 3 Checking facts and ideas

Decide if these statements are true or false Quote from the passage to support your decisions

1 The transformer is the most useful electrical device

2 The apparent value of a capacity, an inductor, or a resistor can be changed by the transformer

3 Transmission of electrical energy and its distribution over great distance are made safely

in factories and homes by means of this useful electrical devices

4 Power transformers are only designed so that their characteristics become an ideal transformer

5 The number of turns on the primary and secondary windings varies, corresponding to their respective voltages

6 The The resulting magnetizing current Im is at least 5000 times smaller than it would be if

an air core were used

7 A transformer is usually reversible in the sense that either winding can be used as the primary winding

8 The small current in a HV winding allows us to use a small size conductor

9 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 4 Making definitions

Make definitions of following concepts:

1) Power Transformer

2) Circuit Breaker

3) Protective Relay

4) Transmission Line

5) Load

II INFORMATION TRANSFER

Task 5 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) …windings…………

2) …primary winding…………

3) …secondary winding…

4) …light bulb…………

5) …transformer…………

IV GUIDED WRITING

Task 6 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

1 When

An AC current is flowing in one of the coils

A similar current is induced into the second coil

2 Even though

No direct electrical connection exists between the two coils

We can induce electrical current in this manner

3 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

4 Since

No current will exist in the secondary coil

The secondary coil is open-circuited.

5 If

More flux were induced in the core

It would cause more voltage to be induced voltage in the primary coil

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

Basic impulse insulation (BIL) of 650kV expresses the transformer’s ability to withstand lightning and switching surges

Answer the following questions

1) What is the another name of voltage transformer (VT)?

2) What is usually the nominal secondary voltage of VT?

3) Compare the constrution of VT to that of conventional transformers

4) Compare the volume of insulation to that of copper or steel

5) What is BIL?

Unit 9: An electrical Substation

I READING AND COMPREHENSION

What is an electrical substation

An electrical substation is a subsidiary station of an electricity generation,

transmission and distribution system where voltage is transformed from high to low or the reverse using transformers

Explanation

Transformation may take place in several stages in sequence, starting at the

generating plant where the voltage is increased for transmission purposes and is then progressively reduced to the voltage required for household or industrial use The range of voltages in a power system varies from 110 V up to 765 kV depending on the country

A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while

increasing the current for domestic and commercial distribution The word substation

comes from the days before the distribution system became a grid The first substations were connected to only one power station where the generator was housed, and were subsidiaries of that power station

Substations generally contain one or more transformers, and have switching, protection and control equipment In a large substation, circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network Smaller distribution stations may use recloser circuit breakers or fuses for protection

of branch circuits Substations do not (usually) have generators, although a power plant may have a substation nearby A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, low voltage switchgear, surge protection, controls, and metering Other devices such as power factor correctioncapacitors and voltage regulators may also be located at a substation

Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings High-rise buildings may have indoor substations Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate

or pollution conditions

Where a substation has a fence, it must be properly grounded to protect people from high voltages that may occur during a fault in the transmission system Earth faults at a substation can cause Earth Potential Rise at the fault location

Transmission substation

A transmission substation is one whose main purpose is to connect together various transmission lines The simplest case is where all transmission lines have the same voltage In such cases, the substation contains high-voltage switches that allow lines to be connected together or isolated for maintenance

Transmission substations can range from simple to complex The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, and a large amount of protection and control equipment (capacitors, relays, switches, breakers, voltage and current transformers)

Distribution substation

150cm

150cm 70cm

50cm

Underground distribution system:

Upgraded predominantly by CAB or C.C.Box methods.

(CAB method〉

Underground distribution system:

C.C.Box method is positively adopted.

(C.C.Box method)〉

A distribution substation is one whose main purpose is to transfer power from the transmission system to the distribution system of some area It is uneconomical to directly connect electricity consumers to the main transmission network (unless they use large amounts of energy); so the distribution station reduces voltage to a value suitable for connection to local loads

The input for a distribution substation is typically at least two transmission or subtransmission lines Input voltage may be, for example, 115 kV, or whatever is common in the area The output is a number of feeders Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility

The feeders will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers at or near the customer premises

Besides transforming the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line

Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the low-voltage side More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side

Task 1 Checking facts and ideas

1 Transformation may happen in several stages in sequence, starting at the

generating plant where the voltage is increased for transmission purposes and is then progressively reduced to the voltage required for household or industrial use

2 The range of voltages in a power system range from 110 V up to 765 kV depending on the country

3 A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer reduces the voltage while increasing the current for domestic and commercial distribution

4 The first substations were connected to only one power station where the generator was located, and were subsidiaries of that power station

5 In a large substation, circuit breakers are used to pause any short-circuits or overload currents that may occur on the network

6 Other devices such as power factor correction capacitors and voltage regulators

may also be placed at a substation.

7 Where a substation has a fence, it must be properly earthed to protect people

from high voltages that may occur during a fault in the transmission system

Task 2 Checking facts and ideas

1 Substations always increases the voltage while decreasing the current for domestic and commercial distribution (F)

2 The minimum level of voltage in a power system is 110 kV and the maximum one is 765 kV depending on the country (T)

3 Circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network in every substation (F)

4 Indoor substations are hardly built in rural areas, but urban ones to decrease the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions (T)

5 Recloser circuit breakers or fuses for protection of branch circuits are dedicatedly used for smaller scale distribution substations

Transmission Substations

6 There is a wide range of transmission substations from simple to complicated one

7 The big transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, and a large amount of protection and control equipment (capacitors, relays, switches, breakers, voltage and current transformers

Distribution Substations

8 It is not cost-effective to indirectly connect electricity consumers to the main transmission network (unless they use large amounts of energy); so the distribution station reduces voltage to a value suitable for connection to local loads (F)

9 The functions of the distribution substation is not only to isolate faults in either the transmission or distribution systems but also to transform the voltage.(T)

10.Complex distribution substations are usually built in the urban areas, with high-voltage switching, and switching and backup systems on the low-high-voltage side

(T)

II GUIDED WRITING Sentence Building

1 If

The frequency of the power system rises about 0.5 [Hz] and more or falls about 1.5 [Hz] and more

The frequency of the power system rises about 0.5 [Hz] and more or falls about 1.5 [Hz] and more

2 In order to

Secure stable power supply in the power system

New installation, extension, improvement, repair and inspection of power facilities are indispensable, and stop of facilities become necessary in the process of these construction works

3 So that

As a result of system supervision, control of power flow is carried out by switching power system or by controlling power generation

The power system can be operated under the condition of standard value for safety operation, if necessary

4 In order to

In the case of loop system, in addition to switching of power system and control

of power generation, series capacitor and phase shifting transformer may be used

Change the power flow distribution

5 So that

In the case of abnormal weather or thunderstorm, switching of power system and control of power generation are carried out to control the power flow supplied for the region

Influence may become as small as possible even when fault occurs

6 When

Transmission line is interrupted by a fault

Recovery operation is performed aiming at power supply as early as possible on the assumption that the fault has disappeared

7 When

Wide-area outage occurs

Comprehensive recovery control of the whole system is needed

8 Since

Reactance of the circuit is larger than resistance in general

Reactive power affects greatly voltage fluctuation rather than active power

9 Which

The system processes load dispatching work automatically

The system is called automatic load dispatching system

10 Since

Large-scale information transmission system and computers are needed

The application of this system is limited to extra-high voltage system or above

in some power companies