Tiếng anh chuyên nghành điện chương 8_9

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
5 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 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
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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
N1

almost exactly equal to
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
25 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
length per turn is greater. Aluminum or copper conductor are used. A transformer is
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reversible in the sense that either winding can be used as the primary winding, where
‘primary’ means the winding connected to the source.
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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.

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1

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
(b) The coil
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(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)
2)
3)
4)
3

Power Transformer
Circuit Breaker
Protective Relay
Transmission Line

3

5)
II

Load
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.

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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.
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.
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)

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?

5


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 correction capacitors 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.
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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.
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7


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)
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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 highvoltage switching, and switching and backup systems on the low-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
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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

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