7-1 TRANSFORMERS
Electrical
transformer is a static device which transforms electrical energy from one
circuit to another without any direct electrical connection and with the help
of mutual induction between to windings. It transforms power from one circuit
to another without changing its frequency but may be in different voltage
level.
7-1-1 USE OF POWER TRANSFORMER
Generation of Electrical
Power in low voltage level is very much cost effective. Hence Electrical Power
is generated in low voltage level. Theoretically, this low voltage leveled
power can be transmitted to the receiving end. But if the voltage level of a
power is increased, the current of the power is reduced which causes reduction
in ohmic or I2R losses in the system, reduction in cross sectional
area of the conductor i.e. reduction in capital cost of the system and it also
improves the voltage regulation of the system. Because of these, low leveled
power must be stepped up for efficient. This is done by step up transformer at
the sending side of the power system network. As this high voltage power may
not be distributed to the consumers directly, this must be stepped down to the
desired level at the receiving end with help of step down transformer. These
are the use of electrical power transformer in the electrical power system.
7-1-2 TYPES OF TRANSFORMER
Transformers
can be categorized in different ways, depending upon their purpose, use,
construction etc. The types of transformer are as follows:
Step Up Transformer & Step Down
Transformer - Generally used for
stepping up and down the voltage level of power in transmission and
distribution power network.
Three phase transformer & Single Phase Transformer - Former is generally used in three phase power
system as it is cost effective than later but when size matters it is
preferable to use three phase transformer as it is easier to transport three
single phase unit separately than one single three phase unit.
Electrical Power Transformer,
Distribution Transformer & Instrument Transformer - Transformer generally used in transmission network
is normally known as Power Transformer, Distribution Transformer is used in
distribution network and this is lower rating transformer and Current
Transformer & Potential Transformer, we use for relay and protection
purpose in electrical power system and in different instruments in industries
are called instrument transformer.
Two Winding Transformer &
Auto-Transformer - Former is
generally used where ratio between High Voltage and Low Voltage is greater than
2. It is cost effective to use later where the ratio between High Voltage and
Low Voltage is less than 2.
Outdoor Transformer & Indoor
Transformer - Transformers designed
for installing at outdoor is Outdoor Transformer and Transformers designed for
installing at indoor is Indoor Transformer.
7-2 CIRCUIT BREAKERS
Circuit Breaker is a
switching device which can be operated manually as well as automatically for
controlling and protection of electrical power system respectively. As the
modern power system deals with huge currents, special attention should be given
during designing of circuit breaker to safe interruption of arc produced during
the operation of circuit breaker
The modern power system
deals with huge power network and huge numbers of associated electrical
equipment. During short circuit fault or any other types of electrical fault
these equipment as well as the power network suffer a high stress of fault
current in them which may damage the equipment and networks permanently. For
saving these equipments and the power networks the fault current should be
cleared from the system as quickly as possible. Again after the fault is
cleared, the system must come to its normal working condition as soon as
possible for supplying reliable quality power to the receiving ends. In
addition to that for proper controlling of power system, different switching
operations are required to be performed. So for timely disconnecting and
reconnecting different parts of power system network for protection and
control, there must be some special type of switching devices which can be
operated safely under huge current carrying condition. During interruption of
huge current, there would be large arcing in between switching contacts, so
care should be taken to quench these arcs in safe manner.
Circuit breaker is the
special device which does all the required switching operations during current
carrying condition.
7.2.1 WORKING PRINCIPLE OF CIRCUIT BREAKER
Circuit breaker mainly
consists of fixed contacts and moving contacts. In normal "ON"
condition of circuit breaker, these two contacts are physically connected to
each other due to applied mechanical pressure on the moving contacts. There is
an arrangement stored potential energy in the operating mechanism of circuit
breaker which is realized if switching signal given to the breaker. The
potential energy can be stored in the circuit breaker by different ways like by
deforming metal spring, by compressed air, or by hydraulic pressure. But
whatever the source of potential energy, it must be released during operation.
Release of potential energy makes sliding of the moving contact at extremely
fast manner.
All circuit breaker have
operating coils (tripping coils and close coil), whenever these coils are
energized by switching pulse, and the plunger inside them displaced. This
operating coil plunger is typically attached to the operating mechanism of
circuit breaker, as a result the mechanically stored potential energy in the
breaker mechanism is released in forms of kinetic energy, which makes the
moving contact to move as these moving contacts mechanically attached through a
gear lever arrangement with the operating mechanism. After a cycle of operation
of circuit breaker the total stored energy is released and hence the potential
energy again stored in the operating mechanism of circuit breaker by means of
spring charging motor or air compressor or by any other means.
Till now we have discussed
about mechanical working principle of circuit breaker. But there are electrical
characteristics of a circuit breaker which also should be considered in this
discussion of operation of circuit breaker.
The circuit breaker has to
carry large rated or fault power. Due to this large power there is always
dangerously high arcing between moving contacts and fixed contact during
operation of circuit breaker.
Again as we discussed
earlier the arc in circuit breaker can be quenched safely if the dielectric
strength between the current carrying contacts of circuit breaker increases
rapidly during every current zero crossing of the alternating current. The
dielectric strength of the media in between contacts can be increased in
numbers of ways, like by compressing the ionized arcing media since compressing
accelerates the deionization process of the media, by cooling the arcing media
since cooling increase the resistance of arcing path or by replacing the
ionized arcing media by fresh gasses. Hence a numbers of arc quenching processes
should be involved in operation of circuit breaker.
7.2.2 TYPES OF CIRCUIT BREAKER
According to different
criteria there are different types of circuit breaker
Classification Based on Arc Quenching Media:
1.
Oil Circuit
Breaker
2.
Air Circuit
Breaker
3.
SF6
Circuit Breaker
4.
Vacuum Circuit
Breaker
Classification Based on Service:
1.
Outdoor Circuit Breaker
2.
Indoor Circuit Breaker
Classification Based on Operating Mechanism of circuit breaker:
1.
Spring Operated
Circuit Breaker
2.
Pneumatic Circuit
Breaker
3.
Hydraulic Circuit
Breaker
Classification Based on Voltage level of installation:
1.
High Voltage
Circuit Breaker
2.
Medium Voltage
Circuit Breaker
3.
Low Voltage
Circuit Breaker
7.3 DISCONNECT SWITCHES/ISOLATORS
In electrical engineering, a disconnector or
isolator switch or disconnect switch is used to make sure that an electrical
circuit can be completely de-energized for service or maintenance. Such
switches are often found in electrical distribution and industrial applications
where machinery must have its source of driving power removed for adjustment or
repair. High-voltage isolation switches are used in electrical substations to
allow isolation of apparatus such as circuit breakers and transformers, and
transmission lines, for maintenance. Often the isolation switch is not intended
for normal control of the circuit and is used only for isolation.
Isolator switches have provisions for a Padlock
so that inadvertent operation is not possible. In high voltage or complex
systems, these padlocks may be part of a trapped-key interlocked to ensure
proper sequence of operation. In some designs the isolator switch has the
additional ability to earth the isolated circuit thereby providing additional
safety. Such an arrangement would apply to circuits which inter-connect power
systems where both end of the circuit need to be isolated.
The major difference between an isolator and a
circuit breaker is that an isolator is an off-load device intended to be opened
only after current has been interrupted by some other control device. Safety
regulations of the utility must prevent any attempt to open the disconnector
while it supplies a circuit.
7-4 LIGHTNING ARRESTER
A lightning arrester (in Europe:
surge arrester) is a device used on electrical power system and communications
systems to protect the insulation and conductors of the system from the
damaging effects lightning. The typical lightning arrester has a high voltage
terminal and a ground terminal. When a lightning surge (or switching surge,
which is very similar) travels along the power line to the arrester, the
current from the surge is diverted through the arrestor, in most cases to
earth.
If protection fails or is absent, lightning that
strikes the electrical system introduces thousands of kilovolts that may damage
the transmission lines, and can also cause severe damage to transformers and
other electrical or electronic devices. Lightning-produced extreme voltage
spikes in incoming power lines can damage electrical home appliances.
A lightning
arrester may be a spark gap or may have a block of a semiconducting material
such as Silicon Carbide or Zinc Oxide. Some spark gaps are open to the air, but
most modern varieties are filled with a precision gas mixture, and have a small
amount of radioactive material to encourage the gas to ionize when the voltage
across the gap reaches a specified level. Other designs of lightning arresters
use a glow-discharge tube (essentially like a neon glow lamp) connected between
the protected conductor and ground, or voltage-activated solid-state switches
called varistors or MOVs.
Lightning arresters built for power system
consist of a porcelain tube several feet long and several inches in diameter,
typically filled with disks of zinc oxide. A safety port on the side of the
device vents the occasional internal explosion without shattering the porcelain
cylinder.
Lightning arresters are rated by the peak current
they can withstand the amount of energy they can absorb, and the break over
voltage that they require to begin conduction. They are applied as part of a
lightning protection system, in combination with air terminals and bonding.
7-5 BATTERIES AND BATTERY
CHARGERS
Supply of
power to protection and control circuits is provided from storage batteries due
to reliability point of view.
The simplest
operating unit to produce emf chemically is called a cell, whereas several
cells constitute a battery. Electrochemical devices consist of two dissimilar
electrodes immersed in a conducting solution, normally known as electrolyte
that is capable of storing electrical energy.
The voltage
of the cell depends upon the material of electrolyte, while the current and
power capacity of a cell depends upon the plate area and weight of active
material in the electrodes.
Main
types of storage batteries are:
- Lead Acid Batteries
- Alkaline Batteries
Active
Parts of Lead Acid Battery:
- Grid (Lead Antimony)
- Positive Plates (Lead Per Oxide- PbO2)
- Negative Plates (Lead- Pb)
- Electrolyte (Sulphuric Acid-H2SO4)
Chemical
Reactions
At
Anode: PbO2 + H2SO4↔
PbSO4 + H2O + ½O2
At
Cathode: Pb + H2SO4↔
PbSO4 + H2O
7-6 STATION GROUNDING SYSTEM
Earthing
or grounding is the term used for electrical connection to general mass of
earth in such a manner as to ensure, at all times, an immediate discharge of
energy without danger. A grounding system to be totally effective must satisfy
the following conditions:
- Provide a low impedance path to ground for personnel and equipment protection and effective circuit relaying.
- Withstand and dissipate repeated fault and surge currents.
- Provide corrosion allowance or corrosion resistance to various soil chemicals to insure continuous performance during the life of the equipment being protected.
Types
of Earthing:
- Solid or Effective Earthing
- Resistance Earthing/Reactance Earthing
Classification
of Earthing
- System or Neutral Earthing: The neutral point of generator, transformer, transmission and distribution system or circuit, rotating machines etc. is connected to earth either directly or through a resistance, or a reactance.
- Equipment Earthing: Equipment Earthing means connecting the non current carrying metallic parts in the neighborhood of electrical circuits to earth.
Resistance
to current through an earth electrode system has the following three
components:
- Resistance of the ground rod itself and connections to it.
- Contact resistance between the ground rod and earth adjacent to it.
- Resistance of the surrounding earth.
7-7 AC & DC Supply System
In any
substation AC and DC supply system plays a very important role for protection,
control and for all auxiliary services.
AC Supply System
For AC
supply, normally a dedicated panel is specified in a substation which is only
for the substation and no external load is connected to it in order to avoid
interruptions on it. On the LT side two transformers are provided exclusively
for the substation auxiliary services. For reliability purposes, load is fed
from one transformer; however in case load can be shifted to the other
transformer either from HT or LT side. Then we have distribution panels, from
where load is distributed throughout the substation through appropriate Circuit
Breakers/ Miniature Circuit Breakers.
DC Supply System
For DC
supply system, Rectifiers, Batteries and Distribution Panels are provided in
the substation. In important substations, normally Two sets of Batteries along
with Three Rectifiers (One as standby) are provided for reliability purposes.
110 Volts
Batteries Two
Sets
110 Volts
Rectifiers Three
Sets
220 Volts
Batteries Two
Sets
220 Volts
Rectifiers Three
Sets
In 500 kV
substations, Four sets of Batteries and Six Rectifiers (One as standby for Two
banks). Even, in case of emergency, loads of the same rating can be coupled
with one Rectifier/Battery.
7.8 POWER CABLES
There are
four main parts of cable:-
1.
Conductor
2.
Insulation
3.
Shield or Sheath
4.
Protective Covering
7.8.1 PURPOSES OF SHIELDING /
SHIELD GROUNDING
The
application of conducting (copper etc) and semiconducting (metabolized paper
tap or containing carbon or silicon etc) materials over the conductor
insulation is called shielding. The main purpose of shield is to keep even
voltage gradient across the insulation in order to avoid damage to insulation
by corona or ionization.
Now shield may have induced voltages in it, so shield
must be grounded in order to discharge these induced voltages. When shield is
grounded, it provides some more advantages as well, which are:
1.
Provides earth
return path in case of phase to ground fault
2.
Human safety
3.
Protects the
cable from external high voltages, produced by lightening etc
Shield must
be grounded at one place only (especially in single phase cable) in order to
avoid flow of current in shield and hence damage to it due to overheating.
Shielding idea was given by Martin Hochsadter in 1915. He gave the idea that put
shield around the conductor of each phase and then ground all shields. Such
cables are called H-cables. Such cable fails phase to ground. In these cables
it is very rare that cable may fail phase to phase.
In a very
long cable, sectionalized are used. In sectionalized shield each section is
insulated from each other and then each section is grounded at one place only.
7.9
BUS-BARS
There are two types of bus bars
used in grid station, which are:
1.
The Flexile or Stranded Bus Bar
2.
The Rigid Bus Bar (may be tubular or solid)
1.
Flexible or Stranded Bus Bar: It is used where:
A. Longer
spans are involved.
B. Where
sufficient clearances are needed to allow for conductor sways and.
C. It is used
as a long drop from horizontal bus to equipment bushing.
In the flexible bus bar sag must be
enough to account for temperature variations without affecting the clearances
between phases and phases to ground.
2. Rigid Bus Bar: It is used where:
A. Heavy
currents are involved
B. Short or
less Spacing is available
To account for thermal expansion /contraction of
rigid bus provision must be made by means of expansion joints and clamps to
permit bus to slide both ways in order to avoid damage to equipment bushing and
isolators etc.
7.9.1 BUS BAR SCHEMES
SINGLE BUS SYSTEM
Single Bus
System is simplest and cheapest one. In this scheme all the feeders and
transformer bay are connected to only one single bus as shown.
Fig (1)
SINGLE BUS SYSTEM WITH BUS SECTIONALIZER
Fig) (2
DOUBLE BUS SYSTEM
Fig (3)
DOUBLE BREAKER BUS SYSTEM
Fig (4)
ONE AND A HALF BREAKER BUS SYSTEM
Fig (5)
MAIN AND TRANSFER BUS SYSTEM
Fig (6)
DOUBLE BUS SYSTEM WITH BYPASS ISOLATORS
Fig (7)
RING BUS SYSTEM
Fig
(8)
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