Power station practice (NEE-702) unit-3

Unit-IIIUnit-III
Sub-stations Layout
• Md Irshad Ahmad
• Irshad.ahmad@jit.edu.in
• Electrical Engineering Department
• Subject: : POWER STATION PRACTICE
• (NEE /NEN–702)

ContentsContents
• UNIT- III: Sub-stations Layout
• Types of substations,
• Bus-bar arrangements, typical layout of substation.
• Power Plant Economics and Tariffs
• Load curve, load duration curve, different factors
related to plants and consumers, Cost of electrical
energy, depreciation, generation cost, effect of Load
factor on unit cost.
• Fixed and operating cost of different plants, role of
load diversity in power system economy. Objectives
and forms of Tariff; Causes and effects of low power
factor, advantages of power factor improvement,
different methods for power factor improvements.

33
THE ELECTRIC SUB STATIONTHE ELECTRIC SUB STATION
THE TRANSFORMER BAY 3
What is a
substation?
An electrical substation takes electricity from a
very high voltage and lowers it to the voltage we
use in our homes & businesses

44
 INTRODUCTION
The present electrical power system is a
complex interconnection of Generating
stations-Transmission systems- Receiving
stations- Distribution systems and Load
points.
In all the above phases of power flow, the
transfer of electrical energy takes place in
the electric sub stations.
4

55
 Basically an electrical sub station consists
of a number of incoming circuits and out
going circuits connected to common bus
bar systems.
 Bus bars are conducting bars to which a
number of incoming or out going circuits
are connected.
5

66
 INTRODUCTION
Each circuit connected to the bus bar will have
certain electrical component such as circuit
breakers, isolators, earth switches, current
transformers and voltage transformers.
These components are connected in a definite
sequence such that a circuit can be switched off
during normal operation by manual command
and also automatically during abnormal
conditions such as short circuits.
6

77
A sub station receives electrical power from
generating station through incoming
transmission lines and delivers electrical
power through the out going transmission
lines.
Sub station is an integral part of a power
system and is an important link between
the generating stations, transmission
systems, distribution systems and load
points.
7

88
 DESIGN CONSIDERATIONS
 The sub station is designed with an
objective to provide maximum
reliability, flexibility, continuity of
service and to meet these objectives
with the lowest investment costs that
satisfy system requirement.
8

99
9
System requirements include the selection of
optimum voltage levels depending on the load
requirements and the transmission distances
involved. Generally, the generating source will be
far away from the load centers. The advantage of
capitalizing on low site costs, availability of ample
cooling water supply, economical fuel supply and
less stringent environmental considerations
compel construction of generating source far away
from load centers, there by, increasing
transmission distances.

1010
 Hence, to transmit power over long distances the
transmission voltage is to be increased and in our
country 400 kV is becoming common and higher
voltages for transmission is being explored. Many
factors such as voltage level, load capacity, site space
limitations, transmission line right of way requirement
and environmental considerations influence the design
of sub stations.
 The system requirement must be met with minimum
costs as the cost of equipment, labor, land and site
treatment is increasing every day.
10

1111
 BUS LAYOUT AND SWITCHING ARRANGEMENT
 Since the major sub station costs are reflected in the power
transformers, circuit breakers and disconnecting switches,
the bus layout and switching arrangement selected will
determine the number of switches and power circuit breakers
required.
 A number of factors must be considered in the selection of
bus layouts and switching arrangements for a sub station to
meet system and station requirements.
 A sub station must be reliable, economical, safe, and as
simple in design as possible.
11

1212
 SUB STATION LAYOUT AND BUS BAR SCHEMES
 The term layout denotes the physical
arrangement of various components in
the sub station relative to one another.
The layout is significant as it influences
the operation, maintenance, cost and
protection of the sub station. These
aspects are considered while designing
the sub station layout.
12

1313
With the given number of incoming lines, out
going lines, transformers, etc., the sub station
can be designed in several alternative ways.
 The physical arrangement of the equipment is
called the layout of the sub station. The layout is
illustrated by means of single line diagrams.
 The design of sub station layout need careful
consideration of several aspects such as: 13

1414
 Switching requirement for normal operation.
 Switching requirement during abnormal
conditions like short circuits and overloads.
 Degree of flexibility in operations, simplicity.
 Freedom from total shutdowns.
 Maintenance requirements, space for
approaching various equipment for
maintenance.
 Road/ rail for transporting main and auxiliary
equipment.
14

1515
 Safety of personnel.
 Protective zones for main and back up
protection.
 Provision for bye pass facilities and for
extensions, space requirements.
 Technical requirements such as ratings,
clearances, earth system, lightning protection.
 Requirement for SCADA and communication.
 Compatibility for local and ambient condition.
15

1616
 The choice of bus bar schemes for ac yards
depend upon several factors mentioned above.
The important bus bar schemes are:
 Single bus bar
 Double bus bar with one breaker per circuit
 Double bus bar with two breaker per circuit
 Main and transfer bus
 Ring bus or Mesh scheme
 Breaker and a half(1 1/2 breaker) arrangement
16

1717
The various schemes are generally compared to
emphasize their advantage and limitations.
 The basis of comparison is generally the
degree of reliability & economic justification.
 The degree of reliability is evaluated by
determining continuity of service under
anticipated operating conditions and possible
faults. 17

1818
THE ELECTRIC SUB STATIONTHE ELECTRIC SUB STATION VARIOUS BUS BAR SCHEMES
 SINGLE BUS BAR SCHEME
BUS
ISOLATOR
BREAKER
CURRENT TRANSFORMER
POWER TRANSFORMER
18

1919
 SINGLE BUS BAR SCHEME
 Advantage –Lowest cost
 Disadvantage:
Maintenance without interruption of supply is not possible.
Sub station can not be extended without completely de-
energizing the sub station
Can be used only where loads can be interrupted or have other
supply arrangements. Least flexibility. 19

2020
 VARIOUS BUS BAR SCHEMES
 SINGLE BUS BAR SCHEME WITH BUS
SECTIONALISER
BUS SECTION-2
ISOLATOR
BREAKER
CURRENT TRANSFORMER
POWER TRANSFORMER
BUS SECTION-1
SECTIONALISER
20

2121
 SINGLE BUS BAR SCHEME WITH BUS SECTIONALISER
Sectionalizing the single bus improves slightly the reliability if
the incoming and out going circuits are distributed evenly on
both the sections.
 Where double feed is provided for any single load it is
preferable to have one circuit from each section.
 In this arrangement each section behaves as a separate bus
bar and any outage can be confined to one section of the bus
bar.
 Only the faulty section will be tripped by bus differential
protection.
21

2222
 DOUBLE BUS BAR WITH ONE BREAKER PER CIRCUIT
BUS-1
BUS-2
BUS COUPLER
BREAKER
POWER TRANSFORMER
BREAKER
22

2323
 DOUBLE BUS BAR WITH ONE BREAKER PER CIRCUIT
 This arrangement has the following advantages:
 Each load may be fed from either bus.
 Operational flexibility may be increased by grouping the
incoming and out going feeders in separate groups.
 Either bus bar can be taken out for maintenance.
 Bus coupler helps in ‘on load change over ‘from one bus to
the other.
 Adopted where load and continuity justify additional cost.
 A major disadvantage is that the breaker can not be taken out
for maintenance without interrupting supply to the concerned
circuit.
23

2424
 DOUBLE BUS BAR WITH ONE BREAKER PER
CIRCUIT
 Bus protection scheme may cause loss of sub
station when it operates if all circuits are
connected to that bus.
 High exposure to bus faults.
 Line breaker failure takes all circuits connected
to that bus out of service.
 Bus tie breaker failure takes the entire sub
station out of service.
24

2525
DOUBLE MAIN BUS & CB BYPASS ISOLATOR SYSTEMDOUBLE MAIN BUS & CB BYPASS ISOLATOR SYSTEM
FEEDER1
BUS-2BUS-2
BUS-1BUS-1
T/F-1
BUSCOUPLER
FEEDER2
T/F-2
FEEDER3 FEEDER4
BAY1 BAY2
BAY3
BAY4
BAY5
BAY6 BAY7
FOR ANY CB PROBLEM OR FOR
PREVENTIVE MAINTANENCE, SUCH
FEEDER CAN BE SHIFTED TO
ANOTHER BUS AND THE BYPASS
ISOLATOR IS CLOSED, THEN
PROTECTION IS TRANSFERRED TO
BUS COUPLER AND THE FAULTY CB
CAN BE ISOLATED.

2626
 MAIN AND TRANSFER BUS
MAIN BUS
TRANSFER BUS
BREAKERTIE BREAKER
LINE LINE
26

2727
 This is an alternative to double bus single
breaker arrangement which provided for change
over to either bus for carrying out maintenance
on other bus. But it provided no facility for
breaker maintenance without interrupting power
supply to the concerned circuit.
 The main and transfer bus works the other way
round.
 This arrangement provides facility for carrying
out breaker maintenance but does not permit
bus maintenance.
27

2828
 Any breaker can be taken out of service for maintenance.
 Wherever maintenance is required on any breaker, the circuit is changed
over to the transfer bus and controlled through the bus coupler breaker.
 Potential devices may be used on the main bus for relaying.
 The cost is increased due to use of an extra isolator for each circuit and
providing interlock for bus coupler and circuit isolators.
 Relaying sensitivity decreases as the same bus coupler is used to
energize the concerned circuit for all the circuit breakers whenever they
are taken out for maintenance.
 Failure of bus or any circuit breaker results in shut down of entire sub
station. 28

2929
DOUBLE MAIN BUS & TRANSFER BUS SYSTEMDOUBLE MAIN BUS & TRANSFER BUS SYSTEM
FEEDER1
BUS-2BUS-2
BUS-1BUS-1
FEEDER2
TRANSFERBUS
COUPLER
T/F-1 T/F-2
BUSCOUPLER
TRANSFER BUSTRANSFER BUS
FEEDER3 FEEDER4
BAY1 BAY2
BAY3 BAY4 BAY5
BAY6 BAY7 BAY8

3030
BUS BAR ARRANGEMENTSBUS BAR ARRANGEMENTS
• Double Bus Bar Arrangement with
Transfer Bus.
This arrangement provides more
additional flexibility, continuity of Power
Supply, permits periodic maintenance
without total shut down as the two main
buses can be operated independently with
the same redundancy.

3131
 DOUBLE BUS BAR WITH TWO BREAKER
PER CIRCUIT
BUS-1
BUS-2
BREAKER
LINE LINE
31

3232
 DOUBLE BUS BAR WITH TWO BREAKER PER
CIRCUIT
 Each circuit has two dedicated breakers.
 Has flexibility in permitting feeder circuits to be
connected to either bus.
 Any breaker can be taken out of service for
maintenance.
 High reliability.
 Most expensive. Used only in large generating
stations where security of connection is paramount.
32

3333
• RING BUS OR MESH SCHEME
ISOLATOR
BREAKER
LINE
LINE
POWER TRANSFORMER
33

3434
 RING BUS OR MESH SCHEME
 In this scheme the breakers are arranged in a ring with
circuits connected between breakers.
 There are the same number of circuits as there are
breakers.
 During normal operation, all breakers are closed. For a
circuit fault, two breakers are tripped, and in the event
one of the breaker fails to operate to clear the fault, an
additional circuit will be tripped by operation of breaker-
failure back up relays.
 During breaker maintenance, the ring is broken, but all
lines remain in service. 34

3535
 RING BUS OR MESH SCHEME
 The circuits connected to the ring are arranged so that
sources are alternated with loads.
 For an extended circuit outage, the line isolator may be
opened and the ring can be closed.
 No changes to protective relays are required for any of the
various operating conditions or during maintenance.
 The ring bus scheme is economical in cost, has good
reliability, is safe for operation, is flexible, and is normally
considered suitable for important sub stations up to a limit of
five circuits.
 It is common practice to build major sub stations initially as a
ring bus; for more than five outgoing circuits, the ring bus is
usually developed to the breaker-and-a-half scheme. 35

3636
• One and a Half Breaker Arrangement.
This arrangement provides three circuit
breakers for every two circuits. It gives
high security against loss of supply but
higher cost is involved. Hence this is
provided for important 400/220 KV Sub
Stations.

Types of LoadsTypes of Loads
Domestic Load
Commercial Load
Industrial Load
Municipal Load
Irrigation Load
Traction Load

Variable load on Power stationVariable load on Power station
The load on a power station varies from time
to time due to uncertain demands of the
consumers and is known as variable load
on the station.
Effects of variable load
Need of additional equipment
Increase in production cost

Load CurveLoad Curve
The curve showing the variation of load on
the power station with respect to time is
known as a load curve

Load duration CurveLoad duration Curve
When the load elements of a load curve are arranged
in the order of descending magnitude ,the curve
thus obtained is called load duration curve.

Important terms and factorsImportant terms and factors
Connected load: It is the sum of continuous
rating of all the equipments connected to
supply system.
Maximum Demand: It is the greatest demand
of load of the power station during a given
period.
Demand factor: it is the ratio of maximum
demand on the power station to its
connected load.

Average load: The average of loads occurring
on the power station in a given period (day
or month or year)is known as average load
or average demand.
Load factor: The ratio of average load to the
maximum demand during a period is known
as load factor.
Diversity factor: The ratio of the sum of
individual maximum demands to the
maximum demand on power station

Coincidence factor: The coincidence factor is
the reciprocal of the diversity factor
Contribution factor: it is the contribution of
a particular load ,in per unit of the
individual demand ,to the group maximum
demand

Plant Capacity factor: it is the ratio of actual
energy produced to the maximum possible
energy that could have been produced
during a given period.
Plant use factor: It is ratio of kWh generated
to the product of plant capacity and the
number of hours for which the plant was
operation.
Plant Utilization factor: it is the ratio of
maximum load to the rated capacity.

TariffTariff
The rate at which electrical energy is supplied
to a consumer is known tariff.
Objectives of tariff
 Recovery of production cost
 Recovery of capital investment cost
 Recovery of operation and maintenance
cost
 A suitable profit

Desirable characteristics of aDesirable characteristics of a
tarifftariff
Proper return
Fairness
Simplicity
Reasonable profit
Attractive

Types of tariffTypes of tariff
Simple tariff
Flat rate tariff
Block rate tariff
Two part tariff
Maximum demand tariff
Power factor tariff
Three part tariff

Simple tariff or uniform rateSimple tariff or uniform rate
tarifftariff
There is a fixed rate per unit of energy consumed.
Advantages:
Simplest tariff method and is readily understood
by the consumers.
Disadvantages:
There is no discrimination between consumers
The cost per unit delivery is high
it does not encourage the use of electricity

Flat rate tariffFlat rate tariff
When different types of consumers are charged
at different uniform per unit rate ,it is called a
flat rate tariff.
Advantanges: it is more fair to different types of
consumers and is quite simple in calculations.
Disadvantages:
 Separate meters are requires for light and
power load.
 A particular class of consumers are charged
at the same rate irrespective of the magnitude
of energy consumed.

Block rate tariffBlock rate tariff
When a given block of energy is charged at a
specified rate and the succeeding blocks of
energy are charged at progressively
reduced rates, it is called a block rate tariff.
Advantages:
Consumer gets an incentive to consume
more energy. It increases the load factor.So
cost of generation reduced.
Disadvantages:
it lacks a measure of consumers demand.

Two part tariffTwo part tariff
When the rate of electrical energy is charged on
the basis of maximum demand of the consumers
and the units consumed ,it is called a two-part
tariff.
In two-part tariff ,the total charges to be made
from the consumer is split in to two
components ,fixed charges and running charges.
The fixed charge depends on the maximum
demand in kW and running cost depends on the
number of units consumed by the consumer in
kWh.
Total charges = b X kW+ c X kWh

Two part tariffTwo part tariff
Advantages:
it is easily understood by the consumers
it recovers the fixed charges which
depend upon the maximum demand
Disadvantages:
The consumer has to pay the fixed
charges irrespective of the fact whether he
has consumed or not.
There is always error in assessing the
maximum demand of the consumer.

Maximum demand tariffMaximum demand tariff
It is similar to two-part tariff with the only
difference that the maximum demand is
actually measured by installing maximum
demand meter in the premises of the
consumer.
Advantages: This removes the objection of two-
part tariff
Disadvantages: Not suitable for small consumers
as a separate maximum demand meter is
required.

Power factor tariffPower factor tariff
The tariff in which power factor of the
consumer’s load taken in to consideration is
known as power factor tariff.
Types of power factor tariff:
1.kVA maximum demand tariff :It is
modification of two part tariff. In this case
fixed charges are made on the basis of
maximum demand kVA and not in kW.

2.Sliding scale tariff : This is also known as
average power factor tariff. In this case, an
average power factor , say 0.8 lagging , is
taken as reference. If the pf of consumer
falls below this factor ,suitable additional
charges are made or if pf of consumer
above the reference ,a discount allowed to
the consumer.
3.kW and kVAR tariff: In this type both kW
and kVAR charged separately.
Power factor tariffPower factor tariff

Three part tariffThree part tariff
When the total charges to be made from the
consumer is split into three parts ,fixed
charge ,semi fixed charge and running charge
,it is known as three part tariff.
Total charge = a + bXkW + cXkWh
a – fixed charge including intrest and
depreciation on the cost of secondary
distribution and labor cost of the collecting
revenues.
b – charge per kW maximum demand
c – charge per kWh of energy consumed.

Power factor and economics ofPower factor and economics of
power factor correctionpower factor correction
Power factor : The cosine of the angle between
voltage and current in an AC circuit is known as
power factor

Disadvantages of low powerDisadvantages of low power
factorfactor
Large kVA rating equipment
kVA = kW/cos ϕ
Greater conductor size
Large copper losses
Poor voltage regulation
Reduced handling capacity of system

Causes of low factorCauses of low factor
AC induction motors
Arc lamps ,electric discharge and
industrial heating furnaces
Varying load on the power system

Power factor improvementPower factor improvement
equipmentequipment
Static Capacitors: The power factor can be improved by
connecting capacitors in parallel with the equipment
operating at lagging power factor.
Advantages:
 They have low losses
 Little maintenance
 Easily installed
 Can work under ordinary atmosphere condition
Disadvantages
 Short service life
 Easily damaged with higher voltage
 Repair uneconomical

Synchronous CondenserSynchronous Condenser
An over-excited synchronous motor running on no
load known as synchronous condenser.
Advantages:
 Power factor can be controlled
 Highly stable
 Faulty can be removed easily
Disadvantages:
 Considerable losses in the motor
 High maintenance cost
 It produces noise
 Not self starting one

Phase advancersPhase advancers
Phase advancers are used to improve the power
factor of induction motors. The low power factor
of an induction motor is due to the fact that its
stator winding draws exciting current which lags
behind the supply voltage by 90 degree. If the
exciting ampere turns can be provided from some
other AC source then the stator wining will be
relieved of exciting current and the power factor
of the motor can be improved.
Advantages: kVAR lagging reduced ,convenient
Disadvantages: Not economical for motors below
200HP

Calculation of Power factorCalculation of Power factor
correctioncorrection

Methods for Power FactorMethods for Power Factor
ImprovementImprovement
• The following devices and equipment are
used for Power Factor Improvement.
• Static Capacitor
• Synchronous Condenser
• Phase Advancer

1. Static Capacitor1. Static Capacitor
• Most of the industries and power system loads are inductive that take lagging current
which decrease the system power factor (See Disadvantages of Low Power factor) .
For Power factor improvement purpose, Static capacitors are connected in parallel
with those devices which work on low power factor.
• Advantages:
• Capacitor bank offers several advantages over other methods of power factor
improvement.
• Losses are low in static capacitors
• There is no moving part, therefore need low maintenance
• It can work in normal conditions (i.e. ordinary atmospheric conditions)
• Do not require a foundation for installation
• They are lightweight so it is can be easy to installed
• Disadvantages:
• The age of static capacitor bank is less (8 – 10 years)
• With changing load, we have to ON or OFF the capacitor bank, which causes
switching surges on the system
• If the rated voltage increases, then it causes damage it
• Once the capacitors spoiled, then repairing is costly

2. Synchronous Condenser
• When a Synchronous motor operates at No-Load and over-exited then it’s called a
synchronous Condenser. Whenever a Synchronous motor is over-exited then it
provides leading current and works like a capacitor.
• When a synchronous condenser is connected across supply voltage (in parallel) then
it draws leading current and partially eliminates the re-active component and this
way, power factor is improved. Generally, synchronous condenser is used to improve
the power factor in large industries.
• Advantages:
• Long life (almost 25 years),High Reliability
• Step-less adjustment of power factor.,No generation of harmonics of maintenance
• The faults can be removed easily
• It’s not affected by harmonics.
• Require Low maintenance (only periodic bearing greasing is necessary)
• Disadvantages:
• It is expensive (maintenance cost is also high) and therefore mostly used by large
power users.
• An auxiliary device has to be used for this operation because synchronous motor has
no self starting torque
• It produces noise

3. Phase Advancer
• Phase advancer is a simple AC exciter which is connected on the main shaft of the
motor and operates with the motor’s rotor circuit for power factor improvement.
Phase advancer is used to improve the power factor of induction motor in industries.
• As the stator windings of induction motor takes lagging current 90° out of phase with
Voltage, therefore the power factor of induction motor is low. If the exciting ampere-
turns are excited by external AC source, then there would be no effect of exciting
current on stator windings. Therefore the power factor of induction motor will be
improved. This process is done by Phase advancer.
• Advantages:
• Lagging kVAR (Reactive component of Power or reactive power) drawn by the motor
is sufficiently reduced because the exciting ampere turns are supplied at slip
frequency (fs).
• The phase advancer can be easily used where the use of synchronous motors is
Unacceptable
• Disadvantage:
• Using Phase advancer is not economical for motors below 200 H.P. (about 150kW)

Load profileLoad profile
• Load profile chart- Load curve
• In electrical engineering, a load profile is a
graph of the variation in the electrical load
versus time. A load profile will vary according to
customer type (typical examples include
residential, commercial and industrial),
temperature and holiday seasons.

Power generationPower generation
• In the electricity generation sector, a load
curve is a chart showing the amount of
electricity customers use over a period of
time.
• Generation companies use this
information to plan how much power they
will need to generate at any given time.

Daily load curveDaily load curve
•
Time Hrs
Load
in
KW
200
700
150

Load curveLoad curve
•
Time Hrs
Load
in
KW
Avarage
Load
Max demand,
peak load
00 24
08 18

Load FactorLoad Factor
• The ratio of the average load supplied during a designated period to
the peak load occurring in that period, in kilowatts.
• Simply, the load factor is the actual amount of kilowatt-hours
delivered on a system in a designated period of time as opposed to
the total possible kilowatt-hours that could be delivered on a system
in a designated period of time.
• Utilities are generally interested in increasing load factors on their
systems.
• A high load factor indicates high usage of the system’s equipment
and is a measure of efficiency. High load factor customers are
normally very desirable from a utility’s point of view.
• Using a year as the designated period, the load factor is calculated
by dividing the kilowatt-hours delivered during the year by the peak
load for the year times the total number or hours during the year.

Load factorLoad factor
• Load factor (electrical) is the average power divided
by the peak power over a period of time.
• in the electricity industry, load factor is a measure
of the output of a power plant.
The two commonest definitions are:
1. the ratio of average load to peak load in a period.
2.the ratio of actual energy produced to The
maximum energy that could have been produced if
the maximum demand stays on for all the time.
A higher load factor is better:
* A power plant may be less efficient at low load
factors.
* A high load factor means fixed costs are spread
over more kWh of output.

Load FactorLoad Factor
Daily LFDaily LF
24_max
__
_
24_max
24_
_
_
_
_
_max
_
_
×
=
×
×
=
=
=
demandimum
producedenergyactual
factorload
demandimum
demandaverage
factorload
demandpeak
demandaverage
factorload
demandimum
demandaverage
factorload

Load FactorsLoad Factors
• Based on the period considered
(Day,Week, Month,or year)
• Daily LF
• Weekly LF
• Monthly LF
• Yearly LF

Diversity factorDiversity factor
• Diversity factor: The ratio of sum of the
individual non-coincident maximum demands
of various subdivisions of the system to the
maximum demand of the complete system.
• The diversity factor is always more than or
equal to 1.
Diversity Factor = ---------------------------------
Sum of Individual max demand
Max demand on Power
station

Power station practice (NEE-702) unit-3

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