EID_Unit 01_000.ppt

ST. VINCENT PALLOTTI COLLEGE OF ENGINEERING &
TECHNOLOGY
1
Prafull Madhukar Tarwatkar

Prafull Madhukar Tarwatkar 3
Part (A)
Electrical Load Assessment

INTRODUCTION
Power conversion systems are commonly referred to by the
function they perform. Power conversion takes place in the load of an
electrical power system. Common types of power system loads are those
which convert electrical energy into heat, light, or mechanical energy.
There are various types of lighting, heating, and mechanical loads used in
industry, commercial buildings, and homes. Each type of electrical load
has a different effect on the electrical distribution system.

CATEGORIES OF LOADS
In general, the types of load can be divided into the following
categories as –
 Domestic – This consists of mainly lights, fans, domestic appliances and
small motors for pumping, various other household appliances such as
heaters, refrigerators, air conditioners, mixers, ovens, heating ranges and
small motors for pumping, various other small household appliances, etc.
The various factors are –
Demand factor = 70 – 100 %;
Diversity factor = 1.2 – 1.3;
Load factor = 10 – 15 %

 Commercial – This mainly consists of lighting for shops and other
advertisements etc., fans, A / C, heating and other electrical appliances
used in commercial establishments, such as shops, restaurants, market
places, etc.
Diversity factor = 1.1 – 1.2;

 Industrial – These loads may be of the following typical power range:
• Cottage industries < 5 kW
• Small scale industries 5 – 25 kW
• Medium scale industries 25 – 100 kW
• Large scale industries 100 – 500 kW
• Heavy industries > 500 kW
For large scale industries -
For Heavy industries -

 Municipal – This load is for street lighting and remains practically
constant throughout night.
Demand factor = 100 %;
Diversity factor = 1;
Load factor = 25 – 30 % (for street lights).
Street lights are required mainly at night but there may be the
small load of traffic signals throughout the day also. Another type of
municipal load is for water supply and drainage.

 Agricultural – This type of load is required for supplying water for
irrigation by means of suitable pumps, driven by electric motors.
Demand factor = 90 - 100 %;
Diversity factor = 1 – 1.5;
 Other Loads – Apart from above mentioned loads, there are other loads
such as bulk supplies, special industries such as paper, textile etc.
traction and government loads which have their own peculiar
characteristics.

VARIOUS FACTORS RELATED TO
LOAD
 CONNECTED LOAD:
The sum of the continuous rating of all the equipment's
connected to supply system.
e.g.: - If a consumer has connections of five 100 Watts lamps
and a power point of 500 Watts, then the connected load of the consumer
is –
(5 * 100) + 500 = 1000 Watts.
The sum of the connected load of all the consumers is the
connected load to the power station.

 Maximum Demand:
It is the greatest demand of the load on the power station during
a given period. The load on the power station varies from time to time.
The maximum demand of all the demands that have occurred
during a given period is maximum demand. The station should be capable
of meeting the maximum demand.

Power factor:
Power factor is the ratio of real power P to the apparent power S or
the cosine of angle between voltage and current in an AC circuit and is
denoted by cos Ø.
Fig. (a) Phasor Diagram Fig. (b) Power Triangle

TIME OF DAY (TOD) TARIFF
What is Time of Day structure?
Time of Day (or TOD) tariff is a tariff structure in which different
rates are applicable for use of electricity at different time of the day. It
means that cost of using 1 unit of electricity will be different in mornings,
noon, evenings and nights. This means that using appliances during certain
time of the day will be cheaper than using them during other times.

Why Time of Day structure?
Electricity grids can be compared to road or highway that can
accommodate only a certain number of vehicles at a time. During peak hours
highways are jammed, similarly during peak hours, electricity grids are
jammed. Drive on highway during off peak hours is like a breeze; similarly
flow of electricity during off peak hours is a breeze. What if people are
charged differently for using highways during different times and also charged
as per size of their vehicles. People with either prefer to go through highway at
a time when traffic is less (off peak) or would like to use a two-wheeler.
Similarly, with TOD tariff, people will either switch to a time when prices are
less or will start using efficient appliances (with lesser electricity
consumption).

Part (B)
Cables, Conductors &
Bus – Bars

CABLES
• INTRODUCTION -
Cable is a general name given to an insulated conductor or group
of conductors and is extensively used in indoor and outdoor distribution
(especially underground) systems.
Nearly 50 – 60 % of total electrical faults occur in overhead
transmission systems, because the wires run overhead which are bare
conductors. This could be brought down to as low as 10 – 20 % if
underground insulated cable system is used.

 PROPERTIES OF INSULATION USED IN UNDERGROUND CABLES:
-
1) Insulation should be having high specific resistance and its
material should be tough but flexible for handling, laying and
bending.
2) The insulation should be corrosion and erosion proof. It should
not get attacked by acids, alkalis, and etching materials.
3) It should be fire proof.
4) Its dielectric strength, insulation resistance should be high, so
that it withstands heavy voltage fluctuations, overload and short
circuit currents etc.

 CLASSIFICATION OF CABLES BASED ON VOLTAGES: -
The cables are classified on the basis of voltage levels are as follows –
1) Low tension (L.T.) cables up to 1.1 kV
2) High tension (H.T.) cable up to 11 kV
3) Super tension cable (S.T. cable up to 33 kV
4) Extra High tension (E.H.T.) cable up to 66 kV
5) Oil filled and gas pressure cables up to 132 kV
6) Radium cured XLPE cables up to 275 kV

CONSTRUCTION OF CABLES
Fig. Diagram showing Armored PVC Insulate Cable

FACTORS CONSIDERED FOR
SELECTION OF CABLES
1. Current Rating of Cable Or Current Carrying Capacity –
Current carrying capacity is an important factor involved in selection of
cables. Current carrying capacity of cable should be checked for permissible
voltage drop and short circuit capacity.
IS: 732 – 1963 permits variation of + 5 % for low and medium voltages
and + 12 % for high voltages. This variation is with respect to declared voltage in
consumer premises.

Voltage drop in 3 ф A.C. circuits –
For calculating voltage drop, line current multiplied by resistance
of cable for any length may be taken.
3. Short Circuit & Earth Fault Rating of Cables –
The cable should be so selected that it should be able to withstand
the short circuit current initially up to the period the protective devices
like circuit breakers, MCB’s etc. operates. This time can be a maximum of
2 seconds.

CROSS LINKED POLYETHYLENE
(XLPE) CABLES
In these cables, insulation material is treated automatically for
having condensed and micro organized molecular structure in such a
manner that all unwanted voids in it are eliminated. These voids actually
carry of all sorts of impurities in any insulating materials like, water,
moisture. Moisture entrapped in the insulation is responsible for growth
of water sponsored electrochemical stress and thus causes premature
cable failures.

XLPE Steam Cured Cables
Fig. Steam Cured XPLE Cable

Best quality EC grade aluminium conductor is provided with
extruded semiconducting XPLE shield. This shield is surrounded with
unfilled cross linked thermosetting Polyethylene insulation. The insulator
molecules are bonded on a 3 dimensional network after the Polyethylene
has undergone vulcanizing under high pressure steam, in presence of
chemical cross linking agent. This insulation is now surrounded with
XPLE shield which is further covered with copper shielding tape lapped.
All 3 – cores are wrapped with taped PVC inner sheath which is further
wrapped with galvanized steel armor wire. Remaining space is filled with
plastic cable fillers. And the entire assembly is covered then with PVC
jacket.

XLPE Radium Cured Cables
Here the cross linking is done by radium curing process instead of steam
curing. In any cross linking process, the main factors which influence the quality
of vulcanization are temperature, time and pressure which are common
parameters in any chemical reaction. The conventional steam cross linking
process uses mainly saturated steam for heating and pressurizing media for the
dielectric. In such a system, steam being at a comparatively high pressure and
high temperature diffuses into the cable insulation during the cross linking
process. Upon cooling, this diffused moisture supersaturated and consequently
condenses and form micro voids.
These micro voids are generally occurring in the central region of the
insulation wall. Radium cured cables are free from such voids and cavities.

METHODS OF LAYING UNDERGROUND
CABLES
1. DIRECT LAYING: -
Fig. Direct Laying system of Cables

Advantages –
1) Simple and less costly method.
2) Gives the best conditions for dissipating the heat generated in cables.
3) It is a clean and safe method as the cable is invisible and free from external
disturbances.
Disadvantages –
1) Extension of load is possible only by a completely new excavation which may
cost as much as the original work.
2) The alterations in the cable network cannot be made easily.
3) The maintenance cost is very high.
4) Localization of fault is difficult.
5) It cannot be used in congested areas where excavation is expensive and
inconvenient.

2. DRAW – IN SYSTEM: -
Fig. Draw - In system of Laying of Cables

Advantages –
1) Repairs, alterations or additions to the cable network can be made
without opening the ground.
2) As the cables are not armored, therefore, joints become simpler and
maintenance cost is reduced simultaneously.
3) There are very less chances of fault occurrences due to strong
mechanical protection provided by the system.
Disadvantages –
1) The initial cost is very high.
2) The current carrying capacity of cable is reduced due to close
grouping of cables and unfavorable conditions for dissipation of heat.

3. SOLID SYSTEMS: -
In this system, the cable is laid in open pipes or through dug out in earth
along the cable route. The troughing is of cast iron, stoneware, asphalt or treated
wood. After the cable is laid in positions, the troughing is filled with bituminous or
asphaltic compound and covered over. Cable laid in this manner is usually plain
lead covered because troughing affords good mechanical protection.
Disadvantages –
1) It is expensive than direct laying system.
2) It requires skilled labor and favorable weather conditions.
3) Due to poor heat dissipation facilities, the current carrying capacity of the
cable is reduced.
In view of these disadvantages, this method of laying underground cables
is rarely used now – a – days.

Table 1. Showing Recommended Depth of Trench for Laying of cables

Table 2. Showing Comparison between Different Methods of Laying of Cables

TESTING OF CABLES
The cables are tested both before and after the installation. Before
installation, the cables are tested for the following.
1. Continuity test
2. Insulation test
Both the tests can be performed with the help of megger. The values of
inductance, capacitance and resistance of the cables are also recorded before
installation. These values help in locating faults in the future when the cables are
installed.

When there are faults in the installed cables, then quick
identification of location of the fault is very much necessary for quick
repairs. For this the following tests are performed on the cables.
1) Murray Loop test
2) Fall of Potential Test
3) D.C. charge and Discharge test

1. MURRAY LOOP TEST: -
In this test, the principle of Whetstone’s bridge is used to locate the
ground faults. In ground faults, one or more cable cover gets earthed. This is the
most accurate test.
In this test, one sound return wire of the same cross – section as that of
the cable is required. The connections are as shown in figure below.
The perfectly sound cable is looped with faulty cable. The balanced of
the bridge is obtained by varying the resistance.

Fig. Murray Loop Test setup

2. FALL OF POTENTIAL TEST: -
Fig. Fall of Potential Test

3. D.C. CHARGE AND DISCHARGE TEST: -
This test is used to locate discontinuity in the core of the cable, with
high resistance to earth.
Fig. Charge and Discharge Test

CURRENT RATING OF CABLES
Once all the thermal resistances are known then the current carrying
capacity of cable can be determined. When the cables carry excessive current, they
get heated up. It is not advisable to operate the cable at excessively high
temperature because of the following reasons:
1) Due to high temperature, oil in the oil filled cables gets expanded and this may
lead to bursting of the sheath.
2) High temperature can cause unusual expansion which leads to the formation of
voids. Such voids may leads to ionization and finally lead to insulation failure.
3) The dielectric losses increase with temperature which also can lead to
breakdown of insulation.
Hence the cable must be operated at a current less than the
maximum current carrying capacity of the cable.

The current rating of cable is dependent on following factors:
1) The maximum permissible temperature at which conductor
insulation can be operated.
2) Heat dissipation arrangement through the cable.
3) The ambient conditions as well as conditions at the time of
installation.
The current carrying capacity can be expressed as:

TRANSMISSION LINE CONDUCTORS
 Factors to be considered while selecting Transmission Line: -
1) Type and size of conductors
2) Voltage level
3) Line regulation and control of voltage.
4) Efficiency of transmission.
5) Corona loss.
6) Power flow capability and stability.
Contd ….

7) Requirement of compensation.
8) Levels of faults at various bus bars and requirement of new
circuit breakers.
9) Grounding needs.
10) Protection schemes for new lines.
11) Co-ordination of insulation.
12) Mechanical design aspects which includes stress and sag
calculations, composition of conductors, spacing of conductors
and configuration for insulators.
13) Design of power system structures.
14) Economic aspects.
Contd ….

 TYPES OF CONDUCTORS: -
1) They should have the low weight.
2) They should have high tensile and fatigue strength.
3) They must have high conductivity.
4) They should have low coefficient of expansion, low corona loss.
5) They should have less resistance and low cost.
Requirements -

The different types of aluminium conductors used in power systems are –
1) AAC – All Aluminium Conductors
2) AAAC – All Aluminium Alloy Conductors
3) ACSR – Aluminium Conductor with Steel Reinforcement
4) ACAR – Aluminium Conductor with Alloy reinforcement

ALL ALUMINIUM CONDUCTOR (AAC)
Fig. All Aluminium Conductors (AAC)
Due to increasing cost of copper, aluminium is used in transmission
systems. For a specific resistance, cross sectional area of aluminium conductor is
greater than that of copper while its weight is about 50 % of that of copper
conductor. This makes transportation and errection of each conductor economical.
Corona effect is reduced due to increased diameter of conductor. These conductors
are more used in distribution where transmission lines are short and voltages are
lower.

ALUMINIUM CONDUCTOR WITH STEEL
REINFORCEMENT (ACSR)
The mechanical strength that is obtained from conductors made up
from all aluminium. This difficulty can be overcome by adding steel core to
the conductor.
Fig. Aluminium Conductor with
Steel Reinforcement (ACSR)
Conductor

The ACSR conductors are more commonly used as they have
following advantages -
1) Due to high mechanical strength and tensile strength, the line span can
be increased. The sag is small. So shorter supports are required for
line. It is also possible to have longer spans for given sag. Due to
smaller supports, breakdown possibility is low. Insulators and other
fittings needed are also low.
2) They have low corona loss.
3) Skin effect is less.
4) Inexpensive as compared to copper conductors having equal resistance
without reduction in efficiency, useful life span and durability.

ALL ALUMINIUM ALLOY CONDUCTOR
(AAAC)
The conductor made from aluminium
alloys is suitable in urban areas as they provide
better strength and conductivity.
One of the alloys of aluminium is known
as silmalec which contains 0.5 % silicon, 0.5 %
of magnesium and rest of aluminium. Due to this
there is an improvement in conductivity and
mechanical strength.

ALUMINIUM CONDUCTOR WITH ALLOY
REINFORCEMENT (ACAR)
In such conductor, the central core is made up from aluminium alloy
which is surrounded by layers of aluminium conductors. The conductivity is better
and strength to weight ratio is equal to ACSR conductor having same diameter. As
compared to ACSR conductors, ACAR conductor is smaller in size and lower in
weight for the same electrical capacity.

BUS – BAR TYPES
Bus – Bars are important components of the substation. The
outdoor bus – bars are of two types viz. the rigid type or the strain type.
Voltage Size
33 kV 40 mm
66 kV 65 mm
132 kV 80 mm
220 kV 80 mm
400 kV 100 mm
The material used in case of rigid type bus bars is aluminium pipes.
The general sizes of pipes commonly used for voltages are given below.

In case of strain type arrangement, material used is ACSR
(Aluminium Conductor with Steel Reinforcement) and All Aluminium
conductors. For high ratings of Bus bars bundled conductors are used. The
commonly used sizes are as below.
66 kV 37/2.79 mm ACSR
132 kV 37/4.27 mm ACSR
220 kV 61/3.99 mm ACSR
400 kV 61/4.27 mm ACSR in duplex

COMPARISON BETWEEN OVERHEAD
AND UNDERGROUND SYSTEM
Sr. No. Parameter Overhead System Underground System
1 Public Safety Less More
2 Initial Cost Less More
3 Flexibility More Less
4 Working Voltage Up to 400 kV Up to 220 kV*
5 Maintenance Cost More Less
6
Frequency of Faults or
Failures
More Less
7 Fault Location and Repairs Easy Difficult
8
Damage Due to Lightning
and Thunderstorm
Yes No
9
Interference to
Communication Circuits
Yes No

BUS-BAR ARRANGEMENTS
When a number of generators or feeders operating at the same
voltage have to be directly connected electrically, bus-bars are used as the
common electrical component.
There are several types of bus bar arrangements. To decide a
particular arrangement, various factors are to be considered. These factors
are system voltage, flexibility, reliability of supply, position of substation
in system and cost.

Following points are to be considered while selecting bus bar
arrangement.
1) The bus bar arrangement should be simple.
2) Without interruption of supply, the maintenance should be
possible.
3) It should not provide any danger to operating personnel while
doing maintenance or repair.
4) The layout should accommodate the future expansion with increase
in load demand.
5) It should be economical one in view of reliability and continuity of
supply.

 SINGLE BUS-BAR SYSTEM -
Fig. Single Bus – Bar Arrangement with 2 Incoming and 2 Outgoing Lines

The single bus bar system has the simplest design and is used for
power stations.
The chief advantages of this type of arrangement are low initial
cost, less maintenance and simple operation.
• Disadvantages.
1. The bus-bar cannot be cleaned, repaired or tested without de-
energizing the whole system.
2. If a fault occurs on the bus-bar itself, there is complete interruption
of supply.
3. Any fault on the system is fed by all the generating capacity,
resulting in very large fault currents.

 SINGLE BUS-BAR SYSTEM WITH SECTIONALIZATION -
Fig. Single Bus-Bar System with Sectionalization

In large generating stations where several units are installed, it is
a common practice to sectionalize the bus so that fault on any section of
the bus-bar will not cause complete shutdown.
Three principal advantages are claimed for this arrangement.
Firstly, if a fault occurs on any section of the bus-bar, that section
can be isolated without affecting the supply to other sections.
Secondly, if a fault occurs on any feeder, the fault current is much
lower than with un-sectionalized bus-bar. This permits the use of circuit
breakers of lower capacity in the feeders.
Thirdly, repairs and maintenance of any section of the bus-bar
can be carried out by de-energizing that section only, eliminating the
possibility of complete shut-down.

 DUPLICATE BUS-BAR SYSTEM -
Fig. Duplicate Bus-Bar System with Single Breaker

In large stations, it is important that breakdowns and maintenance
should interfere as little as possible with continuity of supply. In order to
achieve this objective, duplicate bus-bar system is used in important stations.
Such a system consists of two bus-bars, a “main bus-bar’’ and a “spare” bus-
bar.
• Advantages -
1) If repair and maintenance it to be carried on the main bus, the supply need
not be interrupted as the entire load can be transferred to the spare bus.
2) The testing of feeder circuit breakers can be done by putting them on spare
bus-bar, thus keeping the main bus-bar undisturbed.
3) If a fault occurs on the bus-bar, the continuity of supply to the circuit can
be maintained by transferring it to the other bus-bar.

 MESH SCHEME (RING MAIN) -
Fig. Mesh or Ring Bus Scheme

advantages of this scheme are -
1) As each circuit is double feed, opening of any one breaker for
repair and maintenance does not interrupt supply of other
circuits.
2) It is cheaper than double bus or main and transfer bus scheme.
3) The breaker maintenance is possible without supply interruption.
4) No separate bus protection is required as all sections of
conductors in the station are protected properly.
But the scheme suffers some limitations in its operation and
safety and is not well suited in developing systems.

EID_Unit 01_000.ppt

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