Design of substation (with Transformer Design)

NAME : SAYAN SARKAR
ROLL NO. : 13001616049
REGISTARTION NO. : 161300110709 of 2016-2017
DEPARTMENT : ELECTRICAL ENGINEERING
SEMESTER : 8th
SUBJECT : ELECTRICAL SYSTEM LAB II
SUBJECT CODE : EE 882

GROUP - 3
GROUP MEMBERS:
1) SPANDAN PAUL (13001616037)
2) SOURADEEP MULLICK (13001616038)
3) SIDDHARTHA BASU (13001616042)
4) SHAKYA ACHARYA (13001616046)
5) SAYANTAN KUMAR CHATTOPADHYAY (13001616047)
6) SAYAN SARKAR (13001616049)
7)SAURAV BASAK (13001616051)

INDEX
❖Introduction
❖Classification of Substations
❖Steps of Designing of Substation
❖Single Line Diagram
❖Layout of Substation
❖Substation Transformers
➢ Generator Transformer
➢ Station Transformer
➢ Unit Auxiliary Transformer
➢ Power Transformer
❖Design of Transformer
❖Substation Protection Equipment
➢ Potential Transformer
➢ Current Transformer
➢ Circuit Breaker
➢ Electrical Insulators
➢ Isolator
➢ Lightning Arrester
➢ Relay
❖ Substation Bus Configuration
❖ Primary Design and Protection of 110KV Substation
❖ System parameters of 400/220 KV substation
❖ Conclusion
❖ References

INTRODUCTION
Substations serve as sources of energy supply for the local areas of distribution in which these
are located. Their main functions are to receive energy transmitted at high voltage from the
generating stations, reduce the voltage to a value appropriate for local distribution and provide
facilities for switching. Some substations are simply switching stations where different
connections between various transmission lines are made, others are converting substation
which either convert ac into dc or vice versa or convert frequency from higher to lower or vice
versa. Substations have some additional functions. They provide points where safety devices
may be installed to disconnect equipment or circuit in the event of fault. Voltage on the
outgoing distribution feeders can be regulated at a substation. A substation is convenient place
for installing synchronous condensers at the end of the transmission line for the purpose of
improving power factor and make measurements to check the operation of the various parts of
the power system.
Substations generally have switching, protection and control equipment, and transformers. 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 distribution circuits. Substations themselves do not usually have
generators, although a power plant may have a substation nearby. Other devices such as
capacitors and voltage 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 several 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 metallic fence, it must be properly grounded to protect people from
high voltages that may occur during a fault in the network. Earth faults at a substation can cause
a ground potential rise. Currents flowing in the Earth’s surface during a fault can cause metal
objects to have a significantly different voltage than the ground under a person’s feet; this touch
potential presents a hazard of electrocution. The main issues facing a power engineer are
reliability and cost. A good design attempts to strike a balance between these two, to achieve
reliability without excessive cost. The design should also allow expansion of the station, when
required.

CLASSIFICATION OF SUBSTATION
The substations may be classified in numerous ways such as on the basis of
• nature of duties
• service rendered
• operating voltage
• importance and
• design.
Classifications of Substations on the Basis of Nature of Duties
1. Step-Up or Primary Substations
Such substations are usually associated with generating stations. The generated voltage, which
is usually low (3.3,6.6,11 or 33 kV), is stepped up to primary transmission voltage so that huge
blocks of power can be transmitted over long distances to the load centres economically.
2. Primary Grid Substations
Such substations are located at suitable load centres along the primary transmission lines. In
these substations, the primary transmission voltage is stepped down to different suitable
secondary voltages. The secondary transmission lines are carried over to the secondary
substations situated at the load centres where the voltage is further stepped down to sub-
transmission or primary distribution voltages.
3. Step-Down or Distribution Substations
Such substations are located at the load centres, where the sub- transmission/primary
distribution voltage is stepped down to secondary distribution voltage (415/240 V). These are
the substations which feed the consumers through distribution network and service lines.
Classifications of Substations on the Basis of Service Rendered
1. Transformer Substations
Transformers are installed on such substations to transform the power from one voltage level
to another level as per needs.
2. Switching Substations
Such substations are meant for switching operation of power lines without transforming the
voltage. At Such substations different connections are made between various transmission
lines.
3. Converting Substations
Such substations are meant for either converting ac to dc or vice versa or converting frequency
from higher to lower or vice versa.

Classifications of Substations on the Basis of Operating Voltage
1. High Voltage Substations
HV Substations involving voltages between 11 kV and 66 KV.
2. Extra High Voltage Substations
EHV Substation involving voltages between 132 kV and 400 kV.
3. Ultra-High Voltage Substations
UHV Substations operating on voltage above 400 kV.
Classifications of Substations on the Basis of Importance
1. Grid Substations
These are the substations from where hulk power is transmitted from one point to another point
in the grid. These are important because any disturbance in these substations may cause the
failure of the grid.
2. Town Substations
These substations step-down the voltages at 33/11 kV for further distribution in the towns and
any failure in such substations results in the failure of supply for whole of the town.
Classifications of Substations on the Basis of Design
1. Indoor Type Substations
In such substations the apparatus is installed within the substation building. Such Substations
are usually for a voltage up to 11 kV but can be erected for the 33 kV and 66 kV when the
surrounding atmosphere is contaminated with impurities such as metal Corroding gases and
fumes, conductive dust etc.
2. Outdoor Substations
These substations are further subdivided into:
• Pole Mounted Substations
Such substations are erected for distribution of power in localities. Single stout pole or H-
pole and 4-pole structures with suitable platforms are employed for transformers of
capacity up to 25 kVA,125 KVA and above 125 kVA（but up to 250 kVA) respectively.
• Foundation Mounted Substations
For transformers of Capacity above 250 kVA the transformers are too heavy for pole
mounting. Such substations are usually for voltages of 33,000 volts and above.

STEPS OF DESIGNING OF SUBSTATION
Location Selection
Selection of the location of a substation must consider many factors. Sufficient land area is
required for installation of equipment with necessary clearances for electrical safety, and for
access to maintain large apparatus such as transformers.
Where land is costly, such as in urban areas, gas insulated switchgear may save money overall.
Substations located in coastal areas affected by flooding and tropical storms may often require
an elevated structure to keep equipment sensitive to surges hardened against these elements.
The site must have room for expansion due to load growth or planned transmission additions.
Environmental effects of the substation must be considered, such as drainage, noise and road
traffic effects.
A grounding (earthing) system must be designed. The total ground potential rise, and the
gradients in potential during a fault (called “touch” and “step” potentials), must be calculated
to protect passers-by during a short-circuit in the transmission system.
The substation site must be reasonably central to the distribution area to be served. The site
must be secure from intrusion by passers-by, both to protect people from injury by electric
shock or arcs, and to protect the electrical system from mis operation due to vandalism.
Design Diagrams
The first step in planning a substation layout is the preparation of a one-line diagram, which
shows in simplified form the switching and protection arrangement required, as well as the
incoming supply lines and outgoing feeders or transmission lines. It is a usual practice by many
electrical utilities to prepare one-line diagrams with principal elements (lines, switches, circuit
breakers, transformers) arranged on the page similarly to the way the apparatus would be laid
out in the actual station.
In a common design, incoming lines have a disconnect switch and a circuit breaker. In some
cases, the lines will not have both, with either a switch or a circuit breaker being all that is
considered necessary. A disconnect switch is used to provide isolation, since it cannot interrupt
load current. A circuit breaker is used as a protection device to interrupt fault currents
automatically, and may be used to switch loads on and off, or to cut off a line when power is
flowing in the ‘wrong’ direction. When a large fault current flows through the circuit breaker,
this is detected through the use of current transformers. The magnitude of the current
transformer outputs may be used to trip the circuit breaker resulting in a disconnection of the
load supplied by the circuit break from the feeding point. This seeks to isolate the fault point
from the rest of the system, and allow the rest of the system to continue operating with minimal
impact. Both switches and circuit breakers may be operated locally (within the substation) or
remotely from a supervisory control centre.
With Overhead Transmission Lines (OHTLs), the propagation of lightning and switching
surges can cause insulation failures into substation equipment. Line entrance surge arrestors

are used to protect substation equipment accordingly. Insulation Coordination studies are
carried out extensively to ensure equipment failure (and associated outages) is minimal.
Once past the switching components, the lines of a given voltage connect to one or more buses.
These are sets of busbars, usually in multiples of three, since three-phase electrical power
distribution is largely universal around the world.
The arrangement of switches, circuit breakers and buses used affects the cost and reliability of
the substation. For important substations a ring bus, double bus, or so-called “breaker and a
half” setup can be used, so that the failure of any one circuit breaker does not interrupt power
to other circuits, and so that parts of the substation may be de-energized for maintenance and
repairs. Substations feeding only a single industrial load may have minimal switching
provisions, especially for small installations.
Once having established buses for the various voltage levels, transformers may be connected
between the voltage levels. These will again have a circuit breaker, much like transmission
lines, in case a transformer has a fault (commonly called a “short circuit”).
Along with this, a substation always has control circuitry needed to command the various
circuit breakers to open in case of the failure of some component.
Automation
Early electrical substations required manual switching or adjustment of equipment, and manual
collection of data for load, energy consumption, and abnormal events. As the complexity of
distribution networks grew, it became economically necessary to automate supervision and
control of substations from a centrally attended point, to allow overall coordination in case of
emergencies and to reduce operating costs. Early efforts to remote control substations used
dedicated communication wires, often run alongside power circuits. Power-line carrier,
microwave radio, fiber optic cables as well as dedicated wired remote-control circuits have all
been applied to Supervisory Control and Data Acquisition (SCADA) for substations. The
development of the microprocessor made for an exponential increase in the number of points
that could be economically controlled and monitored. Today, standardized communication
protocols such as DNP3, IEC 61850 and Modbus, to list a few, are used to allow multiple
intelligent electronic devices to communicate with each other and supervisory control centers.
Distributed automatic control at substations is one element of the so-called smart grid.

Insulation
Switches, circuit breakers, transformers and other apparatus may be interconnected by air-
insulated bare conductors strung on support structures. The air space required increases with
system voltage and with the lightning surge voltage rating. For medium-voltage distribution
substations, metal-enclosed switch gear may be used and no live conductors exposed at all. For
higher voltages, gas-insulated switch gear reduces the space required around live bus. Instead
of bare conductors, bus and apparatus are built into pressurized tubular containers filled with
sulphur hexafluoride (SF6) gas. This gas has a higher insulating value than air, allowing the
dimensions of the apparatus to be reduced. In addition to air or SF6 gas, apparatus will use
other insulation materials such as transformer oil, paper, porcelain, and polymer insulators.
Structure
Outdoor, above-ground substation structures include wood pole, lattice metal tower, and
tubular metal structures, although other variants are available. Where space is plentiful and
appearance of the station is not a factor, steel lattice towers provide low-cost supports for
transmission lines and apparatus. Low-profile substations may be specified in suburban areas
where appearance is more critical. Indoor substations may be gas-insulated switchgear (at high
voltages), or metal-enclosed or metal-clad switchgear at lower voltages. Urban and suburban
indoor substations may be finished on the outside so as to blend in with other buildings in the
area.
A compact substation is generally an unmanned outdoor substation being put in a small
enclosed metal container in which each of the electrical equipment is located very near to each
other to create a relatively smaller footprint size of the substation.

SINGLE LINE DIAGRAM
Fig: Single Line Diagram of 220KV Substation, MTPS, DVC

Fig: Single Line Diagram of 220KV Line Protection, MTPS, DVC

LAYOUT OF SUBSTATION
The layout of the substation is very important since there should be a security of supply.
In an ideal substation all circuits and equipment would be duplicated such that following a
fault, or during maintenance, a connection remains available. Practically this is not feasible
since the cost of implementing such a design is very high.
Methods have been adopted to achieve a compromise between complete security of supply and
capital investment.
There are four categories of substation that give varying securities of supply:
• Category 1 – No outage is necessary within the substation for either maintenance or
fault conditions.
• Category 2 – Short outage is necessary to transfer the load to an alternative circuit for
maintenance or fault conditions.
• Category 3 – Loss of a circuit or section of the substation due to fault or maintenance.
• Category 4 – Loss of the entire substation due to fault or maintenance.
Different Layouts for Substations
1.Single Busbar
The general schematic for such a substation is shown in the figure below.
With this design, there is an ease of operation of the substation. This design also places
minimum reliance on signalling for satisfactory operation of protection. Additionally, there is
the facility to support the economical operation of future feeder bays.
Such a substation has the following characteristics:
1. Each circuit is protected by its own circuit breaker and hence plant outage does not
necessarily result in loss of supply.
2. A fault on the feeder or transformer circuit breaker causes loss of the transformer and
feeder circuit, one of which may be restored after isolating the faulty circuit breaker.
3. A fault on the bus section circuit breaker causes complete shutdown of the substation.
All circuits may be restored after isolating the faulty circuit breaker. A busbar fault
causes loss of one transformer and one feeder.
4. Maintenance of one busbar section or isolator will cause the temporary outage of two
circuits.
5. Maintenance of a feeder or transformer circuit breaker involves loss of the circuit.

6. Introduction of bypass isolators between busbar and circuit isolator allows circuit
breaker maintenance facilities without loss of that circuit.
2. Mesh Substation
The general layout for a full mesh substation is shown in the schematic below.
The characteristics of such a substation are as follows. Operation of two circuit breakers is
required to connect or disconnect a circuit, and disconnection involves opening of a mesh.
Circuit breakers may be maintained without loss of supply or protection, and no additional
bypass facilities are required.
Busbar faults will only cause the loss of one circuit breaker. Breaker faults will involve the loss
of a maximum of two circuits. generally, not more than twice as many outgoing circuits as in
feeds are used in order to rationalize circuit equipment load capabilities and ratings.
3.One and a half Circuit Breaker layout
The layout of a 1 1/2 circuit breaker substation is shown in the schematic below.
The reason that such a layout is known as a 1 1/2 circuit breaker is due to the fact that in the
design, there are 9 circuit breakers that are used to protect the 6 feeders. Thus, 1 1/2 circuit
breakers protect 1 feeder.
Some characteristics of this design are:
1. There is the additional cost of the circuit breakers together with the complex
arrangement.
2. It is possible to operate any one pair of circuits, or groups of pairs of circuits.
3. There is a very high security against the loss of supply.

PRINCIPLE OF SUBSTATION LAYOUTS
Substation layout consists essentially in arranging a number of switchgear components in an
ordered pattern governed by their function and rules of spatial separation.
Spatial Separation
1. Earth Clearance – this is the clearance between live parts and earthed structures, walls,
screens and ground.
2. Phase Clearance – this is the clearance between live parts of different phases.
3. Isolating Distance – this is the clearance between the terminals of an isolator and the
connections there.
4. Section Clearance – this is the clearance between live parts and the terminals of a work
section. The limits of this work section, or maintenance zone, may be the ground or a
platform from which the man works.
Separation of Maintenance Zones
Two methods are available for separating equipment in a maintenance zone that has been
isolated and made dead:
1. The provision of a section clearance
2. Use of an intervening earthed barrier
The choice between the two methods depends on the voltage and whether horizontal or
vertical clearances are involved. A section clearance is composed of the reach of a man, taken
as 8 feet, plus an earth clearance. For the voltage at which the earth clearance is 8 feet, the
space required will be the same whether a section clearance or an earthed barrier is used.
HENCE:
Separation by earthed barrier = Earth Clearance + 50mm for barrier + Earth Clearance
Separation by section clearance = 2.44m + Earth clearance
For vertical clearances it is necessary to take into account the space occupied by the equipment
and the need for an access platform at higher voltages. The height of the platform is taken as
1.37m below the highest point of work.
Establishing Maintenance Zones
Some maintenance zones are easily defined and the need for them is self-evident as is the case
of a circuit breaker. There should be a means of isolation on each side of the circuit breaker,
and to separate it from adjacent live parts, when isolated, either by section clearances or earth
barriers.
Electrical Separations
Together with maintenance zoning, the separation, by isolating distance and phase clearances,
of the substation components and of the conductors interconnecting them constitute the main
basis of substation layouts.
There are at least three such electrical separations per phase that are needed in a circuit:
1. Between the terminals of the bus bar isolator and their connections.
2. Between the terminals of the circuit breaker and their connections.
3. Between the terminals of the feeder isolator and their connections.

SUBSTATION TRANSFORMERS
1. GENERATOR TRANSFORMER
The generator transformer is the largest transformer on a power station and connects
the generator output to the switchyard. There is a generator transformer for each generating
unit usually. Generation of electrical power in low voltage level is very much cost effective.
Theoretically, this low voltage level power can be transmitted to the receiving end. This low
voltage power if transmitted results in greater line current which indeed causes more line losses.
But if the voltage level of a power is increased, the current of the power is reduced which
causes reduction in ohmic or I2
R 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 level power must be stepped up for efficient electrical
power transmission. This is done by step up transformer at the sending side of the power system
network.
Fig. Single line diagram showing the location of GT
So, GT’s are step up, unilateral transformers by nature, designed to work 24x7 at full load.
The list of standard equipment required for the design are:
• Marshalling kiosk and signalization
• “Buchholz” relay with two contacts
• Gas relay switch adjusting
• Oil level indicator
• Dehydration with silicone gel
• Safety valve
• Plate tech. characteristic
• The four pockets for thermometers
• Oil drain valve
• Two connections for vacuum and filtering
• Thermostats to control the cooling system
• Indicator of the tap-changer switch
• Sparks gap for bushing
• Two connections for grounding
• Two-foot pedals for mounting transformer
• Thermal Imaging for installation on the transformer
GT
Fig: Generator Transformer at Mejia
Thermal Power Plant

• The current transformers for protection of tank
• Current transformers for protection and measurement
• Automatic valve to prevent oil leakage from the conservative
• Contact thermometer for temperature indication
We find that the minimum cost of the transformer is obtained for the following values of the
design variables:
EMF constant = 0.47; Window height/width ratio = 4.4.
The design details of the optimal machine are given below:
Rating:
MVA-rating of the primary /secondary: 120/160/200
Nominal power factor (assumed) = 0.85
Rated line voltage of primary/secondary/ tertiary: 13600 V / 132000 V / 6600 V
Nominal frequency = 50 Hz.
Connection: Ynd11 Delta winding has been chosen to suppress the triple harmonics as well as
the zero sequence components.
Conductor material: Copper;
Core material- High grade CRGOS.
Dimensions:
No of turns of the primary = 44;
No of nominal turns of the secondary = 427
Additional 10% turns for tapping in the secondary = 47
Total no of secondary turns = 474
No of turns of the tertiary = 37
Cross section of Primary/ Secondary (mm2
): 1870.6; 177.69
Net area of core iron = 0.45427 mm2 3-stepped core has been used.
But higher no of steps should be used for greater economy. It could not be used for non-
availability of data.
Current Primary/ Secondary: 5985.8 A; 568.6 A.
Chosen current density = 3.2 A/mm2
Stacking factor = 0.92;
Gross area of core iron = 0.49377 mm2
Diameter of the core circle = 2.203046×10-02
m
Length of the core sides, m: 0.783; 0.612; 0.367
Area of the window = 1.3195 mm2
;
Window height/width, m; 2.4095; 0.54761
Distance between core centers = 1.3116 m;
Width/height of yoke, m: 0.783; 0.63062
Total length/height of core = 3.6081 m / 3.6707 m
Performance evaluation:
Iron loss = 127908 W;
% Iron loss = 9.0715×10-02

Copper loss = 386165 W;
% Copper loss = 0.27388;
Total % loss = 0.36459
Mean length of turn (m) of Primary/Secondary/: 3.0614; 3.9216
Resistance of Primary/Secondary, Ω: 1.5843E-03 0.20733; 1.5967×10-02
Efficiency at full load & 0.85 lagging p.f = 0.99573
Maximum efficiency of 0 .99631 occurs at a % load of 57.552%
The magnetising current = 0.19128 %;
The core loss current = 9.0715×10-02
%
The no load current = 0.2117 %
Leakage reactance between primary and secondary = 3.7288 %
Voltage regulation at rated power & p.f. between primary and secondary = 2.1967 %
Dimension of the tank (m) length, width, height: 1.712 x 4.437 x 4.171
Total no. of tubes (75 x 25 mm, elliptical) required for the radiators = 3005
No of radiator wings to be chosen accordingly.
Volume, weight and cost:
The weight of tank = 38531 Kg; The cost of tank = Rs. 2311847 /-
The volume of oil = 31672 litter; The cost of oil: Rs. 1900335 /-
Volume of iron = 6.5618 m3
; Weight of iron = 50198; Cost of iron = Rs. 8031674 /-;
Specifications of Generator Transformer (GT) at MTPS
Type of cooling ONAN/ONAF/OFAF
Rating HV (MVA) 120/160/200
Rating LV (MVA) 120/160/200
No load voltage HV (kV) 242.494
No load voltage LV (kV) 21
Line current HV (amps) 824.79
Line current LV (amps) 9523.8
Temperature rise oil (°C) 40 (Over ambient of 50°C)
Temperature rise winding (°C) 45 (Over ambient of 50°C)
Phase 3
Frequency (Hz) 50
Connection symbol YNd11

2. STATION TRANSFORMER:
Certain transformers are required for commencing operation of newly constructed generating
units. Such transformers are called Station transformer which receives power from the grid
itself and feed power to the power station distribution system, when plant is not generating any
power. Rated HV side corresponds to rated value of voltage on the outer busbars, while rated
LV side corresponds to rated value of voltage on the auxiliary bus. These transformers are
placed outdoors.
Fig: Station Transformer at MTPS
Specifications of Station Service Transformer (SST) at MTPS
Type of cooling ONAF/ONAN
Rating HV (MVA) 16/12.50
Rating LV (MVA) 16/12.50
No load voltage HV (kV) 11
No load voltage LV (kV) 3.45
Line current HV (amps) 839.78/656.08
Line current LV (amps) 2677.57/2091.85
Temperature rise oil (°C) 40
Temperature rise winding (°C) 45
Phase 3
Frequency (Hz) 50
Connection symbol Dyn1
Impedance volts % HV-LV 25%

3. UNIT AUUXILIARY TRANSFORMER:
The Unit Auxiliary Transformer is the Power Transformer that provides power to the auxiliary
equipment of a power generating station during its normal operation. This transformer is
connected directly to the generator out-put by a tap-off of the isolated phase bus duct and thus
becomes cheapest source of power to the generating station.
It is generally a three-winding transformer i.e. one primary and two separate secondary
windings. Primary winding of UAT is equal to the main generator voltage rating. The
secondary windings can have same or different voltages i.e. generally 11KV and or 6.9KV as
per plant layout.
The sizing of the UAT is usually based on the total connected capacity of running unit
auxiliaries i.e., excluding the stand by drives. It is safe and desirable to provide about 20%
excess capacity than calculated. The no. and recommended MVA rating of unit auxiliary
transformers are as shown in the table below:
--Unit auxiliary transformer:
MVA: 12.5/16
Manufacturer: Atlanta Electricals
Volts at no load: 15750 (H.V.)
Volts at no load: 6900 (L.V.)
Ampere line value: 458.2/586.5 (H.V.)
Ampere line value: 1045.9/1338.8 (L.V.)
Phase-3, frequency: 50 Hz.
Mass of core and windings: 14300kg.
Mass of oil: 8600kg.
Mass of heaviest package: 25000kg.
Total weight: 30,500 kg.
Specifications of Unit Auxiliary Transformer (UAT) at MTPS
Type of cooling ONAN/ONAF
Rating HV (MVA) 45/36
Rating LV (MVA) 45/36
No load voltage HV (kV) 21
No load voltage LV (kV) 11.5
Line current HV (amps) 1238.64
Line current LV (amps) 2261.87
Temperature rise oil (°C) 40 (Over ambient of 50°C)
Temperature rise winding (°C) 45 (Over ambient of 50°C)
Phase 3
Frequency (Hz) 50
Fig: UAT at MTPS

Connection symbol Dyn1
Impedance volt at 45 MVA Base HV Position on 7/LV (nor tap) – 11.5%
HV Position on 1/LV (max tap) – 10% to 13%
HV Position on 17/LV (min tap) – 10% to 13%
Insulation level (high voltage) L1 125 – AC 50
Insulation level (low voltage) L1 75 – AC 28
Core & winding (kg) 40065
Weight of Oil (kg) 25765
Total weight (kg) 85265
Transformer weight (kg) 50000
Un tanking weight (kg) 41000
4. POWER TRANSFORMER:
The use of power transformer in a switchyard is to change the voltage level. At the sending and
usually step up transformers are used to evacuate power at transmission voltage level. On the
other hand, at the receiving end step down transformers are installed to match the voltage to
sub transmission or distribution level. In many switchyards autotransformers are used widely
for interconnecting two switchyards with different voltage level (such as 132 and 220 KV)

DESIGN OF TRANSFORMER
Design of a 25 kVA, 11kV/433V, 50Hz, 3 phase, delta/star, core type, oil immersed
natural cooled distribution transformer. The transformer is provided with tapings
±𝟐. 𝟓 ± 𝟓% on the hv windings. Maximum temperature rise not to exceed 45℃ with
mean temperature rise of oil 35℃.
Core Design:
Let, k= 0.45 for 3 phase core type distribution transformers.
Voltage per turn Et= k√𝑄= 0.45√25= 2.25V
So, flux in the core ∅m= Et / (4.44×f) =
2.25
4.44×50
= 0.010135 Wb
Hot rolled silicon steel grade 92 is used. The value of flux density Bm is assumed as 1.0 Wb/m2
.
Net iron area AI= 0.010135/1.0 = 0.010135 m2
.
Using a cruciform core, AI= 0.56 d2
Diameter of circumscribing circle d = √0.010135/0.56 = 134.5 mm
a = 0.85 × d = 0.85 × 135.8 = 114.8 mm, b = 0.53 × d = 0.53 × 135.8 = 71.6 mm
The lamination is punched from 750 mm wide plates and the nearest standard dimensions are
a=114 mm and b=73 mm
Window Dimension:
The window space factor of a small rating transformer is kw = 8/(30+kV)
So, kw = 8/(30+71) = 0.195. The value assumed as kw = 0.18.
The current density in the winding is taken as 2.3 A/mm2
.
Output of Transformer Q = 3.33 f Bm kw δ Aw AI× 10−3
So, 25 = 3.33 × 50 × 1 × 0.18 × 2.3 × 108
×Aw× 0.010135 × 10−3
Or window area Aw = 0.0358 m2
Taking the ratio of height to width of window as 2.5,
Hw×Ww
2
= 35.8 × 103
or 2.5 Ww
2
= 35.8 × 103
Therefore, Width of the window Ww is 120 mm and Height of the window is 300 mm
Area of the window provided by Aw = 300 × 120 = 36 × 103
mm2
Distance between adjacent core centres D = Ww + d = 120 + 135 = 255 mm.
Yoke Design:
The area of the yoke is taken as 1.2 times that of limb.
So, flux density in yoke is = 1/1.2 = 0.833 Wb/m2
Net area of yoke = 1.2 × 10.135 × 103
= 12.16 × 103
mm2
Gross area of yoke = 12.16 × 103
÷ 0.9 = 13.5 × 103
mm2
Taking the section of yoke as rectangular
Depth of the yoke, Dy = a = 114 mm and Height of the yoke, Hy = 114 mm.

Overall Dimension of the Frame:
Height of the frame H = Hw + 2Hy = 300 + 2 × 114 = 528 𝑚𝑚.
Width of the frame W = 2D + a =2 × 255 + 114 = 624 𝑚𝑚.
Depth of the frame Dy = a = 114 mm.
L.V Winding:
Secondary line voltage =433V, Connection: Star
Secondary phase voltage Vs=433/√ 3=250V
Number of turns per phase Ts=Vs/Et=250/2.25=111
Secondary phase current, Is = (25*1000)/3*250 = 33.3A
A current density of 2.3 A/mm2
is used.
Area of secondary conductor, as=33.3/2.3=14.48 mm2
Using a bare conductor of 7.7x2.2 mm, Area of bare conductor=a=140 mm2
Current density in secondary winding ∂2=33.4/14.9=2.23 A/m2
The conductors are paper covered. The increase in dimension to be accounted=0.5 mm
So, Dimension of insulated conductor=7.5x2.7 mm2
Using three layers for the winding.
Helical winding is used. Therefore, space has to be provided for (37+1) =38 turns along the
axial depth
Axial depth of L.V winding
Lcs=38*axial depth of conductor=38x7.5=285 mm
The height of the window is 300 mm. This leaves a chance of (300-285)/2=7.5 on each side of
the winding (The minimum clearance should be 6 mm for windings having voltages below
500V)
Using 0.5 mm pressboard cylinders between layers.
Radial depth of L.V winding
bs=number of layers*radial depth of conductor +insulation between layers
=3x2.7+2x0.5=9.1 mm
The adjacent fig. shows cross-section through L.v coil.
Diameter of circumscribing circle=d=135 mm
Using pressboard wraps 1.5 mm thick as insulation between l.v winding and core,
Inside diameter of l.v winding=135+2x1.5=138 mm
Outside diameter of l.v winding=138+2x9.1=156.2 mm

H.V. winding:
Primary line voltage=11000V Connection=Delta
Primary phase voltage Vp =11000V
∴Number of turns per phase Tp=11000*111/250=4884
As ±5% tapings are to be provided, therefore the number of turns is increased to
Tp=1.05*4884=5128
Total voltage per coil is about 1500V ∴ Using 8 coils
Voltage per coil=11000/8=1375V Turns per coil = 5158/8=641
Using 7 normal coils of 672 turns and one reinforced coil of 424 turns
Total h.v. turns provided Tp=7*672+424=5128
Taking 24 layers coil Turns per layer=674/24=28
Maximum voltage between layers = 2*28*2.25=126V which is below the allowable limit
H.V. winding phase current Ip=25*1000/3*11000=0.757A
As the current is below 20A ,cross over coils are used for h.v. winding
Taking a current density of 2.4A/mm2
Area of hv conductor np=0.757/2.4=0.316mm2
Diameter of bare conductor=(4/π*0.316)1/2
=0.635mm
Using paper covered conductors the nearest standard conductor size has
Bare diameter =0.63mm, insulated diameter = 0.805mm with fine covering
Modified area of conductor ap=π/4*(0.63)2
=0.312mm2
Actual value of current density used δp=0.757/0.312=2.42A/mm2
Axial depth of one coil =28*8.05=22.6mm
The spacers used between adjacent coils are 5mm in height
Axial length of h.v. winding:
Lep=number of coils *axial depth of each coil + depth of spacers =8*226+8*5=221mm
The height of window is 300 mm and therefore, the space left between winding and window is
300-221=79mm.
This is occupied by insulation and axial bracing of the coil. The clearance left on each side is
39.5 mm, which is sufficient for 11kV transformers.
The insulation used between layers is 0.3 mm thick paper.
∴Radial depth of h.v. coil bp=24*0.805+23*0.3=26.22mm
The thickness of insulation between h.v. and l.v. winding is =5+0.9kV=5+0.9*11=15mm
This includes the width of oil duct also
The insulation between h.v and l.v winding is a 5 mm thick bakelized paper cylinder .The h.v.
winding is wound on a former 5mm thick and the duct is 5 mm wide ,making the total insulation
between h.v. and l.v. windings 15 mm.
∴Inside diameter of h.v. winding
=outside diameter l.v. winding+2*thickness of insulation
=156.2+2*15=186.2 mm
Outside diameter of h.v. winding =186.2+2*26.22=238.64mm=239mm
Clearance between windings of two adjacent limbs=255-239=16 mm

Resistance:
Mean diameter of primary winding =
186.2+239
2
≅ 212𝑚𝑚.
Length of mean turn of primary winding Lmtp = π x 212 x 10-3
= 0.666m.
Resistance of primary winding at 75 deg. C rp =
𝑇𝑝 𝑥 ƥ 𝑥 𝐿𝑚𝑡𝑝
𝑎𝑝
=
4884∗0.021∗0.666
0.312
= 219.2 Ohm
Mean diameter of secondary winding =
138+156.2
2
=149 mm
Length of mean turn of secondary winding Lmts = π x 149 x 10-3
= 0.468 m
Resistance of secondary winding at 75 deg. C, rs =
111∗0.021∗0.468
14.9
= 0.0732 Ohm
So, total resistance referred to primary side Rp = 219.2 + (4884/111)2
x 0.0732 = 364 Ohm
P.U resistance of transformer €r = (Ip*Rp)/Vp =
0.757∗364
11000
= 0.025
Leakage Reactance:
Mean diameter of windings = (138+239)/2 = 188.5 mm
Length of mean turn Lmt = π x 188.5 x 10-3
= 0.592 m
Height of winding Le = (Lep + Les)/2 = (221+285)/2 253 mm
Leakage reactance of transformer ref. t primary =
𝑋𝑝 = 2 ∗ π ∗ f ∗ μ0 ∗ 𝑇𝑝 ∗ 𝑇𝑝 ∗ (
𝐿𝑚𝑡
𝐿𝑒
) ∗ (𝑎 +
𝑏𝑝+𝑏𝑠
3
)
= 2* π * 50 * 4 * π * 10-7
* 4884*4884*(0.592/0.235) * (15+ (26.22+9.1)/3) *10-3
=590 Ohm
P.U. leakage reactance= 0.757 x 590/11000 = 0.0406
P.U impedance = √ [0.0252
+0.04062
] = 0.0477
Regulation:
P.U. regulation 𝜀 = 𝜀𝑟 ∗ cos 𝜑 + 𝜀𝑥 ∗ 𝑠𝑖𝑛𝜑
P.U regulation at unity p.f 𝜀 = 𝜀𝑟 = 0.025 , 𝑎𝑡 𝑧𝑒𝑟𝑜 𝑝. 𝑓 𝑙𝑎𝑔𝑔𝑖𝑛𝑔 𝜀 = 𝜀𝑥2
= 0.406
At 0.8 p.f lagging 𝜀 = 0.025 * 0.8 + 0.0406 * 0.6 =0.0444
Losses:
• I2R Loss
I2
R Loss at 75 deg. C, = 3Ip2
R = 3*0.7572
*364=626W
Total I2
R Loss taking stray load loss as 15% of above, Pe = 1.15*626 =720W
• Core Loss
Taking the density of laminations as 7.6*1000 kg/m2
Weight of 3 limbs = 3*0.3*0.010135*7.6*103
= 69.3 Kg
The flux density in the limbs = 1 Wb/m2
Corresponding to this density, specific core loss is 1.2W/Kg
So, core loss in limbs = 69.3 * 1.2 =83.2W

Specific core loss = 0.85W
So, core loss in yoke = 115.3 * 0.85 = 93W
Total core losses, Ps= 83.2=98.0 = 181 W
Efficiency:
Total losses at full load=181+720 = 901W
Efficiency at full load and at unity p.f. =
25000
25000+900
∗ 100=96.5%
For max. efficiency, x2
Pe=Ps
So, x= √ (181/720) = 0.0501
Thus max. efficiency occurs at 50.1% of full load. This is a good figure for distribution
transformer.
No load current:
Corresponding to the densities of 1 Wb/m2
and 0.833 Wb/m2
in core and yoke respectively
atc=120 A/m and aty=80 A/m
So, Total magnetizing mmf=3x120x0.3+2x80x0.624=207 A
Magnetizing mmf per phase,AT0/√ 2Tp=62/(√ 2x4884)=5.5 mA
Loss component of no-load current Il=181/3x11000=5.5 mA
No load current, I0=√ (10x10-3
)2
+(5.5x10-3
)2
=111.4 mA
No load current as % of full load current= (11.4 mA/0.757) *100=1.5 %
Allowing for joints etc. the no load current will be about 2.5% of full load current.
Tank:
Height over yoke H=528 mm. Allowing 50mm at the base and about 150 mm for oil. Height
of oil level =528+50+100=728 mm. Allowing another 200 mm height for leads etc., height of
tank Ht=728+200=928 mm. The height of the tank is taken as 0.95 m or Ht=0.95 m.
Assuming a clearance of 440 mm along width on each side
Width of tank Wt=2D+D0+2l=2x255+239+2x40=829 mm
The width of tank Wt is taken as 0.84 m.
The clearance along the length pf the transformer is greater than along it’s width, due to tapings
etc. The clearance used is approx. 50mm on each side.
So, Length of tank=Lt=D0+2b=239+2x50=339 mm
The length of tank Lt is taken as 0.35 m.
Total loss dissipating surface, St=2*(0.84+0.35) *0.95=2.26 m2
Total loss dissipation due to radiation and convection=12.5 W/m2
°C
So, Temperature rise= 901/ (2.26*12.5) =31.9 °C
This is below 35 °C and therefore plain tank is sufficient for cooling and no tubes are required.

Design sheet:
kVA 25 Phase 3 Frequency – 50 Hz Delta/Star
Line Voltage
h.v. 11000V
Phase Voltage
h.v. 11000 V
l.v. 433 V l.v. 250 V
Line Current
h.v. 1.31 A
Phase Current
h.v. 0.757 A
l.v. 36 A l.v. 36 A
Type – Core Type of cooling – ON
Core
1. Material 0.35 mm thick 92 grade
2. Output Constant K 0.25
3. Voltage per turn Et 2.25 V
4. Circumscribing circle diameter d 135 mm
5. Number of steps … 2
6. Dimensions …
a 114 mm
b 73 mm
7. Net iron area Ai 10.135*103
mm2
8. Flux density Bm 1.0 Wb/m2
9. Flux φm 10.135 mWb
10. Weight 69.3 kg
11. Specific iron loss 1.2 W/kg
12. Iron loss 83.2 W
Yoke
1. Depth of yoke Dy 114 mm
2. Height of yoke Hy 114 mm
3. Net yoke area 12.16*103
mm2
4. Flux density 0.833 Wb/m2
5. Flux 10.135 m Wb
6. Weight 115.3 kg
7. Specific iron loss 0.8 W/kg
8. Iron loss 98 W
Windows
1. Number 2
2. Windows space factor Kw 0.18
3. Height of windows Hw 300 mm
4. Width of windows Ww 120 mm
5. Window area Aw 36*103
mm2
Frame
1. Distance between adjacent
limbs
D 255 mm
2. Height of frame H 536 mm
3. Width of frame W 624 mm
4. Depth of frame Df 114 mm

Windings L.V. H.V.
1. Types of winding Helical Cross-over
2. Connections Star Delta
3. Conductor Dimensions
Bare 7.0*2.2 mm2
Diameter- 0.63 mm
Insulated 7.5*2.7 mm2
Diameter- 0.814 mm
Area 14.9 mm2
0.312 mm2
Number in parallel None None
4. Current density 2.23 A/mm2
2.43 A/mm2
5. Turns per phase 111 4884(5128 at -5% tap)
6. Coils
Total number 3 3*8
Per core leg 1 8
7. Turns
Per coil 111 7 of 672 turns, 1 of 424 turns
Per layer 34 28
8. Number of layers 3 24
9. Height of winding 285 mm 221 mm
10. Depth of winding 9.1 mm 26 mm
11. Insulation
Between layers 0.5 mm press
board
0.3 mm paper
Between coils 5.0 mm spacers
12. Coil diameters
Inside 138 mm 186.2 mm
Outside 156.2 mm 239 mm
13. Length of mean turn 0.468 m 0.666 m
14. Resistance at 750
C 0.0732 Ω 219.2 Ω
Insulation
1. Between l.v. winding and core = press board wraps 1.5 mm
2. Between l.v. winding and h.v. winding = bakelized paper 5.0 mm
3. Width of duct between l.v. and h.v. = 5 mm
Tank
1. Dimensions
height Ht 0.95 m
length Lt 0.35 m
width Wt 0.84 m
2. Oil level 0.728 m
3. Tubes Nil
4. Temperature rise 31.90
C
Impedence
1. P.U. Resistance 0.025
2. P.U. Reactance 0.0406
3. P.U. Impedence 0.444
Losses
1. Total core loss 181 W
2. Total copper loss 720 W
3. Total losses at full load 901 W
4. Efficiency at full load and u.p.f 96.5%

SUBSTATION PROTECTION EQUIPMENT
1. POTENTIAL TRANSFORMER
Potential transformer is a voltage step-down transformer which reduces the voltage of a high
voltage circuit to a lower level for the purpose of measurement. These are connected across or
parallel to the line which is to be monitored.
The basic principle of operation and construction of this transformer is similar to the standard
power transformer. In common, the potential transformers are abbreviated as PT.
The primary winding consists of a large number of turns which is connected across the high
voltage side or the line in which measurements have to be taken or to be protected. The
secondary winding has lesser number of turns which is connected to the voltmeters, or potential
coils of wattmeter and energy meters, relays and other control devices. These can be single
phase or three phase potential transformers. Irrespective of the primary voltage rating, these
are designed to have the secondary output voltage of 110 V.
Since the voltmeters and potential coils of other meters have high impedance, a small current
flow through the secondary of PT. Therefore, PT behaves as an ordinary two winding
transformer operating on no load. Due to this low load (or burden) on the PT, the VA ratings
of PTs are low and in the range of 50 to 200 VA. On the secondary side, one end is connected
to the ground for safety reasons as shown in figure.
Similar to the normal transformer, the transformation ratio is specified as
V1/V2 = N1/N2
From the above equation, if the voltmeter reading and transformation ratio are known, then
high voltage side voltage can be determined.
Errors in Voltage Transformer
For an ideal voltage transformer, the voltage produced in the secondary winding is an exact
proportion to the primary voltage and are exactly in phase opposition. But in actual PTs this is
not so because of the presence of voltage drops in primary and secondary resistance and also
due the power factor of the burden on secondary. This causes to occurrence of ratio and phase
angle errors in voltage transformers. Let us know in detail.

Errors in Voltage Transformer
Consider the phasor diagram of potential transformer shown above, where
Io = No load current
Im = magnetizing component of no-load current
Iu = Wattful component of no-load current
Es and Ep = Induced voltages in secondary and primary windings respectively
Np and Ns = Number of turns in primary and secondary windings respectively
Ip and Is = Primary current and secondary current
Rp and Rs = Resistances of primary and secondary windings respectively
Xp and Xs = Reactances of primary and secondary windings respectively
β = Phase angle error
The primary induced voltage or EMF Ep is derived by subtracting the primary resistive (IpRp)
and reactive drop (IpXp) from the primary voltage Vp. And also, secondary terminal voltage
Vs is derived by subtracting secondary winding resistance drop (IsRs) and reactance drop
(IsXs) vectorially from secondary induced EMF Es. Due to these drops nominal ratio of the
potential transformer is not equal to the actual ratio of the PT, hence introduces a ratio error.
Ratio Error
The ratio error of the potential transformer is defined as the variation in actual ratio of
transformation from nominal ratio.
Percentage Ratio Error = (Kn – R) / R × 100
Where
Kn is the nominal or rated transformation ratio and is
Kn = Rated primary voltage / Rated secondary voltage
Phase Angle Error
In ideal PT, there should not exist any phase angle between the primary voltage and reversed
secondary voltage. But in practice, there exist a phase difference between Vp and Vs reversed
(as we can observe in above figure), thereby, introduces phase angle error. It is defined as the
phase difference between the primary voltage and reversed secondary voltage.

In order to reduce these errors such that the accuracy is improved by designing the transformers
in such a way that they windings have appropriate magnitudes of internal resistance and
reactance. In addition to this, the core should require minimum magnetizing and core loss
components of exciting current.
2. CURRENT TRANSFORMER
These transformers used serve the purpose of protection and metering. Generally, the same
transformer can be used as a current or potential transformer depending on the type of
connection with the main circuit that is series or parallel respectively.
In electrical system it is necessary to
a) Read current and power factor
b) Meter power consumption.
c) Detect abnormalities and feed impulse to protective devices.
An effective design of a Ring Type C.T. may be produced using the following procedure,
Principles
In operation the C.T. will induce current in its secondary winding and burden which serves to
completely oppose the magnetising effect of the primary current, except for that small
proportion required to magnetise the core. This core magnetising component will then be the
only source of error if the secondary current is to be used as a measure of the primary current.
Making two assumptions i.e. that the CT has no leakage reactance and that its burden is purely
resistive, the vector diagram for a one-to one ration CT will look like this;
N2 = No. of secondary turns
V2 =Secondary Voltage
Rb = Burden Resistance
I1 = Primary Current
I2 = Secondary Current
Im = Excitation current
Ir = Reactive component of Im
Iw = Watt loss of component Im
e = Ratio Error
From this diagram, the primary current I1 differs from the secondary I2 in magnitude and phase
angle.
The angle error θ is Sin -1
Ir/I1 and the magnitude of I1 = [ (I2 N2 + Iw) 2
+ Ir 2
] ½
θ
Bm
I1
v =2 I2 R2
I2-I2Iw
Im
Ir

In practice, the angle θ is so small as to allow the approximations I1 N2 + Iw and θ = Ir / I1
radians, i.e. the current error is due to the watt loss component of the excitation current and the
phase error is proportional to the reactive component Ir. The ratio error can be corrected by an
amendment to the turns ratio, the secondary winding being reduced by several turns or fractions
of a turn. Because of the non - linearity of the excitation characteristics, such corrections do
not maintain accuracy as the current changes, and a choice must be made which gives good
balance over the whole range of current. Cores can be supplied with drilled holes, enabling the
fractions of a turn to be wound.
The phase angle error, on the other hand, cannot be corrected, being a function of the reactive
component of the excitation characteristics which vary widely over the current range and must
take priority in the design of the transformer and choice of core.
The procedure is best described by considering an example, as follows:
Transformer Specification
Ratio 150/1
50Hz. Burden 2.5Va at Power Factor =1.0
Accuracy BS.3938, Class 0.5 Insulation Level – 11 Kv.
Maximum Permissible Error
From 10% to 20% of rated current Ratio error
Phase displacement
1%
60 minutes
Phase displacement
0.75%
45 minutes
Phase displacement
0.5%
30 minutes
Internal Diameter
The I.D. of the core is fixed by physical consideration of the primary conductor and insulation,
plus allowance for the secondary winding and core insulation. The main insulation is
invariably placed on the primary conductor so that a 20 mm dia. conductor insulated for 11 Kv
will have an overall diameter of about 40mm. The Secondary winding and core insulation for
a nominal 660 volts lead to the choice of core I.D. of 60mm. Assuming a maximum O.D. of
110mm, the mean path length will then be mm.
Flux Density
The requirements of phase displacement and angle error limit the working flux density of the
core. An estimate of the flux density can be made by considering one working condition,

preferably one likely to be most stringent. So, considering the phase displacement at the 20%
full load condition –
amps
From phase diagram,
= 0.4 A
= 1.5A/M
By inspection of resolved component curves for TS grade core material
- when = 60mT.
If the flux density at 20% F. L condition is chosen at 60mT, it will rise to 300mT at full load,
add other points pro-rata which can now be checked for error. If for any condition the phase
displacement is excessive, a lower flux density must be chosen.
Condition
(%Full Load)
120% 100% 20% 10%
Primary current I1
(amps)
Bmax (mT)
Hr (from curves) A/m
Ir (Hr x 0.267 (Sin-
1
Ir/I1)
180
360
4.5
1.2
23’
150
300
4.0
1.068
24’
30
60
1.5
0.4
45’
15
30
0.95
0.307
58.5’
Hw (from curves) A/m
Iw (Hw x 0.267)
E (I
w/I1 x100)
%
1 turn compensation
%
Compensation error e1
%
5.2
1.39
0.77
- 0.67
0.1
4.5
1.20
0.80
- 0.67
0.13
1.05
0.28
0.94
- 0.67
0.27
0.6
0.16
1.07
-
0.67
0.4
Compensation
Assuming the phase angle displacements are within allowable limits, the ratio error is
calculated for each condition as shown above, and a turns ratio correction is chosen which will
make then acceptable. In this case, 1 turn correction is made by reducing the secondary winding
to 149 turns.

Cross Sectional Area
Having chosen the working flux density at full load the required cross-sectional area is
calculated thus: -
Voltage across Burden at full load =2.5 volts
Allowing secondary winding resistance 0.1 ohms
Then additional voltage for internal burden = 0.1 Volts
Total secondary E.M.F. = 2.6 volts
For 149 turn secondary
Volts/Turns = = 0.0175 Volts
At rated condition Bm = 0.3 Tesla
By transformer equation = .0222 x Bm x Afe
Nett C.S.A. Afe
Allowing 0.95 space factor, Gross C.S.A. = 2.77 cm2
Final Dimensions
Before fixing the final dimensions, take account of possible core degradation during winding.
If protected by a case, this will be small, but it is prudent to allow 20% extra area for a core
taped, wound and impregnated.
In this example, a strip width of 20 mm with a build-up of 17 mm gives a final core dimension
of
I.D. – 60 mm
O.D. – 94 mm
Length – 200 mm
3. CIRCUIT BREAKER
In an electrical system if any fault occurs in the system then that part must be isolated from the
remaining part of the system which is healthy and this can be done by the help of circuit
breakers, and it is also helpful
to protect our equipment it will
control the sudden rise of
current or voltage.
A circuit breaker will break the
circuit either manually or
automatically under the
conditions like no-load, full-
load or short circuit and thus it
is very helpful for switching
and protection of various parts
of the power system.

Classification of Circuit Breaker
• Interrupting medium: air, air blast, magnetic blast, vacuum, oil circuit breaker
• Action: automatic and non-automatic circuit breaker
• Method of control: direct control or remote control
• According to service: indoor or outdoor circuit breaker
• Way of operation: gravity opened, gravity closed and horizontal break circuit breaker
I. Air Blast Circuit Breaker
This type of circuit breakers was used from early days and its voltage ranges from 11 to 1100
KV this type of circuit breakers are suitable for high voltages. In this, the compressed air is
used for the arc excitation and thus it is also called a compressed air circuit breaker.
II. Sulfur Hexafluoride Circuit Breaker
Sulfur hexafluoride gas (SF6) has high dielectric strength and good arc extinguishing properties
it is an electronegative gas so it has the ability to absorb the free electrons and its density is
five times that of the air and free heat conversion is 1.6 times than that of the air, it is non-
poisonous and non-inflammable. It could have been the most perfect gas medium for the circuit
breakers but it is not, because it is one of the gases which creates global warming.
III. Oil Circuit Breaker
Oil circuit breaker is such type of circuit breaker which used oil as a dielectric or insulating
medium for arc extinction. In oil circuit breaker the contacts of the breaker are made to separate
within an insulating oil. When the fault occurs in the system the contacts of the circuit breaker
are open under the insulating oil, and an arc is developed between them and the heat of the arc
is evaporated in the surrounding oil. The oil circuit breaker is divided into two categories- i)
Bulk oil circuit breaker and ii) Low oil circuit breaker.
IV. Vacuum Circuit Breaker
A breaker which used vacuum as an arc extinction medium is called a vacuum circuit breaker.
In this circuit breaker, the fixed and moving contact is enclosed in a permanently sealed vacuum
interrupter. The arc is extinct as the contacts are separated in high vacuum. It is mainly used
for medium voltage ranging from 11 KV to 33 KV.
Selection of Type of Circuit Breaker:
The choice of a Circuit Breaker Types for a given power system of known parameters like
operating voltage and the line constants, is a complicated problem. Unless the exact
requirements of the circuit breaker are clearly spelt out, the choice cannot be uniquely made.
For example, in a system where automatic reclosing is not installed, the choice would naturally
be very wide because all the three conventional types of the circuit breakers could perhaps be
utilized in this case.
Apart from the considerations of initial cost we take into account the availability, of properly
trained personnel to operate the breakers and the free supply of high grade insulating oil to the
various substations whenever required. As mentioned earlier in the air-blast circuit breakers
have very high rupturing capacities and can be designed to have extremely small operating

times. Thus at important receiving stations and generating sets of a large interconnected power
system working at voltages of 220 KV and above, the choice of air-blast circuit breakers
becomes almost inevitable. However, in smaller substations working at 110 KV and below
where the duty of the breaker is not very demanding, the oil circuit breakers have as much
possibility as air-blast circuit breakers.
Calculation of circuit breaker parameters:
The bus model of 132kv substation is implemented and all calculations are performed in per
unit system. By imposing the three-phase balanced fault, fault current is calculated.
Different formulae for calculation of circuit breaker parameters are given below:
1. Breaking capacity = √3×V×I×10-6 (MVA)
Where, V is line voltage in volts
I is the rated breaking current in Amps
2. Rated Symmetrical breaking Current = 𝑅𝑢𝑝𝑡𝑢𝑟𝑖𝑛𝑔 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 3 ×rated voltage (kA)
3. Making capacity = 2.55 × symmetrical breaking current (kA)
4. AC component of short circuit current = √2 × symmetrical breaking current (A)
5. DC component of short circuit current = 50% of AC component of short circuit current Or
= 1/2 × AC component of short circuit current (A)
6. Short time rating = √ [(AC component of short circuit current/√2)2
+(DC component of short circuit current)2
] (kA)
Important definitions
• Arc voltage: It is the voltage that appears across the contacts of the circuit breaker
during the arcing period.
• Restriking voltage: It is the transient voltage that appears across the contacts at or near
zero current during arcing period.
• Recovery voltage: It is the normal frequency r.m.s voltage that appears across the
contacts of the circuit breaker after final arc extinction.
• Breaking capacity: It is current r.m.s that a circuit breaker is capable of breaking at
given recovery voltage and under specified conditions.
• Making capacity: The peak value of current during the first cycle of current wave after
the closure of circuit breaker is known as making capacity.
• Short-time rating: It is the period for which the circuit breaker is able to carry fault
current while remaining closed.
• Normal current rating: It is the r.m.s value of current which the circuit breaker is
capable of carrying continuously at its rated frequency under specified conditions.

Standard Parameters
TABLE I: THREE CORE ARMOURED CABLE (COPPER CONDUCTOR)
PARAMETERS AS PER IEC 60502-2.
TABLE II: IMPULSE AND POWER FREQUENCY WITHSTAND VOLTAGE AS PER
IEC 60071 AND 60298
Parameters of SF6 circuit breaker for 220kv substation
Sl.NO Description Requirement
1. Rated system voltage (kV rms), 220
2. Rated Frequency (50 Hz) (+/-5%)
3. Maximum System Voltage kV rms 245
4. Continuous current rating (A) rms 2500
5. Type 1 pole outdoor SF6
6. Mounting Hot dip galvanized lattice steel support
structure, bolted type
7. Number of Poles 1
8. Type of operation Individually operated single pole
9. Required ground clearance from the lowest
live terminal if both the
Terminals are not in the same horizontal
plane. (mm)
4800

11. Minimum height of the lowest part of the
support insulator from ground level (mm). 2550
12. Operating mechanism Spring
13. Auto re-closing duty Single
14. Rated operating duty cycle min. O-0.3 sec.-CO-3min-CO
15. Max. closing time (ms) 120
16. Max. total break time (ms) Less than 3 cycles or 60 ms
17. 1.2/50 microsecond impulse withstand
voltage (kV peak): 1050
18. 1.0-minute power frequency withstand
voltage (kV rms)
460
19. Rated breaking current capacity
i) Line charging at rated voltage at 90
deg. leading power factor (A) rms.
ii) small inductive current (A) rms
iii) Cable charging breaking current (A)
iv) Short circuit current
a) AC component (kA rms)
b) Duration of short circuit in sec.
125
0.5 to 10
Without switching o/v exceeding 2.0
p.u.
250
50
1 sec.
20. Rated short circuit making current capacity
(kA)
125
21. Max. acceptable difference in the instants
of closing/opening of contacts Single Break is required
22. Min. creepage distance of support insulator
(mm) 6125
23. Short time current carrying capability for one
second (kA)
50

24. i) Rating of auxiliary contacts
ii) No. of auxiliary contacts
10 A at 220 V DC
10 NO and
10 NC as spare
25 Breaking capacity of auxiliary contacts 2 A DC with the circuit time constant
not less than 20 ms.
26. Noise level at base and up to 50 meters 75 db (max.)
27. Seismic acceleration (horizontal) 0.1 g
28. Min. Corona extinction voltage (kV rms) 156
29. No. of closing and tripping coils. Two trip coils and one close coil with Anti-
pumping arrangement.
4. ELECTRICAL INSULATORS
Electrical insulators have a very high resistive path through which current flow is not possible.
It is mostly used in the electrical system to avoid the unwanted flow of the current to the earth
from its supporting points such as towers and poles. The poles and towers must be properly
grounded, so insulators are used between the towers and poles and in current carrying
conductors in order to prevent the flow of current from conductor to earth.
I. Pin type Insulator
In the figure, the pin type insulator is shown. It has two parts, i.e. porcelain and galvanized
steel bolt. The galvanized steel bolt is joined at the bottom by cementing. The fixing of pin
insulator is shown in the figure.
There are various methods of securing insulator to the bolts:
i) The insulator has coarse threads and the steel bolt also has coarse threads but is provided
with a soft washer at the top.
ii) The porcelain insulator has coarse threads but they are lined with a soft material into which
the coarse steel pin is screwed. This method is generally adopted.

iii) Into the coarse threaded porcelain, the insulator is screwed the steel bolt with a lead head.
There is a groove on the upper end of the insulator for housing the conductors. The conductors
pass through this grove and is bound by the annealed wire of the same material as the conductor.
Pin type is used for transmission and distribution of electric power at the voltage up to 50 KV.
Insulator are required to withstand both mechanical and electrical stresses. The electrical
breakdown of the insulator can occur either flash-over or puncture. In flash over, an arc occurs
between the line conductor and insulator pin (i.e. earth) and the discharge jumps across the air
gaps, following shortest distance (i.e. a+b+c+d) for the insulator which is shown in the figure.
When the insulators are wet their outer surface is almost conducting so their flashover distance
is = (b+c+d). It is seen that the flashover distance when the insulators are wet is less and to
keep the inner side of the insulator dry. During rain, the rain sheds are made in the order that
these rain sheds should not disturb the voltage distribution they are so designed that their
surface at right angles to the electrostatic lines of force on. The electrostatic lines both when
dry and wet are shown in the figure.
In the case of puncture, the discharge occurs from conductor to pin through the body of the
insulator. When such breakdown is involved, the insulator is permanently destroyed due to
excessive heat. In practice, sufficient thickness of porcelain is provided in the insulator to avoid
puncture strength to flash-over voltage is known as the safety factor.
The value of safety factor is high so that flashover takes place before the insulator gets
punctured. For pin type insulator, the value of safety factor is about 10.

II. Post Insulator
Post insulators are similar to Pin insulators, but post insulators are
more suitable for higher voltage applications.
Post insulators have a higher number of petticoats and a greater height
compared to pin insulators. We can mount this type of insulator on
supporting structure horizontally as well as vertically. The insulator is
made of one piece of porcelain and it has clamp arrangement are in
both top and bottom end for fixing.
III. Suspension Type Insulator
As the line voltage increase, the pin insulators to be used become heavy
and complicated in construction also its cost increase. Further, the
replacement of the damaged insulator will cost more. So, pin insulator
is not an economical problem for higher voltage. For higher voltage
suspension insulator is used, a number of them are connected in series
by metallic links to form a chain and the line conductors are carried by
the bottom-most insulator. The advantage of this type insulator is given
below: Each suspension insulator is designed for 11 KV, so by
connecting a number of such insulators a string of insulators can be designed for any required
voltage. If any one of the insulators in the string fails, it can be replaced easily and at a lesser
cost. The mechanical stresses on the string decrease since the line suspended are flexible. When
the string of the insulator is used in conjunction with the steel towers, the line conductors are
lower than the cross arm which is earthed and acts as a lighting arrestor.
In general, there are three types of suspension insulator:
i) Hewiett suspension type.
ii) Cemented cap type.
iii) Core and link type of insulators.
IV. Strain Insulator
When there is a dead end of the line or the line or there is the corner or sharp curve, the line is
subjected to greater tension. In order to relieve
the line of excessive tension, strain insulators
are used. For low voltage lines (< 11 KV),
shackle insulators are used as strain
insulators. However, for high voltage
transmission lines, strain insulator consists of an assembly of suspension insulators as shown
in Fig. The disc of strain insulator is used in the vertical plane. When the tension in lines is
exceedingly high, as at long river spans, two or more strings are used in parallel.
V. Stay insulator
For low voltage lines, the stays are to be insulated from the
ground at a height not less than 13 meters from the ground. The
insulator used in the stay wire is called as the stay insulator and
is usually of porcelain and is so designed that in a case of
breakage of the insulator the guywire will not fall to the ground.

VI. Shackle Insulator
In early days, the shackle insulators were used as strain insulator.
But, now days, they are frequently used for low voltage distribution
lines. Such insulator can be used either in a horizontal position or in
a vertical position. They can be directly fixed to the pole with a bolt
or to the cross arm. In the fig. shows, a shackle insulator fixed to the
pole. The conductors in the groove is fixed with a soft binding wire.
This is also known as Spool Insulator.
3. ISOLATOR
Isolators are switches which isolate the circuit at times and thus serve the purpose of protection
during off load operation.
Isolator design is considered in the following aspects:
• Space Factor
• Insulation Security
• Standardisation
• Ease of Maintenance
• Cost
Electrical Isolators are of 3 types-
1. Double Break Type Isolator
2. Single Break Type Isolator
3. Pantograph Type Isolator
4. LIGHTNING ARRESTER
A lightning arrester is connected to protect a piece of equipment from lightning and switching
surges.
Over-voltages may cause the burning of insulation of substation equipment if not well
protected. Lightning is one of the most serious causes of over-voltages.
An ideal Lightning Arrester should possess the following characteristics:
• It must not take any current at normal system voltage
• Any transient wave with a voltage peak exceeding the spark over voltage must cause it
to break down.
• After the breakdown, it must be capable of carrying the resulting discharge current
without any damage to itself and without voltage across it exceeding the breakdown
voltage.
• The power frequency current following the breakdown must be interrupted as soon as
the transient voltage has fallen below the breakdown value.

Location of Lightning Arrester
Lightning arrester should be located close to the equipment that it is expected to protect.
In large substations, arrestors should be installed at take-off points of the lines and of the
terminal apparatus.
Many factors like system voltages, basic impulse insulation level, arrestor rating, station layout,
number and arrangement of lines, the position of isolators, the distance between equipment,
etc. have to be taken into account in fixing the location of the arrestors.
The length of the arrester lead should be as low as possible and should not exceed 10M.
The Arresters are installed both on the High Voltage and Low Voltage side of the transformers.
Junction of an OH line and the cable should be protected by LA. Separate earth should be
provided for each LAs. LA ground leads should not be connected to the station earth bus.
Lightning Arrester Ratings
The rating of a lightning arrester is given below,
1. Normal or rated voltage: It is designated by the maximum permissible value of power
frequency voltage which it can support across its line and earth terminal while still
carrying effectively and without the automatic extinction of the follow-up current. The
voltage rating of the arresters should be greater than the maximum sound phase to
ground voltage.
2. Normal Discharge current: It is the surge current that flows through the LA after the
spark over, expressed in crest value (peak value) for a specified wave shape. Example
10, 5, 2.5, 1.5, 1 kA rating.
3. Power frequency spark over voltage: It is the RMS value of the power frequency
voltage applied between the line and earth terminals of the arrester and earth which

causes spark over of the series gap. As per IS 3070, the recommended spark overvoltage
is 1.5 times the rated voltage.
There are also other ratings like maximum impulse spark over-voltage, residual or discharge
voltage, maximum discharge current, etc.
Selection of LA
Here we are selecting an appropriate rating of lightning arresters for the substation.
For the protection of substation above 66kV, an arrester of 10kA rating is used.
Voltage rating of LA = Line to line voltage × 1.1 × coefficient of earthing.
Power frequency spark over voltage = 1.5 ×Voltage rating of LA
(Assuming coefficient of earthing equals 0.8 for the effectively earthed system)
• For 220 KV side:
Voltage rating = 1.1 × 220 × 0.8 = 193.6KV
Power frequency spark over voltage = 1.5 ×193.6 = 290.4KV
Rated discharge current = 10 kA
• For 110 KV side:
Voltage rating = 1.1 × 110×0.8 = 96.8KV
Power frequency spark over voltage = 1.5 × 96.8 = 145.2KV
Rated discharge current = 10kA
• For 66 KV Side:
Voltage rating = 1.1 × 66×0.8 = 58.08kV
Power frequency spark over voltage = 1.5 × 58.08 = 87.12kV
Rated discharge current = 10kA
• For 11 KV side:
Voltage rating = 1.1× 11×0.8 = 9.68KV
Power frequency spark over voltage = 1.5×9.68 = 14.52KV
Nominal discharge current = 5kA
5. RELAY
The relay is the device that open or closes the contacts to cause the operation of the other
electric control. It detects the intolerable or undesirable condition with an assigned area and
gives the commands to the circuit breaker to disconnect the affected area. Thus, protects the
system from damage.
Pole and Throw
The pole and throws are the configurations of the relay, where the pole is the switch, and the
throw is the number of connections. The single pole, the single throw is the simplest type of
relay which has only one switch and only one possible connection. Similarly, the single pole
double throw relay has a one switch and two possible connections.

Construction of Relay
The relay operates both electrically and mechanically. It consists electromagnetic and sets of
contacts which perform the operation of the switching. The construction of relay is mainly
classified into four groups. They are the contacts, bearings, electromechanical design,
terminations and housing.
Contacts – The contacts are the most important part of the relay that affects the reliability. The
good contact gives limited contact resistance and reduced contact wear. The selection of the
contact material depends upon the several factors like nature of the current to be interrupted,
the magnitude of the current to be interrupted, frequency and voltage of operation.
Bearing – The bearing may be a single ball, multi-ball, pivot-ball and jewel bearing. The single
ball bearing is used for high sensitivity and low friction. The multi-ball bearing provides low
friction and greater resistance to shock.
Electromechanical design – The electromechanical design includes the design of the
magnetic circuit and the mechanical attachment of core, yoke and armature. The reluctance of
the magnetic path is kept minimum for making the circuit more efficient. The electromagnet is
made up of soft iron, and the coil current is usually restricted to 5A and the coil voltage to
220V.
Terminations and Housing – The assembly of an armature with the magnet and the base is
made with the help of spring. The spring is insulated from the armature by moulded blocks
which provide dimensional stability. The fixed contacts are usually spot welded on the terminal
link.

SUBSTATION BUS CONFIGURATIONS
The equipment and buses installed in the substation switchyard are arranged and connected in
specific ways to form bus configurations.
The industry has developed several standard bus configurations that vary in complexity, cost,
and reliability.
The standard bus configurations/scheme are
1. Radial bus
2. Sectionalized radial bus
3. Main and transfer bus
4. Single breaker double bus
5. Ring bus
6. One-half breaker
7. Breaker and one-half
8. Double breaker double bus.
The layout of a substation for any particular configuration may vary to accommodate
differences in equipment type, size and arrangement, and site-specific criteria.
1. Radial Bus
The radial bus configuration consists of one main bus. The transmission lines, transformers,
and shunt capacitor banks are connected to the main bus through circuit breakers, circuit
switchers, or motor operated or manually operated disconnect switches.
Radial bus substations are the simplest to operate. But they have the least system reliability and
flexibility of operation.
As shown in the figure, breaker bypass switches can be installed to allow removal of a circuit
breaker from service for maintenance without an outage of the associated circuit, but this leaves
the circuit without relay protection.
When the breaker is isolated for maintenance, the bypass switch is closed, and the circuit
breaker and its associated disconnect switches are opened.
All protective relaying and control for the circuit at the local substation are removed from
service when the circuit breaker is isolated.
A fault on the circuit with its associated circuit breaker bypassed requires an outage of the
complete substation.

2. Sectionalized Radial Bus
The sectionalized or split radial bus is a modification of the radial bus. This configuration
is two radial buses tied together through a sectionalizing or bus tie circuit breaker.
The sectionalizing circuit breaker can be operated normally open or normally closed,
depending on system requirements.
Bus faults or the failure of a breaker (other than the tiebreaker) to operate for a fault requires
an outage of only the affected bus section.
Breaker bypass switches can be applied in sectionalized radial bus substations and operate the
same as in radial bus substations.
3. Main and Transfer Bus
The main and transfer bus is another modification of the radial bus. This configuration consists
of a main bus and a transfer bus.
All circuits are connected to the main bus through circuit breakers and to the transfer bus
through transfer switches. The main and transfer buses are connected through a transfer bus
circuit breaker.
The transfer bus circuit breaker protects a circuit during maintenance of its associated circuit
breaker.
When a circuit breaker is removed from service for maintenance, the transfer circuit breaker
and its associated disconnect switches are closed, the transfer switch for the circuit breaker to
be serviced is closed, and the circuit breaker to be maintained and its associated disconnect
switches are opened. Reliability and protection are not compromised during maintenance.
Considerable attention must be given to the selection of the protective relaying for the transfer
circuit breaker.

4. Single Breaker Double Bus
The single breaker double bus configuration is a modification of the sectionalized radial bus.
This configuration consists of two main buses connected through a tie circuit breaker.
Each circuit has one circuit breaker that can be connected to either the main bus through
disconnect switches.
This configuration allows circuits to be connected to either the main bus to balance the load,
separate critical circuits, or place sources on each bus and allows all circuits to be connected to
one bus in case of an outage on the other bus.
The switching of a circuit from one bus to the other is not automatic and requires manual
switching.
5. Ring Bus
The ring bus configuration is, in reality, a series of sectionalized radial buses connected
together to form a ring. Each bus is called a position.
Sometimes a transmission line and a transformer are paired on one ring position. In this
configuration, only one position is removed from service for a circuit or bus fault.
The circuit breakers which serve the faulted position are opened. The failure of a breaker to
operate for a line or bus fault will cause two positions to be removed from service.

This configuration allows for any circuit breaker to be removed from service for maintenance
without an outage on any circuit.
Line disconnect switches are often installed to allow a line to be removed from service and the
ring to remain intact. The two circuit breakers sourcing the line are opened, the line disconnect
switch is opened, and then the two circuit breakers are closed.
Ring bus substations are highly reliable and ﬂexible to operate. They are generally limited to a
maximum of eight positions to prevent the splitting of the ring. Sources of generation or
redundant circuits should not be terminated on adjacent positions of the ring bus. This prevents
a failed circuit breaker from removing two sources of generation or two feeds to the same load
from service.
6. One-Half Breaker
The one-half breaker configuration is a variation of the ring bus concept on a multiple
substation basis. As with the ring bus, two breakers must be tripped to isolate a faulted line or
transformer.
In the case of the one-half breaker configuration, one of the breakers is usually at the other end
of the transmission line. In the Figure below, Substations A, B, C, D, and E form an extended
ring bus.
The advantages of this conﬁguration are the same as for the ring bus, and on an individual
substation basis, the costs are even lower than for the radial bus.
The one-half breaker configuration is generally applied in substations from 69 kV through 161
kV, and in systems where several substations are located near each other.
7. Breaker and One-Half Bus
The breaker and one-half configuration consist of two main buses. Connected between the
main buses are bays which consist of three circuit breakers. A circuit is terminated between
each two circuit breakers.
In this configuration, each circuit has a dedicated circuit breaker and shares a circuit breaker
with the adjacent circuit, resulting in one and one-half breakers per circuit.
Frequently, a substation is designed to operate initially as a ring bus up through expansion to
six positions. Beyond six positions, the substation evolves to a breaker and one-half
configuration.

There are two types of the breaker and one-half conﬁgurations,
1. conventional
2. folded.
Conventional Breaker and One-Half bus Folded Breaker and One-Half substation bus
In the conventional arrangement transmission lines must pass over one of the main buses,
causing line termination structures to have higher pull-off points. Also, the installation of line
traps, current transformers, and disconnect switches in the lines is difficult.
The folded arrangement locates line termination structures outside the main buses, allowing
conventional pull-off heights to be used. The installation of line traps, current transformers,
and disconnect switches in the lines are relatively easy. Also, the folded arrangement can be
fitted to oddly shaped sites more easily than can the conventional arrangement.
In Breaker and One-Half substation bus configuration, only one circuit, the faulted circuit, is
removed from service for a fault. The main bus fault does not require that circuits be removed
from service.
The failure of a circuit breaker between the main bus and a circuit to operate for the main bus
fault requires that only the circuit adjacent to the circuit breaker be removed from service. The
failure of a circuit breaker between two circuits to operate for a fault requires the two adjacent
circuits to be removed from the service.
This configuration allows any circuit breaker to be removed from service for maintenance
without an outage on any circuit.
Line disconnect switches are sometimes installed to allow a circuit to be removed from service
and all circuit breakers to remain closed.
Breaker and one-half substations are very reliable and flexible in operation. Sources of
generation or redundant circuits should not be connected in the same bay. This prevents a failed
breaker from removing two sources of generation or two feeds to the same load from service.

8. Double Breaker Double Bus Configuration
The double breaker double bus configuration consists of two main buses. Connected between
the main buses are bays consisting of two circuit breakers, and between the circuit breakers, a
circuit. In this configuration, each circuit has two dedicated circuit breakers.
Only the faulted circuit is removed from the service for a fault. A bus fault requires that no
circuits be removed from service. The failure of a circuit breaker to operate for a bus fault
requires only that the circuit terminated in that bay be removed from service.
This configuration allows any circuit breaker to be removed from service for maintenance
without an outage on any circuit. Line disconnect switches are usually not required.
Double breaker double bus substations are the most reliable and are very flexible to operate.
They require no separation of sources of generation or redundant circuits.
Comparison of Bus Configuration
The following tabulation compares the relative constructed costs and levels of reliability of
each configuration for a substation serving six transmission lines.
Usually, some bus configurations can be eliminated from consideration for a particular
substation on the basis of its function.
A radial bus configuration would not be considered for a nuclear generating station, nor would
a double breaker, double bus configuration be considered for a distribution substation.

Typical Bus Configuration Voltage Levels
The selection of a bus configuration for a particular substation should always take into account
the ultimate anticipated development and function of that installation. The figure above shows
at what voltage levels each configuration is typically applied.

PRIMARY DESIGN AND PROTECTION OF 110KV SUBSTATION
Through the analysis of transformer load, the capacity and number of main transformers are
selected, and the main connection modes of 110kV, 35kV and 10kV are determined. By
calculating the short-circuit current, it can be used to select the main electrical equipment to
complete the design of transformer protection and the selection of distribution devices, and
draw the main wiring diagram and the plane layout. Finally, we design a simple relay
protection, and complete the design of the primary electrical part of 110kV substation.
Design principles of main electrical wiring in substations
1. Wiring mode
For substation electrical wiring, when it can meet the operation requirements, its high-voltage
side wiring should be as little as possible with or without circuit breakers. If it can meet the
requirements of relay protection, branch wiring can also be used.
In 110-220kV distribution equipment:
• When the outgoing line is 2 rounds, bridge connection is generally used.
• When the outgoing line is no more than 4 rounds, single bus connection is generally
used.
• When 110-220kV outgoing line is 4 rounds or more, double bus connection is generally
used in hub substation.
2. Circuit breaker settings
According to the electrical wiring mode, each circuit should be equipped with a corresponding
number of circuit breakers to complete the task of switching and closing the circuit.
3. Load parameter settings
⚫ The minimum load is 60-70% of the maximum load.
⚫ The load simultaneous rate is 0.85-0.9, when the feeder is below three cycles and there
are extra heavy loads, it can be 0.95-1.
⚫ The power factor is generally 0.8.
⚫ The average line loss is 5%.
Raw data and plan selection
A 110kV step-down substation is to be built in the suburbs. The system parameters of the
substation to be built are as follows:
• The substation needs to provide three voltage levels: 110kV, 35kV and 10kV to meet
the electricity demand of nearby factories and residents.
• Maximum operation mode: S1 capacity is 200MV.A, S2 capacity is 400 MV.A.
• Minimum operation mode: S1 capacity is 180MW.A, S2 capacity is 300 MV.A.

Load circuit number is as follows:
• High voltage side: feeder 4 times, 2 times standby.
• Medium voltage side: feeder 6 times, 2 times standby.
• Low-voltage side: feeder 12 times, 4 times of standby.
The main electrical wiring of this design is shown in figure 1, 110kV side adopts double bus
connection mode, 35kV side adopts single bus section with bypass bus connection mode, and
10kV side adopts single bus section connection mode.
Figure. Electrical main wiring diagram
Load calculation and main transformer capacity determination
When the substation is equipped with two main transformers, two conditions must be met:
when any one main transformer is shut down, the other main transformer must meet 60% ~
70% of the maximum load, and at the same time it must meet the needs of all primary and
secondary loads.
Maximum load calculation formula:
Smax = Kt (∑
𝑃𝑖𝑚𝑎𝑥
𝑐𝑜𝑠𝜑
𝑛
𝑖=1 )(1+α%) (1)
According to the load data of each voltage level given by the original data, the above
formula is substituted. At this time, the synchronization coefficient is 0.85, and the network
loss rate is 5%, and the total load of each voltage level can be calculated.
• 35kV load calculation:
𝑆35max = 0.85×(2+2+2.5+2.5)÷0.9×(1+5%) = 8.925 MW.
• 10kV load calculation:
𝑆10max = 0.85×(2+4+2.5+2.5+1.5+2.2)÷0.9(1+5%) =15.561 MW.
• Station lighting and power equipment load:
𝑆max = 0.85×82÷0.85×(1+5%) = 86.1 kVA = 0.0861MW.

Short circuit current calculation
The formula for calculating the short-circuit inrush current is:
ish = √2Ip (1+𝑒
−0.01𝜔
𝑇𝑎 ) = √2kshIp (2)
According to formula (2), the short circuit of each voltage level is calculated.
Capacity selection of parallel capacitors
Most shunt capacitors are connected by star connection, triangle connection, double star
connection and double triangle connection. As far as the current situation is concerned,
triangular wiring is seldom used in high voltage field. When any capacitor is short-circuited,
the fault short-circuiting current is very large. If it cannot be cut off in time, the capacitor may
burn or even explode. The star connection can cut off the fault capacitance by fusing the
protective fuse.
For 110kV substation, the power factor in the high voltage side of the substation should be
higher than 0.95 at the maximum load of the main transformer voltage. Power factor should be
controlled from 0.92 to 0.95 in low valley load.
As far as the normal situation is concerned, the general substation generally selects the medium
voltage side or the low voltage side for its reactive power compensation, and the substation
selects the 10kV side compensation.
𝑃ml = 𝑆35max×cosφ = 15.561×0.85 =13.23𝑀𝑊
The formula for calculating the maximum capacitive reactive power required by load is as
follows:
According to the above analysis and calculation, it can be selected in the following capacitor
bank. The specific parameters are shown in table.
Table: BFM11/ -200-1W capacitor technical parameters
N =
𝑄
𝑞
= 2646÷200 = 13.23
Therefore, the reactive power compensation in this design is finally decided to be compensated
at the side of 10kV, and the capacitor of model BFM11/ -200-1W is selected and set as 14
groups.
Rated voltage
（kV）
Rated
capacity(kvar)
Rated capacitance
（μF）
Phase
number
Outline size
width,depth and height（mm）
200 15.79 1 440×180×696

Transformer protection configuration
For transformer, its main protection can adopt the most common longitudinal differential
protection and gas protection. The combination of these two protection modes can achieve
complementary advantages. In this design, the longitudinal differential protection is used as
the main protection of transformer, and the gas protection is mainly used to protect the internal
faults of transformer.
The transformer protections designed in this paper are as follows: gas protection, overload
protection, longitudinal differential protection, over-current protection and some other non-
electrical protection devices, as shown in figure 2., which relevant symbols are described as
follows:
TA1, TA2, TA3------Current transformers for high, medium and low voltage sides,
respectively; TV1, TVR2, TV3------Voltage transformers for high, medium and low voltage
sides, respectively;
1------Gas Protection;
2,3------ Longitudinal Differential Protection;
4, 5, 6------ Over-current Protection;
7, 8, 9------Overload Protection;
10------Non-electrical Protection;

SYSTEM PARAMETERS OF 400/220 KV SUBSTATION
Sr. Description 400 kv 220kv
1. Nominal System Voltage 400 kv 220 kv
2. Max. Operating Voltage 440 kv 245 kv
3. Rated Frequency 50 Hz 50 Hz
4. Number of Phases 3 3
5. Corona Extinction Voltage 320 kv 156 kv
6. Min. Creepage Distance 25 mm/kv 25 mm/kv
7. Rated Short Ckt. Current for 1 sec 40 kA 40 kA
8. Radio Interface Voltage at 1MHz
(for phase to earth voltage)
1000 Mv
(320 kv)
1000 Mv
(156 kv)
9. Rated Insulation Level
i) Full wave impulse withstand voltage
• For lines
• For reactor / X’mer
• For other equipment
1550 kvp
1300 kvp
1425 kvp
1050 kvp
950 kvp
1050 kvp
ii) Switching impulse withstand voltage
(dry/wet)
1050 kvp 1050 kvp
iii) 1 min power frequency withstand
voltage (dry/wet)
• For lines
• For CB / Isolator
• For other equipment
680 kv
520 kv
(line-ground)
610 kv
(open terminals)
630 kv
460 kv
460 kv
(line-ground)
530 kv
(open terminals)
460 kv

CONCLUSIONS
Substation plays the role of transforming and distributing energy, which requires that the
primary design part of substation should be economical and reasonable, and the secondary
design part should be safe and reliable. The operation and capacity of substations directly affect
the power supply of lower loads, and then affect industrial production and people's daily life.
Various protection devices are installed in the high-voltage distribution room, transformer
room, low-voltage distribution room and other parts of the 110KV step-down substation. In
case of failure, the system can automatically make judgment and start corresponding
protection, and the automatic re-closing device in the system will quickly switch on to restore
power supply.
REFERENCES
[1] Chan C C. The State of the Art of Electric, Hybrid, and Fuel Cell Vehicles[J].
Proceedings of the IEEE, 2007, 95(4):704 - 718.
[2] Moursi M E, Joos G, Abbey C. A secondary voltage control strategy for transmission
levelinterconnection of wind generation. IEEE Transactions on Power Electronics, 2008,
23(3):1178-1190.
[3] Erlich I，Wrede H，Feltes C．Dynamic behavior of DFIG-based wind turbines during
grid faults[J]．IEEE in press，2007．
[4] Linder A，Kennel R．Model predictive control for electrical drives[C]．in Proc. IEEE
PESC，2005，Recife，Brazil：1793-1799．
[5] Yap, E.M.; Al-Dabbagh, M.; Kapuduwage, S.K.; Maung, T.O.; Talebi, N. HVDC and
FACTS for improved power delivery through long transmission lines. Power
Engineering Society Inaugural Conference and Exposition in Africa, 2005 IEEE.
[6] Fundamental Concepts in Substation Design.
[7] A Course in Power Systems by J.B.Gupta. S.K.Kataria & Sons publication.
[8] Switchgear Protection and Power Systems by Sunil.S.Rao. Khanna Publishers

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