Substation Network and Load Distribution Substation Network Design Civil Works Specification Various Subsystems in Substation and Their Functions Substation Equipment and Their Functions Design of Capacity of Transmission Lines Calculation of Line Constants and SIL Bus Bar Arrangement Power Transformer Substation Earthing Circuit Breaker Isolator Current Transformer Capacitor Voltage Transformer Lightning Surge Switching Surge Lightning Arrester Surge Absorber

1. ELECTRICAL SAFETY AND PROTECTION
OF EHV SUBSTATION
INCLUDING THE EFFECTS OF POWER SYSTEM
TRANSIENTS
A PROJECT REPORT
Submitted By
PRATAP BHUNIA
(Roll No.)
In partial fulfillment for the award of the degree
of
BACHELOR OF TECHNOLOGY
in
ELECTRICAL ENGINEERING
TECHNO INDIA SALTLAKE
MAULANA ABUL KALAM AJAD UNIVERSITY OF TECHNOLOGY

2. ACKNOWLEDGEMENT
Any accomplishment requires effort of many people and this work is not different. This
satisfaction drives for accomplishment would be with acknowledging the effort of
persons behind it.
During this year long project several people have provided many forms of help and
support. Firstly I would like to thank Prof. S. Pal HOD, Electrical Engineering
department for selecting me to do this project and for continuous guidance throughout
the project. Secondly I would like to thank the other team members who have
provided ideas, been cooperative and made the team work so well. There have been
no occasions where a conflict of opinion has not been resolved successfully.
--------------------------------------
(PRATAP BHUNIA)
ROLL No. -

3. Index
Sl. No. Topic Page No.
1 Substation Network and
Load Distribution
1
2 Substation Network 2
3 Civil Works Specification 4
4 Various Subsystems in
Substation and Their
Functions
10
5 Substation Equipments and
Their Functions
11
6 Design of Capacity of
Transmission Lines
12
7 Calculation of Line
Constants and SIL
14
8 Bus Bar Arrangement 15
9 Power Transformer 17
10 Substation Earthing 20
11 Circuit Breaker 28
12 Isolator 31
13 Current Transformer 34
14 Capacitor Voltage
Transformer
36
15 Lightning Surge 38
16 Switching Surge 41
17 Lightning Arrester 47
18 Surge Absorber 57
19 References 61

4. Electrical Safety and Protection of EHV Substation Including
the Effect of Power System Transients
Substation Network and Load Distribution:
The first step towards the design of 400/220 kV substation is to determine the
load that the substation has to cater and develop it accordingly. The substation is
responsible for catering bulk power to various load centers distributed all around
through six 220 kV feeders and one 400 KV feeder. The substation is fed 950 MVA
power from 2 generating stations through two 400 kV single circuit lines. The power
is received on 400 kV busbar. 350 MVA power is dispatched to a 400 kV load. The
remaining power is fed to 3 400/220 kV 315 MVA transformers and further fed to 220
kV busbar. From 220 kV busbar, 6 loads are supplied. The main 3 transformers are 3
winding transformer. A total loss of 76.47 kVA occurs in 3 315 MVA transformers.
Table 1: Load flow through the 400/220 kV substation
Incoming Power Amount in MVA Outgoing Power Amount in MVA
From 2 generating
stations
950 To 400 kV load 350
To 220 kV loads 599.83
Loss in 3 315
MVA transformers
0.076
Total 950 Total 950

5. Substation Network

6. Substation Design

7. Civil Works Specification
General:
All structures, buildings, foundations etc., layout & other details shall be
designed and developed keeping in view the functional requirement of the line and sub-
station facilities.
Licensed Premises (Substations Sites):
1. Formation Levels: Formation Level (FL) of substations shall be fixed
minimum 600 mm higher than the surroundings on the basis of the drainage conditions
and the Highest Flood Level in the area.
2. Site Preparation: Necessary earth cutting / filling (spreading), leveling,
compaction and dressing should be done to reach the desired formation level.
Backfilled earth shall be free from harmful salts; viz, Sulphates, Chlorides and / or any
Organic / Inorganic materials and compacted to minimum 95% of the Standard
Proctor's Density (SPD) at Optimum Moisture Content (OMC). The subgrade for the
roads and embankment filling shall be compacted to minimum 97% of the SPD at
OMC.
3. Site Surfacing in Switchyard Area: Site surfacing should be carried out to
provide
 A safe & hazard free high earth resistivity working area (switchyard)
 To prevent growth of weeds & grass within the working area.
The site surfacing will be restricted up to 2.0 m beyond the last structure / equipment
foundation. A 100 mm thick base layer of lean concrete of 1:4:8 using coarse aggregate
of 20 mm nominal size shall be provided in the areas with covering with M-20 concrete
layer with minimum thickness of 50mm in the switchyard excluding roads, drains,
cable trenches etc. 30-40 mm Stone / Gravel spreading shall be done in areas presently
in the scope of the scheme. No stone spreading shall for the time being done in the

8. areas (bays) kept for future expansion. To hold the stone (gravel) from spreading out
of the surfaced / gravel filled area, a 115 mm thick and 300 mm deep toe wall 25 mm
above top of gravel shall be provided. All visible portions of toe-wall shall be plastered
& cement painted.
4. Outside Switchyard Area: Areas lying outside the switch yard shall be
landscaped, developed and maintained in a clean and presentable fashion.
Water Supply, Sewerage & Drainage System:
1. Water Supply & Sewerage: Water supply & sewerage system shall be to
meet the total water requirement of the substations, facilities and emergency reserve
for complete performance of the works. The design and construction of septic tanks
and soak pits shall be suitable for a minimum 100 users with a minimum 10 year span.
2. Design of Drainage: The concessionaire shall obtain rainfall data and
design the storm water drainage system including culverts, drains etc. to accommodate
the most intense rainfall that is likely to occur over the catchments area in one hour
period on an average of once per ten years.
 Slope of Drainage System: Invert level of drainage system at outfall point
shall be decided in such a way that any water over flow from water harvesting recharge
shafts can easily be discharged outside the substation boundary wall. For easy drainage
of water, minimum slope of 1:1000 shall be provided from the ridge to the nearest
drain. The above slope shall be provided at the top of base layer of cement concrete.
Roads, Culverts &Pcc Pavement / Parking:
All internal roads, culverts and PCC pavements / parking within the sub-
station area and approach road from main PWD road to the sub-station main entry
gate(s) should be constructed as per transportation of heavy equipments.

9. Transformer Foundations:
1. General Scope:
 RCC foundations & plinths should be designed and constructed having
minimum Grade M-20 laid on base concrete (1:4:8) of minimum thickness 100 mm
along with a pylon support system for supporting the fire fighting system for placing
350 MVA Power Transformers compatible with the type of equipment &fire fighting
systems and manufacturers’ drawings and equipment parameters.
 The foundations of transformers and circuit breakers shall be of block
type. Minimum reinforcement shall be governed by IS: 2974 and IS: 456 suitable for
equipment load requirements of transformer including impact load equivalent to 15%
of total transformer load with oil etc. or total Jacking Load whichever is more.
 An RCC Rail cum Road system shall be provided duly integrated with
the transformer foundation to enable installation of a new unit and the replacement of
any failed unit. The rails shall be fresh, firstquality 52-kg / meter medium manganese
steel as per Indian railway specification T-12-64 and laid to maintain the required rail
gauge.
2. Emergency Oil Evacuation System: Design & construction of
Emergency Oil Evacuation System shall be suitable to the type of fire protection &
emergency oil drainage system selected.
Fire Protection Walls:
1. Fire protection walls in order to protect against the effects of radiant heat
and flying debris from an adjacent fire for 315 MVA Transformers and 100 MVA
Transformers shall be designed & provided in accordance with Tariff Advisory
Committee (TAC) stipulations.
2. A minimum of 2 meters clearance shall be provided between the
equipments and fire walls.

10. 3. The building walls which act as fire walls shall extend at least 1 m above
the roof in order to protect it.
Foundations for RCC Construction Works:
1. General: All the foundations except walls of switch house cum
administrative and fire hydrant building shall be of Reinforced Cement Concrete.
2. Depth of Foundations:All foundations shall rest below virgin ground
level and the minimum depth excluding lean concrete of all foundations below virgin
ground level shall not be less than 500 mm.
3. Height of Foundations: The Switch Yard foundations shall be at least
100 mm above the finished ground level or as per the manufacturers’ design.
Excavation shall extend minimum 150 mm around foundation (from RCC portion
and not from lean concrete).
Buildings:
1. Design Criterion: The buildings shall be designed to withstand the earth
quake pressure as per the requirements of the National Building Code of India.
2. Design Loads:
Building structures shall be designed for the most critical combinations
of dead loads, superimposed loads, equipment loads, crane loads, wind loads, seismic
loads, short circuit loads and temperature loads.
Dead loads shall include the weight of structures complete with finishes,
fixtures and partitions and should be taken as per IS: 1991.
Super-imposed loads in different areas shall include live loads, minor
equipment loads, cable trays, small pipe racks/hangers and erection, operation and
maintenance loads.
Equipment loads shall constitute, if applicable, all load of equipments to
be supported on the building frame.
3. DG Building Cum Fire Fighting Pump House and RCC Water Storage
Tank: The DG and FF buildings designed to accommodate up to 2 DG sets, motors /

11. pumps as per fire fighting requirement and a permanent crane, hoist and service
trucks mounted on suitable steel structure (I-section / RS joist) below the ceiling for
servicing, lifting and maintenance of the heavy equipment shall be constructed
adjacent to each other for convenience of maintenance of equipment.
4. Brick Work: All brickwork shall strictly be done according to the
Haryana P.W.D. specifications.
5. Damp Proof Course: On outer walls horizontal DPC shall be provided at
level with plinth protection and on inner face vertical DPC 20 mm thick shall be
provided. On all inner walls horizontal DPC shall be provided at floor/plinth level.
Switch - Yard Fencing and Gates:
Fencing & Gates shall be provided for Switchyard area as per General
Electrical Layout Plan and any other specified area along the lines shown. Chain link
fence fabric shall have size 75 mm; coated wire shall be of 3.15 mm diameter having
zinc galvanizing after weaving. The barbed wire shall be of 12 SWG galvanized steel
with its weight 155-186 gm/m length of wire. Maximum distance between two barbs
shall be 75mm. The barbs shall carry four points and shall be formed by twisting two
point wires, each two turn tightly round one line wire making altogether 4 complete
turns. The barbs shall have a length of not less than 13 mm and not more than 18 mm.
The points shall be sharp and well pointed and single strand galvanized steel wire
conforming to requirements for fence fabric 4 mm diameter or single strand, high
tensile, galvanized steel wire 4 mm diameter shall be used. Cast aluminum alloy or
galvanized steel malleable iron D-Clamp drop forged with bolt, check nut, thimble
and other clamps & material required shall be supplied and installed for stretching the
Barbed wire complete in all respect.

12. Boundary and Retaining Walls:
A Boundary wall shall be constructed all around the entire substation land.
The front wall shall be 1.4 m high and in addition 0.600 m galvanized iron grill & the
boundary wall on the other three sides shall be 1.8 m with 0.600 m U/C barbed wire
fencing over the wall. All retaining walls and dual purpose boundary walls shall be
designed to withstand sliding and over turning loads.

13. Various Subsystems in
Substation and Their
Functions
Sl
No.
System Functions
1. Substation Earthing System:
 Earth Mat
 Earth Spike
 Earthing Riser
To provide an earth mat for connecting neutral points,
equipment bodies, support structures to earth, for safety of
personnel and enabling earth fault protection.
2. Overhead Earth Wire
Shielding
To protect the outdoor substation equipment from
lightning strikes.
3. Illumination System:
 For Switchyards
 Buildings
 Road etc.
To provide illumination for vigilance, operation and
maintenance.
4. Protection System:
 Relay Panels
 Control Cables
 Circuit Breaker
 CT, PT etc.
To provide alarm or automatic tripping of faulty part from
the healthy part and also to minimize damage to faulty
equipments and associated systems.
5. Control Cabling An underground power cable for protective circuits,
control circuits, metering circuits.
6. Power Cable To provide supply path to various auxiliary equipments
and machines.
7. PLCC System For communications, telemetry, power line carrier
protection etc.

14. Substation Equipments and
Their Functions
Sl No. Equipments Functions
1. Bus Bar Incoming and outgoing circuits connected to
bus bars
2. Circuit Breakers Automatic switching during normal and
abnormal conditions
3. Isolators Disconnection under no-load condition for
safety, isolation and maintenance
4. Earthing Switches To discharge the voltage on deadlines to earth
5. Power Transformers To step down the voltage at constant power
and frequency
6. CT For measurement of high current and relay
operation for protection
7. CVT For measurement of high voltage and relay
operation
8. Lightning Arrester To discharge lightning over voltage and
switch over voltage to earth
9. Wave trap To provide high frequency signals from
entering other zones

15. Design of Capacity of
Transmission Line
Transmission line towers constitute about 28 to 42 percent of the cost of the
transmission line. The increasing demand for electrical energy can be met more
economically by developing different light weight configurations of transmission line
towers. The selection of an optimum outline together with right type of bracing system,
height, cross arm type and other parameters contributes to a large extent in developing
an economical design of transmission line tower.
Description of Tower configuration:
For the present study, 400kV single circuit steel transmission line with a
suspension towers (20˚ angle deviation) is considered. The first model of tower is
triangular base (three legged) self supporting type with angle sections. Thus, for
optimizing the existing geometry, one of these suspension towers is replaced by
triangular base self supporting tower with tube sections (hollow rectangular sections).
The perception of the three legged transmission line top view is shown.
Specifications 400 kV 220kV
Conductor dia 3.177 cm 2.862 cm
Line spacing 13.75 m 5.5 m
Conductor spacing 40 cm (dual conductor) --

17. Calculations for Line
Constants and
Surge Impedance Loading
400 KV Line
Ds (for inductance calculation) = 0.071 m, Dm = 17.32 m,
Ds (for capacitance calculation) = 0.0796 m.
L = 2*10-7
loge (Dm/Ds) = 1.098* 10-6
H/m,
C = 2πe / log (Dm/Ds) = 1.033* 10-11
F/m,
Surge Impedance = Z = √ (L/C) = 325.93 Ώ,
Surge Impedance Loading (per circuit) = V2
/Z = 490.9 MW
220 KV Line
Ds (for inductance calculation) = 0.37m, Dm = 8.66m,
Ds (for capacitance calculation) = 0.43m
L = 2*10-7
loge (Dm/Ds) = 6.3*10-7
H/m,
C = 2πe / log (Dm/Ds) = 1.85*10-11
F/m,
Surge Impedance = Z = √ (L/C) = 184.42 Ώ (Parallel equivalent of both circuit),
So, Surge Impedance per circuit is = 368.85 Ώ
Surge Impedance Loading (per circuit) = V2
/Z = 131.21 MW

18. Bus Bar Arrangement
Bus bar is the term used for a main bar or conductor carrying an electric
current to which many connections may be made. Bus bars are merely convenient
means of connecting switches and other equipment into various arrangements. There
are different types of bus bar arrangements. The choice
of a particular arrangement depends on different factors
like system voltage, reliability of supply, flexibility and
cost. We have chosen double bus bar and transfer bus
arrangement.
In this arrangement two main buses are available
and one more bus is available which is called transfer
bus. Most of the time, the transfer bus is kept uncharged. If a circuit breaker fails then
the feeder is transferred to the transfer bus without affecting the other circuits and
thus keeping the continuity of supply. The main advantages and disadvantages of this
arrangement are listed below.
Advantages:
 Most flexible in operation
 Breaker failure on bus side breaker removes only one circuit from
Service
 Bus fault does not remove any feeder from the service
 All switching done with breakers
 Either main bus can be taken out of service at any time for
maintenance.
 All switching done with breakers
Disadvantages:

19.  The main disadvantage of this arrangement is high cost because of
its three buses.
Details about Number of Bays and
Number of Equipments Requirements
No. of Bays and
Equipments
400 kV Side 220 kV Side
Incoming 2 3
Outgoing 4 6
Bus Coupler 1 1
Power Transformer
(Auto)
3 -
Wave trap 18 21
CVT/PT 18 21
CT 23 32
Circuit Breaker 25 28
Isolator 98 110

20. Power Transformer
A transformer is a static machine used for transforming power from one circuit
to another without changing frequency. This is a very basic definition of transformer.
Since there is no rotating or moving part so transformer is a static device. Transformer
operates on ac supply. Transformer works on the principle of mutual induction.
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. As this high
voltage power may not be distributed to the consumers directly, this must be stepped
down to the desired level at the receiving end with the help of step down transformer.
Electrical power transformer thus plays a vital role in power transmission.
Two winding transformers are generally used where ratio of high voltage and
low voltage is greater than 2. It is cost effective to use auto transformer where the ratio
between high voltage and low voltage is less than 2. Again a single unit three phase
transformer is more cost effective than a bank of three single phase transformers unit
in a three phase system. But a single three phase transformer unit is a bit difficult to
transport and have to be removed from service entirely if one of the phase winding
breaks down.

21. Specifications of 400/220kV power transformer
Rating 315 MVA, 400/220/33 kV
Type 3 phase Auto transformer with tertiary
winding
Rated Capacity (MVA) ONAN - 60% of ODAF
ONAF - 80% of ODAF
ODAF - 100%
Rated voltage (kV) 400/220/ 33
Highest system voltage (kV) 420/245/36.3
System frequency (Hz) 50
Type of cooling ONAN / ONAF / ODAF
Vector Group YNaOd11
Tertiary Winding 33 kV
System of grounding Solidly grounded
Insulation Level
 400 kV
a) 1.2/50μs full wave
impulse voltage withstand
level
b) 1.2/50μs chopped wave
impulse voltage withstand
level
c) Switching impulse
withstand level
 220 kV
a) 1.2/50μs full wave
impulse voltage withstand
level
b) 1.2/50μs chopped wave
impulse voltage withstand
level
1550 𝑘𝑉𝑃
1425 𝑘𝑉𝑃
1425 𝑘𝑉𝑃
1050 𝑘𝑉𝑃
1050 𝑘𝑉𝑃
Power frequency withstand voltage 400 kV 420(L-N) / 680(L-L) 𝑘𝑉𝑅𝑀𝑆
220 kV <460𝑘𝑉𝑅𝑀𝑆
Impedances ( % ) a) HV & MV - 12.5% (Tolerance -
±10%)
b) HV & LV - 45% (Tolerance -
±15%)
c) MV &LV – 30% (Tolerance -
±15%)

22. Tapping range Auto transformer with on load tap changer for
high voltage variation of -10 to +10 % in 16
equal steps, of 1.25% each, provided on
common end of series winding
Type of tap changers ON LOAD TAP CHANGER (Resistance
Transition type)
Connection
 HV & MV
 LV
Star Auto with neutral
Directly earthed delta
Tap control Full capacity On load tap changer suitable for group
/independent, remote /local electrical and local
manual operation and bi-directional power flow
Service Outdoor
Duty Continuous
Overload capacity As per IS:6600 – 1972 / IEC354
Partial Discharge level 500 pico-coulomb
HV/MV winding neutral end
voltage
17.5 kV porcelain without arcing horns
Bushings PF volt Full Chopped
Switching
Impulse Withstand
Voltage
Dry Wet Impulse Impulse Impulse
400
kV
630 630 1425 1425 1050
220
kV
460 460 1050 1050 --
33
kV
95 95 250 250 --
Neutral Bushings 17.5
kV
45 45 95 95 ±6

23. Substation Earthing
Substation earthing system is essential not only to provide the protection of
people working in the vicinity of earthed facilities and equipments against danger of
electric shock but to maintain proper function of electrical system. Reliability and
security are to be taken in considerations as well as adherence to statutory obligations
(IEEE and Indian standards on electrical safety and environmental aspects).
We are concerned with earthing practices and design for outdoor AC substation for
power frequency in the range of 50Hz.
IMPORTANCE:
The earthing system in a plant / facility is very important for a few reasons,
all of which are related to either the protection of people and equipment and/or the
optimal operation of the electrical system. These include:
Equipotential bonding of conductive objects (e.g. metallic equipment,
buildings, piping etc) to the earthing system prevents the presence of dangerous
voltages between objects (and earth).
 The earthing system provides a low resistance return path for earth
faults within the plant, which protects both personnel andequipment.
 For earth faults with return paths to offsite generation sources, a low
resistance earthing grid relative to remote earth prevents dangerous ground potential
rises (touch and step potentials)
 The earthing system provides a low resistance path (relative to remote
earth) for voltage transients such as lightning and surges / overvoltages
 Equipotential bonding helps prevent electrostatic buildup and
discharge, which can cause sparks with enough energy to ignite flammable
atmospheres
The earthing system provides a reference potential for electronic circuits and helps
reduce electrical noise for electronic, instrumentation and communication systems.
TYPES OF EARTHING:
The earthing is broadly divided as
 System Earthing: This is primarily concerned with the protection of
Electrical equipment by stabilizing the voltage with respect to ground (Connection
between part of plant in an operating system like LV neutral of a Power
Transformer winding and earth).
 Equipment Earthing (Safety grounding): This is primarily
concerned with the protection of personnel from electric shock by maintaining the

24. potential of noncurrent carrying equipment at or near ground potential. Connecting
frames of equipment (like motor body, Transformer tank, Switch gear box,
operating rods of Air break switches, etc) to earth. The system earthing and safety
earthing are interconnected and therefore fault current flowing through system
ground raises the potential of the safety ground and also causes steep potential
gradient in and around the Substation. But separating the two earthing systems
have disadvantages like higher short circuit current, low current flows through
relays and long distance to be covered to separate the two earths. After weighing
the merits and demerits in each case, the common practice of common and solid
(direct) grounding system designed for effective earthing and safe potential
gradients is being adopted.
Types of Electrodes:
1. Rod Electrode
2. Pipe Electrode
3. Plate Electrode
Terms And Definitions:
Rod Electrode Pipe Electrode
Plate Electrode

25. A. Step Potential: Step Potential is the difference in the voltage between two
points which are one meter apart along the earth when ground currents flowing.
B. Touch Potential: Touch Potential is the difference in voltage between the
object touched and the ground point just below the person touching the object when
ground currents are flowing.
C. Ground Potential Rise (GPR): The maximum electrical potential that a
sub-station grounding grid may attain relative to a distant grounding point assumed
to be at the potential of remote earth. This voltage is equal to:
Where, IG = Maximum earth grid current
Rg=Earth Grid resistance (‘Earth grid’ i.e. earthing system)
D. Mesh Potential: The maximum touch potential within a mesh of the grid.
E. Transferred Potential: A special case of touch potential is where a
potential is transferred into or out of the sub-station from or to a remote point
external to the sub-station site.
A person standing in a sub-station coming in contact with say rails/water
pipeline/neutral coming from an adjacent sub-station at the time of occurrence of
earth-fault at that sub-station gets exposed to the transferred potential which equals
difference in GPRs of the two sub-stations.
Step and Touch Voltage Criteria:
The safety of a person depends on preventing the critical amount of shock
energy from being absorbed before the fault is cleared and the system de-energized.
The maximum driving voltage of any accidental circuit should not exceed the limits
defined as follows:
For step voltage the limit is
 The tolerable step voltage criteria is
𝐸𝑆𝑡𝑒𝑝 = [1000 + (6 × 𝐶𝑆 × 𝜌 𝑆)]
0.116
√ 𝑡 𝑠
(1)
 The tolerable touch voltage criteria is
𝐸 𝑇𝑜𝑢𝑐ℎ = [1000 + (1.5 × 𝐶𝑆 × 𝜌 𝑆)]
0.116
√ 𝑡 𝑠
(2)
Where,
𝐸𝑆𝑡𝑒𝑝= the step voltage in Volts
𝐸 𝑇𝑜𝑢𝑐ℎ= the touch voltage in Volts
𝐶𝑆= 1 for no protective layer
𝜌 𝑆= the resistivity of the surface material in Ω meters
𝑡 𝑠= the duration of shock current in seconds

26. Sample Calculation:
Weight of a man=70kg
Fault-duration=0.5S
𝜌=100 Ohm-m
𝜌 𝑆=2000Ohm-m
h=10 cm
k=0.09*(1-100/2000) = 0.0855
𝐶𝑆= 1-(0.0855/(2*0.1+0.09)) = 0.705
Tolerable step potential = 222*10−3
*(1000+1.5*0.705*2000) = 691 V
Tolerable touch potential = 222*10−3
*(1000+6*0.705*2000) = 2100 V
• The earth grid conductor size formula is mentioned below
𝐼 = 𝐴√
(𝑇𝐶𝐴𝑃×104)
𝑡 𝑐×𝛼 𝑟×𝜌 𝑟
ln(
𝑘0+𝑇 𝑚
𝑘0+𝑇𝑎
) (3)
Where,
I = rms of current value in kA
A = conductor sectional size in mm²
𝑇 𝑚 = maximum allowable temperature in ˚C for joints (welded or bolted)
Tr = Ref. temperature for material constant in degrees Celsius(C o) = 20°C
𝑇𝑎= ambient temperature for material constants in˚C
𝛼0= thermal coefficient of resistivity at 0˚C
𝛼 𝑟= thermal coefficient of resistivity at reference temperature 20°C
𝜌𝑟= the resistivity of the earth conductor at reference temperature 20°C in μΩ/cm
𝑘0= 1/𝛼0or 1/𝛼0-Tr
𝑡 𝑐 = time of flow of fault current in sec
TCAP = thermal capacity factor
 Spacing factor for mesh voltage (Km)
𝐾 𝑚 =
1
2𝜋
[ln (
𝐷2
16ℎ𝑑
+
(𝑑+2ℎ)2
8𝐷𝑑
−
ℎ
4𝑑
) +
𝐾 𝑖𝑖
𝐾ℎ
𝑙𝑛
8
𝜋(2𝑛−1)
(4)
Where,
D = spacing between conductors of the grid in meters
d = diameter of grid conductors in meter
𝐾 𝑚 = spacing factor for mesh voltage
𝐾𝑖𝑖 = 1 for grids with rods along perimeter
𝐾ℎ = Corrective weighting factor for grid depth
 Spacing factor of step voltage (Ks)

27. 𝐾𝑆 =
1
𝜋
[
1
2ℎ
+
1
(𝐷+ℎ)
+
1
𝐷
(1 − 0.5 𝑛−2)] (5)
Where,
D = spacing between conductors of the grid in meters
h = depth of burial grid conductor in meters
n = number of parallel conductor in one direction
Earth Mat Design:
Primary requirement of Earthing is to have a low earth resistance. Substation
involves many Earthlings through individual Electrodes, which will have fairly high
resistance. But if these individual electrodes are inter linked inside the soil, it
increases the area in contact with soil and creates number of parallel paths. Hence the
value of the earth resistance in the interlinked state which is called combined earth
value which will be much lower than the individual value.
The inter link is made with flat or
rod conductor which is called as Earth
Mat or Grid. It keeps the surface of
substation equipment as nearly as
absolute earth potential as possible. To
achieve the primary requirement of
earthing system, the Earth Mat should be
design properly by considering the safe
limit of Step Potential, Touch Potential and Transfer Potential.
Factors influencing earth mat design are: Magnitude of fault current, duration
of fault, soil resistivity, resistivity of surface material, shock duration, material of
earth mat conductor, earth mat geometry.
Earth Riser:
Earth riser is the connector between structures, equipment bodies and the
earthing mat. Earth risers shall be of high quality stranded copper conductor, yellow
green PVC insulated.
Earthing Conductors:
The earth mat is made from earthing conductors. The design of cross-section
of earthing conductor depends on:
 Fault current through the earth conductor and duration of fault( for main
protection: 0.5 S and for back up protection: 1 S)
 Permitted final temperature for earth conductor
 Permitted voltage drop in each conductor:

28. Type of Conductor Maximum Possible Temperature
Copper 400˚C
Aluminum 200˚C
Steel 500˚C
Calculation of Conductor Cross-section:
Formula: 𝑨 =
(𝑰∗𝒕
𝟏
𝟐)
𝑆
Where: A=cross-section of conductor in mm²
t=Duration of fault current
I=Fault current
S=A factor depending on the conductor material
and insulation and initial and maximum insulation temperature
Sample Calculation:
Type of conductor: Copper
Current carrying capacity: 200A/mm²
Final temperature: 300˚C
Ambient temperature: 30˚C
S: 190
Fault Current: 40kA
Duration: 0.5S
Cross-sectional area of conductor = (40*0.51/2
)/190
= 148.86 mm²
Earth Switch:
Earthing switches are mounted on the base of mainly line side isolator. They
are normally vertically break switches and are kept open normally. It is used to
earth the live parts during maintenance and testing. During maintenance although
the circuit is open still there are some voltages on line, due to which capacitance
between line and earth is charged. Before proceeding to maintenance work the
voltage is discharged to earth, by closing the earth switch.

29. Reference Data for Typical Earthing System:
Earthing electrodes
 25mm/40mm dia steel bars
 2-3cm long
Earthing Mat
 75*10 mm² mild steel placed 3-4m
apart in mesh form
 Distance between parallel strips=2m
 Depth below surface=0.5m
 Joints by electric arc welding joints,
covering by 2 mm thick bitumen
paint.
Earth risers
 75*10mm² MS flats connected to
equipment structures and welded to
Earth Mat
Overhead shielding wire (Earth wire)
 Level 30m above ground level with
adequate clearances
 7/9 SWG steel wire
 Shielding angle=45˚

30. Protection& Effect of
Transients

31. Circuit Breaker
A circuit breaker is a device that interrupts the abnormal or fault currents and in
addition performs the function of a switch. Its basic function is to detect a fault
condition and interrupt the flow. Unlike a fuse which operates once and then must be
replaced, a circuit breaker can be reset to resume normal operation. Circuit breakers
are preferred where continuity of service is required or where frequent fuse
replacement may be expected. When circuit breaker disconnects the two contacts an
arc is produced. The arc produces massive heat and the fault current continues to flow
through the arc. So the arc needs to be extinguished as soon as possible. Different types
of mediums are used for arc extinction like oil, air, vacuum, sulphur hexafluoride etc.
For this substation we have decided to use sulfur hexafluoride circuit breaker.
SF6 CIRCUIT BREAKER:
In SF6 circuit breakers SF6 or sulfur hexafluoride gas is used for arc quenching
medium. SF6 circuit breakers have better properties in the quick extinction of arc
than other circuit breakers. So in high voltage systems SF6 circuit breakers are used.
ADVANTAGES:
 Excellent insulating, arc extinguishing, physical and chemical properties of
SF6 gas.
 Non flammable and chemically stable SF6 gas reduces the chance of
explosion.
 Electrical clearances are very much reduced because of high dielectric strength
of SF6.
 Minimum maintenance is required.
 Its performance is not affected by variations in atmosphere.
 No overvoltage problem. The arc is extinguished at natural current zero
without current chopping.

32. TECHNICAL REQUIREMENTS FOR CIRCUIT
BREAKER:
Sl.no Description 400KV 220KV
01 Service Outdoor Outdoor
02 Type
SF6 SF6
03 Auto Reclosing
1 Ph./3 Ph. 1 Ph./3Ph.
04 Rated frequency (Hz)
50 50
05 i) Nominal system
voltage (KV)
400 220
ii) Rated voltage (KV) 420 245
06 System neutral Earthing Effectively
Earthed
Effectively
Earthed
07 Insulating level (KVp)
1.2/50 micro- Sec impulse
withstand volt.
a) between line
terminals and
ground (KVP)
b) between terminals
with circuit breaker
open
±1425
±1425 impulse on
one terminal and
240KVp Power
frequency voltage
of opposite
polarity on other
Terminal
±1050
-
08 i) 1 min power frequency
withstand voltage (KV
rms)(dry & wet)
a) between line
terminals and
ground
b) between terminals
with Circuit
breaker open
ii) 250/2500 switching
impulse withstand
voltage (dry & wet)
a) between. line
terminal and
ground (KVP)
b) between terminal
with circuit breaker
open
520
610
±1050
900KVp impulse
on one terminal
and 345KVp PF
voltage of
opposite polarity
on other terminal
460
-
08 Rated current (Amps.)
(i) Continuous 2000 2000/1600

33. ii) Short time rating(KA) 50 for 1 sec. 40
for 3 seconds
09
Min. Creepage distance (mm)
bet. ph. to ground and
bet. CB terminals (
Heavily polluted
atmosphere).
10500 in each case 6125
10 Rated Breaking time (m.sec.) 40 m. sec. Not exceeding
60 mS
11 Total Closing time (m.sec.)
Not exceeding 120 mS
12 Rated line charging breaking
current (Amps)
400 125
13 Rated cable charging
breaking current (Amps)
400 250
14 Rated single capacitive
making /breaking current
(Amps) Within permissible
switching over voltage (As
per Table I)
- 250
15 Rated small inductive
making/breaking current
within permissible
switching over voltage (As
per Table I)
Eqvt. to magn.
current of 315
MVA,400/220/33
KV Tr. and
80MVAR Shunt
Reactor
Eqvt. to
magn.
current of
160 MVA,
220/132/33
KV Txf.
16 Rated operating sequence (O
– Operating, C – closing)
O-0.3 sec- CO – 3.0 min-CO
17 Operating mechanism Spring/Spring Spring/Spring
18 Mode of operation Individual
Pole
Operated
Individual
Pole
Operated
19 No. of trip coils 2 per pole 2 per pole
20 Trip coil and closing coil
voltage (DC volt)
220 220
21 Phase to phase clearance of
pipe bus(mm)
7000 4500
22 Minimum clearance of live
parts in air and ground (mm) 8000 5500
23 First pole to clear factor 1.3 1.3
24 Altitude above mean sea
level (meter)
Not exceeding 1000
25 Terminal connectors suitable
for ACSR
connection/Aluminium pipe
Moose/4” Moose/3”

34. Isolators
An isolator switch is used to ensure that an electrical circuit is completely de-
energized for service or maintenance. Such switches are often found in electrical
distribution and industrial applications, where machinery must have its source of
driving power removed for adjustment or repair. High-voltage isolation switches are
used in electrical substations to allow isolation of apparatus such as circuit
breakers, transformers, and transmission lines, for maintenance. The isolator is usually
not intended for normal control of the circuit, but only for safety isolation. Isolator can
be operated either manually or automatically (motorized isolator).
OPERATING MECHANISM:
 The operating mechanism shall be motor operated as well as manually operated
for 420KV, 245 KV and 145 KV Class isolator and shall ensure quick and
effective operation. 36 KV isolators shall be manually operated. The operating
mechanism shall be housed in a weather proof outdoor mechanism box near
the base of the isolator.
 Each isolator/pole of isolator and earth switch shall be provided with a manual
operating handle at a height of 1000 mm. (approx.) from the base of isolator
support structure so that one man can open or close the isolator with ease in
one movement while standing at ground level.
 All operating linkages carrying mechanical loads shall be designed for
negligible deflection. The isolator and earth switches shall be provided with
‘Over Center’ device in the operating mechanism to prevent accidental opening
due to wind, vibration, short circuit forces or movement of the support
structures.
 All rotating parts shall be provided with grease packed roller or ball bearings
in sealed housings designed to prevent ingress of moisture, dirt or other foreign
material. Bearing pressure shall be kept low to ensure long life and ease of
operation. Locking pins whenever used shall be rustproof.
 Signaling of closed position shall not take place unless it is certain that the
movable contacts have reached a position in which rated normal current, peak
withstand current and short time withstand current can be carried safely.
Signaling of open position shall not take place unless movable contacts have
reached a position such that clearance between contacts is at least 80% of the
isolating distance.

35. Technical Requirements For Isolator:
Sl.
No.
DESCRIPTION 400KV 220 KV
1 Nominal System Voltage (KV) 400 220
2 Rated Voltage (KV) 420 245
3 Frequency (Hz) 50
4 No. of Phases 3-phase
5 System Neutral Earthing
Effectively Earthed
6 No of poles 3
7 Location Outdoor
8 Rated Insulation Level
A 1.2/50 micro-sec. lightning Impulse
Withstand Voltage (KVp)
i) Between line terminals and
ground.
ii) Between line terminals with
isolator open.
± 1425
± 1425 KVP
impulse on one
terminal and
240 KVP power
frequency
voltage of
opposite
polarity on
other terminal.
±1050
±1200
B One minute PF withstand
voltage (KVrms)
i) Between line terminals and
ground.
ii) Between terminals when
isolator is open.
520
610
460
530
C 250/2500 micro sec. Switching
surge withstand test voltage (dry &
wet).
i) Between line terminals and
ground.
ii) Between terminals with
isolator open.
± 1050
KVpeak
900 KVP
impulse on one
terminal and
345 KVP power
frequency
voltage of
opposite
polarity on
other terminal.
-
-
D Corona extinction voltage
(KVrms). 320 (min)

36. E Max. RIV at 1.1 Ur √3 at 1.0
MHZ (micro volts)
Less than
1000 at
266KVrms.
9 Rated Normal Current(Amps) 3150 / 2000 2000/1600
10 Rated Short Time withstand current
of Main Contacts and Earth Switch
(KA) and duration (for 3 sec) and
dynamic current (KAP) of isolator
and Earth Switch.
i) 50 for 1
sec.
ii) 125 KAP
i) 40 for 3
sec.
ii) 100 KAP
11 Mounting Condition On
Structure
12 Method of operation Main /
Earthing Switch
Motor / Manua
as well as local
electrical
operation
having motor
Motor /
Manual
13 Number of auxiliary switches for
main isolator
20 NO+20NC
(min.)
10NO+10NC
(min.)
14 Number of Make before break and
break after break auxiliary switches
4 NO + 4 NC
(min.)
2NO+2NC
(min.)
15 Number of auxiliary Switches in
Earth Switch
4 NO+
4NC
16 Rated auxiliary AC Supply (Volt) 400/230
V ±10%
17 Rated auxiliary DC Supply (Volt). 220
±10%
18 Minimum creepage distance of
support insulators (mm) 10500 6125
19 Phase to phase spacing (mm) 7000 4500
20 Operating Time of isolator and
Earth switch.
Less than 12 seconds
21 Mechanical terminal load for
horizontal centre break Isolator
i) Straight Load (N)
ii) Cross Load (N)
1600
530
1000
330
22 Mechanical terminal load for
pantograph Isolator
iii) Straight Load (N)
iv) Cross Load (N)
2000
800
-
23 Rated magnetising / capacitive
current make and break
0.7 Amps at 0.15 PF
24 All Contacts Silver –plated , minimum
20 micron
25 Temperature rise above ambient
temperature of 50 deg C
corresponding to maximum
continuous current (ºC)
Within limit as per table IV of IS :
9921(Pt. II) – 1982

37. Current Transformer
A current transformer (CT) is an electric device that produces an alternating
current (AC) in its secondary which is proportional to the AC in its primary. Current
transformers, together with voltage transformers (VTs) or potential transformers
(PTs), which are designed for measurement, are known as instrument transformers.
When a current is too high to measure directly or the voltage of the circuit is
too high, a current transformer can be used to provide an isolated lower current in its
secondary which is proportional to the current in the primary circuit. The induced
secondary current is then suitable for measuring instruments or processing in
electronic equipment. Current transformers also have little effect on the primary
circuit. Often, in electronic equipment, the isolation between the primary and
secondary circuit is the important characteristic.
Current transformers are used in electronic equipment and are widely used for
metering and protective relays in the electrical power industry.
Current transformers reduce high voltage currents to a much lower value and
provide a convenient way of safely monitoring the actual electrical current flowing in
an AC transmission line using a standard ammeter. The principal of operation of a
current transformer is no different from that of an ordinary transformer.
Unlike the voltage or power transformer looked at previously, the current
transformer consists of only one or very few turns as its primary winding. This
primary winding can be of either a single flat turn, a coil of heavy duty wire wrapped
around the core or just a conductor or bus bar placed through a central hole as shown.
Due to this type of arrangement, the current transformer is often referred to as a
“series transformer” as the primary winding, which never has more than a very few
turns, is in series with the current carrying conductor.
The secondary winding may have a large number of coil turns wound on a
laminated core of low-loss magnetic material which has a large cross-sectional area
so that the magnetic flux density is low using much smaller cross-sectional area wire,
depending upon how much the current must be stepped down. This secondary
winding is usually rated at a standard 1 Ampere or 5 Amperes for larger ratings.

38. Technical Requirements for Current Transformer:
Sr.
No.
Particulars System Voltage (KV
rms) 400-220
1. Nominal system voltage (KV rms) 400-220
2. Highest system voltage (KV rms) 420-245
3. 1.2/50 Microsecond impulse voltage
withstand level
(a) Transformers and Reactors (KVP) 1300-1050
(b) Other equipments and lines (KVP) 1425-1050
4. Switching withstand impulse voltage of all
equipments and lines (KVP)
1050-NA
5. (a) One minute P.F. withstand voltage of arrester
housing (Dty) (KV rms).
630-460
(b) ---Do --- but wet 630-460
5.1 Maximum Continuous Operating
Voltages, kV min.
290/162
5.2 Energyabsorptioncapability,inkj/kV 8
6. Pressure Relief Class (KA rms) 40
7. Anticipated levels of temporary over voltage and
its duration
(a) Voltage 1.3 times rated voltage of
arrester
(b) Duration (Seconds) 1 to 10
8. System frequency (Hz) 50 + 1.5 -
9. Neutral Grounding Effectively earthed
10. Number phase Three
11 Ratio of switching impulse residual voltage
to rated voltage of arrester
Not more than two
12 Long duration discharge class 3 for 220 kV
4 for 400 kV
13 Max RIV when energized at MCOV 1000 micro volts
14 Partial discharge value 50 pc (max)
15 Minimum creepage distance (mm) 10500-6125

39. Capacitor Voltage
Transformer
Capacitor Voltage transformers (CVT) are a parallel connected type of
instrument transformer. They are designed to present negligible load to the supply
being measured and have an accurate voltage ratio and phase relationship to enable
accurate secondary connected metering.
Capacitor Voltage Transformers (CVT), are used for voltage metering and
protection in high voltage network systems. They transform the high voltage into low
voltage adequate to be processed in measuring and protection instruments secondary
equipment, such as relays and recorders).
A Voltage Transformer (VT) isolates the measuring instruments from the high
voltage of the monitored circuit. VTs are commonly used for metering and protection
in the electrical power industry.
A capacitor voltage transformer (CVT) is a transformer used in power systems
to step down extra high voltage signals and provide a low voltage signal, for
measurement or to operate a protective relay.
Capacitor Voltage Transformers also serve as coupling capacitors for coupling
high frequency power line carrier signals to the transmission line.
CVTs in combination with wave traps are used for filtering high frequency
communication signals from power frequency. This forms a carrier communication
network throughout the transmission network.
In an electrical power substation, Capacitor Voltage Transformer in
combination with Wave Trap is placed at the sending and receiving ends of the
substation. At the receiving end they are found just after lightening arrester and
before line isolator.

40. Technical Requirements For Capacitor Voltage
Transformers:
420KV 220KV
a) Highest system voltage KV (rms) : 420 245
b) Rated system voltage KV (rms) : 400 220
c) Rated frequency HZ : 50 50
d) System fault level KA (rms) : 50 40
e) System neutral earthing
:
Effectively
earthed
Effectively
earthed
f) Installation
: Outdoor Outdoor
g) Service condition
:
As per general
condition of
service
As per general
condition of
service
h) Limits of Temperature rise
(immersed in oil)
: 55°C 55°C
i) Voltage factor
:
1.5 for 30 Sec.
1.2 continuous
1.5 for 30 Sec
1.2 continuous.
j) Rated insulation level :
4.1.1 1.2/50 microsecond impulse
withstand voltage KV (peak).
4.1.2 One minute Dry & Wet power
frequency withstand voltage KV (rms)
:
:
1425
630
1050
460
k) Total capacitance (picofarad)
:
4400 + 10% - 5%
l) a) High frequency capacitance for the
entire carrier frequencyrange.
b) Equivalent series resistance over the
entire carrier frequency range (Ohms)
:
:
Within 80% to 150% of rated
capacitance Less than 40
m) Stray capacitance (Pico farads) & stray
conductance (micro Siemens) of the low
voltage terminal of a complete CVT
including Electro Magnetic Unit over the
entire carrier frequency range.
:
:
520 (max)
50
n) One minute power frequency test :
a) Withstand voltage between HF (low
voltage) terminal of intermediate
transformer & earth terminal, KV
(rms).
b) Withstand voltage for secondary
windings & earth terminal, KV (rms).
c) Withstand voltage between HF(LV)
carrier coupling terminal & earth
terminal, KV(rms)
:
:
:
4
3
10
o) Creepage distance Total (mm)
: 10500 6125

41. Lightning Surge
Lightning:
Computers and electronic instruments are essential for processing various kinds of
information in a high speed manner. However, they are often subject to the induced
energy caused by lightning, because of their generally low dielectric strength.
Lightning is a phenomenon in which negative electric charges generated in a
thunderstorm discharge to the ground as a result of dielectric breakdown in the air. A
lightning surge, even an indirect one, causes a surge voltage on the cable lines, and
transmits a momentary high voltage impulse to the sensors/transmitters in the field, or
to the inputs of computers and instruments in the control room.
Lightning surge:
When electric charges are built up in thunderclouds to such level that could break
atmospheric insulation, an electric dis- charge eventually occurs between these clouds
or between the clouds and the ground.
Electric current reaches 20-150 kA. An abnormally high voltage generated by direct
lightning discharge applied to electric power cables or communication cables at that
instance is called ‘Direct Lightning Surge’. Correspondingly, such voltage induced
by electrostatic or electromagnetic induction on those cables located close to the
point where a direct lightning hits, is called ‘Induced Lightning Surge’.
Also, when lightning strikes a lightning rod and the ground potential rises,
instruments’ grounding potential becomes also high. This causes an abnormally great
potential difference between the cables and the ground, which is called ‘lightning
surge caused by increased ground potential’. Direct lightning surge energy is
enormous. A surge protector alone cannot protect the instruments. It is necessary to
share the job by lightning rods and overhead grounding wires to absorb most of the
energy, and by surge protectors to absorb only the rest of the energy.
Here, we explain the mechanism how lightning surges occur, except for the direct
lightning surge.

42. Electrostatic induction:
When thunderclouds located above a power cable or communication cable contain
negative charges at their bottom parts, high level positive charges are induced
electrostatically within the cable and high voltage is developed by electrostatic
induction from thunderclouds (Figure 2-1). At that instance, the negative charge at
the bottom of the thundercloud disappears by discharging between the clouds or
between the clouds and the ground. Then, the positive charge which is trapped by the
cable are freed and led to both directions on the cable as a surge voltage (Figure 2-2).
Electromagnetic Induction:
A discharge between the clouds and the ground occurring near from a power cable or
communication cable generates a magnetic field due to its surge current. When the
magnetic waves propagated within the field reach the cable, a lighting surge is
induced (Figure 3-1)

43. Standard Impulse Wave Shapes:
Experimental Investigation:
 Lightning Surges:
Rise Time: 0.5 to 10 µs
Decay time to 50% of Peak Value: 30 to 200µs
 Wave shapes are arbitrary but mostly unidirectional
 Lightning overvoltage wave can be represented as Double Exponential Wave;
defined by the equation:
V=V₀ [exp (-αt)-exp (-βt)]
Where α and β are constants of micro second value. The above equation is an
UNDIRECTIONAL WAVE which has rapid rise to peak value & slowly falls
to zero value
 Front Time=1.25(0₁t₂-0₁t₁)
 Tail Time=0₁t₄
 Tolerance=3%

44. Switching Surge
The over stresses applied upon the power system, are generally transient in nature.
Transient voltage or voltage surge is defined as sudden sizing of voltage to a high peak
in very short duration. The voltage surges are transient in nature that means they exist
for very short duration. The main cause of these voltage surges in power system are
due to lightning impulses and switching impulses of the system. But over voltage in
the power system may also be caused by, insulation failure, arcing ground and
resonance etc.
The voltage surges appear in the electrical power system due to switching surge,
insulation failure, arcing ground and resonance are not very large in magnitude.
These over voltages hardly cross the twice of the normal voltage level. Generally,
proper insulation to the different equipment of power system is sufficient to prevent
any damage due to these over voltages. But over voltages occur in the power system
due to lightning is very high. If over voltage protection is not provided to the power
system, there may be high chance of severe damage. Hence all over voltage
protection devices used in power system mainly due to lightning surges.
Let us discuss different causes of over voltages one by one.
Switching Impulse or Switching Surge
When a no load transmission line is suddenly switched
on, the voltage on the line becomes twice of normal
system voltage. This voltage is transient in nature.
When a loaded line is suddenly switched off or interrupted
,voltage across the line also becomes high enough current
chopping in the system mainly during opening operation
ofair blast circuit breaker, causes over voltage in the system
.During insulation failure, a live conductor is suddenly
earthed. This may also caused sudden over voltage in the system. If emf wave
produced by alternator is distorted, the trouble of resonance may occur due to 5th
or
higher harmonics. Actually for frequencies of 5th
or higher harmonics, a critical
situation in the system so appears, that inductive reactance of the system becomes
just equal to capacitive reactance of the system. As these both reactance cancel each
other the system becomes purely resistive. This phenomenon is called resonance and
at resonance the system voltage may be increased enough.

45. But all these above mentioned reasons create over voltages in the system which are not
very high in magnitude.
Sources of Surges/Transients
A common source for surges generated inside a building are devices that switch
power on and off. This can be anything from a simple thermostat switch operating a
heating element to a switch-mode power supply found on many devices. Surges that
originate from outside the facility include those due to lightning and utility grid
switching.
Transients can originate from inside (internal sources) or outside (external sources) a
facility:
 Internal Sources:
1) Switching of Electrical Loads
The switching (on and off) and operation of certain electrical loads – whether
due to intentional or unintentional operations – can be a source of surges in the
electrical system. Switching surges are not always immediately recognized or
disruptive as larger externally generated surges but they occur far more frequently.
These switching surges can be disruptive and damaging to equipment over time.
They occur as part of every day operations.
Sources of switching and oscillatory surges include:
 Contactor, relay and breaker operations
 Switching of capacitor banks and loads (such as power factor correction)
 Discharge of inductive devices (motors, transformers, etc.)
 Starting and stopping of loads
 Fault or arc initiation
 Arcing (ground) faults
 Fault clearing or interruption
 Power system recovery (from outage)
2) Magnetic and Inductive coupling :
Whenever electric current flows, a magnetic field is created. If this
magnetic field extends to a second wire, it will induce a voltage in that wire. This is
the basic principle by which transformers work. A magnetic field in the primary

46. induces a voltage in the secondary. In the case of adjacent or nearby building wiring,
this voltage is undesirable and can be transient in nature.
Examples of equipment that can cause inductive coupling include: Elevators, heating
ventilation and air conditioning systems (HVAC with variable frequency drives), and
fluorescent light ballasts, copy machines, and computers.
 External Sources:
The most recognizable source of surges generated outside the facility is
lightning. Although lightning can be somewhat infrequent in certain regions, the
damage it can cause to a facility can be catastrophic. Other areas are subjected to
thunderstorms and lightning much more frequently.
The surges that are the result of lightning can either be from direct contact of the
lightning to a facilities electrical system or, more commonly, indirect or nearby
lightning that induces electrical surges onto the power or communication systems.
Either scenario can be immediately damaging to the electrical system and/or the
connected loads.
Other external sources of surges include utility-initiated grid and capacitor bank
switching. During the operation of the electrical grid, the utility may need to switch
the supply of power to another source or temporarily interrupt the flow of power to its
customers to aid in clearing a fault from the system. This is often the case in the event
of fallen tree limb or small animal causing a fault on the line.
Wave Propagation on Transmission Lines
 Reflection of Traveling waves at a Junction :
When a traveling wave on a transmission line reaches a junction with
another line, or a termination, then part of the incident wave is reflected back, and a
part of it is transmitted beyond the junction or termination.
The incident wave, the reflected wave and the transmitted wave are formed in
accordance with Kirchhoff's laws. They must also satisfy the differential equation of
the line.

47. Consider a step-voltage wave of magnitude E incident at junction J between two lines
of surge impedances Z1 and Z2. A portion ET of this surge would be transmitted and
a portion ER would be reflected as shown in figure.
There is no discontinuity of potential at the junction J.
Therefore, E + ER = ET
There is also no discontinuity of current at the junction.
Therefore, I + IR = IT
Also, the incident surge voltage E is related to the incident surge current I by
the surge impedance of the line Z1. Similarly the transmitted surge voltage ET is
related to the transmitted surge current IT the surge impedance of the line Z2 and the
reflected surge voltage ER related to the reflected surge current IR by the surge
impedance of the line Z1.
However it is to be noted that the reflected wave is a reverse wave. Thus we can
write
E = Z1 I , ET = Z2 IT , and ER = - Z1 IR
Substituting these values gives
E/Z1 - ER/Z1= ET/Z2 = (E + ER)/Z2
This gives on simplification
ER= E* (Z2 - Z 1) /( Z2 +Z1)
Similarly, the transmitted surge may be written as
ET = (2 Z 2 / (Z2+Z1)) * E
 Short Circuit Line fed from an infinite source

48. For this case Z2 = 0
Then a voltage surge E arrives at the junction J,
which is on short circuit, it is reflected with a
change in sign (- E), so as to cancel the incoming
surge. Also, a current surge I of the same sign as
the incident (I) is reflected so that the transmitted
current is doubled (2I). If the line is fed from a
constant voltage source E, then as the reflected
voltage surge (- E) arrives at the generator end, it
send a voltage surge of E back its voltage at E.
Location of Lighting Arrester
Normal practice is to locate the Lightning Arrester as close as possible to the
equipment because of the following reasons:
 Chances of the surges entering into the circuit between the protective
equipment and the equipment to be protected are reduced.
 Suppose that a power Transformer is to be protected by a Lighting Arrester
.Let the inductance of the lead between the two be L. let the residual voltage of
the lighting arrester be IR.
The voltage incident at the transformer terminal will be:
V= IR+L(di/dt)
Where (di/dt) is the rate of change of surge current. If a capacitor is connected
at the terminals of the equipment to be protected (Transformer), it may be
possible to provide some separation between the two, because this reduces the
Steepness of the lighting surge wave and hence reduces the (di/dt) rate.
In case there is some distance between the Lighting Arrester and the Transformer, a
steep fronted wave, after being incident on the lighting arrester, enter the transformer
after traveling over the lead between the two. The wave suffers reflection at the
terminals. The total voltage at the terminal of the transformer is the sum of the
Reflected and incident voltage, which approaches nearly twice the incident voltage.

49. Lighting Arrester
Introduction-
A lightning arrester is a device used on electrical
power systems telecommunications systems to protect the insulation and conductors
of the system from the damaging effects of lightning. The typical lightning arrester
has a high-voltage terminal and a ground terminal. When a lightning surge (or
switching surge, which is very similar) travels along the power line to the arrester, the
current from the surge is diverted through the arrestor, in most cases to earth.
Working Principle:-
The earthing screen and ground wires can well protect the electrical system against
direct lightning strokes but they fail to provide protection against traveling waves,
which may reach the terminal apparatus. The lightning arresters or surge diverts
provide protection against such surges. A lightning arrester or a surge diverted is a
protective device, which conducts the high voltage surges on the power system to
the ground.
The earthing screen and ground wires can well protect the electrical system against
direct lightning strokes but they fail to provide protection against traveling waves,
which may reach the terminal apparatus. The lightning arresters or surge diverters
provide protection against such surges. A lightning arrester or a surge diverted is a
protective device, which conducts the high voltage surges on the power system to
the ground.
Fig below shows the basic form of a surge diverter.
Basic form of a surge diverter
It consists of a spark gap in series with a non-linear resistor. One end of the diverter
is connected to the terminal of the equipment to be protected and the other end is
effectively grounded. The length of the gap is so set that normal voltage is not
enough to cause an arc but a dangerously high voltage will break down the air

50. insulation and form an arc. The property of the non-linear resistance is that its
resistance increases as the voltage (or current) increases and vice-versa.
This is clear from the volt/amp characteristic of the resistor shown in Figure above.
The action of the lightning arrester or surge diverter is as under:
1. Under normal operation, the lightning arrester is off the line i.e. it conducts no
current to earth or the gap is non-conducting.
2. On the occurrence of over voltage, the air insulation across the gap breaks
down and an arc is formed providing a low resistance path for the surge to the
ground. In this way, the Under normal operation, the lightning arrester is off
the line i.e. it conducts no current to excess charge on the line due to the surge
is harmlessly conducted through the arrester to the ground instead of being
sent back over the line.
3. It is worthwhile to mention the function of non-linear resistor in the operation
of arrester. As the gap sparks over due to over voltage, the arc would be a
short-circuit on the power system and may cause power-follow current in the
arrester. Since the characteristic of the resistor is to offer low resistance to
high voltage (or current), it gives the effect of short-circuit. After the surge is
over, the resistor offers high resistance to make the gap non-conducting.
Types of Lightning Arrester:-
There are several types of lightning arrester in general use. They differ only
in constructional details but operate on the same principle, providing low
resistance path for the surges to the ground.
1) Rod gap arrester
2) Horn gap arrester
3) Multi gap arrester
4) Expulsion type lightning arrester
5) Valve type lightning arrester
1) Rod gap arrester:-
It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at
right angles with a gap in between as shown in Fig. One rod is connected to the line
circuit and the other rod is connected to earth. The distance between gap and insulator
(i.e. distance P) must not be less than one third of the gap length so that the arc may
not reach the insulator and damage it.

51. Generally, the gap length is so adjusted that breakdown should occur at 80% of
spark-voltage in order to avoid cascading of very steep wave fronts across
the insulator.
The string of insulators for an overhead line on the bushing of transformer has
frequently a rod gap across it. Fig 8 shows the rod gap across the bushing of a
transformer. Under normal operating conditions, the gap remains non-conducting. On
the occurrence of a high voltage surge on the line, the gap sparks over and the surge
current is conducted to earth. In this way excess charge on the line due to the surge is
harmlessly conducted to earth.
Typical rod gap arrester
Limitations:-
1) After the surge is over, the arc in the gap is maintained by the normal
supply voltage, leading to short-circuit on the system.
2) The rods may melt or get damaged due to excessive heat produced by the
arc.
3) The climatic conditions (e.g. rain, humidity, temperature etc.) affect the
performance of rod gap arrester.
4) The polarity of the f the surge also affects the performance of this arrester.
Due to the above limitations, the rod gap arrester is only used as a back-up protection
in case of main arresters.
2) Horn gap arrester
Fig shows the horn gap arrester. It consists of a horn shaped metal rods A and B
separated by a small air gap. The horns are so constructed that distance between them
gradually increases towards the top as shown. The horns are mounted on porcelain
insulators. One end of horn is connected to the line through a resistance and choke
coil L while the other end is effectively grounded.

52. The resistance R helps in limiting the follow current to a small value. The choke coil
is so designed that it offers small reactance at normal power frequency but a very
high reactance at transient frequency. Thus the choke does not allow the transients to
enter the apparatus to be protected.
The gap between the horns is so adjusted that normal supply voltage is not enough to
cause an arc across the gap.
Typical horn gap arrester
Under normal conditions, the gap is non-conducting i.e. normal supply voltage is
insufficient to initiate the arc between the gap. On the occurrence of an over voltage,
spark-over takes place across the small gap G. The heated air around the arc and the
magnetic effect of the arc cause the arc to travel up the gap. The arc moves
progressively into positions 1, 2 and 3.
At some position of the arc (position 3), the distance may be too great for the voltage
to maintain the arc; consequently, the arc is extinguished. The excess charge on the
line is thus conducted through the arrester to the ground.
(3)Multigaparrester
Fig shows the multi gap arrester. It consists of a series of metallic (generally alloy of
zinc) cylinders insulated from one another and separated by small intervals of air
gaps. The first cylinder (i.e. A) in the series is connected to the line and the others to
the ground through a series resistance. The series resistance limits the power arc. By
the inclusion of series resistance, the degree of protection against traveling waves is
reduced.
In order to overcome this difficulty, some of the gaps (B to C in Fig) are shunted by
resistance. Under normal conditions, the point B is at earth potential and the normal
supply voltage is unable to break down the series gaps. On the occurrence an over
voltage, the breakdown of series gaps A to B occurs.

53. The heavy current after breakdown will choose the straight – through path to earth
via the shunted gaps B and C, instead of the alternative path through the shunt
resistance.
Typical multi gap arrester
Hence the surge is over, the arcs B to C go out and any power current following the
surge is limited by the two resistances (shunt resistance and series resistance) which
are now in series. The current is too small to maintain the arcs in the gaps A to B and
normal conditions are restored.
Such arresters can be employed where system voltage does not exceed 33kV.
(4)Expulsiontypearrester
This type of arrester is also called ‘protector tube’ and is commonly used on system
operating at voltages up to 33kV. Fig shows the essential parts of an expulsion type
lightning arrester.
It essentially consists of a rod gap AA’ in series with a second gap enclosed within
the fiber tube. The gap in the fiber tube is formed by two electrodes. The upper
electrode is connected to rod gap and the lower electrode to the earth. One expulsion
arrester is placed under each line conductor.
Fig shows the installation of expulsion arrester on an overhead line.

54. On the occurrence of an over voltage on the line, the series gap AA’ spanned and an
arc is stuck between the electrodes in the tube. The heat of the arc vaporizes some
of the fiber of tube walls resulting in the production of neutral gas. In an extremely
short time, the gas builds up high pressure and is expelled through the lower
electrode, which is hollow. As the gas leaves the tube violently it carries away
ionized air around the arc.
This deionizing effect is generally so strong that the arc goes out at a current zero
and will not be re-established.
Advantages
1. They are not very expensive.
2. They are improved form of rod gap arresters as they block the flow of power
frequency follow currents
3. They can be easily installed.
Limitations
1. An expulsion type arrester can perform only limited number of operations as
during each operation some of the fiber material is used up.
2. This type of arrester cannot be mounted on enclosed equipment due to
discharge of gases during operation.
3. Due to the poor volt/am characteristic of the arrester, it is not suitable for
protection of expensive equipment
5)Valvetypearrester
Valve type arresters incorporate non linear resistors and are extensively used on
systems, operating at high voltages. Fig shows the various parts of a valve type
arrester. It consists of two assemblies (i) series spark gaps and (ii) non-linear
resistor discs in series. The non-linear elements are connected in series with the
spark gaps. Both the assemblies are accommodated in tight porcelain container.
The spark gap is a multiple assembly consisting of a number of identical spark gaps
in series. Each gap consists of two electrodes with fixed gap spacing. The voltage
distribution across the gap is line raised by means of additional resistance elements
called grading resistors across the gap. The spacing of the series gaps is such that it
will withstand the normal circuit voltage. However an over voltage will cause the
gap to break down causing the surge current to ground via the non-linear resistors.
The non-linear resistor discs are made of inorganic compound such as thyrite or
metrosil. These discs are connected in series. The non-linear resistors have the
property of offering a high resistance to current flow when normal system voltage is
applied, but a low resistance to the flow of high surge currents. In other words, the
resistance of these non-linear elements decreases with the increase in current
through them and vice-versa.

55. Non-linear resistor discs
Under normal conditions, the normal system voltage is insufficient to cause the
breakdown of air gap assembly. On the occurrence of an over voltage, the
breakdown of the series spark gap takes place and the surge current is conducted to
earth via the non-linear resistors.
Since the magnitude of surge current is very large, the non-linear elements will
offer a very low resistance to the passage of surge. The result is that the surge will
rapidly go to earth instead of being sent back over the line. When the surge is over,
the non-linear resistors assume high resistance to stop the flow of current.
Typical arrangement of an arrester in a 400-kV substation

56. Technical Requirements for Lightning Arrestor:
Sl.
No.
Particulars System Voltage (KV
rms) 400-220
1. Nominal system voltage (KV rms) 400-220
2. Highest system voltage (KV rms) 420-245
3. 1.2/50 Microsecond impulse voltage
withstand level
(a) Transformers and Reactors (KVP) 1300-1050
(b) Other equipments and lines (KVP) 1425-1050
4. Switching withstand impulse voltage of all
equipments and lines (KVP)
1050-NA
5. (a) One minute P.F. withstand voltage of arrester
housing (Dty) (KV rms).
630-460
(b) ---Do --- but wet 630-460
5.1 Maximum Continuous Operating
Voltages, kVmin.
290/162
5.2 Energyabsorptioncapability,inkj/kV 8
6. Pressure Relief Class (KA rms) 40
7. Anticipated levels of temporary over voltage and
its duration
(a) Voltage 1.3 times rated voltage of
arrester
(b) Duration (Seconds) 1 to 10
8. System frequency (Hz) 50 + 1.5 -
9. Neutral Grounding Effectively earthed
10. Number phase Three
11 Ratio of switching impulse residual voltage
to rated voltage of arrester
Not more than two
12 Long duration discharge class 3 for 220 kV
4 for 400 kV
13 Max RIV when energized at MCOV 1000 micro volts
14 Partial discharge value 50 pc (max)
15 Minimum creepage distance (mm) 10500-6125
16
(a)
Terminal connector for 132 & 220 kV class LA Bimetallic compression
type Twin Moose ACSR
350mm spacing suitable
for Horizontal and
Vertical takeoff.

57. 16
(b)
Terminal connector for 400 kV class LA i) Bimetallic compression
type Twin Moose ACSR
350mm spacing suitable
for Horizontal and
Vertical takeoff. OR
ii) suitable for 4” IPS
(Type and quantity will
be given during detailed
engineering)
17 Type of mounting Pedestal ( on structure)
18 Arrester rated voltage, kV 360-198 for
400-220kV LA
19 Nominal Discharge current, kA 20–10 for
400–220kV LA

58. Surge Absorber
A surge absorber is a protective device by which the steepness of wave front of a
surge can be reduced by absorbing surge energy.
On the other hand, surge absorber is a protective device that can conduct high voltage
surges to the ground. It is also called surge diverter.
a) Condenser as surge absorber
 A capacitor connected between line & earth acts as a surge absorber. It can
protect the winding of a transformer.
 Reactance of a condenser is inversely proportional to frequency.
Zc=1/wc =1/2πfc
Zc∞1/f
 Capacitor acts as a short circuit at high surge frequency. Capacitor passes the
surge current directly to earth.
 At power frequency, reactance of capacitor is very high & practically no
current flows to earth.
b) Choke as surge absorber
This type of surge arrester consists of a parallel combination of a choke & resistance
connected in series with the line.
The choke offers high resistance at high frequency (xl= 2πfL). So it forces the surge
to flow through the resistance R.
c) Ferranti surge absorber
It consists of an air-core inductor connected in series with the line. The inductor is
surrounded by a metallic sheet called dissipater. It is connected to earth. This
connection (arrangement) is equivalent to a transformer with short circuited
secondary. The energy of the surge is utilized in the form of heat generated in the
dissipater, due to transformer action. This type of surge absorber is mainly used for
the protection of transformer.
Note 1:

59. Surge arrester/Surge diverter
1. It is a protective device that can
conduct high voltage surges to
ground.
2. To eliminate the surge by diverting
the same to ground.
Surge absorber
1. Steepness of wave-front of a surge is
reduced by absorbing surge energy.
2. To eliminate the surge by absorbing
surge energy.
Note 2: Damage caused to power system equipment depends on the steepness of the
travelling waves on the transmission lines as well as the magnitude of the same.
Here surge absorbers play an important role by reducing the steepness of wave-front
of the surge.
Technical Specifications of Surge Absorber
Sl. No DESCRIPTION TECHNICAL PARAMETERS
TYPE OF ARRESTOR STATION CLASS HEAVY DUTY
GAPLESS
i) Nominal system voltage (KV) 400 220
ii) Highest system voltage (KV) 420 245
iii) System Neutral Earthing EE EE
iv) BIL of transformers (KVp) 1300 900
v) System fault level (KA) for 3
sec.
50 for 1 sec.
40
For 3 sec.
vi) Lightning Impulse withstand
voltage for arrestor housing
(KVp)
1425 1050
i) Rated Voltage (KV) 360 or as specified
in the schedule
198
ii) Maxm. Continuous operating
voltage (KVrms)
306 168
iii) Nominal Discharge Current
(KAp) of 8/20 micro second
wave
10 / 20 10
iv) Line discharge class 3 3
v) Minimum Energy Discharge
capability (KJ/KV) at rated
voltage.
10 7.5
vi) Temporary over voltage
withstand capability (KVrms)
for 10.0 secs
360 or as specified
in the schedule
198

60. vii) Insulation Housing withstand
voltages
i) Lightning Impulse(Dry)
ii) Power frequency(wet)
for 10 KA
for 5 KA
As per IEC 60099-4
viii) Minimum creepage Distance
(mm)
10500 6125
ix) Pressure Relief Class A
x) (Minimum) High Current
Impulse withstand (4/10 micro
second wave) KA (peak)
100 100
xi) Maxm. Lightning Impulse(8/20
micro-second Wave) residual
voltage (KVp) 5KA
10KA 800
850
517
550
xii) Maxm. switching surge(30/60
micro-second wave)
protective level (KVp)
500 Amps
1000 Amps
2000 Amps
-
-
750
-
455
-
xiii) Maxm. Steep Impulse(1/20 MS
impulse) residual voltage at 10
KA (KVp)
1050 600
xiv) Partial Discharge(pico-
coulomb) when energized at
1.05 times its continuous
operating voltage.
Not exceeding 10 PC
xv) Rated Frequency (Hz) 50
xvi)
Minm. visible corona
discharge voltage (KVrms) 320
-
xvii) Min. Bending load (kgm) 1000 1000
xviii)
1 min. p.f. withstand (KVrms)
voltage (dry & wet) for arrestor
housing
630 460
xix)
Switching Impulse withstand
voltage (250/2500 micro
second) dry & wet for arrestor
housing (KVp)
±1050 -
xx)
Pressure relief Current
i) High Current (KA rms)
40 40
ii) Low Current (KA rms) As per IEC

61. References
• http://www.wbsetcl.in/SubStation
• http://www.hvpn.gov.in/wps/wcm/connect/HVPN/Home/Procurement/WB+P
rojects
• High Voltage Engineering by M. S. Naidu, V. Kamaraju
• Power Systems by J.B.Gupta
• http://www.powergridindia.com/_layouts/PowerGrid/User/index.aspx?LangI
D=English
• IS:2705 (Part-I-IV) : Specification for current Transformers.
• IS:4201 : Application guide for current transformers.
• IS:5621/2099 : Specification for Bushings/hollow insulators for alternating
voltages above 1000V.
• IS:335 : Specification for insulation oil for transformers and switchgears
• IEC: 60044-1: Current Transformer.
• IEC: 60815: Guide for selection of Insulators in respect of polluted condition.
• IEC: 60296
• IEC: 60376 : SF6 gas
• IEC: 61462 : Silicon Composite Insulator
• IS:2026 (Part I to IV) - Specification for Power Transformer
• IS:2099 & IS:3347 - Bushing for alternating voltage above 1000 volt
• IS : 6600 - Guide for loading of oil immersed transformer
• IS : 335 - Specification for transformer oil
• CBIP - Manual on transformer.
• IEC-60076 - Power Transformer

62. • IEC-60214 - On Load Tap changer.
• IEC-354 - Loading Guide for Oil immersed Transformer
• IEC-551 - Tr. Sound Level.
• IS-13118 - General requirements for circuit breakers for voltages above 1000
V
• IS-9135 - Guide for testing of Circuit Breaker
• IS-2099 - Bushings
• IEC - 376, 376A, 376B - SF6 Gas
• IEC - 62271-100
• IEC - 60694
• IEC – 56
• IEC-99-4: Gapless Lightning Arrestor
• IS 3070 P-III: Metal Oxide Surge Arrestors without gaps for AC Systems.
• IEC 99 P-III: Artificial Pollution Testing of Lightning Arrestor
• IEC 270: Partial Discharge Measurement.
• IS 2071: Methods of H V Testing
• IS 6209: Methods for Partial Discharge Measurement
• IS 5621: Hollow Insulators for use in electrical equipments
• IS: 3156 Part (I-IV): Specification for Voltage Transformer.
• IS: 4146: Application guide for Voltage Transformers.
• IS: 2099 / IS: 5621: Specification for Bushings/hollow insulators for
alternating voltages above 1000 volts.
• IS: 335: Specification for Insulating Oil
• IS 3024 : Specification for Core Materials