power grid design

Power Grid Design

INTRODUCTION

An electrical grid is an interconnected network for delivering electricity from suppliers to

consumers. It consists of three main components; 1) power station that produce electricity

from combustible fuels or non-combustible fuels; 2) transmission lines that carry

electricity from power plants to demand centers; and 3) transformers that reduce voltage

so distribution lines carry power for final delivery.

1: Electricity generation - Generating plants are usually located near a source of

water, and away from heavily populated areas. They are usually quite large to take

advantage of the economies of scale. The electric power which is generated is stepped up

to a higher voltage-at which it connects to the transmission network.

2: Electric power transmission - The transmission network will move (wheel) the

power long distances–often across state lines, and sometimes across international

boundaries, until it reaches its wholesale customer (usually the company that owns the

local distribution network).

3: Electricity distribution - Upon arrival at the substation, the power will be stepped

down in voltage—from a transmission level voltage to a distribution level voltage. As it

exits the substation, it enters the distribution wiring. Finally, upon arrival at the service

location, the power is stepped down again from the distribution voltage to the required

service voltage(s).

WHY WE NEED GRID?

Power is generated at the generating station, and this power could not satisfy to all load,

but by the help of grid the load could be satisfied. The benefits of grid are:-

1: Improvement of reliability

2: Improvement of Power quality

3: Improvement of security and safety

Dept Of Electrical Engineering 1

http://en.wikipedia.org/wiki/Electricity_generation

http://en.wikipedia.org/wiki/Electricity_distribution

http://en.wikipedia.org/wiki/Wheeling_(electric_power_transmission)

http://en.wikipedia.org/wiki/Electric_power_transmission

http://en.wikipedia.org/wiki/Economies_of_scale

http://en.wikipedia.org/wiki/Power_station

Power Grid Design

4: By the help of grid, the entire power system could be controlled

5: Economically power can be transferred from source to load.

INTRODUCTION TO POWER GRID DESIGN:-

For power grid design the most important criteria are selection of sites, bus bar schemes,

bill of material, safety clearance in grid, design of earth mat, control room building plan,

design of switch/relay room, other grid equipments. All these factors play important role

to design a power grid.

SELECTION OF LAND

SELECTION OF SITE:-

Selection of site for construction of a Grid Sub Station is the first and important activity.

This needs meticulous planning, fore-sight, skillful observation and handling so that the

selected site is technically, environmentally, economically and socially optimal and is the

best suited to the requirements.

1. The main points to be considered in the selection of site for construction of a Grid

Substation are given below.

The site should be:

a) As near the load centre as possible.

b) As far as possible rectangular or square in shape for ease of proper orientation of bus

bars and feeders.

c) Far away from obstructions, to permit easy and safe approach / termination of high

voltage overhead transmission lines.

d) Free from master plans / layouts or future development activities to have free line

corridors for the present and in future.

e) Easily accessible to the public road to facilitate transport of material.

f) As far as possible near a town and away from municipal dumping grounds, burial

grounds, tanneries and other obnoxious areas.

g) Preferably fairly leveled ground. This facilitates reduction in leveling expenditure.

h) Above highest flood level (HFL) so that there is no water logging.

i) Sufficiently away from areas where police and military rifle practices are held.


Power Grid Design

2 The site should have as far as possible good drinking water supply for the station staff.

3 The site of the proposed Sub Station should not be in the vicinity of an aerodrome. The

distance of a Sub Station from an aerodrome should be maintained as per regulations of

the aerodrome authority. Approval in writing should be obtained from the aerodrome

authority in case the Sub Station is proposed to be located near an aerodrome.

REQUIREMENT OF LAND / AREA:

The site should have sufficient area to properly accommodate the Sub Station buildings,

structures, equipments, etc. and should have the sufficient area for future extension of the

buildings and / or switchyard.

The requirement of land for construction of Sub Station including staff colony is as

under:

Table-1 Land Area for Respective Voltages

While preparing proposals for acquisition of private land and allotment of Government

land, the area of land for respective Grid Sub Stations shall be taken into consideration as

mentioned in table.

LAY OUT DESIGN

BUS BAR SCHEMES:

The commonly used bus bar schemes at Sub Stations are:

a) Single bus bar.

b) Main and Auxiliary bus bar.

c) Double bus bar.

d) Double Main and Auxiliary bus bar

e) One and a half breaker scheme.

SINGLE BUS BAR ARRANGEMENT:

This is the simplest switching scheme in which each circuit is

provided with one circuit


Power Grid Design

breaker. This arrangement offers little security against bus bar faults

and no switching flexibility resulting into quite extensive outages of

bus bar and frequent maintenance of bus bar isolator(s). The entire

Sub Station is lost in case of a fault on the bus bar or on any bus bar

isolator and also in case of maintenance of the bus bar. Another

disadvantage of this switching scheme is that in case of maintenance

of circuit breaker, the associated feeder has also to be shutdown.

Typical Single Bus Bar arrangement is shown in Fig – 1.

MAIN AND AUXILIARY BUS ARRANGEMENT:

This is technically a single bus bar arrangement with an additional bus

bar called “Auxiliary bus” energized from main bus bars through a bus

coupler circuit, i.e., for ‘n’ number of circuits, it employs ‘n + 1’ circuit

breakers. Each circuit is connected to the main bus bar through a

circuit breaker with isolators on both sides and can be connected to

the auxiliary bus bar through an isolator. The additional provision of

bus coupler circuit (Auxiliary bus) facilitates taking out one circuit

breaker at a time for routine overhaul and maintenance without de –

energizing the circuit controlled by that breaker as that circuit then

gets energized through bus coupler breaker.

As in the case of single bus arrangement, this scheme also suffers

from the disadvantages

that in the event of a fault on the main bus bar or the associated

isolator, the entire substation is lost. This bus arrangement has been

extensively used in 132 kV SubStations.

Typical Main and Auxiliary Bus Bar arrangement is shown in Fig -2.

DOUBLE BUS BAR ARRANGEMENT:

In this scheme, a double bus bar arrangement is provided. Each circuit

can be connected to either one of these bus bars through respective

bus bar isolator. Bus coupler breaker is also provided so that the

circuits can be switched on from one bus to the other on load. This


Power Grid Design

scheme suffers from the disadvantage that when any circuit breaker is

taken out for

maintenance, the associated feeder has to be shutdown.

This Bus bar arrangement was generally used in earlier 220 kV sub

stations.

Typical Double Bus Bar arrangement is shown in Fig – 3.

DOUBLE MAIN AND AUXILIARY BUS BAR ARRANGEMENT:

The limitation of double bus bar scheme can be overcome by using

additional Auxiliary bus, bus coupler breaker and Auxiliary bus

isolators. The feeder is transferred to the Auxiliary bus during

maintenance of its controlling circuit breaker without affecting the

other circuits.

This Bus bar arrangement is generally used nowadays in 220 kV sub

stations.

Typical Double Main and Auxiliary Bus Bar arrangement is shown in Fig

– 4.

ONE AND A HALF BREAKER ARRANGEMENT:

In this scheme, three circuit breakers are used for controlling two

circuits which are connected between two bus bars. Normally, both the

bus bars are in service.

A fault on any one of the bus bars is cleared by opening of the

associated circuit breakers

connected to the faulty bus bar without affecting continuity of supply.

Similarly, any circuit breaker can be taken out for maintenance without

causing interruption. Load transfer is achieved through the breakers

and, therefore, the operation is simple. However, protective relaying is

somewhat more involved as the central (tie) breaker has to be

responsive to troubles on either feeder in the correct sequence.

Besides, each element of the bay has to be rated for carrying the

currents of two feeders to meet the requirement of various switching


Power Grid Design

operations which increases the cost. The breaker and a half scheme is

best for those substations which handle large quantities of power and

where the orientation of outgoing feeders is in opposite directions. This

scheme has been used in the 400 kV substations.

Typical One and a Half Breaker arrangement is shown in Fig – 5.

Fig-2_ MAIN AND AUXILIARY

BUS ARRANGEMENT

Fig-1_SINGLE BUS BAR ARRANGEMENT


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Fig-3 DOUBLE BUS BAR ARRANGEMENT

Fig-4

DOUBLE MAIN & AUXILIARY

BUS ARRANGEMENT


Power Grid Design

Fig-5 ONE & A HALF BREAKER ARRANGEMENT

BILL OF MATERIAL:The lists of material are only typical and cover the general

requirement. Any other equipment / structure / material which may be

required for construction of Sub Station as

per layout.

Lists of material showing, generally required for construction of 132Kv

Substation.


Power Grid Design


Power Grid Design

Table-2 Grid Equipments

ELECTRICAL LAYOUT DRAWING:-

Typical electrical layout drawings and sectional drawings of 132 /33KV

S/S, & 220/132/33KV S/S are shown in Fig-6 & Fig-7 respectively.

Fig-6 132/33KV GRID SUBSTATION


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Fig-7 220/132/33KV GRID SUBSTATION

SAFETY CLEARANCE

SAFETY CLEARANCES:

The various equipments and associated / required facilities have to be so arranged within

the substation that specified minimum clearances are always available from the point of

view of the system reliability and safety of operating personnel. These include the

minimum clearances from live parts to earth, between live parts of adjacent phases and

sectional clearance between live parts of adjacent circuits / bays. It must be ensured that

sufficient clearance to ground is also available within the Sub Station so as to ensure

safety of the personnel moving about within the switchyard.

As per Rule 64 (2) of the Indian Electricity Rules, 1956, the following safety working

clearances shall be maintained for the bare conductors and live parts of any apparatus in

any Sub Stations, excluding over head lines of HV and EHV installations:

The Table below gives the minimum values of clearances required for Sub Stations up

to 765 kV:

TABLE-3 FOR MINIMUM CLEARANCE

“Safety Clearance” is the minimum clearance to be maintained in air between the live

part of the equipment on one hand and earth or another piece of equipment or conductor

(on which it is necessary to carry out the work) on the other.


Power Grid Design

EARTH MAT DESIGN

5.1BASIC REQUIREMENT:

Provision of adequate earthing system in a Sub Station is extremely important for the

safety of the operating personnel as well as for proper system operation and performance

of the protective devices. The primary requirements of a good earthing system in a Sub

Station are:

a) The impedance to ground should be as low as possible but it should not exceed 1.0

(ONE) Ohm.

b) The Step Potential, which is the maximum value of the potential difference possible of

being shunted by a human body between two accessible points on the ground

separated by the distance of one place (which may be assumed to be one metre), should

be within safe limits.

c) Touch Potential, which is the maximum value of potential difference between a point

on the ground and a point on an object likely to carry fault current such that the points

can be touched by a person, should also be within safe limits.

To meet these requirements, an earthed system comprising of an earthing mat buried at a

suitable depth below ground and supplemented with ground rods at suitable points is

provided in the Sub Stations.

All the structures & equipments in the Sub Station are connected to the earthing mat so as

to ensure that under fault conditions, none of these parts is at a potential higher than that

of the earthing mat.

The neutral points of different voltage levels of transformers & reactors are separately

earthed at two different points. Each of these earthed points should be interconnected

with the station earthing mat.

MEASUREMENT OF EARTH RESISTIVITY:

Weather Conditions:

The resistivity of earth varies over a wide range depending on its moisture content. It is,

therefore, advisable to conduct earth resistivity tests during the dry season in order to get

conservative results.


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Test Procedure:

Four electrodes are driven in to the earth at equal intervals s along a straight line in the chosen

direction. The depth of the electrodes in the ground shall be of the order of 30 to 50 cm. The earth

resistance Megger is placed on a steady and approximately level base, the link between terminals

P1 and C1 is opened and the four electrodes are connected to the instrument terminals as shown

in the figure. An appropriate range on the instrument, avoiding the two ends of the scale as far as

possible, is then selected to obtain clear readings.

Fig-8 MEASUREMENT OF EARTH RESISTIVITY

The resistivity is calculated from the equation given below:

ρ = 2 π s R

where

ρ = resistivity of soil in ohm – metre,

s = distance between two successive electrodes in metres, and

R = Megger reading in ohms.

CHAPTER-6 CONTROL & RELAY PANELS

GENERAL INSTRUCTIONS:

Check and ensure that the Control & Relay Panels being installed are meeting the

requirements of DC control voltage (110V or 220V) and CT secondary rating (1A or 5A).


Power Grid Design

Check that there is no physical damage to the relays and other equipment installed in the

C&R Panel.

Event Loggers, Disturbance Recorders, Bus Bar Protection schemes, LBB Protection

schemes, etc. as well as special schemes / equipment for 400 kV GSS should be tested /

got tested as per their schematic diagrams.

ERECTION AND INSTALLATION:Place the panels at their designated locations on the trenches in the Control Room as per

layout. Fix or bolt the panels (as per requirement of installation of the panels) on the

channel / M.S. Angle fitted on the top of the walls of the trench or on the base frame, as

provided, in the Control Room. Level the panels and check their verticality. In the case of

Duplex type of panels, connect the control panel to the relay panel across the corridor

using the fittings provided with the panels. Also fit the covers for the corridor portion.

Where a number of panels are to be placed adjacent to each other to form a Board or

where a panel is to be placed adjacent to an existing Panel / Board, these shall be bolted

together. There shall be no gap between panels which are placed adjacent to each other.

Connect the Bus wiring / interconnecting wiring between the control & relay panels of

the Duplex type. Also connect the similar wiring between control panel to control panel

and / or relay panel to relay panel where a Board formation is made or where panels are

connected to an existing Board / panel as per their relevant schematic drawings.

PRECOMISSION TESTS:


Power Grid Design

Table-4 Precomission Tests For Relay Panels

POST – COMMISSIONING CHECKS:

1 Check phase sequence of the VT supply in the Control & Relay panels.

2 Measure the voltage & current in the relevant circuits, and check their readings in the

relays, protection schemes, meters, etc.

3 Arrange for checking, by the Protection wing, of stability of transformer differential

protection on load.

4 Arrange for checking and verification, by the Protection wing, of directional feature of

over current, earth fault, and distance protection schemes, as applicable.

BATTERY CHARGERS

GENERAL INSTRUCTIONS:

The Battery Room houses lead acid or nickel cadmium batteries for uninterrupted power

supply (UPS) to the substation. In power grid normally 110 no. of batteries are present,

having each capacity of 2.1V to maintain 220V output & the specific gravity of liquid is

1.835. Power House FCBC are designed to supply continuous power to the DC load and

simultaneously charge the batteries connected. Input supply form 415V. AC 3 Phase or

220V. AC 1 Ph. is converted to regulated DC. The charger has two independent systems.

Normally the DC Power is supplied to he load by the Float Charger. It also supplies


Power Grid Design

trickle current to the battery to keep it healthy. If the charging current under Float Mode

exceeds a set level. Boost charger is switched ON. It supplies Quick charging current to

the battery. On battery reaching the set value the Boost Charger is switched OFF.

Maintain a minimum spacing of 15 cm between the Battery Charger and other panels on

both the sides for proper ventilation. During battery boost charging and in float operation,

it should be ensured that the rating of the relevant section is not exceeded. Place the

temperature sensor in the battery room and connect it to the Battery Charger.

GRID EQUIPMENTS:

LIGHTING ARRESTOR:

It is an instrument that protects vital equipments in the grid. When a lightning strikes a

power transmission line, the induced high voltage travels along the line towards both

ends; this arrester will bypass this high voltage to the ground so that the nearby

transformer will not be damaged.

Line Volt.(KV) L.A.Rating(KV)

400 327

220 180

132 108

33 27

Table-5 Lighting Arrestor Rating For Line Voltage

CVT:

It is that type tfr which is used to measure potential. It is the major advantage of PT, &

also used for carrier communication, which replace the coupling capacitor. At first for

carrier communication a coupling capacitor is used with a PT, which is costlier than

CVT. It act as a high pass filter.

POTENTIAL TRANSFORMER -


Power Grid Design

Potential Transformer or Voltage Transformer are used in electrical power system for

stepping down the system voltage to a safe value which can be fed to low ratings meters

and relays. Commercially available relays and meters used for protection and metering,

are designed for low voltage.

CURRENT TRANSFORMER:

A current transformer is used in high voltage circuits where it is not possible to measure

current directly. A CT is a step up transformer with only one turn in primary. There will

be as many cores based on the purposes like metering, protection etc. The secondary of a

CT should never be kept open circuited because very high flux will be developed in the

secondary and hence it may be damaged.

POWER TRANSFORMER:

It is a static device which transforms electrical energy from one ckt to another ckt without

change of frequency, but changing voltage with the principle mutual induction.

Most of the power transformer are in MVA ratings. It is the most costlier equipments in

the grid.

CIRCUIT BREAKER:-

It is protective equipments in the grid. It is the automatic on load switch. There are of 5

medium type of circuit breaker, but SF6 circuit breaker is used for best.

ISOLATORS:-

This is an off load switching device to used open or close for flow of current or not to

flow respectively in the grid.

WAVE TRAP:-

It is the combination of inductance & capacitance, which act as a low pass filter, which

passes low frequencies in to the grid, & this frequency is used in the grid. The Line trap

offers high impedance to the high frequency communication signals thus obstructs the

flow of these signals in to the substation bus bars. If there were not to be there, then

signal loss is more and communication will be ineffective/probably impossible.

INSULATOR:-


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Insulators are used to prevent flow of current from conducting material to non conducting

material. It should be mechanically strong & high dielectric strength. Each insulator

rating is of 11kv to 16kv.

CONCLUSION:

Electrical grid is an interconnected network for delivering electricity from suppliers to

consumers. It consists of three main components; 1) power station that produce electricity

from combustible fuels or non-combustible fuels; 2) transmission lines that carry

electricity from power plants to demand centers; and 3) transformers that reduce voltage

so distribution lines carry power for final delivery. Grid is the nodal point of the entire

power system. It has two objectives i.e.1: Supply Quality Power, 2:Supply the power

from source to load with an economic reasons. AS Grids are interconnected so, there is

an improvement of reliability of can achieved. As grid is the nodal point, if it fails to

work, then entire power system will fails. Grid efficiency is lower i.e. 50-70%. As Grid

has too many equipments so, design of grid is too costlier.

REFERENCES:

Construction Manual for substations by Shreemat Pandey Chairman & Managing

Director Jaipur Rajasthan Rajya Vidyut Prasaran Nigam Ltd.

Albert, R., Albert, I., and Nakarado, G. L. (2004). Structural Vulnerability of the

North American Power Grid. Physical Review E 69 025103(R). 1-4 pgs.

Grid Manual of OPTCL.

http://www.powergridindia.com


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