EQUIPMENTS AND ACCESSORIES OF POWER TRANSMISSION

EQUIPMENTS AND ACCESSORIES IN POWER TRANSMISSION

CLASSIFICATION OF TRANSMISSION LINES

Sr no. Transmission line Length Voltages

1 Short transmission line Upto 50km <20kv

2 Medium transmission line 50-150km >20kv<100kv

3 Long transmission line More than 150km >100kv

Power transmission networkD:\mba\powergrid_map.pdf

Contents

1. Line supports2. Conductors3. Insulators4. Busbars 5. Isolators6. Cross-arms7. Lightening arrestors8. Circuit breakers9. Relays 10. Guy wire & earth wire11.Transformers

1. LINE SUPPORTS or TOWERS

Properties:

• High mechanical strength to withstand weight of conductor•Light in weight•Cheap in cost•Longer life•Easy accessibility of conductor for maintenance

TYPES OF LINE SUPPORTS:-

• Wooden poles• Steel poles• RCC poles• Lattice steel towers

Wooden poles

• Shorter span upto 50 m• Less cost & used for distribution purpose in

rural areas• Pesticides required e.g creosote oil• Used for voltage upto 20 kv• Smaller life(20-25 years)• Less mechanical strength• Made of Sal or Chir • Moderate cross-sectional area

Wooden Poles

Steel Poles•Greater mechanical strength•Longer life•Larger spans•Used for distribution purpose in cities• Three types:

Rail poles

Tubular poles

Rolled steel joints

Steel Poles

RCC(Reinforced concrete poles):-• Greater mechanical strength•Longer life•Longer spans•Good outlook•Little maintenance•Good insulating properties

Two Types:-

Single pole

Double poles

Reinforced Concrete Poles

Lattice Steel towers :-

• Longer life•Longer span•Greater mechanical strength•For long distance at high voltage•Tower footings are usually grounded by driving rods into the earth .This minimizes lightning troubles as each tower acts as lightning conductor.

---By Dhananjay Jha, Engineer (E), SJVN

Aluminium Conductor Steel Reinforced

• Aluminium Conductor Steel Reinforced consists of a solid or stranded steel core surrounded by one or more layers of strands of 1350 aluminium. The high-strength ACSR 8/1, 12/7 and 16/19 standings', are used mostly for overhead ground wires, extra long spans, river crossings, etc. The inner core wires of ACSR is of zinc coated (galvanized) steel.

---By Dhananjay Jha, Engineer (E), SJVN

ACSR Conductors

• Nomenclature :- Al/Steel/dia.

• Panther :- 30/7/3.0 mm

• Zebra :- 54/7/3.18mm

• Snowbird :- 42/3.98 + 7/2.21mm

• Moose :- 54/7/3.53mm---By Dhananjay Jha, Engineer (E), SJVN

Other conductors

• AAC (All Aluminium Conductor)

• AAAC (All Aluminium Alloy Conductor)

• AL-59

• ACCC (Aluminium conductor composite core)---By Dhananjay Jha, Engineer (E), SJVN

Conductors- Comparison

ACSR Moose AAAC Moose Al-59

Dia. (mm) 31.77 31.05 31.50

Cross sectional area (sq-mm)

597 570 586.59

Ambient Temperature (deg.C)

40 40 40

Current carrying capacity(A) at 75 C

728 699 759

At 95 C NA 952 976

SAG (m) at 85 C 13.26 14.15 14.52

---By Dhananjay Jha, Engineer (E), SJVN

3.INSULATORS

OVERHEAD LINE INSULATORS

• The insulators for overhead lines provide insulation to the power conductor from ground.

• The insulators are connected to the cross arm of the tower and the power conductor passes through the clamp of the insulator.

• The insulators have high mechanical strength, high electrical strength, high insulation resistance. They are free from impurities and moisture and can withstand the flashover phenomenon.

TYPES OF INSULATORS

1) PIN TYPE INSULATORS• Pin type insulator consists of a single or a

multiple units. They are secured to the cross arm of the pole.

• Pin type insulator can be used only up to 33kv.• Beyond operating voltage of 33kv, the pin

type insulator become too bulky and hence uneconomical.

2) SUSPENSION TYPE INSULATORS:

• Suspension insulator consists of porcelain disc units mounted one above each other. Each disc is provided with a meta cap at the top and a metal pin under.

• A string of any number of units can be built according to the line operating voltage.

• The conductor is suspended below the point of support by means of insulator string.

• The number of discs in a string depends on the line voltage and the atmospheric conditions.

The usual number of discs are:

Voltage (kv) : 66 132 220 400Number of discs: 4-5 9-10 15-16 22-23• Suspension type insulators are cheaper than

pin type insulator.• If any one string is damaged, it can be

replaced by a sound one. • Each unit is designed for a low voltage.

Depending upon the working voltage, number of units can be connected.

3) STRAIN INSULATORS:

• These are special mechanical strong suspension insulators.

• They are used to take the tension of the conductors at the line terminals, at angle towers and at road crossings.

• The string are placed in horizontal plane• Two or three strings of insulators in parallel

can ne used when the tension in conductor is very high.

4. BUS BAR ARRANGEMENT

SINGLE BUSBAR SYSTEM

Advantages:1.Simple in Design2.Less Expenditure

Disadvantages:1.In case of bus fault or bus bar isolator fault or maintenance total Substation is out of service.2.In case of maintenance of transformer circuit breaker the associated transformer has also to be shut-down. Similarly for Line also.

Single Bus With Bus Sectionaliser

Advantages:1. One complete section can be taken out for Maintenance without disturbing the continuity of other section. 2. If a fault occurs on one section of the Bus, that faulty section alone will be isolated.

Disadvantages:It will be a little more costly with the addition of one isolator and some cases with Circuit breaker, C.Ts and C&R panel.

Duplicate Busbar System

1. Repair and maintenance is carried out on the main bus without interrupting the supply as the whole load can be transffered to the spare bus.

2. Testing of the feeder circuit breakers can be done without power interruption.

3. The continuity of supply can be maintained.

Advantages:

5. ISOLATORS

•Isolators are mechanical switching devices capable of opening or closing a circuit.

•It is operated in the off load condition only.

•Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in the line.

•Isolators are generally used on both ends of the breaker in order that repair or replacement of circuit breaker can be done without and danger.

•Switches can also be classified as SINGLE-BREAK or DOUBLE-BREAK switches.•This refers to the number of places in which the switch opens or breaks the circuit. •The schematic symbol shown in figure shows that this switch breaks the circuit in two places (at both terminals). •The upper part of the schematic symbol indicates that these contacts are in the open position and the circuit will close when the switch is acted upon (manually or automatically). •The lower symbol shows closed contacts. These contacts will open the circuit when the switch is acted upon.

THE PANTOGRAPH ISOLATOR consists of three poles. Each pole con­sists of one support insulator, one rotating insulator, the pantograph mechanism and a counter contact.

6. CROSS-ARMS

•Cross arms are the holder that holds the insulator to the tower structure. •It plays a very important role since the insulators are holding the line directly.•In simple form, Cross­arms are the arm like equipment that used to hold the insulator and the cable to the power tower body. •It is used so that the cable is maintained at certain height.

TYPES OF CROSS-ARMS

1. WOODEN CROSS­ARM 2. STEEL CROSS­ARM

7. LIGHTENING ARRESTORS

TYPES OF LIGHTENING ARRESTORS

1. Rod gap arrestor2. Horn gap arrestor3. Multigap arrestor4. Expulsion type arrestor5. Valve type arrestor

•A lightning arrester is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning.

•The typical lightning arrester has a high­voltage 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

8. CIRCUIT BREAKER

WHAT IS A CIRCUIT BREAKER?

• A circuit breaker is an equipment that breaks a circuit either manually or automatically under all conditions at no load, full load or short circuit.

Operating Principle

Two contacts called electrode remains closed under normal operating conditions. When fault occurs on any part of the system, the trip coil of the circuit breaker get energized and contacts are separated.

TYPES OF

CIRCUIT BREAKER

OIL CIRCUIT BREAKER

AIR BLAST CIRCUIT BREAKER

SF6 CIRCUIT BREAKER

VACCUMCIRCUIT BREAKER

Air Blast Circuit Breaker

• Air blast circuit breakers employ a high pressure air blast as an arc quenching medium.

• Under normal condition the contacts are closed. When a fault occurs contacts are opened and an arc is struck between the them. The opening of contacts are done by a flow of air blast established by the opening of blast valve (located between air reservoir and arcing chamber ).

Oil Circuit Breaker

• Oil circuit breakers (OCB) are such type of the circuit breakers where oil is used as arc quenching media as well as insulating media between current carrying contacts and earthed parts of the breaker. •The oil used here is same as transformer insulating oil.

Disadvantages Of Oil Circuit Breaker

• It is inflammable and there is a risk of fire. • It may form an explosive mixture with air.• It requires maintenance.• Absorbs moisture, so dielectric strength

reduces.• Oil leakage problem.• Oil has to be replaced after some operations

because of the carbonization of oil.

Vacuum Circuit Breaker

• Vacuum is used as an arc quenching medium.

• Have greatest insulating strength.

• 10-7 to 10-5 pressure is to be maintained.

• Used in 11KV panel in control room of grid station.

Advantages

• Compact, reliable and have longer life.• No fire hazards.• No generation of gas during and after

operation.• Can interrupt any fault current.• No noise is produced while operating.• Require less power for control operation.

SF6 Circuit Breaker• Sulphur Hexafluoride (SF6) gas

is used as an arc quenching medium.

• SF6 is an electro-negative gas.• It has strong tendency to absorb

electrons.• When contact are opened in a

high pressure flow of SF6 gas, arc produced.

• Free electron in the arc are captured by the gas which build up enough insulation strength to extinguish arc.

• It is much effective for high power and high voltages services

Advantages

• Simple construction, less cost.• SF6 gas is non flammable, non toxic & chemical

inert gas.• Same gas is recirculated in the circuit.• Maintenance free C.B.• Ability to interrupt low and high fault current.• Excellent Arc extinction.

9. RELAY

What is relay?

A relay is a automatic device which senses an abnormal condition of electric circuit and closes its contacts

PROTECTIVE RELAY

CurrentVoltageInsulationTemperature

Pick up levels Time

Visual indication

Warning alarm

Remove power

output

input

setting

A protection relay is a smart device that receive inputs, compares them to set points, and provide outputs .Inputs can be current ,voltage ,resistance or temperature. Outputs can include visual feedback in the form of indicator lights.A diagram is shown below.

INPUT:- A relay needs information from the system to make decision.

SETTING:- The user programs setting that allow relay to make decision.

PROCESS:- Once input are connected and setting are program, the relay compares there and make decision.

OUTPUT:- The relay will operate a switch to indicate that the input has surpassed a setting. Or the relay can provide notification through visual feedback such as a meter or LED.

TYPES OF RELAY Types of protection relays are mainly based on their characteristics ,logic, on actuating parameter & operation mechanism.

Based on operation of mechanism1. Electromagnetic relay2. Static relay3. Mechanical relay

Based on actuating parameter1. Current relay2. voltage relay3. Frequency relay4. Power relay

Based on characteristics1. Definite time relay2. Inverse time relay with definite minimum3. Instantaneous relays4. IDMT with Instrument5. Stepped characteristics6. Programmed switches7. Voltage restraint over electric current relay

Based on application

1. Primary relay2. Back up relay

Based on logic 1. Differential 2. Unbalance 3. Neutral displacement 4. Direction 5. Restricted earth fault 6. Over fluxing 7. Distance scheme 8. Bus bar protection 9. Reserve power relay 10. Loss of excitation 11. Negative phase sequence relay

10. Guy wire & Earth wires

A guy-wire, guy-line, or guy-rope, also known as simply a guy, is a tensioned cable designed to add stability to a free-standing structure.

Earth wires are bare conductors supported at the top of transmission towers. They serve to shield the line and intercept lighting stroke before it hits the current carrying conductors below.Ground wires normally do not carry current.

11.TRANSFORMERS

What is an Instrument Transformer ?:

•It is a transformer that is used in conjunction with any measuring instrument (i.e., Ammeter, Voltmeter, Wattmeter, Watt-hour-meter, …etc.)or protective equipment (i.e., Relays).

•It utilizes the current-transformation and voltage transformation properties to measure high ac current and voltage.

Types of instrument transformers :

1.Current transformers

2.Potential transformers

Instrument Transformers

What is current Transformer (CT)?: •A current transformer is a transformer, which produces in its secondary winding low current, which is proportional to the high current flowing in its primary winding. •The secondary current is usually much smaller in magnitude than the primary current.• The design of CT depends on which type of instrument is connected to its secondary winding. Measuring instrument OR Protective instrument.

-Measuring instrument CT is expected to give accurate results up to a maximum of 125% of its normal full-load rated current.

-Protective instrument CT is expected to be accurate for up to 20 times of its normal full-load rated current (about 2000% of its full-load rated current!!..??).

Current Transformers

What is a Potential Transformer (PT) or (VT)?:

•A PT or sometimes called VT is a step-down transformer having many primary turns but few secondary turns.

•In a step-down transformer the voltage decreases and the current increases, thus voltage can be easily measured by using a low-range voltmeter instrument.

•The voltage is stepped-down in a known ratio called the voltage ratio.

Potential Transformers

Power transformer

•Power transformers are generally used in transmission network for stepping up or down the voltage level. •It operates mainly during high or peak loads and has maximum efficiency at or near full load.

REFERENCES

• V.K.Mehta• http://powermin.nic.in/• http://www.electrical4u.com/

Thank you..