1. INTRODUCTION

1.1 About RTPP

1.2 Turbo Generator in RTTP

1.3 HT & LT System

1. INTRODUCTION

1.1 ABOUT RTPP

Energy is the basic necessity for the economic development of a

country. Energy may be needed as heat, as light, as motive power etc. The present

day advancement in science& technology had made it possible to convert electrical

energy into any desired form.

Electrical energy is superior to all other forms of energy due to following reasons

Convenient form: Easy to convert into other forms of energy.

Easy control: Electrically operated machines have simple and

convenient starting, for control and operation.

Greater Flexibility: It can be easily transported from one place to

another with the help of conductors.

Cheapness: It is overall economical to use this form of energy for

domestic, commercial and industrial purposes.

High Transmission Efficiency: Electrical energy can be transmitted

from centers of generation to the consumers with high transmission

efficiency.

Electrical energy is produced from energy available in various forms in nature. The

energy due to sun and wind has not been utilized on large scale due to limitations. At

present, the other sources namely water, fuels, and nuclear energy are primarily used

for the generation of electrical energy.

In order to overcome the low voltage problems in Rayalaseema regions, the

government had decided to establish a generating station. Since the climatic and

geographical conditions are not favorable to hydroelectric power stations, the steam

(coal based) power plant was opted and then the “Origin of RTPP” began.

LOCATION:

The Rayalaseema thermal power plant is located at Kalamalla that is

12km from Proddatur. This is popularly known as “THERMAL”. RTPP is spread in a

wide area of about 2800 acres. It is easily approachable by both rail and road. The

National Highway7 runs at 7km from power station. The nearest railway station is

Muddanur. The project envisages the initialization of two thermal generating units

each capacity 210MW

.

1.2 TURBO GENERATOR IN RTPP:

INTRODUCTION:

In RTPP we use a Synchronous ac machine in which the rotor moves

at a speed of 3000rpm such that a constant frequency of 50Hz is maintained. The

nameplate details of turbo generator are

Power : 210MW

Reactive Power : 120MVAR

Apparent Power : 247MVA

Current : 9050A

General Voltage : 15.75KV

Speed : 3000rpm

Power Factor : 0.85

Frequency : 50Hz

Rated Field Current : 2080A

Rated Field Voltage : 270V

STRUCTURE OF GENERATOR:

The 210MW turbo generator having cylindrical rotor uses direct Hydrogen

cooling for the rotor winding and indirect H2 cooling for the stator winding .The

losses in the remaining generator components such as iron losses, friction and

windage losses and stray losses are also dissipated through hydrogen.

The generator frame is pressure resistant up to 10bars and gas tight and

equipped with end oil seals. At each end the hydrogen coolers are arranged

horizontally inside the stator frame. Generator consists of the following components.

A) Stator:

Stator frame

Stator core

Stator winding

Hydrogen coolers

B) Rotor:

Rotor shaft

Rotor winding

Rotor retaining rings

Field connection

PRINCIPLE OF GENERATION:

A steam power plant converts chemical energy of fossil fuel into thermal

energy, thermal energy to kinetic energy, kinetic energy to mechanical energy and

then into electrical energy. Raising the temperature and pressure of steam in boiler

and then expanding it in the turbine achieve this.

Schematic arrangement of coal fired system power plant:

The entire arrangement may be divided into four main parts.

Fuel and Ash circuit

Air and Flue gas circuit

Feed water and steam circuit

Cooling water circuit

1.2.1 EXCITATION SYSTEMS:

The purpose of all excitation system is to provide dc, to rotating

electromagnetic field. The three types of excitation systems are,

1. Static excitation

2. Separate excitation

3. Brush less excitation

Here, we are using Brush less excitation system. It consists of Permanent

Magnet Generator (PMG), AVR, Main Exciter and Diode wheels.

1.2.2 TURBINE:

Steam Turbines are used to convert the steam energy into mechanical

rotational energy. Major steam Turbines are

1. Reaction Turbines

2. Impulse Turbines

Here in RTPP Reaction Turbines are used.

1.2.3 TRANSFORMERS:

Types of transformers generally used when supply is to be taken from grid and

it is used for the plant. Power transformer or generator transformer is used to step up

the generated voltage and give it to transmission line. Unit auxiliary transformer is

used for providing power to auxiliary system inside the plant, taking a part of the

generated voltage.

UNIT AUXILARY TRANSFORMER (UAT):

The various boiler auxiliaries and turbine auxiliaries together are called unit

auxiliaries. The power supplied for the auxiliaries by the same generator via the unit

auxiliary transformer. The auxiliaries of the generator units are supplied power at

6.6KV AC. Each unit has 2 UAT’s of rating 15MVA, 15.75/6.6KV.

STATION TRANFORMER:

The common station auxiliaries like lighting, feed water pumps, air

conditioning and cooling systems, battery-charging systems, oil filtration plants etc

are supplied power through the step down station transformer. The starting power

for unit auxiliaries is usually taken from the main bus via station transformer of

rating 31.5MVA, 220KV/6.6KV connected to each bus.

1.2.4 SWITCH YARD:

INTRODUCTION:

The 220KV RTPP switchyard is located at 17kms away from

Proddutur to evacuate the power from generating transformers to the six outgoing

feeders. The feeders are as given below

CUDDAPAH-1&2

YERRAGUNTLA-1&2

ANANTHAPUR-1&2

These outgoing feeders are connected to a common bus bars named as

Bus A

Bus B

Transfer Bus

RTPP there are two generating units of 210MW, 15.75KV. The 240MVA power

transformer feed to one bus bar. There are two unit auxiliary transformers, two

generators, which are directly connected in parallel with the generator, to supply

power to auxiliary equipment in power plant.

ISOLATORS:

Isolators (disconnecting switches) which operate under no load or no current

condition.

Isolators are not designed to make or break a circuit under load or short circuit

conditions. Isolators do not have rated making current or breaking current capacity.

As the name implies isolator isolates one portion of circuit from another.

Types of isolators based on number of poles

Single Pole Type

Double Pole Type

Three Pole Type

Types of isolators based on types of breaking

Horizontal break type Vertical break type

Single break type Double break type

Mainly two types of isolators are there. They are

Non-Stagger isolator: Earth switch is mounted on the frame of isolator

and the isolator of all phases will be in line.

Stagger isolator: Earth switch is not mounted on the frame of isolator

and the isolator of three phases arranged in diagonal line.

The isolator used in RTPP is double break type and the

maximum design voltage is 245V.

EARTHING SWITCH:

Earthing switch is connected between the line conductor and earth. Normally

it is open when the line is connected, the earthing switch is closed to discharge the

trapped voltage through the line when it is disconnected, and there is some voltage

on the line to which the capacitance between line and earth is charged. This voltage

is significant in high voltage system. Before proceeding with maintenance work

these voltages are discharged to earth, by closing the switch.

CIRCUIT BREAKER:

Circuit breaker essentially consists of fixed and moving contacts called

electrodes under normal operating condition, these contacts remains closed and will

not open automatically unless the system becomes faulty.

Of course, it can be opened manually or by remote control whenever desired

when a fault occurs on any part of system, the trip coils of the circuit breaker get

energized and the moving contacts are pulled apart by some mechanism, thus opening

the circuit. In 220KV RTPP switchyard SF6 circuit breaker is used. Generally 3 to 4

vaccum circuit breakers are presently used in RTPP.

CURRENT TRANSFORMER:

Current Transformer is used to reduce the heavy current flowing in an

element of a power system to low values that are suitable for relay operation. The

circuit rating of protective relay is usually 5 or 1 Amp. Besides reducing the current

level the current transformer also isolates the relay circuit from the primary circuit,

which is a high voltage power circuit, and allows the use of standardized current

rating for relays.

VOLTAGE TRANSFORMER:

Voltage transformers are used for both protection and measuring

purposes. These transformers in generating stations need mainly for protection

purposes rather than to measure parameters. These have an accuracy of 0.2 classes,

but the measuring transformers are uses class 1.

DRY TYPE TRANSFORMER:

Dry type transformer is used to step down the obtained 6.6 Kv to 415V

for LT motors. In these transformer no medium for cooling purpose such as vaccum,

SF6 is used. These transformers have a rating of 250 KVA, 1000KVA, 1600KVA,

2000KVA & 2500KVA.

1.3 HT AND LT SYSTEMS:

High-tension systems having the voltage rating of 6.6Kv supply. These HT

supply is used for operating different auxiliary units. They are turbine and boiler

units. Also, the coal and ash handling plants. Similarly Low Tension systems having

the voltage rating of 415v/230v.These LT supply is used for controlling valves of

above auxiliaries and LDS A/C etc. This system consists of its single line diagrams

and its motors so these are considered as HT&LT systems.

1.3.1 FUEL:

Coal is the most commonly used fuel in thermal station. Coal occurs naturally

in seams and is the result of decay of vegetable matter accumulated in the earth

millions of years ago having got transformed by the action of pressure and heat. As

mined raw coal usually contains impurities such as pieces of slate etc., With the result

that some amount of processing is required at the colliery before it can be shipped.

Coals are classified in increasing order of heat value into the following :

1. Peat

2. Lignite

3. Bituminous

4. Semi-Bituminous

5. Semi-Anthracite

6. Anthracite

Anthracite is fully transformed coal of the best type while Peat is the first stage of this

transformation.

1.3.2 COAL HANDLING PLANT:

Coal from the wagons is unloaded using charger arm and

wagon tripper, which is driven by hydraulic systems. Raw coal is taken on the MCB,

from which it is feed to the crusher. The coal is crushed into a size of 20mm. If any

problem is occurs in one path, the process is diverted to another path from the motor

control cabin itself.

1.3.3 ASH HANDLING PLANT:

A good ash handling plant should have large capacity to handle clinkers,

boiler refuse, soot and dust with minimum attention of operators. It should be able to

handle hot and wet ash effectively. The major dust and ash collectors used in thermal

power plant are electrostatic precipitators. There are four groups into which modern

ash handling systems may be divided.

Those are:

1. Mechanical Handling System

2. Hydraulic System

3. Pneumatic System

4. Steam Jet System

Major emissions from thermal power stations are fly ash, carbon ash

(known as cinder), smoke, dust & irritating vapours like CO, SO2 & Nitrogen Oxides.

These emissions are objectionable if the content exceeds a particular limit.

1.3.4 MILLS:

Mills is one that grinds down pieces of coal into fine powder, which is feed to

the boiler furnace.

1.3.5 FANS:

Fans are provided throughout the steam electric generating unit to supply air

on to exhaust flue gas to meet the needs of various systems. In addition fans are used

for building heat and cold to prevent contamination due to inter leakage & cooling for

a wide variety of equipment from lubricating oil coolers to mechanical draft cooling

towers.

Power plant applications that receive the largest fans for steam generation are:

Forced draft fan (FD)

Primary air fan

Induced draft fan (ID)

FD fans supply combustion air to the steam generator, PA fans normally

handle relative low-flow and very high pressure differentially. ID exhaust combustion

products from the steam generator. ID fans exhaust combustion ID fans control

furnace pressure.

1.3.6 VALVES:

The primary function of stock valves is to provide backup protection for steam

turbine. The primary function of control valves is to regulate the steam flow to the

turbine and thus control the output power of steam turbine generator.

1.3.7 AUXILIARY SYSTEM:

Some of the common auxiliary systems are,

CIRCULATING WATER SYSTEMS:

Most power plants use a circulation water system as the mechanism, which

steam transfers cycle waste heat form steam cycle to the ambient environment.

COOLING POND:

It is the simplest and least expensive alternate method for providing

circulating water, to the plant. It consists of tank of water in which water to be cooled

is distributed by pipe and sprayed through nozzles at a suitable pressure. Water falls

over the pond in a fine spray and has considerable surface of contact with atmospheric

air and this expedites evaporation. But the heat of evaporation is with drawn from the

water itself with the result that it is cooled. A small amount of cooling is due to

conduction and radiation also.

CONDENSER:

The function of the condenser is to condense exhaust steam from the main

power cycle steam turbine. The use of condenser improves the efficiency of the

power plant by decreasing the exhaust pressure of the steam below atmosphere.

Another advantage of condenser is that condensed steam can be recovered and this

provides the source of good and pure feed water to the boiler. There by reducing

considerably the water softening plant capacity. Air and non-condensable gases are

also removed from the steam when it passes through the condenser.

CIRCULATING WATER PUMPS:

Circulating water pumps supply cooling water to the required flow rate and

head to the power plant condenser and the plant auxiliary cooling water heat changes.

CONDENSATE PUMPS:

These pumps pump the condensate water from the condensate hot well to a

deaerating heater.

BOILER FEED PUMPS:

A BFP is a pump, which supplies feed water to steam generator for the

production of steam.

RECIRCULATING COOLING SYSTEM:

In this system, the circulating water to the cooling tower, which rejects the

heat to the atmosphere, carries waste heat removed from the steam turbine exhaust.

COOLING TOWERS:

In a cooling tower the amount of water, which is large is divided in smaller

quantities practically of the size of drops. These water drops fall from a height of 8 –

10 meters to the bottom of the cooling tower. The height of the cooling tower and its

water handling capacity are suitably designed for particular cases. Cooling towers are

classified as atmospheric (or natural draught) and mechanical draught towers.

2. EQUIPMENTS USED FOR PROTECTION

2.1 Need for protection

2.2 Circuit breakers

2.3 Protective Relays

2.4 Isolators

2.1 NEED FOR PROTECTION:

A fault in power system is different as defect in its electrical circuit due to

which the flow of current is diverted from the intended path faults that are caused by

breaking of conductor or failure of insulation. Fault impedance is generally low and

fault currents are generally high. During the faults, the three phase voltage becomes

unbalanced and the supply to the neighbouring circuit is effected, fault current being

excessive they can damage not only the fault equipment, but also the installation

through which the fault current is fed.

Fault in certain important equipment can affect the stability of power systems.

In order to safe guard the equipment representing the systems should be capable of

isolating the faulty section and should be left unprotected. The choice of protection

depends upon several aspects such as type, rating, rating of the protected equipment,

its importance of location, probable abnormal condition cost etc. The protection

relaying senses the abnormal conditions in a part of the system and gives an alarm to

the protected equipment (circuit breaker) so that the faulty section is isolated from the

healthy section.

2.2 CIRCUIT BREAKERS

2.2.1 Introduction:

During the operation of power system, it is often desirable and necessary to

switch on or off the various circuits under both normal and abnormal conditions. In

earlier days this function is performed by a switch and a fuse placed in series with the

circuit. However, such a means of control pocess two disadvantages. Firstly, when a

fuse blows out, it takes quite sometime to replace it and restore supply to the

customers. Secondly, a fuse cannot successfully interrupt heavy fault currents that

result from faults on modern high-voltage and large capacity circuits. Due to these

disadvantages, the use of switches and fuses is limited to low-voltage and small

capacity circuits.

With the advancement of power system, the lines and other equipment operate

at very high voltages and carry large currents. The arrangement of switches along

with fuses cannot serve the desired function of switchgear in such high-capacity

circuits. This necessitates employing a more dependable means of control, which is

obtained by the use of circuit breakers. A circuit breaker can make or break a circuit

either manually or automatically under all conditions viz. No-load, full-load and

short-circuit conditions. This characteristic of the circuit breaker has made it very

useful equipment for switching and protection of various parts of the power system.

A circuit breaker is a piece of equipment, which can

1. Make or break a circuit either manually or by remote control under normal

conditions.

2. Break a circuit automatically under fault conditions.

3. Make a circuit either manually or by remote control under fault conditions.

Thus a circuit breaker incorporates manual as well as automatic control for

switching functions.

2.2.2 OPERATING PRINCIPLE:

A circuit breaker essentially consists of fixed and moving contacts called

electrodes. Under normal operating conditions, these contacts remain closed and will

not open automatically until and unless the system becomes faulty. Of course, the

contacts can be opened manually or by remote control whenever desired. When a

fault occurs on any part of the system, the trip coils of the breaker get energized and

the moving contacts are pulled apart by some mechanism, thus opening the circuit.

When the contacts of a circuit breaker are separated under fault conditions, an

arc is struck between them. The current is thus able to continue until the discharge

ceases. The production of arc not only delays the current interruption process but it

also generates enormous heat, which may cause damage to the system or to the

breaker itself. Therefore, the main problem in a circuit breaker is to extinguish the arc

within the shortest possible time so that heat generated by it may not reach a

dangerous value.

2.2.3 CLASSIFICATION OF CIRCUIT BREAKERS:

There are several ways of classifying the circuit breakers. However, the most

general way of classification is on the basis of medium used for arc extinction. The

medium used for arc extinction is usually oil, Sulphur Hexa Fluoride (SF6) or

vacuum.

Accordingly, circuit breakers may be classified into:

1. Oil circuit breakers that employ some insulating oil for arc extinction.

2. Air-blast circuit breakers in which high pressure air-blast is used for

extinguishing the arc

3. Sulphur Hexa Fluoride circuit breakers in which sulphur Hexa Fluoride

(SF6) gas is used for arc extinction.

4. Vacuum circuit breakers in which vacuum is used for arc extinction.

SULPHUR HEXAFLUORIDE (SF6) CIRCUIT BREAKERS:

In such breakers, sulphur Hexa Flouride (SF6) gas is used as the arc-

quenching medium. The SF6 is an electro-negative gas and has a strong tendency to

absorb free electrons. The contacts of the breaker are opened in a high-pressure flow

of SF6 gas and an arc is struck between them. The gas to form relatively immobile

negative ions rapidly captures the conducting free electrons in the arc. This loss of

conducting electrons in the arc quickly builds up enough insulation strength to

extinguish the arc. SF6 has excellent insulating strength, because of its affinity for

electrons (electro negativity) i.e., the neutral gas molecule absorbs whenever a free

electron collides with the neutral gas molecule to form negative ion, the electron. The

attachment of the electron with the neutral gas molecule may occur in two ways.

SF6+e SF6

SF6+e SF5 + F

The negative ions formed are relatively heavier as compared to free electrons and

therefore under a given electric field the ions don’t attain sufficient energy to lead

cumulative ionization in the gas. Thus these processes represent an effective way of

removing electrons from the space, which otherwise would have contributed to form

electron avalanche. This property therefore gives rise to very high dielectric strength

for SF6 .The SF6 circuit breakers have been found to be very effective for high power

and high voltage service.

Structure of SF6 Molecule:

SF6 CHARACTERISTICS:

1. The gas is neither combustible nor toxic.

2. It is chemically stable and will not age with time.

3. Breaking capacity of SF6-gas is high even at relatively low pressure,

because of its superior dielectric and thermal properties.

4. The interruption in SF6-gas is not forced and thus no over-voltages are

generated. No damping resistor or surge arrester is needed, not even

when controlling small motors.

5. SF6 breaker is silent in operation and moisture ingression into the gas

cycle is almost nil.

6. SF6 breaker performance is not affected due to variation in

atmospheric conditions.

7. SF6 breaker is compact in size and electrical clearances are drastically

reduced.

8. The dielectric strength at the relevant pressure is about 3 times higher

than air and is roughly on par with oil.

9. Any leakage is easily detected. To provide an extra margin of safety,

the breaker is capable of interrupting its rated current at rated voltage

even at atmospheric pressure.

10. SF6 has excellent heat transfer properties because its high molecular

weight together with its low gaseous viscosity enables it to transfer

heat by convection more effectively than the common gases.

11. SF6 breakers can withstand severe RRRV and thus are most suitable

for short line faults without switching resistors and can interrupt

capacitive currents without restriking.

12. Using SF6-gas at low pressure and low velocity, the current chopping

can be minimized.

FIG: SF6 Circuit Breaker

CONSTRUCTION:

It consists of fixed and moving contacts enclosed in a chamber (called arc

interruption chamber) containing SF6 gas. This chamber is connected to SF6 gas

reservoir. When the contacts of breaker are opened, the valve mechanism permits a

high pressure SF6 gas from the reservoir to flow towards the arc interruption

chamber. The fixed contact is a hollow cylindrical current carrying contact fitted with

an arc horn. The moving contact is also a hollow cylinder with rectangular holes in

the sides to permit the SF6 gas to let out thro’ these holes after flowing along and

across the arc. The tips of fixed contact, moving contact and arcing horn are coated

with copper-tungsten arc resistant material. Since SF6 gas is costly, it is reconditioned

and reclaimed by suitable auxiliary system after each operation of the breaker.

WORKING PRINCIPLE:

In the closed position of the breaker, the contacts remain surrounded by SF6

gas at a pressure of about 2.8 kg/cm2. When the breaker operates, the moving contact

is pulled apart and an arc is struck between the contacts. The movement of the moving

contact is synchronized with the opening of a valve which permits SF6 gas at

14kg/cm2 pressure from the reservoir to the arc interruption chamber., the high

pressure flow of SF6 rapidly absorbs the free electrons in the arc path to form

immobile negative ions which are ineffective as charge carriers, the result is that the

medium between the contacts quickly builds up high dielectric strength and causes the

extinction of the arc. After the breaker operation the valve is closed by the action of a

set of springs.

The Puffer Principle:

ADVANTAGES:

Due, to the superior arc quenching properties of SF6 gas, the SF6 circuit

breakers have many advantages over oil or air circuit breakers. Some of them are

listed below:

1. Due to the superior arc quenching property of SF6, such circuit

breakers have very short arcing time.

2. Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such

breakers can interrupt much larger currents.

3. The closed gas enclosure keeps the interior dry so that there is no

moisture problem.

4. The SF6 breakers have low maintenance cost, light foundation

requirements and minimum auxiliary equipment.

DISADVANTAGES:

1. SF6 breakers are costly due to the high cost of SF6.

2. Since SF6 has to be reconditioned after every operation of the breaker,

additional equipment is required for this purpose

2.3 PROTECTIVE RELAYS:

2.3.1 INTRODUCTION:

In a power system consisting of generators, transformers, transmission and

distribution circuits, it is inevitable that sooner or later some failure will occur

somewhere in the system. When a failure occurs on any part of the system, it must be

quickly detected and disconnected from the system. There are two principle reasons

for it. Firstly, if the fault is not cleared quickly, it may cause unnecessary interruption

of service to the customers. Secondly, rapid disconnection of faulted apparatus limits

the amount of damage to it and prevents the effects of fault from spreading into the

system.

Inorder to generate electric power and transmit it to customers millions of rupees

must be spent on power system equipment. This equipment is designed to work under

specified normal conditions. However a short circuit may occur due to failure of

insulation caused by

1. Over voltages due to switching

2. Over voltages due to direct and indirect lightning strokes.

3. Bridging of conductors by birds etc.

The detection of a fault and disconnection of a faulty section or apparatus can

be achieved by using fuses or relays in conjunction with circuit breakers. A fuse

performs both detection and interruption functions automatically but its use is limited

for the protection of low-voltage circuits only. For high voltage circuits (say above

3.3 kV), relays and circuit breakers are employed to serve the desired function of

automatic protective gear. The relays detect the fault and supply information to the

circuit breaker that performs the function of circuit interruption. In this chapter, we

shall focus our attention on the various types of relays and their increasing use for the

protection of power system.

PROTECTIVE RELAYS:

A protective relay is a device that detects the fault and initiates the operation

of the circuit breaker to isolate the defective element from the rest of the system.

The relays detect the abnormal conditions in the electrical circuits by

constantly measuring the electrical quantities, which are different under normal and

fault conditions. The electrical quantities, which may change under fault conditions,

are voltage, current, frequency and phase angle. Through the changes in one or more

of these quantities, the faults signal their presence, type and location to the protective

relays. Having detected the fault, the relay operates to close the trip circuit of the

breaker. This results in the opening of the breaker and disconnection of the faulty

circuit.

CLASSIFICATION OF RELAYS:

Protective relays may be classified depending upon their construction and principle of operation such as:

i) Ordinary Electromagnetic Relays: Moving plunger, Moving Iron,

Attracted Hinged and balanced beam types of relays are various

examples. Such relays are actuated by d.c or a.c quantities.

ii) Electromagnetic Induction relays are simple induction relays, which

use the principle of the induction motor in their operation. Such relays

are actuated by a.c. quantities only.

iii) Electro Thermal Relays- Thermal overload protection using bimetallic

strip.

iv) Physico-Electric relays-Buchholy’s relay is an example of this.

v) Static relays employing thermionic valves, transistors, or magnetic

amplifiers to obtain the operating characteristics.

vi) Electro dynamic relays operate on the same principles as the moving

coil instrument.

The relay having three main parts:

1. First part is the primary winding of a current transformer (C.T.), which

is connected in series with the line to be protected.

2. Second part consists of secondary winding of C.T.and the relay-

operating coil.

3. Third part is the tripping circuit, which may be either a.c. or d.c. It

consists of a source of supply, the trip coil of the circuit breaker and

the relay stationary contacts.

2.3.2 RAMDE RELAY:

The RAMDE is an integrated microprocessor-controlled RMS-measuring

motor protection device. It has different protective functions and characteristics.

They are,

1. Thermal overload relay, which continuously monitors the thermal content

in the stator winding. Issues an alarm signal at 95% and trips the motor at

104% of permissible thermal content.

2. Instantaneous short-circuit protection.

3. Short circuit protection.

4. Protection against locked rotor during operation.

5. Protection against prolonged starting times.

6. Easy commissioning, testing and service.

7. Auxiliary voltage range 48-220v120% independent of dc or ac supply.

Fig: RAMDE RELAY

FUNCTIONAL PRINCIPLE OF RAMDE RELAY:

The three-phase induction motor is the workhorse of industry. With improved

designs the presently day motors are very reliable with high efficiency whose

performance limits can be defined with reasonable accuracy. Protection methods of

by-gone days can no longer provide a proper protection to the present day motors. A

more reliable and accurate microprocessor based protection system can discriminate

more accurately between permissible operation, non-permissible operation and

electrical faults. This means that the number of unwanted interruptions can be

minimized and/or the utilization of the equipment can be increased.

The microprocessor based motor protection relay type RAMDE provides protection

against the following abnormal conditions:

1. Over load.

2. Short circuits.

3. Long starting times.

4. Imbalance.

5. Phase failure.

6. Inter turn short circuits.

7. Stalled rotor.

All current measurement in RAMDE relay is RMS based as against relays

with makes use of the peak value. RMS measurement has been specifically chosen to

take into account the harmonics that are generated and transmitted in the power

system. Practically every industry makes use of direct current (e.g. Electrolysis,

electroplating, DC drives for constant speed etc.) derived by means of thyristor

converters. The harmonics generated by the thyristor converters is given by the

formula h =k p +1

Where ‘h’ is the order of the harmonic.

K is an integer from 1 to n

P is the pulse number

Thus for a three pulse converter the harmonics generated are 2, 4, 5, 8, 10 etc.

however, the industry standard is a six pulse converter and thus the harmonics

generated are 5, 7, 11,13,17,19 etc.

One reason for heating of the motor is due to eddy currents, which are directly

proportional to the frequency. Apart from eddy current heating, fifth harmonic

currents have the peculiar property of producing negative torque. Again if the

motor’crawls’whose speed is in the vicinity of 7th harmonic, the rotor experiences a

negative torque. These negative torques are also partly responsible for the heating of

the motor.

A sensitive short circuit and earth fault protection of the stator is possible due

to the fact a wide range in settings is available.

Protection against long starting times is detected purely by the current levels.

This function is activated when the current is lower than 6 to 10% of the rated motor

current for more than 2 secs. And has later risen above this value. Trip command is

issued if the current does not drop below 112% of the rated motor current during the

set starting time settable from 2 to 60 secs.

To detect unbalance in the supply voltage, the relay employs a novel technique

of measuring the difference between phase currents. Should this difference exceed a

maximum permissible limit, a trip command is issued. The same principle that has

been used for detecting phase unbalance has been extended to provide protection

against single-phase shorts, two-phase shorts and phase failure. The relay type

RAMDE thus has features that can protect a motor for every conceivable fault

encountered in the day-to-day operation of the plant.

PROTECIVE

FUNCTION

BFP MILL CWP ID

Fan

PA

Fan

FD

Fan

BAP CEP BCW ACW COMP CRUSH CONV

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RAMDE RELAY SETTINGS FOR 6.6KV MOTORS

Capacity KW

4000 2100 1650 1600 1250 750 525 300 300 250 200 410 225

FL Current 407 248 188 182.2 130.5 82 58 33 34.5 59 26.4 48.3 25

CT Ratio 500/1/5 300/1/5 200/1/5 200/1/5 150/1/5 100/1/5 70/1/5 40/1/5 40/1/5 30/1/5 30/1/5 50/1/5 30/1/5

2.3.3 ITX RELAYS:

MAIN FEATURES:

1. Extremely large input voltage range.

2. Regulated output voltage.

3. Insensitive to surge voltages.

4. Over voltage protection on the output.

5. High efficiency compared to series stabilization.

Fig: ITX RELAY

FUNCTIONAL PRINCIPLE:

The relay protects the motor against the following hazardous conditions:

1. Inter phase short circuits in the stator winding.

2. Prolonged starting of asynchronous motors.

3. Blocked rotor (restricted).

4. Overload.

5. Earth faults in the stator winding.

The unit receives all required data from the current transformers. The

amplitude of individual phase currents, their phase angle or the proportion of the

reverse-current and zero sequence components are monitored. Three current inputs

are available for connection with the main current transformers. Each on is

individually arranged for a rated transformation ratio of 1A or 5A. For short circuit

and overload protection, the greatest of the phase currents is taken into account

(greatest value tripping).

2.4 ISOLATORS:

Isolators is on of the protective equipment which acts as a switch while

opening the faulty section of the system permanently form the healthy system under

circuit breaker in open condition it can be operated manually or automatically from

the control room.

Isolator operates only when the transient faults occurred that is continuous

occurrence of faults with small time periods. At that time circuit breakers contacts

open and close continuously. This cannot remove fault permanently from the system.

In order to avoid this, isolators are open with circuit breakers contacts in open

condition.

The main constraint in opening of isolators is that the circuit breakers contacts

should be in open condition. Because isolators thus not have any arc quenching

provision so if isolators are open with circuit breakers in closed condition the arc

forms between to contacts of isolators and fault current flows through this into to

healthy part of the system.

3. HT AND LT SYSTEMS

3.1 HT systems

3.2 Single line diagram of HT systems

3.3 Types of HT motors

3.4 Auxiliaries

3.5 LT systems

3.6 Single line diagram of LT systems

3.7 Types of LT motors

3.1 HT AND LT SYSTEMS:

In many industries and substations and at the generating stations HT & LT

systems are required. In case of the generating station, like thermal station or steam

power station which converts heat energy of coal combustion into electrical energy. In

the steam plants steam is produced in boiler by utilizing the heat of coal combustion.

The steam is then expanded in the steam turbine and is condensed in a condenser to

fed boiler again. The steam turbine drive, alternator that converts mechanical energy

into electrical energy.

The whole arrangement can be divided into the following stages:

1. Coal and ash handling arrangements.

2. Steam generating plants.

3. Steam turbine.

4. Alternator.

5. Feed water.

6. Coal arrangements.

1. COAL AND ASH HANDLING PLANTS:

The coal is delivered from coal storage is (crushed into small pieces) in

order to increase its surface exposure, thus promoting rapid combustion without using

large quantity of excess air. The pulverized coal is fed to the boiler by belt conveyors.

The coal is burnt in the boiler and the ash produced after the complete combustion of

coal is removed to the ash handling plant and then delivered to the ash storage plant

for disposal. The removal of the ash from the boiler furnace is necessary for proper

burning of coal.

2. STEAM GENERATING POINT.

The steam generating plant consists of a boiler for the production of

steam and other auxiliary equipment for the utilization of flue gases.

BOILER:

Boiler is a device meant for producing for steam under pressure. Steam boilers are

broadly classified into fire tube and water tube types. Genrally water tube boilers are

used for electric power stations. In the boiler heat transfer takes place through the walls

of the tubes and the drum or drums are protected from direct contact with hot flue

gases. The steam is super heated in a super heater before passing through boiler to the

prime mover. The fuel is burnt in the furnace of the boiler. For efficient combustion

enough air has to be supplied.

ECONOMIZER:

Economizer is a one of the part in steam generating unit, which feeds water for

heating. So by heating the feed water the temperature of the feed water is increased.

AIR PRE-HEATER:

Since the entire heat of the flue gasses cannot be extracted through the

economizers air preheaters are employed to recover some of the heat in these gases.

On an average an increase of 20C in the air temperature results in an increase in the

boiler efficiency by 1%.

The use of air preheatre is more economical with

pulvarised fuel boilers because the temperature of flue gases going out is sufficiently

large and high air temperature is always desirable for better combustion.

SUPERHEATERS AND PREHEATERS:

Their function is to super heat steam to the desired temperature. By

removing the last traces of moisture from the saturated steam coming out of the boiler

and by increasing its temperature sufficiently above saturation temperature there is an

overall increase in cycle efficiency. Besides, too much condensation in the last stages

of turbine is avoided.

STEAM PRIME MOVER:

The steam turbine has several advantages over steam engine as a prime

mover. It has higher thermodynamic efficiency since steam can be expanded to a

lower final temperature than is possible in a steam engine. The basic construction of a

steam turbine is simple. There is no need of piston rod mechanism

And side valves; no fly wheel is needed. Also a steam turbine can be built in large

sizes as much as 1000MW. No wearing action being involved in maintenance of a

steam turbine is comparatively much simple. Problem of vibrations is also much less

since high operating speeds result in a lower weight of rotating parts of same power.

Steam turbines are generally of two types:

1. Impulse

2. Reaction

In an impulse turbine the steam expands in the stationary nozzles and attains a

high velocity. Potential energy in steam due to pressure and internal energy is

converted to kinetic energy when passing through the nozzles. There are a number of

stationary blades and moving blades. A partial drop of pressure is used to allow the

steam into the moving blades. The pressure is gradually reduced in the blades as the

steam passes through them.

FEED WATER:

The condensate from the condenser is used as feed water to the boiler.

Some water may be lost in the cycle, which is suitably made up from external

source. Water heaters and economizer heat the feed water on its way to the boiler.

This helps in raising the overall efficiency of the plant. The condensate from the

condenser is used as feed water to the boiler. So circulating water pump is required.

So from the above discussion the high capacity motors and pumps to

required to operate the different types of the auxiliaries and also low capacity or

ratings of the motors are required for steam value controls at boiler unit and water

values control at feed water pumps and so an. So these high capacity motors are

known as HT or HV motors and similarly low capacity motors are known as LT or

LV motor.

So, the supply is required for to run the HT or HV motor is known as

HT or HV systems. The rating of the HT system is 6.6Kv and obtained by step-downs

form two transformers.

1. Unit auxiliary transformers from 15.75Kv to 6.6 Kv and

2. Station transformers from 220 Kv switchyard to 7.1 Kv.

Similarly the supply is utilized for operating low capacity motor are

know as LT or LV system and is obtained diesel generator and HT system and

batteries etc..

3.2 SINGLE LINE DIAGRAM OF HT SYSTEMS:

FIG:3.1(B)

FIG:3.1(C)

The single line diagram of the HT systems in main plant is given above circuit

diagram. The HT systems panel diagram in rayalaseema thermal power plant (Rtpp)

consists of two generating units. They are named as generator -1 and generator -2.

Each generator generates the voltage range of 15.75 KV and 210 MVA

powers being generating. And two generating transformers are there they are

generating transformer-1 and generating transformer -2.these two generating

transformers are step-up transformers. They steps up the voltage from 15.75 KV to

236KV in RTPP.

Due to transmission loss the voltage is stepped up for transmitting electric

supply. Unit auxiliary transformers are step down transformers. The station equipment

running purpose the voltage is step down from 15.75 KV to 6.9KV for HT supply. In

Rtpp four UAT’S are there. Each generating unit consists of two unit auxiliary

transforms. For generator-1 UAT-1Aand UAT-1B are connected and similarly for

generato-2 UAT-2A and UAT-2B are connected. Also the HT panel diagram consists

of different types of buses for running different auxiliary units.

1. Unit service buses

2. Station boards

1. UNIT SERVICES BUSES:

The supply coming to unit services buses from UAT’S. The Rtpp consists of

two generating units each unit consists of four buses for both turbine and boiler units.

In generator-1 US-1A (T) and US-1B (T) for turbine unit and US-1A (B) & US-1B

(B) for boiler unit.

2 STATION BOARDS:

The Station transformers supply is given to station boards. Each generator

consists of single station board. Station transformer consists of station board A i.e. SA

board and similarly station transformer 2 consists of station board SB. Also bus

coupler and isolator are used for operation the HT panel unit.

At generating unit generator 1generates the voltage of 15.57Kv and is step

ups’ at generating transformer1 from 15.57Kvto 220Kv .The generating transformer

1supply is fed to switchyard .The switchyard is distributes supply to different

stations .In RTPP the switchyard supply is distributed to Anantapur, Kadapa and

Yerraguntla.

The HT supply is taken from UAT”S and Station transformers. If the present

generating unit is working condition the HT supply is taken from UAT’s and

otherwise the HT supply is taken from station transformers. The station transformers

take supply from power grid. Then the unit auxiliary transformer UAT-1A and UAT-

1B are supplies the energy to US-1A (T) and US-1B (T) for turbine unit and similarly

through bus couples to boiler units that is US-1A (B) and US-1B (B). Also turbine

unit and boiler unit are coupled through bus coupler if any one is failure. This

operation is taken place when present generating unit working condition and both

units are operating condition.

Incase the present generating unit is off condition. The HT supply is taken

from station transformer to operate the station equipments for generation of electrical

power .In RTPP two station transformer is there. They are station transformer A and

station transformer B. These two transformers are bypassed and if any one is failure

the supply is taken from other units, through bus coupler.

These station transformers are connecting to both turbine and boiler units. In

RTPP two station transformers are connected to each unit. Station transformer 1

supply is connected to turbine unit and station transformer 2 is supply is connected to

boiler unit. If any one is failure the supply is taken to operate the failure unit from the

other operating unit. In this way station transformer operates the HT system when the

present generating unit is off condition.

3.3 TYPES OF HT MOTORS:

SL NO Motors Description

1 BFP Boiler feed water pump to pump feed water

from dearator to drum

2 MILL To grind the coal

3 CWP Circulating water pump- to circulate cooling

tower water form cooling towers to

condenser

4 ID FAN To create negative draft from boiler to

Chimney and to remove flue gases

5 PA FAN Primary air fan is used to carry coal from

mills to boiler.

6 FD FAN Forced draft fan is used to carry circulating

air

7 BAP Bottom ash pump

8 CEP Condensate extraction pump. Here the water

Is pumped form condenser to Deaerotor

9 BCW Bearing cooling water

10 ACW Auxiliary circulating water

11 COMPRESSOR Pump To develop compressed air

12 CRUSHER To crush the coal

13 CONVEYER To transport the coal

3.4 AUXILARIES:

3.3.1 BOILER AUXILARIES:

Type Number Power

Mills 3 2100KW

ID Fan 2 1500KW

FD Fan 2 500KW

PA Fan 2 1000KW

3.3.2 TURBINE AUXILARIES:

Type Number Power

CEP 3 200KW

BFP 3 4000KW

CWP 3 2500KW

Specifications MILL CWP ID FAN BFP

Rating 2100 KW 1650 KW 1600KW 4000 KW

Class Of

Insulation

F F F F

No Load Current 100 A 82 A 60 A 83 A

Full Load

Current

248 A 188 A 182.2 A 407 A

Starting Current 60% FL

Current

600 % FL

Current

450 % FL

Current

450 % FL

Current

Stator / Rotor

Resistance per

phase

0.0698 0.11 0.102 0.044

Speed 992 RPM 497 RPM 745 RPM 1485 RPM

3.5 LT SYSTEMS DISCRIPTION:

The system which uses LT supply is known as LT supply. LT system

comprises of LT supply, single line diagram and LT motors. The ratings of LT

systems are 415v for three-phase supply and 230v for single-phase supply .The single

line diagram of LT systems describes how the LT supply is given to operate LT

motors continuously. And LT motors are used for lubrication & cooling purpose for

HT motors is running for continuously generating electrical power.

3.5.1 LT BOARDS:

SSS Station Service Switches

EMC Emergency Services

USS-1B Unit Service Switch (Boiler)

USS-1T Unit Service Switch (Turbine)

BVC Boiler Valve Control

TVC Turbine Valve Control

LDS Lightning Distribution Services

3.6 SINLGE LINE DIAGRAM OF LT SYSTEM:

InRTPP the HT motors are used for transporting the coal from coal yard to coal

handling plants also for tripping the coal at WAGON TRIPPLER unit .So it is

necessary to operate these motors continuously to generate electricity. LT motors are

used for lubrication & cooling of HT motors, which are continuously running.

These LT systems supply energy to different auxiliaries in the power plant. For

example turbine unit, boiler unit , coal handling plant , ash handling plant, etc. For

maintenance purpose and valve controlling and domestic purpose the LT supply is

required. LT motors may be fed with 3¢ or 1¢ supply based on the requirement.

TURBINE UNIT:

The LT system is necessary at turbine unit for lubrication of turbine

bearings and for controlling of turbine valve. The LT supply is needed at each

auxiliary specified in the HT system. This is achieved by stepping down voltage to the

operating level with the help of dry type transformer.

The supply is available to operate the motor from US-1A(T) or SA if

the present generating unit operating then supply is taken from UST-1A other wise

incase of failure of UST-1A the supply is taken from SA board .Similarly TVC-2 is

operated by UST-2A or SA-2.

BOILER:

The LT system is required at boiler unit for boiler valve control (BVC)

and boiler circulating water (BCW) etc .For these units supply is taken from UST-

1(B)or SA board buses .If the present generator is under working condition then

supply is taken for operating BCW and BVC fro UST-1A(B) otherwise from SA

board. In case of any source bus failure other one will meet the load demand. In

the similar way BCW-2 and BVC-2 are operated for second generator unit.

COAL HANDLING PLANTS:

The LT supply is required at coal handling plant to supply wagon

Tripler unit and for lighting & distribution services and for A/C operations etc...

The supply for this equipment is taken from CHT-A and CHT-B. These CHT-A

and CHT-B are bypassed. If one of the buses fails the other will meet the total load

demand of both the units.

ASH HANDLING PLANTS:

For operation of the ash handling plant LT supply also required. For

the ash removing purpose ash is mixed with water and that water is pumped to ash

handling plant. Lower capacity motors are required for LDS and A/C and for

controlling of panel equipment. For these purpose the supply is taken from AH-A

and AH-B. These two units AH-A and AH-B are coupled for multiple operations.

STATION BOARDS:

Station board supply is given to LT motors for operation of the

different auxiliaries, like

1. Lighting and distribution services (LDS) AND

2. A/C equipments

3. MISC MCC

4. CLW MCC.

For these operations the supply is taken from two types of boards they

are SA board and SB board. These two boards are also coupled for multipurpose

ELECTRO STATIC PRESIPITATOR:

For the operation of ESP (electro static precipitator), station board

supply is used along with UST-1A or UST-2A. The connectivity to ESP’s such that it

is kept uninterrupted during power failures also. For hitting of hammer in ash

removing unit electrostatic precipitator are used. In this way the LT supply is utilized

for operation of different auxiliaries.

3.7 TYPES OF LT MOTORS:

1. SEAL OIL FAN MOTOR:

The SOF motor (seal oil fan) is used for sealing of powdered coal in coal pipeline.

The coal is transported from coal handling plant to mill at the mill coal is powdered

this pulverized coal (is coal power) is pumped into the boiler for the purpose of

production or generation of steam. In case of coal leakage the temperature of steam is

reduced and coal utilization increases in order to maintain the steam temperature at

constant level. So to coat this pipeline SOF motor is used. To ensure this purpose

6motors are required (3motors for each plant). Among these three motors one is

continuously running and remaining two motors are stand by. If any one of the

running motor fails the other motor will takes the load.

2. HP PUMP MOTOR & LP PUMP MOTOR:

High-pressure pump motors and low pressure pump motor are used for mill

lubrication and bearing cooling purpose at steam generating unit. Due to continuous

operation of motors temperature of the motors increases, so it is necessary to we will

maintain the temperature with in the limits to ensure the safe operation of the motor.

This cooling of motors is required wherever temperature rises. So for this purpose LP

pump & HP motor are required and 2 motors are used for cooling of bearings. Among

these motors one motor runs continuously and other is for stand by

3. LUBRICATION OIL PUMP MOTOR:

Lubrication oil pump motor is for lubrication of bearing and cooling of

bearing motors like PA motors (primary auxiliary motor) and FD fan motor (forced

draught fan motor) and ID fan motor. These are also necessary to keep the motors in

good and running condition. So, it is necessary to cool the bearings and lubricate these

motors also. For this purpose lube oil pump motor is used. Here also 2 motors are

used for lubrication of bearings and cooling of bearings of the motor. Among these

motors one is running continuously and other one is stand by.

4. AOP MOTOR (AUXILIARY OIL PUMP MOTOR):

It is one of the important motors in the LT system. These motors are used to

protect the generating unit from heat. The lubrication of generator and turbine

bearings under barring gear operation is caused by AOP motor to ensure good

operation condition. Barring gear of the generator means the AOP motor is operated

with in the specified limits that is the speed range is above 2800 RPM and pressure

gauge is 6kg/cm 2. If the generator or turbine overcomes the above limit the AOP

motor is not applicable. The two AC motor and 1 DC motor are used instead of AOP

motor. Among these two motors one motor is operated continuously and another

motor is standing by. If both ac motors fails then DC motor is operated.

5. JOP MOTOR (JACKING OIL PUMP MOTOR):

Jacking oil pump motors also run with the same concept as the above AOP

motor. Incase of AOP motor the operating speed range is nearly above 2800 rpm. But

the JOP motor can operate from above 550 rpm .If specified range exceeds then JOP

motor is not applicable for that particular application. Among this single ac motor and

single Dc motor is used instead of JOP motor. If AC motor get fails the Dc motor is

operated.

6. SEAL OIL SYSTEM MOTOR:

The seal oil system motor is used for sealing hydrogen gas inside the generator

casing and also used to avoid any gas leakage into the atmosphere from the generator.

So the cooling is required for the generator. As the generator is running continuously

to generate electric power it should be protected properly. If any leakage of gas occurs

into the atmosphere the atmosphere is polluted. For sealing of ‘H’ gas a seal oil

system motors are used.

For this purpose two AC motors and single DC motor are used. Among this

one Single AC motor is running continuously and another AC motor is standing by.

Both the Ac motors get fails DC motor is used. The pressure capacity of ‘H’ gas used

in the generator is 3kg/cm2.

4. HT AND LT MOTORS PROTECTION

4.1 Induction Motors

4.2 Motor protection

4.3 Details of HT Motors

4.1 INDUCTION MOTORS:

These machines are open circuit cooled 3-phase squirrel cage motors

for high voltage and, in special cases for low voltage. The rated output values apply o

continuous operation at a frequency of 50Hz,a cooling air temperature of 40C and

site altitudes of up to 1000Mts above sea level.

4.1.1 INDUCTION MOTOR CONSTRUCTION:

STATOR FRAME AND WINDING:

The axial cooling tubes are expanded into the end walls. The

laminated stator core is placed centrally in the frame and is secured to percent

rotation or displacement.

The stator winding of the high voltage machine is a double layer coil winding

with class F MICALASTIC insulation. It is a special type of insulation employing

ground Mica and synthetic resin impregnation. Its features are high dielectric

strength, resistance to moisture, oppressive gases and vapors, high rigidly and long

life.

ROTOR WINDING:

The shaft is supported in two bearings and has a parallel shaft extension. On a

two-flow machine, it is a solid shaft. The rotor core is shrunk onto the shaft, clamed

axially and carries the cage winding. The bars of the cage bit tightly in the slots of

the rotor core and are brazed to the end rings. The rotors of the medium

aerodynamically balanced with a half feather key in the shaft extension.

BEARINGS:

The bearings are either grease-lubricated rolling contact bearings or journal

bearings with or without forces oil lubrication. Anti condensation heaters fitted in

electrical machine to warm the air inside the station, any machine above that of the

surrounding. Thus effectively prevents moisture condensation. Bearing and winding

temperature monitoring devices etc are used for monitoring bearing including

temperatures.

COOLING AND VENTILATION:

The basic version machines are self-cooled by the internal fan mounted on the

shaft at the A end .The fans are normally uni directional but can be ordered in

bi-directional form. The cooling air enters the frame radially at the B end, cools the

windings and laminated cores and discharges radially at the A end

4.2 MOTOR PROTECTIONS:

4.2.1 INTRODUCTION:

A fault in its electrical equipment is defined as a defect in its electrical circuit

due to which the flow of current is diverted from the intended path. Breaking of

conductors or failure of insulation causes faults. Fault impedance is generally low,

and faults current is generally high. During the faults, the voltage of the three phases

becomes unbalanced and supply to the neighboring circuits is affected. Fault current

being excessive, they can damage not only the faulty equipment but also the

installation through which the fault current is fed. For example if a fault occurs in

motor, the motor windings are likely to get damaged. Further, if the motor is not

disconnected quickly enough the excessive fault currents can cause damage to the

starting equipment, supply connections, etc.

There are several causes of faults occurring in a particular electrical plant.

Faults can be minimized by improved system design, improved quality of

components, better and adequate protective relaying, better operation and

maintenance, etc…however; the faults cannot be entirely eliminated. Fault statistics

are systematic records regarding number and causes of faults occurring in power

system.

4.2.2 THE ABNORMAL CONDITIONS:

Prolonged overloading:

It is caused by mechanical loading, short time cyclic overloading.

Overloading results in temperature raise of winding and deterioration of insulation

and this in turn results in winding fault. Hence motor should be provided with

overload protection.

Single phasing:

One of the supply lines gets disconnected due to blowing of a fuse or open

circuit in one of the three supply connections. In such cases the motor continues to

run on a single-phase supply. If the motor is loaded to its rated full load, it will draw

large currents.

Excessive currents on single phasing

The winding get overheated and the damage is caused. The single phasing

causes UN balanced load resulting in excessive heating of rotor due to negative

sequence component of unbalanced current. Static single phasing relays are

becoming very popular.

Stator earth faults:

Faults in motor winding are mainly caused by failure of insulation due to

temperature rise.

Phase to phase faults:

These are relatively rare due to enough insulation between phases.

Earth faults are relative more likely. Inter-turn faults: these grow into earth faults.

Separate protections are generally provided against inter=turn faults.

Rotor faults.

1. These are likely to occur in wound rotor motors, due to insulation failure

2. Failure of bearing: this causes locking up of rotor.

3.The motor should be disconnected. Bearing should be replaced.

4.Unbalanced supply voltage this cause heating up of rotor due to negative

sequence current in stator winding.

Supply under voltage:

The under voltage supply cause increase in motor current for the same load.

Fault in starter or associated circuit:

The choice of protection for a motor is depends upon the size of the motor, its

importance in the plant, nature of load.

4.2.3 PROTECTION SCHEMES FOR LARGE MOTORS:

Large motors need protection against various abnormal against various

abnormal conditions. Several types of protective relays are developed to suit various

applications. These relays sense the abnormal conditions and trip the trip circuit of

motor circuit breaker. The protection provided for large 3-phases motors takes into

accounts overloads, short circuits and in some specially developed relays for motor

protection, protection again following:

Faults in windings and associated circuits

Reduction of loss of supply voltage

Excessive overloads

Phase unbalance, and single phasing

Phase reversal.

Switching over voltages-surges

OVERLOAD PROTECTION OF INDUCTION MOTORS:

The current sensing overload protecting devices can sense the following

abnormal conditions:

1. Overloads, under voltage

2. Single phasing

3. Locked rotor, stalling

4. Heavy starting

5. Continuous overload.

6. Heavy breaking.

However, only embedded thermal devices can sense the following conditions.

1. Temperature rise due to higher ambient temperature.

2. Temperature rise due to failure of cooling.

3. Temperature rise due to their causes.

The details about thermal overload protection are described below. The

purpose of thermal over load protection is to protect the motor from excessive thermal

stresses. During full load, the temperature of motor winding reaches almost maximum

permissible unit (dependent on insulation class). During abnormal conditions, the

temperature exceeds the sage limit and the life of insulation is reduced.

The rate of temperature rise is determined by losses and thermal time constant

of the stator. The heat loss from motor to surrounding air depends upon ambient

temperature ventilation and design aspects.

PROTECTION AGAINST UN BALANCE:

The voltage supplied to three-phase induction motor can be unbalanced due to

any of the following reasons:

Single phase loads on distribution service line

Short circuit within or outside the motor

Phase failure by blown fuse.

The unbalanced voltage itself may not be harmful but the negative sequence

currents caused by unbalanced voltage results in rotating magnetic field revolving in

opposite direction. Thus field induces double frequency-induced currents in the rotor

body one continuous giving rise to hear due to copper losses.

The rotor gets heated and the temperature to a motor winding may

reach above safe limit. The unbalanced protection provided to a motor should prevent

prolonged unbalanced condition, but should not disconnect the motor for permissible

unbalanced of short duration. The permissible loading depends upon the percentage

unbalance and the ratio of positive sequence impedance to negative sequence

impedance.

PROTECTION AGAINT SINGLE-PHASING:

A 3-phase induction motor continuous to run even if one of the supply

lines is disconnected. The whole power is then supplied thought the two windings and

they are likely to get overheated. The single phasing causes unbalanced stator

currents. The negative sequence component of unbalanced current causes heating of

rotor and temperature use. For small motors, separate protection against single

phasing is generally not necessary as the thermal; relays sense the increased current in

healthy phases due to single phasing and thereby offer adequate protection.

In case of large motors even a modest unbalance can cause damage of

motor winding due to overheating. Further, if motor is stalled due to losses of one

phase, severe damage to rotor is possible while staring. Therefore, a separate single

phasing protection is desirable.

PHASE REVERSAL RELAY:

The direction of rotation of an induction motor depends upon the phase

sequence of the supply voltage. Phase reversal occurs when the supply connections

are hinged after repairs. Assuming after the repairs (at local load point or supply sub-

station) the phase sequence of supply is reversed; the motor will run in wrong

direction. In some applications, phase reversal is dangerous e.g. elevators, cranes,

hoists, trams etc. in such applications phase reversal relays should be provided the

phase reversal relay may be provided at main incoming substation of industrial works.

The phase reversal relay based on electromagnetic principle comprises

a disc motor driven by magnetic system actuated by secondary of two lines CT's or

VT's.

For correct phase sequence (RYB) the disc exerts torque in positive

direction so as to keep the auxiliary contacts close. When phase reversal takes place,

the torque reverses and the disc rotates in opposite direction to open the contacts.

Thereby the magnetic coil of starter can be de energized or circuit breaker can be

tripped. The solid-state phase reversal relays and phase failure relay senses the phase

reversal or phase failure. Under abnormal conditions it sends tripping command to

output stage

PHASE TO PHASE FAULT PROTECTION:

A phase-to-phase fault short-circuits in stator winding causes burn out

of coils and stampings. Hence the motor should be disconnected from supply very

quickly. Fast over current relays are provided for phase to phase short-circuit

protection.

The relays giving short-circuit protection to the motor should not act

during starting currents. The setting of instantaneous over current relays for phase

faults should not be below the staring characteristic of the motor.

Therefore, the short-circuit protection characteristic of the motor.

Therefore the short-circuit protection characteristic is set just above the maximum

starting current the motor.

STATOR EARTH-FAULT PROTECTION:

Earth-fault protection is set to disconnect the motor from supply as

early as possible so that the damage to winding and laminations is minimum.

Zero sequence current transformer (ZSCT) or core balance type

protection is very convenient method for protection of motors from earth-faults. This

method is especially suitable for system neutral earthed through resistance. In such

systems, earth-fault currents are so low (due to resistance earthling) that phase over

current relays cannot be set to pick-up for earth faults, Core balance (CT)

Where the supply source is earthed, an inverse, very inverse, for

instantaneous induction type relay is connected in the current transformer neutral.

These sources usually have neutral impedance to limit the ground current so that

sensitive ground relay settings are required.

FAULTS IN ROTOR WINDING:

In slip-ring induction motor, rotor faults are possible. The increase in rotor current is

reflected on stator current and the stator over-current protection can thereby act. The

setting of stator over-current relay is generally of the order of 1.6 times full load

current. This enough to detect the rotor faults.

INTER-TURN FAULTS: Inter turn faults are difficult to be detected. the method

adopted for generator stator winding inter-turn faults can be adopted for motors. But it

is too complex and is not practicable.

GROUNDING OR EARTHING: In low voltage circuit the neutral point of supply

should be earthed. In ungrounded 3-phase systems a single line to ground fault on

one line causes increase in voltage of healthy lines with respect of neutral by 3 times.

This cans danger motor insulation.

To avoid this, the neutral point of supply, should be earthed at every

voltage level. Cascade failure of motors can occur if supply neural is nor earthed.

4.3 DETAILS OF HT MOTORS:

SI.NO. 1 2 3 4 5 6 7 8 9 10 11

Name of the Motor

BFP MILL

CWP I D FAN

P A FAN

F D FAN

BAP BCW CEP ACW

COMPRESSOR

Make BHEL

BHEL

BHEL

BHEL

BHEL

BHEL

CROMPTON GREAVES

CROMPTON GREAVES

BHEL

BHEL

KIRLOSKAR

capacity in KW 4000 2100 1650 1600 1250 750 525 300 300 250 200

Stator volts in KV

6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6

Stator Current in Amps

407 246 188 182.2 130.5 82 58 34 33 29 25

Stator connection

Y Y Y Y Y Y Y Y Y Y Y

Rotor connection

SQ.CAGE

SQ.CAGE

SQ.CAGE

SQ.CAGE

SQ.CAGE

SQ.CAGE

SQ.CAGE

SQ.CAGE

SQ.CAGE

SQ.CAGE

SQ.CAGE

No. Of phases 3 3 3 3 3 3 3 3 3 3 3

Frequency 50 50 50 50 50 50 50 50 50 50 50

Amb.Temp. 50 50 50 50 50 50 50 50 50 50 50

Temp.Rise 70 70 70 70 70 70 70 70 70 70 70

R P M 1485 992 497 745 1481 1492 1483 989 1487 986 746

Specification IS-325-78

IS-3578

IS-325-78

IS-325-78

IS-3578

IS-325-78

IS-3578

IS-325-78

IS-325-78

IS-325-78

IS-325-78

Type RC00716-H04WIH

RD00716-H06WIH

RVC00804-HDAIH

LC00912-H08AIH

LC00716-H04A3H

LC00634-H04AIH

TVC500ED

VTVC1080E

LVC0056-ZH04AIH

IRQ-7403-6P

KD450

Weight in KGs 13360

13500 19680

11800

8600 5600 4200 6249 3800 3150 -

Direction of rotation from NDE

clockwise

clockwise

clockwise

Anti clock wise

Anti clock wise

Anti clock wise

Anti clock wise

clockwise

clockwise

clockwise

Clock wise

Efficiency 96.50%

94.70%

96.00%

96.00%

95.30%

95.50%

- - 93.30%

-

Power Factor 0.89 0.78 0.8 0.8 0.88 0.84 - - 0.85 - -Duty Cont. Cont Cont Cont Cont Cont S1 S1 Cont Cont S1

APPENDIX

NAME PLATE DETAILS OF GENERATOR

Rated data and

outputs

Turbo generator Main exciter Pilot exciter

Apparent power 247 MVA ---------------- 15KVA

Active power 210 MW 832KW -------------

Current 9.05KA 2600A 48A

Voltage 15.75KV+- 320V 220V+-

“ 787.5V ---------------- 22V

Speed 50S-1 50S-1 150Hz

Frequency 50Hz ---------------- --------------

Power factor 8.5 ---------------- --------------

Inter connected y-y ---------------- --------------

Stator winding

H2 pressure 2bar ----------------- --------------

Rated field current 2080a ----------------- --------------

For rated output

Rated field voltage 270V ----------------- --------------

NAME PLATE DETAILS OF POWER TRANSFORMER

Type of cooling OFAF ONAF ONAN

Rating

H.V&L.V(MVA)

240 168 120

Rating L.V 240 168 120

Temperature rise of

winding

60c 55c 55c

No load voltage (H.V) 236V

No load voltage (L.V) 15.75kv

Line current (H.V) 587.83A

Line current (L.V) 8808.15A

Temperature rise of oil 40c

Phases 3

Frequency 50Hz

Connection symbol YNdl

%impedance(volt) 15%+-ISTOL

INSULATION LEVEL

H.V 950KVP/395KV

L.V 95KVP/38KV

NAME PLATE DETAILS OF STATION TRANSFORMER:

Type SALOCR

KVA HV 19000/31500

LV 19000/31500

Voltage(at no load) HV 220000

LV 7100

Ampere (line value) HV 50/82.7

LM 1547/2565

Number of phases 3

frequency 50Hz

Winding Impulse test Voltage (KV) Power frequency test

HV line 950 395

HV neutral 95 38

LV 60 20

Type of cooling ONAN/ONAF

Impedance voltage (31.5

MVA base rated tap)

15.20%

Connection symbol Yndl

Mass of core and winding

(kg)

31150

Mass of oil(kg) 18315

Total mass(kg) 71500

Mass of heaviest PKg.(Kg) 41000

Un tanking mass(Kg) 31150

Un taking height(mm) 7450

Volume of oil 20350

Air circulation(m3/mm 8*90

NAME PLATE DETAILS OF VOLTAGE TRANSFORMER:

Make Transformer and Electricals Kerala limited

Type CPUEGLV

Frequency 50Hz

Oil quantity 200L

Insulation level 460/1050kv.

Weight 1200KV

Highest system voltage 245KV

Method if connection Between line and earth in an effectively

earthed neutral system.

No of phase-1

Type of T/F-Earthed

Makers SI.No.730064-5

Year 1993

Secondary winding No

1 2

Measuring/protection Protection

Output 500MVA 100VA

Accuracy class 0.5/3P 3P

Primary terminals AL A2

Secondary terminals 1a1,.1a2 2a1,2a2

Voltage factor 1.2 continuous 1.5/30 sec

Voltage ratio 22/1732KV/110/1.732V 200/1.732KV/110/1.732V

NAME PLATE DETAILS OF CAPACITOR VOLTAGE TRANSFORMER:

.

Make Campton greaves limited. India

Type CVE 245/1050/50

Frequency 50Hz

A-NHF 10-IN 2A1-2n 2a2-2

VOLTS 220000/1.732 110/1.78321 110 110/1.732

VA -------------- 400 100 100

CL -------------- 1.0 / 3 P 3 P 3 P

Total Output / CL 500VA / 1.0

Highest System Voltage 245KV

Equipment Capacitance 4400(+10%-5%)P.F

V.F 1.2 cont / 1.5-30 Sec

Insulation level 460 / 1050 KV

Capacity of oil 50 + 10%KV

NAME PLATE DETAILS OF CURRENT TRANSFORMER:

Make WS Industries (India Ltd) Bangalore

Frequency 50Hz

Highest system voltage 245KV

Basic insulation level 460 / 1050 KV

Oil Weight 360Kg

Total Weight 1250 Kg

Ratio 800 / 1-1-1-1

Core No 1 2 3 4 5

Rated

primary

current (a)

800 800 800 800 800

Rated

secondary

current (a)

1 1 1 1 1

Out put VA ------------ -------------- 50 ------------ -----------

Accuracy

Class

PS PS 0.5 PS PS

Turns ratio ------------- 2/1600 ------------- 2/1600 --------------

Resistance

of C.T. at

750 C

3 3 -------------- 3 3

KVP (V) 1000 1000 ---------- 1000 1000

NAME PLATE DETAILS OF METAL; OXIDE SURGE ARRESTOR:

Make Metal Oxide Surge Arrestor, Obulum

Electrical Industries pvt.Ltd.Hyd.AP.

Rated Voltage66KV

MCOV 56KV

Discharge current 10KA

Model ZAP-198

Rated Frequency 50Hz

Type METOVER

Pressure relief class 40KA (rms)

Position long duration

discharge

Top

Long duration discharge class-111

Year of manufacturing 1993

NAME PLATE DETAILS OF UNIT AUXILIARY TRANSFORMER:

EMCO transformers LTD Bombay

Rating (KVA) 15000

Voltage (No load) HV 15750V

LV 6900V

Line current (amps) HV 549.9

LV 1255.1

Heaviest package 26000

With oil (kg) 20000

With out oil (kg) LV

Insulation level number of phases 3

Frequency 50Hz

Vector symbols Ddo

Diagram DRG No. HT 11/7580

Impedance Tap No.1 8.871%

Volts Tap No.9 191%

Tap No.17 762%

LT 95AC 38

Insulation level

Guaranteed maximum temperature rise Oil 40c

Winding 50

Mass of core and winding (kg)

Mass of tank RTG Acc (kg)

Total oil kg 8000

Liters 9100

Total mass (kg) 34200

NAME PLATE DETAILS OF CIRCUIT BREAKER:

Make Crompton Greaves Limited

Nasil, India

Type 200-SFM-40A

Rated lightning with stand voltage 1050KvP

Rated short-circuit breaking current 40KA

Rated operating pressure 15Kg/cm2-g

First pole to clear factor 1.3

Rated duration of short circuit current LKA_3sec

Gas weight 21Kgs

Sc no 5154C

Year 1992

Rated voltage 245KV

Rated normal current 2550A

Rated frequency 50Hz

Rated closing voltage 220V DC

Rated opening voltage 220V DC

Rated gas pressure 6kg/cm2-g (at 20c)

Rated voltage and frequency for

Auxiliary circuit 415 V, AC, 50Hz

Total weight with gas 3900kgs

NAME PLATE DETAILS OF ISOLATOR:

Make Switch gear of structural, Bhuvaneswar

Current 800A

Rated voltage 220KV

Rated short-circuit current 40KA-3sec

Control supply voltage 220KV

Max.design voltage 245KV

Impulse with stand 1050KV

Frequency 50Hz

CONCLUSIONS

1. The motors of Boiler and Turbine auxiliaries are heavy drive motors and if

we use normal supply then the size of conductor supplying the heavy motors

increases and hence cu losses occur, so to avoid such losses and to have normal

generation of the plant we need to give H.T supply of 6.6 kv, to the motors of

heavy drives.

2. There are some motors whose function is for lubrication of mill, PA fan, FD

fan, I.D fan and making the bearings cool and to control the circulating water

pump valve. These motors require L.T supply of 415v.

3. The plant should be in operation for 24 hrs, for that purpose continuous

supply is required. In case of any break down of supply the generator and motors

should not attain static position suddenly. And some emergency motors of boiler

and turbine auxiliaries should be in continuous service, so for all these purposes

we make use of DIESEL SET GENERATOR (D.G SET GENERATOR).

4. Some of the faults that occur in the motors are short circuit faults, earth

faults, locked rotor, etc. So to avoid such faults we use protection devices like

Relays, Circuit Breakers, and Isolators.

5. For the operation of these protective devices, we need D.C supply. This D.C

supply is obtained from D.C charger. This D.C charger simultaneously supplies

D.C supply to protective devices and also charges batteries through A.C supply.

6. From the above conclusions, we conclude that H.T & L.T systems are

equally important for the successful operation of the generating plant.

BIBILIOGRAPHY

1. Power System Engineering – Black & Veatch

2. Steam, its generation & use – Babcock & Wilcox

3.Elements of Electrical Power Station Design – M.V.

Deshpande

4.A Text book on Power system Engineering – M.L.Soni &

P.V.Gupta