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< 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
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