distribution system and transformers
DESCRIPTION
Distribution System andTransformersTRANSCRIPT
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Energy Saving in Electrical Utilities
K. R. GOVINDAN
Kavoori Consultants
22, Janakiraman Street,
West Mambalam,
Chennai, 600 033.
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Energy management Kavoori Consultants 2
Distribution System and Transformers
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About the faculty
CEO of Kavoori consultants: services
offered:
Energy audit, electrical safety and
installation audit, relay protection and
coordination studies, maintenance,
technical training of executives and
technicians of all trades, in-house as well
as open seminars,
Technical trouble shooting
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ATTITUDE
Half full or half empty?
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Two liters container; 1 liter liquid.
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ENERGY MANAGEMENT More precisely,
EFFICIENT MANAGEMENT OF ENERGY,
THE VITAL RESOURCE.
What is efficient management?
Energy is utilized to do work;
Use only the required minimum
Or optimum requirement
To perform a particular work.
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In the Present day context of:
Depleting energy sources
Spiraling costs
pollution of environment to alarming levels.
Energy management assumes top priority
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ENERGY AUDIT
PRE REQUISITE FOR AN ENERGY MANAGEMENT PROGRAMME
BY ITSELF DOES NOT SAVE ENERGY
HELPS MANAGEMENT IDENTIFY AREAS OF HIGHEST
SAVINGS POTENTIAL
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FUNCTIONS OF AUDIT
Assesses various forms of energy use
Compares with estimated minimum
Provides inputs for budgetary control
MOST IMPORTANT FOR
OLD PLANTS
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SET UP WHEN FUEL COST WAS VERY LOW
NO CONCERN FOR ENERGY EFFICIENCY
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Good energy management is
Increasing utilization efficiency or reducing losses
Or
CONSERVATION OF ENERGY
Let us consider the electrical energy
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THE STORY OF ENERGY
300 BILLION YEARS AGO, ENTIRE
ATMOSPHERE OF OUR EARTH- UNFIT
FOR LIFE SUPPORT
SLOWLY ALGEY AND LIKE PLANTS
APPEARED, CONVERTED CO2 INTO O2
Received and STORED energy from the sun
BY PHOTOSYNTHESIS
Then, animals appeared
Were living on plants
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SEEMS
63 MILLION YEARS AGO
ALL LIVING AND NON LIVING THINGS
SUDDENLY BURIED
THE CAUSE MAY BE A DELUGE OR
THE FALL OF AN ASTEROID
UNDER HIGH PRESSURE FOR LONG TIME
BECAME FOSSILS
THE ENERGY STORED
IN THEM IS THE FUEL WE ARE ENJOYING
NOW!
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Why energy conservation?
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STORY OF DEPLETING ENERGY!
We burn them, exhaust them;
May be after some decades, no
fossil fuel will be available.
We convert atmosphere to CO2 and
other pollutants
May be after a few hundred years
earth may become the old self and
not suitable for any living being
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Energy saved and generated
One kWhr electrical energy saved is
equivalent to a saving of fuel for the
generation of 5kWhrs!
How?
Power from generating stations to the utilization
point passes thro many equipments like
transformers, transmission lines, cable feeders etc.
Thermal efficiency of a turbo generator is only
30%!and other equipments efficiencies are also
involved.
Hence the high figure!
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Power generation, transmission and distribution
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Typical industrial power distribution SLD
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One M.W. used a day Cnsumes 17 M.T. of coal,
pollutes atmosphere by 3.4 tons of coal dust, 0.13 tons of SO2 and
0.18 Tons of oxides per day!
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Control of Atmospheric pollution
Burning of fossil fuels generates
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sulphuric, carbonic, and nitric acids
They fall on Earth as acid rain, affecting both
natural areas and the built environment.
Monuments and sculptures made from
marble and limestone are vulnerable, as
the acids dissolve calcium carbonate.
A liter of petrol, diesel, kerosene used in a vehicle
causes approximately 2.3 kg of CO2 emissions.
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Control of pollution
Energy conserved reduces fuel
consumption
Fossil fuels burnt generates green house
gasses
Also causes acid rain etc.
Some of the solar radiation is reflected
back by the earth and atmosphere and
they escape to the space.
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REMEMBER!
1. GRID POWER -
EACH KW SAVED RESULTS IN REDUCING 6.4 TONNES OF CO2 EMISSIONS/ YEAR
2. DIESEL GENERATORS -
EACH KW SAVED RESULTS IN REDUCING 7.2 TONNES OF CO2 EMISSIONS/ YEAR
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Pollution green house gasses
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Effects of global warming
Will melt polar ice caps and rise the sea levels
there will be about half to one meter increase
in sea level by 2020
at the present levels of global warming
Coastal cities such as Mumbai, Kolkata and
Chennai could go under sea by 2020
could make at least one billion people
homeless between now and 2050
say scientists.
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DO NOT MAKE THE ENTIRE EARTH LOOK LIKE THIS!
PLEASE GIVE A GOOD EARTH TO OUR CHILDREN!
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We have a social responsibility for the future
generation
Leave the world, a wonderful place, as it is-
for the future generation
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It's true that we don't know what we've got until we lose it! Conserve the fast depleting conventional energy
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ENERGY CONSERVATION
Most urgent, top priority Depleting sources
Spiraling cost Cannot have the luxury of
unproductive usage and high demands
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ENERGY CONSERVATION
OPPORTUNITIES IN
NO TWO IDENTICAL FACTORIES ARE
ALIKE
Scientific approach is needed to tackle
unique problems of each industry-
An energy audit
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CONCLUSION
Audit helps in identifying energy conservation opportunities,
Not an one time function;
A continuous activity
Initial phase may provide plenty of opportunities; but
May taper down as the activity continues.
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MISSING THE OBVIOUS
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PARETTO ANALYSIS
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ENERGY CONSERVATION
First let us look at:
What is power,
What is energy and
The sources of energy
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WHAT IS ELECTRICITY?
AMPERES?
VOLTS?
WATTS?
FLOW OF CURRENT AMPERES
WITH POTENTIAL DIFFERENCE VOLTS
ACROSS A RESISTANCE OHMS
FLOW OF CURRENT GIVES POWER WATTS
POWER FLOWING FOR A PERIOD ENERGY
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Simple circuit
4AMPS = 240VOLTS/60 OHMS
VOLTAGE MAKES CURRENT FLOW
THROUGH A RESISTANCE.
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240 V 960W
60
(Heater)
4 AMPS
4 AMPS
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POWER, ENERGY
Power rate of doing work
Energy quantity of work done
Electrical:
Kilo Watt, Kilo Watt Hour
Mechanical:
Horse power, foot pound force (ft lbf)
THERMAL:
British thermal units (BTU)
Joule
Calorie
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Energy equivalents:
1 kilowatt hour = 3.6 10^6 Joules (J) or 3600000 (J)
859.85*10^3 k Calories (kcal) or 859850 cal
2.65 10^6 foot pound force (ft lbf)or 2650000
3412 British thermal units (BTU)
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ENERGY FORMS
Coal
Oil
Gas
Electricity
Steam
Compressed air
Vacuum
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ENERGY SOURCES or FUELS
Material capable of releasing energy
When chemical or physical structure
changed or converted.
Releases energy either by chemical means
-burning,
or by nuclear means, like
nuclear fission or nuclear fusion.
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ENERGY FORMS
Identify source or carriers:
CARRIERS
steam pressure, heat
water potential, Velocity (k.e)
air pressure
electricity? potential difference
DO NOT GET CONSUMED
Energy imparted, carried and delivered.
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ENERGY FORMS Identify source or carriers:
Sources:
Inherent energy expended by irreversible chemical process - burning
Fuels
OIL
GAS
COAL
Gets consumed.
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REDUCE WHAT?
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OUTPUT? USEFUL WORK DONE
NO ! WORK DONE SAME
INPUT? YES.
HOW?
ENERGY INPUT = USEFUL WORK DONE + ENERGY LOST IN CONVERSION / TRANSMISSION.
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ENERGY INPUT = USEFUL WORK + LOSSES
OR
USEFUL WORK +
(LOSSES+WASTAGE+LOW EFFICIENCY)
TO MINIMIZE ENERGY USE:
~IDENTIFY AND MINIMIZE LOSSES.
intrinsic to the system and equipments.
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LOSSES
AVOIDABLE
WASTAGE
LOW EFFICIENCY
UN EVEN DEMAND
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INCREASING POWER FACTOR
REDUCES DEMAND. OK, BUT,
DOES IT REDUCE ENERGY LOSSES?
IF YES, HOW?
AC CIRCUITS POWER NOT = VOLT * AMPS
A PHASE ANGLE EXISTS BETWEEN VOLTAGE
AND CURRENT
POWER = INST VOLTAGE * INST CURRENT
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Power factor
Components of Impedance
(I) Resistance + Reactance (Vectorial sum)
Reactance = Inductive reactance + Capacitive reactance
(Vectorial sum)
These two oppose each other I.e. 180 degrees apart
Almost all circuits, especially in industries inductive I.e,
have low lagging power factor.
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Because
Load consists mainly of:
1. Induction motors
2. Static controls thyristors etc,
3. Power transformers and voltage regulators,
4. Welding machines,
5. Electric-arc and induction furnaces,
6. Choke coils and magnetic systems,
7. Neon signs and discharge lamps.
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Inductive loads
Higher inductive load:
Lower power factor and higher reactive current
Line losses depend directly upon the square of the
current immaterial of its power factor
Losses proportional to Sq of current!
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Lesser current lesser losses!
From the sketch: Inductive component of kVA1 = kW*Tan1 to be reduced to kW * Tan 2. Or to reduce 1 to 2; the demand kVA1 is reduced to kVA2 Possible by supplying a leading RKVA equal to (kW * Tan 1) (kW* Tan 2) Or, the capacitance required in RKVA = kW * (Tan 1 Tan 2)
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R KVA
Capacitive reactance in RKVA
1
2
kW kVA1 `
KVA2`
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Capacitance required for power factor
correction
Capacitance required in kVAr =
Avr. Demand * Avr P.F. * (Tan 1 Tan 2) Or,
Cap. required in kVAr =
M.D * Present P.F. * (Tan(Cos-1 Prsnt P.F) TanCos-1 required P.F.))
Power Factor correction by static capacitors: In most industrial cases, pay back less than 18 months.
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Selection of capacitors
POINTS TO BE CONSIDERED:
1. Reliability of the equipment to be installed
2. Probable life.
3. Capital cost.
4. Maintenance cost.
5. Running costs.
6. Space required and ease of installation.
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LOCATION OF CAPACITORS
Nearest to inductive load or switch board: Reduce current
and I2R loss
INDIVIDUAL CORRECTION
Better across motor terminals
Preferably 7.5 kW and above
Avoids providing separate control gears for capacitors
Improves starting condition voltage drop reduced at start
I.e. Drop across cables, transformers, buses
Reduces I2 or losses
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INDIVIDUAL CORRECTION
Caution
1. Protective equipment of feeders/ equipments
should be properly set
2. Capacitor size dependent on motor
magnetizing current.
3. Motor overload trip setting:
OLTA = OLTA * P.F. without capacitors (With
capacitors) (Without capacitors) power factor with
capacitors
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INDIVIDUAL POWER FACTOR CORRECTION OF MOTORS:
Care necessary in deciding kVAr capacitor in
relation to the magnetizing kVA of the machine.
If rating too high, damage to motor and capacitor.
Motor, still revolving after disconnection from
supply, may act as a generator by self excitation; produce voltage higher than supply voltage.
If motor switched on again before speed fallen to
80% normal speed, high voltage superimposed on
supply circuits; risk of damaging other equipment
connected in same circuit.
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Capacitor location for motors
Location A
Capacitor installed on incoming side of starter, on
line side of O/L relay
(a) Capacitor size dependent on motor
magnetizing current.
(b) Current to starter not reduced.
(c) Motor overload trip setting same as without the
capacitor.
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Capacitor location for motors
Location B
Capacitor installed on load side of starter, line side
of the O/L relay.
(a) Current to the starter reduced.
(b) Motor overload trip setting is the same as
without capacitor.
(3) Location D
Capacitor installed on load side of both starter and
motor O/L relay.
(a) Current to starter reduced.
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Capacitance value
Correct size capacitor in kVAr not to exceed 85% of no-
load magnetizing kVA of machine.
If motor runs, even momentarily, with windings and
capacitor forming a closed circuit, and disconnected
from mains, over-excitation occur if capacitance too
large.
Happens when:
1. Switching off supply to motor.
2. Step changing a star/delta or auto-transformer starter,
3. Breaker trips, or fuses blow on distribution system
such that: Motors with individual capacitors, or
Group of motors and line capacitor, form closed circuits.
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LOSSES IN A CAPACITOR
Capacitor: two conductors, separated by
a dielectric, energized at opposite polarity.
(i) There is no prefect conductor
(ii) There is no prefect dielectric
All conductors have some resistance
All dielectrics have some conductance
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LOSSES IN A CAPACITOR
Current caused by conductance in capacitor draws
a small power.
Known as dielectric loss
Quality of capacitors depends on the dielectric
loss, generally known as Tan loss.
Power loss = VI Cos or, = VI Tan
When angle between actual and the quadrature
current is extremely small.
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LOSSES IN A CAPACITOR
The conductance in the dielectric is also called as
leakage resistance
The current due to this will cause power flow I.e I2R Loss
This Dielectric loss = Capacitor rating in kVA *
Tan
This should be kept at a minimum.
The limits as per standards are: 660 V Capacitors:
(i) Mixed dielectric and film capacitors 0.0025
(ii) Paper Dielectric capacitors 0.005
Above 660 V : not exceeding 0.001
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CASE STUDIES
In a large electrochemical industry, P.F. correction
capacitor 4000 kVAr
Dielectric loss = Tan = .002
Total loss = 4000 * 0.002 = 8 kW
Annual energy lost = 7008 kWhr,
Costing Rs. 315360/- (@Rs 4.5/ kWhr)
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CAUTION IN HANDLING CAPACITORS
Some Capacitors may contain Polychlorinated Biphenyl (PCB) very dangerous to health May cause cancer Should be only buried for disposal Some may contain Isopropyl biphenyl These may be disposed by incineration Always follow EPA (Environmental Protection Agency) requirements or Central/ State government regulations.
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Dielectric Losses In Power Cables
The dielectric power factor of cables for voltage of 33
kV and above is of great importance should have very
low value. The dielectric power factor is
loss in dielectric (watt)
volts * amp
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Dielectric Lose In Power Cables
If cable dielectric is perfect, when voltage is applied,
charging current is in leading quadrature. Should not
have in phase component. But actually has small in
phase component; causes dielectric loss, generating
heat.
The dielectric loss in watts per kilometer per phase is:
2f*C*U02 tan 10-6 watt/km per phase
For paper insulated cables the DLA depends on density
of the paper and the contamination in the oil and paper.
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Properties of different type of capacitors
Sl. No
Details Mixed Dielectric 100% Polypropylene
1 Losses < 2.5 W/ kVAr. 0.5 W/ kVAr
2 Running Costs
Higher 1/5th of MD
3 Life 10 to 15 years Same
4 Temp Rise More Less
5 Reliability Higher temp rise, lower
More reliable
6 Size Very large Much smaller
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Energy meter and leading power factor
Most energy meters erratic for leading power factor CASE STUDIES In a plant in South Madras 100 kVAr capacitor in circuit left Weekend with no load: Meter reads 150 200 units per day Misleading; Capacitors suspected defective; Replaced; No improvement Removed capacitors tested OK Tariff meter should assure accuracy for leading power factor Unnecessarily consumer billed for energy not consumed but shown as consumption by erratic energy meter
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Capacitors and Consumer Problems
Many plants high connected load but power drawn very low Machines intended for different types of production Not all used at one time Low utilization factor around 0.3 to 0.45 EB insists capacitance value based on connected load and some thumb rule! Leads to low leading power factor. Penalty levied for low power factor! Field engineers to be educated with correct method for selection of capacitance, or Listen to the consumer
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CASE STUDIES
Connected load about 100 HP
Power drawn hardly 25 kW
EB insisted 50 kVAr capacitor
Average power factor goes to 0. Lead
Penalty levied per month for low power factor at 20% I.e
Rs.12,496!
While energy consumed is 13.620 kWhrs costing only
Rs.54,905!
Best way is to install automatic power factor correction relays
and controls. Switches on only required capacitance.
But quite expensive for small industries to afford.
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Diesel Generators and Power Factor.
It is believed that average power factor for a DG to
operate is 0.8.
A technically erroneous conclusion.
Alternators rated in Volt Amperes (kVA). To specify maximum current an alternator can deliver.
Power factor specified to specify engine rating; kW
loading and current loading should not be
exceeded.
Hence, power factor of loads supplied can be
improved closer to unity by capacitors.
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CASE STUDIES
DG set rating:
3 phase, 415 V, 50 Hz, 500 kVA; used for
6000 hours/ Year.
Average load 250 kW at 0.65 PF.
Full load copper loss of the alternator =
12 kW
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What is the saving if PF is improved to 0.93?
Energy conservation by improving power factor. Rated Current of Alternator = 695.60 A Current at 0.65 PF = 535 A Copper loss at this current = 7.1 kW Current at 0.93 PF = 374 A Copper loss at this current = 3.5 kW Saving in copper losses = 7.1 3.5 kW = 3.6 kW For 6000 hour operation = 3.6 * 6000 kWh or, 21,600 kWh!
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TO CONSERVE ENERGY: 1. REDUCE LOSSES 2. CUT DOWN WASTE OF ENERGY 3. INCREASE EFFICIENCY OF EQUIPMENTS & SYSTEM 4. REDUCE PEAKING DEMANDS 5. INCREASE POWER FACTOR.
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1. Location of power factor correction capacitor banks
** to be near the load d.B, to reduce i2r loss of cables
2. Major power consuming sectors should be as close as
possible to main sub station
3. Capacitor dielectric losses tan
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REDUCE LOSSES
A. Optimal selection of transformers
* At least loading should be between 40% to 60%
B. Selection of cable sizes
* Generous size to reduce i2r loss
* Warm cable means energy loss
C. Selection of piping sizes optimum reduce pressure losses
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REDUCE LOSSES
D. Optimal selection of equipments to work at max. Efficiency
E. Location of compressors, boilers nearer to consumers.
F. Avoid P.R.Vs, Bends & Unnecessary Circuitous Routes.
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Electrical training Kavoori
Consultants 73
Transformer application in transmission and distribution systems
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Consultants 74
Step-Up Transformers
common and vital electrical tools used in
power transmission.
They are usually the first major
transformer in a transmission system and
are often used in various forms
throughout the system.
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Step-Up Transformers
Based on the same formulas of other transformers
but they step up voltages to higher levels while reducing
amperage
and reduces power loss which is proportional to the
square of the current
Step-Up Transformers ideal in long-distance power
transmission use;
by stepping up voltage and reducing current to reduce
energy lost, which is proportional to the square of the
current.
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Step-up transformer
has more turns on the secondary coil than on the primary coil
the voltage induced in the secondary coil is higher than the primary coil voltage.
number of turns on the primary coil is NP and
on the secondary coil is NS, and
if the respective voltages are VP and VS,
then NS/NP = VS/VP.
Example: the primary coil 200 turns and secondary coil 2,000 turns
the voltage induced in the secondary coil is ten times higher than the primary coil voltage
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Generator step up transformers (GSU)
In all nuclear, thermal or hydro electric
power stations, generator transformers
are step-up transformers with delta-
connected LV windings energized by the
generator voltage, while star connected
HV windings are connected to the
transmission lines.
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Generator step up transformers (GSU)
Subject to voltage changes either due to load rejection
or switching operations,
followed by generator over excitation,
must maintain ability to withstand over-loads.
High currents involved requires control of magnetic
field inside the tank to
avoid localized overheating of associated metallic parts.
All of these situations are taken into account during the
design process of the specific units and tested with
state-of-the-art techniques.
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A typical generator step up transformer
Type :Indoor use, gas cooled three phase on-load tap changing gas insulated transformer
Gas pressure0.5MPa (at 20 deg.-C)VoltagePrimary275kV (tap range: +10% -10%,23taps)
Secondary66kV
Tertiary21kV, 90 MVA
CapacityPrimary300MVASecondary300MVA
Impedance voltage22% (at 300MVA BASE)
Noise85dB
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Oil filled transformers
Generally, transformers are filled with insulating oil, to
provide insulation as the clearances in side the tank and
windings are very small.
also serves as a medium for cooling the windings and
core
Since oil provides electrical insulation between internal
live parts, it must remain stable at high temperatures for
an extended period.
To improve cooling of transformers, the oil-filled tank
have external radiators through which the oil circulates
by natural convection.
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Transformer oil properties
The flash point (min) and pour point
(max) are 140 C and 6 C respectively.
The dielectric strength of new untreated
oil is 12 MV/m (RMS) and
after treatment it should be >24 MV/m
(RMS).
The dielectric strength of air is:3 MV/m
(RMS)
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Transformers
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Heat removal from transformers
When transformers are on line,
considerable amount of heat is produced
in the windings and cores due to:
Copper loss in the windings, I2R loss
Magnetic losses:
Eddy current losses in the magnetic core etc
Hysterises loss in the magnetic core etc
This raises the temperature of the transformer
and is dissipated by various cooling methods
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Magnetic loss due to eddy currents
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Eddy Current Losses in the Core
Alternating flux induces an EMF in the core proportional to flux density and frequency resulting in circulating currents
Depends inversely upon the resistivity of the material and directly upon the thickness of the core.
The losses per unit mass of core material, vary with square of the flux density, frequency and thickness of the core laminations.
By using a laminated core, (thin sheets of silicon steel instead of a solid core) path of the eddy current is broken up without increasing the reluctance of the magnetic circuit. A comparison of solid iron core and a laminated iron core is shown in the sketch.
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Eddy Current Losses in the Core
For reducing eddy current losses, higher resistivity core material and thinner (Typical thickness of laminations is 0.35 mm) lamination of core are employed.
This loss decreases very slightly with increase in temperature.
This variation is very small and is neglected for all practical purposes.
Eddy current losses contribute to about 50% of the core losses.
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Hysterisis losses
when a magnetic field is applied all the grains of the magnetic material will orient in the direction of magnetizing force.
In next half cycle this grains will orient in opposite direction in the direction of magnetizing force.
The energy required to change the orientation of the magnetic grains in the direction of the magnetic field is lost in the form of heat. This loss is called hysterisis loss.
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Transformer magnetic core material
CRGO Steel Laminations Cold Rolled Grain Oriented (CRGO) silicon steels are used for
laminations of the Power Transformers magnetic core.
Properties:
Maximum magnetic induction to obtain high induction amplitude in an alternating field
Core loss will be independent of the load
CRGO steel sheets core loss is low; result in reduction of the constant losses.
Low apparent power input (Low hysterisis loss) results in low no load current
High grade surface insulation
Good mechanical processing properties
Low magnetostriction: results in low noise level
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Typical Losses in a 10 MVA Transformer
Losses in 10 000 kVA 110kV/ 7 kV
transformer are
No load loss or Magnetic losses at rated
voltage :10.5 kW
Load loss or copper loss at rated current
at 75oC : 55 kW
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CLASSIFICATION OF TRANSFORMERS
According to cooling method and permissible temperature rise.
OIL IMMERSED TRANSFORMERS.
Type Oil Circulation Cooling method Symbol
ONAN Natural Air Natural ON
ONAF Thermal Air Blast OB
ONWF Head Only Water OW
OFAN Forced by Air Natural OFN
OFAF Pump Air Blast OFB
OFWF Water OFW
COMBINATION:
ON/OB ON/OFN ON/OFB ON/OFW.
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Oil filled transformers
Double rated transformers and very large
or high-power transformers (with
capacities of thousands of KVA) may also
have
cooling fans, start and stop initiated by
the winding temperature indicators
oil pumps, and
even oil-to-water heat exchangers.
Cooling water pumps
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Consultants 92
Forced air cooled Oil Natural
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Forced air cooled Oil Natural
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Transformers
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Electrical training Kavoori
Consultants 95
Heat removal from transformers
When transformers are on line,
considerable amount of heat is produced
in the windings and cores due to:
Copper loss in the windings, I2R loss
Magnetic losses:
Eddy current losses in the magnetic core etc
Hysterises loss in the magnetic core etc
This raises the temperature of the transformer
and is dissipated by various cooling methods
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Electrical training Kavoori
Consultants 96
Transformer Cooling Methods
Losses in the transformer around 0.5 to 1% of its full load kW rating, converted in to heat;
temperature of the windings, core, oil and the tank rises.
This heat dissipated from the transformer tank and the radiator in to the atmosphere.
cooling arrangements helps in maintaining the temperature rise of various parts within permissible limits.
Cooling provided by the circulation of the oil.
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Electrical training Kavoori
Consultants 97
Typical losses of transformers
Rated
Voltage Combination (kV) No-load loss(kW) On-load loss(kW)
Power
(kVA)
6300
60~150
9.3 45
8000 11.2 54
10000 13.2 63
12500 15.6 74
16000 18.8 90
20000 22.2 106
25000 26.2 126
31500 31.2 149
40000 37.3 179
50000 44.1 213
63000 52.5 255
75000 59.8 291
90000 68.8 333
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Losses comparison : Dry type or liquid filled
Energy management Kavoori Consultants 98
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Comparison of Losses: Oil type and dry type
(Oil Transformer) Losses
Dry Type
Transformer
Losses
KVA Full Load
(W) KVA
Full Load
(W)
500 4930 500 10000
750 7900 750 15000
1000 8720 1000 16400
1500 13880 1500 22500
2000 16310 2000 26400
Energy management Kavoori Consultants 99
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Liquid, resin caste and dry type
Transformers loss comparison
Liquid: Cast: Dry:
Load Losses
(kW) 16.38 21.00 18.52
No Load Losses
(kW) 2.66 7.00 7.55
Total Losses
(kW) 19.04 26.07 28.00
Energy management Kavoori Consultants 100
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Electrical training Kavoori
Consultants 101
Transformer Oil
Forms a very significant part of the
transformer insulation system:
Has the important functions of acting as
an electrical insulation as well as
A coolant to dissipate heat losses.
For small rating transformers heat
removed from the transformer by natural
thermal convection.
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Electrical training Kavoori
Consultants 102
Transformer Cooling Methods
For large rating transformers this is not sufficient;
As size and rating increases, losses increase at a faster rate. oil is circulated by means of oil pumps.
Within the tank the oil is made to flow through the space between the coils of the windings.
Several different combination of natural, forced, air, oil cooling methods are employed
choice of transformer cooling method depends on the rating, size, and location.
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Electrical training Kavoori
Consultants 103
Directed oil flow thro windings
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Electrical training Kavoori
Consultants 104
Power transformer: Name plate details
Make : Hack bridge Hewittic and Easun Ltd.
Rated voltage : 110 kV/ 7 kV.
Rated current : 52.55 A/ 825.76 A
Rated kVA : 10 000 kVA
Connection: Primary Delta; Secondary Star
No load loss at rated voltage :10500W
Load loss at rated current at 75oC : 55000 W
Imp voltage at rated current at 750C: 8.35%
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Electrical training Kavoori
Consultants 105
Allowable temperature Rise
Component Cooling Temp Rise Ambient
C
Winding ON,OB,OW 55 Max 45
(Measured by Resistance) OFN, OFB 60 Daily Average 30 OFW 65 (Yearly average 30)
Oil All 45
(Measured by Thermometer)
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Electrical training Kavoori
Consultants 106
COOLING MEDIUM -LETTER SYMBOLS
Cooling Medium Symbol
Mineral Oil O
Synthetic insulation liquid L
Gas B
Water W
Air A
Solid Insulant S
Natural N
Forced F
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Electrical training Kavoori
Consultants 107
Gas Insulated Power Transformers
Use SF6 Gas as the insulating and cooling medium
instead of insulating oil.
First units produced in 1967.
Several thousand units now in service worldwide.
Transformer applications: GSU, Distribution class units
up to 400 MVA, 345 kV.
Primarily used in substations located in urban
areas (including inside buildings, underground) due
to safety benefits.
-
Electrical training Kavoori
Consultants 108
Gas insulated transformers
Space is becoming an important consideration.
This has resulted in:
large-scale substations to be tucked away underground
in overpopulated urban areas
incombustible and non-explosive , large-capacity gas
insulated transformers for accident prevention and
compactness of equipment.
In line with this requirement, several types of large-
capacity gas insulated transformer have been developed.
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Electrical training Kavoori
Consultants 109
Gas insulated transformers
The gas-forced cooling type was available for up to approximately 60MVA,
gas insulated transformer with higher ratings are liquid cooled.
Disadvantage: complex structure for liquid cooling.
certain manufactures began development of gas forced cooling type transformer,
TOSHIBA has delivered 275kV-300MVA gas cooled and gas insulated transformer,
its structure is as simple as the oil immersed type and is the largest capacity gas insulated transformer in the world.
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Electrical training Kavoori
Consultants 110
Gas insulated transformers
Since heat capacity of SF6 gas is much smaller than that of
insulating oil, the following measures are taken into
account.
1. Raise the SF6 gas pressure to 0.5MPa
2. Produce as large flow as possible by optimizing the
layout of gas ducts in the windings
3. Develop high capacity gas blower with high reliability
4. Apply highly thermal-resistant insulating materials to
raise the limit of winding temperature rise
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Electrical training Kavoori
Consultants 111
Sulfur Hexa Fluorine Gas (SF6)
Physical properties
About five times heavier than air, density 6.14kg /m3.
Colorless, odorless and non-toxic.
Speed of sound propagation about three times less than in air, at atmospheric pressure. Hence interruption of arc less loud in SF6 than in air.
Dielectric strength on average 2.5 times that of air,
Increasing pressure, increases the dielectric strength
Around 3.5 bar, SF6 has the same strength as transformer oil.
Becomes liquid at - 63.2C and in which noise propagates badly.
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Electrical training Kavoori
Consultants 112
Gas insulated transformer
-
Electrical training Kavoori
Consultants 113
Gas insulated substation
Gas insulated transformer does not need conservator,
Height of transformer room reduced.
It has non-flammability and non tank-explosion
characteristics
No need for fire fighting equipment in transformer
room.
So gas insulated transformer, gas insulated shunt reactor
and GIS control panels installed in the same room.
The substation is a fully SF6 gas insulated substation
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Electrical training Kavoori
Consultants 114
Natural-cooled type SF6 gas-insulated transformer
-
Electrical training Kavoori
Consultants 115
Forced-gas-circulated, natural-air-cooled type SF6 gas-insulated transformer
-
Electrical training Kavoori
Consultants 116
Forced-gas-circulated, forced-air-cooled type SF6 gas-insulated transformer
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Energy conservation and transformers
The transformer efficiency is maximum
when loaded at 45-50% of its rated
capacity
Selection of transformers for an industry
Select two transformers of each rating for
the full load of the plant.
In normal times, run them in parallel-
each will be loaded to its 50% capacity, ie.
At its maximum efficiency area.
Energy management Kavoori Consultants 117
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TRANSFORMER LOSSES
Constant loss or no load loss- does not depend upon load condition : about 1kW per 500 kVA Copper losses - proportional to load condition During lean periods, one transformer can be cut out of service - saves about 24 units per day i.e. Rs. 48/- per day per 500 kVA capacity Diagram - transformer losses
Energy management Kavoori Consultants 118
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TRANSFORMER LOSSES
The higher the transformer capacity, the higher the
constant losses
The idle loss of a 5000 kVA transformer is 10 kW!
By prudent switching of transformers, this loss can be
reduced.
Energy management Kavoori Consultants 119
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TRANSFORMER LOSSES
Constant loss or no load loss- does not depend upon load condition : about 1kW per 500 kVA Copper losses - proportional to load condition During lean periods, one transformer can be cut out of service - saves about 24 units per day i.e. Rs. 48/- per day per 500 kVA capacity Diagram - transformer losses
Energy management Kavoori Consultants 120
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TRANSFORMER LOSSES Transformer Load Losses- Model calcuations.
KAVOORI CONSULTANTS, CHENNAI. Energy audit
M/s. *************************** Ltd Table No.
Transformer Load Losses, at the present loading condition:
Transformers with Off Load Tap Changer Make Bharat Bijlee
k.V.A. H.V. L.V. Imp % ge Units
Rating 2000 11000 433 6.25
No load loss 3.3 kWs
Full lload loss at temperature, oC 75 19.8 kWs
Full lload loss at Operating temperature, oC 31.9 17.05
Full load current, L.T. 2669.9 Amps
Full load current, H.T. 175.16 Amps
Cost of electrical energy 5.95 Rs.
No of transformers in Parallel 2
Single transformer in service Two transformers in service
Load Losses, in kW Losses, in kW
%ge Load No Load Load Total No Load Load Total
At an operating temperatur of 31 oC
10.00% 3.3 0.17 3.47 6.6 0.04 6.64
20.00% 3.3 0.68 3.98 6.6 0.17 6.77
30.00% 3.3 1.53 4.83 6.6 0.38 6.98
40.00% 3.3 2.73 6.03 6.6 0.68 7.28
50.00% 3.3 4.26 7.56 6.6 1.07 7.67
60.00% 3.3 6.14 9.44 6.6 1.53 8.13
70.00% 3.3 8.35 11.65 6.6 2.09 8.69
80.00% 3.3 10.91 14.21 6.6 2.73 9.33
90.00% 3.3 13.81 17.11 6.6 3.45 10.05
100.00% 3.3 17.05 20.35 6.6 4.26 10.86
At an operating temperature of oC 65.00 19.16
10.00% 3.3 0.19 3.49 6.6 0.05 6.65
20.00% 3.3 0.77 4.07 6.6 0.19 6.79
30.00% 3.3 1.72 5.02 6.6 0.43 7.03
40.00% 3.3 3.07 6.37 6.6 0.77 7.37
50.00% 3.3 4.79 8.09 6.6 1.20 7.80
60.00% 3.3 6.90 10.20 6.6 1.72 8.32
70.00% 3.3 9.39 12.69 6.6 2.35 8.95
80.00% 3.3 12.26 15.56 6.6 3.07 9.67
90.00% 3.3 15.52 18.82 6.6 3.88 10.48
100.00% 3.3 19.16 22.46 6.6 4.79 11.39
Energy management Kavoori Consultants 121
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Transformers efficiency v.s. load
Energy management Kavoori Consultants 122
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TRANSFORMER LOSSES
Energy management Kavoori Consultants 123
2000 kVA, 6600 /433 Volts Transformer. Total Losses
Single, Two in parallel operation.
(Operating temperature 55 o C)
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
1 2 3 4 5 6 7 8 9 10 11 12
Transformer load in fraction of full load.
Tota
l lo
sses in k
W
(Load +
No L
oad)
Single Transformer
Two transformers parallel
-
TRANSFORMER LOSSES
Constant loss or no load loss- does not depend upon load condition : about 1kW per 500 kVA Copper losses - proportional to load condition During lean periods, one transformer can be cut out of service - saves about 24 units per day i.e. Rs. 48/- per day per 500 kVA capacity Diagram - transformer losses
Energy management Kavoori Consultants 124
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Transformer efficiency VS. Load
Energy management Kavoori Consultants 125
96.50
97.00
97.50
98.00
98.50
99.00
99.50
100.00
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
Perc
en
tag
e e
ffic
ien
cy
Load, fraction of the rating
2000 kVA transformer
2500 kVA Transformer
23500 kVA Transformer
500 kVA Transformer
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TYPICAL EDDY CURRENT LOSS FACTORS FOR OIL-FILLED TRANSFORMERS
126
Transformer size oil
filled transformer
Eddy current loss
factor
Up to 1 MVA 1%
1 MVA TO 5 MVA 1 to 5 %
Greater than 5 MVA 9 to 15%
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SELECTION OF CABLE SIZE
CONSIDER, SAY, A BULK LOAD OF 207 HP + 5 kW
CONNECTED BY 1000 M OF CABLE FROM THE SUB
STATION.
Cable selected was 3-1/2 * 240 sq.Mm aluminum
conductor p.V.C insulated armored cables
I2r loss in the cable = 7626 w
Energy management Kavoori Consultants 127
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SELECTION OF CABLE SIZE
If a 3-1/2 * 300 sq.Mm cable is used, the loss will be only
6100 w
Difference in loss of power = 1525 w
Difference in loss of energy
in one year = 13,360 units
cost saved @ Rs. 3 = Rs. 40,000/- unit
Energy management Kavoori Consultants 128
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Power distribution systems
Power Factor Improvement Capacitors Location
Assume a sectional load of 155 kW located at about
1000m from the main substation and connected by an
aluminum cable of size d * 240 sq.mm cable.
Energy management Kavoori Consultants 129
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Power Factor Improvement Capacitors Location
DC resistance of the cable 0.125/km
load of the remove section 155 kW
power factor of the load = 0.8
Consider the power factor capacitor at this main
substation bus.
Current drawn by the load at 0.8 pf = 240a
Power loss in the cable = 2702 * 0.125 = 9.082 kW
Energy management Kavoori Consultants 130
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Power Factor Improvement Capacitors Location
If the power factor correction capacitor is
connected at the load section distribution board:
For a corrected power factor (of say 0.97)
The current drawn will be 222.3 a power loss in the
cable for this current = 222.3 * 0.125 = 6.177 kW
Energy management Kavoori Consultants 131
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Power Factor Improvement Capacitors Location
Saving in power loss= 9.082 - 6.177= 2.905 kW or 3 kW
Saving in one year of operation
= 3 * 24 * 365 = 26,680 kW
Energy cost saved per year
= 26,280 * 3 = Rs. 78,840/-
to minimize the power loss and save energy and its
cost, always locate capacitors at the section using
maximum power, as close as possible to the respective
substation panel.
Energy management Kavoori Consultants 132
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Load location - Cable losses
Suppose the sub-station is close by and only 100 m cable is used. Loss in cable = 610 w Energy saving in one year = 48,092 kWhr Cost of energy saved @ Rs. 1.4/kWhr = Rs. 1,44,000/- To minimize power loss and save energy and its cost always locate the section using maximum power as close as possible to the main substation.
Energy management Kavoori Consultants 133
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ENERGY UTILIZATION EFFICIENCY
IN HARMONIC ENVIRONMENT
SHRI. K.R. GOVINDAN,
KAVOORI CONSULTANTS,
New No: 22, JANAKIRAM STREET, WEST
MAMBALAM, CHENNAI 600 033.
PH:24846139.
134
-
POWER UTILIZATION
All alternating current equipments and
power distribution systems and elements
Designed to work from a power source with voltages
of 50 HZ frequency and a sinusoidal waveform
Their behavior, energy utilization efficiency and other
characteristics are much affected when supplied with
distorted wave forms.
Incandescent lamps, heaters, etc draw current
proportional to the voltage following sinusoidal
waveform
Hence these loads are called linear loads
135
-
POWER DRAWN BY A LINEAR RESISTIVE LOAD
Both current and voltage rise and fall together
Hence current is in phase with the voltage
The power drawn at any instant is I X V
during a negative half cycle, voltage and current
are negative
Since the power is the product of voltage and
current it becomes positive
Hence a positive power is drawn thorough out the
cycle
136
-
POWER UTILIZATION
All alternating current equipments and
power distribution systems and elements
Designed to work from a power source with voltages
of 50 HZ frequency and a sinusoidal waveform
Their behavior, energy utilization efficiency and other
characteristics are much affected when supplied with
distorted wave forms.
Incandescent lamps, heaters, etc draw current
proportional to the voltage following sinusoidal
waveform
Hence these loads are called linear loads
137
-
Unity power factor Voltage, current and power wave forms
Energy management Kavoori Consultants 138
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POWER DRAWN BY A LINEAR INDUCTIVE LOAD
Induction motors also draw current proportional to
voltage
Since current drawn is inductive, lags the voltage-
but still follow sinusoidal waveform
Same hold good for capacitors, but current leads
the voltage
In phase or out of phase, the current drawn is
proportional to the voltage
Hence these are also linear loads
139
-
POWER UTILIZATION INDUCTIVE LOAD
Though current follows voltage waveform, the peak
and the zero value of the current is displaced by an
angle from the peak and zero point of voltage
waveform
With respect to the instantaneous voltage value,
the current value becomes a function of the Cos of
the angle (between voltage and current)
Hence power at any instant is equal to Voltage X
Current X Cos of the angle between them
140
-
LINEAR INDUCTIVE LOAD
141
-
POWER UTILIZATION INDUCTIVE LOADS
If the load is totally inductive like a reactor or a induction coil the current drawn lags the voltage by 90 degrees
Since power is the product of instantaneous voltage and current, its frequency is double of the voltage frequency
It also passes through the negative half of the cycle
Since the negative half and the positive half of the waveform are identical I.e. positive power and negative power, total power drawn by the load is zero
142
-
POWER UTILIZATION INDUCTIVE LOADS
No net power flows
143
-
POWER CONTROL
In the past, a resistance or an auto transformer was employed to regulate power
It controls the peak value of the voltage applied
But still the voltage follows a sinusoidal waveform but with lesser amplitude
Since power is a product of voltage and current, the power follows sinusoidal waveform
With reduction in peak value the power drawn is also reduced
But, involves wastage of power in the controlling element
144
-
LINEAR POWER CONTROL
145
1
2
1. Line voltage, 2. Controlled voltage
-
SOLID STATE POWER CONTROL
To eliminate the losses in the controlling elements.
Solid state or thyristor controls employed.
These follow different technique to control power
Chops off a portion of the wave so that the volume
of power to the load is reduced
Now the current is not following the voltage
waveform; it is like interrupted impulses of current
This is a non sinusoidal distorted waveform
146
-
SOLID STATE CONTROL OF POWER DISTORTS WAVEFORM
147
-
HARMONICS AND ENERGY LOSS
Harmonic currents are just circulating in the
network
They do not contribute to the power delivered
But causes I2R losses
In addition the magnetic effect of harmonics
creates other problems which also results in
considerable losses
Alternating current passing though a conductor
sets up alternating magnetic field.
Create varying magnetic field around the conductor
148
-
HARMONICS AND ENERGY LOSS SKIN EFFECT
Center of the conductor enveloped by more varying
magnetic flux than on the outside.
They push the current to the periphery of the conductor as
the center is subjected to higher intensity of magnetic field
This concentration at surface is the skin Effect
Increases conductor effective resistance
This is more pronounced if the conductors are associated
with magnetic material as the flux density is much higher
149
-
HARMONICS AND ENERGY LOSS CONDUCTORS, CABLES ETC.
SKIN EFFECT
These effects are proportional to the frequency
of the alternating current
Hence very high for higher frequency harmonic
currents
Since effective area of cross section is
reduced, higher resistance offered to the
current flow
Very high I2R losses are involved
For closely placed conductors another factor
comes in to play I.e.Proximity Effects
150
-
HARMONICS AND ENERGY LOSS CONDUCTORS, CABLES ETC.
PROXIMITY EFFECT
Conductor halves in close proximity cut by more
Flux than the remote halves.
Current distribution not even throughout the
Cross-section,
Greater portion carried by remote halves.
When currents are in opposite directions,halves in
closer proximity carry more current.
Overall effect- increase in effective resistance.
151
-
EFFECTIVE AREA OF CONDUCTORS
FOR HARMONIC CURRENTS
152
Cross sectional area of a round conductor available for conducting DC current
DC resistance
Cross sectional area of the same conductor available for conducting normal-frequency AC
AC resistance
Cross sectional area of the same conductor available for conducting high-frequency AC
AC resistance
-
HARMONICS AND ENERGY LOSS CONDUCTORS, CABLES ETC.
Proximity effect decreases with increase
In spacing between cables.
At certain harmonics the combined effect results
in twice the I2R loss
153
A.C/D.C resistance
ratio
Frequency Harmonic of 50 Hz
1.01 50 1
1.21 250 5
1.35 350 7
1.65 550 11
-
HARMONICS AND INDUCTION MOTOR
When the power supplied to the stator of the motor
contains harmonics,
The stator winding affected by skin effect
The rotor is severely affected, as the conductors are
subjected to magnetic field of varying frequencies.
1.5 Hz to 300 Hz.
In the motor the rotating magnetic field developed by
the fundamental frequency voltage only develop
necessary torque delivers shaft power
154
-
HARMONICS AND INDUCTION MOTOR
With motor designed for 3% slip, the rotor currents
have a frequency of 1.5Hz;
The rotor is designed to have the reactance and
DC resistance nearly equal at this frequency to get
optimum efficiency.
But, different types of Rotating Magnetic fields are
setup by individual harmonic currents
While fields created by forward magnetic fields
subtract on the rotor field, negative ones added up
to the rotor field
155
-
HARMONICS AND INDUCTION MOTOR
5th harmonic creates 250 Hz frequency while 11th and
13th pair together to induce 500 Hz in the rotor
These high frequency harmonics snow balls the skin
effect and the rotor I2R loss becomes very high
The rotor have currents at 6,12,18,12 etc times the
stator frequency
High frequency means higher eddy current and
hysterisis loss
The negative torques will affect the shaft horse power;
some times create very bad vibration
At certain level the efficiency drops down about 10%
156
-
HARMONICS AND INDUCTION MOTOR
Harmonic fields rotating relative to each other
produce torque pulsations
Needs re-examination of torsional characteristics of
entire shaft system
Leakage flux set up in stator and rotor end windings
added to the losses
With skewed rotor bars, high frequency produce
substantial iron loss;
Depends upon amount of skew and iron loss
characteristics
157
-
HARMONICS AND INDUCTION MOTOR
Case Study:
Test on a 15 kW motor at full out put
With 50 Hz fundamental sinusoidal voltage loss at
full load = 1303 Watts
With Quasi-square wave voltage 1600 Watts
Losses up by 23%
158
-
HARMONICS AND TRANSFORMER Transformers essentially comprises of current
carrying conductors encircled by iron core
Hence harmonics effects results in:
Higher eddy current and hysterisis losses
Skin effects due to harmonic current
High copper losses
This effect more important for converter transformers
Filters do not neutralize harmonic current in these transformers; due to higher losses develop unexpected hot spot in tanks
159
-
NO LOAD CURRENT OF A STAR/STAR TRANSFORMER
HARMONIC RESOLUTION
160
Harmonic analysis of peaked no load current wave of i0 = 100 sin + 31.5 sin
5+
-
HARMONICS AND TRANSFORMER
Third harmonics-Important for power transformers; circulation of triplen zero sequence current in delta windings
These extra currents over heat the windings
The RMS value of pure sine wave is 0.707 of peak value
340 V peak value has an RMS voltage of 240
But this ratio is not true for a distorted waveform
RMS value is the measure of the heat generated by an equivalent DC current
Hence, heat produced by harmonics are much higher
161
-
THIRD HARMONICS IN PHASE WITH FUNDAMENTAL
162
-
THIRD HARMONICS OUT OF PHASE WITH FUNDAMENTAL
163
-
Third harmonics phase relation ship
164
-
HARMONICS AND POWER FACTOR
Since harmonic currents are neither in phase nor follow supplied voltage they do not deliver any power
In a pure sinusoidal waveform the displacement angle between the current and the voltage decides the power factor, known as displacement power factor or apparent power factor
This does not hold good in case of harmonic currents as they do not have any such angular relation
Hence power factor is kW/Volts X Ampere
Actually this is the true power factor in a circuit which has harmonic currents
165
-
HARMONICS AND TRANSFORMER
The losses in a transformer are a combination of
1. Excitation (No load loss) I.e. Eddy current,
hysterisis, stray losses
2. Load losses mainly due to I2R loss in the
conductor
3. Both the losses increase as the square of the
frequency but does not contribute to the power
transfer
4. Heats transformer; increases the temperature
resulting in premature failure apart from wasting
energy!
166
-
DERATING FACTOR FOR TRANSFORMERS
167
-
HARMONICS AND INSULATED CABLES
A cable is essentially a conductor surrounded by an
insulation
These two components create losses;
The conductor develops I2R loss due to the current
flow
If the current passing through contains higher
harmonics this loss is increased due to the
increased skin and proximity effects as shown
earlier
168
-
HARMONICS AND INSULATED CABLES
The insulation is subjected to dielectric loss
This loss is
= 2 f C U02 tan 10 -6 (watt/km per phase)
For a specified design,
C and U02 are constant; therefore, loss is
proportional to the frequency
Higher the harmonics higher the losses
169
-
BALANCED LOAD Neutral Current
170
Y
B
N
R 5 A 5 A 5 A
5 A 5 A 0 A
-
BALANCED LOAD WITH THIRD HARMONICS Neutral Current
171
Y
B
N
R 5 A 5 A 5 A
5A 10A 15A
-
THIRD HARMONICS AND NEUTRAL CURRENTS
172
-
ELECTRICAL FAILURE MECHANISM
All protective systems are based on Current2 & Time
Rarely Mechanical Damage.
Resistance Current 2 Power
Loss
Time
Energy
Loss Heat
Temperature Insulation
Failure
-
ELECTRICAL FAILURE
Power loss is proportional to the square of
the current;
Immaterial, whether the current is in
phase with voltage or of fundamental
frequency
Harmonic currents are no exception to this;
They do not deliver power, but circulate in
the system, contributing to energy loss.
result: higher temperature
-
ELECTRICAL FAILURE
Most of the protective schemes are based on this,
I.e. I2t, resistance being almost constant.
But added disadvantage with harmonics is
They increase the resistance also, by skin and
proximity effects.
Hastens failure, reduce useful life
-
CAPTIVE POWER GENERATORS AND
HARMONICS
Generators for large lighting installations:
discharge lamps with inductive chokes etc generate 30% 3rd harmonics
If generated voltage contains 3% harmonics, with harmonic loads, waveform may worsen
Even in a well balanced three phase lighting system 20% 3rd harmonic may exist in each phase.
3rd harmonics are of additive nature; in the neutral it will be 60%..
will heat up the machines and neutral conductors
but the fundamental current may be zero
-
CAPTIVE POWER GENERATORS AND
HARMONICS
Eg: A carefully balanced 250 kVA fluorescent lighting load in a warehouse.
fed from the public utility - 13% of full load line current observed in the neutral
fed from 320 kVA stand by generator - current increases to 250 Ampere (72% full load current)
due to high third harmonic content in the generator output waveform
The solution was to replace 11/12 pitch of the stator winding by 2/3 pitch
The neutral current was below than the value when supplied from the utilities
-
CAPTIVE POWER GENERATORS AND
HARMONICS
Sizing generators for non linear loads:
Simple rules of the thumb is to oversize the
standard generators for the load to be catered
Some allow 50% non linear loads
But, manufacturers should be given full information
of non linear loads while ordering
The crux of the problem - one of the generating
impedance
Current harmonics of non linear loads are constant
do not depend upon the power supply
-
CAPTIVE POWER GENERATORS AND
HARMONICS
But voltage distortion is a direct function of generating impedance
The stator pitch configuration have varying reactance for each harmonics
Hence evaluating the voltage distortion for all harmonics individually is necessary
These distorted voltages affect the performance of AVRs affecting their stability
PMG excitation system has improved this situation
The power to the AVR is constant irrespective of generated output
-
CAPTIVE POWER GENERATORS AND
HARMONICS
Designing generators with specific winding pitches
and low reactance is not quite commercially viable
Hence practical solution is to derate the standard
industrial generators
Some reputed manufacturers select a 0.12 p.u.
subtransient reactance as a good practical solution
The basics; 6 pulse VFD motor drive with 26%
current distortion
-
CAPTIVE POWER GENERATORS AND
HARMONICS
POWER FACTOR:
Conventional power factor is Watts/Volt amp is =
Cosine of the angle between current and voltage
This is really a displacement power factor
But with harmonic currents, power factor as Cos
does not hold good
Because there are many harmonic currents flowing in
the circuit
If the total RMS value of the current is taken into
consideration, the power factor value may become
worse
-
CAPTIVE POWER GENERATORS AND
HARMONICS
The power drawn is a function of the fundamental
current only
Harmonic current increase the total RMS current
without increasing the power
Discrepancy arise between ammeter reading and
voltmeter reading
Standard power factor meter measures
displacement power factor only
They may show a unity power factor while infact
the real power factor may be as low as 0.70
-
ELECTRICAL FAILURE MECHANISM
All protective systems are based on Current2 & Time
Rarely Mechanical Damage.
Resistance Current 2 Power
Loss
Time
Energy
Loss Heat
Temperature Insulation
Failure
-
ELECTRICAL FAILURE
Power loss is proportional to the square of
the current;
Immaterial, whether the current is in
phase with voltage or of fundamental
frequency
Harmonic currents are no exception to this;
They do not deliver power, but circulate in
the system, contributing to energy loss.
result: higher temperature
-
ELECTRICAL FAILURE
Most of the protective schemes are based on this,
I.e. I2t, resistance being almost constant.
But added disadvantage with harmonics is
They increase the resistance also, by skin and
proximity effects.
Hastens failure, reduce useful life
-
CAPTIVE POWER GENERATORS AND
HARMONICS
Generators for large lighting installations:
discharge lamps with inductive chokes etc generate 30% 3rd harmonics
If generated voltage contains 3% harmonics, with harmonic loads, waveform may worsen
Even in a well balanced three phase lighting system 20% 3rd harmonic may exist in each phase.
3rd harmonics are of additive nature; in the neutral it will be 60%..
will heat up the machines and neutral conductors
but the fundamental current may be zero
-
CAPTIVE POWER GENERATORS AND
HARMONICS
Eg: A carefully balanced 250 kVA fluorescent lighting load in a warehouse.
fed from the public utility - 13% of full load line current observed in the neutral
fed from 320 kVA stand by generator - current increases to 250 Ampere (72% full load current)
due to high third harmonic content in the generator output waveform
The solution was to replace 11/12 pitch of the stator winding by 2/3 pitch
The neutral current was below than the value when supplied from the utilities
-
CAPTIVE POWER GENERATORS AND
HARMONICS
Sizing generators for non linear loads:
Simple rules of the thumb is to oversize the
standard generators for the load to be catered
Some allow 50% non linear loads
But, manufacturers should be given full information
of non linear loads while ordering
The crux of the problem - one of the generating
impedance
Current harmonics of non linear loads are constant
do not depend upon the power supply
-
CAPTIVE POWER GENERATORS AND
HARMONICS
But voltage distortion is a direct function of generating impedance
The stator pitch configuration have varying reactance for each harmonics
Hence evaluating the voltage distortion for all harmonics individually is necessary
These distorted voltages affect the performance of AVRs affecting their stability
PMG excitation system has improved this situation
The power to the AVR is constant irrespective of generated output
-
CAPTIVE POWER GENERATORS AND
HARMONICS
Designing generators with specific winding pitches
and low reactance is not quite commercially viable
Hence practical solution is to derate the standard
industrial generators
Some reputed manufacturers select a 0.12 p.u.
subtransient reactance as a good practical solution
The basics; 6 pulse VFD motor drive with 26%
current distortion
-
CAPTIVE POWER GENERATORS AND
HARMONICS
POWER FACTOR:
Conventional power factor is Watts/Volt amp is =
Cosine of the angle between current and voltage
This is really a displacement power factor
But with harmonic currents, power factor as Cos
does not hold good
Because there are many harmonic currents flowing in
the circuit
If the total RMS value of the current is taken into
consideration, the power factor value may become
worse
-
Lamp Characteristics: efficacy, life and colour rendering index.
Lamp type Previous
coding
ILCOS coding Lamp
efficacy
(lumens/
Watt)
Quoted lamp life
(hours)
Colour rendering Index
compared to Inc lamp
Tungsten
filament
GLS I 10 to 18 1000 to 2000 100
Tungsten
halogen
TH HS 15 to 25 2000 to 4000 100
High pressure
mercury
MBF QE 30 to 60 14000 to 25000 47
Low pressure
mercury
(fluorescent)
MCF FD (tubular)
FS (compact)
65 to 95
65 to 95
6000 to 15000
8000 to 10 000
11
Metal halide MBI M 65 to 85 6000 to 13000
Low pressure
sodium
SOX LS 70 to 150 11000 to 22000
High pressure
sodium
SON S 55 to 120 12000 to 26000 23
Induction XF 70 to 80 60000
Energy management Kavoori Consultants 192
-
CAPTIVE POWER GENERATORS AND
HARMONICS
The power drawn is a function of the fundamental
current only
Harmonic current increase the total RMS current
without increasing the power
Discrepancy arise between ammeter reading and
voltmeter reading
Standard power factor meter measures
displacement power factor only
They may show a unity power factor while infact
the real power factor may be as low as 0.70
-
CONCLUSION
Harmonics are created in a power system by the consumer and also by the supplier
But major portion by consumer
Harmonics creates lot of problems, destroys equipments
All energy efficient equipments essentially creates harmonics;
These result in added energy losses
Hence harmonics are to be limited
While selecting energy efficient equipments these points are to be given greater attention
194