Distribution System and Transformers

Energy Saving in Electrical Utilities

K. R. GOVINDAN

Kavoori Consultants

22, Janakiraman Street,

West Mambalam,

Chennai, 600 033.

Energy management Kavoori Consultants 2

Distribution System and Transformers

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


ATTITUDE

Half full or half empty?


Two liters container; 1 liter liquid.

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.


In the Present day context of:

Depleting energy sources

Spiraling costs

pollution of environment to alarming levels.

Energy management assumes top priority


ENERGY AUDIT

PRE REQUISITE FOR AN ENERGY MANAGEMENT PROGRAMME

BY ITSELF DOES NOT SAVE ENERGY

HELPS MANAGEMENT IDENTIFY AREAS OF HIGHEST

SAVINGS POTENTIAL


FUNCTIONS OF AUDIT

Assesses various forms of energy use

Compares with estimated minimum

Provides inputs for budgetary control

MOST IMPORTANT FOR

OLD PLANTS

?

SET UP WHEN FUEL COST WAS VERY LOW

NO CONCERN FOR ENERGY EFFICIENCY


Good energy management is

Increasing utilization efficiency or reducing losses

Or

CONSERVATION OF ENERGY

Let us consider the electrical energy



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


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!

Why energy conservation?


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

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!


Power generation, transmission and distribution

14

Typical industrial power distribution SLD

15


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!

Control of Atmospheric pollution

Burning of fossil fuels generates


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.

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.


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


Pollution green house gasses


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.



DO NOT MAKE THE ENTIRE EARTH LOOK LIKE THIS!

PLEASE GIVE A GOOD EARTH TO OUR CHILDREN!


We have a social responsibility for the future

generation

Leave the world, a wonderful place, as it is-

for the future generation


It's true that we don't know what we've got until we lose it! Conserve the fast depleting conventional energy


ENERGY CONSERVATION

Most urgent, top priority Depleting sources

Spiraling cost Cannot have the luxury of

unproductive usage and high demands

ENERGY CONSERVATION

OPPORTUNITIES IN

NO TWO IDENTICAL FACTORIES ARE

ALIKE

Scientific approach is needed to tackle

unique problems of each industry-

An energy audit


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.


MISSING THE OBVIOUS


PARETTO ANALYSIS


ENERGY CONSERVATION

First let us look at:

What is power,

What is energy and

The sources of energy


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

31

Simple circuit

4AMPS = 240VOLTS/60 OHMS

VOLTAGE MAKES CURRENT FLOW

THROUGH A RESISTANCE.

32

240 V 960W

60

(Heater)

4 AMPS

4 AMPS

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


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)


ENERGY FORMS

Coal

Oil

Gas

Electricity

Steam

Compressed air

Vacuum


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.


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.


ENERGY FORMS Identify source or carriers:

Sources:

Inherent energy expended by irreversible chemical process - burning

Fuels

OIL

GAS

COAL

Gets consumed.


REDUCE WHAT?


OUTPUT? USEFUL WORK DONE

NO ! WORK DONE SAME

INPUT? YES.

HOW?

ENERGY INPUT = USEFUL WORK DONE + ENERGY LOST IN CONVERSION / TRANSMISSION.

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.


LOSSES

AVOIDABLE

WASTAGE

LOW EFFICIENCY

UN EVEN DEMAND


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


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.



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.

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!


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)


R KVA

Capacitive reactance in RKVA

1

2

kW kVA1 `

KVA2`

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.


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.



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

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


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.


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.


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.


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.


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



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.



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


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)


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.


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


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.


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


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


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


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.


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.


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


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!


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.



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


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

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.


Electrical training Kavoori

Consultants 73

Transformer application in transmission and distribution systems


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.


Consultants 75

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.


Consultants 76

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


Consultants 77

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.


Consultants 78

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.


Consultants 79

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


Consultants 80

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.


Consultants 81

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)


Consultants 82

Transformers


Consultants 83

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


Consultants 84

Magnetic loss due to eddy currents


Consultants 85

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.


Consultants 86

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.


Consultants 87

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.


Consultants 88

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


Consultants 89

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


Consultants 90

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.


Consultants 91

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


Consultants 92

Forced air cooled Oil Natural


Consultants 93

Forced air cooled Oil Natural


Consultants 94

Transformers


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


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.


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

Losses comparison : Dry type or liquid filled


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


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



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.


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.


Consultants 103

Directed oil flow thro windings


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%


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)


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


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.


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.


Consultants 109


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.


Consultants 110


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


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.


Consultants 112

Gas insulated transformer


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


Consultants 114

Natural-cooled type SF6 gas-insulated transformer


Consultants 115

Forced-gas-circulated, natural-air-cooled type SF6 gas-insulated transformer


Consultants 116

Forced-gas-circulated, forced-air-cooled type SF6 gas-insulated transformer

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.


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


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.


TRANSFORMER LOSSES



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


Transformers efficiency v.s. load


TRANSFORMER LOSSES


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



Transformer efficiency VS. Load


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

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%

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


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


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.



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



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



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.


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 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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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



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

Documents

Distribution System and Transformers