07 Electricity

7/22/2019 07 Electricity

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Electricity

Basic Concepts


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Contents

General Electric Current

Voltage

Resistance

Ohms law

DC/AC Frequency

Power, Power factor

Energy

Power System Generation: Generators, switchyard, transformers, control andprotection

Transmission: Components, parameters and design

Distribution: Substations, switchgear and protection, design


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Current

Rate of flow of electricity or electricalcharge (electrons) through a conductor

Measuring Unit: Amperes (mA, A, KA)

One ampere of current represents one coulomb of electrical charge

(6.24 x 1018charge carriers) moving past a specific point in one

second.


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Voltage

Voltage, also called electromotive force, is a quantitativeexpression of the potential difference between two points.

Voltage (or potential difference) between two points canalso be understood as the driving force which causes

electric charge (or current) to flow between the points. The greater the voltage, the greater the flow of electrical

current through a conducting medium for a givenresistance to the flow.

Measuring Unit of Voltage: Volt (mV,V,kV)

(One volt will drive one coulomb (6.24 x 1018) charge carriers, such as

electrons, through a resistance of one Ohm in one second).


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Resistance

It is the property of a material which opposes the flow ofelectricity (electric charge) through them.

Its SI unit is (Ohm).

Resistance of a conductor depends upon the resistivityof the material, length of conductor under consideration,and its cross sectional area.

Mathematically,

R= x L/A ------------ (i)where, - resistivity (meter)

L- length of conductor

A- cross section area of conductor


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Ohms Law

It states that the current flowing between twopoints is directly proportional to the potential

difference (voltage) between them and inversely

proportional to the resistance between them. Mathematically,

I=V/R ------------- (ii)

Where, R- resistance, OhmV- voltage, Volt

I- Current, Ampere


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DC/AC

Electric current and voltage can be eitherdirect or alternating, on the basis ofpolarity and direction of flow.

Direct current (DC) flows in the samedirection at all points in time.

In an alternating current (AC), the flow of

charge carriers and the polarity reversedirection periodically.


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Alternating Current


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Frequency

The number of cycles per second is also called the frequency

The standard unit of frequency is the hertz (Hz)


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Power Power is the rate of transfer of energy through a point.

Apparent power (S) is the absolute value of total power, and is given by,

S=V*I

Its SI unit is VA (volt-ampere)

Real power (P) is the total useful power flowing in a circuit, and is given by,

P=V*I*Cos

Its SI unit is Watt (W)

Reactive power (Q) is the power which does not do useful work and ismostly lost in the form of heat, or consumed by inductive loads. It is given

by,

Q=V*I*Sin

Its SI unit is VARHere, V denotes Voltage

I denotes current and

denotes the phase angle between voltage

and current vectors. Cos is generally known as Power Factor.


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Power factor

It is the ratio of Real power to Apparentpower.

Mathematically, it is given as the Cosine of

phase angle between current and voltagevectors. i.e.,

Cos

where, is the phase angle between currentand voltage waveform.

Power factor plays important role in the quality ofelectricity supply


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Energy

Energy is the total power transferred incertain amount of time.

Generally electrical energy is measured inWatt Hour (WH).

1 kWH is also known as 1 unit ofelectricity.


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Power System

It consists of three major components: Generation: Generates electricity

Transmission: Transmits electricity throughlong distances to substations

Distribution: Distributes electricity amongutilities/consumers


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Power System


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Three Phase System

Three phase system is generally adoptedand economical method of electricity

generation, transmission and distribution.

In this system there are three conductorscarrying AC current, each out of phase to

others by 120(Their instantaneousvalues reach maximum value after 1/3 and2/3 of a cycle)


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Alternators (AC Generators)

Generators are the machines whichconvert mechanical energy into electrical

energy.

Parts of generator: Rotor: Rotating part of generator

Stator: Stationary part of generator

Armature: Power producing part, generallypresent in stator.

Field: Magnetic field component of generator


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Alternator operating principle

When field current is provided in the rotor winding,rotating magnetic field is established, and hence flux is

linked to the stator consisting of armature winding. Due

to the interaction of armature winding with the rotating

field, an electromotive force (EMF) is generated causingflow of current through the armature winding of the

generator.

The principle is based on Farradays law of

electromagnetism which says that an EMF is generatedin a conductor when it is placed in a varying magnetic

flux.

The rotor is rotated by mechanical input such as rotation

of turbine.


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Alternator operating principle

The output frequency of an alternator depends on thenumber of poles and the rotational speed.

The speed corresponding to a particular frequency iscalled the Synchronous speed for that frequency.


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Generator parts


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Poles in Generator


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Generator types

Synchronous generators They operate at synchronous speed

Self excited- through exciter

Voltage regulated through built in AVR Can operate at stand alone or island mode

Induction generators They operate at higher than synchronous speed

Require external excitation- such as from the grid

Frequency & voltage governed by grid

Can not run in island modewhen grid is off system

will be down


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Generation

Governor: It is a device used to measure and regulate

the speed of turbines.

Automatic Voltage Regulator (AVR):An automatic voltage control device controls

the field current to keep output voltage

constant.


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Generator sizing

Generator rating factors.

Max. ambient

temperature in oC20 25 30 35 40 45 50 55

ATemperature

Factor1.10 1.08 1.06 1.03 1.00 0.96 0.92 0.88

Altitude 1000 1500 2000 2500 3000 3500 4000 4500

B Altitude Factor 1.00 0.96 0.93 0.90 0.86 0.83 0.80 0.77

C ELC Correction Factor 0.83

D Power Factor When load is light bulbs only 1.0

When load includes tube light and other

inductive loads0.8

Power Output in kWGenerator KVA = -----------------------------

A x B x C x D


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Transformers They are the devices which are used in the power

system to change the voltage level of electrical supply.

The generation voltage is stepped up to higher level forthe purpose of transmission. Similarly the transmission

voltage level is stepped down to lower level for

distribution purpose. The generation voltages generally used are 400V, 6.3kV,

6.6kV and 11kV.

Standard transmission voltages are: 33kV, 66kV, 132kV

and 220kV. Primary distribution voltage levels: 33kV, 11kV

Secondary distribution voltage level: 400V


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Powerhouse and Switchyard equipment

Circuit Breakers: They are the devices which are used to make or break circuitsmanually, automatically and by remote control in case of any

faulty condition and also in normal condition.

Disconnectors

Disconnectors are the devices which are used to isolate acertain segment of a circuit, mainly for the purpose of

maintenance.

Instrument transformers

Current transformers (CT) step down the value of current to alevel at which control, protection and metering equipment can beused. (generally 5A or 1A)

Potential transformers (PT) step down the value of voltage tolevel at which control, protection and metering equipment can be

used. (generally 110V)


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Powerhouse and switchyard equipment Lightening arrestors (Surge arrestors)

They are the devices which protectthe insulation on the system from

lightening.

They divert the surges directly to

ground, protecting othercomponents of the power system.

Protective relays

They are the devices which sense

the faulty or abnormal conditionson a system and actuate the

operation of circuit breakers.


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Powerhouse and switchyard equipment

Communication system:

SCADA (Supervisory Control And Data Acquisition) isused for the control and supervision of the Integrated

Power System (IPS)

With the help of SCADA, data from all the powerstations and substations of the whole system is

collected at the Load Dispatch System (LDC).

Optical Fiber system or Power Line CarrierCommunication system is used as the communication

medium.


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Transmission System

Transmission of power from generating stationsto distribution stations through long distances can

incorporate huge amount of losses.

So, power is transmitted at high voltage level inorder to minimize the loss.


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Transmission System

We know, P = V * I

where, P is power,

V is voltage and

I is current

So for constant power, when V increases, Idecreases.

The loss in transmission line is given by,

Pl = I2* R (single phase)

Pl= 3*I2* R (three phase)

where, Pl is power lost in line,

and R is the line resistance.

Hence the line loss decreases with the increase in voltage level.


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Transmission line components Conductors

Aluminum Core Steel Reinforced (ACSR) conductors are generally used.

Based on their cross section diameters, code names representing differentanimals and birds are given to ACSR conductors.

Support (Tower) Up to 33kV lines, steel tubular, wooden or RCC poles are used, where as for

higher voltage transmission lines, rigid, self supporting lattice towers (pylons)

are used.

Insulators and fittings Insulators are required to support the line conductor and provide clearance from

ground and structure.

Toughened glass insulators are generally used in high voltage transmission lineswhere as porcelain type are mostly used up to 33kV.

Earth wire They are strung above phase conductors, in order to provide shielding or

protection from lightening strokes.

They are connected to ground at each tower.


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Transmission line design

Standard Voltage 66, 132, 220, 400 KV (11kV and 33kV areused for transmission by small hydropower projects for short

lines)

Selection Criterion of Economic Voltage

Power to be evacuated

Length of line

Voltage regulation

Power loss in Transmission

Initial and operating cost

Present and future voltage in neighborhood


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Voltage level selection

The most economical voltage level selected among standardvoltage levels

Use of empirical formula (Stills formula) for estimation ofmost economical voltage level

V=5.5 * (0.6*L + P/100)

Where,

V is Voltage (kV)

L is Length (km)

P is Power to be transmitted (kW)

The standard voltage level closest to the result of above

calculation is selected.

Note: Still's formula is one of the empirical formulas used for selection of the most economical voltage level


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The size of conductor should be such that it can carry the rated

current continuously without excessive rise in temperature. The line loss should be kept to a minimum (generally less than

5%)

The voltage regulation should be within specified limits(generally within 10%)

The larger the conductor diameter, the lesser the resistance andthe line loss and higher is the current carrying capacity.

So, larger conductors are used for evacuation of large amountof power.

Conductor selection


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Properties of some ACSR conductors

Code

Name Eq. Cu Area Eq. Al Resistance Inductive reactance

Currentcarrying

capacity

for a

temperatu

re .rise of

30 C (A)

Current Diameter

Sq mm Sqare inch Area at 20C Ohm/km in still air

Square inch ohm/km Double Ckt SingleCkt (A) (cm)

Ferret 25 0.04 41.87 0.6795 0.19988 0.4134512 170 115 0.9

Rabbit 30 0.05 52.21 0.5449 0.19641 0.4065152 192 135 1.005

Otter 50 0.08 82.85 0.3434 0.18931 0.3923023 250 185 1.266

Dog 65 0.1 103.6 0.275 0.18555 0.3847883 280 205 1.414

Wolf 95 0.15 154.3 0.1844 0.1918434 0.4047098 400 305 1.813

Lynx 110 0.175 179 0.1589 0.17542 0.3645309 440 335 1.953

Panther 130 0.2 207 0.1375 0.17325 0.3601944 470 370 2.1

Lion 140 0.215 232.5 0.1223 0.17137 0.3564191 525 405 2.26

Bear 160 0.25 258.1 0.1102 0.17053 0.3547503 570 430 2.345


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Name Current rating in still air(Amp)

Resistance

(/km)Squirrel 76 1.374

Gopher 85 1.098

Weasel 95 0.9116

Rabbit 135 0.5449

Dog 205 0.2745

Aluminum 4mm2 23 7.15

Aluminum 6mm

2

30 4.76Aluminum 10mm2 40 2.86




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Performance of a Transmission line

Efficiency: It is the ratio of output power to input power.

Mathematically, = (Pr/Ps) * 100%

=(Pr/Pr+loss) * 100% and,

Loss = 3 * I2* R (for three phase)

Where, is efficiency

Pr is receiving end (output) power

Ps is sending end (input) powerR is line resistance


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Sizing of transmission line

(3 phase)

Decide on transmission line voltage

Then find resistance of ACSR based online length and size/type of cable

Or

Power loss is

VICosP 3

VCos

PI

3

RIPloss

23


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Transmission line design exampleLet us consider, a 40MW plant has to evacuate its power through 30km

at 132kV at a power factor of 0.8 lagging. Generally allowable line lossshould be limited to 5%

First we try using Rabbit conductor and get the following result:

Line loss= 5.8% > 5%

Since the result is unsatisfactory, we choose a higher sized

conductor.Using Wolf conductor, we get the following result:

Line loss= 2.%

Since the performance is good, Wolf is satisfactory. But is this tooexpensive?? Try lower size conductor.


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Distribution System

It is the part of the power system which isresponsible for distributing electricity among the

utilities.

It mainly consists of distribution substations,transformers (step down), distribution lines

consisting of conductors, cables, poles, protection

system etc.

Distribution is done primarily at 33kV andsecondarily at 11kV and then at 400V (220V,

230V or 110V)


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Substation Part of electrical power system responsible for switching,

voltage transformation and regulation . Components of a substation:

Transformers

Bus bars

Circuit breakers Disconnectors

Lightening arrestors

Instrument transformers etc.

Busbars are flat strips or hollow tubes of copper oraluminum.

They carry large currents to multiple devices.


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Distribution System

3 phase 4 wire system is generally used in a distributionsystem.

The three wires are the phase wires and the fourth one is aneutral wire.

Among the four wires, a domestic electrical supply isprovided by tapping from one phase wire and the neutral

wire.


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Distribution System

The domestic electric supply is done at 400V line- to-line (voltage between phase wires) which is equivalent to

230V line- to- neutral (voltage between a phase wire and

the neutral wire), according to the relation,

Vll=3*VlnWhere, Vll is the line to line voltage and

Vln is the line to neutral voltage

Large (bulk) consumers like industries, hospitals etcrequire three phase supply at 11kV and even 33kV.


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Distribution line design exampleLets consider, 100kW of power is to be delivered to a load center at a

distance of 500m, using Dog conductor, Assume power factor = 0.8First try transmitting the power at 400V and we get the followingresult:

Line loss= 13.3%

Since the result is very poor, higher voltage level will be required.

At 11kV, we get the following result:Line loss= 0.02%An excellent result is achieved. But still the cost can be reduced by using smaller

sized conductor. So we try using smaller sized conductor

Using Ferret conductor, we get the following result:Line loss= 0.04%

Since the performance is good even with Ferret (smaller) conductor,we design the line using Ferret conductor at 11kV.

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07 Electricity