P FE 4 Generation Transmission and Distribution of Electrical Power

8/13/2019 P FE 4 Generation Transmission and Distribution of Electrical Power

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Fundamentals of

Electrical Engineering

4. Generation, Transmissionand Distribution of Electrical

Power

Dr Houssem BOUCHEKARA

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Power

Generation

Power

Transimission

Power

Distribution

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Prior to the discovery of Faraday’s Laws of electromagnetic discussion, electrical

power was available from batteries with limited voltage and current levels.

Although complicated in construction, D.C generators were developed first to

generate power in bulk. However, due to limitation of the D.C machine to generate voltage

beyond few hundred volts, it was not economical to transmit large amount of power over a

long distance.

For a given amount of power, the current magnitude ( = /), hence section o

the copper conductor will be large.

Thus generation, transmission and distribution of D.C power were restricted to area

of few kilometer radius with no interconnections between generating plants.

Therefore, area specific generating stations along with its distribution networks had

to be used.

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In later half of eighties, in nineteenth century, it was proposed to have a power

system with 3-phase, 50 Hz A.C generation, transmission and distribution networks.

Once A.C system was adopted, transmission of large power (MW) at higher

transmission voltage become a reality by using transformers. Level of voltage could be

changed virtually to any other desired level with transformers –

which was hitherto

impossible with D.C system.

Nicola Tesla suggested that constructionally simpler electrical motors (induction

motors, without the complexity of commutator segments of D.C motors) operating from 3-

phase a.c supply could be manufactured.

In fact, his arguments in favor of A.C supply system own the debate on switching

over from D.C to A.C system.

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A modern power station has more than one generator and these generators are

connected in parallel. Also there exist a large number of power stations spread over a region

or a country.

A regional power grid is created by interconnecting these stations through

transmission lines. In other words, all the generators of different power stations, in a grid

are in effect connected in parallel.

One of the advantages of interconnection is obvious; suppose due to technical

problem the generation of a plant becomes nil or less then, a portion of the demand of

power in that area still can be made from the other power stations connected to the grid.

One can thus avoid complete shutdown of power in an area in case of technical

problem in a particular station.

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Hydroelectric power generation involves the storage of a hydraulic fluid, water,

conversion of the hydraulic (potential) energy of the fluid into mechanical (kinetic) energy in a

hydraulic turbine, and conversion of the mechanical energy to electrical energy in an electricgenerator.

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Thermal generating plants are designed and constructed to convert energy from fuel

(coal, oil, gas, or radiation) into electric power. The actual conversion is accomplished by a

turbine-driven generator.

A turbine is used to rotate the generator.

Turbine may be of two types, namely steam turbine and water turbine. In a thermal

power station coal is burnt to produce steam which in turn, drives the steam turbine hence the

generator (turbo set).

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In figure the elementary features of a thermal power plant is shown. In a thermal powe

plant coil is burnt to produce high temperature and high pressure steam in a boiler. The steam is

passed through a steam turbine to produce rotational motion. The generator, mechanically

coupled to the turbine, thus rotates producing electricity.

Chemical energy stored in coal after a couple of transformations produces electricalenergy at the generator terminals as depicted in the figure. Thus proximity of a generating

station nearer to a coal reserve and water sources will be most economical as the cost o

transporting coal gets reduced. However, these plants pollute the atmosphere because o

burning of coals.

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As coal reserve is not unlimited, there is natural threat to thermal power plants based

on coal. It is estimated that within next 30 to 40 years, coal reserve will exhaust if it is consumed

at the present rate. Nuclear power plants are thought to be the solution for bulk power

generation.

The present day atomic power plants work on the principle of nuclear fission of

235

U.

When235

U is bombarded by neutrons a lot of heat energy along with additional neutrons are

produced. These new neutrons further bombard235

U producing more heat and more neutrons.

Thus a chain reaction sets up. However this reaction is allowed to take place in a controlled

manner inside a closed chamber called nuclear reactor.

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The wind is a free, clean, and inexhaustible energy source. It has served humankind

well for many centuries by propelling ships and driving wind turbines to grind grain and

pump water. Wind power is the conversion of wind energy into a useful form of energy, such

as electricity, using wind turbines.

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Photovoltaics are best known as a method for generating electric power by using sola

cells to convert energy from the sun into electricity. The photovoltaic effect refers to photons o

light knocking electrons into a higher state of energy to create electricity.

Solar cells produce direct current electricity from light, which can be used to powe

equipment or to recharge a battery.

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The purpose of the electric transmission system is the interconnection of the electric

energy producing power plants or generating stations with the loads. A three-phase A

system is used for most transmission lines. The operating frequency is 60 Hz in the U.S. and

50 Hz in Europe, Australia, and part of Asia.

The three-phase system has three phase conductors. The system voltage is defined

as the rms voltage between the conductors, also called line-to-line voltage. The voltage

between the phase conductor and ground, called line-to-ground voltage, is equal to the line-

to-line voltage divided by the square root of three.

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Highvoltage and extra-high-voltage (EHV) transmission

lines interconnect power plants and loads, and form an

electric network.

The physical arrangement of most extra-high-voltage

(EHV) lines is similar. Figure shows the major components of

an EHV, which are:

1. Tower: The figure shows a lattice, steel tower.

2. Insulator: V strings hold four bundled conductors in each

phase.

3. Conductor: Each conductor is stranded, steel reinforced

aluminum cable.

4. Foundation and grounding: Steel-reinforced concrete

foundation and grounding electrodes placed in the

ground.

5. Shield conductors: Two grounded shield conductors

protect the phase conductors from lightning.

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High-voltage DC lines are used to transmit large amounts of energy over long

distances or through waterways. One of the best known is the Pacific HVDC Intertie, which

interconnects southern California with Oregon. Another DC system is the +400 kV Coal

Creek-Dickenson lines. Another famous HVDC system is the interconnection between

England and France, which uses underwater cables. In Canada, Vancouver Island is supplied

through a DC cable.

In an HVDC system the AC voltage is rectified and a DC line transmits the energy. At

the end of the line an inverter converts the DC voltage to AC. A typical example is the Pacific

HVDC Intertie that operates with +500 kV voltages and interconnects Southern Californiawith the hydro stations in Oregon.

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A short-circuit fault takes place when two or more conductors come in contact with

each other when normally they operate with a potential difference between them. Short-circuit

faults in three-phase systems are classified as:

1. Balanced or symmetrical three-phase faults.

2. Single line-to-ground faults.

3. Line-to-line faults.

4. Double line-to-ground faults.

Generator failure is caused by insulation breakdown between turns in the same slot orbetween the winding and the steel structure of the machine. The same can take place in

transformers.

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Power System Protection – Main Functions

1. To safeguard the entire system to maintain continuity of supply.

2. To minimize damage and repair costs.

3. To ensure safety of personnel.

Power System Protection – Basic Requirements

1. Selectivity : To detect and isolate the faulty item only.

2. Stability : To leave all healthy circuits intact to ensure continuity of supply.

3. Speed : To operate as fast as possible when called upon, to minimize damage,production downtime and ensure safety to personnel.

4. Sensitivity : To detect even the smallest fault, current or system abnormalities

and operate correctly at its setting.

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Power System Protection – Speed is Vital!!

The protective system should act fast to isolate faulty sections to prevent:

Increased damage at fault location. Fault energy = I2Rf t, where t is time in

seconds.

Danger to the operating personnel (flashes due to high fault energy sustainingfor a long time).

Danger of igniting combustible gas in hazardous areas, such as methane in coal

mines which could cause horrendous disaster.

Increased probability of earth faults spreading to healthy phases.

Higher mechanical and thermal stressing of all items of plant carrying the fault

current, particularly transformers whose windings suffer progressive andcumulative deterioration because of the enormous electromechanical forces

caused by multi-phase faults proportional to the square of the fault current.

Sustained voltage dips resulting in motor (and generator) instability leading to extensive

shutdown at the plant concerned and possibly other nearby plants connected to the system.

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Power System Protection – Qualities

1.

Dependability : It MUST trip when called upon.2. Security : It must NOT trip when not supposed to.

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Power System Protection – Basic Components

1. Voltage transformers and current transformers: To monitor and give accurate

feedback about the healthiness of a system.

2. Relays: To convert the signals from the monitoring devices, and giveinstructions to open a circuit under faulty conditions or to give alarms when

the equipment is protected, is approaching towards possible destruction.

3. Fuses: Self-destructing to save the downstream equipment being protected.

4. Circuit breakers: These are used to make circuits carrying enormous currents,

and also to break the circuit carrying the fault currents for a few cycles based

on feedback from the relays.5. DC batteries: These give uninterrupted power source to the relays and

breakers that is independent of the main power source being protected.

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Figure 3. 1: Availability is a subset of reliability and reliability is a subset of power quality. Power quality deals

with any deviation from a perfect sinusoidal voltage source. Reliability deals with interruptions. Availability

deals with the probability of being in an interrupted state.

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Figure: Common power quality problems. Perfect power quality corresponds to an undistorted sinusoidal voltage

with constant amplitude and constant frequency.

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Interruptions — Interruptions are the loss of service voltage to a customer and can

be momentary or sustained in nature.

Sags — Voltage sags are temporary RMS reductions in voltage typically lasting from a

half cycle to several seconds. Sags result from high currents, typically due to faults or

starting motors, interactingwith system impedances.

Swells — Voltage swells are temporary RMS increases in voltage typically lastin

from a half cycle to several seconds. Swells are commonly caused by the de-energizing o

large loads or asymmetrical faults (a line to ground fault will cause a voltage rise in the othe

two phases). Swells can cause insulation breakdown in sensitive electronic equipment i

voltage increases are high enough for a long enough period of time. Equipment tolerance to

swells, like sags, is described by voltage tolerance envelopes like the ITIC Curve.

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Transients — Voltage transients are sudden nonrecurring changes in voltage

magnitude. An impulse transient, most commonly caused by lightning, is described by the

time to reach its peak value and the time to decay to half of its peak value.

Noise — Noise can be broadly defined as unwanted voltage signals with broadband

spectral content. Common causes include power electronics, control circuits, arcinequipment, loads with solid state rectifiers, and switched mode power supplies. Noise

problems are often exacerbated by improper grounding.

Flicker — Voltage flicker refers to low frequency variations in RMS voltage that cause

visible changes in the brightness of incandescent lighting. These voltage variations are

caused by the cycling of large loads such as refrigerators, air conditioners, elevators, ar

furnaces, and spot welders.

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In this lesson, a brief idea of generation, transmission and distribution of electrical

power is given - which for obvious reason is neither very elaborative nor exhaustive.

Nonetheless, it gives a reasonable understanding of the system for a beginner going to

undertake the course on electrical technology. If you ever get a chance to visit a substation

or power station – don’t miss it.

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P FE 4 Generation Transmission and Distribution of Electrical Power

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Transcript of P FE 4 Generation Transmission and Distribution of Electrical Power