Basic Structure of a Power System

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BASIC STRUCTURE OF A POWER SYSTEM

Biruk EyasuBiruk EyasuBiruk EyasuBiruk Eyasu

December, 2013

Basic structure of a power system

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Introduction Electric power systems are real-time energy delivery systems. Real time means that power is

generated, transported, and supplied the moment you turn on the light switch. Electric power

systems are not storage systems like water systems and gas systems. Instead, generators

produce the energy as the demand calls for it.

Figure 1 shows the basic building blocks of an electric power system. The system starts with

generation, by which electrical energy is produced in the power plant and then transformed in

the power station to high-voltage electrical energy that is more suitable for efficient long-

distance transportation. The power plants transform other sources of energy in the process of

producing electrical energy. For example, heat, mechanical, hydraulic, chemical, solar, wind,

geothermal, nuclear, and other energy sources are used in the production of electrical energy.

High-voltage (HV) power lines in the transmission portion of the electric power system

efficiently transport electrical energy over long distances to the consumption locations. Finally,

substations transform this HV electrical energy into lower-voltage energy that is transmitted

over distribution power lines that are more suitable for the distribution of electrical energy to

its destination, where it is again transformed for residential, commercial, and industrial

consumption.

A full-scale actual interconnected electric power system is much more complex than that is

shown in Figure 1; however the basic principles, concepts, theories, and terminologies are all

the same.

Figure 1: Electric power system structure


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Balancing the grid

One of the main difficulties in power systems is that the amount of active power consumed plus

losses should always equal the active power produced. If more power would be produced than

consumed the frequency would rise and vice versa. Even small deviations from the nominal

frequency value would damage synchronous machines and other appliances. Making sure the

frequency is constant is usually the task of a transmission system operator.

Generation All power systems have one or more sources of power. For some power systems, the source of

power is external to the system but for others it is part of the system itself. Direct current

power can be supplied by batteries, fuel cells or photovoltaic cells. Alternating current power is

typically supplied by a rotor that spins in a magnetic field in a device known as a turbo

generator. There have been a wide range of techniques used to spin a turbine's rotor, from

steam heated using fossil fuel (including coal, gas and oil) or nuclear energy, falling water

(hydroelectric power) and wind (wind power). The speed at which the rotor spins in

combination with the number of generator poles determines the frequency of the alternating

current produced by the generator. All generators on a single synchronous system, for example

the national grid, rotate at sub-multiples of the same speed and so generate electrical current

at the same frequency. If the load on the system increases, the generators will require more

torque to spin at that speed and, in a typical power station, more steam must be supplied to

the turbines driving them. Thus the steam used and the fuel expended are directly dependent

on the quantity of electrical energy supplied.

Figure 2: Hydro power generation station


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Electricity grid systems connect multiple generators and loads operating at the same frequency

and number of phases, the commonest being three-phase at 50 or 60 Hz. However there are

other considerations. This range from the obvious: How much power should the generator be

able to supply? What is an acceptable length of time for starting the generator (some

generators can take hours to start)? Is the availability of the power source acceptable (some

renewable are only available when the sun is shining or the wind is blowing)? To the more

technical: How should the generator start (some turbines act like a motor to bring themselves

up to speed in which case they need an appropriate starting circuit)? What is the mechanical

speed of operation for the turbine and consequently what is the number of poles required?

What type of generator is suitable (synchronous or asynchronous) and what type of rotor

(squirrel-cage rotor, wound rotor, salient pole rotor or cylindrical rotor)?

As I have stated above, power is generated from different kinds of sources which are renewable

and non- renewable. Next, we will see some of the most frequently used ones.

Hydro power Stations

In a hydro power station, water head is used to drive water turbine coupled to the generator.

Water head may be available in hilly region naturally in the form of water reservoir (lakes etc.)

at the hill tops. The potential energy of water can be used to drive the turbo generator set

installed at the base of the hills through piping called pen stock. Water head may also be

created artificially by constructing dams on a suitable river. In contrast to a thermal plant, hydro

power plants are eco-friendly, neat and clean as no fuel is to be burnt to produce electricity.

While running cost of such plants is low, the initial installation cost is rather high compared to a

thermal plant due to massive civil construction necessary. Also sites to be selected for such

plants depend upon natural availability of water reservoirs at hill tops or availability of suitable

rivers for constructing dams. Water turbines generally operate at low rpm, so number of poles

of the alternator is high. For example a 20-pole alternator the rpm of the turbine is only 300

rpm.

Solar radiation

With the exclusion of nuclear and geothermal energy, most energy resources on and in this

world originate from the sun. The sun can be seen as a huge nuclear reactor with a radiating

power of 300 million exawatt. When this is fully written out, it is

300,000,000,000,000,000,000,000,000 watt. The surface of the earth, being just a very small

part in the total radiation sphere of the sun, receives only about a half of a billionth of this

energy, equaling some 160 PW. In the year 2008, the total primary energy supply to the world’s

economies was 12,267 megaton oil equivalent, equaling 514 exajoule. Averaging this amount of

energy over a full year means a continuous energy flow of 16.3 terawatt (TW). This is only


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0.01% of the 160 PW of energy that the surface of the earth receives from the sun. The fact is

that capturing this amount of energy is not easy. Geologists have calculated that it took the

earth some million years to build up the fossil fuel resources that are used now in one year. The

current use of fossil fuels resembles a rapid discharge of the earth as a battery, while charging it

took an aeon.

Vegetation has a low efficiency of capturing solar energy. Forests and wheat have a capturing

efficiency of only 0.25%, while straw is relatively much better with about 2%. Everybody will

agree that even 2% is still a very low number since one cannot cover the world with straw

producing plants. As an illustration, if all grain produced in the world would be converted into

liquid bio fuel; it would cover only 10% of all current petrol use. In this respect, photo voltaic

cells with their solar-energy capturing efficiency of 10 to 15% perform much better than most

plants. However, plants will reproduce, whereas solar cells must be replaced regularly. A major

problem in producing electricity from direct solar radiation is the unpredictability and variability

of sunshine. Countries in the higher latitudes, such as Canada and Finland, lack the necessary

sunshine at those times of the day and year when electricity is needed most. Fortunately, in

areas such as the Middle East and Mexico, the peak in solar radiation practically coincides with

the maximum need for cooling buildings.

Figure 3: Solar Panel


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Wind power

Wind, a renewable energy source indirectly caused by solar radiation, also has problems of

variability. At an optimum location, generally offshore, a wind-mill-driven generator will only

run at its nominal (= name plate) power during 30% of the time, while at most land-based

locations that will take place may be 20% of the time. Because wind speed varies in time, the

output of a wind park can have a distribution during the year. A capacity factor of 25% to 35% is

the best that can be expected. During a large part of the year, individual wind turbines have no

output at all. Therefore, wind parks always need backup power.

Figure 4: Wind farm

Although 24-hour wind level forecasts are quite accurate, wind speeds can change quickly and

unpredictably so that the back-up capacity should have the ability to react very fast. Difficult

situations arise especially when a wind park is operating at its nominal output and the wind

speed suddenly increases to values where the wind turbines have to be shut down to prevent

damage. Such events require a substantial amount of rapid back-up generating capacity.


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Transmission system The huge amount of power generated in a power station (hundreds of MW) is to be

transported over a long distance (hundreds of kilometers) to load centers to cater power to

consumers with the help of transmission line and transmission towers.

Figure 5: Transmission tower

To give an idea, let us consider a generating station producing 120 MW power and we want to

transmit it over a large distance. Let the voltage generated (line to line) at the alternator be 10

kV. Then to transmit 120 MW of power at 10 kV, current in the transmission line can be easily

calculated by using power formula circuit (which you will learn in the lesson on A.C circuit

analysis) for 3-phases follows:

Instead of choosing 10 kV transmission voltage, if transmission voltage were chosen to be 400

kV, current value in the line would have been only 261.5 A. So sectional area of the

transmission line (copper conductor) will now be much smaller compared to 10 kV transmission

voltages. In other words the cost of conductor will be greatly reduced if power is transmitted at

higher and higher transmission voltage. The use of higher voltage (hence lower current in the

line) reduces voltage drop in the line resistance and reactance. Also transmission loss is


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reduced. Standard transmission voltages used are 132 kV or 220 kV or 400 kV or 765 kV

depending upon how long the transmission lines are.

Therefore, after the generator we must have a step up transformer to change the generated

voltage (say 10 kV) to desired transmission voltage (say 400 kV) before transmitting it over a

long distance with the help of transmission lines supported at regular intervals by transmission

towers. It should be noted that while magnitude of current decides the cost of copper, level of

voltage decides the cost of insulators. The idea is, in a spree to reduce the cost of copper one

cannot indefinitely increase the level of transmission voltage as cost of insulators will offset the

reduction copper cost. At the load centers voltage level should be brought down at suitable

values for supplying different types of consumers. Consumers may be (1) big industries, such as

steel plants, (2) medium and small industries and (3) offices and domestic consumers.

Electricity is purchased by different consumers at different voltage level. For example big

industries may purchase power at 132 kV, medium and big industries purchase power at 33 kV

or 11 kV and domestic consumers at rather low voltage of 230V, single phase. Thus we see that

400 kV transmission voltages are to be brought down to different voltage levels before finally

delivering power to different consumers.

Substations

Substations are the places where the level of voltage undergoes change with the help of

transformers. Apart from transformers a substation will house switches (called circuit breakers),

meters, relays for protection and other control equipment. Broadly speaking, a big substation

will receive power through incoming lines at some voltage (say 400 kV) changes level of voltage

(say to 132 kV) using a transformer and then directs it out wards through outgoing lines. At the

lowest voltage level of 400 V, generally 3-phase, 4-wire system is adopted for domestic

connections. The fourth wire is called the neutral wire (N) which is taken out from the common

point of the star connected secondary of the 6 kV/400 V distribution transformer.

Figure 6: Single line representation of power system


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Distribution system

Figure 7: Power distribution scheme

The loads of a big city are primarily residential complexes, offices, schools, hotels, street

lighting etc. These types of consumers are called LT (low tension) consumers. Apart from this

there may be medium and small scale industries located in the outskirts of the city. LT

consumers are to be supplied with single phase, 220 V, 40 Hz. Power receive at a 33 kV

substation is first stepped down to 6 kV and with the help of underground cables (called feeder

lines), power flow is directed to different directions of the city. At the last level, step down

transformers are used to step down the voltage form 6 kV to 400 V. These transformers are

called distribution transformers with 400 V, star connected secondary. You must have noticed

such transformers mounted on poles in cities beside the roads. These are called pole mounted

substations. From the secondary of these transformers 4 terminals (R, Y, B and N) come out. N

is called the neutral and taken out from the common point of star connected secondary.

Voltage between any two phases (i.e., R-Y, Y-B and B-R) is 400 V and between any phase and

neutral is 230 V (=400/ ). Residential buildings are supplied with single phase 230V, 50Hz. So

individual are to be supplied with any one of the phases and neutral. Supply authority tries to

see that the loads remain evenly balanced among the phases as far as possible. Which means

roughly one third of the consumers will be supplied from R-N, next one third from Y-N and the

remaining one third from B-N. The distribution of power from the pole mounted substation can

be done either by (1) overhead lines (bare conductors) or by (2) underground cables. Use of

overhead lines although cheap, is often accident prone and also theft of power by hooking from

the lines takes place. Although costly, in big cities and thickly populated areas underground

cables for distribution of power, are used.

Basic Structure of a Power System

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