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Understanding Electricity

In South Africa, our most abundant source of energy is coal. Most of our coal is low quality with a low heat value and a high ash content. The majority of our coal deposits which are suitable for cheap power generation are found in eastern and south-eastern Gauteng and in the northern Free State. In Gauteng it is generally found at shallow depths and in thick seams, whereas in KwaZulu-Natal, the seams are deeper and thinner, but of a higher quality.

Eskom relies on coal fired power stations to produce approximately 90% of its electricity. Eskom uses over 90 million tons of coal per annum. Coal mining in South Africa is relatively cheap compared to the rest of the world. These low costs have had an important effect on the nation's prosperity and potential for development. In Europe, by contrast, costs are almost four times higher.

How electricity is made

In the series, Energy, we have discussed the various "ingredients" for electricity, the basics of electricity as a form of energy. But how is it in actual fact generated?

Michael Faraday discovered in 1831 that magnets and moving wire had strange effects on each other when they moved close together. In fact, Faraday found that the mechanical energy used to move a magnet inside a coil of wire could be changed into electrical energy which flowed ill the wire. It is this simple discovery which has led to modern power stations.

In large power stations, huge magnets are turned inside vast coils of insulated metal wire. It is here that the primary sources of energy are used.

There are a number of ways of utilising primary sources of energy to "drive" a generator. In South Africa we use mainly thermal energy to generate the electricity we need.

A thermal power station

Coal, oil, gas and nuclear fuels can be used to heat water and convert it into steam at high temperatures and pressures. This is done in boilers or reactors. The very hot steam, at temperatures of between 500 C and 535 C, is released and turns a large turbine, connected to the rotating magnet and electricity is generated. In this way the energy in the fuel has been converted into electricity. A Power Station can therefore be defined as a converter of energy.

There are of course many other methods by which electricity can be generated, for example, by harnessing nature.

Wind power

The force of wind, used for centuries for pumping water and grinding corn, is the most promising . renewable energy source for making electricity.
 
The traditional Dutch windmill has been redesigned to be a most advanced aerodynamic machine with blades designed to capture the wind as efficiently as possible. These windmills are connected to generators where electricity is being generated. 
 

Hydro-power generating plant

In mountainous countries, hydro electricity is an important source. However, in South Africa its most important role is the storage of "electricity" to meet unexpected demands or a sudden breakdown at a baseload power station. These hydro-generating plants are also referred to as peaking power stations.

We have two systems in operation in South Africa. These are the conventional hydro and the pumped storage systems. In the conventional system, water is stored behind a dam wall. The water can be released to drive huge turbines, which are connected to generators for power generation. The power station is normally situated close to the dam wall.

The other system utilizes pumped storage plant. This is the only practical way at present of storing "electricity" on a large scale. The idea is simply to use surplus electricity -for example, at night or week-ends when we are using less electricity (off-peak periods) - to pump water to a mountain top reservoir. This water can also be used as a supplement for other water schemes.

 In an event of a shortage of supply of electricity from other power generating stations, the top reservoir can be emptied very quickly back down through the turbine to regenerate electricity. In other words, the motor, which was driving a pump, becomes a generator driven by a turbine.

Power generation from these plants is limited as they rely on the water level of the dams or rivers which in turn is affected by the rainfall in its catchment area.

 Geothermal power

The Earth is a virtually inexhaustible reservoir of natural heat, which reaches the surface in some places as springs, geysers and volcanoes in certain countries. Hot springs are formed by underground water coming up through deep faults in the earth. In few places underground springs are hot enough to produce steam at or near the surface of the Earth and this may be worth tapping for electricity generation, as happens in Iceland, Italy, New Zealand and Kenya.

 Solar power

Solar energy is captured, concentrated and stored by green plants to create fuels, but there are possibilities for harnessing it directly to make electricity.

The success of solar cells, which convert sunlight directly into electricity, has encouraged the idea of solar energy as a clean and free source of electricity. Solar cells provide the electricity needs for most satellites in orbit around the Earth.

On the Earth's surface, our power requirements are more substantial and the atmosphere reduces the intensity of the sun. Very large areas of solar panels are required to produce useful amounts of electricity. The investment cost, although falling, is still very high. But solar cells are finding many applications in sunny countries in powering warning beacons, microwave Repeaters, water pumping and weather stations etc. A limited domestic use is also possible. Eskom and other energy providers work together in supplying an energy source to its customers, i.e. gas for cooking and electricity through solar systems for lights, radio and TV.

Tidal power

The pull of gravity from the sun and moon raises and lowers the sea around our coast twice a day and give tides of up to 8 metres - a huge resource of natural energy if it can be harnessed. Electricity can be generated through a barrage, a special type of dam built across an estuary, which admits the raising tide to build up a head of water then releases the water through turbines in the barrage. These turbines are also linked to a generator.

Wave power

 Ocean waves, generated by a combination of wind effects and rotation of the Earth, represent an enormous reservoir of natural energy.

To convert the heave, up-and down motion of waves, into the smooth rotation of a generator, requires considerable ingenuity. An air turbine with two sets of blades, to rotate a shaft in the same direction whichever way the air is flowing, has been developed. This is used in oscillating column wave power devices in which the wave motion, up and down, forces air in and out of a large steel or concrete chamber.

Generators 

A power station generator, the equivalent of Faraday's bar magnet, is a powerful electromagnet a coil energised by direct current to produce a magnetic field. This is mounted on the central rotating shaft, and is called the rotor. Around the rotor is a series of coils called the stator in which the electrical voltage is generated by the rotating magnetic field. Both rotor and stator may weigh several hundred tons.

 The rotor, connected to a turbine, turns at 3000 revolutions per minute - 50 cycles per second -to produce alternating current with a frequency of 50 hertz (cycles per second). Modern generators (in thermal power plants) typically produce 500- 600 megawatts of power - enough to light 5 -6 million lOO-watt bulbs. Other power generating plants as mentioned, can produce between I KW -250 MW of electrical energy, e.g. wind, tidal, wave etc.

 How electricity gets to your home When you next switch on the electric light or the television, stop to think for a moment of all the work which has been done to generate (make) electricity and to get it to your home.

 Power stations all over South Africa are linked by transmission lines and towers called pylons. Transmission is a word from the verb "to transmit" which means to send from one place to another. Transmission lines send the electricity through thick aluminium and copper wires. The network of transmission lines is called the National Grid.

 In order for the electricity to be transmitted safely and efficiently, it must be at a high voltage (pressure) and a low current. This is because if the current is too high, the cable would heat up too much and even melt and if the voltage were too low, hardly any energy would be carried. Remember that we need the pressure volts to enable us to transmit electricity over vast distances. The generators in the power stations produce electricity at 20000 volts. This voltage is raised or transformed before it is sent out at 132000, 275000, 400000 or even 765000 volts onto the transmission grid. These very high voltages are necessary to push the required flow of electricity through the wires and keep costs down.

 The electricity is transformed down to 11 000 volts for local distribution and then further reduced according to the need - for example, 240 (220) volts for domestic use. The electricity entering your home at 240 volts has had an eventful journey. From the initial high voltage transmission grid to a lower voltage distribution network. Travelling over ground and (probably) underground for great many kilometers, it has been transformed many times on the way.

 You've probably seen some of the equipment, which performs these operations in your local area. They are known as substations, which can be found in many sizes - small transformers mounted on wooden poles, larger transformers sitting behind high fences and huge arrays of strangely shaped devices on sites occupying several hectares.

 (Refer: How Electricity is Generated)

 Transformers

A transformer is basically a very simple device. The alternating current is led through primary coil of wire, which produces an alternating magnetic field in the ring-shaped core of soft iron. This in turn creates a voltage in a secondary coil, from which the output current can be drawn. If the secondary coil has more turns than the primary, the output voltage is higher than the input voltage. This is a step-up transformer.   A step-down transformer has more turns in the primary coil than in the secondary coil to reduce the voltage.

 (Refer:How Electricity is Transmitted)

(Refer: How electricity is Distributed)

 Supply and demand

 Electricity must be generated as needed batteries are not capable of storing the enormous quantities.

 There is no realistic way to store large quantities of electricity required for distribution to the user. So, the amount being fed into the grid must always match what the customers are taking out. This varies not just from day to day, but from minute to minute.

 As the demand increases, more stations must be brought into play. This is planned in advance, because for many types of power station the starting-up and shutting-down operations are slow and complicated. Economics are important too, because some stations generate (supply) electricity more cheaply than others do.

 But the general consideration is always that the electricity supply should be consistent and reliable - a quality product. Much of the electricity and electronic equipment we use depends on voltage and frequency remaining accurate and constant.

The pattern of the daily demand can be predicted very accurately, unless something unexpected happens such as a sudden deterioration in the weather.

 The main peaks usually occur at about 06:00 in the mornings and lasts until about midday. A second peak period is normally from about 17:00 until 21:00. The increase in the demand in the morning is due to many main industries such as mining, iron and steel smelters, railways etc. which comes into production.

 In South Africa, the national control centre at Simmerpan, Germiston, controls the power transmission network throughout the country. They are aware of what the basic demand is for South Africa and its neighbouring states, to which we also supply some electricity.

Each power station on the Eskom grid will keep national control informed of their capabilities ensuring that power is available when the customers want it. Thus, national control will at all times ensure that we meet the demands of the customer by ensuring a continuity of supply.

Electricity supplied by all different generating stations must enter the grid at precisely the correct voltage and frequency. Before being switched into the grid, a generator is run up to the correct speed to match the system frequency. Turbines rotate at speeds of up to 3000 revolutions per minute. The incoming generator is synchronized onto the grid with the alternating current already flowing. The generated power (electricity) for public supply is in the form of an alternating current (AC).

 

The grid (system) frequency (50-hertz) is maintained to very close limits - system time must be in step with standard time. System frequency is also an indicator to system control what the demand for electricity is at any given point of time.