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How is electric power generated at DEM?

All the hydroelectric power plants, except for the channel-types Zlatoličje HPP and Formin HPP, are fundamentally built so that the riverbed is impounded by reinforced concrete dam. Turbines and generators are installed in the pier. Each vertical turbine is hooked up to a generator.

The dams also contain spillways and gates serving to release water over a dam when the water level is too high. Behind the dam, a reservoir is formed and, simultaneously, the head required to drive the turbines is determined based on the height of the dam. A turbine’s power is dependent on the size of the available head and the quantity of water flowing through the turbine. Water drives the turbine, which rotates an electric generator, which generates electric power according to the principle of electromagnetic induction.

Generation of electric power according to the principles of electromagnetism

Electromagnetic induction is the generation of voltage across a conductor situated in a changing magnetic field. Electromagnetic induction is based on the fact that all matter is composed of atoms that contain electrically charged particles. The atomic nucleus is made up of protons and neutrons, around which electrons revolve. Electrons are negatively charged, whereas the protons in the core, equal in number to the quantity of electrons, are positively charged. Atoms having the same number of protons and electrons are electrically neutral. A number of electrically charged particles attract while an equal number repel each other. Materials are distinguished by how strongly the electrons of their atoms are bound to the core. Substances whose electrons are strongly bound are electric insulators, meaning they do not possess any free electrons and thus do not generate electric current.

On the other hand, electric conductors are substances that possess free electrons, meaning that their electrons can move freely. If such a conductor (usually a copper wire) is placed in a magnetic field and then moved transversely in the direction of the field, forces act on the electrons in the wire, pushing them to one side of the wire (depending on the direction of movement) and resulting in a surplus of electrons on this side. Since they are negatively charged, we call this negative electric potential. On the opposite side of the wire, a scarcity of a comparative number of electrons therefore arises, resulting in positive potential. The difference between the potentials is called the electrical potential difference and is commonly called induced voltage. This generates electric current or power if both ends of the wire are connected with a conductive wire.

Electric current is thus the directed movement of electrons along a conductive wire from the area of surplus electrons to the other side with the deficit of electrons. The strength of the current is dependent on the size of the induced charge and the electric resistance of the connecting wire. The longer the conductive wire is, the stronger the magnetic field, and higher the speed of movement of the electrons, the higher the induced voltage. Here it is not important if the conductive wire moves in the magnetic field or if it remains still and the magnetic field moves. This principle of electromagnetic induction is used in electric generators to produce electric power.

An electric generator is made up of a stator (which remains still), a rotor (which rotates), and electromagnetic fields placed on the periphery of the rotor. The stator is made of iron. Electric conductors are placed within it, connected to each other so that the induced electric voltages in individual conductors add up among each other. This system of connected conductors is called a stator coil. Magnetic fields are produced on the periphery of the rotor. Alternating, the north and south poles switch depending on which end has the surplus of electrons. A magnetic field is generated between the north and south poles concluding via air slits and the stator, so that the coil lies in the magnetic field. When the rotor turns, the magnetic field begins moving depending on the wiring. Electrical voltage is induced in the coil, which can be measured between the two ends of the coil. The two ends of the coil are called the generator’s terminals. Electrical circuits are hooked up to these terminals, and the ensuing electric power is transmitted to users.

Synchronous generator

Transmission of electric power

At the beginning of the use of electric power, power plants were small in comparison with today's plants, and electric generators directly provided electricity to users in the vicinity. Power was generated from voltage that could not be too high, due to the danger to people. The first generators produced so-called direct current in which the current always flows in the same direction.

The use of electric power, due to a number of advantages, quickly grew, and the need for more powerful plants that would be constructed near suitable resources (rivers, coalmines) became evident; however, the problem of how to supply electric power to distant users remained, for enormously large sections of power transmission lines would be required to ensure an acceptable loss of energy during transmission.

With the invention of alternating current, in which the direction of current changes (this happens 50 times a second in our network), and multi-polar generators, which also operate according to the described principle of electromagnetic induction, the use of electric power has greatly expanded. Alternating current makes it possible to change the voltage using a transformer relatively easily. The higher the voltage, the less energy is lost during transmission. Nowadays the electric currents produced by generators are transformed into higher voltages and transmitted via power transmission lines with voltages of 110 kV, 400 kV and higher over long distances. In order to supply power to the users, the voltage is then transformed into lower values, down to the voltage we refer to as household voltage (220 V, or 280 V for three-phase connections).