Topic 12.3 Transmission of Electrical Power 1 hour Power Losses in Transmission Lines • • • • There are a number of reasons for power losses in transmission lines such as: Heating effect of a current Resistance of the metal used Dielectric losses Self-inductance Power Losses in Transmission Lines • The main heat loss is due to the heating effect of a current. By keeping the current as low as possible, the heating effect can be reduced. • The resistance in a wire due to the flow of electrons over long distances also has a heating effect. If the thickness of the copper wire used in the core of the transmission line is increased, then the resistance can be decreased. However, there are practical considerations such as weight and the mechanical and tensional strength that have to be taken into account. The copper wire is usually braided (lots of copper wires wound together) and these individual wires are insulated. • The insulation material has a dielectric value which can cause some power loss. Some of the power from the lines goes into changing the orbits of the electrons in the insulating material. • Finally, the changing electric and magnetic fields of the electrons can encircle other electrons and retard their movement on the outer surface of the wire through self-inductance. This is known as the ‘skin effect’. The size of the power loss depends on the magnitude of the transmission voltage, and power losses of the order of magnitude of 105 watts per kilometre are common. Power Loss in a Real Transformer • • • • • • • Power losses in real transformers are due to factors such as: Eddy currents Resistance of the wire used for the windings Hysteresis Flux leakage Physical vibration and noise of the core and windings Electromagnetic radiation Dielectric loss in materials used to insulate the core and windings. Eddy Currents • As already mentioned, any conductor that moves in a magnetic field has emf induced in it, and as such current, called eddy currents, will also be induced in the conductor. This current has a heating effect in the soft iron core of the transformer which causes a power loss termed an iron loss. There is also a magnetic effect in that the created magnetic fields will oppose the flux change that produces them according to Lenz’s Law. This means that eddy currents will move in the opposite direction to the induced current causing a braking effect. Eddy currents are considerably reduced by alloying the iron with 3% silicon that increases the resistivity of the core. To reduce the heating effect due to eddy currents, the soft-iron core is made of sheets of iron called laminations that are insulated from each other by an oxide layer on each lamination. This insulation prevents currents from moving from one lamination to the next. Copper Loss • Copper wire is used as the windings on the softiron core because of its low resistivity and good electrical conductivity. Real transformers used for power transmission reach temperatures well above room temperature and are cooled down by transformer oil. This oil circulates through the transformer and serves not only as a cooling fluid but also as a cleaning and anticorrosive agent. However, power is lost due resistance and temperature commonly referred to as ‘copper loss’. Hysteresis • Hysteresis is derived from the Greek word that means “lagging behind” and it becomes an important factor in the changes in flux density as a magnetic field changes in ferromagnetic materials. Transformer coils are subject to many changes in flux density. As the magnetic field strength increases in the positive direction, the flux density increases. If the field strength is reduced to zero, the iron remains strongly magnetized due to the retained flux density. When the magnetic field is reversed the flux density is reduced to zero. So in one cycle the magnetization lags behind the magnetizing field and we have another iron loss that produces heat. Hysteresis is reduced again by using silicon iron cores. • The capacity for the primary coil to carry current is limited by the insulation and air gaps between the turnings of the copper wire and this leads to flux leakage. This can be up to 50% of the total space in some cases. • Because the power is being delivered to the transformer at 60Hz, you can often hear them making a humming noise. Minimal energy is lost in the physical vibration and noise of the core and windings. • Modern transformers are up to 99% efficient. Power Transmission • For economic reasons, there is no ideal value of voltage for electrical transmission. Electric power is generated at approximately 11 000 V and then it is stepped-up to the highest possible voltage for transmission. Alternating current transmission of up to 765 kV are quite common. • For voltages higher than this, direct current transmission at up to 880 kV is used. A.C. can be converted to D.C. using rectifiers and this is what is done in electric train and tram operations. D.C. can be converted to a.c. using inverters. • There a number of D.C. transmision lines such as the underground cross-channel link between the UK and France. �The New Zealand high-voltage direct current scheme has around 610 km of overhead and submarine transmission lines. • There are 3 conductors on a transmission line to maximize the amount of power that can be generated. Each high voltage circuit has three phases. The generators at the power station supplying the power system have their coils connected through terminals at 120° to each other. • When each generator at the power station rotates through a full rotation, the voltages and the currents rise and fall in each terminal in a synchronized manner. • Once the voltage has been stepped-up, it is transmitted into a national supergrid system from a range of power stations. As it nears a city or town it is stepped-down into a smaller grid. As it approaches heavy industry, it is stepped down to around 33 – 132 kV in the UK, and when it arrives at light industry it is stepped-down to 11-33 kV. Finally, cities and farms use a range of values down to 240V from a range of power stations. • When the current flows in the cables, some energy is lost to the surroundings as heat. Even good conductors such as copper still have a substantial resistance because of the significant length of wire needed for the distribution of power via the transmission cables. To minimize energy losses the current must be kept low. Extra Low Frequency EM Fields • We have all seen though the media patients being given shock treatment through 2 electrodes to try and get the heart beat at its natural frequency. The human body is a conducting medium so any alternating magnetic field produced at the extra-low frequency will induce an electric field which in turn produces a very small induced current in the body. Using a model calculation in a human of body radius 0.2 m and a conductivity of 0.2 S/m (sieverts per metre), it has been shown that a magnetic field of 160 μT can induce a body surface current density of 1 mA m-2. • It is currently recommended that current densities to the head, neck and body trunk should not be greater than 10 mA m-2. Possible Risks of High Voltage Power Lines • Current experimental evidence suggests that low-frequency fields do not harm genetic material in adults but there is some evidence that there could be a link to infant cancer rates due to low-frequency fields. The risks attached to the inducing of current in the body are not fully understood. It is likely that these risks are dependent on current (density), frequency and length of exposure.