Read the text and do the tasks Electric power systemsElectric power is the product of two quantities: current and voltage. These two quantities
can vary with respect to time (AC power) or can be kept at constant levels (DC power). Most
refrigerators, air conditioners, pumps and industrial machinery use AC power whereas most
computers and digital equipment use DC power. AC power
has the advantage of being easy to transform between
voltages and is able to be generated and utilized by brushless
machinery. DC power remains the only practical choice in
digital systems and can be more economical to transmit over
long distances at very high voltages. The ability to easily transform the voltage of AC power is important for two reasons.
Firstly, power can be transmitted over long distances with less loss at higher voltages. So in
power systems where generation is distant from the load, it is desirable to step-up the voltage
of power at the generation point and then step-down the voltage near the load. Secondly, it is
often more economical to install turbines that produce higher voltages than voltages used by
most appliances, so the ability to easily transform voltages means this mismatch between
voltages can be easily managed. Solid state devices make it possible to transform DC power to different voltages, build
brushless DC machines and convert between AC and DC power. Nevertheless devices
utilizing solid state technology are often more expensive than their traditional counterparts,
so AC power remains in widespread use. One of the main difficulties in power systems is that the amount of active power
consumed plus losses should always be equal to the active power produced. If there is more
power produced than consumed, the frequency would rise. Even small deviations from the
nominal frequency value would damage synchronous machines and other appliances. 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 a part of the system itself - it is
these internal power sources. 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 by using fossil fuel or nuclear
energy, falling water and wind. 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 in 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. Power systems deliver energy to loads. These loads range from household appliances to
industrial machinery. Most loads expect a certain voltage and, for alternating current devices,
a certain frequency and number of phases. The appliances in your home will typically be
single-phase operating at 50 or 60 Hz with a voltage between 110 and 260 volts. Centralized
air conditioning systems are an exception as they are now typically three-phase because this
allows them to operate more efficiently. All devices in your house will also have wattage;
this specifies the amount of power the device consumes. At any one time, the net amount of
power consumed by the loads on a power system must equal the net amount of power
produced by the supplies to reduce power lost in transmission.Complete the sentences, using the words and word combinations from the text