The generators at Lay Yang B Power Station generate at an EMF of 20 kV and have a current of 17.1 kA when supplying power of 500 Megawatts with a rotating electromagnet drawing a direct current of 5000 amperes.
At a step-up transformer, the voltage is stepped up to 400kV for transmission. The current used varies from time to time but it’s kept to a minimum as possible for smaller power losses through the cables.
After transmission, the voltages are stepped down to 66kV at a terminal station and then down to 11kV at a substation. Pole transformers in different suburbs further steps it down to 415 volts and 240 volts for domestic use. Factories and some public places such as schools are transmitted with high voltages, e.g. 11kV and they step it down using their own transformers.
In alternating current (AC) the direction of flow of the current changes continuously, as opposed to direct current (DC), which flows in one direction only. A changing magnetic flux is required for an EMF to be induced and thus can only be done by alternating current.
The great advantage of AC over DC is that transformers can be used to increase or decrease the transmission voltage. DC cannot be transformed easily.
Other advantages of AC over DC:
- AC generators are more cheaper, simpler and more reliable than DC generators.
- AC can be readily switched by circuit breakers at any voltage, whereas switching of DC is only possible at the lowest voltages.
- AC motors and other electrical appliances are cheaper, simpler and more reliable.
- The frequency can be very precisely controlled and so AC is useful in motors which require accurate speed e.g. clocks, taper recorders, record players.
- AC can be easily and cheaply transformed up from generator voltages (typically 11-22kV) to main transmission voltages and down again to lower voltages as required.
The transmission system is required to transmit the electrical power generated in power stations to industrial, commercial and domestic customers.
Voltage is stepped-up by transformers
A transformer transforms electrical energy from one circuit to another by electromagnetic induction between two coils. The step-up transformer immediately increases the voltage to 400kV for transmission. In a step-up transformer, the number of secondary turns exceeds that of the primary coil and therefore the output voltage is greater than the input voltage.
The electrical power is given by, P=VI, where P = Power (P)
V= Voltage (V)
I = Current (I)
In-Line Cable transmission
Some of the power being fed into a transmission line is wasted before it reaches the consumer. Power loss in any wire is due to the heating effect of the electric current in them.
Power loss in wires = I^2R, where
I = Current (A)
R = Resistance ®
P = Power (W)
The power loss is kept minimum by keeping I and R to a minimum. R is kept low by using wire with large cross-sectional areas. Aluminium alloy conductors are used for the power transmission cables because they are cheaper and lighter than copper. TO increase the strength of the conductors, the aluminium strands are wound around a core of galvanised steel wires. To reduce power loss, disturbances etc. bundle conductors are used, where they are separated by spacers.
Due to some power lost through the cables,
P = I^2R
P
So,
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Voltage is stepped-Down
After transmission, voltage is stepped-down at a terminal station to 66kV and then down to 11kV at a substation. In a step-down transformer, when power is a constant value and when the voltage is decreased by a factor, the current will be increased by the same factor.
P = VI
Ideally, The output power = the input power
In a stepped-up thansformer, when power is a constant value and when the voltage is increased by a factor, the current will be decreased by the same factor.
P = VI
High currents aren’t used when stepping-up the voltage because it would result in greater power losses. Transmitting at high voltages will reduce the current, thus reducing high power losses.
The current in the generator can also be calculated by:
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The electricity supplied to houses is 240V (peak 340V) 50 Hz, single phase. There are two wires in the cable to the house – one is the active or phase wire and the other is the neutral. These come to a mains connection box that contains a fuse in the active wire. From there the cables go to the fuse box or switchboard.
The active lead is connected to a metal bar called the neutral bar which is earthed usually to a metallic pipe underground.
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From the fuse box, a cable that contains three wires is connected around the house to the power points and lights. The power points and lights are connected in parallel.
Appliances are connected to the power point through a three-pin plug unless the appliance is double insulated. The active wire is connected to the appliance switch and then to the relevant circuit. The conducting path is completed with the wire back to the neutral. The earth wire is connected to the metal frame of the appliance so that, is a fault develops, there will be a conducting path to the earth.
The morning peak in demand usually occurs about 10.00 am in the morning. This is due to the fact that people are getting ready to go to work or are already there or where factories have already begun functioning. A majority of factories, schools and shops open at this time.
The evening peak occurs roughly at 6.00 p.m. The reason behind this is, many factories are closing, as well as schools and shops. However a fair amount of power is still in use by household and perhaps factories that go on to function at night.
The demand for power is usually lowest at about 6.00 a.m. This could be because many people are still asleep, thus not a lot of electricity is being used in the household. Also public venues such as schools and shops would still be closed at this hour.
Demand varies with seasons, depending on the requirement of the public. It can be seen that on clod winter days and hot summer days, the demand for electricity is very high compared to mild summer and winter days. The reason for this, is that people use a lot of electrical equipment such as heaters, ovens etc. in winter and air-conditioners and other equipment to keep their body temperature at a steady in summer.
The SEC charges a lower tariff for electricity used by off-peak appliances in order to level demands of power. In doing so it also saves enormous amounts of money because it reduces the number of power stations needed to meet each day’s peak demand for electricity. It also allows for maximum use of all plant, which is built. Examples of off-peak appliances include hot water services and heat banks. Such off peak appliances convert electricity into heat, which can be stores and used anytime during the day without adding to peak electricity demands.
It is important that SEC accurately forecasts demand for electricity because this way the public knows when to use electrical appliances at the correct time. If this is done incorrectly there could be many problems at power stations.
The fluctuating demand is met by Victoria’s hydro-electric plant and Snowy Mountains entitlement (as Victoria is entitled to one-third of production) as well as the gas turbine stations at Jeeralong in the Latrobe Valley and at Newport.
