Page 393 - Power Electronic Control in Electrical Systems

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9

Examples, problems and

exercises

9.1 Simple exercises 1

1. Draw a diagram of a transmission tower for a high-voltage overhead transmission
line.
2. What is the function of a transformer?
3. What parameters are used to describe `power quality'?
4. What is the minimum number of conductors required for a three-phase overhead
transmission line?
5. How is power factor defined when the voltage or current is not sinusoidal?
6. Thomas Edison is credited with the invention of `the electric light bulb'. In the
early days of the electricity supply industry (roughly 100 years ago), when the new
electric companies were expanding and promoting the use of electric power,
Edison believed that DC was superior to AC. On the other hand, George Westing-
house took the opposite view. For low-voltage (i.e. residential) and medium-
voltage (industrial) supplies, the argument is obviously well settled in favour of
AC, but for very high voltage transmission it is not always a straightforward
choice.
Moreover, in the case of small isolated power systems such as the power system
of a car or an aircraft, the argument is being re-opened. Whereas most cars use
12 VDC and most commercial aircraft use 115-V three-phase AC at 400 Hz, these
standards may be replaced within the next decade or two.
Table 9.1 summarizes some of Edison's and Westinghouse's arguments, along
with some additional ones that are relevant today. An additional column is
provided for you to add your own comments.

1
The exercises in this section can be used for classwork, either with a whole class or in small groups.