Chapter 7
Electrical systems
This Chapter assumes that the designer has access to a recognised reference book
on high voltage systems such as the ABB Switchgear Manual or the Siemens Power
Engineering guide.
The single line diagram is the key drawing for switchgear and protection. Much thought
needs to be put into it to make sure that it represents the simplest and lowest cost
arrangement of switchgear, transformers cabling and protection equipment that will
provide what is needed. It must be updated with every change because it is the key
drawing long into the future. Drawing E1 on the CD for the Paerau scheme is a good
example of a single line diagram.
The first thing that needs to be decided upon is the electrical arrangement of the
generators, switchgear and transformers. Decisions need to be made on transformer
voltage and rating, where the auxiliary supply will be fed from and how much redun-
dancy will be provided. For generator ratings above about 100 MVA it is normal
practice to directly connect the generator to the transformer and do the switching on
high-voltage side because this eliminates the need for very expensive high current gen-
erator voltage switchgear. This is seldom an economic solution for smaller schemes
because standard low cost generator voltage switchgear is widely available.
If there is a need to supply a local distribution system and a transmission system
then the arrangement adopted for Paerau and illustrated in Figure 80 is attractive. The
transformers have a natural rating of 7.5 MVA and a force cooled rating of 15 MVA
so that a single transformer can carry the full output if needed.
In most circumstances, a generator voltage bus section circuit breaker is not
needed because it is not difficult to have a complete shutdown at a convenient time.
The rule should always be “the less switchgear the better’’. Complexity is no index of
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142 Small hydroelectric engineering practice
Figure 80 Paerau single line diagram.
7.1.1 Transformers
If there is a need to supply power to a transmission system and a sub transmission
system for instance 33 kV and 132 kV then a three winding transformer is an
excellent solution. Many engineers are nervous about using three winding transformers
but, provided there is a good understanding of the requirements and the winding
options available, they are easy to apply and they can lead to large savings and help
with the management of fault levels. There is a paper I wrote on the advantages and
application of three winding transformers on the CD.
Although on load tap-changers are often used at hydro stations, analysis and expe-
rience tells us that they are seldom needed. For instance in Iran the 400 kV transformers
connected to 250 MW machines only have off-load taps and no problems have been
reported. In most cases, the hydropower station needs to maintain a constant volt-
age at the high-voltage export connection. This seldom requires more than a +/5%
change in generator terminal voltage that is within the standard range of generator
capability. Note that the +/5% limits only apply when the generator is delivering its
rated MVA. The upper end of the range the limit is set by saturation of the generator
“The advantages and application of three winding transformers’’.

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