Small Hydroelectric Engineering Practice

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.

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7.1 SINGLE LINE DIAGRAM

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

performance!

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Available from http://www02.abb.com/global/seitp/seitp161.nsf/0/26ed7fc021b62000c1256

f570051b81a/$file/index_abb_de.html and http://www.energy.siemens.com/hq/en/energy-

topics/publications/power-engineering-guide/ or http://www.abb-shb.de/index_abb_en_start.

html.

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.

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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

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“The advantages and application of three winding transformers’’.