Estimation of Plant Electrical Load ( Part 1 )

One of the earliest tasks for the engineer who is designing a power system is to estimate the normal
operating plant load. He is also interested in knowing how much additional margin he should include
in the final design. There are no ‘hard and fast’ rules for estimating loads, and various basic questions
need to be answered at the beginning of a project, for example,
• Is the plant a new, ‘green field’ plant?
• How long will the plant exist e.g. 10, 20, 30 years?
• Is the plant old and being extended?
• Is the power to be generated on site, or drawn from an external utility, or a combination of both?
• Does the owner have a particular philosophy regarding the ‘sparing’ of equipment?
• Are there any operational or maintenance difficulties to be considered?
• Is the power factor important with regard to importing power from an external source?
• If a generator suddenly shuts down, will this cause a major interruption to the plant production?
• Are there any problems with high fault levels?
1.1 PRELIMINARY SINGLE-LINE DIAGRAMS
In the first few weeks of a new project the engineer will need to roughly draft a key single-line
diagram and a set of subsidiary single-line diagrams. The key single-line diagram should show the
sources of power e.g. generators, utility intakes, the main switchboard and the interconnections to
the subsidiary or secondary switchboards. It should also show important equipment such as power
transformers, busbars, busbar section circuit breakers, incoming and interconnecting circuit breakers,
large items of equipment such as high voltage induction motors, series reactors for fault current
limitation, and connections to old or existing equipment if these are relevant and the main earthing
arrangements. The key single-line diagram should show at least, the various voltage levels, system
frequency, power or volt-ampere capacity of main items such as generators, motors and transformers,
switchboard fault current levels, the vector group for each power transformer and the identification
names and unique ‘tag’ numbers of the main equipment.
The set of single-line diagrams forms the basis of all the electrical work carried out in a
particular project. They should be regularly reviewed and updated throughout the project and issued



in their final form at the completion of the project. They act as a diary and record the development
of the work. Single-line diagrams are also called ‘one-line diagrams’.
At this stage the engineer can begin to prepare a load schedule for each subsidiary switchboard
and motor control centre, and a master schedule for the main switchboard. The development of the
single-line diagrams during the project is discussed in sub-section 1.7.
The master load schedule will give an early estimate of the total power consumption. From
this can be decided the number of generators and utility intakes to install. The kW and kVA ratings
of each generator or intake will be used to determine the highest voltage to use in the power
system. Table 1.1 shows typical voltages used throughout the world for generation, distribution and
transmission of power at oil industry plants, see also sub-section 3.7.