Page 26 - Handbook of Electrical Engineering

ESTIMATION OF PLANT ELECTRICAL LOAD 5

Table 1.4. Diversity factors for load estimation

Type of project D c for C sum D i for I sum D s for S sum
Conceptual design of a new 1.0to1.1 0.5to0.6 0.0to0.1
plant
Front-end design of a new 1.0to1.1 0.5to0.6 0.0to0.1
plant (FEED)
Detail design in the first half of 1.0to1.1 0.5to0.6 0.0to0.1
the design period
Detail design in the second half 0.9to1.0 0.3to0.5 0.0to0.2
of the design period
Extensions to existing plants 0.9 to 1.0 0.3 to 0.5 0.0 to 0.2

The above method can be used very effectively for estimating power requirements at the
beginning of a new project, when the details of equipment are not known until the manufacturers can
offer adequate quotations. Later in a project the details of efficiency, power factor, absorbed power,
rated current etc. become well known from the purchase order documentation. A more accurate form
of load schedule can then be justified. However, the total power to be supplied will be very similar
when both methods are compared.
The total load can be considered in two forms, the total plant running load (TPRL) and the
total plant peak load (TPPL), hence,

n

TPRL = (D c C sumj + D i I sumj ) kW
j=1
n

TPPL = (D c C sumj + D i I sumj + D s S sumj ) kW
j=1
Where n is the number of switchboards.

The installed generators or the main feeders to the plant must be sufficient to supply the TPPL
on a continuous basis with a high load factor. This may be required when the production at the plant
is near or at its maximum level, as is often the case with a seasonal demand.

Where a plant load is predominantly induction motors it is reasonable to assume the overall
power factor of a switchboard to be 0.87 lagging for low voltage and 0.89 lagging for high voltage
situations. If the overall power factor is important with regard to payment for imported power, and
where a penalty may be imposed on a low power factor, then a detailed calculation of active and
reactive powers should be made separately, and the total kVA determined from these two totals. Any
necessary power factor improvement can then be calculated from this information.

1.2.1 Worked Example

An offshore production and drilling platform is proposed as a future project, but before the detail
design commences it is considered necessary to prepare an estimate of the power consumption. The
results of the estimate will be used to determine how many gas-turbine driven generators to install.