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.
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.
1.2 LOAD SCHEDULES
Each switchboard will supply power to each load connected to it and in many cases it will also supply
power to switchboards or distribution boards immediately downstream. Hence the input power to a switchboard will have the possibility of two components, one local and one downstream. Hereinafter
the term switchboard will also include the term motor control centre, see sub-section 7.1.
Each local load may be classified into several different categories for example, vital, essential
and non-essential. Individual oil companies often use their own terminology and terms such as
‘emergency’ and ‘normal’ are frequently encountered. Some processes in an oil installation may
handle fluids that are critical to the loss of power e.g. fluids that rapidly solidify and therefore must
be kept hot. Other processes such as general cooling water services, air conditioning, sewage pumping
may be able to tolerate a loss of supply for several hours without any long-term serious effects.
In general terms there are three ways of considering a load or group of loads and these may
be cast in the form of questions. Firstly will the loss of power jeopardise safety of personnel or
cause serious damage within the plant? These loads can be called ‘vital’ loads. Secondly will the loss
of power cause a degradation or loss of the manufactured product? These loads can be called the
‘essential’ loads. Thirdly does the loss have no effect on safety or production? These can be called
the ‘non-essential’ loads.
Vital loads are normally fed from a switchboard that has one or more dedicated generators
and one or more incoming feeders from an upstream switchboard. The generators provide power
during the emergency when the main source of power fails. Hence these generators are usually
called ‘emergency’ generators and are driven by diesel engines. They are designed to automatically
start, run-up and be closed onto the switchboard whenever a loss of voltage at the busbars of the
switchboard is detected. An undervoltage relay is often used for this purpose. Testing facilities are
usually provided so that the generator can be started and run-up to demonstrate that it is ready to
respond when required. Automatic and manual synchronising facilities can also be provided so that
the generator can be loaded during the tests.
Low voltage diesel generators are typically rated between 100 and 500 kW, and occasionally
as large as 1000 kW. High voltage emergency generator ratings are typically between 1000 and
2500 kW. The total amount of vital load is relatively small compared with the normal load and, in
many situations, the essential load. Consequently the vital load is fed from uninterruptible power
supplies (UPS), as AC or DC depending upon the functions needed. The vital loads are usually fed
from a dedicated part of the emergency switchboard. The UPS units themselves are usually provided
with dual incoming feeders, as shown in Figure 17.3.
Some of the vital and essential loads are required when the plant is to be started up, and there
is no ‘normal’ power available. In this situation the starting up of the plant is called ‘black starting’.
The emergency generator must be started from a source of power, which is usually a high capacity
storage battery and a DC starter motor, or a fully charged air receiver and a pneumatic starter motor.
In many plants, especially offshore platforms, the vital and essential loads operate at low
voltage e.g. 380, 400, 415 volts. Large plants such as LNG refrigeration and storage facilities require
substantial amounts of essential power during their start-up and shut-down sequences and so high
voltage e.g. 4160, 6600 volts is used. The vital loads would still operate at low voltage. Tables 1.2
and 1.3 shows typical types of loads that can be divided into vital and essential loads.
All of the vital, essential and non-essential loads can be divided into typically three duty categories:
• Continuous duty.
• Intermittent duty.
• Standby duty (those that are not out of service).
Hence each switchboard will usually have an amount of all three of these categories. Call
these C for continuous duty, I for intermittent duty and S for the standby duty. Let the total amount
of each at a particular switchboard j be ( Cjsum, Ijsum and Sjsum ). Each of these totals will consist of
the active power and the corresponding reactive power.
In order to estimate the total consumption for the particular switchboard it is necessary to
assign a diversity factor to each total amount. Let these factors be ( Dcj for Csumj , Dij for Isumj ). Oil companies that use this approach have different values for their diversity factors,
largely based upon experience gained over many years of designing plants. Different types of plants
may warrant different diversity factors. Table 1.4 shows the range of suitable diversity factors. The factors should be chosen in such a manner that the selection of main generators and main feeders from
a power utility company are not excessively rated, thereby leading to a poor choice of equipment in
terms of economy and operating efficiency.
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,
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.
This in turn will enable an initial layout of all the facilities and equipment to be proposed. Since this is
a new plant and the preliminary data is estimated from process calculations, mechanical calculations
and comparisons with similar plants, it is acceptable to use the following diversity factors, Dc = 1.0,
Di = 0.5 and Ds = 0.1.
Tables 1.5, 1.6, 1.7 and 1.8 show the individual loads that are known at the beginning of
the project.
The total power is found to be 12,029 kW. At this stage it is not known whether the plant is
capable of future expansion. The oil and gas geological reservoir may not have a long life expectation,
and the number of wells that can be accommodated on the platform may be limited. The 4000 kW
of power consumed by the drilling operations may only be required for a short period of time e.g.
one year, and thereafter the demand may be much lower.
During the detail design phase of the project the load schedules will be modified and additional
loads will inevitably be added. At least 10% extra load should be added to the first estimate i.e.
1203 kW. The total when rounded-up to the nearest 100 kW would be 13,300 kW.
Sufficient generators should be installed such that those that are necessary to run should be
loaded to about 80 to 85% of their continuous ratings, at the declared ambient temperature. This
subject is discussed in more detail in sub-section 1.3. If four generators are installed on the basis that
one is a non-running standby unit, then three must share the load. Hence a reasonable power rating
for each generator is between 5216 kW and 5542 kW.
