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 aswitchboard 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 and
Dsj for Ssumj . 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.
