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Transformer Riddle No.39 - Transformer Load and cable size calculation
Load calculation for t/f and cable capacity determination of /cable size for any load.
Please let me know the best formulas for the subject cited.
Author : Gajendra - From: India
Thu, February 18th, 2010 - 14:37
Each local load may be classified into several different categories for example, vital, essential and non-essential. Individual industry often use their own terminology and terms such as ‘emergency’ and ‘normal’ are frequently encountered.
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 jeopardize 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.

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)

Vital AC loads:

UPS supplies
Emergency lighting
Emergency generator auxiliaries
Helicopter pad lighting
Control room supplies
Vital LV pumps

Essential AC loads:

Diesel fuel transfer pumps
Main generator auxiliaries
Main compressor auxiliaries
Main pump auxiliaries
Diesel fire pump auxiliaries
Electric fire pumps
Living quarters
Air compressor
General service water pumps
Fresh water pumps
Equipment room HVAC supplies
Life boat davits
Anti-condensation heaters in panels and switchboards
Security lighting supplies
Control room supplies
UPS supplies
Radio supplies
Computer supplies
Battery chargers for engine starting systems
Instrumentation supplies

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 below 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.


Because of the sensitive nature of the vital and essential consumers with regard to personnel safety and production continuity, it is established practice to supply their associated switchboards with dual, or occasionally triple, feeders. For non-essential switchboards it may be practical to use only one feeder.
For switchboards other than those for the generator or intake feeders it is established practice to add some margin in power capacity of their feeders so that some future growth can be accommodated. The margin is often chosen to be 25% above the TPPL.
If the feeders are plain cables or overhead lines then it is a simple matter to choose their cross-sectional areas to match the current at the 125% duty.
For transformer feeders there are two choices that are normally available. Most power transformers can be fitted with external cooling fans, provided the attachments for these fans are included in the original purchase order. It is common practice to order transformers initially without fans and operate them as ONAN until the demand increases to justify the fan cooling. Thereafter the transformer is operated as ONAF. Adding fans can increase the capacity of the transformer by 25% to 35%, depending upon the particular design and ambient conditions. The alternative choice is simply to rate the ONAN transformer for the 125% duty, and initially operate it at a lower level. The decision is often a matter of economics and an uncertainty about the future growth.
When standby or future capacity is required for transformers it is necessary to rate the secondary cables or busbars correctly at the design stage of the project. Likewise the secondary circuit breakers and switchgear busbars need to be appropriately rated for the future demand. The decision to over-rate the primary cables or lines may be made at the beginning of the project or later when demand increases. Again this is a matter of economics and forecasting demand.
Author : Hamid - From: Iran
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