Transformer design procedure

[STONE85]

Hello,
I am enquiring as to the procedure that has to be followed to size a transformer for a project after all loads that will come off the transformer have been calculated. . If anyone could point me in the direction of standards ( uk) / list of steps to be taken. Primary is 11kv secondary is 400v

Thank you

 

[waross]

It depends.
The answer may be more regulatory than technical.
In Canada, transformers owned by the end user are typically sized according to the Canadian Electrical Code.
The utilities are not bound by the code and use their own sizing criteria.
Example:
Planning for a lumber mill, we were faced with a decision as to whether to buy the main transformer and buy power at the primary voltage or to contract for power at the secondary voltage with the utility owning the transformer.
The difference between a Code sized transformer and a secondary sized transformer was exactly 2:1.
Check your local codes and conditions.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[ElecEE8]

Hi. I share here in attached file for your information. Hope it helps.
It based IEC std.

 

[waross]

ElecEE8; Thanks for the information.
Utilities can and do use those or similar guidelines for transformer loading.
The North American electrical codes do not generally allow transformer overloading.
Utilities are specifically exempted from those codes.

Transformers sized to code in new construction are generally oversized due to the diversity factor. (In North America)
In existing systems, load may be added if it may be demonstrated that the transformer will not be overloaded.
EG: A transformer sized according to code is 100 KVA. If it may be demonstrated that the maximum demand has been 80 KVA, then an additional 20 KVA of load may be added.
The most commonly accepted demonstration of the maximum demand is the maximum demand shown on the electrical billing invoices over the previous year.
If that information is not available, the inspection department may, at its discretion, accept the information from temporarilly installed demand meters for a shorter period.
This exception allows added load after one year of operation and cannot be used for new construction.
The code does allow a conservative diversity factor for some equipment and loads.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[STONE85]

Thank you for replies.

 

[prc]

https://www.linkedin.com/pulse/rated-power-transformers-p-ramachandran

 

[ElecEE8]

Hi Bill,
The procedure i shared is mainly applicable for power distribution sector in Oil& Gas, Petrochemical & chemical plant only. For utilities may be some criteria different approach from what i shared which is i am not fully aware of.
Thanks & Regards

 

[waross]

I don't know what code you are working under.
In North America, we are bound by the Canadian Electrical Code or by the National Electric Code.
1. Electrical equipment in general must be used withing the approved ratings. Transformers must be rated to carry the calculated load.
Our issue is the Calculated Load.

(3) The calculated load in a consumer’s service, feeder, or branch circuit shall be considered a continuous load
unless it can be shown that in normal operation it will not persist for
(a) a total of more than 1 h in any two-hour period if the load does not exceed 225 A; or
(b) a total of more than 3 h in any six-hour period if the load exceeds 225 A.
(4) A load of a cyclic or intermittent nature shall be classified as continuous unless it meets the requirements
of Subrule (3).

Also, when loads are mutually exclusive such as electric heating and electric air conditioning, only the greatest need be considered.

I have mentioned an exception for additional loads when it is shown that the demand over a period of time justifies a lesser demand.
In a hypothetical case:
You are designing similar or identical pumping stations.
You are designing a new station and you have historical data from existing pumping stations that demonstrates that a lesser demand factor is justified.
If you came to me as the AHJ and asked for an exception to the rules based on available data, I would be inclined to grant an exception subject to the following conditions.
1. My ability under the local code to grant an exception.
2. Assurances that the flow, density, pressure difference and loading of the proposed station will be equal to or less than the conditions of loading of the example stations.
3. Any exemption will be based on information supplied by yourself and you will be solely responsible if there are future issues with the installation and the AHJ will be held blameless.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!