Transformer Sizing

Most transformer manufacturers would be thrilled to help you determine the most appropriate size for your application.

You absolutely must double the mva of your transformer. You must consider start up load, and consider thermal overloading of a too small transformer, the problem of having to re set the overloads, re setting the mills that were mid operation when the compressors kicked on. A 6mva transformer is not enough.

STOP!

Do not proceed.

Contact 3 manufacturers and give them your requirements.

They, alone, can help you size your transformer!

No one here can help you.

Some may try but they may just kill you and others with their misguided kindness.

You don’t say what starting method is used for each motor.

If you already have one transformer, do you have the space to install a second transformer and split the loads between them?

This sounds like a job for a contractor or consultant.

We should all know our limitations or somebody can get hurt.

The 6MVA transformer may well be large enough. The load factor for your equipment is often less than 50%, for design a conservative rule of thumb is to use 75% of nameplate. The value of the heat lost switching to a larger transformer is rarely if ever justified, because most end up underloaded in actual operation, the value of the incremental heat is very small, and not easily predicted.

The transformer manufacturers are not the technical resource you want. Your starting methods and the starting torque requirements of your loads/motors are required to know if you can start the loads, without lowering the transformer voltage so low you cannot develop the accelerating torque for the motor you are trying to start. The transformer manufacturers have no knowledge of this, and by the time you give them this information, you could have done it yourself, with no chance of giving them the wrong information.

Since this has only motor load, light flicker from voltage dips while starting a motor will not be objectionable, and once one of the motors is running, subsequent voltage drops from other motors starting will likely not be enough to make them slip out of synchronism (pull out).

The detailed calculations are now easily done by computer, and that is your job, as you alone have access to the technical information required to perform the modeling.

If I was to size this based on experience, with no calculation, I would choose the 10mVA option, the 6MVA could be marginal or not work at all, depending on your power system source impedance before the transformer, your speed/torque curves of the machinery, the type of motor, synchronous, induction, wound rotor, and your starting method. You run all of the connected loads, and then start the largest motor, to see if the system will survive. You need to know the upstream power system source impedance, and the impedance of your proposed transformer, use a standard value around 5-6%, check with your tentative transformer manufacturer.

My rule of thumb is you can start a 200hp motor across the line with no ill effects with a 1000kVA transformer, 5.75% impedance, and maybe less than 250MVA short circuit availability to the primary of the transformer. Starting a medium voltage motor is less critical, since the other machines on the transformer will tolerate voltage dips during starts.

1) Pl provide the Working/ Standby loads.

2) Pl give the sequence of operation for the various motors. Which is the last motor to start ?

3) What is the mode of operation ie starting for these motors ? Any VFD's are present in the circuit ?

4) What are the other loads ( tie feeders or trafo feeders ) on the 5.5 kV board ?

You have to give the type of motor,Starting method, acceleration time (Voltage should not fall below 80% and trf should be capable)Starting sequence etc