Technical Papers +Technical Questions

Technical Papers

Technical Papers

Following are some of the technical papers OPS Programs has published to the public:

How Core Losses Affect Operating Costs for Transformers Delivering Non-Sinusoidal Loads

Presented at the EIC/EMCW Exposition

  How Core Losses Affect Operating Costs (18.5 KiB, 5,081 hits)

Payback Times for Premiums Paid for High-Efficiency Dry-Type Transformers

Presented at the EMCW Conference

  Payback Times for Premiums (24.2 KiB, 3,524 hits)

Loss Evaluation – Will Non-Utility Transformers Be Next?

presented at the EEIC/EMCW Conference

  Loss Evaluation (18.5 KiB, 3,710 hits)

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Efficiency Values for the European Market

CLASP Transformers Design Report 

Technical Questions

The following can help clarify some of the questions engineers might pose when working on certain designs through OPS programs.


Cooling Duct Location During Optimization

Cooling duct location during optimization – we are still unclear on what occurs to ducts during this process.  In the transformer program, it appears that their number and location is inputted.  In optimization, it appears that their number is defined, but not their location – is this correct?

In Transformer we specify how many different locations duct are going to be present, for example: a winding with 4 layers and 2 location of ducts the program place the ducts between layer 1 and 2 and layer 2 and 3.

In Optimization we specify how many different locations duct are going to be present – the same as Transformer in example above if during optimization the number of the layers may drop to 2 then although you specify 2 locations optimization will reduce the number of locations to 1. If the number of the layers during the optimization for this example increased to let’s say 6 layers optimization will continue to consider only 2 locations for ducting.

DG Core

How much different are the interlayer and impulse stresses in the rectangular windings compared to the stacked (cruc.) core?

This question has come up because of the stresses on the external part of the winding around the corners of the paper.

The stresses in the corners would be slightly greater that on the sides but not necessarily any greater than those that would be found on the cruciform arrangement. The reasons for this are two- fold: The corner shapes (especially on the inner layers) create more stresses and secondly, the winding in the corners would generally be tighter than on the sides. So the bulge on the sides, which would generally be composed of insulation and films of oil (with about the same dielectric constant), would contribute to slightly longer dielectric paths and therefore less stresses.

As far as I can remember there should be some extended insulation to overlap in the window of the wound core!

Right or wrong?  This we gather would be put in place due to creepage and clearance required!? Please clarify.

Generally, all overlaps are in the ends both to reduce space and provide insulation uniformity in the window.

Is the phase to phase clearance in our design have sufficient space for cooling?

Yes. One caveat: This distance will also have to be re-considered for different BIL values.

Does the phase to phase space have a solid board inserted?

If you can assure the accuracy of the space, it would not. However, to assure spacing a board would be necessary but at the expense of reducing the cooling surface of the coils at that location.

Does the oil temperature rise differ in the circular and rectangular winding.

The difference would not be discernible.

Does the oil temperature rise differ in the wound and cruc. core?

For given similar total losses, no. Individual winding gradients may be slightly different depending on overall construction.

With regards to production manufacture, does the inner part of the rectangular form of the coil round a lot on the corners and edges once removed off the former when winding is finished?

The general answer is generally no. The rounding magnitude depends on the size of the coil and obviously how tightly it is wound.

If so, what means can be used to keep and maintain this form specially when the wound core needs to be assembled in the winding window?

Some people use spreader bars on the corners as they are assembling the coil. Some others –    with larger coils – make a form on the inside of the coil and varnish and bake (corners only) to assure dimensional compliance. To repeat, this rounding is not an overwhelming occurrence.

Ferrite Core Material

If we ask you to add a series of ferrites to the data in your software, do the ferrite vendors end up at all of the other user’s databases?

The core loss data is not stored in external files. It is part of the calculation, and therefore will be available to all users. However, if you have a special core material, you can enter the data as mw/cm^3 or kw/m^3. We recently added the following core losses to the programs: 3C91, 3C94, 3C96, 3C97, 3F36, and 3F46.

If we have a custom core, shape and size does everyone end up seeing it or can it be only on the “BH Version”?

It does not have to be for everyone. We can set up a specific core data file just for you.

Is it possible to enter data from and actual build and see the results?


Would it get handled as a “new” design adjusting the input until the results of the actual design are reached?



Is there any way to know the inrush value in OPS, assuming that a Delta-Wye transformer will be back-fed (connected in reverse)? –

If the transformer was connected in reverse, the magnitude of the inrush current would be almost proportional to the exact kVA as the primary is. The only difference comes from the relative location of the winding i.e. if it is outside it would have a greater air core inductance and vice-versa. OPS can calculate this current by reversing P1 and S1.

The Absolute Peak Current (Phase Current) and Practical Max. Value the OPS calculate when we run Impedance in the Extended Analysis. Are these rms or peak current? Do you know the duration of each one?

The absolute peak current and practical max. value that OPS calculates are peak current. Please see attached for more information.The duration depends on the flux density of core and other factors, so we can not define it.

Two Winding Inductor

Question: Can I run inductor program so that I have rms current in the winding for winding losses and a much smaller current seen by the core? For example if a 3 phase inductor had two windings on each core leg and they were connected in opposite polarities. I would run the same current through each and have full rms current losses in the winding but no net flux so no core loss. Then be able to adjust or unbalance the winding currents so that the resultant unbalance would give flux. Is this a function for which you know a way to model?

The way that I have created a flux on the core is to use the transformer program and make the currents of the primary and secondary the same, use the same core and enter a voltage that would create the flux desired.

Windings in Different Coil Heights

 1.  We would like explanation of how the program deals with windings of different coil heights.  We have designs that the first [typically low voltage] secondary layer is longer than the second [high voltage] layer.  This causes additional radial flux losses that must be addressed, do the OPS routines do any calculation of these additional losses?

In all its calculations TRANS takes into consideration the “Electrical Length” of each winding.  Our definition of Electrical Length Corresponds to your definition of “Coil Height”.  The ‘Flux” losses can be accounted for in the entries for ‘stray Losses’


When designing transformers with OPS software, users may run into specific questions regarding input of data.  For the best results and for the software to perform at optimal levels, users must enter valid and relevant data.  If you have specific questions regarding your transformer design, please email us at