3 Phase Transformer Saturation

What is, "a static load of a high resistance"?

Why go to all the bother to make a heater?

Are there any electricians, electrical technicians or electrical engineers on staff?

I have some suggestions, but, I'll save them for later.

What kind of "soft start" (and it's really a power controller if you are reducing voltage continuously) did you use? There are two kinds of SCR based power controllers sold for the heating industry, phase angle controlled, and zero-cross variable time based. The cheaper ones are the zero-cross version, but they ONLY work directly on resistive loads. If you have anything inductive like, oh say a transformer?, then you MUST use a phase angle controlled SCR controller.

Then within that realm, there are some that are half-wave, meaning 3 SCRs and 3 diodes and some that are full wave, meaning 6 SCRs. If you bought the cheaper half-wave versions, again, no bueno for a transformer fed system.

Suggestion: System diagram. Analyzing a fault condition under these conditions is put'near impossible.

Because you are rectifying output of transformer - probably it is getting saturated by DC current at Secondary.

For such application, the transformer shall be Delta/Zig-Zag Star.

In Zig-Zag Transformer - winding on each limb is divided in two parts.

Each phase consist of two half windings in series from different limbs.

Say for R phase is formed using lower part (near to star point) of Phase R limb and upper part of Phase B limb. the two halves are in series.

The series connections are such that the Load Current (DC current) flows in two parts of same limb in opposite directions. This way it nullifies the saturation effect of DC current (direction of flux due to DC component of load current is opposite).

You can google for Zig -Zag connections of 3 phase transformers or in Wikipedia.

It is very common practice to use Zig Zag connection on output winding of transformers used in arc furnaces and other Rectifier Converters to get rid of core saturation due to DC component of current in load current.