Isolation transformers and short circuits

Question

For some quick background and info: I'm a relatively fresh (~2y/ experience) EE in the solar field. I work in the R&D department of my company. My country operates in a 230/400V 50Hz grid.

We are currently doing some tests to see electrical equipment reactions to grid failures. We had no problem doing grid disconnection tests, simply breaking contacts on some of or all of the phases and neutral.

Now we want to conduct short circuit tests on that same equipment. The problem for us is that the test station is simply a branch from one of the electrical cabinets supplying parts of our factory. Short circuit conditions may pop the breakers that we really badly want to stay on.

So after a quick discussion with some of our more tenured EE colleagues, a suggestion was made to isolate our test station through an isolation (1:1) transformer.

Now I might be missing something, or misremembering what I learned back in uni, but I am skeptical of this solution.

Let's assume we get that setup, then the question is this:

Once a short-circuit condition is supplied on the circuit that is fed from the secondary winding, we will have a breaker pop, which will stop the short circuit. How does the isolation transformer protect the equipment on the primary side of it? Won't the breakers on the primary side have just as much time to pop as the ones on the secondary? And if so, what would be a better course of action to make sure that the short circuit tests only pop the breakers within our test setup?

Answer 1

Upstream = factory side of transformer.
Downstream = tested equipment side of transformer.

IF the transformer used is rated at far lower short circuit current handling ratings than the factory breaker immediately upstream of it AND at a higher current rating than the equipment breaker downstream of it
THEN the short current mAY be limited by the transformer.

You would need to know how the transformer "looked" up stream with a short applied downstream. A possible problem is if the breaker of the equipment under test has a trip rating too close to the transformer's downstream maximum short current.
eg if the equipment breaker was 100A rated and the transformer SC current was 150A the breaker may not trip or may trrip very slowly. This MAY damage the transformer.

Answer 2

As mentioned an ideal 1:1 isolation transformer won’t solve your problem. It will pass short circuit currents directly.

A real life isolation transformer may solve your problem if its leakage inductance provides sufficient impedance to prevent you from overloading your breaker.

Check the specs of your transformer to verify its short circuit behavior.

It’s also possible your colleagues are recommending a transformer to step up the current and allow higher current testing on the secondary.

Answer 3

So after a quick discussion with some of our more tenured EE colleagues, a suggestion was made to isolate our test station through an isolation (1:1) transformer

An isolation transformer is missing the point. Maybe you didn't properly understand what your more tenured EE colleagues were saying or, maybe you chose the wrong person to ask.

What I would do is use a local breaker/fuse on your test rig. The local breaker would be chosen so that it always tripped before your main building circuit breaker tripped.