Sets are not self defence.
They are fixed pattern, and serve to train accuracy, positioning, timing and movement.

The skills they teach can be taken into self defence if they manifest under pressure without contrivance.

In practice, the sets are about habit.
They encourage a certain way of moving, a habitual approach to attack.
This is what you take into self defence.

When facing a single tree, if you look at a single one of its red leaves, you will not see all the others. When the eye is not set on one leaf, and you face the tree with nothing at all in mind, any number of leaves are visible to the eye without limit. But if a single leaf holds the eye, it will be as if the remaining leaves were not there.

 (Takuan Soto) 

The conventional use of strength involves the application of force at a given, specific moment in time. If the aim is to break an arm, the individual exerts for a moment and the effect occurs.

The problem with this is that it is tiring. It wears you out.
Exerting the muscles is not very energy efficient because most of the effort accomplishes nothing; it feeds back into you when resistance is encountered.

'Internal power' is altogether different. Exerting never occurs.
The limbs are imbued with strength at all times, so an arm break would be performed with no more force than raising a glass of water to your mouth.

Where does the power come from?

It comes from unifying the body and projecting a wave of kinetic energy (jing) throughout the entire structure as and when necessary.
It is the wave that breaks the arm, not the local muscle strength.

If you lack composure, you will not see what is right in front of you.
Being calm is utterly essential.
You need to remain detached, emotionally aware and at ease.

This is the real reason why white belt syllabus students practice 'escapes'.

As soon as you use tension, you have failed.
Even if you eventually prevail, it has cost you an unnecessary amount of effort and is not tai chi.

Correct use of alignment, timing, pressure and positioning will enable you to skilfully defend yourself.