One does not need to know or find out the breakdown to model a limited range, especially if the goal is not to decide whether the cap is good or not. Note the OP's / topic's limited goal to just find more measurement results on the non-linear leakage behavior vs. some ideas that leakage would be caused by a simple (ideal) resistor with linear response to voltage. (And there is no demand on the voltage range of the measurements, though the larger the better).
Ignoring the breakdown will just result in a model that will not be as useful above the measured range (extrapolation of a model is always a risky thing). Even if the breakdown (or anything else for that matter) would start to affect leakage well below its voltage (or at any range), a simple model vs. measurements might reveal how the model starts to drift at higher voltages (or another range), leading naturally to limit models usable range. Just like every model should list the limitations. (E.g. "Note that this simple model is only valid upto 85% of breakdown voltage, whatever that voltage happens to be.")
Of course, it would make the model better to also study the behavior near and up to breakdown, so as to know how it would affect the model, and especially, how far below the breakdown it has an effect on the leakage current. This is a sort of win-win possibility: if the breakdown affects leakage only near the breakdown voltage, then the bad side is that it can not be predicted beforehand, but the good side is that the simpler model is valid for larger range. On the other hand, if the breakdown affects leakage further below the breakdown voltage, the bad side is the simpler model has lesser useful range, but it may also become possible to predict the breakdown beforehand. It would then depend on how the breakdown affects the leakage, if or how well it could be predicted (with a better model that includes breakdown effects).
EDIT: Personally I'd be happy just for a model that handles the non-linearity below breakdown effects better than the typical "parallel ideal resistor".
A tester must give reliable and reputable results ...
Otherwise, no need to waste time testing the capacitor, just check the circuit correctly, it is sufficient and more efficient than an unreliable test.
The tester must give 100% certainty or at least one result equal to the old high voltage testers ...
If we are not able in 2018, with all the technical means at our disposal, to do better than we did in 1950, we can eat our diploma and change our profession.
That is why I asked to do the tests both at low voltage and at the rated voltage on the damaged (they have been stressed, but are they really damaged ? For exemple, a X2 capacitor is not damaged by surges) capacitors and to see if the results are concordant or not.
Whether or not you can use a low voltage test to detect a faulty capacitor is well a Joeqsmith's goal in this topic, he wrote:
"I can't tell the the good from the bad with my RLC meter. Time to see if my old equipment can detect which ones are damaged using only a 24V DC source."
Can a faulty capacitor be detected using just a 24V power supply?
If you want to use models to predict a capacitor leakage current:
- It is necessary to know up to how much voltage this models can be used , it is an essential information .... Clearly, it is essential to know the breakdown voltage of the capacitor.
In a faulty capacitor, there is no guarantee that the capacitor will be able to block the rated voltage.
- the different types of capacitors certainly will not follow the same model, and there are hundreds of different types of dielectrics.
If it is necessary to wait for the models of all types of capacitors, it will be too early for ever to draw conclusions.