Which of the harmonics is mostly undesired and must be eliminated? Why 2nd order is not considered as dangerous as the 3rd order?

I read that triplens (3rd, 6th, 9th) are the most dangerous of all because they have zero sequence and don't cancel each other unlike positive and negative sequence that cancel each other and considered insignificant. Is this true? But, on the other hand, as the order increases the impedance of the circuit... show more I read that triplens (3rd, 6th, 9th) are the most dangerous of all because they have zero sequence and don't cancel each other unlike positive and negative sequence that cancel each other and considered insignificant. Is this true?

But, on the other hand, as the order increases the impedance of the circuit increases. this means 3rd order is more severe than 5th order because 3rd order is offered less inductive impedance. Am i right?
If so, why 2nd order is not considered as dangerous as the 3rd order?
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