Transformer Overexcitation and the 5th Harmonic

[flickstar]

Hey Guys,

Can anybody please offer an insight into the relationship between transformer overexcitation and the 5th harmonic? I know that transformer overexcitation produces odd-order harmonics but can offer no explanation as to why. Also, why the 5th harmonic? - is this simply because it is the lowest order odd-harmonic that is not a multiple of 3?

Any assistance in broadening my understanding is greatly appreciated.

 

[busbar]

Excerpt from IEEE Std C37.91-2000 §6.2.8 Effect of overexcitation on differential relays

When the primary winding of a transformer is overexcited and driven into saturation, more power appears to be flowing into the primary of the transformer than is flowing out of the secondary. A differential relay, with its inputs supplied by properly selected CTs to accommodate ratio and phase shift, will perceive this as a current differential between the primary and secondary windings and, therefore, will operate. This would be an undesirable operation, as no internal fault would exist, with the current imbalance being created from the overexcitation condition…

Since overexcitation manifests itself with the production of odd harmonics, and since the third harmonic (and other triplens) may be effectively cancelled in ? transformer windings, use the level of fifth harmonic as a restraining quantity in the differential relay…