Triplen harmonics through Transformer

I have built a model with a three-phase Ynyn transformer and trying to pass through it triplen harmonics (zero sequence). Normally when the neutral of both primary and secondary side are grounded the zero sequence currents should pass from the primary to the secondary side. However this doesn t happen in my case. Three times the line current is dumped to the ground through the neutral at the primary side but this is not getting back to the secondary. Can somebody give me any tip about how to pass triplen harmmonics to the secondary side of a transformer. Thanks!!