Autotransformer with delta tertiary Sub is serving a load though a wye-grounded wye-grounded transformer.  However, the secondary of the service transformer is only loaded on two of the phases.  

After doing analysis of it, the open phase results in an over voltage, and the other two phases result in an undervoltage. Why though?

Is it because the delta source is effectively floating?  Obviously balancing the load would resolve the issue, but what else would? Delta-wye transformer instead? Grounding transformer?

Thanks!