Three phase modelling of an a.c. transmission system is presented for harmonic penetration studies. Circuit coupling and impedance unbalance are incorporated in a simulation programme which models an 86 bus equivalent of the New Zealand South Island system. A comparison of measured and simulated results at the current injection busbar is used to select system component models; then results of impedances and sequence voltages are presented for selected busbars when the system is subjected to current unbalance and circuit configuration changes.