A balanced load which has a grounded neutral point, i.e. with the phase technology 3PH PH-E or 3PH YN, can have a zero sequence current flow. This is obvously the case, if a zero sequence voltage is applied at the load's terminal.

But even without a zero sequence voltage component present at the terminal, a zero sequence current flow can appear, depending on the voltage dependency of the balanced load.

If the load has the behaviour of a constant impedance, the current in each phase is proportional to the applied voltage. If the voltage is unbalanced, but contains only positive and negative sequence components, the current contains only positive and negative sequence components as well.

However, if the voltage dependency of the load is different (for example if the load has a constant power behaviour), a zero sequence current component appears if the voltage contains contains a positive and negative sequence components, even if no zero sequence component is present. This is due to the fact that the current in each phase is not proportional to the applied voltage, in case the load is not a constant impedance.

The attached document describes this fact with equations and vector diagrams.