PI control challenges in 4-leg inverter neutral point

Hello everyone,

I’m working with a 4-leg inverter topology (artificial neutral) supplying unbalanced / asymmetric phase currents.
The objective is to stabilize the neutral point even under intentionally asymmetric phase currents.

System overview:

• 4-leg inverter with active neutral leg
• Neutral controlled via feedback (neutral current / voltage)
• Classical PI controller
• Digital control implementation

Observed problem:

Under unbalanced phase currents, the neutral point shows periodic oscillations at 150 Hz.
Despite extensive PI tuning:

1.The controller either reacts too slowly
2.Or becomes overly aggressive
3.Yet the 150 Hz oscillation persists

Adjusting gains improves performance only marginally. The behavior seems structurally limited, not just a tuning issue.

What was tried:

• Different PI gains
• Integrator limiting
• Increased sampling rate

Oscillatory behavior persists under asymmetric load conditions.

Question:

Is this a fundamental limitation of PI control for stabilizing an artificial neutral in a 4-leg inverter with unbalanced phase currents?

Have you encountered similar behavior or used alternative control structures for this? Any insights would be appreciated.