Control Strategies for Four-Leg Inverter Systems

A four-leg inverter adds a neutral leg to the conventional three-phase topology, enabling independent management of zero-sequence voltages and currents. Such systems have become pivotal in applications requiring unbalanced load compensation, renewable energy integration and microgrid stability. A variety of control strategies has been developed to ensure rapid transient response, low harmonic distortion and reliable power sharing. Multiloop configurations typically employ inner current loops to regulate phase and neutral currents, and outer voltage loops to maintain stable amplitude and frequency. Space vector modulation techniques optimise switching patterns, while discontinuous pulse-width modulation schemes minimise switching losses by disabling one leg per switching cycle. Advanced predictive control methods leverage real-time models to anticipate system dynamics and enforce constraints, yielding improved performance under varying load conditions. In grid‐forming and grid‐following modes, droop control facilitates decentralised power sharing among multiple inverters without communication, whereas hierarchical and cooperative controls coordinate groups of converters to balance energy flow and voltage unbalance. Recent innovations include adaptive algorithms that adjust control gains online, decoupled per-phase frameworks that simplify controller design and power decoupling schemes for enhanced resilience against voltage sags. Together, these strategies underpin four-leg inverters’ global deployment in electric vehicle chargers, uninterruptible power supplies and active power filters, ensuring high power quality and system reliability in the face of unbalanced loads and renewables variability.

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Control Strategies for Four-Leg Inverter Systems publication trend

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Technical terms

Four-leg inverter: A power electronic converter with an additional neutral leg to manage zero-sequence currents and unbalanced loads.

Discontinuous PWM (DPWM): A modulation technique that suppresses switching in one leg per cycle to lower switching losses and reduce harmonic distortion.

Space Vector Modulation (SVM): An advanced PWM method mapping voltage reference vectors to optimal switching states to improve waveform quality.

Zero-sequence current: The component of three-phase current that flows through the neutral conductor under unbalanced or single-phase conditions.

Droop control: A decentralised strategy for sharing active and reactive power among parallel inverters by simulating a virtual impedance between frequency and active power.

References

1. A Comprehensive AC Current Ripple Analysis and Performance Enhancement via Discontinuous PWM in Three-Phase Four-Leg Grid-Connected Inverters. Energies (2020).
2. An Overview of Four-Leg Converters: Topologies, Modulations, Control and Applications. IEEE Access (2022).
3. Minimum Loss Discontinuous Pulse-Width Modulation Per Phase Method for Three-Phase Four-Leg Inverter. IEEE Access (2020).
4. SiC-Based Improved Neutral Legs With Reduced Capacitors for Three-Phase Four-Wire EV Chargers. IEEE Transactions on Transportation Electrification (2021).