access icon free Mitigation of the low-frequency neutral-point current for three-level T-type inverters in three-phase four-wire systems

© The Institution of Engineering and Technology
Received 18/09/2017, Accepted 12/03/2018, Revised 21/02/2018, Published 12/03/2018

Large electrolytic capacitors are normally applied to maintain a stiff DC-bus in uninterrupted power supply systems. However, the low-frequency currents flow through them, which can reduce their lifespan and risk the system reliability. Therefore, this study investigates the neutral-point current and corresponding suppression scenarios for three-phase four-wire three-level T-type inverters. First, the neutral-point current for three-level T-type inverters is analysed and the mathematical expression is obtained. With the mathematical model, the neutral-point currents in cases of different load conditions are investigated. In order to reduce the neutral-point current and extend the lifespan of DC-bus capacitors, a neutral-point current suppression control strategy is proposed. The basic concept of the proposed control strategy and its effectiveness in cases of different load conditions are presented. Finally, a 30 kW T-type three-level inverter platform is built and the experimental results are presented to verify the theoretical analysis.

Inspec keywords: mathematical analysis; electric current control; invertors; power capacitors

Other keywords: mathematical model; neutral-point current suppression control strategy; low-frequency neutral-point current mitigation; reliability; electrolytic capacitor; DC-bus capacitor; three-level T-type inverter; three-phase four-wire system; power 30 kW; uninterrupted power supply system

Subjects: DC-AC power convertors (invertors); Mathematical analysis; Mathematical analysis; Control of electric power systems; Current control

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