Mitigation of the low‐frequency neutral‐point current for three‐level T‐type inverters in three‐phase four‐wire systems

Abstract

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.

Active DC‐link balancing and voltage regulation using a three‐level converter for split‐link four‐wire system

“…In [13], the authors have presented the method to calculate the filter inductors and capacitors value for the three‐phase split link four‐wire inverters. In [14], the authors have presented the low‐frequency neutral current for three‐level T‐type inverters in three‐phase four‐wire systems. The proposed method reduces the size and improves the lifespan of large electrolytic capacitors.…”

Switching frequency variation based on current ripple analysis of a 3‐L PMSM drive system considering neutral point balance

“…Three-Level (3-L) inverters are widely used in permanent magnetic synchronous motor (PMSM) drives due to less voltage/current harmonics, lower common-mode voltage, and reduced switching loss [1][2][3][4]. Various topologies of 3-L inverters have been developed in the past decades, such as neutral point clamped (NPC), T-type, cascaded H-bridge and flying capacitor, and they have different advantages and applications [5][6][7][8]. Among these topologies, the 3-L T-type inverter has high penetration in the low and medium voltage PMSM drives due to the simple topology and low power loss [9].…”

Neutral point potential balancing method for three‐level power converters in two‐stage three‐phase four‐wire power conversion system

“…Analytic expression of the neutral point current components for the three‐phase four‐wire three‐level neutral point clamped (3P4W‐NPC) inverter at fundamental and triple fundamental frequency is obtained in [6]. Zhang and Ding [7] present the peak‐to‐peak value of the NPP ripple for the three‐phase four‐wire T‐type inverter. However, the analytical expression of other components (besides fundamental and triple fundamental frequencies) in the NPP ripple of the 3LT 2 3L3P4W inverter has not been reported.…”

“…ZSV injection can avoid the distortion of line‐line voltage in three‐level 3P3W inverter, but will result in distortion of phase voltage in 3LT 2 3L3P4W inverter. In summary, the NPP balancing methods for the three‐level 3P3W inverter cannot be applied well in 3LT 2 3L3P4W inverter directly. (2) Three‐level inverter (3P4W) modulation [7, 12–17]. In [13], the modulation sub‐space in a‐b‐c coordinate system is expanded in terms of factor of neutral‐point balance.…”

“…In [13], the modulation sub‐space in a‐b‐c coordinate system is expanded in terms of factor of neutral‐point balance. Methods based on O level disassembling in CB‐PWM are presented in [7, 14–17]. However, the introduction of ‘transient vector’ and O level disassembling lead to an increase of switching times of semiconductor devices in the arms participating in NPP balancing during a switching cycle, which augments the switching losses.…”