# Reducing neutral-point voltage fluctuation in NPC three-level active power filters

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## Abstract

Shunt active power filters (SAPFs) have been widely used to improve power quality of the grid by mitigating harmonics injected from nonlinear loads. This paper presents a new method for improving the performance of SAPFs using neutral-point-clamped (NPC) three-level inverters. NPC three-level inverters often suffer excessive voltage fluctuations at the neutral-point of DC-link capacitors, which may damage switching devices and cause additional high harmonic distortion of the output voltage. In order to solve the problem, two compensating schemes are proposed to restrict voltage fluctuation in the inverters. The first is voltage dependent, adopting a voltage compensation method, while the second is current dependent, using a current compensation method. The paper describes the respective circuit architectures and principles of operation. Corresponding models are mathematically formulated and evaluated under typical balanced and unbalanced working load conditions. The results show that both schemes are able to alleviate considerably voltage oscillations and hence harmonic distortions, and the current-compensated NPC inverter outperforms the voltage-compensated NPC inverter. Consequently, it is shown that the proposed approaches are effective and feasible for improving power quality of the grid when connected to nonlinear loads.

## Keywords

Shunt active power filter (SAPF) Three-level inverter Neutral-point-clamped (NPC) DC-link Voltage compensation Current compensation## Abbreviations

- SAPF
Shunt active power filters

- NPC
Neutral point clamped

- DG
Distributed generation

- PV
Photovoltaics

- THD
Total harmonic distortion

- SVPWM
Space voltage vector pulse-width modulation

- VNPCI
Voltage-dependent NPC three-level inverter

- CNPCI
Current-controlled NPC three-level inverter

- IGBT
Insulated gate bipolar transistor

- \(u_\mathrm{sa}, u_\mathrm{sb}, u_\mathrm{sc}\)
Three-phase alternating-current supply

- \(i_\mathrm{sa}, i_\mathrm{sb}, i_\mathrm{sc}\)
Grid currents

- \(i_\mathrm{La}, i_\mathrm{Lb}, i_\mathrm{Lc}\)
Load currents

- \(i_\mathrm{ca}, i_\mathrm{cb}, i_\mathrm{cc}\)
Compensation currents provided by the SAPF

- \(u_\mathrm{o}\)
Neutral-point voltage of the three-phase bridge arm

- \(u_\mathrm{N}\)
Neutral-point voltage of the DC-link capacitors

- \(u_\mathrm{c}\)
Voltage of the primary side of the transformer

- \(u_\mathrm{b}\)
Compensating voltage

- \(u_\mathrm{s}\)
DC-link voltage applied across the capacitors

- \(S_{1}, S_{2}, S_{3}, S_{4}\)
IGBT power switches

*T*Coupling transformer

*L*, \({C}_{f}\)LC filter

- \(u_\mathrm{dc}\)
DC voltage applied to the single-phase full-bridge inverter

- \(u_{i}\)
Voltage output of the single-phase inverter

- \(i_{1}\)
Current through the filter inductor

- \(i_{2}\)
Transformer primary side current

- \(i_\mathrm{cf}\)
Current through the filter capacitor

- \({D}_{1}\)
Diode used to prevent reverse current flow

- \({L}_{2}, {C}_{3}\)
LC filter

- \(u_\mathrm{dc1}\)
Voltage input of the boost DC/DC converter

- \(u_\mathrm{dc2}\)
Voltage output of the boost DC/DC convertor

- \(i_{\mathrm{c3(off})}\)
Current in capacitor \({C}_{3}\) when switching device \({S}_{5}\) is turned off

- \(i_{3}\)
Current in the inductor \({L}_{1}\)

- \(i_{4}\)
Compensating current

- \(u_{\mathrm{L1(off})}\)
Voltage across the inductor \({L}_{1}\) when switching device \({S}_{5}\) is turned off

- \(u_{\mathrm{L1(on})}\)
Voltage across inductor \({L}_{1}\) when switch \({S}_{5}\) is turned on

*D*Duty cycle of the switching device \({S}_{5}\)

- \(K_{p}, K_{i}\)
Parameters of the PI controller

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