Electrical Engineering

, Volume 100, Issue 2, pp 721–732 | Cite as

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

  • Peng Qian
  • Xiandong Ma
  • Guohai Liu
  • Zhaoling Chen
Original Paper


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.


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



Shunt active power filters


Neutral point clamped


Distributed generation




Total harmonic distortion


Space voltage vector pulse-width modulation


Voltage-dependent NPC three-level inverter


Current-controlled NPC three-level inverter


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


Neutral-point voltage of the three-phase bridge arm


Neutral-point voltage of the DC-link capacitors


Voltage of the primary side of the transformer


Compensating voltage


DC-link voltage applied across the capacitors

\(S_{1}, S_{2}, S_{3}, S_{4}\)

IGBT power switches


Coupling transformer

L, \({C}_{f}\)

LC filter


DC voltage applied to the single-phase full-bridge inverter


Voltage output of the single-phase inverter


Current through the filter inductor


Transformer primary side current


Current through the filter capacitor


Diode used to prevent reverse current flow

\({L}_{2}, {C}_{3}\)

LC filter


Voltage input of the boost DC/DC converter


Voltage output of the boost DC/DC convertor


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


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


Compensating current


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


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


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

\(K_{p}, K_{i}\)

Parameters of the PI controller


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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  • Peng Qian
    • 1
  • Xiandong Ma
    • 1
  • Guohai Liu
    • 2
  • Zhaoling Chen
    • 2
  1. 1.Engineering DepartmentLancaster UniversityLancasterUK
  2. 2.College of Electrical and Information EngineeringJiangsu UniversityZhenjiangChina

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