Three-level NPC Inverter Research Articles

Deadbeat Control with a Repetitive Predictor for Three-Level Active Power Filters

Three-level NPC inverters have been put into practical use for years especially in high voltage high power grids. This paper researches three-level active power filters (APFs). In this paper a mathematical model in the d-q coordinates is presented for 3-phase 3-wire NPC APFs. The deadbeat control scheme is obtained by using state equations. Canceling the delay of one sampling period and providing the predictive value of the harmonic current is a key problem of the deadbeat control. Based on this deadbeat control, the predictive output current value is obtained by the state observer. The delay of one sampling period is remedied in this digital control system by the state observer. The predictive harmonic command current value is obtained by the repetitive predictor synchronously. The repetitive predictor can achieve a better prediction of the harmonic current with the same sampling frequency, thus improving the overall performance of the system. The experiment results indicate that the steady-state accuracy and the dynamic response are both satisfying when the proposed control scheme is implemented.

A Control Method for Active Power Filters with Three-Level NPC Inverter

This paper researches control problem for active power filters with three-level NPC inverter and proposes a novel PI control algorithm for tracking harmonic command current. This novel PI control algorithm can suppress the periodic error in the whole system to achieve zero steady error tracking. In this scheme, the state variables are estimated with a state observer to cancel the delay of one sampling period in this digital control system. Harmonic current is predicted with a repetitive algorithm simultaneity, which makes use of the repetitive nature of load current. The controller is analyzed and designed in the paper, and the experiment results illustrate that this APF can be controlled in a satisfactory way.

Two Different Schemes for Three-Level Voltage Space Vector Generation for Induction Motor Drives with Reduced DC-Link Voltage

In this paper, two different schemes for three-level voltage space vector generation for induction motor drives are proposed. In a conventional 3-phase four-pole induction motor, there exist two identical winding coil groups per phase around the stator, which are connected in series and spatially apart by two pole pitches. In this work, these two identical voltage profile pole pair winding coils in each phase of the induction motor are disconnected and reconnected appropriately to achieve a drive scheme with reduced dc-link voltage. Two different schemes are proposed in the present work. In the first scheme the identical pole phase winding groups are connected in two star groups and fed from the independently controlled (decoupled control) two two-level inverters from both sides of the motor with the same dc-link voltage with half the magnitude compared to conventional NPC three-level inverter. The phase windings are connected in two star groups, so there will not be any path for zero sequence currents. Thereby requirement of isolated voltage sources is eliminated compared to the previous open-end winding multilevel inverter schemes. In the second scheme the identical pole phase winding groups are connected in parallel and fed from the two two-level inverters from both sides of the motor with one fourth the dc-link voltage (compared to the conventional NPC three-level inverter) resulting in a three-level voltage space vector across the motor phase windings. The schemes presented in this paper, does not require any special design modification for the induction machine. For the present study, a four pole IM drive is used, and the scheme can be easily extended to induction motors with more than four poles. The proposed scheme is experimentally verified on a four pole, 5HP induction motor drive.

Research on neutral-point potential balancing and control method for three-level NPC inverter

Research on neutral-point potential balancing and control method for three-level NPC inverter

Improved design of three-level NPC inverters in comparison to two-level inverters

As voltage source 3-level NPC inverters are suitable for AC drives in average voltage and FACT applications in distribution systems, this report introduces optimized design of 3-level NPC inverter ...

Virtual-Flux Direct Power Control for Mains Connected Three-Level NPC Inverter Systems

This paper proposes a control strategy which extends the Virtual-Flux Direct Power Control traditionally employed for the conventional two-level VSI to a three-level NPC inverter. This topology generates a higher number of output voltage levels, increasing the flexibility for selecting an appropriate voltage vector. The mid-point potential is controlled according to the direction of the mid-point current and the sign of the mid-point voltage deviation. The method is adapted to be used with an LCL output filter, where some undesirable characteristics, such as filter resonance, have to be compensated. Further investigation concerning the dependency of the mid-point voltage with the current phase-shift is given. Theoretical analysis is presented and the performance of the proposed method is verified by simulation.

A novel SVPWM strategy considering neutral-point potential balancing for three-level NPC inverter

This paper proposes a novel SVPWM (space vector pulse width modulation) strategy for the three-level neutral-point-clamped voltage source inverter, based on the particular disposition of all the redundant voltage vectors. The new modulation approach shows superior performance for harmonic voltage and balancing control of neutral-point potential compared to the popular eight-stage centered SVPWM. It realizes suppression of inverter neutral-point potential variation by accurately modifying redundant factor of small vectors pairs, only requiring information of DC-link capacitor voltages and three-phase load currents. This is convenient to apply and is compatible of digital computer realization. Feasibility of the proposed control approach is verified by simulation and experimental results.

The nearest three virtual space vector PWM - a modulation for the comprehensive neutral-point balancing in the three-level NPC inverter

This letter presents a new modulation approach for the complete control of the neutral-point voltage in the three-level three-phase neutral-point-clamped voltage source inverter. The new modulation approach, based on the virtual space vector concept, guarantees the balancing of the neutral-point voltage for any load (linear or nonlinear) over the full range of converter output voltage and for all load power factors, the only requirement being that the addition of the output three-phase currents equals zero. The implementation of the proposed modulation is simple according to the phase duty-ratio expressions presented. These expressions are only dependent on the modulation index and reference vector angle. The performance of this modulation approach and its benefits over other previously proposed solutions are verified experimentally.