Three-level Neutral Point Clamped Voltage Research Articles

Influence of switching sequences on the neutral point voltage balance in a three-level voltage source invertor

BACKGROUND:Ensuring the voltage balance of the neutral point of the DC link within acceptable limits is one of the mandatory requirements during the operation of a three-level autonomous neutral point clamped voltage source inverter. It is known that neutral point voltage imbalance can adversely affect the operation of the inverter and the load at all, lead to the failure of both power switches and capacitors in the DC link. Neutral point voltage imbalance mostly often occurs due to the asymmetry of the nominal values of DC capacitors, inconsistent properties of switching devices, asymmetric three-phase load, and also due to the imperfection of the converter control algorithm.
 AIMS:Selection of the optimal switching sequence that ensures the smallest deviation of the neutral point voltage and the number of switching of power switches in a three-level voltage source inverter.
 METHODS:For an objective comparison of the considered switching sequences and their influence on the neutral point voltage balance, a simulation model of a three-level neutral point clamped voltage source inverter was developed in theMATLAB/Simulinksoftware. To control this inverter, a space vector modulation method was used with three different switching sequences: five-segment, seven-segment and standard.
 RESULTS:The topology of a three-level neutral point clamped voltage source inverter is presented. The experimental dependencies of the neutral point voltage deviation error was taken with a change in the modulation coefficient, the frequency of the fundamental harmonic at the inverter output, and load characteristics for three different switching sequences.
 CONCLUSIONS:In this paper, the influence of the basic vectors and the influence of switching sequences on the neutral point voltage balance are studied. Combinations of states of large and zero basic vectors do not affect the neutral point voltage. For the medium basic vectors, the neutral point voltage can increase or decrease depending on the operating conditions of the inverter. The small basic vectors significantly affect the neutral point voltage. Considering the abovementioned, the seven-segment sequence that ensures both the optimal balance of the neutral point voltage and the level of switching losses should be considered as the best switching sequence for a three-level inverter.

An Improved Overmodulation Strategy for a Three-Level NPC Inverter Considering Neutral-Point Voltage Balance and Common-Mode Voltage Suppression

The three-level neutral-point clamped voltage source inverter (3L-NPC-VSI) is widely used in the maglev traction systems due to its high output voltage, large output capacity and low output current harmonics. In order to improve the utilization of the DC-bus voltage, an overmodulation strategy is necessary. This paper proposes an improved overmodulation strategy based on the minimum amplitude error method for a 3L-NPC-VSI. Compared with the conventional overmodulation strategy based on the minimum amplitude error method, the utilization of the DC-bus voltage is higher. Meanwhile, a virtual space vector modulation strategy is adopted for inverter neutral-point (NP) voltage balance and common-mode voltage (CMV) suppression. Furthermore, the suppression of leakage current also has been verified. Furthermore, the implementation details of the proposed overmodulation strategy based on minimum amplitude error method is elaborated. The effectiveness of the proposed method is verified by simulation and experimental results.

Indirect power control of grid-connected photovoltaic system using fuzzy control with a three-level inverter

This paper proposes an enhanced study of a photovoltaic generator (PVG) connected to the grid. A fuzzy maximum power point tracking (MPPT) is used to extract maximum power. The control strategy adopted gives the possibility to control separately the active and reactive power. the voltage control at the input of the three levels inverter allows fixing the reference currents in the DQ frame to control indirectly the power injected into the grid. The use of a three-level neutral point clamped voltage source inverter (3L-NPC-VSI) with space vector pulse width modulation (SVPWM) control and the RL filter allows to have a quality current and voltage wave with a minimum total harmonic distortion (THD). The simulation results on MATLAB/Simulink confirm the performance of this proposed strategy in the steady-state with high efficiency.

Disturbance Observer Based Sensorless Predictive Control for High Performance PMBLDCM Drive Considering Iron Loss

The control of motor drive without considering iron loss affects the phase back electromotive force and the torque performance. This article investigates the performance of permanent magnet brushless direct current motor (PMBLDCM) modeling taking impact of iron loss into account. A disturbance observer based sensorless scheme is proposed for PMBLDCM model with precise estimation of the rotor angular position. The effect of unmodeled nonlinear parameters on iron loss of PMBLDCM are regarded as disturbances for the proposed sensorless approach. A linear feedback control law with optimal current vector formulation is introduced to minimize the current ripple and torque ripple resulting in reduced copper loss. In order to select optimum switching vector considering the effect of iron loss, a model predictive control technique is proposed for the three-phase three-level neutral point clamped voltage source inverter driven PMBLDCM. The proposed sensorless predictive control (SPC) algorithm is verified both by simulation and experiments. The SPC approach demonstrates improved performance compared to the conventional approach in which the effect of iron loss is not taken into account.

Improved DTC Technique for THL-NPC VSI fed Five Phase Induction Motor Drive Based on VVs Assessment Over a Wide Speed Range.

A three-level neutral point clamped voltage source inverter fed five-phase induction motor (FPIM) drive is assessed by analyzing common-mode voltage (CMV). The presence of CMV leads to motor bearing fault and phase winding insulation failure due to additional voltage stress. To nullify the effect of CMV, this article proposes a modified direct torque control (DTC) based on the virtual vector (VV) concept. The proposed DTC utilizes the appropriate nonvertex voltage vectors to form VVs that neutralize the average volt-second in the $xy$ plane, maintain dc-link capacitor voltage balancing, and limits the switching voltage stress. The theoretical analysis is carried out to investigate the VVs effect on change in flux and torque response. Based on this assessment, a modified lookup table is designed for a wide range of speed operations. For the low-speed operation, i.e., below 25% of rated speed, the small VVs provide better drive performance instead of zero VV. The proposed DTC method is verified at steady-state and dynamic conditions through experimental investigations. Comparative results are provided for the assessment of the proposed DTC method in comparison to existed methods. It is concluded from the findings that the proposed DTC method is a promising approach to improving FPIM performance at high- and low-speed operations with zero CMV output.

A Novel Zero Dead-Time PWM Method to Improve the Current Distortion of a Three-Level NPC Inverter

This paper proposes a novel pulse width modulation (PWM) for a three-level neutral point clamped (NPC) voltage source inverter (VSI). When the conventional PWM method is used in three-level NPC VSI, dead time is required to prevent a short circuit caused by the operation of complementary devices on the upper and lower arms. However, current distortion is increased because of the dead time and it can also cause a voltage unbalance in the dc-link. To solve this problem, we propose a zero dead-time width modulation (ZDPWM) which does not require dead time used in complementary operation. The proposed technique applies the offset voltage to the space vector pulse width modulation (SVPWM) reference voltage for the same modulation index (MI) as the conventional SVPWM, but any complementary switching operation needs dead time. In addition, the proposed method is divided into four operation sections using the reference voltage and phase current to operate switching devices which flow the current depending on the section. This ZDPWM method is simply implemented by carrier and reference voltage that reduce the current distortion, because complementary operation that needs dead time is not implemented. However, the operation section is delayed due to the sampling delay that occurs during the experiment. Therefore, in this paper, we conduct a modeling of sampling delay to improve the delay of operation section. To verify the principle and feasibility of the proposed ZDPWM method, a simulation and experiment are implemented.

Comparative analysis of 3L-NPC converter output voltage with selective harmonic elimination PWM

The main purpose of this article is to show the importance of a comparative analysis of a three-level neutral point clamped (3L-NPC) output voltage with selective harmonic elimination PWM (SHEPWM). As compared to other PWM technique, the SHEPWM method has a lower dynamic power loss and a better THD at the same switching frequency. The objective of this analysis is to provide a modelling results of THD and a voltage/current spectrum of the output voltage with a various SHEPWM implementation for the 3L-NPC converter. A mathematical model for this research was developed in Matlab/Simulink. A practical use of the comparative analysis results can be applied to power electronic systems of high power wind generators.

Control and stability analysis of VSC-HVDC based transmission system connected to offshore wind farm

Offshore wind farms (WFs) with significant capacities have been installed recently all over the world. In order to transmit the WF power to the onshore grid, high voltage direct current (HVDC) transmission system is appropriate technology. This paper analytically studies the impact of system parameters, controllers and operating conditions on the dynamic behavior of HVDC transmission systems based on three-level neutral point clamped voltage source converters (VSC). Also, it investigates modeling, control and stability analysis of VSC-HVDC system connected to the offshore wind farms. The VSC-HVDC system comprises offshore and onshore converters and high voltage dc transmission lines. The paper extracts VSC-HVDC system dynamics at the dc-side and argues the interaction between onshore converter control and HVDC transmission line dynamics. Moreover, the paper presents controller design for the dc-link voltage regulation by the onshore converter and examines the impacts of HVDC line length and dc voltage control bandwidth (BW) on the system stability by the modal analysis and time domain simulations.

Nonlinear Effects of Three-Level Neutral-Point Clamped Inverter on Speed Sensorless Control of Induction Motor

In the model reference adaptive speed observer, the induction motor supply voltage is used as the input of the reference model. However, measuring the supply voltage complicates the system and increases the cost, so the command voltage calculated by the controller is generally used instead of the actual supply voltage in the drive system. However, due to the nonlinear effects of the inverter, the voltage calculated by the controller is different from the actual supply voltage, resulting in a speed observation deviation. This paper analyzes the multiple effects that cause the three-level neutral-point clamped (TL-NPC) inverter output voltage and command voltage deviation. A voltage deviation compensation measure based on the volt-second balance principle is proposed. In this context, the expression of the rotational speed deviation caused by the voltage deviation is derived rigorously and in detail. Finally, the effectiveness of the voltage compensation measure is verified by experiments. The experimental results are basically consistent with the theoretical derivation expressions. The method and analysis in this paper is applicable to induction motor speed sensorless control systems driven by two-level and other multilevel inverters.

Commutation Behavior Analysis of a Dual 3L-ANPC-VSC Phase-Leg PEBB Using 4.5-kV and 1.5-kA HV-IGBT Modules

Three-level neutral-point clamped voltage source converter (3L-NPC-VSC) is widely used in high-power and medium-voltage (MV) applications and the most used multilevel VSC topology. 3L-Active-NPC-VSC (3L-ANPC-VSC) was introduced to overcome the unequal losses distribution as main structural drawback of the 3L-NPC-VSC. During the switching event of semiconductor devices, the stray inductance takes a special role in the commutation path of the switching current. Mutual coupling inductance between the components participating in the commutation path has to be considered for a theoretical assessment of the total stray inductance of each commutation path. Besides, the measurement of the commutation path stray inductance has to be carried out during the turn- on of the semiconductor device, instead of during the turn- off , to consider the reverse blocking voltage of the opposite switching semiconductor device. This paper shows how a dual 3L-ANPC-VSC phase-leg has been carefully designed and assembled to build a ±10 MVAr five-level VSC for MV-static synchronous compensator applications. The minimization of the stray inductance, the equalization of the total switching power losses for all commutation paths and power density maximization with snubberless operation are within the main technical design requisites. On the basis of the theoretical analysis, the stray inductance of all the commutation paths has been extracted, considering mutual coupling effect, by means of three-dimensional finite-element-analysis simulations. Once the design concept is theoretically validated, the estimated values have been compared with the experimental results obtained from the performance of double-pulse tests for each commutation path. This paper presents, for the first time, all the analysis details of the 3L-ANPC-VSC commutation behavior using 4.5-kV and 1.5-kA trench high-voltage insulated-gate bipolar transistor modules.

Long-Horizon Direct Model Predictive Control: Modified Sphere Decoding for Transient Operation

In this paper, we present modifications to the sphere decoder initially introduced in the work of Geyer and Quevedo and modified in the work of Karamanakos et al. that significantly reduce the computation times during transients. The relative position of the unconstrained solution of the integer quadratic program underlying model predictive control (MPC) with respect to the convex hull of the (truncated) lattice of integer points is examined. If it is found that the unconstrained solution does not lie within the convex hull—a phenomenon that is observed mostly during transients—then a projection is performed onto the convex hull. By doing so, a new sphere that guarantees feasibility and includes a significant smaller number of candidate solutions is computed. This reduces the computation time by up to three orders of magnitude when solving the optimization problem at hand. Nonetheless, the reduction of the computational burden comes at a cost of (mild) suboptimal results. The effectiveness of the proposed algorithm is tested with a variable speed drive system consisting of a three-level neutral point clamped voltage source inverter and a medium-voltage induction machine. Based on the presented results, the sphere decoding algorithm with the proposed refinements maintains the very fast transient responses inherent to direct MPC. Moreover, it is observed that the occasional implementation of suboptimal solutions does not lead to a deterioration of the system performance.

Performance analysis of zero common‐mode voltage pulse‐width modulation techniques for three‐level neutral point clamped inverters

This study surveyed three zero common-mode voltage (CMV) pulse-width modulation (PWM) methods for three-phase three-level neutral point clamped voltage source inverters, investigates their performance characteristics, and provides a comparison with the standard three-level and two-level space vector PWM methods. DC-link voltage utilisation, harmonic distortion factor, DC-link current ripple factor and power losses of each PWM method are thoroughly investigated by analysis, simulations and experiments. The research results illustrate the advantages and disadvantages of the compared methods and provide the basis for the selection and application of appropriate PWM methods in low CMV requirement fields.

Common-Mode Voltage and Vibration Mitigation of a Five-Phase Three-Level NPC Inverter-Fed Induction Motor Drive System

Common-mode (CM) voltage is one of the main reasons for the flow of bearing currents in induction motors (IM) that ultimately leads to the bearing failure and causes severe vibrations. For three-phase IM drives, many works have been reported either on the reduction or on the elimination of CM voltages that are generated due to pulse width modulation (PWM) action of the feeding power converters. However, for five-phase IM drives, very few studies are available with the perspective of the CM voltage elimination. Also, the effect of CM voltage on the mechanical vibrations of a five-phase IM has not been reported in the literature. Hence, in this paper, firstly, the modeling of CM current generated in the five-phase IM is done and its expression based on admittance transfer function is derived. Then the CM voltage is eliminated from a five-phase three-level neutral point clamped (NPC) voltage source inverter. The space vector pulse width modulation (SVPWM) technique is developed for the elimination of CM voltages. Only those switching states are selected which give zero CM voltage. The same selected switching states also give the balanced dc link capacitor voltages. Its effect is observed on the mechanical vibration of the motor and it is correlated with the CM current. The mechanical vibration is observed for both, with and without CM voltage elimination. Both the simulation and experimental results are given to validate the concept proposed.

해상풍력단지에서의 PMSG 풍력발전기를 활용한 계통연계점 불평형 전원 보상

This paper proposes a control algorithm for permanent magnet synchronous generators with a back-to-back three-level neutral-point clamped voltage source converter in a medium-voltage off-shore wind power system under unbalanced grid conditions. Specifically, the proposed control algorithm compensates for unbalanced grid voltage at the PCC (Point of Common Coupling) in a collector bus of an off-shore wind power system. This control algorithm has been formulated based on symmetrical components in positive and negative synchronous rotating reference frames under generalized unbalanced operating conditions. Instantaneous active and reactive power is described in terms of symmetrical components of measured grid input voltages and currents. Negative sequential component of AC input current is injected into the PCC in the proposed control strategy. The amplitude of negative sequential component is calculated to minimize the negative sequential component of grid voltage under the limitation of current capability in a voltage source converter. The proposed control algorithm enables the provision of balanced voltage at the PCC resulting in the high quality generated power from off-shore wind power systems under unbalanced network conditions.

계통 불평형시 과도 응답 특성이 개선된 고압 이중여자 유도형 풍력발전 시스템의 제어 전략

【This paper investigates control algorithms for a doubly fed induction generator with a back-to-back three-level neutral-point clamped voltage source converter in a medium-voltage wind power system under unbalanced grid conditions. Negative sequence control algorithms to compensate for unbalanced conditions have been investigated with respect to four performance factors: fault ride-through capability, instantaneous active power pulsation, harmonic distortions, and torque pulsation. The control algorithm having zero amplitude of torque ripple indicates the most cost-effective performance in terms of torque pulsation. The least active power pulsation is produced by a control algorithm that nullifies the oscillating component of the instantaneous stator active and reactive power. A combination of these two control algorithms depending on operating requirements and depth of grid unbalance presents the most optimized performance factors under generalized unbalanced operating conditions, leading to a high-performance DFIG wind turbine system with unbalanced grid adaptive features.】

Minimization of Active Power and Torque Ripple for a Doubly Fed Induction Generator in Medium Voltage Wind Power Systems under Unbalanced Grid Conditions

This paper investigates control algorithms for a doubly fed induction generator with a back-to-back three-level neutral-point clamped voltage source converter in medium voltage wind power systems under unbalanced grid conditions. Three different control algorithms to compensate for unbalanced conditions have been investigated with respect to four performance factors; fault ride-through capability, instantaneous active power pulsation, harmonic distortions and torque pulsation. The control algorithm having a zero amplitude of torque ripple shows the most cost-effective performance concerning torque pulsation. The least active power pulsation is produced by the control algorithm that nullifies the oscillating component of the instantaneous stator active and reactive powers. A combination of these two control algorithms depending on the operating requirements and the depth of the grid unbalance presents the most optimized performance factors under generalized unbalanced operating conditions leading to high performance DFIG wind turbine systems.

Sliding Mode Control of Energy Recovery Transformer-Less Power Accumulator Battery Pack Testing System

The main objective of the present work is to apply sliding mode controller to medium voltage and high power output energy recovery power accumulator battery pack testing systems (ERPABPTS), which is composed of a three-level neutral point clamped three-phase voltage source inverter (NPC-VSI) and a non-isolating DC-DC converter. A decoupled scheme for the proposed system is put forward, based on that, two sliding mode plane for active and reactive current component control are illustrated. The ERPBPTS is used to integrate discharging energy to the power grid with high efficiency when performing current test, active and reactive power references for the grid-connected inverter are determined based on the discharging energy from the DC-DC converter. Without using the isolation transformer, the proposed scheme could greatly reduce the total volume of the system, offer less total harmonic distortion, and meanwhile maintain higher DC-link voltage utilization. The proposed method is implemented and verified both by simulation and experimental results on a laboratory hardware platform using 175kW IGBT-based ERPABTS using an embedded DSP controller

An advanced static var compensator based on a three level IGBT inverter modelling analysis and active power filtering

This paper presents the dynamic performance analysis of an Advanced Static Var Compensator (ASVC) using three-level neutral point-clamped voltage source inverter. The paper presents the principles of operating and the method of reference currents generation. The dynamic behaviour of the system is further analysed using Matlab/Simulink with SimPower Systems toolbox through a set of simulation tests. The results obtained have been applied to an active power filter which might lead to the design of a robust controller for current harmonics and reactive power applications

Voltage stabilisation of SMES VSI DC-link and compensation of reactive power by SMES using optimised switching strategies

This paper presents a novel and optimised switching strategy and control approach for a three-level two-quadrant chopper in a three-level neutral point clamped (NPC) voltage source inverter (VSI) superconducting magnetic energy storage (SMES). Using the proposed switching strategy, the voltage of the inverter capacitors in SMES can be independently controlled. Also, the minimum power and switching losses as well as the proper convection can be achieved using this switching strategy. The simulation results indicate that the proposed switching strategy along with a proportional-integral (PI) control approach can be easily implemented in the power networks, and it can balance and stabilise the capacitors’ voltage level of the multi-level inverters while satisfying the IEEE standard specifications. Considering the fact that the space vector pulse width modulation (SVPWM) is highly effective in decreasing the low order harmonics, in this article, this type of modulation combined with the most optimised switching strategy is utilised. In addition, in this study, a new algorithm is presented for SMES to compensate the voltage sag in the power networks.

Voltage stabilization of VSI SMES capacitors and voltage sag compensation by SMES using novel switching strategies

Abstract This paper presents a novel and optimized switching strategy and control approach for a three-level two-quadrant chopper in a three-level Neutral point clamped (NPC) voltage source inverter (VSI) superconducting magnetic energy storage (SMES). Using the proposed switching strategy, the voltage of the inverter capacitors in SMES can be independently controlled; also, the minimum power and switching losses – as well as the proper convection – can be achieved using this same strategy. The simulation results indicate that when combined with a proportional-integral (PI) control approach the proposed switching strategy can be easily implemented in the power networks and can balance and stabilize the multi-level inverters’ capacitor voltage level. The voltage variation of the capacitors in the steady state condition is less than (0.062%) which is 15 times better than the IEEE standard requirement (1%). To investigate the effectiveness and reliability of the proposed approach in stabilizing capacitor voltage, SMES performance using the presented approach is compared with that of SMES when the capacitors of the three-level inverter are replaced with equal and ideal voltage sources. This comparison is carried out from the power-quality point of view and it is shown that the proposed switching strategy with a PI controller is highly reliable. Considering that the Space Vector Pulse Width Modulation (SVPWM) is highly effective in decreasing low order harmonics (LOH), this article utilizes this type of modulation when it is combined with the most optimized switching strategy. In addition, this study proposes a new algorithm for SMES to compensate the voltage sag in the power networks. Simulation results show that the VSI SMES, when combined with the proposed algorithm, is able to compensate the voltage sag and phase voltage in less than one cycle, which is 5 times better than other voltage sag compensators.