Neutral Point Clamped Voltage Source Research Articles

Sensorless speed controller of an induction motor with MRAS-based model predictive control

Speed sensors play critical role in induction motor (IM) control systems. However, the use of these sensors cause several drawbacks such as increasing the overall cost, decreased reliability on control and increased noise problems in data acquisition. The sensorless control methods also have been proposed in control systems of IM to increase the overall performance. On the other hand, model predictive controllers (MPCs) which is one of the most recently used controller methods require the speed measurement to perform speed and torque control of an IM. In addition to control method, the topology of voltage source inverter (VSI) affects the overall efficiency of the IM drive because of its losses and total harmonic distortion (THD) ratios delivered by the topology. The main proposal of this study is to implement an IM drive built with a three-level active neutral point clamped (3L-ANPC) voltage source inverter (VSI) and to propose an MPC that eliminates sensor requirement in speed control of IM. The proposed IM drive takes benefits of multilevel inverter topology, which reduces the torque ripple, enables low switching frequency and provides decreased harmonic distortion. The switching pulses of ANPC VSI are generated by the proposed sensorless MPC method for controlling the torque of IM. The modelling and experimental verification of the ANPC based IM drive have been carried out and the sensorless MPC controller has been validated in this study. There two types of model reference adaptive system (MRAS) have been proposed to ensure precise estimation of IM speed that should be supplied to MPC controller. These MRAS methods are titled as the rotor flux based model reference adaptive system (MRASF), and the stator current based model reference adaptive system (MRASCC) according to estimation references. The proposed MPC-based IM drive has been analyzed under variable speed and torque circumstances to detect the efficiency and reliability of MRASF and MRASCC algorithms. The efficiency indicators such as flux and torque responses, and speed control of the suggested MPC IM drive are validated with experimental analysis. The results of revealed that the IM effectively tracks the reference speed and produces the required torque depending to the precise control of proposed MPC algorithm.

Capacitor sizing of three‐level neutral point clamped voltage source inverter for electric vehicles: Effects of modulation and motor characteristics

AbstractA three‐level neutral point clamped (3L‐NPC) voltage source inverter (VSI) topology can be advantageous in electric vehicles with a high DC‐link voltage and a high switching frequency. A bulky DC‐link capacitor is not an option; thus, the DC‐link capacitor's sizing considering the traction system characteristics is an important design step. This paper investigates how the DC‐link capacitor size of a 3L‐NPC VSI gets affected by the combination of the interdependent characteristics of an electric drive, such as its power factor, modulation index, current, and fundamental frequency, with the effects of the modulation methods. Five pulse width modulation (PWM) methods, of which three of them have an active neutral point potential control, are compared in terms of their neutral point potential (NPP) oscillations. Then, the size of the DC‐link capacitor is determined for each PWM method so that the NPP ripple is kept under desired limits at all operating conditions. It is shown that both the modulation technique and the electric machine characteristics influence the capacitor size. For example, electric machine design modifications can introduce more than a 30% reduction in capacitor size. Finally, DC‐link and NPP oscillations with different PWM methods are experimentally validated in a scaled‐down 3L‐NPC inverter.

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.

Duty-Cycle Correction-Based Model Predictive Current Control for PMSM Drives Fed by a Three-Level Inverter With Low Switching Frequency

In this paper, a duty-cycle correction-based model predictive current control (DC-MPCC) is proposed for permanent magnet synchronous motor (PMSM) supplied by a neutral-point clamped three-level voltage source inverter (NPC-3LVSI). Unlike the conventional MPCC, which evaluates the impact of basic voltage vectors on the concerned state variables, the proposed DC-MPCC modifies the output voltage levels with optimized duty-cycle corrections. Firstly, the last three-phase voltage levels are assumed to be kept during the next control period. Then, the current tracking error and neutral-point potential are predicted. After that, the voltage levels are modified with zero, one, or two state changes, which formulate seven candidate solutions. Subsequently, the duty-cycle corrections of the modified voltage levels are computed based on the principle of minimizing current tracking error and neutral-point voltage drift. Finally, the optimal switch sequence is generated by evaluating and sorting a cost function with a penalty on switch actions. The proposed DC-MPCC features variable switching instants, relatively lower sampling frequency, and satisfactory performance under low switching frequency. Experimental tests carried out on a NPC-3LVSI fed PMSM drive, with 100 Hz fundamental frequency and 400 Hz switching frequency, accompanied by a video demonstration, validate the effectiveness of the proposed method.

Single Loop Voltage Control Strategy Based on Model Predictive Control for NPC-UPS

UPS can provide stable electrical energy for sensitive loads when severe grid disturbance happens. A neutral point clamped (NPC) three-level voltage source converter is used as the circuit topology for UPS. In this study, the discrete mathematical model of UPS is derived and a single voltage loop control strategy based on model predictive control is presented. The proposed method has a simple control structure and a fast response speed and omits the current inner loop and modulator. Furthermore, it can achieve ac voltage control and the balance of dc-side voltages. A three-level UPS simulation model is set up in PSCAD, and the efficacy of the proposed method is verified by means of simulation results under various types of loads.

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.

Performance of Neutral Point Clamped Five Level Inverter Using Space Vector Modulation Control Fed by DPC-VF-SVM Rectifier

The purpose of this paper is to develop a control and regulation method for the input DC voltage of a five level neutral point clamping (NPC) inverter feeding a induction motor. In this context, the authors propose in the first part, the control strategy of three-phase pulse width modulation rectifier, they propose the virtual flux-based direct control (DPC_VF) with vector modulation (SVM), this method is applied for the enhancement of network power quality by compensation of harmonic currents produced by an non linear load, and good regulation of DC-bus voltage. The second part of the paper is dedicated to the presentation of the model of the three phase, five-level NPC voltage source inverter with its space vector modulation (SVM) control method. The performance of the proposed strategy in terms of out-put voltage and THD has studied successfully and shown using MATLAB/Simulink.

Dynamic performance analysis of grid-connected PMSG based on nonlinear control

This paper presents nonlinear control of a Wind Energy Conversion System (WECS) based Permanent Magnet Synchronous Generator (PMSG) and Five-Level Neutral Point Clamped Voltage Source Converter (5L-NPC-VSC). For WECS, the Maximum Power Point Tracking (MPPT) is a very important requirement in order to maximise the effectiveness. Therefore, the MPPT approach and grid-connected control methods are discussed. On the other hand, the proposed control techniques are based on Sliding Mode Approach (SMA) and Vector Control (VC) to achieve the MPPT, to keep the dc-link voltage constant and to the control active and reactive powers. The stability of the regulators is obtained using Lyapunov analysis. An extensive simulation study, using MATLAB/Simulink, is conducted on a 4-MW wind farm composed of two 2 MW PMSG based wind turbines.

State feedback control of advanced static var compensator using a five-level NPC inverter topology

This paper deals with the modeling and control of an advanced static var compensator (ASVC) using a five-level neutral point-clamped (NPC) voltage source inverter (VSI). The nonlinear state space model of the five-level ASVC is obtained from the d-q axis frame. The effectiveness of this compensator highly depends on the choice of the control strategy. The proposed state feedback control (SFC) technique is applied to adjust the ASVC Var flow with the AC transmission network and achieve DC voltage capacitor balance. The dynamic performance of the ASVC based SFC controller is evaluated under several operating conditions. The simulation results demonstrate that the proposed SFC control strategy is highly robust compared to the conventional Proportional-Integral (PI) control.

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.

MPPT Control for Grid Connected Wind Energy Conversion System Based Permanent Magnet Synchronous Generator (PMSG) and Five-Level Neutral Point Clamped Converter

This paper presents nonlinear control of a Wind Energy Conversion System (WECS) based Permanent Magnet Synchronous Generator (PMSG) and Five-Level Neutral Point Clamped Voltage Source Converter (5L-NPC-VSC). For WECS, the Maximum Power Point Tracking (MPPT) is a very important requirement in order to maximize the effectiveness. Therfore, MPPT approach and grid-connected control methods are discussed. On the other hand, the proposed control techniques are based on Sliding Mode Approach (SMA) and Vector Control (VC) to achieve the MPPT, to keep the dc link voltage constant and to the control active and reactive powers. The stability of the regulators is obtained using Lyapunov analysis. An extensive simulation study, using MATLAB/Simulink, is conducted on a 4-MW wind farm composed of two 2 MW PMSG based wind turbines.

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.

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.

Particle Swarm Weighting Factor Optimization for Predictive Control of Three Level Inverter with Balanced Voltages

The paper presents a design procedure of weighting factor optimisation for finite states model predictive direct torque and flux control of an induction machine (IM). The multilevel converter feeding the machine is a three level neutral point clamped voltage source inverter (3LNPC-VSI). The flexibility of the predictive method permits to control simultaneously the torque, the flux and the DC link input voltage of the source. The cost function uses at least one weighting factor adjusted to satisfy desired performance. The chosen value of this factor may not be suited for all ranges of operating points and finding these values are very time consuming and complex. The main aim of this paper is to propose an online optimisation of the weighting factor by the particle swarm optimisation (PSO) approach well known by its robustness and fast convergence to the global optimum. Simulation results show that PSO strategy is very efficient to design accurately and quickly these weighting factors.

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.

Two‐Stage optimization based finite state predictive torque control for 3L‐NPC inverter Fed IM drives

The computational burden in Finite State Predictive torque control (FS-PTC) increases with the number of voltage vectors and number of variables in the cost function. The conventional FS-PTC for three level neutral-point clamped voltage source inverter (3L-NPC VSI) fed motor drive has a disadvantage of long execution time since all available vectors are evaluated in the prediction stage. This paper proposes a new method for reducing the number of voltage vectors from 27 to 17 in the prediction stage, which reduces the computational burden of conventional FS-PTC. The performance of the proposed voltage selection method is investigated for an induction motor drive in terms of electromagnetic torque and stator flux responses (transient and ripple), total harmonic distortion of stator currents, neutral point voltage variations, average switching frequency reduction and robustness of the drive. Experimental results confirm that the execution time is reduced by 30% compared to conventional FS-PTC while similar dynamic and steady-state performances are preserved.

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.