Three-level NPC Research Articles

New strategy for eliminating zero-sequence circulating current between parallel operating three-level NPC voltage source inverters

New strategy for eliminating zero-sequence circulating current between parallel operating three-level NPC voltage source inverters

Hybrid Energy Storage System Microgrids Integration for Power Quality Improvement Using Four-Leg Three-Level NPC Inverter and Second-Order Sliding Mode Control

Rising demand for distributed generation based on renewable energy sources (RES) has led to several issues in the operation of utility grids. The microgrid is a promising solution to solve these problems. A dedicated energy storage system could contribute to a better integration of RES into the microgrid by smoothing the renewable resource's intermittency, improving the quality of the injected power and enabling additional services like voltage and frequency regulation. However, due to energy/power technological limitations, it is often necessary to use hybrid energy storage systems (HESS). In this paper, a second-order sliding mode controller is proposed for the power flow control of a HESS, using a four-leg three-level neutral-point-clamped (4-Leg 3L-NPC) inverter as the only interface between the RES/HESS and the microgrid. A 3-D space vector modulation and a sequence-decomposition-based ac-side control allow the inverter to work in unbalanced load conditions while maintaining a balanced ac voltage at the point of common coupling. DC current harmonics caused by unbalanced load and the NPC floating middle point voltage, together with the power division limits, are carefully addressed in this paper. The effectiveness of the proposed technique for the HESS power flow control is compared to a classical PI control scheme and is proven through simulations and experimentally using a 4-Leg 3L-NPC prototype on a test bench.

High Performance Multistring Converter Topology for Three-Phase Grid Tied 200 kW Photovoltaic Generating System

Under various external conditions, grid connected PV system performance is strongly affected by the topology that is used to connect a PV system with grid. This research aims to design a multistring based converter topology for three-phase grid connected 200 kW PV system that has a high performance in various operating conditions. Research was done by a simulation method using Matlab-Simulink with performance being evaluated including the generated power, efficiency, power quality in accordance with grid requirements, as well as the power flow. In the simulation, multistring converter topology was designed using two dc-dc boost multistring converters connected in parallel to a centralized of three-phase three-level NPC inverter with the size of the string being shorter and more parallel strings as well as the maximum voltage of the PV array of 273.5 V close to dc voltage reference of 500 V. Each dc-dc boost multistring converter have individual MPPT controllers. The simulation results showed that this multistring converter topology had a high performance in various operating conditions. This due to more power generated by the NPC inverter (> 190 kW) at the time of high power generation on the STC conditions (1000 W/m2, 25 oC), the lowest efficiency of the total system is 95.08 % and the highest efficiency of the total system is 99.4 %, the quality of the power generated in accordance with the requirements of grid, as well as the inverter put more active power to the grid and less reactive power to the grid. The response of the inverter slightly worse for loads with greater reactive power and unbalanced.

Harmonic Analysis and Controller Design of 15 kV SiC IGBT-Based Medium-Voltage Grid-Connected Three-Phase Three-Level NPC Converter

Cascaded converters are generally used for medium-voltage (MV) grid-connected applications due to the limitation in the voltage rating of available silicon (Si) power devices. These converters find application in active power filters, STATCOM or as the active front end converters for solid state transformers at the distribution voltage levels. The high voltage wide bandgap semiconductor devices have enabled the grid connected operation of noncascaded converters. This results in high power density, less number of switching devices, and high efficiency for three-phase MV grid interface. This also results in control simplicity without the need for complex dc bus balancing algorithms otherwise needed for cascaded converters. However, such noncascaded, grid-connected converters introduce challenges in maintaining power quality at low currents. This paper investigates the harmonic performance and current distortion of the grid-connected, three-level neutral point clamped converter using 15 kV silicon carbide Insulated Gate Bipolar Transistor (IGBTs). A suitable control scheme for stable harmonic compensation is proposed. The challenges and control performance are explained through frequency domain analysis, simulations, and experimental validation on a developed prototype of the three-phase converter up to 4.16 kV, three-phase MV grid-connected operation.

계통연계형 3-레벨 NPC 인버터의 6차 고조파 제어 기법을 이용한 계통 전류 고조파 저감

계통연계형 3-레벨 NPC 인버터의 6차 고조파 제어 기법을 이용한 계통 전류 고조파 저감

Direct Current Control for Grid-Connected Diode-Clamped Inverters

The accurate control of the line current, robust and dynamic behavior even under distorted grids and other system faults are the most important requirements for grid-connected inverters. Applying direct current control to multilevel inverters combines the advantages of both and is an enabling step towards improved inverter performance. Direct current controllers suffer from high complexity at increased level count, which is resolved by using geometrical principles and simple analytical calculations for switching vector selection within the space vector diagram. The simplified, novel parametric controller concept is scalable to inverters with arbitrary level count, which does not require any switching tables and the new fully symmetric setup guarantees equal switching frequencies among the three phases. The hypotheses are further confirmed on a real hardware test setup on the example of a three-level NPC inverter hardware and a Xilinx development platform. Experimental results prove the expected behavior of the direct current control under various conditions, which shows a general approach to balance the dc-link capacitors of diode-clamped inverters within the theoretical limits and demonstrates the benefits of using field programmable gate arrays as a controller platform.

Multiple Fault Diagnosis for Three-level Inverter Via Wavelet Analysis and Support Vector Machine

Inverter is an important part in the locomotive traction system. In this paper a fault diagnosis method of three-level NPC inverter with two IGBT failures concurrently in different bridges was introduced. With the Simulink toolbox in Matlab, We obtained the output voltages’ difference between faulty and fault-free cases, get Ua、Ub、Uc , d-q transformation on Ua、Ub、Uc , put three-phase into two-phase, Ud and Uq. Making use of wavelet analysis to process Ud and Uq, extracted the fault feature and construct the feature vector. The samples which contain the fault information were accessed finally. With the LIBSVM toolbox, the multi-fault classifiers were established. Finally, the simulation samples were trained and tested. Experimental results showed that the classification result was fine and achieved the desired objective.

Three-Level Inverter-Fed Direct Torque Control of the Synchronous Motor

Synchronous motors and back-to-back converters with IGCT three-level NPC active front end (AFE) rectifiers are widely used in up-to-date large adjustable speed drives. Thanks to the development of the microprocessor technology and power electronics, Direct Torque Control (DTC) became possible and available. The structure and the formation of the switching table for switching devices of the inverter are of a great importance for the DTC system. The quality of the transient, particularly, fast response and precision of the motor variables control, depends largely on the table structure and formation. The paper describes the main principles of the inverter control. Twenty-four voltage vectors were divided into seven different variants. These variants can be represented in the form of two adjacent groups; each group contains two variants of short vectors, one variant of long ones and middle vectors are common for the both groups. As a result of the conducted research, a switching table was formed where the switchover from one variant of voltage vectors to the other one is provided by the switching of one switching device only. The simulation results obtained in the Matlab Simulink environment confirmed the efficiency and acceptable characteristics of the transients quality of the formed switching table. During the steady state operation the main control variables, such as torque and stator flux, are maintained with an error which does not exceed 5 %.

A Comparative Study Between the Nearest Three Vectors and Two-Level Hexagons Based Space Vector Modulation Algorithms for Three-Level NPC Inverters

Multi-level power converters are nowadays the most appropriate solution for medium-voltage grid-connected Renewable Energy Sources (RES) such as variable speed wind-turbines and photovoltaic (PV) generators. Basically, there exist two space vector modulation (SVPWM) schemes which are commonly used to modulate the output voltages provided by three-level inverters . For the first method, namely the nearest three vectors (NTV), the position of the space vector is identified in one of the triangles forming the space vector diagram. As for the second method that is referred to as the hexagon technique, the opportune position of the space vector is determined within a two-level hexagon. This paper is therefore focused on a comprehensive comparative study between the performances of the two aforementioned techniques. The evaluation is performed in terms of Common Mode Voltages (CMV), Total Harmonic Distortion (THD), conducted emissions, and efficiency. The proposed comparison is based on numerical simulations and experimental tests. The obtained results confirm that the NTV technique provides the better performances in terms of THD and CMV for low modulation index values. It also provides less common mode emissions when the modulation index is near its maximum value. As for the efficiency, both methods provide the same performance for a modulation index superior or equal to 0.8.

Finite-State Predictive Torque Control of Induction Motor Supplied From a Three-Level NPC Voltage Source Inverter

Finite-state predictive torque control (FS-PTC) of an induction motor (IM) drive has been widely investigated for two-level voltage source inverter recently. This control method suffers from high and variable switching frequencies in a wide range, due to the limited number of available voltage vectors of the power converter. In order to take advantage of multilevel inverter drives which offer the benefits of low harmonic distortion of the stator currents, torque ripple, and low switching frequency, this paper proposes the integration of the FS-PTC with a three-level neutral-point clamped (3L-NPC) inverter driven IM drive. The drawback inherited from the topology of 3L-NPC voltage source inverter (VSI), such as neutral-point voltage, is easily handled by treating it as a variable to the cost function. Similarly, apart from the inverter topology itself, the average switching frequency is reduced further, and is maintained almost constant over a wide speed range. The effectiveness of the proposed FS-PTC in terms of torque and flux responses, capacitor voltage balancing, and low average switching frequency is validated through experimental results.

A Method for the Suppression of Fluctuations in the Neutral-Point Potential of a Three-Level NPC Inverter With a Capacitor-Voltage Loop

This paper investigates the problem of fluctuation of the neutral-point potential (NPP) in a three-level NPC inverter with a capacitor-voltage loop. The phase pulse width duty cycle disturbance pulse width modulation (PWM) method is proposed to suppress the NPP fluctuation efficiently. Based on the basic carrier-based phase-disposition PWM method, the average pulse neutral-point current model is established. Then, the frequency, amplitude, and equivalent initial phase of the NPP fluctuation are analyzed based on the current model. According to the alternating error of the dc-link capacitor voltages, a capacitor-voltage loop with a quasi-proportional resonant (PR) controller is presented. The control variable, which varies with the modulation index, phase current, load power factor, etc., can be obtained from the quasi-PR controller. Finally, an experimental three-level NPC inverter is described and the validity and feasibility of the proposed method are verified by experimental results.

Space Vector Modulation Technique for 3-Level NPC Converter with Constant Switching Frequency

This paper presents a simple Space Vector Modulation (SVM) methodology for a three-level NPC converter. Nearest three vectors (NTV) and corresponding duty cycles are deduced through simple generic mathematical expressions. Extra degrees of freedom of NPC converter are used to fully benefit from SVM advantages and to control the switching frequency. Simulation and experimental results are presented and discussed to validate the proposed methodology.

New Operation Strategy for a Grid-Connected Three-Phase Three-Level NPC qZS Inverter Based on Power Losses

This paper proposes a new operation strategy for a three-phase three-level neutral-point-clamped quasi-impedance source inverter in a grid-connected mode. The strategy is intended for any inverter based on impedance source networks where the shoot-through states allow a wider input operation range. This feature is commonly requested in photovoltaic applications due to variations in the level of irradiance. Converters of that family have different control variables or degrees of freedom that can be used to boost the input voltage. In the first step, focus was on possible control variables for the design of the control strategy to regulate the active/reactive power. We used a power losses analysis to determine the most suitable variables in each input voltage interval. Our detailed description for the design of the control strategy is based on the well-known relationship in an electrical system between P - d and Q - V . Good performance of the full system was validated by simulations. DOI: http://dx.doi.org/10.5755/j01.eie.22.3.15316

Virtual flux direct power control of the three-level NPC shunt active power filter based on backstepping control

In this paper, a novel control strategy of multilevel shunt active power filter is presented. The idea behind the proposed control scheme is to combine the backstepping control and direct power control with virtual flux concept into one controller able to improve the performance of active power filter. The choice of the multilevel topology leads to a clear enhancement in the compensation of the polluting load harmonics. To take full benefit of this choice, the task of balancing voltages of dc-link capacitors is assigned to the multilevel space vector modulation with balancing ability. For this reasons, the adopted multilevel active power filer is proposed to reduce harmonic currents flowing into the power system and to draw a nearly sinusoidal current with unity power factor. Complete simulation of the resultant active filtering system validates the efficiency of the proposed backstepping control law. Compared to the traditional control, the used of backstepping control allows to exhibit excellent transient response during large load variations.

Comparison Study for SVPWM based CMV Elimination between Two- Level Inverter, Three-Level NPC Inverter and Dual Two-Level Inverter fed Induction Motor

Comparison Study for SVPWM based CMV Elimination between Two- Level Inverter, Three-Level NPC Inverter and Dual Two-Level Inverter fed Induction Motor

스위치 손실 감소를 위한 단상 3레벨 NPC 인버터의 새로운 스위칭 방법

This paper proposes a method of switching to improve power loss for the single-phase three-level NPC inverter. The conventional switching methods, which are called as the bipolar and unipolar switching methods, are used for single phase inverters using three-level topology. However, these switching method have disadvantage in the power loss. Because all of the switch are operated. To reduce the power loss of the three-level NPC inverter, clamp switching method is introduced in this paper. This way, one of the lag is fixed that switching loss is reduced. This paper analyzes and compares power losses of unipolar method and clamp method. The validity of the power loss analysis is verified through the simulation and experimental results.

Carrier Phase-Shift PWM to Reduce Common-Mode Voltage for Three-Level T-Type NPC Inverters

Common-mode voltage (CMV) causes overvoltage stress to winding insulation and damages AC motors. CMV with high dv/dt causes leakage currents, which create noise problems for equipment installed near the converter. This study proposes a new pulse-width modulation (PWM) strategy for three-level T-type NPC inverters. This strategy substantially eliminates CMV. The principle for selecting suitable triangle carrier signals for the three-level T-type NPC is described. The proposed method can mitigate the peak value of CMV by 50% compared with the phase disposition pulse-width modulation method. Furthermore, the proposed method exhibits better harmonic spectrum and lower root mean square value for the CMV than those of the reduced-CMV method on the basis of the phase opposition disposition PWM scheme with modulation index higher than 0.5. The proposed modulation can easily be implemented using software without any additional hardware modifications. Both simulation and experimental results demonstrate that the proposed carrier phase-shift PWM method has good output waveform performance and reduces CMV.

Comprehensive predictive control of neutral-point voltage balancing for back-to-back three-level NPC converters

A comprehensive predictive strategy was proposed for the neutral-point balancing control of back-to-back three-level converters. The phase currents at both sides and the DC-link capacitor voltages were measured for the prediction of the neutral-point current. A quality function was found to balance the neutral-point, and a metabolic on-times distribution factor was used as a predicator to minimize the quality function at each switching state. Simulation results show that the proposed method produces smaller ripples in tested signals compared with the established one, namely, 9.15% less in a total harmonic distortion (THD) of line-to-line voltage, 1.08% less in the THD of phase current, and 0.9 V less in the ripple of the neutral-point voltage. The obtained experimental results show that the main harmonics of the line-to-line voltage and the phase current in the proposed method are improved by 10 dB and 6 dB, respectively, and the ripple of neutral-point voltage is halved compared to the established one.

Decoupling Control Strategy for Conergy Three-Level Three Phase Grid Converter

Conergy three-level NPC topology is often used in grid connected inverter due to its high efficiency, fewer switching devices and better EMI performance. Traditional half-bridge topology can be transformed to Conergy NPC topology by adding bi-direction switches. This paper proposed a novel control strategy for Conergy three-level NPC three phase grid converter with real and reactive power closed-loop controller in the synchronous d-q rotating frame. The power control loop is based on the feed forward decoupling of d-axis and q-axis components of the output current, thus the active and reactive power can be controlled separately. Additionally a proportional-integral controller is added to enhance the robustness of the power controller. Simulation results verify the effectiveness of the proposed strategy.

3-레벨 NPC 인버터에서 클램핑되지 않는 스위치 전압의 간단한 보상기법

3-레벨 NPC 인버터에서 클램핑되지 않는 스위치 전압의 간단한 보상기법