T-type Neutral Point Clamped Research Articles

Enhancing Wind Power Plant Integration Using a T-NPC Converter with DR-HVDC Transmission Systems

This paper introduces an innovative control technique tailored for the seamless integration of wind power plants (WPP) into onshore grids. Our approach utilizes Diode-Rectifier-based High Voltage Direct Current (HVDC) system and incorporates a T-type Neutral Point Clamped (T-NPC) converter on the HVDC back, providing solutions for current challenges in wind power integration. While the use of Diode Rectifier (DR) as the Offshore converter for HVDC transmission has proven advantageous in terms of cost and loss reduction, it also introduces new challenges, such as maintaining AC grid voltage and frequency on the Offshore side and achieving the synchronization of the wind energy conversion system (WECS). This paper proposes a comprehensive control strategy to adresse these challenges by developing frequency and AC voltage control loops, ensuring the synchronization of WECS with the DR AC side. A notable distinctive feature of our proposed technique is to ensure the regulation of the DC voltage of the HVDC cable, accomplished through the onshore inverter. This crucial aspect ensures stable and efficient power transmission from wind power plants to the onshore grid.

Design of 3-Level T-type NPC Inverter for EV Applications

In recent years, there has been a significant demand for lightweight converters that provide exceptional control performance while generating minimal acoustic noise. This has resulted in higher switching signals for hard- switched 2-level low voltage 3-phase inverters. However, the increased switching frequencies have a negative impact on the inverters’ efficiency, requiring the use of expensive switch technology to maintain high efficiency. This paper aims to tackle these challenges by introducing the design and implementation of a extremely effective 3-Level T-Type Neutral Point Clamped (NPC) inverter designed for Electric Vehicle (EV) applications. 3-Level T-Type NPC inverter associates the strengths of both NPC and T-Type inverters. Similar towards the NPC, it offers a lower breakdown voltage for the switching devices, thereby reducing switching losses. Additionally, it shares the characteristics of the T-Type inverter, resulting in fewer conduction losses. To validate the feasibility of the converter, simulation is performed using MATLAB software, followed by the development and testing of a lab-scale prototype. The proposed three-level T-type NPC inverter have several merits over traditional 2-level inverters, including reduced distortion, switching losses, and improved efficiency. These benefits contribute to enhanced motor drive performance, reduced electromagnetic interference (EMI), and extended battery life in EVs.

Improved Space Vector Based PWM Technique for Three Phase Z-Source Inverter

This article proposes a new space vector pulse width modulation (SVPWM) switching technique for three phase three level Z source inverter (3L-ZSI). The main feature of the proposed maximum boost control technique is that it eliminates the sixth frequency inductor current ripple, which is normally present in the conventional maximum boost control technique. Additionally, the proposed technique facilitates to reduce the switching frequency of the impedance network and common mode voltage, resulting in reduced size of the passive components. The vectors, “- - -” and “+ + +” of three level switching vectors have been implemented after proper modification for the concept of 3L- ZSI. The proposed technique is analyzed with respect to neutral point clamped (NPC), T-type NPC, and controlled diode bridge clamped 3L-ZSI. The efficacy of the proposed technique has been verified by both simulation and experimental results.

A zero‐current‐transition soft‐switching technique for T‐type NPC three‐level inverter

Based on the commutation property of the T-type neutral point clamped (T-NPC) three-level inverter, a novel zero-current-transition (ZCT) soft-switching topology is proposed. The proposed topology employs only two auxiliary switches and one inductor and capacitor (LC) resonant tank on each phase leg. The commutation process of the soft-switching technique is analyzed. The results show that all switches on main leg and auxiliary leg can achieve soft commutation without changing pulse width modulation (PWM) strategy. The control family of T-NPC three-level ZCT soft-switching topology are analyzed respectively, and then the resonant tank and timing schemes of different modes are given. Using two auxiliary switches in resonance can produce greater resonance current due to the combination of external excitation sources, which is more conducive to realize the soft-switching commutation of the main switch. At last, a half bridge prototype of T-NPC three-level inverter is built. The influence of equivalent internal resistance of power switches, diode reverse recovery and stray inductance for soft commutation is analyzed. The advantages of ZCT soft-switching technology is verified.

계통 연계형 3레벨 TNPC 인버터의 LVRT & HVRT

With the increase in renewable power generation facilities, strict regulations on grid connection standards are being presented. The standards of Low Voltage Ride Through(LVRT) and High Voltage Ride Through(HVRT), which are representative grid connection standards to improve grid stability by maintaining grid connection in low and high voltage situations when a grid failure occurs, are gradually being subdivided. In this paper, the LVRT and HVRT control algorithms suggested in the transmission and distribution system connection regulations of KEPCO(Korea Electric Power Corporation) are applied to the grid-connected 3-level TNPC(T-Type Neutral Point Clamped) inverter. And in the VRT situation, the active and reactive current injection strategies were applied to the grid connected inverter algorithm. In addition, various simulation cases were analyzed, such as LVRT & HVRT conditions of transmission and distribution system. In the distribution system LVRT simulation, the grid voltage operated at 150ms and 1500ms, respectively, within 0.5p.u. and 0.75p.u. In the transmission system simulation, the LVRT operation and the HVRT operation were also simulated. By analyzing various simulation cases, the characteristics of the VRT cases was analyzed in detail and VRT control performance of the designed TNPC inverter was verified.

Comparison of T-Type Converter and NPC for the Wind Turbine Based on Doubly-Fed Induction Generator

A three-phase T-type converter topology proposes for wind turbine based on doubly fed induction generator (DFIG). The proposed converter consists of only twelve power switches without flying capacitors or any clamping diodes. The proposed system is implemented to grid side converter of wind turbine. Fuzzy logic controllers are more widely used in wind turbines compared to traditional control systems. Fuzzy logic PI controller has been applied to T-type and the conventional neutral-point clamped (NPC) converters control circuits. Total harmonic distortion (THD) value of current of presented system is measured as 1.76%. Also, DC-link voltage utilization can be maintained. Comparisons are conducted with T- type converter and a 3L-NPC) method under same operation conditions. The proposed scheme has been carried out to verify in simulation results.

Neutral Point Clamped Transformer-Less Multilevel Converter for Grid-Connected Photovoltaic System

Transformer-less (TL) inverter topologies have elicited further special treatment in photo-voltaic (PV) power system as they provide high efficiency and low cost. Neutral point clamped (NPC) multilevel-inverter (MLI) topologies-based transformer-less are being immensely used in grid-connected medium-voltage high-power claims. Unfortunately, these topologies such as NPC-MLI, full-bridge inverter with DC bypass (FB-DCBP) suffer from the shoot-through problem on the bridge legs, which affect the reliability of the implementation. Based on the previous above credits, a T type neutral point clamped (TNP)—MLI (TNP-MLI) with transformer-less topology called TL-TNP-MLI is presented to be an alternate which can be suitable in the grid-connected PV power generation systems. The suggested TL-TNP-MLI topologies free from inverter bridge legs shoot-through burden, switching frequency common-mode current (CMC), and leakage current. The control system of the grid interface with hysteresis current control (HCC) strategy is proposed. The effectiveness of the proposed PV connected transformer-less TNP-MLI topology with different grid and PV scenario has been verified through the MATLAB/Simulink simulation model and field-programmable gate area (FPGA)-based experimental results for a 1.5 kW system.

A Neutral Point Voltage Balancing Scheme With Improved Transient Performance for 5-Level ANPC and TNPC Inverters

In this paper, a neutral point (NP) voltage balancing scheme suitable for three phase 5-level active NP clamped and T-type NP clamped (TNPC) inverters is discussed. Carrier-based pulsewidth modulation is used for the balancing of dc-link capacitors and floating capacitors (FC) in the inverter. The FC voltages are regulated with the help of redundant switching states of pole voltage levels and the dc-link capacitor voltages are controlled using zero-sequence voltage injection. Though the balancing approaches are different for dc-link capacitors and FCs, they are not decoupled from each other. This is due to the effect of certain redundant switching states used for FC balancing causing NP voltage variation. To address this issue, the redundant state time ratio is estimated online every sample and based on this dynamic time ratio, an optimal relationship between neutral current and zero-sequence voltage is established. It is found that the use of dynamic time ratio helps in achieving better transient performance than that of generally adopted fixed time ratio. Simulation and experimental results prove that the proposed balancing scheme works well under all operating conditions and exhibits fast and stable transient performance.

Simplified model predictive control of a three‐phase T‐type NPC inverter

This study presents a simplified model predictive control (SMPC) strategy for three-phase T-type neutral-point-clamped (NPC) inverters to reduce the computational effort while achieving the current control, capacitor voltage equalisation and common-mode voltage (CMV) minimisation targets. The approach involves three stages. In the first stage, the number of switching states is reduced from 27 to 19 for reducing the peak CMV. In the second stage, a graphical approach is proposed to identify candidate control set (CCS) based on the present switching state, which helps in reducing the number of computations further. The number of elements in CCS may be 7, 10, 13 or 19. A look-up table with present applied state and its corresponding CCS is formed based on this graphical approach for all the 19 switching states. The third stage involves modifications to the MPC algorithm. Two versions of SMPC schemes (SMPC-I and SMPC-II) are proposed. SMPC-I can restrict the CMV to one-sixth of DC input voltage. SMPC-II can achieve near zero CMV. The performance of the SMPC strategy is analysed with parameter uncertainty. Computational effort of SMPC schemes is compared with other MPC methods. The feasibility of SMPC algorithm is verified experimentally on T-type NPC inverter prototype.

Hybrid PWM Strategy for Power Efficiency Improvement of 5-Level TNPC Inverter and Current Distortion Compensation Method

This paper proposes a pulse width modulation (PWM) strategy for improving the efficiency of a 5-level H-bridge T-type neutral point clamped (TNPC) inverter. In the case of the proposed PWM strategy, unlike the conventional PWM strategy in which both of the switching legs of the H-bridge inverter operate at a high frequency, one switching leg of the inverter operates at a low frequency. As the switching frequency is lowered, the switching loss is reduced, this improving the efficiency of the system. The duty references for the switching legs and the operating principle of the inverter are described in detail. The proposed PWM strategy, however, causes distortion of the output filter inductor current. The cause of the distortion has been analyzed and a compensation method is proposed to mitigate the distortion of the current. The effect of the proposed PWM strategy can be predicted through the loss calculation of the inverter for each modulation strategy. Furthermore, current distortion mitigation obtained by compensation method is confirmed through the simulation. In order to verify the effectiveness of the proposed strategy, a 2 kW H-bridge TNPC inverter prototype is implemented and tested. The simulation and experimental results show that the efficiency of the inverter is improved when the proposed PWM strategy is applied.

Model Predictive Current Control of a Three-Phase T-Type NPC Inverter to Reduce Common Mode Voltage

This paper presents a reduced control set model predictive control (RCSMPC) method for three-phase T-type neutral-point-clamped (NPC) inverter. The whole control set (WCS) consists of all the 27 switching states of T-type NPC inverter. The reduced control set (RCS) with 19 switching states is formed from WCS by excluding the switching states with common mode voltage (CMV) value higher than one-sixth of input DC voltage [Formula: see text]. With RCS, single-objective model predictive current control method can restrict the CMV peak value to [Formula: see text]. To further reduce the CMV below this threshold, a cost function with the weighted sum of two control targets is formulated in the RCSMPC method. The two control targets of RCSMPC method are CMV mitigation and load current control. The weight for CMV is called bias factor. The RCSMPC method is computationally efficient, as the number of switching states is less than that of WCSMPC. To further reduce the computational burden, CMV values corresponding to all the switching states are calculated offline and stored in memory. Robustness of both the methods is investigated with parameter deviations at different bias factors and reference currents. The proposed method is validated using simulation and experimental results and compared with the existing methods.

Study of Passivity-Based Decoupling Control of T-NPC PV Grid-Connected Inverter

For the purpose of improving the performances of T-type neutral point clamped (T-NPC) photovoltaic (PV) grid-connected inverter under large grid, a passivity-based decoupling control scheme is proposed in this paper. First, the Euler–Lagrange (EL) mathematical model of a T-NPC PV grid-connected inverter with an LC filter is built based on the power circuit and operating principles. Second, with this EL mathematical model and passivity of the inverter, the passivity-based controller is designed by the damping injecting method. Third, by adopting a proportional integral regulator to directly control direct current (dc) voltage, together with passivity-based indirect control, better performances of the dc voltage control can be further obtained. With the proposed passivity-based controller, the PV grid-connected inverter can achieve both dynamic current decoupling under the synchronous rotating dq coordinate and dc side voltage dynamic decoupling, which can improve the quality of the current connected into the power grid. The passivity-based PV grid-connected inverter is with strong robustness against parameters perturbations of resistors and attenuating the influence of delay in the inverter. Moreover, the passivity-based decoupling control scheme proposed in this paper for the T-NPC PV grid-connected inverter has a simple structure, and is easily debugged and realized in engineering. Experimental results under 10-kW prototype show that the proposed control scheme is effective.

Design and Implementation of a Three-Phase Active T-Type NPC Inverter for Low-Voltage Microgrids

This paper presents the design and implementation of a 3 kVA three-phase active T-type neutral-point clamped (NPC) inverter with GaN power devices for low-voltage microgrids. The designed inverter is used in a battery-based energy system (BESS) for power conversion optimization in applications to low-voltage microgrids. A modular design method has been developed for the design and implementation of the AT-NPC inverter. Experimental verification has been carried out based on a 3-kW three-phase T-Type NPC grid-connected inverter. FPGA based digital control technique has been developed for the current control of the three-level three-phase grid inverter. A maximum efficiency of 98.49% has been achieved within a load range from 50% to 75%.

A Dual Three-Level T-NPC Inverter for High-Power Traction Applications

This paper proposes a dual three-level, T-type, neutral-point clamped (T-NPC) inverter fed dual permanent magnet synchronous motor topology for low-voltage, high-power traction applications. The switching pulses of the T-NPC inverters are interleaved and controlled, such that they produce equal and opposite three-phase currents at their output. This maintains balanced dc-link voltages in steady-state and transient conditions, even with slight mismatches between two sets of three-phase loads. Thus, the proposed configuration decouples the modulation strategy from the capacitor balancing control. Simulation and experimental results demonstrate that in comparison to the conventional single T-NPC inverter, the proposed topology reduces dc-link capacitors’ voltage deviation and current stress by more than 90% while avoiding additional switching instances to balance capacitor voltages.

Study on Passivity-Based Control of TNPC PV Grid-Connected Inverter

Euler-Lagrange (Euler-Lagrange, EL) mathematical model was established according to the topology of TNPC (T-type Neutral Point Clamped) PV grid-connected inverter. Based on the mathematical models and passivity-based control theory, the approach called injecting damping was adopted to design the passivity-based controller of the inverter. The passivity-based controller can decouple d-q axis current at AC side of the grid and achieve unity power factor, the total harmonic distortion of grid-connected current is low, and the passivity-based controller also make the inverter have good dynamic and static performances. Simulation results show that the designed passive controller of TNPC PV grid-connected inverter is feasible.