Three-level Neutral Point Clamped Converter Research Articles

A review of single stage and multistage power converters for doubly fed induction generator integrated with power grid

The doubly fed induction generator (DFIG) is the most popular induction generator for onshore wind energy conversion system (WECS). Therefore, the purpose of research is to optimize power transfer from DFIG to power grid which requires frequency and voltage stability in power converters. Hence, the single-stage and multistage power converter topologies associated with grid-integrated DFIG are thoroughly examined in this work. In the single stage, a matrix converter and cycloconverter are used. Whereas, the multistage topologies include two-level back-to-back (2L-B2B) converter, Z-source converter, and multilevel converters. Furthermore, three-level neutral point clamped (3L-NPC) voltage source converters with battery energy storage systems (BESS) and modified superconducting fractional order terminal sliding mode controllers (MSTFOTSMC) have been used with multilevel converter both at rotor side converter (RSC) and grid side converter (GSC) for stabilized generated voltage and higher power output. The MATLAB simulation results demonstrate that the system effectively controls fluctuations in voltage and current thereby improving the power quality in unbalanced situations.

A Three-Level Neutral-Point-Clamped Converter Based Standalone Wind Energy Conversion System Controlled with a New Simplified Line-to-Line Space Vector Modulation

Wind power systems, which are currently being constructed for the electricity worldwide market, are mostly based on Doubly Fed Induction Generators (DFIGs). To control such systems, multilevel converters are increasingly preferred due to the well-known benefits they provide. This paper deals with the control of a standalone DFIG-based Wind Energy Conversion System (WECS) by using a three-level Neutral-Point-Clamped (NPC) converter. The frequency and magnitude of the stator output voltage of the DFIG are controlled and fixed at nominal values despite the variable rotor speed, ensuring a continuous AC supply for three-phase loads. This task is achieved by controlling the DFIG rotor currents via a PI controller combined with a new Simplified Direct Space Vector Modulation strategy (SDSVM), which is applied to the three-level NPC converter. This strategy is based on the use of a line-to-line three-level converter space vector diagram without using Park transformation and then simplifying it to that of a two-level converter. The performance of the proposed SDSVM technique in terms of controlling the three-level NPC-converter-based standalone WECS is demonstrated through simulation results. The whole WECS control and the SDSVM strategy are implemented on a dSPACE DS 1104 board that drives a DFIG-based wind system test bench. The obtained experimental results confirm the validity and performance in terms of control.

Quantitative Analysis of Balancing Range for Single-Phase 3L-NPC Converters

Multiple techniques have been suggested to achieve control balance in single-phase three-level neutral-point clamped (3L-NPC) converters. Nevertheless, there is a deficiency of quantitative calculations related to the extent of balancing. Operating beyond the balancing range may result in a sequence of safety incidents. This paper presents a conceptualization of the 3L-NPC converter as two cascaded H-bridges. By employing power conservation principles, the balancing range for the NPC converter is derived, and two novel methods are investigated to broaden the balance range in accordance with the calculated balance range. A comparison is made among the balancing ranges under different balancing control methods. This study establishes a theoretical foundation to ensure the secure and stable operation of the NPC converter.

An Optimal Switching Sequence Model Predictive Control Scheme for the 3L-NPC Converter with Output LC Filter

In some applications of microgrids and distributed generation, it is necessary to feed islanded or stand-alone loads with high-quality voltage to provide low total harmonic distortion (THD). To fulfil these demands, an LC filter is usuallyconnected to the output terminals of power electronics converters. A cascaded voltage and current control loop with pulse-width modulation schemes are used to regulate the voltages and currents in these systems. However, these strategies have some drawbacks, particularly when multiple-input–multiple-output plants (MIMO) are controlled using single-input–single-output (SISO) design methods. This methodology usually produces a sluggish transient response and cross–coupling between different control loops and state variables. In this paper, a model predictive control (MPC) strategy based on the concept of optimal switching sequences (OSS) is designed to control voltage and current in an LC filter connected to a three-level neutral-point clamped converter. The strategy solves an optimisation problem to achieve control of the LC filter variables, i.e., currents and output voltages. Hardware-in-the-loop (HIL) results are obtained to validate the feasibility of the proposed strategy, using a PLECS–RT HIL platform and a dSPACE Microlab Box controller. In addition to the good dynamic performance of the proposed OSS–MPC, it is demonstrated using HIL results that the control algorithm is capable of obtaining low total harmonic distortion (THD) in the output voltage for different operating conditions.

Design and control of utility grid-tied bipolar DC microgrid

Abstract This paper explains in detail the design and control of a utility grid-connected bipolar DC microgrid, which consists of a solar photovoltaic system (SPV), a wind energy conversion system (WECS), a battery energy storage system (BESS) at the DC bus, and a three-level neutral point clamped (NPC) converter to connect the DC microgrid to the utility grid. A three-level bidirectional DC–DC converter at the battery energy storage system (BESS) is controlled with model predictive current control (MPCC), and a three-level NPC converter is controlled with a voltage-oriented control based proportional resonant (VOC-PR) controller. The proposed MPCC scheme for the three-level bidirectional DC–DC converter and the VOC-PR control scheme for the three-level NPC converter coordinate with each other to balance and regulate the voltage at the bipolar DC microgrid, control the power flow to the loads at the DC microgrid and the load at the point of common connection (PCC), and also inject or draw power from the utility grid and BESS based on the availability of Renewable Energy Sources (RES). Simulation and experimental results are presented to demonstrate the effectiveness of the proposed MPCC control scheme.

Capacitor current analysis of a three-level neutral point clamped converter under unbalanced loading conditions

A neutral point clamped (NPC) converter is considered a forefront in industrial applications. Supplying a typically balanced stand-alone load is one of those applications. However, the loading may become unbalanced which can impact the capacitors’ current and voltage ripple. In this work, an approach is proposed that analyze the capacitor current under unbalanced loading. The proposed method is based on a combination of two-dimensional Fourier series and symmetrical components. Since, two-dimensional Fourier series sectorize the spectrum into harmonics that are either defined by multiples of, fundamental, carrier or both frequencies, the method derives the Fourier coefficients for each sector, corresponding to a sequence current. Therefore, based on the presented approach, each harmonic amplitude in the spectrum sector is defined by three Fourier coefficients reflecting zero, positive and negative sequence current. The capacitor current spectrum is obtained by vector summing sequence coefficients. The method is tested on unbalanced load conditions with (out) a ground path. Results verify feasibility of the proposed method in deriving Fourier coefficients of capacitor currents that accurately reflects the loading status of the NPC. This is vital for converter design in terms of proper sizing of DC capacitor and can help in avoiding components failure.

An improved model predictive control of back-to-back three-level NPC converters with virtual space vectors for high power PMSG-based wind energy conversion systems

In this study, an improved virtual space vector (VSV)-based two-stage model predictive control (MPC) scheme is presented for neutral point clamped (NPC) converters in high power-rated permanent magnet synchronous generator (PMSG)-based wind energy conversion system applications. The presented MPC scheme utilizes the VSVs and involves two-stage prediction for effectively reducing computation complexity. In stage-I prediction, a virtual space vector is identified based on the optimum cost function. Then, a set of sub-sector vectors are placed in stage II prediction based on the capacitor voltage levels. From this, the cost function predicts the optimum switching state. At the same time, the conditional selection of small voltage vectors in stage II prediction eliminates the neutral point voltage balancing constraint. Finally, the presented method is verified by the simulation of the back-to-back NPC converter of a high-power-rated PMSG-based wind energy conversion system. Further, the adaptability of the presented scheme for maximum power point tracking, reactive power control, and dc-link voltage control is also verified by simulation.

An Improved Reference Voltage Decomposition Method Based on Three-Level NPC Converters With Neutral-Point Voltage Balancing and Common-Mode Voltage Reduction

In this paper, an improved reference voltage decomposition method (RVDM) that takes account of neutral-point (NP) voltage balancing and common-mode voltage (CMV) reduction for neutral-point-clamped three-level converter (NPC-TLC) is proposed. Based on the analysis of space vector diagram (SVD), CMV is simply reduced by injecting zero-sequence voltages at two-level SVD. Then, according to the principle of adjusting zero-state duty cycles to change the NP current, the switching states of a certain phase or two phases are adjusted from two to three. Therefore, two improved methods that take the balancing control of NP voltage and the reduction of CMV into account are introduced and named method I and method II respectively. Both two proposed improved methods are simple in principle and easy to implement. Considering reliability and output performance, method II is selected as the improved strategy. Finally, the superiority of the improved strategy is verified by experiments, and experimental results are consistent with the theoretical analysis.

Advanced sensorless nonlinear control and stability analysis of single-stage PV systems connected to the grid via a 3L-NPC inverter

This paper proposes a novel control strategy for a single-stage photovoltaic (PV) system consisting of two PV panels connected to the grid via a three-level neutral point clamped converter topology with an LCL filter. This control method is based on a developed 5th order model containing sum and difference terms of the voltages of the two input capacitors. This allows decoupling the issues of maximum power point tracking and power factor correction from the issue of balancing the power exchange generated by the panels, which facilitates control design and improves system performances. The control problem under consideration is dealt with using a non-linear controller composed of three loops: (i) an inner loop is developed, based on backstepping and Lyapunov approaches, to correct the power factor by forcing the grid current to be sinusoidal and in phase with the grid voltage; (ii) an outer loop is designed, using a filtered proportional-integral controller, to regulate the DC bus voltage to a climate-dependent reference; (iii) a balancing loop is designed, using a proportional-integral controller, to cope with the neutral point voltage balancing problem. The proposed controller also includes a state observer that provides on-line estimation of the network state variables that are not accessible to measurements. Another important aspect of this work is the development of a formal, complete and rigorous analysis in order to describe the performance and analyse the stability of the closed-loop system using various analytical tools, including averaging theory, Routh criteria and indirect Lyapunov stability. A simulation in MATLAB/SIMULINK environment shows, on the one hand, the efficiency and robustness of the proposed nonlinear controller against changing climatic conditions and, on the other hand, the superiority of this control strategy compared to the one based on a PI linear inner loop controller for the studied system with an l- filter and an LCL filter.

Three-level ANPC rectifier loss calculation and balance control method

Traditional three-level neutral-point clamped converter has the problem of the uneven loss distribution, which restricts the increase of converter capacity. Three-level active neutral-point clamped (3L-ANPC) converter can reduce the imbalance of power device loss distribution. At present, three traditional SPWM modulation strategies are used in single-phase three-level ANPC modulation. Considering the loss distribution, it is generally required to input the current and voltage sampling values in real-time and calculate the temperature of the switching device online for control or collect the temperature of the switching device in real-time for feedback control, which is more complicated to realize. This paper proposes a modulation scheme based on two carrier cycles, combined with traditional SPWM1 and 2, to analyze the switching conditions and loss characteristics of each switching device in this scheme. The simulation of a single-phase three-level ANPC rectifier was built, and the experiment was carried out on the Yuankuan HIL semi-physical platform. The results show that this scheme is simple, easy to implement, and can effectively reduce the loss distribution imbalance.

Design of a Three-Phase Three-Level Back-to-Back Bridge Interconnection-Based Filter Scheme

This article proposes a systematic design methodology for a back-to-back bridge structure to be used with three-phase bidirectional AC-AC converters (also called an active front end) feeding a motor with long unshielded cables. The bridge provides a path for circulating common-mode (CM) harmonics, resulting in low motor-bearing voltage, suppression of motor winding over-voltage, superior low-frequency conducted emissions attenuation, and wide band performance. The proposed filter structure is designed using simplified sets of derived analytical attenuation pertaining to different groups of harmonics. Consequently, each filter component is optimized for low weight using specific core materials depending on current-ripple content. Following the physical design, parasitic models for the converter, filter, motor, and long cables are developed in the time domain to allow prediction of CM and differential-mode (DM) voltages as well as reducing the filter’s impact on high-frequency emissions. The overall design methodology is validated through emissions characterization using a 12-kW rated three-level neutral point clamped (NPC) converter in a back-to-back configuration.

Power quality enhancement of distribution grid using a photovoltaic based hybrid active power filter with three level converter

This paper employs a second-order sequence filter (SOSF) and proportional–integral (PI) controller to control a hybrid active power filter with photovoltaic (HAPF-PV) through three-level neutral point clamped (NPC) converters. The PV system uses perturbation and observation (P&O) method to track the maximum power point and drive a boost DC/DC converter. A reference signal of a shunt active power filter (SAPF) is generated by estimating the real (not imaginary) term of the polluted load current. The estimation of this term relies on adopting a SOSF and sampling that its input at points the load voltage crosses the zero. SAPF is responsible for compensating grid current harmonics. The suggested approach helps obtain fundamental real term of the polluted unbalanced load current with good accuracy and minimal mathematical computations. In addition to improve power quality, this system can produce green output using the PV arrays connected to the HAPF via a DC-link. HAPF-PV has advantages of power quality enhancement and distributed generation simultaneously. Also, in this system, three-level NPC converters are used in order to reduce harmonic currents injected into the grid by HAPF-PV. Escalating the converter output voltage had led to obtaining output sinusoidal waveform of the converter, and so in this paper, a three-level NPC converter with a neutral point is used to reduce harmonics due to PWM switching. Besides, a series active power filter (SeAPF) is presented for series compensation such as voltage sag/swell. The scheme was implemented in MATLAB considering various types of disturbances including the PCC voltage sag/swell, unbalanced load and changes in irradiance.

Research on Low Switching Loss Control Strategy of High-Speed FESS Based on NPC Three-Level Converter

The high-speed flywheel energy storage system (FESS) has been used in urban rail transit system to provide network stability and regenerative braking energy recovery due to its merits of high-power density, almost infinite charging–discharging cycles, nonexistent capacity deterioration, and environmental friendliness. The electrical fundamental frequency of the FESS motor/generator will be significantly increased when the rotor is mainly composed of carbon fiber composites to achieve higher speed. In addition, in view of the 1500-VDC power supply of the urban rail transit system, the switching loss of the power switching devices will be significantly increased or the system failure may be occurred when the high switching-to-fundamental frequency ratio (SFFR) control strategy is applied. Therefore, a low SFFR <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$=$ </tex-math></inline-formula> 1 control strategy of the I-type neutral point clamped (NPC) three-level converter for the high-speed FESS with brushless direct current (BLDC) motor in two-phase conduction mode is designed, and the matching midpoint voltage balance strategy is proposed. The effectiveness of the proposed control strategy is verified by simulation and experiments. The stable and low switching loss operation of the high-speed FESS rated at 1500 VDC, 330 kW, and 36 000 r/min is realized, and the midpoint voltage balance is strongly controlled.

Reactive Power Circulation Based Fault Tolerance Schemes for Multi-Megawatt 3L-NPC Paralleled Converters in Variable Speed Hydro Applications

Parallel connected multi-MW three-level neutral point clamped power converters (3L-NPC) are widely preferred in large-rated variable speed pumped storage unit as it improves operating efficiency and reliability. During open circuit fault conditions/gate drive malfunction, the entire unit must be shut down to ensure the safety of other healthier components. This article proposes reactive power circulation-based fault tolerance control strategies for grid side converter (GSC) during inner and outer switch fault conditions. In case of an open-circuit fault in outer switches, the normal sinusoidal currents are achieved in GSC by circulating reactive power among the healthy to faulty power converters. In case of an open-circuit fault in inner switches, the reactive power is circulated between healthy converters and a doubly fed induction machine (DFIM). In addition, neutral-point (NP) voltage oscillations are suppressed by injecting a zero-sequence signal into the reference voltage. The feasibility study and performance analysis of the proposed fault tolerance control strategies are carried out in the MATLAB/Simulink 2014a environment for the 250 MW DFIM unit. Furthermore, experimental validation is carried out in 3HP DFIM scale-down laboratory prototype with three parallel connected converters.

Power quality improvement of large power induction motor drive using three-level front-end converter

PurposeThe purpose of this study is to provide the enhancement of power quality of a high power-rated voltage source inverter driven induction motor with a three-phase, three-level neutral point clamped converter placed at the front end, while a passive power filter is connected in shunt with it. The improvement in power quality can be achieved by reducing the total harmonic distortion in source current. The controllers were designed for the linearization of the high-power induction motor drive. A control method is presented for the regulation of the common DC-link voltage.Design/methodology/approachThe induction motor is modeled using its dynamic equations, and a decoupling controller is designed to linearize the nonlinear dynamics of the drive through feedback. The common DC-link voltage of the proposed front-end connected converter is monitored and controlled through a control method which feeds the pulse width modulated inverter that drives the induction motor. A passive power filter is designed to meet the reactive power requirement of the system in addition to improve the power quality.FindingsSimulations were carried out for the proposed topology of the drive mechanism, and the outcomes were analyzed by a comparative analysis of the drive system both in the presence of the passive filter as well as in the absence of the filter. The total harmonic distortion is found to be reduced enough to meet the standards with the designed filter, and the reactive power is also compensated considerably. The input power factor at the supply side is maintained almost to unity, and the DC-link voltage of the proposed circuit topology is maintained at the desired level. The overall performance of the drive system was found to be useful and economical.Originality/valueA new topology of a front-end connected three-level neutral point clamped converter to a high power-rated induction motor drive is proposed. The drive is fed by a pulse width modulated inverter with a common DC-link with the front end connected converter. A passive filter is designed with respect to the reactive power requirement of the system and connected in shunt to the converter at the supply side. Control schemes are designed and used for the drive system and also for the regulation of the common DC-link voltage of the proposed front end connected converter.

MPC-based three-phase unbalanced power coordination control method for microgrid clusters

The microgrid cluster group-level bus voltage is high, the capacity is large, and the interconnection bus is connected to various microgrid subsystems. It is challenging for the microsource inverters in the subsystems to directly participate in the imbalance regulation of the group-level bus voltage, so it is currently difficult to solve the three-phase imbalance of the group-level bus voltage. In this paper, a particular structure of a series microgrid is applied to the cluster system, and a three-phase unbalanced control strategy based on model predictive control for the cluster system is proposed. First, the grid-connected current prediction control model of the series microgrid inverter using an LCL filter is established, a medium-voltage high-capacity three-level neutral point clamped (NPC) power electronic converter is introduced into the system and used as the central inverter of the DC microgrid, and the model prediction control method is applied to this central inverter. Second, considering possible load asymmetry operating conditions of the group bus, the p-q-o method is used to calculate the load imbalance current component that must be compensated, which is used as a part of the output current of the series microgrid or DC microgrid, based on which the three-phase unbalanced power coordinated control method between two heterogeneous microgrids is studied. The simulation results show that the series microgrid and DC microgrid exhibit good three-phase unbalance suppression under load unbalanced conditions, and the coordinated control between them can effectively improve the three-phase unbalance suppression capability of the system, which confirms that the proposed control strategy can achieve high-quality voltage output of the microgrid cluster system in multiple scenarios.

Detailed Power Loss Analysis of T-Type Neutral Point Clamped Converter for Reactive Power Compensation

The paper presents an analysis of power losses in a three-phase T-type neutral point clamped converter with insulated gate bipolar transistors. The paper’s main aim is to perform a detailed analysis of power losses in the converter operating as an active power filter. The study is based on the use of characteristics of semiconductor devices provided by the manufacturer in the module datasheet. Thanks to the analysis, it is possible to recognise the value of power losses, which facilitates the design of the converter cooling system. Identifying how power losses are distributed between the module switching devices is also possible. Power losses are shown as functions of the output current and module temperature. The analysis results were successfully verified by measuring power losses using a laboratory model of the converter with rated currents of 10 and 20 A. The obtained results indicate relatively low power losses and a relatively even distribution of power losses between the semiconductor devices. This is a superior feature of the three-level T-type neutral point clamped converter topology.

Improved Model Predictive Current Control of NPC Three-Level Converter Fed PMSM System for Neutral Point Potential Imbalance Suppression

For the neutral point clamped (NPC) three-level converter fed permanent magnet synchronous motor (PMSM) system, the performance of the conventional model predictive current control (MPCC) algorithm will be deteriorated if the amplitude of the neutral point potential (NPP) is large. Additionally, the adjustment process of the weighted coefficients of the conventional MPCC algorithm is complex because of numerous control terms in the cost function. To solve the above issues, an improved MPCC algorithm is proposed in this paper. Firstly, Newtonian iteration is used to transfer the stator current into stator voltage in the cost function. Then, the NPP term in the conventional cost function can be eliminated by introducing the partition control of the NP potential, which also eliminates the whole adjustment process of weighting coefficients. Finally, based on the amplitude of the NPP, the amplitude and phase angle of medium and small vectors are modified to improve the control performance of the torque and flux. Experimental results show that the fluctuation of the neutral point potential can be suppressed rapidly. Meanwhile, the performance of the torque, flux and current are also improved compared with the conventional MPCC.

An Indirect Model Predictive Control Method for Grid-Connected Three-Level Neutral Point Clamped Converters With $LCL$ Filters

This article presents a model predictive control (MPC) algorithm for a three-level neutral point clamped converter connected to the grid via an <i>LCL</i> filter. The proposed long-horizon MPC method, formulated as a multicriterion quadratic program (QP), simultaneously controls the grid and converter current as well as the filter capacitor voltage, while meeting the relevant grid standards. To achieve the latter, a carrier-based pulsewidth modulation stage is employed. Finally, soft constraints are included to ensure operation of the system within its safe operating limits, particularly with regards to a potential overcurrent or overvoltage trip during transient operation. The presented simulation results based on a medium-voltage system as well as experimental studies based on a scaled-down prototype verify the effectiveness of the proposed method.

Bypass Inductor Type LCL Filter Parameter Optimization for Three-Level Grid-Connected Converter

In the application of the three-level grid-connected system, passive damping is the most common method to keep the LCL filter working stably. However, in the case of high power density, the low switching frequency of the grid-connected converter results in the complexity of filter parameter design, and the increase in passive components leads to lower equipment utilization efficiency. To solve the above problems, based on the optimization of resonant frequency and system loss, this study proposes a set of LCL filter parameter design processes of a three-level neutral point clamped (NPC) converter, which can switch components freely and is easy to achieve. This study explores the theoretical evidence and application value of the proposed design, considering the influence of the current ripple and the reactive power limit. The design adopts the improved passive damping method to select the appropriate inductance ratio and impedance ratio to make the resonance frequency of the whole system and the extra loss of the system smaller. The simulation and experiments show that compared with the conventional method, the improved design method reduces the current THD of the grid side by 1.5% and the damping resistance loss by 0.17%.