Dc-bus Capacitor Voltage Research Articles

Three-Phase Open-End Induction Motor Drive Interfaced to Higher Voltage DC Bus

In applications that require a three phase induction motor to be driven by an inverter from a DC bus input of much higher voltage than needed, the option is to use a step down DC-DC converter before the inverter or use high voltage rating switches in the inverter along with low modulation index. In either case, disadvantages include poor efficiency, high voltage rated switches, etc. In open end winding three phase induction motor drives, the three separated windings are driven by 3-phase inverters from both sides. The arrangement is equivalent to three 1-phase inverters driving each winding separately from the same DC bus in parallel. This paper proposes an inverter scheme for open end induction motor drives, using a 1-phase inverter for each winding supply, but all three inverters being connected in series by their DC bus to the higher applied DC voltage. Unlike existing techniques for interfacing an induction motor drive to a higher voltage DC bus, the proposed scheme requires lower voltage switches with lower voltage bus capacitors to drive the lower voltage motor from a higher voltage DC bus. The series connected DC bus capacitor voltages are shown to remain balanced during normal operation.

Sensorless integral backstepping control of a single-stage photovoltaic system connected to the single-phase grid using a multi-level NPC inverter with an LCL filter

In this paper, the control problem of a single-phase, grid-tied photovoltaic (PV) system consisting of a three-level neutral point clamped (3L-NPC) inverter and an LCL filter is considered. The proposed controller aims at achieving threefold control objectives: (i) guaranteeing the PF correction (PFC) objective by enforcing the current of the grid to track a sinusoidal reference signal in phase with the grid voltage; (ii) controlling the DC bus capacitor voltages to follow a reference value provided by the maximum power point tracking (MPPT) algorithm to achieve the maximum available PV power; (iii) ensuring a balanced power exchange through the regulation of the two input capacitor voltages to the same values. To achieve these aims, a multi-loop nonlinear controller is synthesized, including three control loops, specifically the inner PFC requirement loop designed through the integral backstepping approach, the outer voltage control loop formulated using a filtered proportional integral (PI) regulator, and the power balance loop built up using a PI regulator. The control problem under consideration entails several difficulties as the existence of numerous state variables that are inaccessible to measurements. This challenge is addressed by adding a high-gain observer to estimate the current of the solar panels. The performance of the proposed controller against climate changes is demonstrated via numerical simulations using the MATLAB/Simulink platform.

Three-Level NPC Inverter-Fed IM Drives under PTC, Minimizing the Involved Voltage Vectors and Balancing the DC Bus Capacitor Voltages

The paper presents a comparative study of the steady-state and transient behaviors of three-level neutral-point clamped (3L-NPC) inverter-fed induction motor (IM) drives under the control of three predictive torque control (PTC) schemes: the conventional one (C-PTC) and two new strategies involving selected stator voltage vectors (SVs), namely: (SV-PTC1) and (SV-PTC2). Compared to the C-PTC, the latter enable a reduction of the number of voltage vectors as well as the weighting factors. The introduced PTCs mainly differ by the cost function, which is more simple in the case of SV-PTC2. However, SV-PTC1 allows a systematic clamping of each stator phase to the DC bus voltage, at a low level of 60 degrees, and a high level at 60 degrees per cycle, while such a clamping is arbitrary in the case of SV-PTC2. Simulations prove the higher performances of the introduced SV-PTCs over the C-PTC ones, in terms of the reduction of the current distortion and of the torque ripple. Simulation results were validated by the experiments.

Performance of Reduced-DC-Source-Based Three-Phase High-Resolution Multilevel Inverter With Optimal Asymmetry

Asymmetrical cascaded bridge multilevel converters use voltage sources of different magnitudes to generate a higher number of levels. However, the availability of sources of different ratings feeding the bridges is a limitation. This article overcomes such a limitation by utilizing the concept of capacitor voltage balancing using the level doubling network (LDN) principle in the space-vector plane. With the use of optimal asymmetry, the presented topology can generate 43 levels at the output line voltage by using only 12 switches per phase. Hence, the most notable attribute of the presented topology is that it does not require isolated auxiliary power supplies to develop such a high-resolution output voltage. LDNs in this topology are self-balanced, and the auxiliary full-bridges require a closed-loop controller to balance their dc bus capacitor voltage. To validate the proposed concept, MATLAB simulations and experimental results are presented in this article.

A Cascaded Controller for a Grid-Tied Photovoltaic System With Three-Phase Half-Bridge Interleaved Buck Shunt Active Power Filter: Hybrid Control Strategy and Fuzzy Logic Approach

This paper focuses on the control development of a grid-tied photovoltaic system coupled with a shunt active power filter. The considered power system includes a PV panel, a single-capacitor three half-bridge interleaved buck converter connected to a three-phase power grid and a nonlinear load. This study aims to accomplish simultaneously three objectives: i) to ensure a high compensation level of current harmonics and reactive power consumed by the nonlinear load in order to perform power factor correction at the grid side; ii) to extract the maximum power from the PV panel regardless the climatic conditions; iii) to regulate the dc bus capacitor voltage to its reference provided by the MPPT controller. To this end, a controller using a two-loop cascade strategy is designed. First, the inner loop is developed with the hybrid automaton approach to formulate the switching control signals with the aim of guaranteeing a unitary power factor. Second, the outer loop is designed using fuzzy logic control to ensure the regulation of the photovoltaic voltage, while the perturb and observe algorithm is employed to perform maximum power point tracking. Finally, the system with the developed control scheme is simulated using MATLAB/SimPowerSystems environment. The obtained results confirm that the proposed controller is able to achieve the previous objectives under varying environmental conditions.

Advanced Switching Sequences Based Model-Predictive Control for Single-Phase NPC Converters

This article presents a continuous control set model-predictive control (MPC) technique based on advanced switching sequences (SSs) for a grid-connected single-phase three-level neutral-point-clamped converter. The SSs are developed considering the tracking performance of the current feed to the utility grid, the dc-bus capacitor voltage balancing, and the switching loss of the semiconductor devices. In the proposed strategy, with the same sampling frequency, the number of pole state changes (SCs) per control cycle is only two, while in the optimal SS and the modulated MPC (pulse pattern similar to the conventional space vector modulation technique) schemes, the number of pole SCs is four. The reduction in the number switching SCs per control cycle considerably reduces the switching loss of semiconductor devices. Moreover, to avoid the challenges of utilizing the weighting factor, a single-objective cost function is adopted to track the reference current, and the dc-bus capacitor voltages are balanced using the redundancy nature of the voltage vectors. Furthermore, the proposed scheme can ensure a fixed-switching-frequency operation, which is advantageous for selecting filter parameters during real-life implementation. The effectiveness of the proposed strategy is validated by the results obtained from both the simulation in MATLAB/Simulink and the experiment carried out using a downscaled laboratory test rig.

Evaluation and Analysis of Soft Computing Techniques for Grid Connected Photo Voltaic System to Enhance Power Quality Issues

The major problem in grid connected transmission and distribution networks are the power quality issues due to the unbalance loads. The power quality issues relate to the current and voltage based issues generated at the point of common coupling. Some of the current based power quality issues relate to reactive power extraction, poor zero voltage regulation, harmonics in the load currents undesired neutral current and power factor and. Several DSTATCOM compensation techniques has been introduced for compensation of reactive power and power quality issues in the three phase, four wire system. The proposed system focus on DSTATCOM soft computing techniques such as: Widrow–Hoff least mean square, vectorial filter, discrete adaptive filter and quasi Newton control algorithms. The proposed algorithms controlled the switches pulses to drive the voltage source converter section in the DSTATCOM network thereby mitigating power quality issues and reactive power generation in the grid connected photovoltaic system. The soft computing techniques has maintained constant DC bus capacitor voltage and common coupling voltage along with unity power factor, zero voltage regulation under linear and non-linear load conditions, eliminating the harmonics in the load currents. The proportional integral controller is used for error correction between the reference and capacitor voltage. The novel drift-free maximum power point technique has shown better results under varying temperature and irradiances conditions. The DSTATCOM soft computing techniques has been implemented and simulated in MATLAB/SIMULINK.

A Single Phase, Single Stage AC-DC Multilevel LLC Resonant Converter With Power Factor Correction

Single stage LLC resonant converters with inherent power factor correction are getting popularity in AC-DC converters due to its reduced size and weight. However, single stage topologies are usually less efficient in regulating the dc bus capacitor voltage pertaining to line and load transients. This paper proposes a multi-level flying capacitor based single stage AC-DC LLC topology to address the issue of voltage balancing of dc-bus capacitor and to reduce the voltage stress of the switching devices. The proposed three-level inverter topology guarantees zero voltage switching, less circulating currents, reduced switching stress and losses. The converter uses bridgeless rectification scheme for better efficiency and the power factor is made nearly unity by operating the source-side inductor in discontinuous current conduction. Variable switching frequency control is used to regulate the output voltage of the converter and pulse width modulation is used to control the dc-bus voltage. This dual control scheme is very effective to keep the dc-bus voltage nearly constant over a wide range of line and load variations. The proposed topology and control scheme have been validated by hardware results on a 250W resistive load.

DC-Bus Capacitor Maximum Power Discharge Strategy for EV-PMSM Drive System With Small Safe Current

To prevent the passengers from electric shock, the DC-bus capacitor voltage of the high-voltage permanent magnet synchronous machine (PMSM)-based drives in electric vehicles (EVs) is required to decline to safe voltage as quickly as possible when emergency occurs. Considering that the discharge time for small safe current limit powertrain systems will be longer than required, this paper proposed a maximum power discharge strategy to accelerate the dissipating process and meanwhile avoid the voltage surge. Firstly, the PMSM discharge model as a generator is established on the basis of analyzing the discharge course. Secondly, the feasible reference current trajectory is presented by drawing the trajectories of the voltage, current, and power constraints. Then the maximum power discharge strategy is achieved by following the extreme points in the reference trajectory. Finally, simulation and experiment are conducted on a three-phase SPMSM powertrain system to validate the proposed algorithm can effectively shorten the discharge process.

Robust Control of Winding-Based DC-Bus Capacitor Discharge for PMSM Drives in Electric Vehicles

Active discharge circuit is of great significance for discharging the dc-bus capacitor voltage to safe voltage in the electric vehicles (EVs) based PMSM drive system when EVs encounter an emergency such as a crash event. However, when electric vehicle occurs a collision, the active discharge circuit may be damaged or failed, which poses a great electric threat to the passengers and succors. In order to resolve the problem, this article proposes a winding-based dc-bus capacitor discharge method. To quickly discharge the dc-bus capacitor energy, the machine windings are used as discharge resistance with active discharge circuit fault. For the purpose of reducing the bus voltage to safe voltage as soon as possible, the discharge method can be divided into two phases, rapid discharge stage and bus voltage regulation stage. Firstly, the dc-bus voltage will be dropped to safe voltage due to the large flux-weakening current applied to the d-axis. Secondly, in order to stabilize the bus voltage at safe voltage, an extended observer state (ESO) is designed to estimate and compensate the total power loss that is regarded as disturbance in bus voltage regulation stage. The parameters of the proposed ESO are designed and the tracking performance is analyzed. Finally, the proposed discharge technique is validated by both simulation and experiments, which are conducted on a three-phase SPMSM drive platform.

An Improved Space Vector Modulation With Capacitor Voltage Control for T-Type Five-Level Nested Neutral Point Piloted Converter

In this article, a novel control method is proposed to control capacitor voltages for the T-type five-level nested neutral point piloted (5L-NNPP) converter. Due to the diversity of switching states for NNPP topology, there is no guarantee that the redundant vectors have the opposite effect on the dc bus neutral voltage. Meanwhile, the voltage balance controls between the dc bus neutral voltage and three-phase flying capacitor voltage are coupled and interacted with each other in the 5L-NNPP converter. This has severe voltage balance problems with the conventional space vector modulation (SVM), which limits its wide applications, especially under low output frequency. To solve the voltage balance problems, an improved SVM and voltage balance control (VBC) function are proposed to balance dc bus capacitor voltages and flying capacitor voltages, which realizes decoupling of voltage control. The switching states, switching principle, and dwell time calculation are developed, and the flowchart of VBC function is also elaborated on. The performance of the proposed improved SVM is investigated and compared with conventional SVM. The effectiveness and validity of the improved SVM with a VBC function is verified by simulation and experimental results of the 5L-NNPP converter.

Winding-Based DC-Bus Capacitor Discharge Technique Selection Principles Based on Parametric Analysis for EV-PMSM Drives in Post-Crash Conditions

When the permanent magnet synchronous machine (PMSM) based electric vehicles (EV) are in the postcrash situations, the dc-bus capacitor voltage requires to be reduced quickly for the sake of electric shock risk prevention. Currently, due to the advantages of low cost, compact structure, and high reliability, the pure-winding-based discharge strategies are attracting increasing attentions. However, there are no theoretical criteria for judging whether a pure-winding-based method is suitable for bleeding a particular PMSM-based EV drive, not to mention the rules for opting for an effective discharge strategy among the instant nonzero d-axis, zero q-axis (NDZQ), long-cycle NDZQ, and the nonzero d-axis, nonzero q-axis (NDNQ) current control techniques. This article discusses the applicable occasions of the instant NDZQ, long-cycle NDZQ, and NDNQ discharge methods based on parametric analysis. On this ground the principles for determining whether an EV drive can adopt a pure-winding-based method for discharge are given. Meanwhile, the implementation procedures of the proposed criteria are detailed clearly. Finally, the proposed selection criteria are used for evaluating five different EV drives so as to select the corresponding winding-based discharge strategy for each system, and the effectiveness is verified by simulation or experiments.

A hybrid fuzzy with feedback integral phase locked loop-based control strategy for unified power quality conditioner

This paper proposes a hybrid fuzzy control strategy comprising both Takagi-Sugeno fuzzy and Mamdani fuzzy along with novel feedback integral phase-locked loop based modified synchronous reference frame for unified power quality conditioner for power quality enhancement in power distribution network. The hybrid fuzzy controller can vary the gain nonlinearly for controlling dc-bus capacitor voltage of unified power quality conditioner, which limits the dc-bus voltage deviations during the load and supply voltage turbulences. Moreover, the proposed novel feedback integral phase-locked loop system is employed to improve the grid synchronization performance of unified power quality conditioner by considering the feedback integral loop and nonlinear adaptive-filter type phase detector approach. As a consequence, the proposed feedback integral phase-locked loop system can extract the positive sequence signal perfectly during power system disturbances. Additionally, the novelty of modified synchronous reference frame control technique relies upon nonintervention with low pass filter as well as high pass filter. Hence, the proposed control technique quickly and accurately extracts the reference signal from the distorted power system. The efficacy of the proposed control technique is justified through a real-time Opal-RT Hardware underneath different states of power system and compared with both conventional modified phase locked loop system and Takagi-Sugeno fuzzy with synchronous reference frame-based control system.

Variable nonlinear gain fuzzy with improved synchronous reference frame control strategy for performance enhancement of unified power quality conditioner

This paper proposes a variable nonlinear gain fuzzy (VNGF) controller and negative-feedback based phase-locked loop (NFPLL) with improved synchronous reference frame (ISRF) control strategy for unified power quality conditioner (UPQC). Several power quality (PQ) problems present in the power distribution network can be exterminated through the proposed VNGF-NFPLL-ISRF control method. The VNGF method employs Takagi-Sugeno Fuzzy (TSF) type numerical consequent rule with a nonlinear alteration of variable control gain for controlling dc-bus capacitor voltage of UPQC, which limits the voltage deviances through the power system perturbations and delivers quick settling time for dc-bus voltage. The scheme of NFPLL based synchronization is built upon presenting a negative-feedback control action in the interior loop of conventional PLL and hence a greater extent of protection to grid as well as load disturbances, unresponsive to noise, harmonics and additional contaminations existing through the signal can be attained. Moreover, the novelty of ISRF control method relies upon nonintervention with low pass filter (LPF) and high pass filter (HPF). As a consequence, the proposed VNGF-NFPLL-ISRF control technique quickly and accurately extracts the reference signal during power system perturbations. The effectiveness of the proposed approach is experimentally validated and a comparative survey has been accomplished with conventional TSF controller based SRF method (TSF-SRF).

Transformerless UPS System Based on the Half-Bridge Hybrid Switched-Capacitor Operating as AC–DC and DC–DC Converter

A single-phase transformerless online uninterruptible power supply (UPS) is proposed in this article. This UPS is based on the new half-bridge doubler-boost with switched-capacitor operation. It is capable of operating with ac and dc input sources. The proposed converter provides a high power factor and improves power efficiency for line voltage applications. The switched capacitor concept allows a natural balance of the dc bus capacitor voltages, both in the ac–dc mode and the dc–dc mode. It presents a common point between the line voltage source, load, and batteries. This characteristic makes it possible to eliminate the common mode current, reducing problems with electromagnetic interferences and allowing the use of a simple bypass system. The feasibility of the proposed UPS is verified by experimental results from a 2.5-kW prototype with an efficiency of up to 94.9% and unity power factor.

An Adaptive Multistep Balancing Modulation Technique for Multipoint-Clamped Converters

In this article, a new pulsewidth modulation technique is proposed for the output voltage control and the dc-bus capacitors voltage balancing of a multipoint-clamped multilevel converter. The voltage equalization is achieved by allowing each converter's leg voltage to switch through multiple levels in a single modulation period (multistep operation). The approach is generalized with respect to the number of levels and phases and can be applied regardless of the converter operating conditions. Nevertheless, the multistep behavior generally leads to an increase of the average switching transitions rate. In the proposed method, this drawback is mitigated with an online adaptive selection of the number of switching levels. The algorithm is validated through an extensive hardware-in-the-loop testing and compared to other approaches.

A New Variable Frequency Control of 49-Level Cascaded Packed U-Cell Voltage Source Inverter

Requirement of large number of levels with lower number of switching devices has made asymmetrical converters more popular than the symmetrical ones. Asymmetrical cascaded multilevel inverters (ACMLI) can achieve high efficiency by combining switching devices with different voltage ratings and technologies. The proposed ACMLI cascades two or more units of packed U-Cell (PUC) inverters using two or more isolated dc link supplies. In this article, one of the PUC unit is controlled using high switching frequency while the other PUCs are operated in a step mode at low switching frequencies, thus operating them in a variable frequency control mode. The cascading of two 7-level PUC inverters with dc link voltage ratio of 1:7 can produce an output voltage with 49 (7x7) levels. The multilevel output voltage waveform is nearly sinusoidal with very low THD content, and the low switching frequency operation leads to lower power dissipation and greater system efficiency. However, each PUC module requires two dc voltage sources. To address this concern, in this article, each PUC module consists of one dc voltage source and one dc bus capacitor. With the cascaded PUC topology and proposed control algorithm, load current and dc bus capacitor voltage control is achieved simultaneously. The proposed converter and its control technique lead to the breaking of the design tradeoff rule between switching frequency (efficiency) and filter size. This is very useful in various applications such as uninterruptible power supplies and grid-tie inverters. The converter and its control technique are simulated using MATLAB/Simulink software and simulation results for both open loop and closed loop are discussed. Hardware results are obtained by developing a 1-kW experimental prototype. Simulation and experimental results confirm the usefulness and effectiveness of the proposed topology and its control technique.

Reliable Winding-Based DC-Bus Capacitor Discharge Technique Over Full-Speed Range for IPMSM Drive in Electric Vehicles Without Position Sensor

When electric vehicles encounter emergency, the dc-bus capacitor voltage in the motor drive system needs to be reduced as quickly as possible to prevent the passengers from secondary electrical injury. For the purpose of compactness and low cost, a novel discharge scheme by using the machine windings has come forth. Yet, the relevant research works are incomprehensive without considering the standstill cases and sensor fault issues. This article proposes a position sensor fault-tolerant winding-based dc-bus capacitor discharge method over the full-speed range. To achieve voltage discharge at high speed without using position sensor, an improved second-order sliding-mode observer based on continuous sigmoid function is presented. Moreover, an adaptive sine-wave-based position observer with high immunity to voltage change is proposed for discharge in the low and zero speed cases. In terms of the discharge procedures, deceleration-based and acceleration-based algorithms are designed and optimized aiming at the spinning and standstill situations, respectively. More specifically, they include voltage, current, and speed regulation methods. The proposed fault-tolerant discharge technique is verified by both simulation and experiments, which are conducted on a three-phase interior permanent magnet synchronous machines drive system.

A Disturbance Rejection-Based Control Strategy for Five-Level T-Type Hybrid Power Converters With Ripple Voltage Estimation Capability

This article proposes a robust control strategy for five-level T-type (5L-T) hybrid power converters to achieve superior dynamic performance for effectively regulating the dc-bus voltage under external disturbances and generating high-quality grid current at the same time. A new filter-less dc-bus ripple voltage estimation method and a simple technique to remove this ripple component from the measured dc-bus voltage of a single-phase converter are developed. A sliding-mode control (SMC) incorporated with an extended-state observer (ESO) is employed for the outer voltage control loop, and to dynamically calculate the input (i.e., the active power reference) for the inner current-tracking controller. The proposed SMC-ESO approach presents a high disturbance rejection capability and robustness against the dc-bus load variation, and thus, significantly improves the dynamic and steady-state performance during system uncertainties. Moreover, a finite control set-model predictive control algorithm is derived as the inner current controller to track their references while balancing the dc-bus capacitor voltages. The effectiveness of the proposed controller is demonstrated and verified through measurement results.

A Series-Connected Multilevel Converter: Topology, Modeling, and Control

Cascaded H-bridge (CHB) is a promising topology for medium-voltage high-power applications. CHB consists of low-power H-bridge modules in each phase, and these modules are connected in a cascade manner to achieve multilevel operation. However, CHB requires a phase-shifting transformer with multiple secondary windings to generate an isolated dc source for each module, and this requirement increases the overall system cost. In this paper, a series connection of low-power modules is employed to realize a new power converter called a series-connected multilevel converter. Each phase of the proposed topology requires a single isolated dc source. Hence, the required number of secondary windings is considerably reduced. This approach also minimizes the complexity and cost of the overall system. The operation of the proposed topology is presented by using three-level neutral point clamped modules in each phase. Model predictive control (MPC) is employed to control the dc-bus capacitor voltage and output currents of the proposed topology. A discrete-time mathematical model of the proposed topology is also developed to predict the future behavior of control variables. The performance of the proposed topology with MPC is verified through MATLAB simulations and a scaled-down laboratory prototype.