Variable DC Link Voltage Research Articles

Multi‐channel VSI fed large variable speed asynchronous hydro‐condenser: fault analysis, fault diagnosis and fault tolerant control

Multi-level voltage source inverter (VSI) fed doubly fed induction machine (DFIM) has gained prominence in large rated hydro-generating unit (HU) since it provides part load operation with reduced power converter rating and high dynamic stability. Power converters connected in rotor side control real and reactive power of the unit. This study discusses the condenser operation of variable speed HU (250 MW), to be commissioned in Tehri plant (India), with the power flow diagram. Further, it provides dynamic behaviour and survivability status of the DFIM unit during power converter and control circuit faults where power converter redundancy is not available in large rated DFIM unit. Economic analysis of 250 MW HU under power converter and control circuit failures are also investigated. The present work also explores fault tolerant operation of a 250 MW DFIM unit at open switch fault in converters to increase the continuity of the unit operation which provides additional 2% of reactive power support to the grid compared to the DFIM without power converter redundancy operation. Open switch fault is detected through Park's vector phase currents technique and variation in DC link voltage. The computer simulation results are validated in an experiment carried on 2.2 kW DFIM in the laboratory.

Reduced Capacitance Battery Storage DC-Link Voltage Regulation and Dynamic Improvement Using a Feedforward Control Strategy

This paper proposes a feedback/feedforward control strategy to attenuate the dc-link voltage variations in a reduced-capacitor battery energy storage system. It also identifies the impact of dc-link voltage variations on a battery energy storage system's response dynamics and power capabilities. It shows that addition of a feedforward controller to the existing feedback controller improves the dc-link voltage regulation and consequently the response dynamics of the reduced-capacitor battery energy storage system. It formulates a power setpoint change as a disturbance to the battery energy storage control system. The battery energy storage system is analyzed using an averaged-value modeling technique. Small-signal analysis is then used to tune feedforward controller parameters. The performance of the proposed controller is evaluated via an electromagnetic transient software (PSCAD/EMTDC), and the findings are experimentally validated using a laboratory-sized test bench.

Maximum Efficiency Tracking of an Integrated Two-Staged AC–DC Converter Using Variable DC-Link Voltage

This paper presents an approach to achieve maximum conversion efficiency of a two-staged-cascaded ac–dc converter. In this study, the integrated ac–dc converter consists of a three-phase boost-type power factor correction rectifier and a phase-shifted full-bridge dc–dc converter. The detailed loss modeling of the integrated stage implies that the total power loss varies in a nonmonotonic fashion with intermediate dc-link voltage. In order to ensure maximum conversion efficiency, an optimum dc-link voltage level can be set depending on the load power level. A comparison between the efficiency variation under two different cases is carried out with extensive mathematical analyses: 1) fixed dc bus and 2) variable dc-bus voltage control. In addition, the experimental results obtained from a 6-kW-integrated ac–dc hardware prototype show a unity power factor along with a peak efficiency of 97.2%, which is 2.5% more than the fixed dc-link control approach.

A Hybrid Moth-Flame Fuzzy Logic Controller Based Integrated Cuk Converter Fed Brushless DC Motor for Power Factor Correction

This research work deals with a hybrid control system based integrated Cuk converter fed brushless DC motor (BLDCM) for power factor correction. In this work, moth-flame optimization (MFO) and a fuzzy logic controller (FLC) have been combined and a moth-flame fuzzy logic controller (MFOFLC) has been proposed. Firstly, the BLDC motor modeling is composed with the power factor correction (PFC) based integrated Cuk converter and BLDC speed is regulated using variable DC-Link inverter voltage which results in a low switching operation with fewer switched losses. Here, with the use of a switched inductor, the task and execution of the proposed converter is redesigned. The DBR (diode bridge rectifier) trailed by a proposed PFC based integrated Cuk converter operates in discontinuous inductor conduction mode (DICM) for achievement of better power factor. MFO is exhibited for gathering of a dataset from the input voltage signal. At that point, separated datasets are sent to the FLC to improve the updating function and minimization of torque ripple. However, our main objective is to assess adequacy of the proposed method, but the power factor broke down. The execution of the proposed control methodology is executed in the MATLAB/Simulink working platform and the display is assessed with the existing techniques.

A 1-MHz Series Resonant DC–DC Converter With a Dual-Mode Rectifier for PV Microinverters

The photovoltaic (PV) output voltage varies over a wide range depending on operating conditions. Thus, the PV-connected converters should be capable of handling a wide input voltage range while maintaining high efficiencies. This paper proposes a new series resonant dc–dc converter for PV microinverter applications. Compared with the conventional series resonant converter, a dual-mode rectifier is configured on the secondary side, which enables a twofold voltage gain range for the proposed converter with a fixed-frequency phase-shift modulation scheme. The zero-voltage switching turn- on and zero-current switching turn- off can be achieved for active switches and diodes, thereby, minimizing the switching losses. Moreover, a variable dc-link voltage control scheme is introduced to the proposed converter, leading to a further efficiency improvement and input-voltage-range extension. The operation principle and essential characteristics (e.g., voltage gain, soft-switching, and root-mean-square current) of the proposed converter are detailed in this paper, and the power loss modeling and design optimization of components are also presented. A 1-MHz 250-W converter prototype with an input voltage range of 17–43 V is built and tested to verify the feasibility of the proposed converter.

Design of solar‐powered agriculture pump using new configuration of dual‐output buck–boost converter

This paper proposed a new structure of single switch dual output Zeta derived DC-to-DC converter for an efficient harvesting of photovoltaic (PV) power for a 4-phase switched reluctance motor (SRM) driven agriculture pump. The system is essentially developed to minimise both cost and complexity, while synchronously guaranteeing optimal operation of the PV array. The proposed converter is derived from a hybrid of a ‘Zeta’ and Landsman converters and so it contains all the benefits of both converters including balanced outputs. Due to an identical instantaneous duty cycle, the switching node is shared by both Zeta and Landsman converters. The proposed converter offers continuous power to drive, a low ripple in output current, and balanced dual output supply. The PV power optimisation and to deliver smooth start of SRM, are the two major objectives of proposed DC-to-DC converter. A new maximum power point tracking (MPPT) technique is also used in the system to optimise the performance of PV array. Moreover, the proposed MPPT control has an additional advantage of regulating the flow rate of water pumping by adjusting the duty cycle of DC-to-DC converter. The motor speed is smoothly regulated through the variable DC link voltage of a mid-point converter.

A High-Efficiency High-Density Wide-Bandgap Device-Based Bidirectional On-Board Charger

This paper proposes a novel two-stage topology for a 6.6-kW on-board charger. The first stage, employing an interleaved bridgeless totem-pole ac/dc in critical conduction mode to realize zero-voltage switching, is operated at over 300 kHz. A bidirectional CLLC resonant converter operating at 500 kHz is chosen for the second stage. A variable dc-link voltage is adopted to track the wide battery voltage range, so that the CLLC resonant converter can always operate at its most efficient point. The 1.2-kV SiC devices are adopted for the ac/dc stage and the primary side of dc/dc stage, while 650-V GaN devices are used for the secondary side of dc/dc stage. In addition, PCB winding coupled inductors and integrated transformer are implemented in ac/dc stage and dc/dc stage, respectively, for the purpose of high density and manufacture automation. The proposed structure is demonstrated to have 37-W/in3 power density and above 96% efficiency over the entire battery voltage range, which far exceeds the current practice.

Implementation of a Novel Control Technique in Landsman Converter Using Bumble Bee Optimization

This paper focuses on a solar PV array based BLDC motor employing Landsman converter under partial shading condition (PSC). A converter acts as an interface between the SPV array under PSC and Voltage Source Inverter (VSI) feeding the Brushless DC (BLDC) motor. BLDC motor incorporating the merits of higher efficiency, high reliability, high ruggedness, easy-to-drive, capability to operate successfully at low voltage and excellent performance over a wide range of speed. The speed control of BLDC motor by variable DC-link voltage. This eliminates the additional phase current sensing, DC-link voltage sensing, additional control and associated circuitry. The proposed system includes simplicity control, compactness, and soft starting of the BLDC motor. The operation of Landsman converter in CCM results reduced stress on devices. To optimize the operating point of the SPV array in order to get maximum possible power output by means of the better maximum power point tracking (MPPT) technique. The technique is Bumble Bee Mating Optimization (BBMO) based MPPT. The novel technique is compared to the conventional techniques. The simulated results are executed in MATLAB/SIMULINK.

Primary Control Design Based on Capacity Curves for Inverter-Based Microgrids

In this paper, a multi-loop controller for an inverter-based microgrid (MG) system is proposed in order to control the voltage of point of common coupling (PCC) and to perform accurate active and reactive power sharing. Inner current and voltage loops, that are designed based on the dynamic model of the MG system, are used to obtain appropriate switching functions for interfaced inverters and the reference values for currents of DG units. Also, a modified droop controller is introduced to enhance the performance of the designed voltage control loop. In order to improve the dynamic operation of the proposed controller in supplying demanded power for harmonically distorted loads, the extraction of harmonic components of PCC voltages is accomplished. Upper and lower limits of voltage amplitude and frequency for droop curves are analyzed using capacity curves of each DG unit. The effect of renewable power generation uncertainty on DC link voltage variations is also compensated using an additional control loop in multi-loop structure. The performance of the proposed control technique has been verified by simulation using MATLAB/SIMULINK environment which shows a proper steady state and transient performance during changes in generation and non-linear harmonically distorted loads.

Wear-Out Failure Analysis of an Impedance-Source PV Microinverter Based on System-Level Electrothermal Modeling

In this paper, the wear-out performance of an impedance-source photovoltaic (PV) microinverter (MI) is evaluated and improved based on two different mission profiles. The operating principle and hardware implementation of the MI are first described. With the experimental measurements on a 300-W MI prototype and system-level finite-element method simulations, the electrothermal models are built for the most reliability-critical components, i.e., power semiconductor devices and capacitors. The dependence of the power loss on the junction/hotspot temperature is considered, the enclosure temperature is taken into account, and the thermal cross-coupling effect between components is modeled. Then, the long-term junction/hotspot temperature profiles are derived and further translated into components’ annual damages with the lifetime and damage accumulation models. After that, the Monte Carlo simulation and Weibull analysis are conducted to obtain the system wear-out failure probability over time. It reveals that both the mission profile and the thermal cross-coupling effect have a significant impact on the prediction of system wear-out failure, and the dc-link electrolytic capacitor is the bottleneck of long-term reliability. Finally, the multimode control with a variable dc-link voltage is proposed, and a more reliable dc-link electrolytic capacitor is employed, which results in a remarkable reliability improvement for the studied PV MI.

Reduced Multilevel Converter: A Novel Multilevel Converter With a Reduced Number of Active Switches

This paper presents a new multilevel converter topology based on a variable or multilevel dc-link stage. This stage is shared by all inverter output phases, thus increasing the number of output voltage levels while reducing the overall number of active devices compared to traditional topologies; therefore, it is called a reduced multilevel converter (RMC). The converter is not capable of reducing the blocking voltage of the devices; hence, unlike other multilevel converters aimed at medium-voltage applications, this converter is interesting for low-voltage- and high-power-quality-demanding applications such as photovoltaic inverters, wind energy conversion systems, and uninterruptible power supplies. The main novelty behind the proposed concept is the basic dc cell used to generate the variable dc-link voltage, which includes a controlled path through the floating capacitors to provide the necessary degree of control to enable a shared multilevel dc link for all the output phases of the converter. The dc cells are connected in a multicell structure to increase the number of levels of the converter. A five-level version of the RMC topology is briefly compared to other five-level converters such as the five-level active neutral-point-clamped converter and the five-level flying capacitor converter. This paper presents the structure of the new topology with its operating principle, switching states, main characteristics, and experimental validation using finite-control-set model-predictive control.

Variable DC-Link Control Loop Design for an Integrated Two-Stage AC/DC Converter

This paper presents an innovative technique of synthesizing variable dc-link voltage control loop for a two-stage integrated ac-dc converter, which is a cascaded combination of a single-phase continuous conduction mode interleaved totem-pole power factor correction and a half-bridge CLLC resonant converter. Although the variable dc-link control exhibits a better overall efficiency than the fixed dc-link control, it faces issues of: 1) loss of output voltage regulation and 2) low-frequency oscillations in the dc voltages and inductor current. In order to address and analyze these issues, both time-domain and frequency-domain studies of the control loop are performed, which led to two different voltage loop controller configurations, resolving the oscillation and regulation issues. As verification to the proof-of-concept, a hardware prototype of an integrated ac-dc converter is developed and tested up to 2 kW. The experimental results exhibit a power factor of 0.992, peak efficiency of >94% and a total harmonic distortion of <5%, with the elimination of low-frequency oscillation of the dc voltage waveforms.

An Integrated Dual-Output Isolated Converter for Plug-in Electric Vehicles

This paper proposes a unique integrated and isolated dual-output dc–dc resonant converter, which can interface both HV traction batteries and LV loads. In addition, the proposed topology is bidirectional, capable of delivering power from HV traction batteries to the grid for vehicle-to-grid (V2G) applications. To increase the power density of the converter, the dual-output dc–dc resonant converter combines magnetic components of resonant networks into a single three-winding electromagnetically integrated transformer. The resonant converter uses a half-bridge topology with split capacitors as the resonant network components to further reduce the size of a converter. The variable dc-link voltage strategy is utilized to enhance the efficiency over the entire output voltage range. A 3.3-kW converter is designed and developed for validation of various operation modes, including grid-to-vehicle, V2G, and HV-to-LV charging.

FPGA implementation and analysis of model predictive current control for three-phase voltage source inverter

A detailed investigation of the model predictive current control (MPCC) for a three-phase voltage source inverter (VSI) with an output RL filter is examined in this manuscript. The investigation on VSI and its operating conditions such as dynamic, steady state, transient state condition with balanced and unbalanced loads are explained. The parameter variations such as sampling time, filter inductance and DC-link voltage variations are also explained in terms of load current Total Harmonic Distortion (THD) and inverter switching frequency. During each sampling period, the discrete mathematical model of the system is used to predict the load current for all the inverter switching state. These predictions are estimated using cost function, and so, the switching state which gives minimum cost function is selected and applied to the inverter. The effectiveness of the control technique is verified in MATLAB software and also in real-time hardware setup using Xilinx XCS500E Spartan 3E FPGA board. The detailed simulation and hardware results are included to investigate the implementation.

FPGA implementation and analysis of model predictive current control for three-phase voltage source inverter

A detailed investigation of the model predictive current control (MPCC) for a three-phase voltage source inverter (VSI) with an output RL filter is examined in this manuscript. The investigation on VSI and its operating conditions such as dynamic, steady state, transient state condition with balanced and unbalanced loads are explained. The parameter variations such as sampling time, filter inductance and DC-link voltage variations are also explained in terms of load current Total Harmonic Distortion (THD) and inverter switching frequency. During each sampling period, the discrete mathematical model of the system is used to predict the load current for all the inverter switching state. These predictions are estimated using cost function, and so, the switching state which gives minimum cost function is selected and applied to the inverter. The effectiveness of the control technique is verified in MATLAB software and also in real-time hardware setup using Xilinx XCS500E Spartan 3E FPGA board. The detailed simulation and hardware results are included to investigate the implementation.

Sliding Mode Control for Three-Phase Quasi-Z-Source Inverter

In this paper, a novel multiobjective control strategy for the three-phase quasi Z-source inverter (q-ZSI) is proposed. A sliding mode control (SMC)-based controller is proposed to regulate inductor current through shoot-through ratio, and DC-link voltage reference is considered as an additional control input to keep q-ZS network capacitor voltage at a desired constant level. The load current is regulated by a proportional resonant controller whose output is then divided by DC-link voltage reference to obtain a modulation signal. With these three control inputs, the system is no longer underactuated, thus to eliminate probable steady-state error, which always happens in the underactuated system. The SMC-based controller for inductor current has the advantages of easy implementation, strong robustness, fast response, and low current ripple. The capacitor voltage is kept constant by the proportional integral-based variable DC-link voltage reference theme with negligible steady-state error and load current is pure sinusoidal with low THD. Simulations and experimental results both verify the effectiveness of the proposed control strategy.

차량용 IPMSM의 고속 영역에서 가변 전압에 강한 고정자 자속 기준 제어

This paper proposes an efficient control strategy for IPMSM in a high speed region by determining the operation range with the actual measured values of voltage, current, and torque in the DC-link voltage fluctuation condition. The proposed flux weakens the control method’s acquisition of the maximum stator flux linkage via the motor speed and DC-link voltage of the inverter and the optimal dynamic operating point is determined according to the torque command and the maximum stator flux linkage. In addition, the maximum torque is analyzed using the amplitude and phase angle of the stator current variation for minimizing the practical operating point error. In addition, the flux weakening control region is determined by the amplitude of the stator flux linkage, which is related to the inverter output voltage. A 2 [kW] IPMSM is employed to investigate the torque control performance according to the speed and DC-link voltage variation in the experiment.

High-Efficiency High-Density Critical Mode Rectifier/Inverter for WBG-Device-Based On-Board Charger

A widebandgap-based bidirectional on-board charger (OBC) has been proposed with a variable dc-link voltage, which tracks the battery voltage fluctuation. Although this approach is deemed most efficient for the resonant dc/dc stage, it posts significant challenges for the rectifier/inverter stage, which operates in the critical mode to realize zero-voltage switching (ZVS), while subjecting to the large variations of input and output voltages. Design considerations of the ac/dc stage are presented in this paper, including: the evaluation of 1.2-kV SiC MOSFETs; the ZVS extension techniques to realize ZVS under all input/output variations; and a novel universal control strategy for both the rectifier mode and the inverter mode. A prototype is built which achieves 98.5% efficiency at a switching frequency higher than 300 kHz. Furthermore, a 6.6-kW OBC system is demonstrated, using both SiC and GaN devices with 37-W/in3 power density and above 96% efficiency.

A New Asymmetric Multilevel Inverter Topology Suitable for Solar PV Applications With Varying Irradiance

A new asymmetrical multilevel inverter topology is reported that is capable to operate satisfactorily with wide variation in dc-link voltage, while feeding power to the ac grid. A topological building block is first introduced that has one full-bridge inverter connected in series with a level doubling network. Following this, the interconnection of such building blocks is attempted to increase the number of levels at the output voltage waveform. The investigation reveals that for a three-phase system, a converter configuration with two such building blocks is capable to generate a nominal asymmetry of 14:7:2:1 using only four voltage sources. In solar Photovoltaic (PV) applications, one main source may be fed by PV array and the other three auxiliary sources may be fed through separate dc/dc converters, each having power rating of 3.2% of the peak power rating of PV arrays. The proposed converter can generate 3097 space vectors. Asymmetrical hexagonal decomposition is modified (to ensure satisfactory operation of LDN and to eliminate any dc component in the phase voltage waveform) to control such converter. The converter is extensively simulated in MATLAB/Simulink. A solar PV system of 9.4 kWp available in the laboratory is used to feed power to the grid at different irradiance. Simulation results match well with the prototype experiments confirming the usefulness of the proposed topological alternative for solar PV applications.

Control model design to limit DC-link voltage during grid fault in a dfig variable speed wind turbine

Abstract This paper presents a control model design capable of inhibiting the phenomenal rise in the DC-link voltage during grid- fault condition in a variable speed wind turbine. Against the use of power circuit protection strategies with inherent limitations in fault ride-through capability, a control circuit algorithm capable of limiting the DC-link voltage rise which in turn bears dynamics that has direct influence on the characteristics of the rotor voltage especially during grid faults is here proposed. The model results so obtained compare favorably with the simulation results as obtained in a MATLAB/SIMULINK environment. The generated model may therefore be used to predict near accurately the nature of DC-link voltage variations during fault given some factors which include speed and speed mode of operation, the value of damping resistor relative to half the product of inner loop current control bandwidth and the filter inductance.