High Dc-link Voltage Research Articles

DC-Link Voltage Fluctuation Suppression Method for Modular Multilevel Converter Based on Common-Mode Voltage and Circulating Current Coupling Injection under Unbalanced Grid Voltage

Grid voltage imbalance conditions often occur. Modular multilevel rectifiers (MMCs) have high DC-link voltage fluctuation under an unbalanced grid, which affects the normal operation of DC-side equipment. To suppress voltage fluctuation under an unbalanced grid, a coupling injection strategy composed of third zero-sequence common-mode voltage (TZCV) and secondary circulating current (SCC) was designed in this paper. In this paper, we calculated the coupling time-domain expression of the TZCV and SCC under an unbalanced grid voltage. Then, the influence of an SCC and TZCV coupling injection on DC-link voltage fluctuation was analyzed. The converter power flow of different system control objectives under an unbalanced grid was calculated, and the overall control method of the converter based on the arm current was proposed. The advantage of the method proposed in this paper is that it can realize online control under different grid voltages and input power conditions in real time and effectively suppress DC-link voltage fluctuation. The simulation was carried out on the MATLAB/Simulink platform, and a hardware-in-the-loop experimental platform was built; the results verify the effectiveness of the proposed strategy.

A Rapidly Reconfigurable DC Battery for Increasing Flexibility and Efficiency of Electric Vehicle Drive Trains

Traditional electric vehicle (EV) battery drive trains comprise hard-wired batteries forming a fixed high-voltage DC link, and a main inverter. This paper proposes a novel topology to break hard-wired batteries into smaller subunits interfaced by low-voltage field effect transistors (FET). The new DC link, named <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">reconfigurable DC battery</i> , can offload a great portion of the switching duty from the main inverter. Several benefits follow: <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1)</i> The proposed topology reduces the total loss despite the added components. The net loss is reduced because the reconfigurable DC battery can generate an optimal high-frequency voltage waveform to spare up to 2/3 of the switching actions of the main inverter—i.e., delegating the modulation duty from the main inverter to the reconfigurable DC battery. The added semiconductor loss is negligible compared to that of the main inverter, thanks to small voltage steps of multilevel conversion and the latest low-voltage transistors. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2)</i> The main inverter’s output waveform has less distortion, e.g., down to 50% compared to conventional space-vector modulation. This distortion reduction is a consequence of large segments of the overall output voltage (particularly the apex) being entirely formed by the reconfigurable DC battery with multilevel precision. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3)</i> The multilevel output qualities substantially reduce the voltage transients d <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</i> /d <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> of the inverter output, which is known to reduce insulation and bearing stress in motors. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4)</i> The topology eliminates the vulnerability of large hard-wired battery packs to the weakest cells. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5)</i> The constant presence of the high voltage in conventional hard-wired batteries and the associated issues, e.g., during manufacturing, maintenance, and crashs are avoided because of normally-off FETs. We demonstrate the proposed motor drive on a 3-kW setup with eight battery modules.

Bidirectional DC - DC Converter Topology for Electric Vehicles Using Fuzzy Logic Controller

This project organizes an application of hybrid electric vehicle systems operated with novel designed bidirectional dc-dc converter (BDC) which interfaces a main energy storage (ES1), an auxiliary energy storage (ES2) and dc bus of different voltage levels. Proposed BDC converter can operate both step up and step-down mode. In which step up mode represents low voltage dual source -powering mode and step-down mode represents high voltage dc link energy –regenerating mode, both the modes are operated under the control of bidirectional power flow. This model can independently control power flow between low voltage dual source buck/boost modes. Here in, the circuit configuration, operation, steady-state analysis, and closed-loop control of the proposed BDC are discussed according to its three modes of power transfer. In this project fuzzy logic controller is used and also system results are validating through MATLAB/SIMULINK software. Index Terms—Bidirectional dc/dc converter (BDC), dual battery storage, hybrid electric vehicle, Fuzzy logic controller.

Study of different parameters influencing IGBT and diode robustness under short-circuit type III conditions

The purpose of this work is to study the influence of different parameters on short-circuit type III (SC-III) robustness limit of the IGBT and freewheeling diodes (FWD) beyond the specified datasheet values. SC-III measurements without gate-emitter voltage (VGE) clamping show a reduced SC-III safe operating area (SOA) at higher DC-link voltages because of high collector current peaks (IC,peak) induced by transient gate-emitter overvoltage and subsequent high negative diC/dt values. By applying a VGE clamping circuit, SC-III last-pass collector-current (IC,sat,Lastpass) can be fully recovered to the level of SC-I, even though the IGBT and diode encounter harder conditions in the SC-III event. Parameters such as parasitic inductance (Lpar), load current (Iload) and temperature (Tstart) have an influence on the SC-III robustness limit of the IGBTs and FWDs. Also, the second reverse-recovery peak of the FWD during an SC-III event is strongly influenced by whether the gate-emitter voltage is applied across the sense or load-emitter terminal.

A Modular Magnetic Linked Converter Station for Offshore Power Transfer Through HVDC Link

This paper proposes a modular magnetic linked converter station (CS) for offshore power transfer through a high voltage dc (HVDC) link. The proposed converter station can collect medium voltage ac and dc power simultaneously and transform the power to HVDC suitable for offshore power transmission. The proposed CS adopts multiple half-bridge (HB) converters to form a front-end interface that can collect both medium voltage ac and dc power. Moreover, several high-frequency magnetic linked multiport converters (MLMCs) are employed in the CS to isolate and step up the voltage. The MLMC is operated at a unity step-up voltage ratio which matches the turn ratio in multiple windings shared by a common magnetic link. This condition facilitates minimum current stress on the switches and inherent soft-switching operation. In the proposed CS, the hard switching operation of the front-end HB converters does not affect the switching condition of the MLMCs. Furthermore, the application of the switched capacitor-based HVDC converter enables discretized dc-link capacitor arrangement to limit high inrush current under dc fault. A controller structure is also proposed for the CS to balance the electrical parameters. The results from a simulation study of the proposed CS, both in steady-state and dynamic conditions, show that the proposed CS performs satisfactorily with the proposed controller, which is validated by the laboratory scale experiment.

A novel solar photovoltaic powered grid interactive battery charging system for electric vehicle applications

ABSTRACT In this work, a novel Solar Photo Voltaic (SPV) powered grid interactive Electric Vehicle (EV) battery charging system has been proposed and validated. The objective of the proposed system is to provide seamless battery charging facility that includes a high capacity station battery system. The grid interactions like integration of SPV energy, station battery charging and reactive power support are included through the DC link of aShunt Active Power Filter (SAPF). The SAPF maintains unity power factor on the source side grid current. The objectives of the proposed system are ensuring the source side power factor nearly unity with minimal THD of source current, an SPV grid integration system with Maximum Power Point Tracking (MPPT), a comfortable charging point for EVs with a reliable charging station supported by a back up station battery. The novelty of the proposed work is that the common DC bus bar maintains a high DC link voltage when the utility source is available and the SAPF is active. The regulation of the DC link voltage is achieved by a PI controller tuned by the Gravitational Search Algorithm. The proposed system has been validated using simulations in the MATLAB SIMULINK environment as well using an experimental prototype. The proposed GSA controller outperforms in terms of THD (%) of source and load currents are 2.4 and 17.87, respectively.

A Survey on the State-of-the-Art and Future Trends of Multilevel Inverters in BEVs

All electric vehicles are the only way to decarbonize transport quickly and substantially. Although multilevel inverters have already been used in some transportation modes, they are rarely used in road transportation, especially in light-duty passenger BEVs. With the transition to a high 800-V DC link to extend the driving range and enable extreme fast charging, the possibility of using multilevel inverters in commercial light-duty passenger BEVs becomes feasible. Higher efficiency, higher power density, better waveform quality, lower switching frequency, the possibility of using low-rated switches, and inherent fault tolerance are known advantages of multilevel inverters that make them an efficient option for replacing 2-level inverters in high DC link passenger BEVs. This paper discusses high DC link voltage benefits in light-duty passenger BEVs, presents the state-of-the-art of different conventional multilevel inverter topologies used in BEVs, and compares them with conventional 2-level inverters from different aspects and limitations. Based on commercial upper-class passengers’ BEV data and a review of multilevel inverters on the market, future trends and possible research areas are identified.

Analytical DC-side stabilizing conditions for hybrid HVDC links based on dominant frequency model reduction

Incorporating the advantages of line commutated converters (LCCs) and voltage-source converters (VSCs), hybrid high voltage DC (HVDC) links have bright prospects in bulk power transmission. For this new technique, however, there is a risk of oscillatory instability in the DC link, and the mechanism behind the instability is still unclear. This paper derives analytical DC-side stabilizing conditions for hybrid HVDC links by using dominant frequency model reduction. The small-signal model of the LCC-VSC link is first truncated by reserving only the state variables highly relevant to the dominant mode so that the expression of the dominant oscillation frequency can be obtained. Dynamics of other state variables are reintroduced and then simplified while leaving their properties nearby the dominant frequency intact. Based on the reduced model, an analytical stability criterion is obtained, which reveals that the DC-side stability of hybrid HVDC links will deteriorate with reduced DC voltage operation, a heavy load, a small DC-link capacitor, slow inner loop dynamics, a small proportional and a large integral gain of the DC voltage regulator. In addition, simplified sufficient stabilizing conditions of hybrid HVDC links are further derived for control parameter design. Case studies validate the accuracy of dominant frequency model reduction and the derived stabilizing conditions.

Design-oriented DC-side stability analysis of VSC-HVDC systems based on dominant frequency analysis

This paper derives analytical DC-side stabilizing conditions of voltage-source-converter-based high voltage DC (VSC-HVDC) links for control design. The case of a single modular multilevel converter (MMC) connected to an infinite DC bus system is first explored to determine the frequency of the dominant mode (dominant frequency) in the DC link. Then, the small-signal model of a generic two-terminal HVDC system is formulated. Since stability discrimination only focuses on the behaviors of the dominant mode, nondominant modes are collapsed in a way that leaves their dynamic properties nearby the dominant frequency intact. Accordingly, concise DC-side stabilizing conditions based on key parameters of physical systems and controllers can be derived, which indicates that the instability mechanism of VSC-HVDC links under DC-side perturbations is that the DC-voltage-controlled converter and the transmission line cannot provide sufficient positive damping to offset the negative damping induced by the power-controlled inverter. It is also interesting to find that increasing the proportional gain of the DC voltage control diminishes the damping of the converter, thereby endangering system stability. The maximum value of the proportional gain is further determined to guarantee sufficient system stability margins under various operating points. Finally, simulation studies validate the proposed DC-side stabilizing conditions for two-terminal VSC-HVDC systems.

A Wide Bandwidth GaN Switching Power Amplifier of Active Magnetic Bearing for a Flywheel Energy Storage System

A switching power amplifier (SPA) provides the driving current for an active magnetic bearing (AMB) to achieve magnetic suspension (MS) control. The performances of the current bandwidth and current ripple should be significantly considered in the SPA of the AMB. The SPA can obtain a wide bandwidth by increasing the dc-link voltage. However, a high dc-link voltage will cause a high current ripple. To achieve a wide bandwidth and low current ripple, a new GaN-device-based SPA for the AMB of a flywheel energy storage system (FESS) is proposed in this article. The GaN SPA can operate at a high switching frequency and high dc-link voltage. Thus, the GaN SPA can improve the performance of the current bandwidth and current ripple compared to the conventional SPA. According to the characteristics of the GaN SPA, a low-loss PWM method and an optimal dead time strategy is proposed to increase the efficiency of the SPA. Furthermore, a high-response control approach of dead-beat model predictive control is proposed to further improve the current bandwidth. The proposed GaN SPA is applied in a 500-kW MS FESS prototype. The effectiveness of the SPA is verified experimentally.

A Family of High Step-Up Magnetically Coupled Impedance Source Inverters With Clamped DC-Link Voltage and Low Shoot-Through Current

The use of coupled inductors in impedance source inverters improves the voltage gain performance at the expense of high dc-link voltage spikes and shoot-through (ST) currents. The result is an increase in voltage and current stresses on semiconductors, as well as reactive element capacity and overall losses. Thus, to overcome these problems, this letter proposes a new family of magnetically coupled impedance source inverters with a smooth dc-link voltage, a low ST current, and a higher voltage gain that are confirmed through experimental tests.

A Nine-Level Transformerless Boost Inverter With Leakage Current Reduction and Fractional Direct Power Transfer Capability for PV Applications

This article describes an integrated voltage-boosting technique, which is essential to achieve compatibility between low-voltage photovoltaic (PV) panels and high-voltage dc links needed for dc-to-ac conversion. To achieve the twin objectives of voltage-boosting and multilevel inversion, this article proposes a transformerless T-type nine-level hybrid boost inverter. To improve the efficiency in the boosting stage of the proposed converter system, a significant portion of the PV energy is directly transferred from the PV source to the load, while the other part is processed through an interleaved converter, which is fused with the inverter. Thus, the proposed converter ensures a higher power density and reduces the power ratings of the semiconductor devices. This article also introduces a modified zone-based pulsewidth modulation (PWM) technique, which achieves a complete elimination of the switching frequency voltage transitions in the total common-mode voltage (TCMV). Thus, this technique mitigates the generation of leakage current while preserving the advantages of conventional multilevel inverters such as low <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{d}v/\text{d}t$ </tex-math></inline-formula> and inductive power capability. Besides that, a rigorous mathematical analysis for the common-mode equivalent circuit of the proposed configuration is also presented in this article. Detailed simulation and experimental studies are carried out to validate the feasibility of the proposed configuration.

Double-Vector-Based Finite Control Set Model Predictive Control for Five-Phase PMSMs With High Tracking Accuracy and DC-Link Voltage Utilization

Finite control set (FCS) model predictive control (MPC) has been widely emerged for the double-subspace control problem of five-phase permanent magnet synchronous motor (PMSM) drives thanks to the principles of the virtual voltage vectors (V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> s), which improve the steady-state performance with low computational burden. However, the V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> s are designed offline to include one-subspace components, which is not considered as an optimized solution. Therefore, this article presents an improved double-vector-based FCS-MPC scheme for five-phase PMSM drives, considering the control objectives of the two subspaces. The proposed scheme remedies the limitations of the conventional V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> s using online selection of the two vectors combination and duty ratio calculation. To keep low computational burden, ten possible double-vector combinations are examined during every sampling interval for each fundamental <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">α–β</i> sector, which is defined using the deadbeat calculations. Compared with the standard FCS-MPC and the traditional FCS-MPC with V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> s schemes, the proposed double-vector-based FCS-MPC scheme can achieve improved tracking accuracy and high dc-link voltage utilization. The potentials of the proposed scheme over these existing FCS-MPC schemes are verified by simulation analysis and experimental test on a five-phase PMSM drive prototype.

Deciphering the Effect of Corrugated p-Base on Reverse Blocking IGCT

The corrugated p-base (CB) structure has been acknowledged as an effective method to improve the maximum controllable current (MCC) of asymmetric integrated gate-commutated thyristor (IGCT), but never clarified in symmetrical devices such as reverse blocking IGCT (RB-IGCT). This study focuses on the influence of the CB structure on the MCC and the failure mechanism of RB-IGCT. First of all, by employing the CB structure in the RB-IGCT samples, it is verified that the CB structure can improve MCC of the symmetrical devices by more than 50%, no matter under high or low dc-link voltages. Moreover, regarding the scenario under the high dc-link voltage, the effect of the CB structure on the dynamic avalanche is analyzed via simulation, and the improvement of MCC and the positive trend of MCC upon temperature are further clarified. As to the scenario under the low dc-link voltage, distinct failure phenomenon and negative temperature dependence of MCC are observed evidentially on the symmetric devices with CB structure. To explain these failure characteristics that are conflicting with the well-acknowledged dynamic avalanche failure, a novel failure mode named cathode retrigger failure is proposed in this study. It is believed that this study not only deciphers the fundamental mechanisms of the CB structure in RB-IGCT, but also provides practical strategy to optimize the symmetric IGCT devices.

Performance Enhancement of Doubly Fed Induction Generator–Based Wind Farms With STATCOM in Faulty HVDC Grids

In this study, an investigation of different faults for a wind turbine–based doubly fed induction generator (DFIG) system is studied and the performance using a static compensator (STATCOM) is observed. The DFIG network is connected to a voltage source converter high-voltage dc link with a fault occurring near the wind generator network. The ride through capability of DFIG is promising with STATCOM using the proposed control strategy. The ac and dc voltage and torque oscillations are damped effectively, and improved power flow is observed. The low voltage AC grid fault occurs for an HVDC transmission, and the DFIG performance without and with STATCOM is compared, where the DFIG converter control schemes are developed using the proposed improved field-oriented control (IFOC) method. In this, the reference rotor flux value alters to a new synchronous speed value or a slighter value or a standstill depending on the stator voltage dip due to grid disturbance. This speed variation leads to introducing rotor current at that new rotor slip frequency as there is a change in the rotor speed because of the fault, which further decreases the stator flux dc component. Hence, this dc-offset constituent in the stator flux is alleviated and decays rapidly in scheming the divergence of the speed of the rotor to a new orientation speed with decay in the rotor flux. This operation is done in the inner control scheme of the rotor converter, which is quicker in response to the faults. Apart from this, the stator’s real and reactive power also changes accordingly based on the lookup table mechanism–based closed-loop control action of the pulse generator, and this power change is done in the outer loop. The analysis for DFIG and HVDC operation is verified under different faults without and with STATCOM.

Impact of RFB and PLL Dynamic on Combined ALFC-AVR Regulation of Multiarea Multisource System Under Deregulated Environment with AC/Accurate HVDC link

ABSTRACT The paper discusses the coordinated effect of redox flow battery (RFB) and accurate high voltage dc (AHVDC) link on the combined voltage and frequency control of two-area systems under deregulated environment. Each area has three generating companies (GENCOs) and three distribution companies. GENCOs in Area-1 are thermal, realistic dish-Stirling solar thermal system (RDSTS) and solar thermal power plants, and in Area-2 GENCOs are hydro, RDSTS, and geothermal power plants. Appropriate nonlinearity constraints are equipped with both hydro and thermal units. A maiden attempt has been made to employ a cascaded fractional-order PD with filter and PID with filter (CFOPDN-PIDN) as a secondary controller for the considered system. Artificial flora algorithm is used to optimize the controller parameters under various system conditions. The proposed CFOPDN-PIDN controller yields better system dynamic responses than the PIDN controller for the Poolco, bilateral, and contract violation power transactions. The investigations reveal that the parallel operation of the AC/AHVDC links deteriorates the system dynamics having more oscillations and peak deviations compared to individual AC and AHVDC links. The dynamic effect of Phase-Lock-Loop (PLL) is important for the realistic operation of AHVDC link. A novel approach for analyzing the dynamic effect of the AHVDC links considering PLL effects during the coordination with the AC system is presented. By adding the dynamic model of PLL to the parallel AC/AHVDC link, the system's performance is improved. The analysis reflects that the RFB-AHVDC links with PLL dynamics provided better system damping than the RFB-AC tie-line and AC tie-line alone.

Technical Review of Dual Inverter Topologies for More Electric Aircraft Applications

Electric drives are an essential part of more electric aircraft (MEA) applications with ever-growing demands for high power density, high performance, and high fault-tolerant capability. High-speed motor drives can fulfill those needs, but their speeds are subject to the relatively low dc-link voltage adopted by MEA. The power inverters are thus expected to efficiently and effectively manage that limited voltage. A recently popular topology is represented by the dual inverters. They are featured by inherited fault tolerance, a high dc-link voltage utilization, and an excellent output power profile. This article aims to present a comprehensive review of different structures based on the dual inverter. To meet the stringent requirements of MEA applications, three performance aspects, including the voltage utilization, the inverter output quality, and the fault-tolerant capability, are selected. Based on the chosen performance metrics, the key features of adopting dual inverter topologies against other converter selections are explicitly demonstrated. Finally, a practical guideline for choosing suitable dual inverters for different MEA applications is provided.

Multi-Period AC/DC Transmission Expansion Planning Including Shunt Compensation

Due to the increasing penetration of large-scale renewable energy generation at transmission level, hybrid AC/DC transmission expansion is becoming more relevant by the day, especially if the considered renewable generation is far away from existing AC grid infrastructure. This paper presents a multi-period hybrid AC/DC network expansion planning model that incorporates node voltages, reactive power and network losses. Since the operation of a high voltage DC link is influenced considerably by the converter stations, converter transformer, filter, and phase reactor are modeled in each converter station. Since the AC power flow equations render a non-convex problem whose solution algorithms do not always guarantee convergence or optimality, a second-order conic relaxation is utilized. The resulting model is a mixed-integer convex problem with a convergence guarantee that embodies a better approximation of system operation than its DC linear counterpart. Three case studies are considered to illustrate the proposed model.

Nine-level Cascaded H-Bridge Multilevel Inverter for Photovoltaic Sources Based on Hybrid Active Filter

Nowadays, a multilevel inverter is one of the important devices that provides a practical approach device in power system industries because of its features which are less switching losses, lower electromagnetic interference, reduced harmonic distortion, higher DC link voltages, and improved output voltage and current waveforms. Among all existed multilevel inverter, cascaded H-bridge multilevel inverter is the most attractive because of the technology development with megawatt power level. So, the cascaded H-bridge multilevel inverter has been tested and proven as a suitable DC/AC device for medium and high power applications such as renewable energy systems, electric vehicles, and motor drive applications. In this paper, the project is focusing on the combination of a nine-level cascaded H-bridge multilevel inverter connected to photovoltaic (PV) sources and a hybrid powerfilter. This project aims to study and to analyze the results of the proposed system. Thebehaviour and the performance of the designed system have shown that the system offered330V peak output AC voltage with an efficiency of 93.96% and a 5% reduction of THD.This project will be carried out only in Matlab/Simulink software.

A REVIEW ON POWER CONVERTERS WITH DIFFERENT CONTROL TECHNIQUES FOR PV APPLICATION

Nowadays, in most of the industrial applications, power converters are required because it is very essential to convert the fixed voltage dc source into variable voltage dc source for some applications and for some more applications, it is required to convert the variable voltage dc source into fixed voltage dc source based on the application. In this paper, different types of dc-dc converters are studied based on their applications. In recent days, dc-dc converters are implemented in photovoltaic applications like solar panel, wind energy integration, dc grid system, etc. In order to obtain the regulated output dc voltage, different control techniques are also studied. In this paper comparison of different control techniques have been done. In photovoltaic systems, dc-dc converters are required in order to link the low voltage PV panel and high voltage dc link. MPPT control technique is also employed in the photovoltaic system to extract the maximum power from the PV panel. In PV DC grid system, DC-DC converters are more essential as it converts the unregulated and unstable output power to regulated, stable output. By using dc-dc power converters, use of bulky and heavy low frequency step-up coupling transformer can be avoided.