High Dc-link Voltage Research Articles

A Double-Modulation-Wave PWM for Dead-Time-Effect Elimination and Synchronous Rectification in SiC-Device-Based High-Switching-Frequency Converters

The adoption of fast-switching silicon carbide (SiC) devices enables power converters to operate at high switching frequencies (e.g., 100 kHz). However, as a limiting factor at high switching frequencies, the dead-time in the conventional pulsewidth modulation (PWM) can cause significant voltage losses, bring serious low-frequency voltage/current harmonics, and reduce the linear modulation region with lower dc-link voltage utilization. Using the dead-time compensation can effectively eliminate the dead-time effect but further reduce the linear modulation region. The dead-time elimination PWM essentially has no dead-time effect. But due to the abandon of drive pulses for generating synchronous rectification, it has a lower converter efficiency, as well as current jumps at zero-crossings. In this article, to overcome the drawbacks of the conventional PWM and the dead-time elimination PWM, a double-modulation-wave PWM for dead-time-effect elimination and synchronous rectification is therefore proposed, where extra drive pulses for generating synchronous rectification are added based on the dead-time elimination PWM. The added drive pulses can also help with mitigating current jumps at zero-crossings. At last, the proposed PWM is experimentally validated and compared with the conventional PWM and the dead-time elimination PWM at 100 kHz on a three-phase converter with SiC mosfets and SiC Schottky diodes, showing the merits of low voltage losses, low output harmonics, high dc-link voltage utilization, large linear modulation region, and high efficiency.

DC-link loop bandwidth selection strategy for grid-connected inverters considering power quality requirements

DC-link voltage and output current control loops are two cascaded loops in the control structure of grid-connected inverters. A high DC-link voltage loop bandwidth (DCL-BW) provides more disturbance rejection capability for the control loop and is preferred from control system perspective. However, for stability issues, this BW is limited and must be sufficiently less than that of the current control loop. Among the different control schemes, instantaneous active reactive control (IARC) method provides the highest possible DCL-BW (i.e., 0.02 × switching frequency). Having this degree of freedom in the controller design, a proper methodology should be defined for selection of DCL-BW. In this paper, different factors including the output current harmonic distortion, low-order harmonics on DC link voltage, and the inverter nominal rating are introduced as affecting parameters for DCL-BW selection. Accordingly, a DCL-BW selection strategy based on output current harmonic distortion is proposed. The proposed method enhances the power quality indices of the grid-connected inverters, which especially is useful for few kVA power inverters. Finally, to validate the conducted analysis, experimental and simulation results are provided.

Investigation Study of Multi-Mode Multi-Speed Operation Method for Surface-Mounted Permanent Magnet Synchronous Machines

Typically, a surface-mounted permanent magnet synchronous machine (SPMSM) has a poor flux-weakening performance due to its low synchronous inductance; hence, its speed is proportional to the supply voltage. A relatively high DC-link voltage is required to operate an SPMSM at higher speeds. This paper proposes a multi-mode multi-speed operation method to overcome this issue. With this method, the total line-to-line back electromotive force (EMF) is modified using the winding switching. Each phase of the stator winding is divided into two equal coils. There are four modes of operation of the machine. In mode I, the three-phase stator winding is connected in a wye configuration. In modes II, III, and IV, the winding is reconfigured such that the sum of the individual back EMFs of the coils exhibits a difference in voltages between the line terminals. The back EMF in these modes decreased by 1.74, 3.67, and 6.49 times, respectively, compared with that in mode I. This resulted in a four-speed operation, with base speeds of 1, 1.97, 3.68, and 7.47 pu in modes I-IV, respectively. Herein, the analytical model of the machine and drive topology are explained and demonstrated. Simulation results obtained using the 2-D finite element method are presented along with the experimental measurements to verify the feasibility of the proposed method.

Study of interactions between close HVDC links inserted in an AC grid: A mixed nonlinear and modal analysis approach

This paper focuses on interaction between two closed-connected high-voltage DC (HVDC) lines. This interaction is studied by employing high-fidelity nonlinear modeling in MATLAB R /Simulink R software environment. In order to describe the mechanism behind the HVDCs interaction, both nonlinear time-domain simulations and modal analysis of the coupled HVDC links, have been performed. System small-signal stability has been assessed and the path of interactions has been identified by computing the participations of various states in the oscillatory modes; this sets the preliminaries for a global, grid-oriented HVDCs control design approach. After the detailed analysis of newly-identified coupling oscillations , the minimal modeling requirements to put them into evidence have also been studied; this significantly facilitates the analysis in a realistic large-scale grid context.

Novel Active-Neutral-Point-Clamped Inverters With Improved Voltage-Boosting Capability

Conventional active-neutral-point-clamped (ANPC) inverters exhibit low voltage gain that inherently leads to a high dc-link voltage requirement. An improved ANPC inverter that is capable of generating five voltage levels has recently reduced the dc-link voltage twofold to achieve unity gain. This led to the development of a single-stage dc–ac power converter with no frontend boost dc–dc converter. This article proposes novel ANPC inverters capable of unity or boosted voltage gain while generating higher voltage levels. The first topology can provide a voltage gain of 1.5 and extends the number of levels to seven by incorporating only one additional switch. The topology can also be extended by adding three switches and one floating capacitor to generate nine levels with unity voltage gain, or 11 levels with a voltage gain of 2.5. The proposed ANPC inverters and their operations are comprehensively discussed. Experimental results are provided to validate the feasibility of the proposed ANPC inverters.

2354 Published By: Blue Eyes Intelligence Engineering & Sciences Publication Retrieval Number J1160881019/2019©BEIESP DOI: 10.35940/ijitee.J1160.0981119 Journal Website: www.ijitee.org High Voltage DC Transmission Systems for Offshore Wind Farms with Different Topologies

Theoretical review of various topologies of high voltage DC links in application to off shore wind forms has been studied and analysed. In addition to that, various types of high voltage DC links such as back to back, two terminal, multi-terminal systems has been covered under this study. The Line-Commutated Converters, Voltage Source Converter, Modular Multi-Level Converter as well as some of advanced hybrid high voltage DC topologies in application to off shore wind forms has been reviewed. This study covers complication arising from large-scale wind power generation. The review paper also points out the scope of future research in high voltage DC converters.

Input Impedance Modeling and Verification of Single-Phase Voltage Source Converters Based on Harmonic Linearization

The impedance-based stability analysis of single-phase voltage source converters (VSCs) emphasizes the precise impedance modeling. Considering the complexity of $dq$ -frame impedance modeling and measurement, this paper proposes a mirror-frequency impedance modeling and measurement approach of single-phase VSCs in the stationary frame based on harmonic linearization. First, the mirror frequency definition is put forward and the dynamics of the second-order generalized phase-locked loop considering the mirror-frequency effects is validated by simulations. Next, the impedance model considering the mirror-frequency effects and dc-link voltage control loops is compared with the conventional one-dimension impedance model and a mirror-frequency impedance measurement method is presented, which are verified by experiments on the hardware-in-the-loop platform. The results indicate that the mirror-frequency impedance model considering the dc-link voltage control is more accurate and better captures the cross-coupling dynamics of VSCs. Finally, the different bandwidths of dc-link voltage control loops and different capacitances are designed to study the impacts of dc-link voltage control loops and ripples on impedance. The experiments and impedance-based analysis illustrate that the wider bandwidth of dc-link voltage control loops or the less capacitance with high dc-link voltage ripple will expand the frequency range of negative impedance in the low frequency.

Dual-T-Type Seven-Level Boost Active-Neutral-Point-Clamped Inverter

The conventional three-level active-neutral-point-clamped (ANPC) inverter requires high dc-link voltage at least twice the peak of ac output. To reduce the dc-link voltage, a recent topology that enhanced the voltage gain from half to unity has been presented. In this letter, an alternative ANPC topology is established by using two T-type inverters. Two floating capacitors with self-voltage balancing capability are integrated to achieve a voltage-boosting gain of 1.5. In addition, the proposed topology is capable of generating seven voltage levels. Its operation is validated through circuit analysis followed by experimental results of a prototype.

Load Frequency Control of HVDC Link Interconnected Power System Using Genetic Algorithm

Advances in power electronics have improved grid support functions such as tie-line power control and frequency control, making renewable generation and High Voltage DC (HVDC) links more common in power system applications. Load Frequency Control (LFC) systems handle the complex interactions between the distributed generator and the control area with the HVDC link. In this work, LFC of a two-zone system including parallel AC/DC transmission links has been analysed. The parameters of this system are optimised using advanced genetic algorithm resulting in improved performance of system in terms of reduction in peak overshoots and settling time of frequency measurement, tie line power flow and area control error signals in an interconnected power system. The advantage of having parallel HVDC links is also demonstrated when performance is compared to system interconnected with only AC tie line.

Evaluation of Strength Measure for Static Voltage Stability Analysis of Hybrid Multi-Infeed DC Systems

This paper presents an evaluation method of strength measure for the static voltage stability analysis of the hybrid multi-infeed dc (HMIdc) system, which comprises both the line-commutated converter-based high voltage dc (LCC-HVdc) and voltage source converter-based HVdc (VSC-HVdc) links. For this purpose, the coupled single-port model proposed originally to assess the static voltage stability of ac power systems is first extended in the HMIdc system. In particular, how the LCC-HVdc and VSC-HVdc links should be modeled in the extended coupled single-port model is further investigated. Then, a generalized effective short-circuit ratio (GESCR) index is proposed as a strength measure for the static voltage stability analysis of the HMIdc system using the extended coupled single-port model. The GESCR can resolve the ambiguity encountered by earlier indices arising from empirical reasoning; thus, it is able to indicate the static voltage stability of the HMIdc system more accurately. Finally, the simulated results based on a classical simplified HMIdc system and a modified IEEE 118-bus system, both including two LCC-HVdc links and a VSC-HVdc link, validate the proposed GESCR.

High Step-Up Y-Source Inverter With Reduced DC-Link Voltage Spikes

Impedance-source inverters using coupled inductors have been investigated as alternatives for providing high step-up voltages. However, leakage inductances of the coupled inductors have commonly led to lower overall effectiveness, in addition to generating high dc-link voltage spikes. The latter raises voltage stresses of switches, which in turn, may reduce the power levels of the inverters. A high step-up Y-source inverter has therefore been proposed in this paper to provide a high boost with a smooth dc-link voltage ensured by proper recycling of the leakage energy. These features have been verified by comparing simulation and experimental results of an existing Y-source and the proposed inverters. Factors compared are their respective boost ratios, voltage stresses, current stresses, and dc-link voltage spikes.

A 300-kHz 6.6-kW SiC Bidirectional LLC Onboard Charger

The reverse LLC voltage gain is lower than unity. The dc-link voltage is then lower than the peak grid voltage so that the buck type dc–ac converters cannot be grid-tied directly, which causes the LLC reverse operation difficult in bidirectional application. This paper proposes a silicon carbide (SiC) bidirectional LLC charger architecture to achieve high efficiency and high power density. The first stage is an interleaved bridgeless totem pole power factor correction (PFC) to achieve unity power factor. The second stage is a 300-kHz LLC taking advantage of wide zero voltage switching (ZVS) range and magnetic integration. Thanks to extra control freedom of high dc-link voltage, the dc–dc voltage gain regulation is shared by the dc–ac stage taking advantage of high voltage SiC MOSFETs. A reverse LLC voltage gain compensation control by regulating dc-link voltage is proposed to enable the LLC bidirectional operation. A digital adaptive synchronous rectification driving scheme is proposed based on the LLC primary driver signals, which immunes to the circuit oscillation caused by high dv/dt and parasitic elements. A 6.6 kW SiC bidirectional LLC charger is build. The power density is 3.42 kW/L with 3 kW/kg and increases 55.6% over the reference design. The charging efficiency is above 96% through the battery voltage from 240 to 420 V, and 2% higher than the state-of-the-art products. The peak discharging efficiency under 6.6 kW is 96%, and 1% higher than the state-of-the-art efficiency.

A Transformerless Three-Port Nonagonal MMC for the Grid Connection and Local Consumption of Distributed Generation

While large-scale distributed generation (DG) grid connection requires its grid-connected converter to be able to boost the output voltage and connect it to the host grid, its priority is still to consume the power locally so as to increase the energy utilization rate. Using a common ac bus to transmit the distributed power to local load requires two separate converters, resulting in large size and multistage power conversion. Therefore, to lose the transformer and increase the compactness, a three-port nonagonal modular multilevel converter (MMC) is proposed in this paper, which consists of nine bridge arms connected from end to end. From the vertexes, three 3-phase ac ports protrude, connecting DG, ac grid, and local load together. By reusing power modules, with only nine bridge arms, the proposed nonagonal MMC realized three-port direct ac/ac power conversion in a single converter. Furthermore, the topology can operate without a transformer or high-voltage dc link, and under low frequency. It is then compared with other topologies to demonstrate its advantage in applications that require high compactness and flexibility. Finally, the proposed topology and its corresponding control are verified in RTlab to be effective for DG grid connection and local consumption.

Control Approach to Enhance the Performance of a Current-Fed Switched Inverter

Current-fed switched inverter (CFSI), a derivative of Z-source inverter, has the advantages of lower component count, single-stage conversion, high gain, and inherent shootthrough protection. But due to the limitation of modulation index, CFSI is forced to operate at higher dc-link voltage. As a result, voltage stress across the switches and capacitors increases. To mitigate this problem, a new pulsewidth modulation (PWM) technique is proposed. With the proposed scheme, CFSI can be operated at a lower dc-link voltage by increasing the modulation index. But operating at lower dc-link voltage increases peakto-peak ripple in the inductor current and leads to condition of early discontinuous conduction mode. Therefore, this paper also implemented a variable duty cycle (VDC) scheme, which reduces the peak-to-peak ripple in the input inductor current and increases the continuous conduction mode (CCM) operating range of CFSI. The proposed PWM scheme along with VDC scheme is validated for CFSI, which helps to: 1) reduce the voltage stress form 280 to 180 V; 2) reduce the input current ripple from 53.33% to 15.15%; 3) enlarge CCM range from 56% to 82%; and 4) improve the efficiency by 3.5%.

Circulating current control scheme of modular multilevel converters supplying passive networks under unbalanced load conditions

Load imbalances in the supply of passive loads with modular multilevel converter (MMC)-based medium (MVDC) and high voltage (HVDC) dc-links are inevitable. In such applications, the maximum-allowable voltage ripples and peak arm currents define the operating limits of the MMCs. This paper demonstrates the impact of circulating current control in capacitor voltage ripples of sub-modules and peak arm currents under unbalanced load conditions when supplying passive networks, comparing the effects of three circulating current control methods on internal variables of the MMCs. The efficacy of the circulating current control is validated through real-time digital simulations of an 800 MVA benchmark MMC-based HVDC transmission system.

High Voltage DC Transmission with Power Synchronization Control using VSC

The main scope of this paper is a propose suitable power synchronization control scheme for high voltage dc links based on voltage source converter (VSC), operating with ac grid. The proposed of power synchronization control (PSC) concept, where grid synchronization achieved without use of dedicated synchronization unit in MATLAB/Simulink. PSC concept used, where detect the power at the sending side and meet the system power demand at receiving side through high voltage. PLL system is also implemented for synchronize converter with grid and current control loop system is perform to minimize the active reactive power demand in grid. We can develop the VSC-HVDC link model in MATLAB/Simulink to observer our system response.

Frequency stability control method considering limited EDCPS

Aiming at the problem of frequency stability of the asynchronous AC/DC hybrid power grid, a prediction model of post-disturbance steady frequency is proposed, which considers the emergency DC power support (EDCPS) and the frequency response of AC grids on both sides of the high-voltage DC (HVDC) link. This post-disturbance steady frequency of both AC grids can be predicted quickly by utilising the wide area measurement system data, and the desired amount of EDCPS can be calculated directly by using the setting value of the steady frequency for the disturbed AC grid. Then this study considers the fact that the voltage level of the AC buses at both ends of the HVDC system will affect the operation modes of the HVDC system, resulting in the limited EDCPS. Therefore, a calculation method of the actual EDCPS command for steady frequency control is proposed. It can automatically identify the actual operation mode of the post-disturbance HVDC system and calculate the actual EDCPS command corresponding to the desired EDCPS value. Simulations of a typical two-area AC/DC hybrid power grid are carried out to validate the effectiveness of the proposed method.

Hybrid‐wave modulation for modular multilevel high‐frequency DC converter in DC distribution application

Compared with AC distribution networks, DC distribution networks reduce the utilisation of power converters, power loss, and construction cost. In DC distribution network, the high-frequency modular multilevel DC converter (MDCC), which connects the low-voltage DC link with medium- or high-voltage DC links, becomes a practical equipment to realise the bus connection, voltage conversion, and electrical isolation. Especially, high-frequency link (HFL) modulation plays an important role in converter performance. The square-wave (SW) modulation obtains better power transfer capability and higher DC voltage utilisation. However, the SW modulation leads to a large dv/dt stress upon HFL transformer. The sinusoidal, triangle and quasi-square-wave (QSW) modulations reduce dv/dt and harmonics significantly. However, the power transfer capability and DC voltage utilisation are also reduced. To address dv/dt stress problem, ensure high DC-voltage utilisation and maintain a strong power transfer capability simultaneously, a hybrid-wave modulation strategy, which composed of QSW and SW modulation, is proposed for MDCC. The experimental results verify the analysis and proposed scheme.

A Nonlinear Controller Based on a High Gain Observer for a Cascade Boost Converter in a Fuel Cell Distributed Power Supply System

This study aims to design a controller for a cascade boost converter connected to a fuel cell in a distributed generator system. Knowing that the fuel cell voltage is too low to satisfy load requirement, and it varies with a nonlinear manner depending on the load current with uncertainties lying on fuel cell operating conditions, we use a DC converter to step up the low voltage and find out a control law to regulate the converter output voltage. Here, a lyapunov stability technique is used to design the nonlinear controller. An average current mode control method is adopted to satisfy the above objective. Moreover, a high gain observer is developed to estimate the converter output current instead of to be captured. Besides, the cascade boost converter is proved to be more suitable for high DC link voltage compared to a single boost converter. Theoretical studies and simulation using Matlab Simpower System tools show that the adaptive controller meets the objectives for what it is designed.

Power system decarbonisation with Global Energy Interconnection – a case study on the economic viability of international transmission network in Australasia

The electric energy system in Indonesia is undergoing with the challenges of fast-increasing electricity demand, carbon constraints, and rising costs. Using our model of the Australian and Indonesian electrical grids (either separately or interconnected) that incorporates operational flexibility in capacity expansion planning, we first show that meeting the projected demand for Java and Bali—the main Indonesian grid, with 100% locally integrated renewables by 2050 would be challenging. However, a submarine high-voltage DC (HVDC) link connecting Indonesia’s Java-Bali power grid to the Australian National Electricity Market (NEM) grid through the Northern Territory would help alleviate this situation, given Australia’s abundant renewable energy resources. Then, our model reveals that the Australian NEM could also profit from additional renewables if connected to the Northern Territory through a ground HVDC transmission line to gather intermittent wind and solar generation, which would be curtailed otherwise if unused by Indonesia through the submarine link. Despite the expensiveness of long HVDC links, the wholesale electricity cost of the integrated 100% renewable Australasia power system could be reduced by over 16%, from $AUD177/MWh with only local renewables to $AUD148/MWh with integrated HVDC transmission. The model retrieved the optimal international HVDC link with capacity of 43.8 GW, and the optimal regional HVDC transmission line with a capacity of 5.5 GW. To the best of our knowledge, this is the first detailed model on power system decarbonisation planning for both Australian NEM and Indonesian Java-Bali power grid considering HVDC interconnections.