High Dc-link Voltage Research Articles

Half- and Full-Bridge Modular Multilevel Converter Models for Simulations of Full-Scale HVDC Links and Multiterminal DC Grids

This paper presents an improved electromagnetic transient (EMT) simulation model for the half- and full-bridge modular multilevel converters (MMCs) that can be used for full-scale simulation of multilevel high-voltage dc (HVDC) transmission systems, with hundreds of cells per arm. The presented models employ minimum software overhead within their EMT parts to correctly represent MMCs behavior during dc network faults when converter switching devices are blocked. The validity and scalabilities of the presented models are demonstrated using open-loop simulations of the half- and full-bridge MMCs, and closed-loop simulation of a full-scale HVDC link, with 201 cells per arm that equipped with basic HVDC controllers, including that for suppression of the second harmonic currents in the converter arms. The results obtained from both demonstrations have shown that the presented models are able to accurately simulate the typical behavior of the MMC during normal, and ac and dc network faults.

Three-Phase AC-Side Voltage-Doubling High Power Density Voltage Source Converter With Intrinsic Buck–Boost Cell and Common-Mode Voltage Suppression

The three-phase two-level voltage source converter (VSC) is widely employed in power conversions between ac and dc for its four-quadrant operation and control flexibility. However, it suffers from the low output voltage range with a peak value of half dc-link per phase, which necessitates the use of either high dc-link voltage or bulky step-up transformer to enable the medium voltage operation. Additionally, the high common-mode (CM) voltage between ac loads neutral points and ground may reduce the service life and reliability of electric machinery. In this paper, a three-phase ac-side voltage-doubling VSC topology with intrinsic buck–boost cell is analyzed. By this configuration, the ac-side voltage is doubled with the phase peak value equal to dc link. That is, only half of the dc-side capacitor bank is needed to generate the same output voltage. The proposed converter uses its buck–boost cell as a virtual voltage source to synthesize negative half of the output voltage by modulating its output ac phase voltage around the negative bus (which is the real zero when grounded). This permits the average CM voltage to be suppressed to zero, and loads connected to converter ac side not to withstand any dc voltage stress (reducing the insulation requirement). Modeling and control design for both rectifier and inverter modes of this converter in synchronous reference frame have been investigated to ensure a four-quadrant three-phase back-to-back system. Experimental results have verified the feasibility and the effectiveness of the proposed configuration and the designed control strategies.

Stochastic Multiperiod OPF Model of Power Systems With HVDC-Connected Intermittent Wind Power Generation

This paper presents a new model for a stochastic multiperiod optimal power-flow (SMP-OPF) problem which includes an offshore wind farm connected to the grid by a line-commutated converter high-voltage dc link. The offshore wind farm is composed of doubly fed induction generators (DFIGs), and the DFIG's capability curve is considered in order to obtain a more realistic dispatch for wind farms. The uncertainties of wind power generation are also taken into account using a scenario-based approach which can be adopted by the system operator to obtain the optimal active and reactive power schedules for thermal and wind power generation units. To illustrate the effectiveness of the proposed approach, it is applied on the IEEE 118-bus test system. The obtained results demonstrate the capability of the proposed SMP-OPF model for the determination of the optimal operation of power systems.

Offshore Wind Integration to a Weak Grid by VSC-HVDC Links Using Power-Synchronization Control: A Case Study

In this paper, the application of a recently invented power-synchronization control is proposed for integrating a doubly fed induction generator (DFIG)-based offshore wind farm to a weak ac grid through a voltage-source converter (VSC)-based high-voltage dc link. The control strategy, along with the antiwindup techniques and the bumpless transfer between two different control modes, is elaborately discussed. Two different fault cases, namely, onshore and offshore faults are considered and the fault-ride through techniques are presented. In case of the onshore fault, both with-chopper and without-chopper solutions are investigated. For an offshore fault, a coordinated fault-ridethrough scheme is proposed when the offshore HVDC converter and the wind farm are in voltage-control modes. The entire study is carried out in a real-time digital simulator (RTDS) platform.

Performance Analysis of a Novel Reduced Switch Cascaded Multilevel Inverter

Multilevel inverters have been widely used for high-voltage and high-power applications. Their performance is greatly superior to that of conventional two-level inverters due to their reduced total harmonic distortion (THD), lower switch ratings, lower electromagnetic interference, and higher dc link voltages. However, they have some disadvantages such as an increased number of components, a complex pulse width modulation control method, and a voltage-balancing problem. In this paper, a novel nine-level reduced switch cascaded multilevel inverter based on a multilevel DC link (MLDCL) inverter topology with reduced switching components is proposed to improve the multilevel inverter performance by compensating the above mentioned disadvantages. This topology requires fewer components when compared to diode clamped, flying capacitor and cascaded inverters and it requires fewer carrier signals and gate drives. Therefore, the overall cost and circuit complexity are greatly reduced. This paper presents modulation methods by a novel reference and multicarrier based PWM schemes for reduced switch cascaded multilevel inverters (RSCMLI). It also compares the performance of the proposed scheme with that of conventional cascaded multilevel inverters (CCMLI). Simulation results from MATLAB/SIMULINK are presented to verify the performance of the nine-level RSCMLI. Finally, a prototype of the nine-level RSCMLI topology is built and tested to show the performance of the inverter through experimental results.

Design of New Multilevel Inverter Topology for Various Unipolar Triangular Carrier PWM Strategies

Multilevel inverters have been widely used for high power and high voltage applications as their performance is highly superior to that of conventional two level inverters due to reduced harmonic distortion, lower electromagnetic interference, and higher dc link voltages but it has some disadvantages such as increased number of components, complex pulse width modulation control method and voltagebalancing problem. In order to compensate the above described disadvantages a new topology with a reversing voltage component is proposed to improve the multilevel performance. This topology requires fewer components compared to existing inverters (particularly in higher levels) and requires fewer carrier signals and gate drives. Therefore, the overall cost and complexity are greatly reduced particularly for higher output voltage levels. Finally, a power circuit of the seven-level proposed topology is developed and simulated with different modulation strategies to show the performance of the inverter by simulation results using MATLAB-SIMULINK. By comparing the various Unipolar Pulse Width Modulation (UPWM) techniques, it is observed that UPDPWM provides less THD and UCOPWM techniques provide higher fundamental RMS output voltage. Compared to the conventional MLI’s like DCMLI, FCMLI and CMLI the proposed topology will reduce the number of switches, clamping diodes and clamping capacitors. The proposed topology will have the advantages if the number of levels is increased. The total harmonic distortion will be within IEEE standards. The main object of the proposed topology is to reduce the number of switches compared to conventional MLI’s and to achieve the THD within IEEE standards without any additional filter.

Interconnection of Direct-Drive Wind Turbines Using a Series-Connected DC Grid

This paper presents a “distributed high-voltage dc (HVDC) converter” for offshore wind farms. The proposed converter topology allows series interconnection of wind turbines with the need of neither ac transformer nor offshore platform at the sending end. Each wind turbine is equipped with a 5-MW permanent-magnet synchronous generator and an ac-dc-dc converter. The converter topology is a diode rectifier (ac-dc) cascaded with a single-switch step-down converter (dc-dc). The dc-dc stage allows the current to flow at all times in the dc link while simultaneously regulating generator torque. The inverter station, located onshore, is a thyristor-based converter that performs dc link current regulation. It also regulates the HVDC link voltage through supervisory inverter controls. A complete wind farm is simulated using the PSCAD/EMTDC software package. The 150-MW wind farm is modelled using six units of 25 MW with a rated dc link voltage of 125 kV at 1.2 kA. The simulation demonstrates the stable operation of the proposed configuration where each turbine is able to independently perform peak power tracking.

Simulation Model of Single-Phase Simplified Eleven-Level Inverter

paper presents simulation model of a single phase simplified eleven-level inverter (SELI). Multilevel inverter offers high power capability. Its performance is highly superior to that of conventional two-level inverter due to reduced harmonic distortion, lower electromagnetic interference and higher dc link voltage. The inverter is capable of producing eleven levels of output voltages (Vdc, 4Vdc/5, 3vdc/5, 2Vdc/5, Vdc/5, 0, -Vdc/5, -2Vdc/5, -3Vdc/5, -4Vdc/5, -Vdc) from the DC supply voltage. Theoretical predictions are validated using MATLAB Simulink tool box. Keywordsclamped, diode clamped, multilevel inverter, H- bridge, Simplified Eleven-level Inverter (SELI).

8200호대 전기기관차 객차전원공급장치(HEP)의 출력전압품질향상을 위한 최적화된 PWM 방법

HEP(Head Electric Power) system, supplying 3-phase service power to the coach vehicles, is a kind of special auxiliary power equipment which is boarded on 8200 series electric locomotives in KORAIL. This equipment shares high voltage DC link with a main propulsion converter/inverter systems. It was difficult to use high frequency PWM technique so that GTO has been used as a power device same like the main power system. Due to low PWM frequency(300㎐) of HEP inverter, the output voltage has less power quality comparing to normal SIV(Static Inverter) system. In this paper, an optimal PWM strategy is presented for new IGBT type HEP inverter system. Several PWM techniques were investigated to improve output voltage quality under fixed lower filter inductance and not high PWM frequency. Finally PC simulations have been done to clarify its availability.

Magnetically Coupled Impedance-Source Inverters

Z-source inverters are a new class of inverters proposed with output voltage or current buck-boost ability. Despite their general attractiveness, there are some present limitations faced by existing Z-source inverters, most of which are linked to their requirement for low modulation ratio at high input-to-output gain and the presence of an impedance network. The former means a high dc-link voltage, which can stress the semiconductor switches unnecessarily. The latter leads to increases in cost and size, which similarly are undesirable. To lessen these concerns, an interesting approach is to use magnetically coupled transformers or inductors to raise the gain and modulation ratio simultaneously, while reducing the number of passive components needed. A study of the approach is now presented to show how various existing magnetically coupled inverters can be derived by applying a generic methodology. The same methodology is then applied to develop more magnetically coupled Z-source inverters with advantages that have not been identified in the literature. These findings have already been proven in experiments.

Improved Pulse-Width Modulation Strategies for Diode-Assisted Buck–Boost Voltage Source Inverter

The recently proposed diode-assisted buck–boost voltage source inverter (VSI) enhances the voltage boost capability greatly without extreme large boost duty ratio, which is more suitable for wide range voltage regulation in dc/ac power conversion. This paper mainly explores pulse-width modulation (PWM) strategies for diode-assisted buck–boost VSI and their relationship of voltage gain versus voltage boost duty ratio. In view of variable intermediate dc-link voltage, two kinds of improved PWM strategies with high dc-link voltage utilization are presented to obtain the maximum linear voltage gain in theory, as well as to minimize the voltage stress of switching devices. The operation principle, and the relationship of voltage gain versus voltage boost duty ratio and switching device voltage stress versus voltage gain are analyzed in detail. The related simulation and experiments are implemented to verify the theoretical analysis. Compared with existing counterparts, diode-assisted buck–boost VSI with improved PWM strategies demonstrates good performances: less switching device requirement and higher efficiency at high voltage gain. Owing to improved performance, diode-assisted buck–boost VSI is more promising and competitive topology for wide range dc/ac power conversion in a renewable energy application. Furthermore, the idea of improved modulation strategies can also be extended to the control of diode-assisted buck–boost current source inverter.

Sliding Mode Control of Energy Recovery Transformer-Less Power Accumulator Battery Pack Testing System

The main objective of the present work is to apply sliding mode controller to medium voltage and high power output energy recovery power accumulator battery pack testing systems (ERPABPTS), which is composed of a three-level neutral point clamped three-phase voltage source inverter (NPC-VSI) and a non-isolating DC-DC converter. A decoupled scheme for the proposed system is put forward, based on that, two sliding mode plane for active and reactive current component control are illustrated. The ERPBPTS is used to integrate discharging energy to the power grid with high efficiency when performing current test, active and reactive power references for the grid-connected inverter are determined based on the discharging energy from the DC-DC converter. Without using the isolation transformer, the proposed scheme could greatly reduce the total volume of the system, offer less total harmonic distortion, and meanwhile maintain higher DC-link voltage utilization. The proposed method is implemented and verified both by simulation and experimental results on a laboratory hardware platform using 175kW IGBT-based ERPABTS using an embedded DSP controller

Design and Implementation of a New Multilevel Inverter Topology

Multilevel inverters have been widely accepted for high-power high-voltage applications. Their performance is highly superior to that of conventional two-level inverters due to reduced harmonic distortion, lower electromagnetic interference, and higher dc link voltages. However, it has some disadvantages such as increased number of components, complex pulsewidth modulation control method, and voltage-balancing problem. In this paper, a new topology with a reversing-voltage component is proposed to improve the multilevel performance by compensating the disadvantages mentioned. This topology requires fewer components compared to existing inverters (particularly in higher levels) and requires fewer carrier signals and gate drives. Therefore, the overall cost and complexity are greatly reduced particularly for higher output voltage levels. Finally, a prototype of the seven-level proposed topology is built and tested to show the performance of the inverter by experimental results.

Improvement of the Low-Voltage Ride-through Capability of Doubly Fed Induction Generator Wind Turbines

So far, active crowbars are a preferred technique for doubly fed induction generator (DFIG) wind turbines, which is used to protect the power converter against over-current and undesirably high dc link voltage during voltage dip. However, its main drawbacks are that (1) the DFIG absorbs reactive power from the grid during grid voltage dips, (2) the crowbar activation increases the acceleration of the rotor and so, deteriorates the dynamic stability of DFIG, and (3) the control is not flexible for long-time voltage sags. In the paper, three different initiating logic control methods of crowbar protection are compared, and how low-voltage ride-through (LVRT) characteristics of DFIG wind turbines with active crowbar are affected by different switching logic control modes of the crowbar are investigated. According to the comparison results, an improved crowbar switching control strategy is proposed to reduce its operation time and improve the LVRT capability of DFIG wind turbines. In addition, an emergency pitch blade angle control scheme to reduce the acceleration of the rotor and prevent the over-speeding of rotor is presented in detail, and as a result, the LVRT capability of DFIG wind turbines is enhanced even during long-time voltage sags. Finally, the presented control strategies are validated in simulation tool Matalab/Simulink for a 1.5MW generator.

Experimental Validation of High-Voltage-Ratio Low-Input-Current-Ripple Converters for Hybrid Fuel Cell Supercapacitor Systems

Electric vehicle technology has been adopting fuel cells (FCs) for hybrid applications over the past few years. Therefore, the development of advanced power electronic systems for the integration of fuel cells with on-board energy management is fundamental for achieving high-performance systems. An FC for vehicular applications is usually a low-voltage current-source like device that produces electricity and heat directly from input hydrogen and oxygen. Most often, it is required that the FCs be stacked for high-voltage dc-link in order to supply the input power for the drivetrain and electric motor drive system. The FC has a nonlinear nature, and it must be controlled to operate in the high-efficiency operating range. Hybrid electric vehicles have physical constraints such as volume and weight under limited cost and expected lifetime. There is a need for high-voltage input/output ratio of dc-dc boost converters to be connected between the FC to the motor drive dc-link. In addition, it is necessary to have low input ripple at the dc-dc boost converter in order to maximize the FC lifetime, and the traditional dc-dc boost converter topologies have poor performance on these specifications. This paper proposes a new dc-dc converter family of topologies aimed at improving the application to electric vehicle power control. This family is defined as floating-interleaving boost converters (FIBCs). The paper will thoroughly show analysis and experimental verification of FIBC's, and they will be compared with conventional boost converter characteristics. The paper supports how performance figures related to the passive components, i.e., the inductor and capacitor, will have better volume and weight, extremely low input current ripple, and improved efficiency and transfer ratio. The analysis presented in this paper shows how to choose the most suitable topology in order to achieve the desired specifications. The selected topology is fully validated experimentally using advanced nonlinear sliding mode control, which has the additional feature of operating even in faulty conditions.

Optimal Automatic Generation Control of Interconnected Power System with Asynchronous Tie-lines under Deregulated Environment

This article presents the design of optimal automatic generation control regulators for an interconnected power system operating in a deregulated environment. An extra-high-voltage AC transmission line in parallel with a high-voltage DC link is considered as an area interconnection between the two areas. The dynamic model of the power system under consideration is developed by incorporating various types of possible market transaction environments. The designed optimal automatic generation control regulators are implemented, and dynamic system responses for various system states are obtained considering load perturbation in one of the areas. The patterns of open-loop and closed-loop eigenvalues are also determined to ensure system stability with and without optimal automatic generation control regulators. From the investigations carried out in the work, it is inferred that the dynamics performance of the system under consideration has an appreciable improvement when a high-voltage DC link is incorporated in parallel with an extra-high-voltage AC line as an interconnection between two areas as compared to that when only an extra-high-voltage AC line is considered as an area interconnection. The frequency deviations are more in the area in which distribution companies violate the contracts. Moreover, optimal automatic generation control regulators are able to demonstrate the matching of demand with generation under a deregulated environment with different market transactions. The power system model under investigation is marginally stable in open-loop operating mode, while it attains good stability margins in closed-loop mode.

A Power Tracking Control Method for Single-Phase PWM Rectifier

Power control method had been widely used because of its simplicity, good dynamic performance, decoupled active and reactive power control for three-phase PWM rectifier. But it is not adapted to single-phase rectifier as power calculating problem. A power tracking control method is proposed for single-phase PWM rectifier. In new method, voltage and current with harmonics are decomposed of Fourier series. Their fundamental components are only extracted in order to calculate active power and reactive power and track requirement value. So the system not only realizes high power factor, sinusoidal current and stable DC link voltage, but also meet the application demand the capability of power regeneration to the power supply. The results of experiment show that the DC link voltage fluctuations should be minimal and the power factor is high. The proposed method is effective and practicable.

PV Power-Generation System with a Phase-Shift PWM Technique for High Step-Up Voltage Applications

A PV power-generation system with a phase-shift pulse-width modulation (PWM) technique for high step-up voltage applications is proposed. The proposed power-generation system consists of two stages. In the input stage, all power switches of the full-bridge converter with phase-shift technique can be operated with zero-current switching (ZCS) at turn-on or turn-off transition. Hence, the switching losses of the power switches can be reduced. Then, in the DC output stage, a voltage-doubler circuit is used to boost a high dc-link bus voltage. To supply a utility power, a dc/ac inverter is connected to induce a sinusoidal source. In order to draw a maximum power from PV arrays source, a microcontroller is incorporated with the perturbation and observation method to implement maximum power point tracking (MPPT) algorithm and power regulating scheme. In this study, a full load power of 300 W prototype has been built. Experimental results are presented to verify the performance and feasibility of the proposed PV power-generation system.

Photovoltaic panel integrated power conditioning system using a high efficiency step-up DC–DC converter

This paper presents a high efficiency photovoltaic (PV) panel integrated power conditioning system (PCS) by proposing a high efficiency step-up DC–DC converter. The suggested PCS consists of a high efficiency DC–DC converter and a single-phase DC–AC inverter. Each PV panel has its own DC–AC converter, performing the maximum power point tracking (MPPT) function and increasing its flexibility and expandability. Moreover, the proposed DC–DC converter converts the low PV panel voltage into a high DC-link voltage with a high step-up voltage conversion ratio. It can reduce the switching power losses, increasing the power conversion efficiency. The performance of the suggested PCS has been verified through a 180 W prototype of the PCS for a 60 Hz/120 V ac power grid. The proposed DC–DC converter achieves a high efficiency of 96.0%. The PCS including the DC–DC converter and DC–AC inverter achieves an efficiency of 93.1% with an almost unity power factor.

Decoupled Space-Vector PWM Strategies for a Four-Level Asymmetrical Open-End Winding Induction Motor Drive With Waveform Symmetries

In this paper, two space-vector-based pulsewidth modulation (PWM) (SVPWM) strategies named equal- and proportional-duty SVPWMs are described, which are used to synthesize a four-level waveform from an open-end winding configuration of an induction motor. Two isolated dc-link voltages, which are in the ratio of 2 : 1, are employed to achieve this objective. Both of these PWM strategies achieve the avoidance of overcharging of the dc-link capacitor of an inverter operating with lower dc-link voltage by its counterpart with higher dc-link voltage. Implementation of these PWM strategies requires only the instantaneous three-phase reference voltages, eliminating the need of sector identification or lookup tables. The numbers of samples per cycle for individual inverters are selected in such a way that the quarter-wave, half-wave, and three-phase symmetries are achieved for the dual-inverter drive despite unequal dc-link voltages for the constituent inverters. It is also shown that one of the two PWM techniques, called the proportional-duty SVPWM, results in a better spectral performance and lower switching power loss in the overall dual-inverter system compared to the equal-duty SVPWM.