Three-level Voltage Source Converter Research Articles

Grid-friendly Control Strategy with Dual Primary-Side Series-Connected Winding Transformers

High-power three-level voltage-source converters are widely utilized in high-performance AC drive systems. In several ultra-power instances, the harmonics on the grid side should be reduced through multiple rectifications. A combined harmonic elimination method that includes a dual primary-side series-connected winding transformer and selective harmonic elimination pulse-width modulation is proposed to eliminate low-order current harmonics on the primary and secondary sides of transformers. Through an analysis of the harmonic influence caused by dead time and DC magnetic bias, a synthetic compensation control strategy is presented to minimize the grid-side harmonics in the dual primary side series-connected winding transformer application. Both simulation and experimental results demonstrate that the proposed control strategy can significantly reduce the converter input current harmonics and eliminates the DC magnetic bias in the transformer.

Steady - State and Transient Performance of HVDC Link Based 3-Level vsc Supplying a Passive Load = Performances en Régime Stationnaire et Transitoire de la Liaison CCHT Utilisant le VSC 3 Niveaux Alimentant une Charge Passive

This paper investigates the steady-state and transient performance of high-voltage DC (HVDC) transmission systems connected to passive network. The VSC HVDC tie employing PWM may well represent the ultimate FACTS device. Besides controlling the through power flow, it can supply reactive power and provide independent dynamic control at its two terminals. The control systems for rectifier and inverter are discussed in DC (HVDC) transmission systems based on three-level voltage source converters. The study involves analysis of active-reactive power capabilities (P-Q envelope) including active power flow and provision of voltage support to AC networks. The transient performance is explored by examining the VSC_HVDC response to external AC faults. Finally, the models and results are presented and tested by simulations using Matlab Simulink and its toolbox SimPowerSystems.

Three-Level 48-Pulse STATCOM with Pulse Width Modulation

In this paper, a new control strategy of a three-level 48-pulse static synchronous compensator (STATCOM) is proposed with a constant dc link voltage and pulse width modulation at fundamental frequency switching. The proposed STATCOM is realized using eight units of three-level voltage source converters (VSCs) to form a three-level 48-pulse STATCOM. The conduction angle of each three-level VSC is modulated to control the ac converter output voltage, which controls the reactive power of the STATCOM. A fuzzy logic controller is used to control the STATCOM. The dynamic performance of the STATCOM is studied for the control of the reference reactive power, the reference terminal voltage and under the switching of inductive and capacitive loads.

Multistep Finite Control Set Model Predictive Control for Power Electronics

For direct model predictive control with reference tracking of the converter current, we derive an efficient optimization algorithm that allows us to solve the control problem for very long prediction horizons. This is achieved by adapting sphere decoding principles to the underlying optimization problem. The proposed algorithm requires only few computations and directly provides the optimal switch positions. Since the computational burden of our algorithm is effectively independent of the number of converter output levels, the concept is particularly suitable for multi-level topologies with a large number of voltage levels. Our method is illustrated for the case of a variable speed drive system with a three-level voltage source converter.

High-Frequency Operation of a DC/AC/DC System for HVDC Applications

Voltage ratings for HVdc point-to-point connections are not standardized and tend to depend on the latest available cable technology. DC/DC conversion at HV is required for interconnection of such HVdc schemes as well as to interface dc wind farms. Modular multilevel voltage source converters (VSCs), such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), have been shown to incur significantly lower switching losses than previous two- or three-level VSCs. This paper presents a dc/ac/dc system using a transformer coupling two modular multilevel VSCs. In such a system, the capacitors occupy a large fraction of the volume of the cells but a significant reduction in volume can be achieved by raising the ac frequency. Using high frequency can also bring benefits to other passive components such as the transformer but also results in higher switching losses due to the higher number of waveform steps per second. This leads to a tradeoff between volume and losses which has been explored in this study and verified by simulation results with a transistor level model of 30-MW case study. The outcome of the study shows that a frequency of 350 Hz provides a significant improvement in volume but also a penalty in losses compared to 50 Hz.

Improved Model Predictive Control of Three-level Voltage Source Converter

—In conventional model predictive control, 27 switching states for three-level power converters are evaluated online, and the state in which the cost function is the smallest will be applied in the next sampling period. The amount of calculation, however, is too high, and it may cause too high of a jump for the phase voltage or line voltage in the AC side of the three-level converter, which means that each switching action causes voltage variation to exceed Udc/2. To solve these problems, an improved model predictive method is proposed in this article. The proposed method only needs a subset of all the three-level states, which is obtained by adding and subtracting two-level state groups in the current switching state, for prediction and optimization. The high jump of the AC-side voltage is effectively avoided, and computational effort is greatly reduced. In addition, reduced switching frequency is obtained. The experiments are carried out by the two methods, and the results verify the correctness and feasibility of the proposed control strategy.

Sliding Mode Control of Three Levels Back-To-Back VSC-HVDC System Using Space Vector Modulation

In this study, a sliding mode strategy proposed to control a three levels Back-to-Back High Voltage Direct Current (HVDC) system based on the three-level voltage source converter (VSC). The voltage-balancing control of two split DC capacitors of the VSC-HVDC system is achieved using three-level space vector modulation with balancing strategy based on the effective use of the redundant switching states of the inverter voltage vectors. Finally, a complete simulation of the VSC-HVDC system validates the efficiency of the proposed strategy law. Compared to the conventional control, Sliding Mode Control scheme for the VSC-HVDC system shows the attractive advantages such as offering high tracking accuracy, fast dynamic response and good robustness. DOI: http://dx.doi.org/10.11591/ijpeds.v4i2.5575

Simulation of three‐level 24‐pulse voltage source converters‐based static synchronous compensator for reactive power control

The proposed STATCOM (static synchronous compensator) consists of two sets of three-level 24-pulse VSC (voltage source converter) gated at the fundamental frequency. The dc-link voltage of VSC is maintained at a constant value unlike the common operation of the STATCOM with a variable dc-link voltage. The phase angle between the ac voltages produced by two sets of three-level 24-pulse VSCs is varied for controlling the magnitude of the converter ac voltage which in turn controls the reactive power of the STATCOM. The proposed STATCOM has the advantage of maximum utilisation of the converter as it is operated at a constant dc-link voltage. The dynamic and steady-state performances of STATCOM are evaluated by varying the reference reactive power, the reference terminal voltage and switching on inductive and capacitive loads.

Dynamic Averaged and Simplified Models for MMC-Based HVDC Transmission Systems

Voltage-source converter (VSC) technologies are rapidly evolving and increasing the range of applications in a variety of fields within the power industry. Existing two- and three-level VSC technologies are being superseded by the new modular multilevel converter (MMC) technology for HVDC applications. The computational burden caused by detailed modeling of MMC-HVDC systems in electromagnetic transient-type (EMT-type) programs complicates the simulation of transients when such systems are integrated into large networks. This paper develops and compares different types of models for efficient and accurate representation of MMC-HVDC systems. The results show that the use of a specific type of model will depend on the conducted analysis and required accuracy.

The New High-Efficiency Hybrid Neutral-Point-Clamped Converter

This paper introduces a novel three-level voltage-source converter (VSC) as an alternative to known three-level topologies, including the conventional neutral-point-clamped converter (NPCC), many T-type VSCs, and active NPCC. It is shown that, operating in the low converter dc-link voltage range, this new solution not only can achieve higher efficiency than many typical three-level structures but also can overcome their drawback of asymmetrical semiconductor loss distribution for some operating conditions. Therefore, a remarkable increase of the converter output power capability and/or system reliability can be accomplished. The switching states and commutations of the converter, named here as hybrid NPCC (H-NPCC), are analyzed, and a loss-balancing scheme is introduced. Five- and seven-level H-NPCC topologies with loss-balancing features are also presented. Finally, a semiconductor-area-based comparison is used to further evaluate many three-level VSC systems. Interestingly, the total chip area of the proposed H-NPCC is already the lowest for low switching frequencies.

Modeling and Simulation Studies of Dual-Stage Three-Phase Grid-Connected Photovoltaic Systems

Abstract This paper proposes a full detailed mathematical model of a dual-stage, three-phase, gridconnected photovoltaic (PV) system. In this work, the system structure and the modeling techniques of each part of the PV system have been discussed, based on the MATLAB/Simulink software environment, including the PV array, the power-electronic converters (the DC-DC boost converter and the threelevel voltage source converter (3L-VSC)), and the utility interface (the low-pass filter and the coupling transformer). These are followed by introduction of the control schemes (the maximum power point tracking (MPPT) and 3L-VSC control schemes), principles of operation, and modeling steps for gridconnected PV system. A comprehensive set of simulation cases have been conducted to demonstrated the effectiveness of the mathematical model of the PV system. This work can assist power engineers in developing circuit-based simulation models for impact assessment studies, analysis, and identification of potential issues with respect to the grid integration of PV systems.

Analysis and Performance Comparison of Different Power Conditioning Systems for SMES-Based Energy Systems in Wind Turbines

Suitability of energy systems based on Superconducting Magnetic Energy Storage (SMES) has been widely tested in the field of wind energy, being able to supply power in cases such as low wind speeds or voltage dips, and to store energy when there are surpluses. This article analyzes and compares the performance of three SMES-based systems that differ in the topology of power converter: a two-level Voltage Source Converter (VSC), a three-level VSC and a two-level Current Source Converter (CSC). Their performance has been improved by means of an appropriate modulation strategy. To obtain a high reliability and accuracy, a co-simulation between MATLAB/Simulink® (running the control system) and PSIM® (running the power system) has been executed.

Harmonic optimised 24-pulse voltage source converter for high voltage DC systems

The harmonic reduction, control and design of 24-pulse three-level voltage source converters (VSCs) with fundamental frequency switching for a high-voltage DC transmission system is dealt with here. The level of harmonics of the converter is optimised by selecting a proper dead angle (β) of the three-level VSC. Simulation results are presented for the designed three-level multipulse voltage source converter to demonstrate its improved steady-state and dynamic performance.

Test Bench for Grid Code Simulations for Multi-MW Wind Turbines, Design and Control

The hardware configuration and control methodology for a test bench for power grid simulations for multi-MW wind energy applications are described. The variable voltage is generated using two (four in the final stage) medium voltage, three-phase, three-level voltage source converters with press-pack insulated gate bipolar transistors and a specially designed step-up transformer. This system differs fundamentally from known grid simulation test benches that almost always use switched inductances. Simulation results using MATLAB/Simulink and measurements as well as constraints to the system are discussed.

Estimation and Measurement of Junction Temperatures in a Three-Level Voltage Source Converter

The design of a power converter must guarantee that the operating junction temperatures thetavj of all devices do not exceed their limits under all specified operating conditions. Usually, this is ensured by a simulative or analytical junction temperature estimation based on simple electrical and thermal models and semiconductor datasheet values. This paper discusses the difficulties and quantifies the limitations of this approach on the example of a three-level neutral point clamped voltage source converter (NPC VSC) with insulated gate bipolar transistors. The calculations are compared to the results of direct junction temperature measurements with an infrared camera. The paper also provides the experimental proof for the unequal loss and junction temperature distribution in the three-level NPC VSC

Reduce Beat and Harmonics in Grid-Connected Three-Level Voltage-Source Converters With Low Switching Frequencies

This paper investigates low-frequency beat and harmonics in grid-connected three-level pulsewidth-modulated (PWM) voltage-source converters with low switching frequencies. The impact of switching frequencies and switching sequence, as well as mitigation techniques, are studied. Different from some recent results, the analysis confirms the advantages of selecting switching frequency, as an odd-triplen multiple of the grid operating frequency, for eliminating imbalance and undesirable even-order harmonics, which can negatively impact the dc-link neutral-point balance in a three-level converter. Tests and analysis show that low-frequency subharmonic beat can occur in a carrier-based PWM converter when the switching frequency is low and is an imperfect multiple of the grid frequency. Recognizing the beat as interaction of the switching sequence and switching frequency, mitigation techniques to reduce the beat and associated harmonics are investigated and verified through experiments. A simple displacement of switching frequency from the odd-triplen multiple of the grid frequency can effectively suppress the beat with an asynchronous PWM scheme.

Design and Performance Evaluation of Subsynchronous Damping Controller With STATCOM

A long transmission line needs controllable series as well as shunt compensation for power flow control and voltage regulation. This can be achieved by suitable combination of passive elements and active FACTS controllers. In this paper, series passive compensation and shunt active compensation provided by a static synchronous compensator (STATCOM) connected at the electrical center of the transmission line are considered. It is possible to damp subsynchronous resonance (SSR) caused by series capacitors with the help of an auxiliary subsynchronous damping controller (SSDC) on STATCOM. The objective of this paper is to investigate the SSR characteristics of the system and propose a new design procedure for SSDC based on nonlinear optimization to meet the specifications on the damping torque in the range of critical torsional frequencies. The SSDC uses the Thevenin voltage signal to modulate the reactive current reference of STATCOM. The Thevenin voltage signal is derived from the locally available STATCOM bus voltage and reactive current signals. The STATCOM configurations considered in this paper are 12 pulse, two- and three-level voltage source converter with Type-2 and Type-1 control, respectively. The controller regulates either reactive current (supplied by the STATCOM) or the bus voltage. The 3-phase model of the STATCOM is based on switching functions. By neglecting harmonics in the switching function, D-Q model is derived which is combined with similar models of the other system components for linear analysis. The results of the linear analysis are validated by carrying out transient simulation based on the detailed nonlinear models. The study is performed on the system adapted from the IEEE First Benchmark Model.

10 kV IGCTs

This article describes the requirements, semiconductor design, and the characteristics of the first 10-kV integrated gate commutated thyristors (IGCT) for 6- to 7.2 kV, three-level NPC voltage source converters. It is shown that the use of 10-kV IGCTs enables the reduction of the total number of the main power components and also the reliability of the converter is increased. Finally the tradeoff curve and concept specifications of 10-kV IGCTs are presented.

High dynamic control of a three-level voltage-source-converter drive for a main strip mill

A high dynamic control system for the Alspa VDM 7000 medium-voltage drive was implemented, which provides fast torque response times of a few milliseconds despite the typically low switching frequency of gate-turn-off thyristors which is necessary to achieve high efficiency. The drive system consists of a three-level voltage-source converter with active front end and a synchronous motor. The drive has most recently been applied to a main strip mill. It provides a maximum of 8.3-MW mechanical power with a rated motor voltage of 3 kV. Besides motor torque as the main control objective, the control system has to comply with a number of additional objectives and constraints like DC-link voltage regulation and balancing, current and torque harmonics, motor flux, and excitation.