Pulse-width Modulated Voltage Source Converter Research Articles

Broad Comparison of Multilevel Inverter Topologies Operating in Hostile Environments: DC Transmission Applicability and Feasibility

Especially in the mining industry, pulsewidth modulation voltage source converters driving induction motors located far away are often present. In this case, due to the long cable used for the converter-to-motor connection, several unwanted high-frequency phenomena are generated. In this context, a new configuration for motor drive systems was proposed, in which the rectifier is kept in its original position and the inverter bridge is installed close to the motor terminals, being the long cable used as a dc transmission line. Since this environment is always more hostile and can compromise the inverter lifetime, new concerns regarding the system reliability arise, despite the various technical-economic advantages of this new configuration. In this way, this article presents a comprehensive comparison of commercially widely accepted multilevel inverter topologies, searching for the best technical features for the aforementioned dc transmission applicability/feasibility. Simulations were performed assessing several essential quantities/indexes, with a major focus on reliability. Then, a discussion was made seeking those sets that could minimize the implementation costs and improve the system reliability, concerning the insulated gate bipolar transistor (IGBTs) and the metalized film and aluminum electrolytic capacitors of the dc link.

Distributed energy resources integration in single-phase microgrids: An application of IDA-PBC and PI-PBC approaches

This paper presents a unified Hamiltonian formulation for controlling distributed energy resources (DERs) in ac single-phase microgrids (SP-MGs) via proportional-integral passivity-based control (PI-PBC), and interconnection and damping assignment passivity-based control (IDA-PBC). The proposed Hamiltonian formulation allows us to consider both pulse-width modulated voltage source converters (PWM-VSC) and pulse-width modulated current source converters (PWM-CSC) under a unified model. Renewable generation and supercapacitor energy storage systems are integrated via PWM-VSC technologies, while superconducting coils are integrated through PWM-CSC technologies. IDA-PBC and PI-PBC theories enable us to design control strategies begin that consider Lyapunov’s stability theory combined with the well-known advantages of proportional and integral control actions. Our simulation’s results corroborate the applicability of the proposed control approaches under stability paradigm. MATLAB/Simulink is employed for computational implementations via begin the SimPowerSystems toolbox.

Steady-state assessments of PMSGs in wind generating units

The number of fully-rated converter wind turbine generators equipped with a Permanent Magnet Synchronous Generator (PMSG) has sensibly increased in the late years. The optimal utilization and controllability of a PMSG is achieved using a Pulse Width Modulation – Voltage Source Converter (PWM-VSC) that allows interfacing the electric machine with the distribution network by means of a DC link. The generator/Machine-Side Converter (MSC) controls the operation of the PMSG. This paper proposes a methodology to assess the feasibility of the steady-state working points of PMSG based wind turbines without running a set of time-consuming time-domain simulations. Three control objectives for MSC are usually considered: stator voltage control, unitary power factor control and torque control. The aim of the present paper is that of providing an analytical methodology to check the feasibility of the steady-state working points dictated by different control strategies of the MSC starting from the specific operational constraints provided by the wind power plants characteristics. The effectiveness of the proposed methodology is demonstrated comparing numerical results of a MATLAB® implementation against dedicated time-domain simulations. Simulations results demonstrated the appropriate performance of the proposed methodology characterized by the almost negligible computational time required.

Solid‐state control for reactive power compensation and power quality improvement of wound field synchronous generator‐based diesel generator sets

This paper presents the use of pulse-width modulated voltage source converter (PWM-VSC) for power quality improvement of a wound field synchronous generator-based diesel generator (DG) set realising standalone supply system. The system consists of a diesel engine driven wound field synchronous generator, a three-leg PWM-VSC with a capacitor on the DC link and linear/non-linear loads. The PWM-VSC supplies the leading kilo volt-amphere reactive (kVAR) to the loads and maintains the unity power factor of the DG set. In addition, it eliminates the harmonics from the source currents and provides load balancing. The terminal voltage is controlled by field excitation control of the generator and the supply frequency is controlled by the speed regulating system of the diesel engine. A variable step size filter-x least mean square-based control algorithm is used for estimation of reference currents for control of VSC.

Particle Swarm Optimization-based Superconducting Magnetic Energy Storage for Low-voltage Ride-through Capability Enhancement in Wind Energy Conversion System

— This article presents a novel application of the particle swarm optimization technique to optimally design all the proportional–integral controllers required to control both the real and reactive powers of the superconducting magnetic energy storage unit for enhancing the low-voltage ride-through capability of a grid-connected wind farm. The control strategy of the superconducting magnetic energy storage system is based on a sinusoidal pulse-width modulation voltage source converter and proportional–integral-controlled DC-DC converter. Control of the voltage source converter depends on the cascaded proportional–integral control scheme. All proportional–integral controllers in the superconducting magnetic energy storage system are optimally designed by the particle swarm optimization technique. The statistical response surface methodology is used to build the mathematical model of the voltage responses at the point of common coupling in terms of the proportional–integral controller parameters. The effectiveness of the proportional–integral-controlled superconducting magnetic energy storage optimized by the proposed particle swarm optimization technique is then compared to that optimized by a genetic algorithm technique, taking into consideration symmetrical and unsymmetrical fault conditions. A two-mass drive train model is used for the wind turbine generator system because of its large influence on the fault analyses. The systemic design approach is demonstrated in determining the controller parameters of the superconducting magnetic energy storage unit, and its effectiveness is validated in augmenting the low-voltage ride-through of a grid-connected wind farm.

Active Mitigation of Subsynchronous Interactions Between PWM Voltage-Source Converters and Power Networks

Pulse-width-modulated (PWM) voltage-source converters (VSCs) are gaining widespread acceptance in modern power systems. It has been shown recently that full-scale high-power PWM VSCs can induce negative electrical damping at subsynchronous frequencies. However, active reshaping of the VSC incremental output impedance to minimize the negative impacts of a VSC on subsynchronous damping is not reported. To fill out this gap, this paper presents: 1) an extended analysis of the output impedance of a PWM-based two-level VSC; and 2) more importantly, three simple and robust active reshaping techniques to maximize the positive electrical damping in the subsynchronous frequencies without affecting the converter control performance. The first reshaping technique uses the grid voltage and an active-damping controller to generate active impedance that modifies the VSC impedance in the subsynchronous range. The second reshaping technique uses an internal active damping controller to modify the dynamics of the phase-locked loop, which has significant contribution to the negative impedance of the VSC. The third reshaping technique combines the first and second techniques. The proposed active mitigation methods show excellent performance in reshaping the VSC impedance and inducing positive electrical damping to mitigate possible subsynchronous interactions between the VSC and the power network. Further, the proposed compensators show robust control performance at different output power levels of the VSC without significant impact on the converter control performance. A theoretical analysis and comparative time-domain simulation and experimental results are presented to verify the validity and effectiveness of the proposed techniques.

Fuzzy logic control of wind energy conversion system

This paper proposes a variable speed control scheme of grid-connected wind energy conversion system, WECS, using permanent magnet synchronous generator. The control algorithm tracking the maximum power for wind speeds below rated speed of wind turbines (WTs) and ensure the power will not exceed the rated power for wind speeds higher than the rated speed of wind turbine. The control algorithm employed fuzzy logic controller (FLC) to effectively do this job. The WT is connected to the grid via back-to-back pulse width modulation-voltage source converter (PWM-VSC). Two effective computer simulation software packages (PSIM and SIMULINK) have been used to carry out the simulation effectively where psim contains the power circuit of the WECS and MATLAB/SIMULINK contains the control circuit of the system. The control system has two controllers for generator side and grid side converters. The main function of the generator side controller is to track the maximum power from wind through controlling the rotational speed of the turbine using FLC. In the grid side converter, active and reactive power control has been achieved by controlling d-axis and q-axis current components, respectively.

Novel Control Strategy for Voltage Source Converters Based on Energy Function

This paper proposes a nonlinear control strategy for pulse-width modulated voltage source converters (VSCs). A passivity-based approach, resulting from the Lyapunov direct method, is presented for designing an effective nonlinear VSC controller to improve their dynamic behaviors under perturbed conditions. The differential equations of the converters are obtained and then transformed into a d − q frame. Considering the nonlinearity of the transformed equations, the Lyapunov direct method is utilized to find a solution to improve the transient response. The time derivative of the Lyapunov function guarantees that increasing its dissipative term will result in the trajectory of the VSC system returning to the equilibrium point more quickly. The proposed control strategy can be simultaneously applied to multiVSC devices, such as a unified power flow controller (UPFC). Moreover, it is shown that the proposed strategy meets the steady-state requirements without using any other control loop. As an example, a 3-machine system with a UPFC is studied to verify the effectiveness of the proposed strategy.

New topology for management of bi-directional power flow between vehicle and grid with reduced ripple current at unity power factor

In this paper, a new topology of single-phase pulse width modulated (PWM) AC-DC converter is proposed for bidirectional power flow between plug-in hybrid electric vehicle (PHEV) and the grid. This converter consists of a PWM voltage source converter (VSC) and a ripple reduction circuit. The ripple reduction circuit consists of a switching leg and an inductor. The inductor is connected between the midpoints of the second and third legs of VSC. An AC voltage of 230 V, 50 Hz is converted into 400 V DC using a single-phase VSC with ripple compensation circuit. A buck-boost DC-DC converter charges and discharges the battery of the electric vehicle (EV) or PHEV with this 400 V DC. While in discharging mode, it feeds energy back to the grid. The zero voltage regulation (ZVR) and power factor correction (PFC) modes of operation at PCC are also achieved with reactive power compensation and harmonics elimination of other connected loads at PCC with modified control in order to improve the power quality at the grid.

The Application of Resonant Controllers to Four-Leg Matrix Converters Feeding Unbalanced or Nonlinear Loads

Matrix converters (MC) have some advantages when compared to conventional back-to-back pulsewidth modulation voltage-source converters. The MC may be considered more reliable and is smaller because the bulky dc capacitor is eliminated from the topology. Therefore, when MCs are used in ac-ac power conversion, the size and weight of the whole generation system is reduced. To interface a MC-based generation system to an unbalanced three-phase stand-alone load, a four-leg MC is required to provide an electrical path for the zero-sequence load current. Moreover, to compensate for the voltage drops in the output filter inductances, nonlinearities introduced by the four-step commutation method and voltage drops in the semiconductor devices, closed-loop regulation of the load voltage is required. In this paper, the design and implementation of a resonant control system for four-leg MCs is presented. The application of this control methodology when the four-leg MC is feeding, a linear/nonlinear unbalanced load is also presented in this study. High-order resonant controllers are also analyzed. Experimental results, obtained from a small prototype, are discussed.

Input Harmonic Estimation and Control Methods in Active Rectifiers

Active rectifiers using pulsewidth-modulation voltage-source converters are popular choices for adjustable-speed drives to provide high input power quality. However, input current harmonics are often generated as a result of input voltage source disturbances. Since the frequencies of the fifth-, seventh-, and higher-order harmonics can easily exceed the current controller bandwidth in high-power applications, the elimination of these harmonic disturbances imposes difficult control challenges. Cost-effective observer-based harmonic estimation and mitigation methods are introduced in this paper that compensate delay effects caused by the digital signal processing. These delays can significantly limit the current controller bandwidth if not compensated. Analytical, simulation, and experimental results are developed to illustrate the effectiveness of the new harmonic estimation and control techniques.

A Theoretical Analysis of the Harmonic Content of PWM Waveforms for Multiple-Frequency Modulators

This paper theoretically identifies the harmonic components of a carrier-based pulsewidth-modulated (PWM) voltage-source converter (VSC) output voltage when the modulating wave includes fundamental and baseband harmonic components. This occurs, for example, when a VSC is used as an active power filter. The general analytical solution provided can be applied with a minimum additional mathematical effort to any harmonic combination in the modulation signal. The analysis undertaken in this paper determines how the fundamental and low-order harmonics (in the modulation signal) interact with the carrier to produce the low (and high) order frequencies in the PWM VSC output voltage. The analysis is based on a double Fourier series expansion in two variables. This approach to harmonic identification is evaluated by a comparison with a fast Fourier transform analysis of simulated PWM waveforms.

Transient Stability Enhancement of Wind Generator by PWM Voltage Source Converter and Chopper Controlled SMES Unit

In order to investigate the impacts of the integration of wind farms into utility networks, transient stability should be analyzed before connecting wind turbine generator system (WTGS) to the power system. This paper proposes a robust controller for Superconducting Magnetic Energy Storage (SMES) unit to enhance the transient stability of a grid connected fixed speed wind generator system. In the proposed controller, both SMES active and reactive powers are controlled to decrease the fluctuations of output power and terminal voltage of the wind generator during transient condition. The power conversion system (PCS) of SMES unit used in this paper is composed of a sinusoidal Pulse Width Modulated Voltage Source Converter (PWM-VSC) and a two-quadrant DC-DC chopper using Insulated Gate turn-off Bipolar Transistors (IGBT). Stability during symmetrical and unsymmetrical faults in the network system is analyzed.  The effects of the faults on the generator dynamics are also discussed clearly. Simulation results demonstrate that the proposed SMES controller is very effective for stabilizing wind generator as well as the entire power system. Keywords:  Fixed speed wind generator system; Transient analysis; System faults; PWM-VSC; DC-DC chopper and SMES unit.© 2009 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved. DOI: 10.3329/jsr.v1i2.2281

Improvement of Wind-Generator Stability by Fuzzy-Logic-Controlled SMES

This paper proposes a fuzzy-logic-controlled superconducting magnetic energy storage (SMES) scheme for the stabilization of grid-connected wind-generator systems. The control scheme of SMES is based on a sinusoidal pulsewidth-modulation voltage-source converter and a two-quadrant dc-dc chopper using an insulated-gate bipolar transistor. A comparative study is carried out between the proposed fuzzy-logic-controlled SMES and the fuzzy-logic-based pitch controller to improve the wind- generator stability. Simulation results demonstrate that the performance of the proposed fuzzy-logic-controlled SMES is better than that of the fuzzy-logic-based pitch controller in order to stabilize the wind generator.

Support Tools for Simulation-Based Optimal Design of Power Networks With Embedded Power Electronics

This paper presents a gradient-based nonlinear optimization wrapper for an electromagnetic transient simulation program to assist in the design of hardware and control parameters of flexible AC transmission system (FACTS) apparatus. A new second-order sensitivity analysis method is introduced that quantifies the sensitivity of the optimal solution to parameter uncertainties. Using this approach, the sensitivity of the harmonic elimination capability of a pulsewidth-modulated voltage-source converter to variations in switching angles is investigated. This paper also adapts the method of Pareto optimization to the simulation-based design of FACTS apparatus when multiple design objectives are to be satisfied. The developed Pareto optimization tool generates a Pareto frontier plot to visualize the design tradeoffs between the multiple objectives. The effectiveness of the tool is demonstrated through a design example that selects optimal values for the DC-bus capacitor and the control settings of a STATCOM.

New Optimized PWM VSC Control Structures and Strategies Under Unbalanced Voltage Transients

Control structures and strategies have a critical influence on a power electronic converter's behavior during disturbances. Most of the previous works in the field of pulsewidth modulation voltage-source converter (VSC) operation under unbalanced conditions propose dual vector controllers with dc bus voltage optimization strategies, which have been proven to be well adapted for rectifier applications. In this paper, two major contributions are made. On the one hand, a new optimized operation strategy based on exchanged power maximization is proposed for vector control structures, which permits the extension of optimized operation to other VSC applications (e.g., flexible alternating current transmission system and distributed generation interfaces). On the other hand, a scalar control structure is proposed based on resonant controllers, together with three adapted optimized operation strategies, namely: 1) dc bus voltage optimization; 2) power exchange maximization; and 3) a hybrid strategy (an intermediary mode between the other two strategies). Their main advantage is their simplicity and the lack of real-time symmetrical component extraction techniques. This scalar controller and the proposed optimized operation strategies are compared to conventional vector controllers. It is proven through experimental analysis that the proposed scalar controller offers very good performances with simpler structures and bigger flexibility in terms of operation modes.

Nonlinear Control of Voltage-Source Converter Systems

The nonlinear control method, associated with the books of A. Isidori, H. Nijmeijer, and A. J. van der Shaft, has so far been applied to the STATCOM (system order, N = 3). There have been two impediments to progress in applying the method: 1) difficulty in synthesizing the nonlinear transformation which is the key to the method; 2) the advanced mathematical language which makes the aforesaid books generally inaccessible. This paper describes the successful application of the nonlinear control method to control systems based on back-to-back pulsewidth modulation voltage-source converter. This paper is written in a tutorial form to explain how the nonlinear control method can be applied without using the advanced mathematical language. The SSSC and the UPFC (system order, N = 5) are chosen as illustrative examples.

Modeling and control of unbalanced three-phase systems containing PWM converters

This paper proposes two complex vector models to characterize unbalanced three-phase four-wire systems that contain pulsewidth-modulated voltage-source converters (PWM-VSCs). In the first case, the three-phase system is decomposed in orthogonal odq components. A complex vector model is then built from the dq and o components. In the second case, a complex vector model is introduced by decomposing the system in three single-phase systems. In both cases, an orthogonal, fictitious circuit is introduced to handle a single-phase system, such as the homopolar system or each one of the three single-phase systems that compose a three-phase four-wire system. From the vector models, two digital controllers, which employ two different reference frames, are proposed. Also, the use of the same gains for both controllers simplifies the equations of the controller in the stationary reference frame. As an example, current control in a PWM-VSC system is presented.

Recent developments of high power converters for industry and traction applications

The introduction of new high power devices like integrated gate commutated thyristors (IGCTs) and high voltage insulated gate bipolar transistors (IGBTs) accelerates the broad use of pulse width modulation (PWM) voltage source converters in industrial and traction applications. This paper summarizes the state-of-the-art of power semiconductors. The characteristics of IGCTs and high voltage IGBTs are described in detail. Both the design and loss simulations of a two level 1.14 MVA voltage source inverter and a 6 MVA three-level neutral point clamped voltage source converter with active front end enable a detailed comparison of both power semiconductors for high power PWM converters. The design and the characteristics of a commercially available IGCT neutral point clamped PWM voltage source converter for medium voltage drives are discussed. Recent developments and trends of traction converters at DC mains and AC mains are summarized.