Grid Converter Research Articles

Analysis of the Passive Damping Losses in LCL-Filter-Based Grid Converters

Passive damping is the most adopted method to guarantee the stability of LCL-filter-based grid converters. The method is simple and, if the switching and sampling frequencies are sufficiently high, the damping losses are negligible. This letter proposes the tuning of different passive damping methods and an analytical estimation of the damping losses allowing the choice of the minimum resistor value resulting in a stable current control and not compromising the LCL-filter effectiveness. Stability, including variations in the grid inductance, is studied through root locus analysis in the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</i> -plane. The analysis is validated both with simulation and with experiments.

System Frequency Support Through Multi-Terminal DC (MTDC) Grids

Control of the converter stations in a multi-terminal DC (MTDC) grid to provide frequency support for the surrounding AC systems is the subject matter of this paper. The standard autonomous power sharing control loop for each converter is modified with a frequency droop control loop. The objective is to minimize the deviation from nominal AC system frequency and share the burden of frequency support among the converter stations of the MTDC grid. The effectiveness of the frequency support is demonstrated through nonlinear simulation of a test system consisting of three isolated AC systems interconnected through an MTDC grid with four converter stations. An averaged model of the MTDC grids is developed to carry out modal analysis of combined multi-machine AC-MTDC systems. Modal analysis is used to characterize and substantiate the time domain behavior in presence of frequency droop control. It is established that appropriate droop control loop for the MTDC grid converters could be effective in reducing the deviation from nominal AC system frequency provided the sensitivity of the system eigen-values to changes in control parameters (e.g., droop coefficients) is accounted for a priori through modal analysis.

Electrohydrodynamic injection converters

This study is the continuation of the work devoted to electrochemical electrohydrodynamic (EHD) converters. The goal of this study is the computer simulation of the formation and development of the EHD flow in a symmetric and grid electrode system located in a channel with dielectric walls. Three types of EHD grid converters, namely, defocusing, symmetric, and focusing systems were investigated. The physical causes of the appearance of return vortices in defocusing electrode systems are explained. This is due to the formation of the charge plug in the behind-the-electrode region, which retards the through pumping of the fluid. It is shown that, in order to eliminate the charge plug, it is necessary to arrange one counter electrode along the flow’s axis and to select the impurity composition of the fluid so that the injection also occurs on the surface of the passive electrode rather than the active one only. In this case, the bipolar charge structure compensating for the return retarding effect is formed in the behind-the-electrode region. It was originally suggested to use focusing electrode systems to enhance the effect of the through pumping.

Control strategy for distributed integration of photovoltaic and energy storage systems in DC micro-grids

Abstract The interest on DC micro-grid has increased extensively for the more efficient connection with DC output type sources such as photovoltaic (PV) systems, fuel cells (FC) and battery energy storage systems (BESS). Furthermore, if loads in the system are supplied with DC power, the conversion losses from sources to loads are reduced compared with AC micro-grid. This paper proposes operation and control strategies for the integration of PV and BESS in a DC micro-grid. The proposed control enables the maximum renewable energy utilization during different operating modes of the micro-grid i.e., grid connected, islanded or transition between these two modes, whilst making an allowance for the DC voltage control and DC-loads supply. When the system is grid connected and during normal operation, active power is balanced by the AC grid converter to ensure a constant DC voltage. In order to achieve the system operation under islanding conditions, a coordinated strategy for the BESS, PV and load management including load shedding and considering battery state of charge (SoC), are proposed. Seamless transition of the PV converter control between maximum power point tracking (MPPT) and voltage control modes, of the battery converter between charging and discharging and that of grid side converter between rectification and inversion are ensured for different grid operation modes by the proposed control methods. The DC bus voltage level is employed as an information carrier to distinguish different modes and determine mode switching. MATLAB/SIMULINK simulations are presented to demonstrate the robust operation performance and to validate the proposed control system during various operating conditions.

Control and Operation of a DC Microgrid With Variable Generation and Energy Storage

Control and operation of a dc microgrid, which can be operated at grid connected or island modes, are investigated in this paper. The dc microgrid consists of a wind turbine, a battery energy storage system, dc loads, and a grid-connected converter system. When the system is grid connected, active power is balanced through the grid supply during normal operation to ensure a constant dc voltage. Automatic power balancing during a grid ac fault is achieved by coordinating the battery energy storage system and the grid converter. To ensure that the system can operate under island conditions, a coordinated strategy for the battery system, wind turbine, and load management, including load shedding, are proposed. PSCAD/EMTDC simulations are presented to demonstrate the robust operation performance and to validate the proposed control system during various operating conditions, such as variations of wind power generation and load, grid ac faults, and islanding.

Grid Converters and Power Electronics (review of "Grid Converters for Photovoltaic and Wind Power Systems: Teodorescu, R., et al.; 2011) [Book News

The book?s concept originated during biannual industrial/Ph.D. courses organized in Aalborg University, Institute of Energy Technology. These courses started in 2005 and lasted until the end of 2010. The book consists of twelve chapters that cover some of the following topics: PV inverter structures; grid requirements for PV; grid synchronization; islanding detection; grid converter structures; grid synchronization; grid filter design; and grid current control.

Providing Ride-Through Capability to a Doubly Fed Induction Generator Under Unbalanced Voltage Dips

This paper analyzes the effect of unbalanced voltage over doubly fed induction generators (DFIGs) and presents a novel control strategy, named dynamic programming power control plus (DPPC+), based on dynamic programming control. The high penetration of wind energy in the electrical grids demands for new requirements for the operation of wind energy conversion systems (WECSs). DFIG is the most employed WECS, and the DPPC+ guarantees their operation under unbalance conditions achieving the required objectives. Although the technique can be implemented to control both rotor and grid converters, we hereby expound the former, which regulates stator active and reactive power. The validation of the results obtained with DPPC+ has been performed through the use of experimental tests on a 20-kW test bench, consisting of a DFIG and induction motor drive. The obtained results show that the DPPC+ is suitable for achieving a good dynamic response while controlling current distortion and power and/or torque oscillations for both steady-state conditions and unbalanced voltage dips, showing the low-voltage ride-through capability.

A DSP- and FPGA-Based Industrial Control With High-Speed Communication Interfaces for Grid Converters Applied to Distributed Power Generation Systems

New energy concepts such as distributed power generation systems (DPGSs) are changing the face of electric distribution and transmission. Power electronics researchers try to apply new electronic controller solutions with the capacity of implementing new and more complex control algorithms combined with internal high-speed communication interfaces. Thus, it is possible to monitor, store, and transfer a large number of internal variables that can be sent online to local or remote hosts in order to take new set points of different generation units. With this objective, this paper presents the design, implementation, and test of an industrial multiprocessor controller based on a floating-point digital signal processor (DSP) and a field-programmable gate array, which operate cooperatively. The communication architecture, which has been added to the proposed electronic solution, consists of a universal serial bus (USB), implemented with a minimum use of the DSP core, and a controller area network (CAN) bus that permits distributed control. Although the proposed system can be readily applied to any DPGS, in this paper, it is focused on a 150-kVA back-to-back three-level neutral-point-clamped voltage source converter for wind turbine applications.

Direct Power Control Applied to Doubly Fed Induction Generator Under Unbalanced Grid Voltage Conditions

This paper analyzes the effects of unbalanced voltage on doubly fed induction generators. It also presents a novel control strategy based on direct power control (DPC+) applied to this type of generators, predominant in wind energy applications, that enables them to work under perturbed conditions and achieve optimum results. Although the technique can be implemented to control both rotor converters and grid converters, we will hereby exemplify the former which regulates stator active and reactive power. The results obtained with DPC+ are then compared through experimental tests to indicate that the technique is suitable and achieves good dynamic responses while controlling current distortion, power or torque oscillations. The validation of results has been performed through experimental tests on a 20-kW generator.

Dynamic Programming Power Control for Doubly Fed Induction Generators

This paper presents a novel control strategy, dynamic programming power control (DPPC), to be applied to doubly fed induction generators most commonly used in wind energy applications. Although the technique can be implemented to control both rotor and grid converters, it will hereby be expounded the former, which regulates stator active and reactive powers. The results obtained are compared with those from other techniques, such as direct torque/power control (DTC/DPC) through the use of experimental tests, and indicate that DPPC achieves gains in dynamic response and considerable improvements in terms of ripple reduction and frequency spectrum as a result of constant switching frequency operation. The validation of results has been performed through experimental tests on a 6-kW generator.

Modeling and Control of AWS-Based Wave Energy Conversion System Integrated Into Power Grid

In this paper, the model of Archimedes wave swing (AWS) based wave energy conversion (WEC) system is proposed where a new coordinate transformation for the linear permanent magnet generator between the abc frame of reference and the dq0 frame of reference is proposed and the model of the LPMG in dq0 reference frame suitable for power system dynamic and stability analysis is derived for the first time. Since the output power and the induced voltage both vary, full-scale back-to-back converters are employed to connect the WEC to the power grid in order to maintain a constant active power and terminal voltage at the system side. The controllers for the generator side converter and the grid side converter of the back-to-back converters are also developed. Simulations are performed, when WEC operates under conditions such as no-load, supplying an isolated R-C load and integration with a power system, to simulate the dynamics of the WEC and verify the effectiveness of the model derived and the controllers developed. Comparison between the dynamic responses simulated and those obtained in the field test demonstrates that using the proposed dq0 model, the dynamics of the WEC system can be simulated properly. Using the generator side converter controller, the maximum power can be extracted from the wave while the loss in the LPMG can be minimized. Furthermore, the WEC can keep the output power at the power grid converter side constant and maintain the terminal voltage constant with the grid side converter controller. These are the desired features for the WEC to operate in parallel with the power grid. The effectiveness of the controller proposed is also validated under small disturbance.

Study of the Effects of Inductor Nonlinear Behavior on the Performance of Current Controllers for Single-Phase PV Grid Converters

Resonant and repetitive controllers are well suited for single-phase grid-connected inverters due to their optimum tracking and harmonic rejection capability. In particular, in this paper, their application for single-phase photovoltaic systems is considered. The effects of nonlinear inductance on the performance of current controllers designed to track periodic signals and/or to compensate periodic disturbances are investigated. When the inductance has a nonlinear behavior, a distorted current waveform is produced. Two different nonlinearities have been considered: saturation for high currents and a light nonlinearity, which occurs in the first portion of the magnetization curve, for low currents. A current-dependent model of the nonlinear inductance has been developed. It is mathematically based on the Volterra-series expansion, and it allows us to prove how the harmonic compensation provided by resonant and repetitive controllers can also mitigate the effects of the inductance saturation. This result is the main contribution of this paper, and it is also substantiated with experimental evidence. Moreover, the repetitive controller is able to comply with the harmonic limits reported in IEEE 1547 and IEC 61727, even in very hard saturation conditions.

Multiple harmonics control for three-phase grid converter systems with the use of PI-RES current controller in a rotating frame

Voltage source inverters connected to the grid in applications such as active rectifiers, active filters, uninterruptible power supplies, and distributed generation systems need an optimal ac current control. To obtain zero steady-state error at the fundamental frequency (i.e., unity power factor), the use of a standard integrator in a rotating frame is as effective as the use of a resonant controller in a stationary frame. However, the grid voltage harmonics influence the current controller and generate current harmonics unless several integrators in multiple rotating frames or resonant compensators in a stationary frame are adopted. In this letter, a hybrid system consisting of a proportional integral (PI) controller plus a generic hth harmonic resonant controller implemented in a frame rotating at the n th harmonic frequency is discussed in detail. The hth harmonic controller is able to decrease both the (h - n)th and (h + n)th harmonics, while the PI controller is able to decrease other harmonics if the synchronization phase signal adopted for the frame transformation is unfiltered. It is demonstrated that the use of a PI and sixth harmonic resonant compensator is effective for both positive and negative sequence fifth and seventh harmonics; hence, four harmonics are compensated with the proportional integral-resonant (PI-RES) controller implemented in a synchronous frame. Simulation and experimental tests validate the proposed analysis

06/00919 Identification and properties of the fundamental expansion functions for neutron transport in an infinite homogeneous medium: Milgram, M. S. Annals of Nuclear Energy, 2005, 32, (11), 1167–1190.

This study deals with a new nonlinear backstepping sliding mode control for Photovoltaic (PV) based grid interactive voltage source converter (VSC) system. The VSC dynamic design is incorporated with a low pass LCL filter prior to utility grid connectivity, which is expressed in terms of active (P1) and reactive (Q1) power. Low pass LCL filter is introduced as passive ac element and hence a reduced harmonic profile is obtained in terms of total harmonic distortion (THD). The aim of the work is to arbitrate between dynamic performance and filtering effect of the proposed PV-VSC system to provide better reliability with superior power quality. To design a robust control theory with grid uncertainty eradication capability, a new finite time backstepping sliding mode control (FTBFSMC) is proposed in this paper. The control method is implemented in terms of reactive power at PCC (Q1) and dc side converter voltage (Vpv), where the converter dynamics are being calculated directly from instantaneous values of converter current (Ii,abc) and bus voltage (V1,abc) to avoid PLL angle calculation at the beginning. A new small signal stability phenomenon is depicted by multivariable dynamic model of the proposed PV based grid converter system, where PV penetration as well as nonlinear control parameter optimization practices is considered. The performance of the proposed control is verified through various case studies in MATLAB/script environment, which are being extended to MATLAB/Simulink model as well as PSCAD/EMTDC platform for validation.