Stability Analysis of Distributed Multi-Converter System

This paper deals with a wind energy conversion system (WECS), based on a doubly fed induction generator (DFIG) and an electromechanical active power filter (APF). The objective is to respond simultaneously to electric energy requirement and power quality concerns. The proposed WECS is able not only to track the maximum power point of the wind energy but also to dampen harmonic currents from the grid line. To achieve these purposes, back-to-back pulse width modulation (PWM) voltage source converters (VSCs) are employed to excite the DFIG rotor. To control the power converters, two regulation loops, following the field oriented control (FOC) method, have been conceived. The first loop, dedicated to the rotor converter, enables a decoupled control of active and reactive powers flows. As well, it permits the cancellation of the most noteworthy harmonics from the utility current. The second loop, devoted to the front-end converter (FEC), makes it possible to regulate the DC-bus voltage shared between the two converters. Furthermore, the paper provides a study of the DFIG sub and super-synchronous operating modes which can be reached by acting on the wind speed. To demonstrate the performance of aforementioned WECS, a Matlab/Simulink model was established and simulated. Results seem to be very promising.

Transient stability enhancement of wind farms connected to a multi-machine power system by using an adaptive ANN-controlled SMES

This paper presents a three-phase four wire grid interactive multiple solar photovoltaic (PV) and a battery based microgrid system. The DC links for the main voltage source converter (VSC) and the ancillary VSC are interfaced individually, to two solar PV arrays using a different maximum power point (MPP) tracking control i.e. incremental conductance (InC) method, in an inexpensive and effectual single stage topology. These VSCs are interfaced in parallel linkage to the point of common coupling (PCC) with distinct nonlinear loads, to enhance microgrid rating for distributing the active power on grid side and the system is reliable in an islanded mode. In normal situations, the current control is used for the main VSC in the grid intertied mode and at the disconnection from the grid, this system operates with the voltage control. The main VSC's mode transition performance is free from transitory and is obtained by a power electronic switch (PES). At the main VSC DC link, a DC-DC converter with the buck-boost operation is linked in series with the battery, manages the load network demand and sustains the DC link's voltage to MPP. At the PCC, frequency and voltage are regulated via voltage control and gives incessant power to the loads and good quality voltage is provided to the ancillary VSC to perform through the current control. The current control is utilized for power factor correction, harmonics eradication and enhancement of system's dynamics with PV feed-forward part. The microgrid operation is found satisfactory for scenarios like, grid outage, solar irradiance alteration, load unbalance and neutral current compensation and authenticated through simulated results obtained in MATLAB / SIMULINK platform.

The number of voltage levels available in pulsewidth modulation (PWM) voltage source inverters can be increased by using a split-wound coupled inductor within each inverter leg and interleaved PWM switching of the upper and lower switches. The magnetizing inductance of the symmetrical split-wound inductor filters the high-frequency PWM voltage differences between the upper and lower switches. The same inductor presents a three-level PWM voltage at the inverter output terminals, with the winding leakage inductance being located in series with the low-frequency output current. Deadtime PWM signal delays can be reduced as DC-rail short circuits are not possible: as a result, the quality and voltage range of the PWM output is improved. Since the inductor windings are technically exposed to high-frequency PWM AC voltages with no DC components, device voltage drops help to reduce the buildup of winding DC currents. Theoretical analysis and a sample design case are presented to illustrate how to design suitable inductors for the various topologies. Simulation and experimental results are used to illustrate the operation of the proposed inverter structures.

The shuffled frog leaping algorithm (SFLA) is presented, to optimally design multiple proportional–integral (PI) controllers of the static synchronous compensator (STATCOM) system with the purpose of improving the transient stability of a grid‐connected wind farm. The control strategy of the STATCOM system depends on a sinusoidal pulse width modulation voltage source converter, which is controlled by the cascaded PI control scheme. The response surface methodology is used to establish a second‐order fitted model of the transient voltage responses at the point of common coupling in terms of PI controllers’ parameters. For realistic responses, a three‐mass drive train model is used for the wind turbine generator system because of its great influence on the dynamic analyses. The SFLA code is built using MATLAB software. The effectiveness of optimised PI‐controlled STATCOM by the SFLA is then compared to that optimised by genetic algorithms technique taking into consideration symmetrical and unsymmetrical fault conditions. The proposed control scheme is applied to a real grid‐connected wind farm system at Hokkaido Island, Japan. The validity of the proposed system is verified by the simulation results, which are performed using PSCAD/EMTDC software environment. With the proposed SFLA‐based STATCOM, the transient stability of a grid‐connected wind farm can be enhanced.

This paper proposes an improved-second order generalized integrator (SOGI) based decoupled voltage and frequency controller (DVFC) for permanent magnet synchronous generator (PMSG) based pico-hydro system with power quality improvement. The proposed pico-hydro system consists of a voltage source converter (VSC) used for harmonics mitigation, regulating voltage at point of common coupling (PCC), and DC-link voltage control. The decoupled frequency controller consists of a conventional electronic load controller (ELC) using uncontrolled three-phase diode bridge rectifier along with an insulated gate bi-polar transistor (IGBT) switch-based DC-DC converter connected in series to an auxiliary load. The frequency is regulated by dumping excess energy into the auxiliary load. The VSC and decoupled ELC, are connected in parallel to the load. The improved-second order generalized integrator (SOGI) based control algorithm is used to generate switching pulses for the VSC. The addition of damping factors, provides faster convergence, good dynamic response and improves steady-state performance of the given system. The proposed ISOGI based control algorithm with decoupled frequency controller for PMSG, is validated on a laboratory prototype with experimental implementation.

The solar photovoltaic (PV)‐based microgrid is one of the most ideal renewable energy resources. This paper presents a utility grid intertie multi‐PV‐inverter‐based microgrid (MG) control for the solar rooftop application. The main and ancillary voltage source converters (VSCs) DC links are assimilated with PV arrays of independent maximum power point tracking (MPPT) algorithms for the economical and efficacious single‐stage configuration. At the point of common coupling (PCC), parallel VSCs arrangement increases the MG's power rating with distinctive local loads. In normal circumstances, the current control methodology is employed by the main VSC under the grid‐interactive mode. Conversely, in some grid contingency conditions under the isolated mode, its control transfers to the voltage control, which regulates the frequency and the voltage at PCC and acts like a grid‐forming inverter. The ancillary VSC operation is implemented with the current control algorithm. The main VSC supplies the active power towards the grid and accomplishes the load requirement, whereas the ancillary VSC retains its load demand in a grid‐interactive mode and also regulates the load requirement in an islanded mode of operation. The utilization of feed‐forward components for PV arrays powers in the VSCs current control techniques improves the MG dynamic performance. The seamless mode transfer performance of the main VSC is implemented via a power electronics switch. The MATLAB software is used to model a microgrid and its performance is analysed via RT‐LAB in real time through the OPAL‐RT (OP4510) controller for various scenarios, that is, varying solar insolation, load alterations and unbalanced non‐linear loads. The utilization of the modified Kwong's algorithm‐based current control enhances the power quality such as harmonics current elimination and improvement in the power factor. The voltage profile of PCC voltages is enhanced via using a modified Kwong's algorithm‐based filter in the voltage controller. The modified Kwong's algorithm has fast convergence, reduces misadjustments and gives very good performance compared to the least‐mean‐square (LMS) and Kwong's approaches. The total harmonic distortion (THD) of grid currents and PCC voltages THD in an isolated mode is found according to the limits of the IEEE‐519 standard.

When calculating the permanent magnet (PM) eddy current loss (ECL) in the interior permanent magnet synchronous machine (IPMSM) under pulse width modulation (PWM) voltage source inverter (VSI) supply, it is very important to take the high frequency harmonic voltages (HFHVs) in the input voltage into consideration because most of the PM ECL in IPMSM with distributed windings is caused by them [1–2]. However, common finite element analysis (FEA) with sinusoidal current source supply isn’t able to take the HTHVs into consideration and the coupled field-circuit time-stepping FEA under PWM VSI supply with fine time step is needed for the calculation, which is very time-consuming especially when conducting 3D FEA to take the end effect and axial segmentation into consideration. A hybrid method with the combination of 2D FEA and analytical method has been proposed in [3] for fast calculating the PM ECL considering axial segmentation. However, the 2D time-stepping FEA with fine time step, which takes several hours for one case study, is still needed for the PM ECL calculation under one load condition with the method proposed in [3]. In this paper, the analytical method in [3] is further developed and applied for the PM ECL calculation of an IPMSM applied in the HEV application [4]. Rather than calculating the PM ECL with the average flux density passing through the PM as input [3], the relationship between arbitrary HFHV and the corresponding PM ECL is established. Hence, the total PM ECLs can be calculated with the FFT results of the input line-line voltage directly without time-stepping FEA at a given load condition.

Brushless doubly-fed induction generators have higher reliability, making them an attractive choice for not only offshore applications but also for remote locations. These generators are composed of two back-to-back voltage source converters, a grid side converter and a rotor side converter. Existing techniques use the rotor side converter for reactive current control; however, it is more suitable for stabilizing steady state behavior. In order to stabilize the voltage fluctuations at the point of common coupling (PCC) due to sudden inductive load introduction, the grid side converter may be a better choice due to faster response and higher control bandwidth. Therefore, this paper proposes a control scheme for the grid side converter to suppress the PCC voltage fluctuations when a large inductive load is suddenly connected. The proposed technique is based on an analytical model of the transient behavior of the voltage drop at the PCC. The analysis shows that reactive current control using the grid side converter introduces a double fundamental frequency component to the PCC voltage. To block this harmonic, we designed a notch filter. The simulation results in Matlab/Simulink show that the proposed technique can not only significantly reduce the voltage drop but also results in an 82% reduction in voltage distortion at the PCC.

This study presents design and implementation of a dynamic electronic load controller (DELC) for voltage and frequency control of a three‐phase self‐excited induction generator (SEIG) driven by constant power unregulated small‐hydro turbine. This DELC is a combination of insulated‐gate bipolar transistor‐based voltage sourced converter (VSC) and variable frequency drive (VFD) which is connected across the DC link of VSC. The VFD operates in direct torque control mode and is coupled with a pumping system. The DELC is connected at point of common coupling in parallel with consumer loads. The VSC provides instantaneous control of the SEIG voltage through reactive power compensation. The VFD consumes the power in excess of consumer loads to maintain constant generator output power which in turn regulates the frequency. The excess generated power consumed by the VFD is used to pump water either to an overhead tank or for irrigation purposes. The performance of the proposed DELC of SEIG is demonstrated under various types of loads such as linear/non‐linear, balanced/unbalanced loads and so on. Test results recorded on a developed DELC of SEIG exhibit simultaneous voltage and frequency control capabilities of the DELC under varying loads.

One of the most common fault in Pulse Width Modulation (PWM) Voltage Source Inverter (VSI) feeding an induction motor is open gate drive of the switching device (IGBT). In addition to these extreme fault cases, there may be less severe faults cases due to improper contact points, problematic solder joints, poor connections etc. Main objective of this work is to develop a rule base that not only can segregate these two different types of fault cases but also can identify the power switch in which such a fault has occurred. Rigorous simulation results using PSIM software for creating all variations of the above mentioned fault cases in a PWM VSI driven induction motor is presented for diagnosing the condition of an inverter. The three phase line currents feeding the motor are recorded and transformed to d-q reference frame using Park's Transformation for further analysis. From d and q current components thus obtained, the mean current vector is calculated. The loci of the mean current vector are plotted on the d-q plane and their patterns are observed. Shapes of these loci have been found to be effective in classifying different fault modes.

This paper proposes a new position sensorless control for interior permanent-magnet synchronous motor (IPMSM) drives fed by overmodulation mode pulsewidth modulation (PWM) voltage source inverter. It is attractive to expand the speed range of the IPMSM for application to electric vehicles, electrical household appliances, and so on. An overmodulation mode PWM voltage-source inverter makes it possible to solve problems such as efficiency, wide speed range operation, and so on. Most position sensorless controls based on inverter output voltage knowledge, however, cannot directly be utilized because the voltage reference obtained by the controller is not equal to the inverter output voltage. This paper proposes unknown input observer to estimate rotor position without voltage knowledge, and investigates robustness to inductance variation.

This paper describes the basic concept of an integrated virtual learning system as an instructional tool for the development of digital signal processor (DSP)-based control schemes for motor drive applications. The system is comprised of a graphical user interface (GUI) front-end and a hardware-in-the-loop custom DSP for rapid prototyping and efficient testing of digital control algorithms. The circuit design and control algorithm development of a pulse width modulation (PWM) voltage source inverter (VSI) for 3-phase brushless DC (BLDC) motor control applications is used to describe the functions of the system. It is shown that the virtual learning system provides a cost-effective learning tool for students or engineers in training and can serve as a supplement to a conventional laboratory based system.

This paper presents a simple core loss modeling approach for an inductor of three-phase voltage source converter (VSC) under pulse width modulation (PWM) technique. The inductor core loss calculation in PWM VSCs is challenging since the current ripple and its duty cycle are varying in each switching cycle. To calculate the core loss, however, generating the loss map as a common method is complicated and time consuming. This paper presents a new modeling method in which the minor B-H loops are categorized according the average dc bias and ac current ripple. The core loss is then modeled via calculating the power loss of averaged minor B-H loop in each category. On the other hand, unlike the loss map method, the presented model needs less iteration to find the core loss. Moreover, in this paper, the effect of dc pre-magnetization is investigated by measuring its influence on the core. As a case study, the core loss of an inductor has been calculated and the experimental results are presented. The inductor is designed with a toroidal Kool Mu powder core for a 5 kW three-phase PWM VSC.

In this work, a novel control of single‐phase solar photovoltaic (PV) fed unified active power filter (PV‐UAPF) is proposed for use in low‐voltage distribution system. The shunt voltage source converter (VSC) of PV‐UAPF mitigates current power quality issues such as current harmonics, reactive current etc. while the series VSC of PV‐UAPF protects the sensitive load against point of common coupling (PCC) voltage disturbances. The shunt VSC control is based on extraction of fundamental load active current using recursive least squares with a variable forgetting factor. The proposed method improves the dynamic response of the system without sacrificing the steady‐state performance. The series VSC is controlled using a proportional‐resonant controller and it injects voltage in/out phase with PCC voltage during voltage sags/swells, respectively. The PV array which is integrated at the DC link of the PV‐UAPF through a reverse blocking diode, provides the load power demand and also supplies remaining power into PCC. The system performance is evaluated using Matlab‐Simulink software and is then experimentally verified on a prototype developed in the laboratory under steady‐state and dynamic conditions such as irradiation variation, PCC voltage fluctuations and load disturbances to validate the performance of the system.