Pedram Sotoodeh

Design and Implementation of an 11-Level Inverter With FACTS Capability for Distributed Energy Systems

In this paper, a new single-phase wind energy inverter (WEI) with flexible AC transmission system (FACTS) capability is presented. The proposed inverter is placed between the wind turbine and the grid, same as a regular WEI, and is able to regulate active and reactive power transferred to the grid. This inverter is equipped with distribution static synchronous compensators option in order to control the power factor (PF) of the local feeder lines. Using the proposed inverter for small-to-medium-size wind applications will eliminate the use of capacitor banks as well as FACTS devices to control the PF of the distribution lines. The goal of this paper is to introduce new ways to increase the penetration of renewable energy systems into the distribution systems. This will encourage the utilities and customers to act not only as a consumer, but also as a supplier of energy. Moreover, using the new types of converters with FACTS capabilities will significantly reduce the total cost of the renewable energy application. In this paper, modular multilevel converter is used as the desired topology to meet all the requirements of a single-phase system such as compatibility with IEEE standards, total harmonic distortion (THD), efficiency, and total cost of the system. The proposed control strategy regulates the active and reactive power using power angle and modulation index, respectively. The function of the proposed inverter is to transfer active power to the grid as well as keeping the PF of the local power lines constant at a target PF regardless of the incoming active power from the wind turbine. The simulations for an 11-level inverter have been done in MATLAB/Simulink. To validate the simulation results, a scaled prototype model of the proposed inverter has been built and tested.

A Method of Seamless Transitions Between Grid-Tied and Stand-Alone Modes of Operation for Utility-Interactive Three-Phase Inverters

A method for the seamless transition of three-phase inverters switched between grid-tied and stand-alone modes of operation is presented in this paper. In this method, only the inverter current and voltage sensors are utilized, and no control over the grid-side static transfer switch is needed. The presented method contains two strategies for grid-tied-to-stand-alone and stand-alone-to-grid-tied transitions. In the stand-alone-to-grid-tied transition strategy, a novel algorithm is presented for estimating the grid angle nearly instantaneously, which allows the three-phase inverter to respond very quickly if the grid and point-of-common-coupling voltages are out of phase. This fast response allows the inverter to effectively eliminate the transient overcurrent that would normally occur if it was connected to the grid without first being synchronized. The fast response also allows the inverter to return to normal operation very quickly after such an event. The strategy for the seamless transition from grid-tied to stand-alone mode is also presented. These strategies have been verified through experiments, and the results are presented in this paper.

Design and control of a single-phase D-STATCOM inverter for wind applications

Application of renewable energy systems has become very popular. Since most utilities do not track the end points of their distribution lines carefully, where most of the wind turbines are connected to the grid, increasing the application of renewable energies in utilities can result in problems for the whole system dynamics. This paper presents the design and control of a D-STATCOM inverter for small to mid-sized wind turbines (10kW-20kW) to solve the problem of power factor correction of the grid. The proposed D-STATCOM Inverter can control the VARs on each single feeder line while the output of the renewable energy source, especially wind, is varying. Active power is controlled by shifting the phase angle while reactive power control is achieved by modulation index control. Also, the inverter is able to eliminate a large amount of harmonics using the optimized harmonic stepped waveform (OHSW) technique. The proposed inverter utilizes the hybrid-clamped topology. All simulations were done in MATLAB/Simulink environment.

A single-phase D-STATCOM inverter for distributed energy sources

Currently many utilities are resistant to the idea of increasing the penetration of distributed energy sources on distribution systems. The majority of distribution systems in the United States are radial and provide utilities with no communication or feedback on the low-voltage side of the substation. This makes the dynamic control of feeder lines very limited with time steps that are well above the variable power output of wind turbines and solar installations. This is in part due to the added technical difficulties associated with maintaining compliance with IEEE standards. This paper presents the design of a unique inverter with D-STATCOM capability for small to mid-sized (10kW-20kW) permanent magnet wind installations. The proposed inverter can actively regulate VARs on individual feeder lines at a programmable output while providing the variable output power of the renewable energy source. The aim is to provide utilities with distributive control of VAR compensation and power factor correction on feeder lines. The designed inverter utilizes a multi-level voltage-source converter (VSC) topology. Reactive power control is achieved by modulation index control. Active power control is achieved by phase-shift-angle control and VSC harmonics are eliminated by the optimized harmonic stepped waveform technique (OHSW). All simulations were done in MATLAB.

A technique for voltage-source inverter seamless transitions between grid-connected and standalone modes

A technique for the seamless transition of voltage-source inverters switched between grid-tied and standalone modes of operation is presented in this paper. In this technique, only the inverter current and voltage sensors are utilized and no control over the grid-side static transfer switch is needed. The presented technique contains two strategies for grid-tied-to-standalone and standalone-to-grid-tied transitions. In the standalone-to-grid-tied transition strategy, a novel algorithm is presented for estimating the grid angle nearly instantaneously, which allows the three-phase inverter to respond very quickly if the grid and point of common coupling voltages are out of phase. This fast response allows the inverter to effectively eliminate the transient overcurrent that would normally occur if it were connected to the grid without first being synchronized. The fast response also allows the inverter to return to normal operation very quickly after such an event. The strategy for the seamless transition from grid-tied to standalone mode is also presented. These strategies have been verified through experiments, and the results are presented in this paper.

A single-phase 5-level inverter with FACTS capability using modular multi-level converter (MMC) topology

Renewable energy systems have become a major part of the modern power systems. Clearly, to connect renewable energy systems to the main grid, a power inverter is needed. There are two major topologies for renewable energy inverters, namely conventional two-level topology and multi-level topology. The modular multilevel converter (MMC) is one of the most advanced multi-level topologies that have gained increasing attention recently. In addition to an inverter in a renewable energy system, capacitor banks or STATCOMs are needed to compensate the reactive power of the grid. In this paper a novel single-phase MMC-based inverter with STATCOM capability for grid connection is proposed. The proposed inverter is designed for grid-connected wind turbines in the mid-sized range. In the proposed control strategy, active and reactive power is controlled by the power angle and the modulation index, respectively. The function of the proposed inverter is to transfer active power to the grid as well as keeping the power factor of the local grid constant at a target power factor regardless of the incoming active power from the renewable energy source, especially from a wind turbine. Generally, the main goal of this paper is to present a new inverter with FACTS capability in a single unit without any additional cost. The simulations have been done in MATLAB/Simulink for a 5-level inverter. A scaled prototype model of the proposed inverter has been built and tested to verify the simulation results.

A new single-phase inverter with D-STATCOM capability for grid-connected small wind turbines

This paper presents the design of a novel multi-level D-STATCOM inverter for renewable energy systems using modular multi-level converter (MMC) topology. The aim of the work is to design a new type of inverter with FACTS capabilities to provide utilities with more knowledge about the distribution systems, specifically on end points. The inverter is placed between the renewable energy source, specifically a wind turbine, and the distribution grid in order to regulate the active and reactive power required by the grid. This inverter is capable of controlling active and reactive power by controlling its phase angle and modulation index, respectively. The unique contribution of the proposed work is to combine the two concepts of inverter and D-STATCOM using a novel voltage source converter (VSC) multi-level topology in a single unit without any additional cost. Simulations of the proposed inverter, with 5 levels, have been completed in Matlab/Simulink. The simulation results validate the performance of the proposed control strategy.

A Novel Single-phase Inverter with Distribution Static Compensator Capability for Wind Applications

Abstract The modular multi-level converter is an attractive topology for high-voltage DC/flexible AC transmission systems. In this article, a new single-phase modular multi-level converter based distribution static compensator inverter for grid connection is proposed. The proposed inverter is designed for grid-connected wind turbines in the small- to mid-sized (10–20 kW) range using the most advanced multi-level inverter topology. The proposed modular multi-level converter distribution static compensator inverter controls the DC-link voltage as well as the active and reactive power transferred between the renewable energy source, specifically the wind turbine, and the grid in order to regulate the power factor of the grid regardless of the input active power from wind turbine. The goal of this study is to present a new inverter with flexible AC transmission system capability in a single unit without any additional cost. The five-level distribution static compensator inverter is simulated, and the results are presented to verify the operation of the proposed system. The simulation studies are carried out in the MATLAB/SIMULINK environment (The MathWorks, Natick, Massachusetts, USA). To validate the simulation results, an experimental configuration of a five-level modular multi-level converter distribution static compensator inverter has been built and tested.

Dynamic model for Brushless Doubly-Fed Machine with stator winding faults

In this paper, a detailed analytical model for dynamic analysis of the Brushless Doubly-Fed Machine is presented. The model is capable to predict the machine behavior under different modes of operation including: simple induction mode, cascade mode and synchronous mode, both with and without winding faults and unbalanced excitations. The proposed approach employs the basic principles of the generalized harmonic theory for calculating self inductances and mutual inductances of circuits with any distribution of conductors. Dynamic equations of modified coupled-circuit method are then solved to compute the currents flowing in the stator windings and rotor loops.

Modeling of a BDFM-based wind turbine under unbalanced grid voltage dips

Brushless Doubly-Fed Machine (BDFM) is a machine which incorporates the robustness of the squirrel cage induction machine while having the speed and power factor control of a synchronous machine. A special kind of rotor called Nested-Loop rotor plays an important part in the BDFM, coupling the two stator fields. In future BDFM will share a large part in wind power market. This paper presents a mathematical model of Brushless Doubly-Fed Machine (BDFM) based on Stator Flux Orientation (SFO) in the positive and negative reference frames under unbalanced grid voltage conditions.