Ruth Douglas Miller

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.

Using GENI for experimental evaluation of Software Defined Networking in smart grids

Abstract The North American Electric Reliability Corporation (NERC) envisions a smart grid that aggressively explores advance communication network solutions to facilitate real-time monitoring and dynamic control of the bulk electric power system. At the distribution level, the smart grid integrates renewable generation and energy storage mechanisms to improve the reliability of the grid. Furthermore, dynamic pricing and demand management provide customers an avenue to interact with the power system to determine the electricity usage that best satisfies their lifestyle. At the transmission level, efficient communication and a highly automated architecture provide visibility in the power system and as a result, faults are mitigated faster than they can propagate. However, such higher levels of reliability and efficiency rest on the supporting communication infrastructure. To date, utility companies are moving towards Multiprotocol Label Switching (MPLS) because it supports traffic engineering and virtual private networks (VPNs). Furthermore, it provides Quality of Service (QoS) guarantees and fail-over mechanisms in addition to meeting the requirement of non-routability as stipulated by NERC. However, these benefits come at a cost for the infrastructure that supports the full MPLS specification. With this realization and given a two week implementation and deployment window in GENI, we explore the modularity and flexibility provided by the low cost OpenFlow Software Defined Networking (SDN) solution. In particular, we use OpenFlow to provide (1) automatic fail-over mechanisms, (2) a load balancing, and (3) Quality of Service guarantees: all essential mechanisms for smart grid networks.

Simulation of Electromechanical Interactions of Permanent-Magnet Direct-Drive Wind Turbines Using the FAST Aeroelastic Simulator

Two detailed models of permanent-magnet direct-drive (PMDD) wind turbines with full converters are presented in this paper: one for a 10-kW turbine, and one for a 5-MW turbine. The models are verified by comparing the power curves found through simulation with field test data. Other results are also presented that show the unprecedented detail of the models. The mathematical representations include switching models for the full converters, circuit models for permanent-magnet synchronous generators, realistic aerodynamics, tower and blade vibrations, and many other variables. The models are valuable tools for wind turbine design and research and can be used for a wide range of purposes including control system design, sensitivity analysis, and interactions between the electrical and mechanical parts of a PMDD wind turbine. Simulation of the models is carried out in the MATLAB/Simulink environment using the FAST aeroelastic simulator.

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.

Basic design and cost optimization of a hybrid power system for rural communities in Afghanistan

Afghanistan is a mountainous country with a significant amount of snow during the winter and once it melts the water runs into rivers, lakes and streams. Therefore it does not face any shortage of running water during the year. Also, Afghanistan has plentiful wind and solar energy potential. Therefore, small hydro-power, wind turbines and solar energy are attractive renewable energy sources for remote communities. The development of such a hybrid power system is a complex process. This paper will give an insight into design, cost-effectiveness and feasibility of a hybrid power system using Hybrid Optimization Model for Electric Renewable (HOMER) with two different scenarios in order to encourage private investors and local community people to take advantage of this potential available in Afghanistan and ensure sustainability of investments in micro-hydro power, wind and solar.

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 complex networks approach for sizing and siting of distributed generators in the distribution system

The use of distributed energy is gaining more importance with the advent of the smart grid, challenges of power transfer over long distances and the need to be secure and independent in energy production. In this paper, we present an analytical method, using electrical centrality, to determine the locations and sizes of distributed generators to be placed in the distribution system. Electrical centrality is a metric used in the topological analysis of power systems, that differentiates the electrical structure of the system from its topological structure. It uses the Zbus matrix of the distribution system to determine which nodes are electrically more central to the system and indicates them as the best locations for the placement of distributed generators, with the size of the generator related to the centrality of the node but decided by exhaustive search. It is assumed that all the generation is supplied through distributed generators. We obtain the results for the 12-, 33-, and 69-node distribution systems using this method. The results indicate that the locations indicated by electrical centrality are either the end nodes or nodes closer to the end nodes in the different branches of the networks. Generally, the end nodes are the ones where the voltage drops. As a result, this placement of distributed generators definitely corrects the voltage profile. This placement successfully makes the overall system losses very small as is seen from the optimal power flow solution obtained before and after the distributed generator placement.

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.

Active Control of Wind Turbine Rotor Torsional Vibration

Transient and harmonic stresses in wind turbine rotor shafts contribute to gearbox failure. This paper investigates the reduction of rotor shaft torsional vibrations through active control of the generator torque. A 5 MW turbine model is used to test the procedure. A model of a permanent magnet synchronous generator is included as part of the wind turbine simulation. The simulations are carried out using the software FAST from the National Renewable Energy Laboratory (NREL). The PI and feedback linearized controller for the generator is derived together with the means for vibration isolation. Examples of steady, time varying, and turbulent wind are presented which all show significant reduction in the torsional oscillations.© 2013 ASME

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.