Mahesh S. Illindala

Operation and control of a dynamic voltage restorer using transformer coupled H-bridge converters

The dynamic voltage restorer (DVR) as a means of series compensation for mitigating the effect of voltage sags has become established as a preferred approach for improving power quality at sensitive load locations. Meanwhile, the cascaded multilevel type of power converter topology has also become a workhorse topology in high power applications. This paper presents the detailed design of a closed loop regulator to maintain the load voltage within acceptable levels in a DVR using transformer coupled H-bridge converters. The paper presents system operation and controller design approaches, verified using computer simulations, and a laboratory scale experimental prototype.

Torque Ripple Reduction in Interior Permanent Magnet Synchronous Machines Using Stators With Odd Number of Slots Per Pole Pair

This paper develops analytical principles for torque ripple reduction in interior permanent magnet (IPM) synchronous machines. The significance of slot harmonics and the benefits of stators with odd number of slots per pole pair are highlighted. Based on these valuable analytical insights, this paper proposes coordination of the selection of stators with odd number of slots per pole pair and IPM rotors with multiple layers of flux barriers in order to reduce torque ripple. The effectiveness of using stators with odd number of slots per pole pair in reducing torque ripple is validated by applying a finite-element-based Monte Carlo optimization method to four IPM machine topologies, which are combinations of two stator topologies (even or odd number of slots per pole pair) and two IPM rotor topologies (one- or two-layer). It is demonstrated that the torque ripple can be reduced to less than 5% by selecting a stator with an odd number of slots per pole pair and the IPM rotor with optimized barrier configurations, without using stator/rotor skewing or rotor pole shaping.

Microgrids and sensitive loads

This paper discusses the issues related to the operation of a microgrid being called to feed reliable electric power to sensitive electrical loads. Challenges and solutions involved in terms maintaining adequate voltage quality and load tracking capabilities are discussed in the paper.

Reducing Harmonic Eddy-Current Losses in the Stator Teeth of Interior Permanent Magnet Synchronous Machines During Flux Weakening

Interior permanent magnet (IPM) synchronous machines can experience large harmonic eddy-current losses in the stator teeth under flux-weakening operation, significantly depressing the efficiency of these machines at high operating speeds. This paper presents a new analytical/finite-element hybrid design approach to reduce the harmonic eddy-current losses in IPM machine stator teeth during flux-weakening operation. The proposed technique achieves this objective by three steps: 1) developing an analytical index for the harmonic eddy-current losses in IPM machine stator teeth; 2) designing the spatial distribution of the rotor MMF to minimize the analytical index; and 3) synthesizing the rotor geometry to implement the desired rotor MMF function while maintaining the basic machine characteristics unchanged. It will be shown that two-layer rotors, if properly optimized, are significantly more effective than one-layer rotors for the purpose of reducing the harmonic eddy-current losses in IPM machine stator teeth during flux-weakening operation at high speeds.

The Influence of Inverter-Based DGs and Their Controllers on Distribution Network Protection

The ever growing penetration of distributed generation (DG) in a distribution network has a profound impact on network protection and stability. Traditional protection schemes and algorithms need to be extensively investigated as more and more DGs get introduced into the network. The current version of IEEE Standard 1547 does not present a comprehensive solution for fault current detection in the presence of various kinds of DGs. Power electronic inverter-based DGs (IBDGs) are of special concern in distribution network protection as they are often incapable of providing sufficient fault current and their controllers play a principal role in the DG behavior. In this paper, the effects of voltage and current controllers for IBDGs on industrial and commercial power system protection schemes are investigated. It is shown that the inverter control mode has a direct impact on its fault current levels and duration. A simplified distribution network model with IBDG operating under voltage and current control modes was tested to verify the effects of these controllers. This paper also proposes an adaptive relaying algorithm to detect the faults in the presence of IBDGs with various types of controllers.

Small Signal Dynamics of Inverter Interfaced Distributed Generation in a Chain-Microgrid

Distributed generation, or DG, involves utilization of small generators that are distributed in a power network, to supply the electric power demands of utility customers. This paper presents the analysis of droop based generation control schemes for DG inverters comprising of active power-frequency and reactive power-voltage controllers. This enables decentralized operation with load sharing in a microgrid containing several interconnected DGs. Small-signal models are developed for microgrids consisting of several DGs connected in a chain topology. Mathematical propositions for stability are developed and guidelines are provided for design of controllers to meet the IEEE P1547 performance specifications.

Protection Strategies for Medium-Voltage Direct-Current Microgrid at a Remote Area Mine Site

This paper presents protection strategies for a medium-voltage dc (MVDC) microgrid at a remote area mine site. The microgrid is operated to provide high power quality and reliability to sensitive loads and to improve the energy efficiency of the mining equipment. In the MVDC microgrid, various local distributed energy resources have been used, including photovoltaic arrays, wind turbines, a fuel-cell stack, an energy storage system, and mobile diesel generators. For the protection of transmission lines, a communication-based differential protection scheme with solid-state electronic relays is employed to isolate the faulted part of the MVDC microgrid. This is further reinforced by dc overcurrent protection as a backup. Earlier research work had neglected the backup protection for dc systems. In addition, communication-based dc directional overcurrent protective relays are used for both source protection and load protection to support a bidirectional power flow. MATLAB/Simulink modeling and simulation results are presented and discussed to illustrate the proposed systems dependability and security.

Graphical and Analytical Methods for Stalling Analysis of Engine Generator Sets

Reciprocating engine driven generator sets (or “gensets”) are a leading class of distributed energy resources (DERs) from the low kilowatt up to hundreds of kilowatt ratings. Natural-gas-based DERs are more likely to be favored by both industries and utilities in the near future because of their low cost due to the newly found shale deposits at many places in the world. The DERs such as gensets when operating under islanded conditions are susceptible to stalling and can bring down the entire industrial power system, particularly if they are loaded beyond 80% of the rated engine capacity. This paper presents graphical and analytical methods to determine the stalling conditions of the most common type of gensets that have a synchronous generator as the utility interface. A systematic approach is developed from the first principles of generator electromagnetic energy conversion. The engine fuel map limits have been integrated into the generator speed versus active power characteristics for this purpose. The stalling prevention by applying an underfrequency load relief (V/Hz) scheme is also discussed.

Modeling and control of a natural gas generator set in the CERTS microgrid

The recent addition to the CERTS microgrid was a commercial off-the-shelf natural gas engine generator set, also referred as gen-set. With this incorporation, the CERTS microgrid that earlier consisted of the same kind of inverter-based combined heat & power (CHP) distributed energy resources (DERs), has got transformed into a diverse/mixed microgrid. As the gen-set came equipped with an isochronous speed governor and ac brushless type exciter, a few enhancements became necessary for augmenting it with CERTS controls that enable capabilities of plug-and-play operation, and seamless islanding and reconnection with the utility grid. This paper describes the modeling and control of gen-set. The models developed are validated with experimental testing against large step changes in load. Performance of gen-set with CERTS controls is demonstrated in different modes of operation of the mixed microgrid.

Flexible Distribution of Energy and Storage Resources

Flexible Distribution of EneRgy and Storage Resources (FDERS) is a new framework of integrating Distributed Energy Resources (DERs) and energy storage devices. It was inspired by the V-shape formation of flocks of birds and peloton formation of cycling racing teams that extend the range of traveled distance significantly by way of composite energy savings. For example, the V-shape formation consisting of 25 birds could result in a 70% increase in the range of distance flown by them as compared with a bird flying solo. Similar ideas when applied in integrating DERs and energy storage devices offer the benefits of increased DER lifetime, optimal energy storage deployment, enhanced controllability and improved system robustness. FDERS is an enabling technology to achieve this goal through reconfiguration and the use of variable interface reactances for interconnecting multiple DERs with the utility and load. The values of interface reactances are adjusted depending on the energy resource availability, response characteristics and fuel pricing of individual DERs. This paper presents the basic concept of FDERS supported by energy savings analysis of DER formations in series and parallel configurations.