Vijay Bhavaraju

Microgrid Generation Capacity Design With Renewables and Energy Storage Addressing Power Quality and Surety

Microgrids are receiving attention due to the increasing need to integrate distributed generations and to insure power quality and to provide energy surety to critical loads. Since renewables need to be in the mix for energy surety, a high renewable-energy penetrated microgrid is analyzed in this paper. The standard IEEE 34 bus distribution feeder is adapted and managed as a microgrid by adding distributed generation and load profiles. The 25 kV system parameters are scaled down to 12 kV and renewable sources including solar PV and wind turbines, an energy storage system, and a diesel generator for islanded mode have been added to the 34-bus system. The distribution generations (DG) and renewables are modeled in detail using PSCAD software and practical constraints of the components are considered. The monitoring of the microgrid for measuring power quality and control requirements for these DGs and storage are modeled to maintain the power quality of the system when loads are varied. Renewable sources are modeled with seasonal variation at different locations. The microgrid is monitored at number of buses and the power quality issues are measured and indexes are calculated. This paper proposes a generalized approach to design (determine the capacity requirements) and demonstrates the management of microgrids with metrics to meet the power quality indexes.

Transition Management of Microgrids With High Penetration of Renewable Energy

Microgrids are receiving attention due to the increasing need to integrate distributed generations and to ensure power quality and to provide energy surety to critical loads. Some of the main topics concerning microgrids are transients and stability concerns during transitions including intentional and unintentional islanding and reconnection. In this paper, the standard IEEE 34 bus distribution feeder is adapted and managed as a microgrid by adding distributed generations and load profiles. Supervisory power managements have been defined to manage the transitions and to minimize the transients on voltage and frequency. Detailed analyses for islanding, reconnection, and black start are presented for various conditions. The proposed control techniques accept inputs from local measurements and supervisory controls in order to manage the system voltage and frequency. An experimental system has been built which includes three 250 kW inverters emulating natural gas generator, energy storage, and renewable source. The simulation and experimental results are provided which verifies the analytical presentation of the hardware and control algorithms.

The Role of Energy Storage in a Microgrid Concept: Examining the opportunities and promise of microgrids.

A Microgrid is a cluster of distributed generation (DG), renewable sources, and local loads connected to the utility grid. A microgrid provides a solution to manage local generations and loads as a single grid-level entity. It has the potential to maximize overall system efficiency, power quality, and energy surety for critical loads. The Microgrid Exchange Group, an ad hoc group of expert and implementers of microgrid technology, has defined a microgrid as a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island mode.

A New Framework for Microgrid Management: Virtual Droop Control

Microgrids can provide the most promising means of integrating large amounts of distributed sources into the power grid and can supply reliable power to critical loads. A generalized control framework is required to regulate microgrid voltage and frequency, maintain system stability, maintain power quality, and manage distributed generations. A microgrid control framework for power management and voltage and frequency regulation is proposed in this paper. The proposed method, virtual droop control, is described and formulated and compared with the existing natural droop control technique. Unit commitment algorithm has also been implemented to manage nonrenewable sources to improve the system efficiency. The proposed technique operates the microgrid at a constant voltage and frequency, and uses communication for power sharing. It also provides the means to operate the microgrid in case of lost communication and sabotage on communication network. The proposed method has been applied to Fort Sill microgrid and the modeling results have been compared with natural droop control technique. A laboratory setup, which consists of a 100 kW natural gas generator, 56 kWh Li-ion based battery with 250 kW inverter, and 100 kW load bank, has been built and tested. The results of the setup have been provided, which confirms the viability of the proposed technique.

Managing intermittent renewables in a microgrid

Renewable energy sources connected to the utility grid can achieve enough penetration to affect the distribution system since they are intermittent. An aging electric distribution system infrastructure, with an increasing demand for power quality, reliability, energy surety, and security, needs solutions for managing increasing penetration of renewables. By 2015, the global wind and solar Photovoltaic production capacities are expected to reach 430.6 GW and 125.9 GW, respectively. The potentials of DGs can be better realized in the concept of a microgrid rather than as individual sources. In this paper, the standard IEEE 34 bus is used as a feeder and managed as a microgrid. The system parameters are scaled down to 12kV level and two renewable sources including 500kW solar PV and 1MW wind turbine have been added to the system. The distribution system and the renewables are modeled using PSCAD software and practical constraints of the renewables are considered. The monitoring requirements of the microgrid and control requirements for these DGs are managed to maintain the power quality of the system when loads are varied and the renewable sources are modeled with annual variation at different locations.

Microgrids: Architectures, Controls, Protection, and Demonstration

Abstract—In the recent years, there has been a growing interest in the concept of microgrids to integrate distributed generation systems and to provide higher reliability for critical loads. Several microgrid demonstration projects have been implemented to investigate further and advance this emerging concept. This article provides a detailed review of microgrid systems. It describes different architectures, including AC, DC, and hybrid systems. Various microgrid components, including sources, converters, and loads, are illustrated. Microgrid management and controls are discussed, and a modified natural droop control is described in detail. Both physical layers and standard protocols are explained for communication in the microgrid structure. The unique protection complexities have been raised and discussed in the presence of distributed generations and bidirectional power flow. A demonstration of a military microgrid system at Fort Sill is illustrated, and the experiment of a typical microgrid operation scenario is provided.

Generation capacity design for a microgrid for measurable power quality indexes

Microgrids are receiving a lot of attention to utilize distributed generations in a sub-system and provide higher efficiency and reliability and support local loads. A high renewable-energy penetrated microgrid is studied in this paper. The distribution system and the loads in the microgrid are represented by a properly scaled 12kV IEEE 34 bus system. The central power is replaced with a 1.875MVA base-load diesel generator and a 250kW solar PV and a 500kW wind generations are added at nodes 848 and 890, respectively. The major loads in this microgrid are then modeled with a load profile. Special variable kW and kVAR loads were modeled to include load demand variations. The wind and solar PV plants are modeled with wind and solar power profiles. The microgrid system including the source variations and demand variations is simulated using PSCAD software. The microgrid is monitored at number of buses and the power quality issues are measured and indexes are calculated. This system can be used to determine the capacity requirements for the non-renewable generators to maintain the power quality indexes.

Evaluation of commercial scale transformerless solar inverter technology

As the transformerless solar inverters have been pushed to an impressive 99% efficiency in residential and small commercial applications (less than 20 kW), high efficiency medium and large commercial scale (50-250 kW) transformerless solar inverters are gaining more and more attentions. However, due to the differences of inverter power ratings, grid voltages and standard requirements, there are new challenges and problems for the commercial scale photovoltaic system to eliminate the isolation transformers and achieve the competitive efficiency. This paper reviews and evaluates the state-of-art technologies of todays U.S. commercial scale PV inverters, and proposes the future trends for next generation commercial scale transformerless solar inverters. The transformerless inverter topologies, leakage current and existing standards are discussed and assessed. The goal of this paper is to address the new challenges of moving transformerless solar inverters into the commercial scale PV system with 99% efficiency.

A new control method for microgrid power management

Microgrids are receiving tremendous attention due to increasing need to integrate Distributed Generations (DG) and to provide reliable power to critical loads. Proper power management in microgrid is required to regulate the system voltage and frequency, maintain power quality, and manage DGs. A microgrid system has been studied in this paper, which includes two natural gas generators, an energy storage device, solar PV, and a wind turbine. The DGs are modeled in details using PSCAD software and the practical constraints of the components are considered. The Virtual Droop Control (VDC) method for microgrid power management is proposed and described in details. The proposed technique has been applied to adjust the active and reactive power of each component in order to manage the system. Unit commitment algorithm has also been implemented to improve the system efficiency.

Hybrid DC switch for solar array fault protection

Photovoltaic (PV) systems have the intrinsic nature of distributed DC power generation that requires additional protection to reduce the fire and shock risks from various PV faults. Due to the current limited nature of PV generation, the short circuit current of a PV panel is typically 1.2 times of its rated current. This poses a unique overcurrent protection challenge, because traditional overcurrent protection, such as fuses and breakers, are designed for cable protection and they provide protection only for high reverse current faults in solar arrays. This paper proposes a solar fault protection system, including a hybrid DC switch based overcurrent protection device and a protection and coordination scheme. An optimized switch timing control is proposed for the hybrid DC switch. The proposed control method utilizes the parallel electromechanical contactors arc voltage to determine the solid state devices switch timings, thus significantly reduces the solid state device rating and size. A protection and coordination scheme of using hybrid DC switches in PV system is also proposed and implemented. The proposed scheme guarantees the hybrid DC switches only need to interrupt forward fault current, so that the hybrid DC switches do not need to be sized according to the high reverse fault current. Experimental results verify that the hybrid DC switch with the proposed protection and coordination scheme can effectively protect the solar array faults.