Brandon M. Grainger

Ship to Grid: Medium-Voltage DC Concepts in Theory and Practice

Corporate research centers, universities, power equipment vendors, end users, and other market participants around the world are beginning to explore and consider the use of dc in future transmission and distribution system applications. Recent developments and trends in electric power consumption indicate an increasing use of dc-based power and constant power loads. In addition, growth in renewable energy resources requires dc interfaces for optimal integration. A strong case is being made for intermeshed ac and dc networks, with new concepts emerging at the medium-voltage (MV) level for MV dc infrastructure developments.

Survey of battery energy storage systems and modeling techniques

Grid level energy storage systems are a cornerstone of future power networks and smart grid development. Better energy storage systems are one of the last hurdles hindering the integration of renewable generation. There are currently many methods of implementing energy storage, ranging from pumped hydro storage to sodium-sulfur battery storage. All energy storage technologies share a common disadvantage which is high initial installation costs. This survey was undertaken with the intent of identifying the technological state of battery energy storage for power systems, as well as providing a background on the modeling and simulation of those battery technologies.

A comparative study of MPPT methods for distributed photovoltaic generation

Photovoltaic (PV) energy generation is becoming an increasingly prevalent means of producing clean, renewable power. PV is renewable, reliable, and domestically secure. One of the most important components of PV systems is the inverter technology that converts the direct current (DC) power output from the PV panel or array to alternating current (AC) used on both the individual end-user and centralized grid levels. The large variety of inverters share the same general goal: to allow for the most efficient and stable transfer of as much power as possible. One specific means of accomplishing this goal is the inclusion of a Maximum Power Point Tracking (MPPT) DC-DC converter. The purpose of MPPT is to ensure that the PV panel or array is always producing power as near to the knee of its I-V curve as possible. This extracts the maximum amount of power at any given time. In constantly sunny situations, there is little impact on overall performance of a particular MPPT design on the PV system, as only small voltage differences due to the particular construction of each panel effects the overall voltage outputs. However, cloud cover changes the output from a PV panel drastically with reduced solar irradiation causing the current of the solar panel to drop. It is postulated herein that the stability and quality problems created by central MPPT during periods of differing solar irradiation on various panels could be solved with a system of MPPT distributed on each panel. These would then feed collectively to a central inverter. To test these systems, a PSCAD model was developed for both centralized and distributed MPPT systems, and the solar irradiation was randomly varied. This allowed for observation of the stability and quality of the output voltage for each system.

Analysis of high capacity power electronic technologies for integration of green energy management

The rate of growth in US electrical demand necessitates the development of new generation resources and the expansion of electrical infrastructure. With respect to this growing demand and the current trends of energy policy development, policy makers, corporations, and private investors continue to push the development and integration of green and renewable generation, including nuclear, wind, and solar power. The integration of novel and existing generation sources within the US electric grid presents new requirements and challenges for large scale resource management and interconnection. The sheer magnitude of new generation being added to the grid, as well as the diversity of generation sources, will drive expansion and investment within the transmission & distribution (T&D) infrastructure sector and in enabling technology developments. In this paper, descriptions and solutions are presented for many of the foreseeable scenarios that will emerge from the T&D perspective regarding integrated generation management (IGM). By analyzing several primary classes of transmission technology - conventional AC transmission, high voltage DC transmission (HVDC), and FACTS compensation techniques as applied to both AC and DC transmission - this work presents future applications, advantages, and development requirements for power electronics and control technologies in a diversified generation environment and with respect to power system dynamic performance.

Power Electronics for Grid-Scale Energy Storage

Power electronic conversion units will serve as a key enabling technology for assisting in the continued growth of grid-scale energy storage. This paper presents existing and future power electronic conversion systems and components that aid the interconnection of grid-scale energy storage or utilize storage to minimize grid disruption at all voltage classes including transmission, distribution, and future grid architectures such as the microgrid. New R&D solutions to aid the interconnection process including efforts in bidirectional charger design and potentially solid-state transformers (SSTs) are emphasized. The role of energy storage to support microgrid research and growth, while highlighting power electronic behavior within this environment, is considered. Last, an example that bridges the microgrid and energy storage theme is given through the design and operation of a direct current (dc) electric vehicle (battery) charging station. When appropriate, manufacturer solutions and success stories of utilizing latest battery technologies interfaced via power electronic solutions at the utility scale are provided.

A Secure Communication Architecture for Distributed Microgrid Control

Microgrids are a key component in the evolution of the power grid. Microgrids are required to operate in both grid connected and standalone island mode using local sources of power. A major challenge in implementing microgrids is the communications and control to support transition from grid connected mode and operation in island mode. Here, we propose a secure communication architecture to support microgrid operation and control. A security model, including network, data, and attack models, is defined and a security protocol to address the real-time communication needs of microgrids is proposed. The implementation of the proposed security scheme is discussed and its performance evaluated using theoretical and co-simulation analysis, which shows it to be superior to existing protocols.

A microgrid co-simulation framework

Microgrids have been proposed as a key piece of the Smart Grid vision to enable the potential of renewable energy generation. Microgrids are required to operate in both grid connected and standalone island mode using local sources of power. A major challenge in implementing microgrids is the communications and control to support transition to and from grid connected mode and operation in island mode. Microgrids consists of two interdependent networks, namely; the power distribution and data communication networks. To accurately capture the overall operation of the system, we propose a co-simulation model driven by embedded power controllers. Further, we propose a novel co-simulation scheduler taking into account events from both the power and communication network simulators, as well as the timing of each embedded controllers execution loop to adaptively synchronize both simulators efficiently. The approach ensures minimal synchronization error while still providing the ability to simulate extended operational scenarios. The numerical results illustrate the novelty of the propose co- simulation to study the microgrid power and communication networks interactions, and the effect on the power stability.

Design and simulation of a DC electric vehicle charging station connected to a MVDC infrastructure

Medium Voltage DC (MVDC) infrastructure serves as a platform for interconnecting renewable electric power generation, including wind and solar. Abundant loads such as industrial facilities, data centers, and electric vehicle charging stations (EVCS) can also be powered using MVDC technology. MVDC networks are expected to improve efficiency by serving as an additional layer between the transmission and distribution level voltages for which generation sources and loads could directly connect with smaller rated power conversion equipment. This paper investigates an EVCS powered by a MVDC bus. A bidirectional DC-DC converter with appropriate controls serves as the interface between the EVCS and MVDC bus. Two scenarios are investigated for testing and comparing EVCS operation: 1) EVCS power supplied by the interconnected MVDC model and 2) EVCS power supplied by an equivalent voltage source. Comparisons between both are discussed. The CCM/DCM buck mode operation of the bidirectional DC-DC converter is explored as well as the system isolation benefits that come with its use.

Analysis of an Offshore Medium Voltage DC Microgrid Environment - Part II: Communication Network Architecture

The microgrid is a conceptual solution proposed as a plug-and-play interface for various types of renewable generation resources and loads. The high-level technical challenges associated with microgrids include (1) operation modes and transitions that comply with IEEE1547 and (2) control architecture and communication. In Part I, the emphasis is on the design of an electrical control architecture for an offshore oil drilling platform powered by wind generation. Engineering a distributed control system having safety critical features, requiring real-time performance is challenging. In this follow-up article we introduce the communication framework for the microgrid scenario under investigation. In all communication networks, scholastic delays and performance are inherent. The only feasible approach is to put bounds on the random processes, qualitatively define the worst cases, and build the distributed control system to be resilient enough to tolerate those behaviors. This is the approach taken by this paper. We propose a communication architecture, discuss performances requirements of the sub-systems, and layout network solutions meeting those specifications.

Analysis of an offshore medium voltage DC microgrid environment — Part I: Power sharing controller design

The AC microgrid is proposed as a plug-and-play interface for various types of renewable generation resources. The fundamental microgrid requirements include the capability of operating in islanding mode and/or grid connected modes. The high-level technical challenges associated with microgrids include (1) operation modes and transitions that comply with IEEE1547 and (2) control architecture and communication. Proposed control layers and respective functions have been defined in the microgrid literature. These layers include the primary, secondary and tertiary control. In this work, the focus is on the design of a power management secondary control for an offshore DC microgrid serving induction motor drives. The power is managed through a secondary controller to adjust the power delivered through bidirectional DC/DC converters, which intertie with the inverters connected to the machine loads. Simulations in Matlab/Simulink demonstrate the system performance under a mechanical torque step change for one motor drive unit with other conditions fixed. Part I of the concept emphasizes the electrical layout and part II, a communication layout of the offshore platform is described.