Microgrids Integrating renewable energy sources - (RES) to improve reliability

by David G. Hart, Steven A. Kunsman, ABB Power Systems Automation & Communications, North America and Brad Luyster, ABB Power Systems - Microgrids, North America

Distributed energy and advanced grid automation systems can play a huge role in the restoration of electric power by isolating the faulted section of a power system and islanding small communities where distributed generation resources can be utilized to restore power in these “micro” grids.
Since the devastation of Sandy, power industry stakeholders including utilities, regulators, and communities are reviewing technology and modernization solutions as a means to minimize power disruption and improve reliability.

Historically, power has been provided by centralized generation transmitted via lines to regional distribution substations. Microgrids are small scale versions of the traditional grid that rely on some local generation from distributed energy sources such as industrial co-generation, solar or wind.

Microgrids may be entirely self-sufficient and act independently in cases where access to the traditional grid is not possible, but most will typically be interconnected to the traditional grid. In this case, under normal conditions, the microgrid’s distributed energy resources may supply excess power back to the traditional grid. However, during power disruptions, the microgrid can become an autonomous entity operating in “island-mode”, where isolation is made from utility interconnection, until the main grid’s power is restored.

Drivers for the deployment of microgrids include expansion of renewable energy sources, local generation in remote areas and the ability to isolate and island from the main electric grid. In all cases, the ability to improve the reliable delivery of energy is a key factor.
Microgrids serve a group of local consumers such as a remote region, critical infrastructure, university campus, military base, or municipality and offer reliable power during disruptions on the main power grid. However, when discussing microgrids, it is often other intermediary “consumer advocates” such as political leaders, regulators, business executives and others who serve the end consumer, that initiate the microgrid discussion as they search for ways to ensure a reliable and safe supply of power to their constituencies.

While the microgrid concept is not new, generation using traditional and RES energy storage and control and automation technologies available today make microgrids a cost-effective grid automation solution suitable for everyday energy needs as well as a reliable alternative during main grid disruptions. This article reviews the use of microgrids for stand-alone island applications as well as highlights the benefits to the traditional utility power grid.

Stand-Alone Microgrids
Providing Power to Remote Areas As difficult as it may be for people living in more developed parts of the world to imagine, there are areas that cannot be reliably addressed by the existing grid infrastructure. For example, a remote mining operation in the Antarctic or an island separated from the mainland by many thousands of miles of water may not be able to access the main grid at all. Obviously, these cases require local generation to meet power needs. However, to include renewable energy sources as a major part of the solution requires overcoming some problems. Historically, the greatest obstacle to microgrid adoption by utilities has been the risk to grid stability particularly with a high penetration of renewables.

A Case Study:
Such was the situation on Flores Island in the Azores. Remote from the central grid in Portugal, the islands are powered by heavy fuel oil (HFO) generators and a hydro power plant. By necessity, the local utility company on Flores Island had already established a microgrid that operated independently from the main grid.

In an effort to reduce the cost of diesel power generation, the utility wanted to increase the penetration of renewable energy sources with the installation of a number of wind turbines. Grid advancements to enable these remote regions to maintain some self-sufficiency as well as incorporate readily available sources of renewable energy can bring huge economic benefits.

However, some renewable energy sources, most notably wind and solar, are intermittent by nature, resulting in unreliable energy generation and leading not only to fluctuation and surges but potentially to blackouts. It soon became apparent the amount of wind power injected into the system had to be limited in order to stabilize the grid from fluctuations in frequency and voltage. Stabilizing the grid requires dynamic power injection and absorption for a short amount of time to mitigate the impact of the renewable energy variations. While common energy storage solutions using batteries, pump storage or similar technologies can help store energy for use during peak times or low energy availability, they don’t effectively stabilize the grid due to slower response times and the longer period of time required to discharge. To solve these challenges, the local utility implemented ABB PowerStore™, a compact and versatile grid stabilizing generator.

The PowerStore consists of:

  • Flywheel spinning mass including motor/generator
  • AC-DC-AC converter system
  • Operator interface

It is a flywheel based technology and includes AC-DC-AC converter system hardware based on customized IGBT power converters. An operator interface is used to monitor the flywheel and converter components and to provide access to historical data. The flywheel technology allows the system to compensate for power output fluctuations from RES.
The project on Flores Island was successful in smoothing out wind power fluctuations such that the operator can operate the power station without maintaining an online diesel generator. In addition, the utility has expanded the project to neighboring Graciosa Island and is considering increasing the number of wind turbines on the islands.

Interconnection of the Microgrid to the Traditional Grid
RES Integration & Reliability- The Driving Forces for Microgrids

In scenarios like the Azores, the remote nature of the load represents a clear problem for the high penetration of renewable energy sources – there is no main grid to lean on. However, the same type of microgrid established on Flores Island can also be developed in conjunction with the traditional power grid. When connected to the traditional grid, the compelling driver for the discussion is most often reliability. How can we ensure our community, place of business, or entity is protected from an extended power outage? The reliability benefits of the microgrid are clear. When the main grid loses power, the microgrid automation systems detect the external loss of power and manage the microgrid as an islanded system. As long as there are adequate local resources (diesel generator, wind turbine, fuel cell, PV, etc.), the power continues to flow within the microgrid while those outside of the microgrid experience a power outage.

Microgrid’s Hidden Benefits As utilities and community leaders progress in their microgrid discussions, a number of additional benefits are identified:

  • Meeting renewable mandates − Many regions around the world have set aggressive renewable portfolio targets. For example, in the U.S., the state of Illinois has determined by law that it will obtain 25 percent of its energy from renewable sources by 2025. According to the U.S. Department of Energy, Illinois currently derives only 11.54 percent of its energy from renewable sources. It's almost certain that aggressive targets like these won't be reached through utility-scale renewable power generation alone. Support will be needed from the private sector to enable the deployment of distributed resources managed by microgrid solutions
  • Security - A 2003 major outage in North America showed just how vulnerable the grid can be. Reportedly, a faulted power line in Ohio caused a cascade of failures throughout the northeastern U.S. and Canada, eventually affecting more than 50 million people. Experts agree, any prolonged disruption of power, regardless of the cause, represents a threat to security and economic stability. We must find ways to protect the grid from disasters to insure reliable power to consumers
  • Improved public relations (PR) - As many utilities are discovering, one of the greatest benefits to supporting microgrids is improved PR. Perhaps consumers and the media don't pay as much attention as they should when a utility restores power quickly, but they sure notice it when it takes weeks to turn the lights back on after a storm. In cases such as these, having a segment of consumers that can operate independently allows the utilities to minimize the impact on consumers and spread their emergency resources more effectively to focus efforts on those areas that aren't supported by a microgrid. The overall outage duration is shorter and fewer consumers are impacted. The improved PR resonates with the local community offsetting the potential revenue loss. Microgrids allow commercial and industrial entities to make pragmatic investments in renewable energy, starting with a low penetration of renewables but expanding as their microgrid evolves
Power. Flexible. Easergy.
BeijingSifang June 2016