Back-up power system in the form of microgrid

Smart Controls for Renewable Resilient Microgrids

March 18, 2019
Microgrids are attractive financially and present a range of possibilities.

Critical facilities, such as hospitals, emergency service providers, transportation hubs and military bases, require back-up sources of power in the event of electric grid outages, especially in areas prone to natural disasters like hurricanes, earthquakes and wildfires. Most often, emergency back-up power is provided by diesel-fueled generators, but with the costs of renewable power generation and storage continuing to fall there has been an increase in creating back-up power systems in the form of microgrids made up of renewable distributed energy resources (DER). 

DER offers multiple possibilities. In addition to providing critical emergency back-up power, renewable DER microgrids can provide multiple benefits including reducing carbon-based fuel powered electricity consumption, decreasing customer electric utility costs and potentially generating revenue for customers through the provision of retail and wholesale electric services, for example, demand response programs and ancillary services. To ensure that customers maximize the benefits of renewable microgrids during the majority of the time they are not providing emergency back-up power requires thoughtful, thorough planning and control schemes using innovative optimization algorithms. 

A microgrid is often defined as an electric system with generation, storage and the ability to power loads when grid power is not available. There are various scales of microgrid considered here: single or multiple loads within a building, an entire building, several buildings, a campus or military base. The focus here is on behind the meter or customer microgrids as opposed to utility microgrids that would be in front of the meter.

There are several values to consider, including: utility bill savings with time-of-use (TOU) rates and peak shaving and load shifting, ancillary services, and reduced facility down-time through emergency back-up power. TOU peak periods will change in the future to address changing net load curves, for example, the duck curve where net load is lower when PV generation peaks in mid-day and ramps sharply in early evening as PV generation decreases and residential loads increase as people return home from work. Microgrid control systems must be optimized for future rates or better yet optimized for a range of rates that may vary seasonally, and not only for a customer’s current TOU rate structure.

For a relatively small additional cost to a PV and battery storage system, islanding capability can be included to provide valuable resilience in the event of a grid outage and turn a PV and battery system into a “microgrid”. Seamless islanding with no interruption in power during a grid outage requires relatively costly additional hardware. If several minutes of power outage can be tolerated, islanding can be provided at a lower cost. Factoring in emergency power service makes a microgrid a more attractive financial investment. To further increase the value of a microgrid, incorporate control capabilities that minimize overall electric utility costs while reserving sufficient energy to sustain whatever length of outage may be needed. In addition, microgrids can be used to participate in wholesale markets to generate revenue or participate in community resilience and reliability efforts.

Renewable microgrids are great for lowering a customer’s electric utility costs, reducing the customer’s overall carbon footprint, and providing critical electric reliability and resiliency. To provide resiliency for longer periods may require sizing generation and storage systems that exceed what is needed for normal operation or even peak operation. Ensuring that this renewable power does not go to waste during grid-tied periods requires a supervisory control layer – one that can participate in electricity markets and provide grid services. 

Renewable generation and storage can provide days of power for a facility at any time of the year, but will be wasted if not used for grid support or community or regional power sharing.

A microgrid designed for long outages, such as weeks rather than days, will have generation and storage capabilities that exceed what can be used by the facility or community on most days.  Electric interconnection rules often limit the amount of power a microgrid can export or at least limit the amount that will be compensated.  Installing additional storage and participating in retail and wholesale grid services markets will minimize the loss of resources whose primary purpose is for resilience. Creating a smart microgrid that optimizes performance for multiple functions requires advanced controls. Lawrence Berkeley National Laboratory’s DER-CAM (DER-Customer Adoption Model) maximizes the benefits to cost ratio of microgrids through both planning and design and operational control.

DER-CAM is a flexible decision-support tool designed by LBNL that optimizes the type, size, and operation schedules of DER to meet one or more performance objectives, including reducing time-of-use (TOU) electricity costs, maximizing renewable resource utilization, ensuring adequate back-up capacity for reliability, and maximizing revenue from demand response or ancillary services. It has two modes: planning and operations.

Planning mode uses historical energy consumption and facility activity data for DER planning and scheduling. In operations mode, DER-CAM.OS uses real-time data and load and weather forecasting algorithms to create detailed operations scheduling of existing building or DER components on an hour-ahead, day-ahead or week-ahead basis. It uses a multi-layered distributed architecture following IEEE 1547.4 standards, in which control tasks are distributed across hierarchical layers.

The multi-layered control system allows distributing controls at multiple levels. For example: there’s device-level control with extremely fast response times; network level real-time control that keeps the network stable; higher levels of control primarily focusing on energy management and market/utility interactions. This type of architecture makes the system modular and scalable. Thus, additional DER and loads can be easily integrated without replacing the entire system.

To increase the uptake of microgrids for resilience requires smart modular controls that enable more cost-effective and affordable phased installation plans and that maximize DER values through optimized self-consumption, demand shifting based on TOU rates and provision of grid services.

About the Author

Douglas Black

Douglas Black, Ph.D.
Grid Integration Group
Energy Technologies Area
Lawrence Berkeley National Laboratory

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