As long as there has been a commercial distribution grid, non-utility-owned distributed generators (DGs) have presented operating challenges. Relatively small point generation sources connected at low or medium voltage make grid operators worry about things like synchronization, system protection and safety. But for the past 100 years or so, DGs have been few and far between and consequently their impact has been limited. That is of course, until the smart grid era, when distributed energy resources (DERs) like solar and small wind generators combined with energy storage systems began to proliferate. Meanwhile, micro-turbine technology has improved rapidly, making super-efficient combustion combined heat and power (CHP) units economically attractive. All that DG/DER growth is a fact of life in the modern, diverse grid, but distribution system operators are still wary of the operating impact of a large number of widely dispersed point sources. Enter the microgrid, a concept that is gaining momentum in the utility community, both on the grid and customer sides.

Figure 1 – The Modern, Diverse Grid
(click to enlarge)
 

Microgrids hold promise as a way to organize distributed generation into a beneficial supplemental power source for consumers and the grid, reducing the cost of peak power while enhancing grid reliability. Utilities need to achieve these benefits while simultaneously managing the distribution network efficiently, safely and reliably. That means that grid operators must overcome the hurdle of integrating microgrids within the traditional grid, and that requires advanced tools. One solution that has attracted the attention of leading utilities is the Advanced Distribution Management System (ADMS), a powerful combination of advanced analytics and real-time control. Implemented in the microgrid-enriched network, ADMS enables this advanced form of DG to deliver on its promise of enhanced reliability and reduced cost.

Microgrids: Think Globally, Grid Locally
A microgrid is a small-scale version of the centralized power grid. It generates and distributes electricity on a localized basis, usually at distribution (low or medium) voltage. Microgrids are designed to operate either in isolation from (as an island) or interconnected with the ‘big’ grid, often referred to as the area electric power system or AEPS to avoid confusion. They may be as small as a single residential or commercial load, or may be configured to provide power to a larger commercial property such as a shopping mall, college campus or military installation, or even an entire neighborhood. Most microgrids consist of one or more generators, matched to the load in an area that can be isolated from the AEPS. Figure 2 illustrates a fairly complex, but complete, microgrid.

This particular configuration is an entirely low voltage cell connected behind a single transformer to the AEPS through a medium voltage interconnection point. In the illustration several kinds of low voltage distributed generators are represented within the microgrid, although multiple units of a single type of generator would be more typical. Note that some renewable sources, such as photovoltaic solar and wind energy are depicted, along with an energy storage mechanism. As one of the key business drivers for implementing a microgrid is to improve reliability, microgrid configuration is a great means of coupling energy storage (like lithium-ion or flow batteries) with intermittent renewable sources.

A well-designed microgrid can enhance reliability, seamlessly interconnecting with and separating from the AEPS when it experiences an outage, allowing critical loads to be served continuously. Critical loads are connected on low voltage feeders with local generators, while other (non-critical) loads within the microgrid can be isolated and dropped as required during an outage. Typically, the sizing of the generators would, at a minimum, match the requirements of the critical loads within the microgrid.

Figure 2 – Microgrid Configuration
 

Beyond emergency disruptions, microgrids also afford consumers the opportunity to disconnect from AEPS when the cost of power (for example, during system peak) exceeds the cost of local generation. Because microgrids deliver power in close proximity to its generation point, they avoid much of the overhead cost associated with transmitting and distributing energy, including the losses inherent in long distance energy transport. A grid cell with that capability benefits its owner and the utility alike, by making it possible to reduce the cost of peak power for both. The utility also may enjoy the opportunity to shave peak power without having to resort to the unpopular practice of curtailing other loads to do so.

Other, less common reasons for implementing microgrids include serving a remote or otherwise logistically difficult area, typically one that is dependent on a single, vulnerable grid connection.

As microgrids gain momentum, it seems clear that the industry believes they have the potential to enhance reliability and reduce cost. Yet, on the grid side, operating issues still remain, and will have to be overcome for microgrids to become mainstream.

First, there is the problem of system protection. When distributed generators provide power beyond the needs of the local loads, that power will flow back into the grid. But unlike the transmission grid that connects large, centralized power plants, the distribution system was not designed for bi-directional power flow. When a fault occurs in the medium or low voltage part of the grid, all protective gear assumes that power is flowing in a single direction, from a known source to all end points. That assumption greatly simplifies and reduces the cost of the distribution system’s protective scheme, but the existence of distributed generation can nullify the principles of design, and cause the scheme to fail when the grid needs it most – under a fault. Even with reverse power relays in place to isolate the microgrid for a medium voltage fault, the coordination of the distribution system scheme can be violated.

System stability is another central concern. Although one core concept of microgrids is to match local generation to local loads, system operators may be justified in worrying about large amounts of both generation and load cycling into and out of the grid. Managing real and reactive power and frequency of interconnected generators can, at times, be challenging. Voltage set points and reactive power requirements are not static but vary with the changing conditions of the system. Intermittency due to the nature of renewable resources, or to potential problems on the consumer side of the switch could easily result in major swings in demand or capacity for a fairly limited area of the system. It doesn’t take much of an imagination to conceive of a scenario where the distribution grid might not be able to withstand a shift of significant size without impact to other customers in the immediate network vicinity.

Beyond the issues of system protection and stability, a lack of visibility into and control of significant microgrid presence could lead to inefficiencies in the system. There are times when it makes sense to monitor and control the grid in a holistic, centralized fashion. A bigger picture view is helpful to optimize voltage and VARs, for example, and to dynamically reconfigure the network for various operating constraints. In short, planning for and operating the grid in the presence of distributed generation, even when it is packaged in microgrid configuration, is not trivial. And that’s where a better network management tool like an Advanced Distribution Management System can help to ensure safe and reliable operation.

A Smarter Grid Calls for Smarter Distribution Management Systems
Microgrids can add to the reliability and efficiency of the grid, but they also add complexity. A tool like ADMS provides the platform for modeling, monitoring and managing the microgrid-enhanced power system. But before we look at how ADMS might help, a step back to describe the tool might be helpful. In the often-hyped environment of the smart grid, it isn’t unusual for a product or system to be called ‘advanced’. But in the world of operational control of the grid, ADMS has some very specific characteristics:

• Convergence of SCADA, DMS, and OMS functionality: In addition to real-time network analysis, the ADMS allows the user to operate all SCADA monitoring and control functions, as well as presenting complete OMS functionality for managing outages and dispatching crews. The right user experience is critical; the ADMS presents an integrated flow of information in a single, straightforward user experience, simplifying the operations and analysis of the distribution grid for the operator.
   
• Scale of data management and analysis: To manage the smarter grid, ADMS must account for hundreds of thousands, or even millions of real-time data points. And accounting for a variety of new devices and end point types, the ADMS must be able to adapt as new kinds of distribution and customer devices come on line.
   
• Scope of feature function: Starting with closed loop control, or the ability to analyze, execute commands, and then re-analyze the network to determine impact, the ADMS must be able to provide functions that drive critical business value for the smarter grid. Optimizing volts/VARs to increase efficiency and reduce peak load, enabling distributed generation and other forms of DERs, supporting demand response analysis and execution, and automating the distribution switching process to enable a faster response time, are a few of the many important groups of functions that support smart grid processes and value.

These capabilities make ADMS the key tool for getting a handle on the increasingly complex distribution grid, especially one containing one or more microgrids. The functions required are many, but the shortlist below provides a good overview of the features that ADMS brings to the microgrid integration problem:

- Load Forecasting – Detailed load profiling and weather integration
- Renewable Forecasting – Using weather integration to forecast microgrid renewable generation
- Network Planning – Model voltage changes, flicker, fault currents, and contribution from microgrids
- Network Reconfiguration – Optimize network to minimize losses
- Relay Protection – Adaptive control and recloser operation
- Fault Calculation – Model DG effect on fault current
- Volt/VAR Control – Manage load taps, capacitors, voltage regulators with closed loop control
- FLISR – Closed loop control (automated) switching
- Harmonic Indices – Report harmonic distortion in voltage and current levels
- DER Operation – managing DG and energy storage systems for balancing load
- Load Shedding – Sustain power system stability during disturbances
- Isolated Operation – Islanding support with load balancing and frequency, and voltage stability

A modern ADMS has the power to analyze and manage large, complex networks quickly, which enables system operators to evaluate scenarios both with and without microgrids to assess their impact. And using real-time state estimation and load flow calculations, the ADMS can provide the key operating tool to help manage the grid during steady state and emergency operations, while allowing microgrids to interoperate and contribute to grid reliability and efficiency.

ADMS and Microgrids Working Together for a Smarter Grid
Microgrids are likely to become standard in the modern, diverse grid and they can provide big benefits to both customer and utilities. But they bring with them a set of challenges that will have to be met. ADMS provides the means to model, monitor, and manage the microgrid-enhanced grid, allowing both the distribution system and interconnected microgrids to perform at their best.

About the Author

Jeff Meyers, P.E., smart grid strategy and development, Smart Infrastructure, came to Schneider Electric as the former president of Telvent Miner & Miner. Leveraging his experience in utility transmission, substation and distribution design, Meyers works with development teams and utility users of Schneider Electric technology, helping them to understand the Smart Grid and how the use of integrated technology can bring energy efficiencies to the industry.