Please take a look at an article recently published in Utility Horizons International, written by Thierry Godart.
In this thought leadership article, Godart explores the concept of the ‘grid of grids,’ a smart grid powered by a collection of microgrids.
Full Article below:
Building the Grid of Grids with Utility Microgrids
If the electric grid did not exist, would we rebuild it today? Would we create utilities to produce and deliver electricity? If so, what would their business model be? In many parts of the world, where access to energy is scarce, these questions are valid and challenge the concept of the utility, as we know it. In other parts of the world, where electricity is readily available and sometimes taken for granted, we cannot conceive of a different utility model – but we must consider it. Our current model should be evolving toward a “grid of grids” – and the tool that will further this evolution is the microgrid.
Benefits of Utility-owned Microgrids
Traditionally, there are three types of microgrids. All of these types share the common characteristic of being capable of connecting to or disconnecting from the grid and are integrated via an intelligent data link into the overall control scheme of the utility distribution system.
The first type of microgrids is a single facility with one point of connection to the grid and one meter. The facility typically has local generation, which usually includes gas-powered combined heat and power (CHP), fuel cells, microturbines, and photovoltaic panels, as well as a flexible load that can be modulated locally through a facility energy management system. These facilities can range from an industrial factory or plant, commercial building, high-rise multi-tenant apartment building, or even large houses. In the near future, residences can become their own microgrid. With the cost of technology decreasing, end users can combine rooftop PV, gas-powered microturbines, smart appliances, electric vehicles and battery storage systems to generate their own power.
The second type of microgrids is a privately owned electrical system, which often serves an enclosed community like a university campus, industrial complex, military base, or even a residential area such as a subdivision or planned community. While this type of microgrid includes its own miniature distribution grid and can rely on a sub-metering system to bill its tenants, it still appears as a single flexible load to the utility distribution system operator.
The final type of microgrids is the utility microgrid, which is a section of the utility distribution grid that has purposely been reconfigured as a microgrid. To create the utility microgrid, a group of feeders and distribution substations are meshed into a standalone grid, containing enough generation to sustain the local load. For example, eco-districts within cities, community solar projects enhanced with storage and demand response for critical areas of urban and suburban neighborhoods, remote communities including island communities, can each be formed into autonomous grids, thus transforming the existing distribution grid into a set of connected microgrids.
Creating a utility microgrid is also the most efficient way to incorporate other microgrids in the distribution grid because the necessary (utility) supporting services such as frequency control, voltage stability, power quality, storm management, etc. can be mutualized across the microgrids. Typically, there is one main substation serving this grid of grids. At that substation, the utility operator and the utility distribution management system see the utility microgrid as a fully capable power plant (e.g., four quadrant PQ generator) that operates according to a schedule of load/generation exchange. In normal mode, the utility microgrid is connected to the substation, but looks like a constant load to the rest of the grid. Ideally, this load is a net zero load viewed from the grid. That is, all load and generation variability due to supply and demand unbalance is managed within the grid of grids itself.
The utility microgrid is generally the result of a business partnership between the utility and the microgrid owners. The utility signs a service level agreement with the owners of the connected microgrids, which specifies the cost of the services required to operate the Utility Microgrid. This new “rate” can be considered an evolution of interruptible load rates or demand response aggregation contract.
More stakeholders readily benefit from privately owned microgrids because they can be efficiently controlled by the utility. Not only does the individual microgrid serve its owner, but its extra capacity can also help mitigate energy and voltage unbalance in its immediate neighborhood. Also, if the Utility Microgrid is connected to other renewable-powered microgrids, it will contribute decarbonization of the current grid by substituting the load with locally produced clean power and without the utility rate payers) having to subsidize those renewable sources. Plus, during major outages, the restoration process will use a black start process that depends only on the Utility Microgrid’s generated power rather than having to wait for the complete transmission and distribution grid to be placed back in service.
Designing the Grid of Grids
In today’s distribution grid, there are often sub-networks of highly redundant meshed grids, especially in dense urban areas. For example, the secondary network of Manhattan (operated by Con Edison) could be considered a Utility Microgrid if it had significant distributed generation connected to it. The same principle of highly redundant meshed networks applies for the design of Utility Microgrids capable of supplying loads from multiple sources rather than a unidirectional radial network.
Thus, designing utility microgrids starts with planning the reconfiguration of feeders connected to substations within the same vicinity where there is a significant potential for high penetration of distributed resources. The feeders form a meshed network connected to fewer substations. This newly formed topology of the Utility Microgrid creates a grid that is capable of operating in normal mode without power from the substation and that relies solely on its local generation to supply its local load. Planning energy storage within the Utility Microgrid is key to mitigating variability of generation, ensuring stability of frequency and voltage, and supplying energy reserve.
Under the Utility Microgrid model we are dispatching and modulating flexible loads much more dynamically than was previously the case using the traditional distribution grid. Fast communications between the connected microgrids will be necessary for real-time forecasting, monitoring, and the efficient operation of control systems. Moreover, the protection schemes and devices will need to be upgraded to take into account the electrical characteristics of the newly formed grid. For microgrids relying heavily on renewables, forecasting the weather in a very granular way (e.g., clouds, wind speed, etc.) will be a key function of the Utility Microgrid operator. They will incorporate weather forecasting into the real-time scheduling of distributed resources to continuously maintain an accurate picture of the state of the system at any given time.
While this seems complex, Utility Microgrids are more an evolution of the power system than a complete disruption. The key to success will be to standardize the means by which protection and automation systems interoperate with the local microgrids. By doing so, this will ensure that the evolution of the Utility Microgrid – including the addition and replacement of distributed resources – is cost effective. Repetitive designs with open and common operating protocols across the utility will ensure shorter learning curves and faster integration of Utility Microgrids in the broader grid infrastructure. The network operating center is designed to present the information consistently across all Utility Microgrids in three distinct functions: 1) the daily forecast and scheduling of supply/demand; 2) the real-time supervision and control; and 3) the coordination with the utility distribution operation.
Operating the Grid of Grids
To truly benefit both the end user and the utility, Utility Microgrids require an adaptation of both operating procedures and systems currently in use by the distribution system operator. This concept pushes the current tiered operation of the grid one step further and closer to the customer, creating a new organized tier, which will avoid inefficient operation of the grid.
Today, the bulk system operator dispatches large power plants into the transmission system. The bulk system operator does not directly dispatch energy resources and does not control switches in the distribution system. Respectively, the distribution system operator dispatches the power from the transmission substation to the load modeled as an aggregated load at the distribution substation. The distribution operator does not dispatch energy resources behind the meter. Now, however, facing the high volume of variable and unpredictable distributed energy resources, the operator needs a new way to supervise them to maintain control of the grid.
The grid of grids improves the predictability and controllability of the distribution grid. By grouping individual microgrids into a single Utility Microgrid, we isolate the complexity of multiple control points and a highly variable net load. Designing the distribution grid as a grid of grids will make it scalable and reliable even with a very large amount of connected distributed resources.
Utility Microgrids are the most efficient way to manage our twenty-first century Smart Grid – they respect the physical constraints of electric systems by modeling the microgrid as a power plant connected to a substation and delegating the control and supervision of localized distributed resources.
The Utility Microgrid is both supervised and operated by the utility. Supervision is the concept of dispatching set points and monitoring the Utility Microgrid as one single power plant. There are three modes of supervision for a Utility Microgrid:
1. Normal Mode: The load and generation are dispatched according to the daily forecast where local generation, storage and demand are balanced with no power exchange at the Utility Microgrid substation.
2. Contingency Mode: When the day-ahead schedule cannot be met, hourly contingency modes are planned using reserves from within the Utility Microgrid. This can be in the form of storage dispatch or emergency demand response. A Utility Microgrid will identify critical and non-critical load to operate in contingency mode where non-critical loads can be disconnected.
3. Critical Mode: this is when the utility microgrid requires support from the grid to meet its served load. Failure of inverters, microturbines, or major network equipment failure will happen and will require emergency reserve supply from the grid through the substation.
Beyond these three supervision modes, the Utility Microgrid will also contribute to the overall resiliency of the distribution grid. In case of major outages – storms, transmission equipment failures, power plant failure, etc. – the utility may call on the microgrid to contribute to other loads as emergency reserve power. Following a blackout, for instance, the microgrid can help restore power as part of a black start restoration scheme. This also drives a generalization of our current bulk power system, where utilities call on independent power generators to supply reserves and to participate in resolving emergency situations. Now, the same concept can be applied to the distribution system with Utility Microgrids acting as dispatchable power plants.
Operating the Utility Microgrid will be a new function for the distribution utility. Fundamentally, it is about maintaining voltage and frequency within a set of feeders connected to a substation. Thus, it is not that different from operating the distribution grid, so utility grid operators are best qualified for the job. However, there are significant differences as well.
In a Utility Microgrid, the load follows the available generation and energy storage plays a significant role. Also, supply and demand are very dynamic as one node can behave as a load and suddenly change into a generator. The mindset of traditional utility operators who are used to calling on more generation to meet a predefined load forecast and connectivity model will need to change drastically. Better automation systems and real-time visualization will be required to manage these changes. Also, dealing with the Utility Microgrid customers who are becoming “prosumers” – those consumers who are proactively managing their own energy consumption and production – will require new skills for a utility operator as well.
Still, operating a Utility Microgrid will be similar to operating a distribution grid and should be a rewarding experience for utility operators. With the concept being generalized across the utility territory, it is envisioned that utilities will invest in network operating centers manned by newly trained utility operators dedicated to Utility Microgrids. While separate from the distribution control center, the network operating center will be tightly integrated, such that seamless dispatch and control can be performed between the centers.
The Ultimate Demand Response Tool
The most flexible load – from a distribution grid operator’s perspective – is the load that can be dispatched in real time to follow generation constraints. Flexible load is what will contribute to the decarbonization of electric power and, that will allow us to avoid building more fossil generation plants. It will also lower the coincidental peak demand and mitigate the variability of renewable power.
Beyond energy efficiency measures and manual demand response, automated demand response (ADR) is the most efficient way to achieve deferral of generation build up. It is also the most effective way to mitigate the variability of renewable power. However, ADR is often constrained by the industrial process for industrial loads, general business activity for commercial loads, or personal comfort for residential loads, so it is not necessarily a perfect solution for all circumstances.
Customer-generated energy will become the ultimate tool for automated demand response. We now have systems capable of generating or storing the required energy and preventing automated demand response events from disturbing the activity of the electricity user. Thanks to these advances, we have achieved the ultimate stage of demand flexibility. The microgrid adds another level of flexibility – plus it satisfies the end user’s need for local power consumption.
The ultimate evolution of demand response is flexible distributed power generation. The fastest and most efficient way to reach a high penetration of renewables with safe and reliable delivery of electricity is to implement the concept of grid of grids. The Utility Microgrid encapsulates the benefits of multiple distributed energy resources by physically regrouping them into a newly form meshed network connected to a distribution substation. The utility acting as the reliability coordinator supervises Utility Microgrids as standalone power plants, thus indirectly dispatching the underlying distributed energy resources.
With the right regulatory framework, this hierarchical organization will scale to a sustainable grid of grids, benefitting all of society and creating a new growth opportunity for the smart utility as a Utility Microgrid operator. Several initiatives have already started in the US and abroad regarding the adaption of utilities to the world of distributed energy. It is time to accelerate the concept of interoperable distributed energy resources so that these resources can be further integrated within Utility Microgrid to create an efficient grid of grids.