New ‘ad-hoc’ microgrid design approach guarantees stability

Technology News | November 15, 2017

By Rich Pell

Microgrids – small-scale power systems that supply local energy to localized consumers – are increasingly being used as an alternative source of electricity in areas with unreliable or no access to a main power grid. Being more vulnerable to small disturbances, such systems are typically designed from scratch with a particular community’s power needs in mind, and in a way that limits the amount of power that any one appliance can draw from a network.

This typically means simple, centralized configurations with thick cables and large oversized capacitors that increase a microgrid’s reliability, but add to its cost. In contrast, the new microgrid design process developed by the MIT researchers ensures a network’s stability without relying on its particular configuration of transmission lines and power sources, offering a more reliable and lower-cost approach.

Instead, the researchers calculated the minimum capacitance on a particular load that is required to maintain a microgrid’s stability, given the total load, or power, that a community consumes. Using this approach, microgrid designers do not have to start from scratch in designing power systems for each new community.

As long as the load units include capacitors of the appropriate size, say the researchers, the system is guaranteed to be stable, no matter how the individual components are connected. Such a modular design could be easily reconfigured for changing needs, such as additional households joining a community’s existing microgrid.

“What we propose is this concept of ad hoc microgrids: microgrids that can be created without any preplanning and can operate without any oversight,” says Konstantin Turitsyn, associate professor of mechanical engineering at MIT. “You can take different components, interconnect them in any way that’s suitable for you, and it is guaranteed to work. In the end, it is a step toward lower-cost microgrids that can provide some guaranteed level of reliability and security.”

To come up with their solution, the researchers used a general mathematical theory – called Brayton-Moser potential theory — that characterizes the dynamics of the flow of energy within a system comprising various physical and interconnected components. In this case, they applied the theory to systems whose main goal is the transfer of power, which enabled them to look at the disturbances caused in a system when there was a variation in the loading, such as when a device was plugged in or an appliance turned off.

From the calculations, the researchers were able to develop a framework that relates a microgrid’s overall power requirements, the length of its transmission lines, and its power demands to the specific capacitor size required to keep the system stable.

“Ensuring that this simple network is stable guarantees that all other networks with the same line length or smaller are also stable,” says Turitsyn. “That was the key insight that allowed us to develop statements that don’t depend on the network configuration. This means you don’t have to oversize your capacitors by a factor of 10, because we give explicit conditions where it would remain stable, even in worst-case scenarios.”

Looking ahead, the researchers hope to use a similar approach to alternating current (AC) microgrids, which are mostly used in developed countries.

“In the future we want to extend this work to AC microgrids, so that we don’t have situations like after Hurricane Maria, where in Puerto Rico now the expectation is that it will be several more months before power is completely restored,” Turitsyn says. “In these situations, the ability to deploy solar-based microgrids without a lot of preplanning, and with flexibility in connections, would be an important step forward.”