Cross-layer Design and Control for Smart Grid
- Written by Yukio Hiranaka
Power systems can be seen as having vertical technical layers and horizontal management layers; for overall optimization, each set of layers needs a cross-layer control mechanism. In designing such controls for a variety of functions, care must be taken to respect the integrity and independence of the vertical and horizontal stacks.
If you plug an electric kettle that consumes more than 1 kW into a crowded circuit in your home, you may cause the breaker to open, shutting down all the electric appliances connected to the circuit. As a counter measure, we designed a smart power outlet (SPO) that connects the kettle to a circuit selected from multiple breaker circuits, which can handle the current. Of course that requires numerous additional wirings to the SPO. The point is to isolate usage (which circuit to connect the kettle to) from situations on the supply side (which circuit has what current margin).
To put it abstractly, we are trying to protect the integrity and independence of the various vertical technical layers: in this case, an upper layer consisting of appliances, and the lower layer constituting the power supply.
However, if we have no margin on any of the circuits connected to SPO, or we are exceeding the margin on the current the SPO has selected for use, the SPO has to reject the newly activated appliance or keep it waiting. We think of this kind of operation as modifying the usage layer on the basis of supply layer conditions and, thus, an example of cross-layer control—a kind of control that crosses the layer into another layer's internals.
In addition to the vertical technical layers stated above, power management has to deal with the horizontally arranged layers comprising management entities such as power suppliers, distributors and end customers. Demand response is an example of cross-layer control from one horizontal layer to another.
There are critical issues in cross-layer controls. Creating such controls disrupts efforts to divide the system into easily manageable sub-systems, and may make the system too complicated and uncontrollable. The issue of stability also arises because control loops are enlarged. Any cross-layer control aims to optimize the relevant part of the system to its desired status. And multiple separate controls—for example, one suppressing the total power usage with direct load control by the distributor, another maximizing revenue with dynamic pricing by the suppliers—may trigger control contention.
Thus, only a single cross-layer control can be safely operated in any one system.
Just as important, there should be a stable solution for any cross-layer control. For example, if we want to cut the whole power usage by some percentage, we should set the upper limit for each customer so that the sum of the limits is within the available amount. Otherwise, unwanted phenomena such as oscillation or chain reaction may appear when the customer responds in the opposite direction to the cross-layer control—say, the customer starts a secondary air-conditioner when the central air-conditioner was remotely stopped by a distributor's controller.
As any cross-layer control needs communication between the relating control layers, it needs a protocol commonly understandable by them. Formulation of such a protocol requires compromise among layers, and will impose restrictions on the layers continuously into the future, making it harder to design new cross-layer controls. Still, by addressing these difficulties, we can open a new world of intelligent power usage. For instance, we can find new ways to trigger safe-stop procedures before a power outage event, develop new methods of power usage reservation and negotiation, and implement systems in which volunteer appliances are recruited to reduce their current usage by a certain amount.
To make a solution to the aforementioned problem of cross-layer protocol, we are trying to figure out the basic requirements for it by designing practical communication between our SPOs. We hope that the results will promote intelligence in power usage including on the part of end user's appliances. Direct-load control schemes, in which appliances take commands from the distribution supply layer, represent a kind of cross-layer control. So do dynamic pricing systems, where distributed components are constrained to achieve a certain optimization.