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Scrupulous power management—namely, knowing when, where, and how much electricity is being consumed—is receiving attention worldwide.
To create an electricity-management system that provides a more stable power supply, energy saving, and convenience, Hitachi has developed a wireless communication technology intended for an "advanced metering infrastructure" (AMI). This technology enables stable data communication in a large-scale and ever-changing environment.
IGARASHIYes, smart grids have started to gain attention in countries that suffer numerous power blackouts. When widespread, long-term blackouts occur, it is desirable to keep supplying power in a stable manner through, for example, pinpointing the places affected by blackouts and shortening the time taken until power restoration. Under the backing of governments, smart grids have all of a sudden expanded in an attempt to satisfy that desire.
In Japan, although smart grids have received attention, they have been considered a future need. Since Japan's electricity system is very stable, there has been no sense of urgency in regard to promptly introducing smart grids.
Meanwhile, with the connection of dispersed power sources (such as solar-power generators) to the electricity system, it has become necessary to ensure stable voltages. In a manner analogous to the flow of water from upstream to downstream, electricity flows from generators to households. When solar-power generators (which feature large power fluctuation) are connected to the system in great numbers, voltage fluctuation due to backflow of electrical current occurs on a regular basis. So we want to control voltage under such conditions.
Following the occurrence of the Great East Japan Earthquake in 2011, planned power outages were imposed in areas of eastern Japan. Spurred by those events, challenges such as stabilizing the electricity system and supply-and-demand measures have come to the fore, and the amount of attention given to smart grids and smart meters has increased in a single burst. The circumstances under which smart grids should be introduced—under the aim of remotely controlling how much and where power is being consumed—have arisen in Japan too. In other words, we have become enlightened in regard to energy saving.
IGARASHIA smart meter, which is one component of a smart grid, is a metering device equipped with a communication function. Although there are many ways to provide that communication function, we have been developing wireless communication technology.
As for this wireless communication, the first challenge to be raised was the fact that radio waves are sometimes not received. Meter data recorded by a smart meter is collected on a piece of equipment called a "gateway" (located along a communication pathway) and sent en block to the relevant power company. However, in consideration of the cost of each gateway, it is not possible to deploy a great number of them. Accordingly, it is necessary to study ways of configuring communication paths in consideration of the case that radio waves do not arrive at the gateway after being sent from a smart meter.
HARADAWhen the scale of communication gets bigger, solving such problems becomes an even bigger challenge. Since the transmission speed of wireless communication is lower than that of fixed-line communication, transmission time gets longer. Besides, when various meters generate data randomly, their radiowaves interfere and don't reach the receiver side. Moreover, as a typical problem with wireless communication, the so-called "hidden node problem"—namely, communication becomes impossible because a receiver is positioned between two transmitters—can occur.
As a result of these problems, since the probability that radiowaves will collide is significant, the collection ratio of transmitted data is not 100%. To allow power companies to collect data with certainty, it is therefore necessary to devise a means of avoiding this collision of radiowaves.
IGARASHIAs for wireless communication, it is a problem that sometimes radiowaves cannot be received. As a solution to that problem, "multi-hop routing" is applied. Multi-hop routing is a way of transmitting information in the manner of a "bucket relay."
Many routing methods are available around the world. As well as standard technologies, numerous in-house methods are applied by different companies. We have chosen standard technologies as the basis of our routing methods. This is because our methods will be used as an infrastructure and because standard technologies are being adopted in the USA.
Although a multitude of standard technologies are available, we required one that is suitable in terms of its ability to be scaled (namely, they provide good scalability). So we adopted a protocol called "RPL" (short for IPv6 Routing Protocol for Low Power and Lossy Networks). Communication paths are configured in accordance with rules specified by RPL.
Figure 1: Overview of RPL
Figure 2: Network configuration
IGARASHIAs for wireless communication, definitive data-collection ratio changes in accordance with day-to-day variations of transmission quality. For example, since people and vehicles come and go constantly and elevators operate in a stop-and-go manner, noise during transmissions goes up and down and connection strength gets stronger and weaker. In this manner, wireless communication depends heavily on changes in the environment. With those circumstances in mind, we are attempting to add technologies for investigating continuously varying communication quality and selecting "substantially good areas" for communication.
IGARASHIIt means not simply choosing good places but "balancing" the switching of communication paths. When a communication path changes, at first, information indicating that "the path has switched" is transmitted. As a result, when the communication path switches excessively, communications about the path configuration are carried in great number, and those communications adversely affect the primary communications, namely, the meter data.
In other words, we have to deal with a trade-off between the two kinds of communications. If communication quality becomes poor, a better area is found, and the communication path is switched to that area. If the path is switched too many times, however, the communication band gets compressed, and information cannot be collected. But if the path is not completely switched, communication quality will keep getting worse. As the scale of the network increases, the balance between switching and not switching paths becomes more important.
Our technology features "loosely switching paths." While several bits of past information are used, "that path is no good" is not immediately decided; instead, paths are switched more loosely. If it becomes clear that a path is no good, it is switched at once. From our past experience in researching sensor networks and multi-hop routing, we have a lot of know-how concerning path switching.
HARADACompared to the transmission speed of wired communication, that of wireless communication is slower, so communication time is longer. And if data is generated randomly, collision and congestion will occur, and communication will become impossible. Given that issue, we came up with a solution—namely, controlling the timing of communications.
Figure 3: Method for allocating time slots
HARADAAs shown in Figure 3, the most compact solution is to schedule communications by setting time t as the shortest time between the gateway and a directly connectable meter, and setting a length of time for meters with number of hops, h, as h×t. However, even if a given meter could hitherto communicate directly with the gateway, buildings sometimes go up between the two, cars pass between them, and so on; consequently, direct communication becomes impossible. If the communication is routed through another meter, the communication time with the gateway is doubled. Since rescheduling communications at such times is difficult, we set up the scheduling so that time slots with a certain amount of leeway are allocated to each meter, and communication is ended within that time even if the communication path is switched.
This solution also involves a trade-off. That is to say, rather than pursuing performance; we have developed a communication-scheduling solution in consideration of the operational side.
HARADAThat's right. As shown in Figure 4, it is necessary to not only collect data but also send data (in the opposite direction) to meters and report firmware updates. If all the communications required are taken into account, it is clear that time will be lacking. Given that problem, we attach a level of priority to data in the manner of "required data" and "data that is good even with some loss." We worked hard to think up a method that could meet various communication requests within a finite period of time.
Figure 4: Required communications by automated-metering system
HARADAAppealing technology through desk-top calculations is not enough; we must try out developed technology in the real world. To construct an evaluation environment, we installed 200 wireless devices at the Yokohama Research Laboratory and experimentally evaluated the proposed wireless-network technology in that environment. The tests were performed under a condition in which data is collected from all the wireless devices once every 30 minutes. As a result, it took a whole day to collect data from all the meters. And initial failures of the arranged devices occurred, and the failed devices had to be retrieved and reset. So it took a lot of time and hard work until all data was collected.
We are planning to scale up the tests to 500 devices because we think that is the number of devices that the technology must be run on. In this work, it is theoretically possible to collect information from 2000 devices. However, actually using that many devices is difficult, so we are thinking about shorten the data-collection interval. In the simplest terms, if we imagine cutting the data-collection interval to one quarter of the present interval, and if transmission with 500 devices is possible, transmission would also be possible with 2000 devices.
IGARASHIThe locations of the wireless devices are determined in consideration of the environment in which the meters are actually set. For example, in the case of individual houses, distance between the meters is wide to some extent; however, in the case of apartment buildings, all meters may be concentrated in one place. These are the kinds of environments that we considered. If the devices aren't set at a certain spacing, communication won't be possible, and if they are set too far apart, communication won't be possible. Our know-how—namely, where to locate the wireless devices for multi-hopping—has grown gradually in this manner.
IGARASHIIn the case of buildings, meters are often located in the basement. Aggregation equipment (i.e. gateway) is basically fixed to utility poles; however, wireless (radio) waves do not pass through concrete very well, so transmission becomes difficult. Moreover, in the case that there are no utility poles, the equipment is installed in metal boxes above the road. However, if the devices are clad in metal, they cannot easily receive radiowaves. In circumstances such as these, communication won't work well no matter what.
As for future usage as an infrastructure, it looks like adopting 100% wireless technology will be difficult; accordingly, another technology will have to be used in conjunction with wireless. Even so, we would like to see wireless as the main technology and another technology supplementing it.
HARADAThe first thing I noticed was that operations management is vital. In the case of wireless communication, as we expected, it is difficult to collect 100% of the data. When considering operations management, we take into account how to recover the remaining data.
And we must be aware that the conditions under which radiowaves are received might be tougher than the experimentally predicted conditions. So I think we must brush-up the proposed technology by thinking about that possibility a bit more deeply.
Since I've always liked making things, I'd like to push our research forward with enjoyment. I am far from having all the right know-how, but by continuing to study, I want to acquire knowledge and skill so that I can soon create the things I imagine existing in the distant future.
IGARASHI"Demand response"—that is, varying electricity charges in accordance with the time of day—is becoming a hot topic. With the introduction of smart meters, it becomes possible to know how much electricity is being used at certain time slots. As a result, when rates change at peak times, users will be prompted to cut their energy consumption voluntarily, and a specified power load can be maintained. From both the supplier side and the consumer side, various measures will become possible.
Although this wireless technology was created with AMI (advanced metering infrastructure) in mind, that is not to say that it is solely for AMI. I want to apply it in other fields.
Given that I originally researched sensor networks, I think it would be advantageous if various kinds of information could be collected by our wireless technology. For example, since various sensors are utilized in the farming industry, "agricultural information" could be collected. In addition, since information such as building strength can be extracted by sensors, "earthquake-resistance information" would become available. I am glad that we can create an infrastructure that can acquire information that has been unavailable up until now. These challenges will motivate our research from now onwards.
(Publication:January 25, 2013)