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In Rukubji, like many other mini-grids, the power supply is sufficient during off-peak times. However, during preparation of morning and evening meals, the use of high-power kitchen appliances regularly causes electrical demand of the village to exceed the available supply and brownouts to occur. The lowered voltage that characterizes a brownout causes lights to dim, televisions to flicker, and electrical appliances, such as rice cookers, not to function properly; the corresponding drop in frequency may cause flickering in magnetic-ballast fluorescent lights and reduced speed in electric motors (refrigerators, power tools).
The GridShare system involves installing a device in every household and business connected to the mini-grid that provides two mechanisms to alleviate brownouts: indication of the current state of the grid and an enforcement mechanism to limit load when the grid is overburdened (figure 1). The implementation of this project in Rukubji has provided a more reliable electrical service to the village.
Figure 1. BPC electrician installs a GridShare and breaker box near the electrical meter (a). LED indicator lights with an instructional sign are installed near the rice cooker (b). Borrowing from familiar power-indicating graphics on cell phones, the yellow bars next to the green LED remind users that when the green light is lit, the grid is at ''full power'' and any appliances may be used, while the empty bars next to the red LED suggest that the red light means the grid electricity is limited and only low power appliances can be used.
Isolated mini-grids deliver electricity service to populations that are inaccessible by centralized electrical grids due to rough terrain and/or remote locations (ESMAP 2000, Martinot et al 2002, Terrado et al 2008, Modi et al 2005, REN21 2005, Palit and Chaurey 2011). Because mini-grids have finite generation resources, when consumers have unrestricted access, overloading and brownouts are common (Dorji 2007, Greacen 2004, Dorji et al 2012). This problem occurs worldwide; for example, in a survey of Thai mini-grid systems, respondents in 48 of 59 villages surveyed complained of low voltages (Greacen 2004).
Hydroelectric mini-grid systems are a common source of electricity in rural Bhutan, with 10 such systems in operation and the potential for more (Chhetri 2012, Dorji et al 2012, Uddin et al 2007). In some of these systems, rice cookers and electric water boilers account for 50%–70% of the peak load (Dorji 2007). Primarily due to these appliances, the electrical load of these villages exceeds the power generated for a few hours each day. During all other times, the hydroelectric generator supplies excess power, which is often rejected as heat via a dump load.
The GridShare system is designed to restrict electricity use only when the demand on the system exceeds the supply. This adaptive approach offers a distinct advantage over simple load limiters, which is particularly important for hydroelectric mini-grids, where the power produced throughout the day is relatively constant while demand varies greatly. In addition, unlike simple load limiters, the GridShare provides information to users about the status of the grid and whether they are being restricted. By combining these features with an education program focused on load-shifting, the GridShare enables more efficient use of the available generation resource.
The GridShare device is installed at the electrical service entrance of every residence and business connected to the mini-grid, while the LED indicator box is installed indoors near the high-power cooking appliances. Each GridShare device acts independently; there is no communication between devices or with the utility.
Figure 2. Functional block diagram of the GridShare device. (Reproduced from Quetchenbach (2011).)
When the supply voltage is above a programmable threshold (200 V), the GridShare is in normal mode and only the green light illuminates. When the voltage drops below the threshold, if no large appliance (above 400 W at the nominal 230 V) is in use, the GridShare enters brownout mode and only the red light illuminates. If a large appliance is in use when the voltage drops, the GridShare allows the customer to continue cooking with no power restriction for 1 h before entering brownout mode. During this ''timer mode'', both lights are on.
When the GridShare is in brownout mode, if the current exceeds the large-appliance threshold, the relay opens, disconnecting power from the house for 30 s. The relay then closes for 10 s to allow the GridShare to make another current measurement. If the current has dropped below the threshold, indicating that the large appliance has been turned off, the relay remains closed; otherwise, the power is disconnected for another 30 s and the process repeats. Since the peak load is mainly resistive, the current threshold is computed as the current at which a resistive load would dissipate 400 W at 230 V.
All voltage thresholds, power limits and timers on the GridShare are programmable for flexibility. The thresholds mentioned in this letter are those used in the Rukubji pilot project. Reprogramming the GridShare microcontroller requires a computer and a compatible microcontroller programmer. Though modifying the software requires advanced knowledge of computers and programming, microcontroller chips can be programmed off-site and delivered to a local technician to install in each GridShare, or local technicians or engineers can be trained to program the microcontrollers with compiled code provided by an off-site software developer.
Figure 3 illustrates the action of the GridShare in two hypothetical households, one in which a rice cooker is already in use when the grid enters a brownout (House 1) and a second in which the users turn on the rice cooker during a brownout (House 2). The GridShare in House 1 enters timer mode, illuminates both the red and green lights, and allows the rice cooker to finish cooking. In House 2, the users attempt to turn on the rice cooker during a brownout while the red light is on. All power is immediately cut to the house until the rice cooker is turned off. After the brownout ends, only the green light is lit and residents of both houses can use high-power appliances.
Figure 3. A hypothetical timeline of two homes in a brownout to clarify the indication and enforcement aspects of the GridShare. A rice cooker is a ''large appliance'' unless on warming mode, which for a typical 600 W rice cooker requires approximately 40 W.
The GridShare pilot project site is the village of Rukubji, along with the neighboring villages of Tsenpokto and Bumiloo, which are all connected to the Rukubji micro-hydro power plant. The villages lie near the eastern border of Wangdue Phodrang district of central Bhutan, a journey of about 125 km (80 miles) or 6 h by car from the national capital, Thimphu. In June of 2011, GridShare devices were installed to assess the effectiveness of the GridShare and residents'' satisfaction with the system.
Successful implementation of the pilot installation in Rukubji required partnerships with the DoE of Bhutan, the BPC and the village of Rukubji. The DoE helped identify the pilot village and provided advisory and logistic support throughout the pilot. The BPC owns and maintains the micro-hydro system and mini-grid in Rukubji.
BPC engineers and electricians provided technical support for the installation and continue to oversee monitoring and maintenance of the GridShares. Additionally, the BPC employs an operator who lives in the village and performs system maintenance and other operational tasks, including repairing power lines, installing meters for new customers, reading meters, distributing bills, and collecting payments.
The support of the Rukubji community for the pilot project was secured through verbal agreements with each household and a signed agreement with the village leaders.
In January of 2010, the implementation in Rukubji began with an initial visit to the village to introduce the GridShare concept and determine residents'' interest in the project. Following the villagers'' positive response, the team returned in July of 2010 and held community meetings, surveyed each household, and installed voltage and current loggers to monitor the electric system for the duration of the project. Additionally, an installation and monitoring agreement was signed with the village leaders during this visit. Each household verbally agreed to participate prior to the July 2010 survey and all subsequent surveys.
Following the GridShare installation, DoE and BPC representatives visited the village several times to monitor the system, ensure GridShares were operating properly, and inspect for signs of tampering. The team returned in January of 2012 to perform an initial assessment of the project through surveys and a final community meeting. The project continues to be managed by the BPC.
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