Figure 1. Microgrid Architecture
Microgrids provide a powerful solution for enabling the conversion of the electric grid to a more distributed architecture and providing greater flexibility of electricity service to customers. Microgrids are small, independent electric-power grids, including both sources and loads. Microgrids have the capability to connect and disconnect effortlessly from the traditional grid. A microgrid comprises distributed generation, energy storage, loads, and a control system that is capable of operating in grid-tied mode and islanded mode. The concept of microgrid including it's architecture, components, types, mode of operation, various projects, benefits, and the challenges of microgrids have been summarized in this paper.
Microgrids are increasing universally, driven by technological, regulatory, economic, and environmental factors. Siemens have help build and get the best from these modern energy systems. To a greater or lesser amount, every business needs access to consistent and economical sources of power. It is an additional bonus for some if that electricity can be generated using renewable sources. Modern technology allows business to meet these needs themselves, producing energy as well as consuming it locally, creating flexible networks known as “microgrids”. At the beginning of the electrical age, every grid was a microgrid – a locally limited system in which power was generated and distributed to users. Gradually these were included into larger networks, becoming national or even crossing frontiers. Economy of scale dictated ever larger, usually fossil-fuelled generation plants, supplying often distant cities and industrial centers via transmission at high voltage. But now the trend has reversed toward generation that is again dispersed, potential renewably sourced, and often within flexible modern microgrids, able to attach or disconnect from the wider system at will. Microgrid operators can take advantage of management systems able to tell them exactly when it makes sense to generate, when to draw power, and when to sell power to the local utility company. Business of various sizes, whether they use less than 1 or more than 100 MW are changing from being passive consumers to becoming local voltages, provide increased efficiency through the use of waste heat, voltage sag correction, or provide uninterruptible power supply functions to name a few. Generally, microgrids deals with Low Voltage (LV) distribution systems with Distributed Energy Resources (DERs), such as low hydro power turbines, bio mass, photovoltaic (PV) arrays, etc., along with storage devices (i.e., batteries, flywheels, and energy capacitors) and controllable loads, providing significant control capabilities over the network process. These low voltage systems are usually tied to the Medium Voltage (MV) distribution network, but can also operate islanding mode in case of faults in the upstream network. From the customer point of view, microgrids offer both thermal and electricity needs. Microgrids can also increase local reliability, reduce emissions, get better power quality by enhancing voltage and reducing the voltage dips, and as a result there is a possibility to lower the cost of energy. From the grid's point of view, a microgrid can be viewed as a controllable entity within the power system that can be operated as a single combined load or generator and given attractive payment, as a small source of power or subsidiary [1] .
The main objective of this paper is to describe the microgrid concept including it's architecture, types, modes of operation, benefits, challenges, and various projects implemented in India.
Figure 1 shows a microgrid schematic diagram distribution substation, and it includes a variety of DER units and different types of end users of electricity. DER units include both Distributed Generation (DG) and Distributed Storage (DS) units with different capacities and characteristics. The electrical connection point of the microgrid to the utility system at the low-voltage bus of the substation transformer creates the microgrid Point of Common Coupling (PCC). The microgrid serves a variety of customers, for example, residential buildings, commercial entities, and industrial parks. The microgrid normally operates in a grid-connect mode through the substation transformer. However, it is also estimated to provide sufficient generation capacity, controls, and operational strategies to supply at least a portion of the load after being disconnected from the distribution system at the PCC and remain operational as an autonomous (islanded) entity. The existing power utility practice often does not permit accidental islanding and automatic resynchronization of a microgrid, primarily due to the human and equipment safety concerns. However, the high amount of penetration of DER units potentially necessitates provisions for both islanded and gridconnected modes of operations and smooth changebetween the two to enable the best consumption of the microgrid resources [2].
Figure 1. Microgrid Architecture
Microgrid components are classified as follows [3] .
The primary essential component required in any microgrid system is a power source. The energy source is often controlled for the demands on the microgrid, such as generating desired capacity, as well as other considerations. Solar power has been gradually becoming more popular energy source for remote microgrid providing energy entrance to rural areas. Continuous reduction in the cost of photovoltaic solar power modules have resulted in solar based microgrids. The ease of accessibility made these solar microgrids more favorable. Maintenance of the power modules, due to the moisture content in the air is the major problem of the solar microgrids. The other problem is storage requirement which is imposed by the intermittent source. This is particularly important in economic considerations, as storage systems usually make up the bulk of microgrid costs and as solar capacity increases, storage requirements also increase. Generally, small hydro power stations and wind power stations act as power sources to provide energy to the remote rural areas. Natural gas power plant is also be used as an alternative power source for microgrids. Barcaldine power station with a capacity of 55 MW in Australia is using natural gas as source. Developing countries are using natural gas power plants in the microgrids and these plants are energy efficient too.
The power management system handles the transfer of electrical power from the power source to the consumer. This kind of electric load management usually requires converting the power generated from the source with an inverter that transforms the electricity to the form required for most devices, such as uninterrupted power supplies or cooking equipment and interfacing with the storage components of the microgrid to balance the supply and demand loads of the microgrid. Recent microgrid systems regularly incorporate software and control system strategies, such as smart meters that can handle theoperation of the grid in a competent and trustworthy manner.
Storage systems are almost mandatory for any microgrid since they allow the microgrid to balance the electrical energy and hence make electricity available when it is needed by the consumer. Few of the common energy storage technologies for various capacities are discussed here. In the case of remote microgrids, batteries are the most commonly used storage technology because the storage capacity required for the microgrid does not justify higher cost of other storage technologies. Implementation of large scale storage technologies, such as hydro based storage or thermal storage is quite challenging in remote microgrids.
Microgrids are classified as follows [4].
When the source of generation and multiple loads are located in small radial distance like in educational institutions or university campuses, then campus microgrids are more efficient and beneficial. Campus microgrids are getting more into the limelight of the microgrid market due to the fact that a single owner or management is managing both generation and multiple loads and thus avoiding the regulatory problems found in other microgrid markets. The advantage of campus microgrids is that when cost of energy supplied by the utility is less than cost of generation of campus microgrid, it draws energy from the utility grid. On the other hand, campus microgrid exports power to utility grid when cost of generation is less in campus microgrids compared to the utility grid.
Utility microgrid is a sector of controlled grid. It is also called as community microgrid or milligrid. While technically, they are not different from microgrids, and are fundamentally different from a supervisory and business model perception, primarily because they include traditional utility infrastructure. The consequence of this feature is that utility regulation comes much more significantly into play. In other words, any milligrid must comply with existing utilitycodes or accommodation must be made in the code.
Virtual microgrids wrap distributed energy resources at different places but are harmonized, such that they can be accessible to the grid as a single controlled entity. Virtual microgrids can provide energy to the terminating consumers while achieving synchronized balance between supply and demand. Utilization of renewable sources can be maximized using virtual microgrids and at the same time, one can minimize negative effects of the output fluctuations caused by the commercial grids.
Remote power systems are clearly not able to operate gridconnected, isolated power systems that involve similar technology and are closely related. So close to that research point of view, they are commonly described as microgrids.
Microgrids can operate in the interconnected mode or islanding mode. At most care must be taken such that the phase, voltage, frequency, and phase angles of the renewable energy sources, batteries and system loads are synchronized to the grid, when connected to the grid. The following are the different modes of operation of the microgrid [5].
The microgrid is coupled to the distribution system and is supplying energy to the grid using renewable solar or wind power. Stored electricity from the Sodium Nickel Chloride, Lithium Ion Battery and Lead Acid Battery Systems can also be used to supply energy to the grid. During the supply to grid operating mode, the natural gas generator will not be operated [4].
The microgrid is connected to the distribution system and is absorbing energy from the grid to power its load. Electricity can also be stored in the Sodium Nickel Chloride, Lithium Ion Battery and Lead Acid Battery Systems for upcoming consumption. During the supply from grid operating mode, the solar photovoltaic system and wind turbine system may also be powering the load and charging the batteries, butthe natural gas generator will not be operated.
The microgrid is designed to operate in isolation from the distribution grid with the Islanding (Generator) operating mode. During this mode, the natural gas generator will be the primary source of electricity with the renewable solar and wind generators providing additional power. Electricity stored in the Lead Acid, Sodium Nickel Chloride and Lithium Ion Battery Systems can also be used at this time.
The microgrid is operated in isolation from the distribution grid with the Islanding (No Generator) operating mode. During this mode, non-conventional solar and wind generators will be a chief source of power. Electricity stored in the Lead Acid, Sodium Nickel Chloride, and Lithium Ion Battery Systems can also be used at this time. Since all generation sources are irregular with this operating mode, low priority microgrid loads may be disconnected depending on the amount of generation available.
The microgrid is designed to have black start capability that involves using backup systems to help launch the microgrid's generation sources. During this mode, the microgrid is not connected to the distribution system and does not have electricity serving its load. The microgrid will use the backup systems to initiate the renewable generation sources and connect the battery systems to help restore power to system loads.
The microgrid is designed to operate in the event of an outage on the distribution system and provide seamless service to its loads. In this scenario, the microgrid will automatically disconnect from the grid and start drawing electricity from the natural gas generator, renewable energy sources and battery systems.
The microgrid is designed to operate in the event of an outage on the distribution system and provide continuous service to its loads. In this scenario, the microgrid will automatically disconnect from the grid and start drawing electricity from renewable energy sources and battery systems. Since only intermittent generation sources are available, low priority microgrid loads may be disconnected depending on the amount of generation available.
The microgrid is designed to operate in the event of an outage of the distribution system and provide continuous service to its loads. Utilities from time-to-time have planned outages to allow for maintenance and servicing. In this scenario, the microgrid will automatically disconnect from the grid and start drawing electricity from the natural gas generator, renewable energy sources, and battery systems. Since this operating mode involves a planned outage, the battery systems can be fully charged ahead of time to maximize the amount of power for loads during the outage.
The microgrid is designed to operate in the event of an outage on the distribution system and provide continuous service to its loads. Utilities from time-to-time have planned outages to allow for maintenance and servicing. In this scenario, the microgrid will automatically disconnect from the grid and start drawing electricity from renewable energy sources and battery systems. This operating mode involves a planned outage and hence the battery systems can be fully charged ahead of time to maximize the amount of power for loads during the outage. However, since only intermittent generation sources are available, low priority microgrid loads may be disconnected depending on the amount of generation available.
Microgrid components such as renewable or fossil-fueled generators, Point-of-Common-Coupling breaker and its control, loads, energy storage systems, and others must meet following requirements [6].
Several techniques are used to enhance the efficiency of the system. Unnecessary conversions to and from AC are eliminated because most alternative energy sources (solar, wind, fuel cell, bio or fossil fuels driving a Permanent Magnet Alternator (PMA), etc.) fundamentally produce DC. Fuel-based spinning generators are no longer synchronized to the AC frequency, saving significant fuel in the predominant less-than-full-load conditions. The burned fuel is used to create energy used by the loads, not to maintain the spinning RPM, providing over a 50% power savings when driving very light loads. An additional 20% fuel savings is achieved using PMAs by not having to energize the stator. With the addition of battery storage, generators can be sized to supply the average load instead of the peak load, e.g. to support motor start currents. This results in considerable cost and weight savings, and allows the generators to run closer to maximum efficiency.
Distributed generation by itself is more reliable since other sources can pick up the slack if any one source goes offline, but AC-only minigrids are prone to sags and surges when this happens. This self-regulating architecture continuously and transparently adjust to changes, for example when a source suddenly goes offline. For situations requiring high-reliability, parallel distribution lines can be used, either AC or DC, in case one is cut or short develops. Fly Back Energy's microgrid products use highly derated components and do not use high frequency switching but rather are designed to operate at 60 Hertz.
This architecture allows for both flexibility in power (kilowatts to megawatts) and geographical area while optimizing the use of available renewable energy sources. Based on modular building blocks, scalability is easily achieved, overcoming the size limitations of a DC-only microgrid. The system is capable of operating standalone or connected to the grid. Not only are new builds supported, but the technology works in retrofits using existing infrastructure (including home wiring and loads). Many legacy AC loads can run directly off of DC, and many more loads are being introduced that are improved for DC operation for even more efficiency. For those loads that cannot be operated on DC, Fly Back Energy offers both load-specific controllers and regulated inverters that generate utility quality AC.
Microgrids can simply grow through the additionalinstallation of generators, storage, and loads. Such an extension usually requires an incremental new planning of the microgrid and can be performed in a parallel and modular manner in order to scale up to higher power production and consumption levels.
According to market research studies, economics of heat recovery and its application by CHP systems is very important to the evaluation of microgrids. In addition, the utilization of renewable energy resources will help to reduce fuel costs and CO emissions.
Microgrids can support a true for operation, control, and energy trade. In addition, interactive energy transactions with the centralized utility grid are also possible with this model. The proposed concept does not dictate the size, scale, and number of peers and the growth rate of the microgrid.
As the central grid is still far from reaching all remote villages, a number of pilot projects have been successfully implemented using microgrids in India.
Table 1 shows various microgrid projects implemented in India. This section will make the case that microgrids are not just a theoretical solution, but have actually translated to successful small-scale projects. This section will present three case studies of microgrids that have already been deployed. Through these case studies, this paper illustrates the diversity of contexts in which microgrids can be used and the various implementation and deployment strategies that developers are using [7].
Table 1. Case Studies Summary
One example of electrification using microgrids is Gram Oorja's installation of a solar powered microgrid in Darewadi, which is north of Pune in Maharashtra. Despite being about 1 km from a nearby village that is connected to the grid, Darewadi has not been connected to the grid due to the hilly and uneven terrain separating it from the grid-connected village. Gram Oorja's project has installed solar capacity of about 10 kW for 40 households. The project was initially setup through a Corporate SocialResponsibility (CSR) partnership with Bosch Solar Energy. For this project, about 70% of the total energy has been earmarked for productive activities (i.e., a pump and a flour mill). This project has been implemented since October 2012 and it is Gram Oorja's first remote electrification project. Bosch covered the upfront costs for the microgrid, but the villagers are charged a rate of about 20/kWh that ensures they can pay for their own battery placement costs in 5 years. The villagers end up spending anywhere from 120 to 150 per month on electricity. Though electricity is available for 24 hours a day, the villagers still have to manage their power use at night due to limitations on the battery storage. Gram Oorja helped the villagers set up a local committee, which is currently composed of three women and four men, and they are responsible for making any major decisions concerning the microgrid.
Another small enterprise that operates microgrids is MeraGao Power. Currently, MGP serves a number of villages in the state of Uttar Pradesh. MGP provides DC electricity produced by solar photovoltaic panels. The specifics of electricity provision, like timing and duration of daily provision are decided primarily by MGP on the basis of a fairly standardized model with limited involvement of villagers. Electricity from their microgrids is available for seven hours in the evening, and the electricity provided issufficient for two light bulbs and a cell phone charger. This unit offering was fixed by MGP based on inputs gathered through community engagement during pilot projects. MGP provides LED light bulbs compatible with the DC connection and cell phone chargers to their customers. Installation of MGP's microgrid system is simple, and it takes three to four technicians only a day to complete the installation in a village or hamlet. MGP estimates that it should be able to recover the capital cost of each project from villager payment in less than three years. MGP does not rely on any government funding and use private funding for all projects thus far. It has received a $300,000 grant from Development Innovation Ventures, U.S. Agency for International Development (USAID) and has raised $1 million in equity to expand operations in the coming years [7].
The Sagar Islands in the Sundarbans region is an example of collection of many hybrid DG microgrid projects, with the unique characteristic that the electricity provision is linked with water provision to the community. Until 1996, most of the island was powered for a few hours each evening using diesel generation with 300 kW of total capacity. Because these diesel units both required high levels of maintenance and resulted in large amounts of pollution, MNRE in 1996 identified the area for new solar projects and set up a 26 kW solar PV microgrid. This project has now expanded toinclude 300 kW of generation capacity through solar PV, along with 400 kW from diesel generation, and 500 kW from wind-diesel hybrid power to meet expanded energy needs. The system provides about six hours of electricity every evening for residential consumers for a total of 30 kWh per month. The community is involved primarily through the local cooperative members taking responsibility for the collection of electricity tariffs.
Microgrids today provide new capabilities that offer great promise, and have sparked significant interest. Here are three major benefits offered by modern microgrids [8] .
Microgrids can offer the ability for localized groupings of loads to be served by either local generation resources or through the central grid. Modern microgrids go a step further because they can offer dynamic islanding capability, where even smaller increments of these local groupings of loads can be connected to local electricity supplies. Such capability makes it possible to maximize available energy supply and ensure the most critical loads are served during extended outages [8].
With more local control, the served microgrid loads can decide how and when to be served by the central grid versus local electricity generation. Factors that can influence this decision include the price of electricity at particular times as well as availability of renewable energy resources. Depending on how ownership of microgrid assets is structured, local customers also have more choice about what technologies are included as part of the microgrid, like self-healing solutions that can offer a higher level of power reliability and can help attract new business to an area [8].
Another way in which customers served by a microgrid can exert greater control over their energy use, by opting to use local renewable energy sources so they have more control over the types of electric generation used to meet demand. Renewable energy resources can also offer a more secure energy supply in the event of a natural disaster that might disrupt fuel supplies for other types of generation, such as smaller diesel generators or even centralized utility generating plants.
Microgrid must be able to operate safely either connected to the main grid or in an 'islanded' fashion and in the latter case it needs to actively manage generation and consumption [9].
Microgrid must coexist with the main grid, the microgrid customers need to share some of the costs associated with the main grid's operation. Also there needs to be ownership and accountability when it comes to providing safe operation and security of energy supply, responsibilities that are currently borne by the utility in the existing system [9].
The areas where the utility assets have been fully depreciated, the cost of energy from the distributed resources will be higher. None of those is intractable, but it will take time to overcome them, and, with the utilities feeling that their business model is threatened, there will be additional pushback for microgrid adoption in countries with developed electrical grids. The proposition is much more clearly beneficial in developing countries with nonexistent or very weak central grid [10].
Microgrid is a small scale powergrid which can operate independently or in combination with main electrical grid. It can provide onsite generation to fulfill the local demand of the consumers. Addition of microgrid to the main grid increases the reliability and power quality of supply. Microgid can be served as backup power in the event of main grid failure. The modular structure of microgrids could make the main grid less susceptible to localized disaster. Modularity also means that microgrids can be used, piece by piece, to gradually modernize the existing grid. Microgrids encourage using of renewable energy sources so that they can avoid depletion of fossil fuels. Finally this paper presents architecture, types, modes of operation, features, benefits and challenges. In addition it alsopresents various microgrid projects implemented in India.
International standards should be formed for microgrid inter operability, control, communication and Islanding, and reconnection issues. Power system operators, power system planning engineers, and public should be provided more information regarding microgrids and their advantages. More incentives should be given to the Research and Development sectors which are looking after microgrid issues.