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RECENT TRENDS IN DISTRIBUTED GENERATION- TECHNOLOGY ABSTRACT: Distribution generation also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy generates electricity from many small energy sources. For a large and dispersed rural country, decentralized power generation systems, where in electricity is generated at consumer end and there by avoiding transmission and distribution costs, offers a better solution. Distributed energy resource (DER) systems are small-scale power generation technologies (typically in the range of 3 kW to 10,000 kW) used to provide an alternative to or an enhancement of the traditional electric power system. The paper discusses the recent trends in distributed generation (DG). Technological trends like fuel cells, micro turbines and wind power are summarized. The operational requirements are discussed beyond the trend of increasing numbers of DGs in particular areas which leads to micro grid operations. Protection of DG is summarized as well. 1 INTRODUCTION Thomas Edison was a man of great foresight. When he set up his first heat-and-electricity plant near Wall Street in 1882, he imagined a world of micro power. Edison thought the best way to meet customers' needs would be with networks of decentralized power plants in or near homes and offices. After a century with power stations getting bigger and transmission grids needed to transmit wider, the idea of local generation for local consumption is back. While the developed countries enjoy highly uninterrupted power supply, many countries have acute power shortages as compared to the demand. Most undeveloped countries have had a monopolistic concept over their national energy supply. Energy is necessarily for sustainable development for it is needed to provide the basic energy needs as well as for industrialization processes. Distributed generation (DG) technologies, being one of such modern systems, make use of small-scale power generation technologies (fuel cells, wind, photovoltaic cells...). They are located close to the load being served and on the local distribution system with the primary aim of generating electricity and back-up sources. There are several reasons for this One is market liberalization. Small, local power plants offer a cheap way into market and cause only low investments. They do not suffer huge transmission losses. The surplus heat they generate can be employed for useful purposes. Therefore local power becomes economically competitive. Another reason is the demand for reliable, uninterrupted power. The situation in California is the best motivation for distributed power generation. The distributed generation (DG) framework is moving away from traditional large-scale power generation plants (100 MW to GW) located near the natural resource converted to electricity, to small power generators (kW to MW) sited directly at the loads. Enabling this concept is more of a regulatory than a technological issue. However, there are still a couple of technical hurdles facing it. The gradual shift from centralized to distributed generation means changes not only in the kinds of power plants used, but in the way electricity is transported from the point of production to the point of consumption. The old, centralized model of transporting electricity made extensive use of long-distance high-voltage transmission. This usually meant building unsightly overhead power lines. And although there have been continual improvements in long-distance transmission efficiency, a significant proportion of the power transmitted is still lost. DG allows us to place power generation facilities much closer to end-users, reducing transmission losses. If DG-units are rare, the system operator can tolerate them. If their number is growing they need to be coordinated with the system's control center. The aggregation of several DG-units to a micro grid opens up a large field of customer oriented and optimized operation. This paper points out the recent trends in technology, grid integration and operation of DG. Section 2 underlines some technological trends, which are supporting the wide area use of DG now and in the near future. Beside the network integration, the operation of DG is a fast developing trend. The increasing number of DG in a particular area of a power system leads to new solutions for operation and protection. The requirements and basic trends are presented in section 3. 2 TECHNOLOGICAL TRENDS DG comprises several technologies. Internal combustion engines are old technologies, which have many moving parts, are maintenance intensive and are not environmental friendly. Today, Photovoltaic is marketable only for small-scale applications. On the other hand there are recent technological developments, which are marketable today or in the near future. Micro turbines and Fuel Cells are economically feasible due to combined heat and power production and small-scale applicability. The well-established wind power is going in the direction of larger scales with onshore and offshore wind-farms providing higher economical efficiency. 2.1 Micro turbine Micro turbines are one of the most promising DG products currently on the market. The key technical features are: • High efficiency (as high as 85%) • Low emission (< 15ppm NOx and CO) • Fuel flexibility: oil, diesel, natural gas, biogas, methanol, hydrogen, etc. • Low maintenance cost (high speed single shaft engine and static power electronics converter)• • Remote control of power production available Typical applications for micro turbines are industrial, commercial and public buildings, large residential buildings, fun parks etc. Micro turbines are currently sold on the market. They always require a power electronics interface to connect them to the distribution grid. Most attractive applications for micro turbines make use of the thermal energy as well for heating or cooling. Figure 1: ABB MT100 micro turbine. 2.2 Fuel Cell A bright vision in the area of DG is the wide area introduction of fuel cells for private households. Fuel cells are providing simultaneously electricity and heating and shall be a replacement for nowadays gas heating in many countries. This application is of special interest because the production is as close as possible to the demand. A couple of activities from several big companies are ongoing for immobile as well as mobile use. Different technologies in the power ranges up to 5 kW, up to 250 kW or as backup power are available as pilot installations. 2.3 Wind-Power There are two recent trends concerning wind power. The first one is the introduction of permanent magnetized direct driven machines and the second one is wind farm with DC interconnections. Figure2: A typical Wind Generator 2.3.1 Permanent magnetized direct driven generators Direct driven generators are on the market since several years. Without the need of a gearbox the design has less moving parts and is more environmental friendly because of less oil, which is especially important for offshore usage and less noise. Usually electrically excited machines are used. With the recent introduction of permanent magnetized generators the construction can be further simplified while reducing the rotor losses. This reduces the vulnerability, operation and maintenance costs. The nowadays power electronic converters are allowing to fulfill all guidelines for the grid connection in a very flexible way. An additional trend is a higher output voltage up to 4 kV, which e.g. reduces the energy transmission losses and allows simplified setups of wind farms. 2.3.2 Wind farms The second trend concerns wind farm configurations with low cost profile and high efficiency. There are several solutions existing in AC and/or DC technology. Due to the existing power electronic converters within the today's windmills, there are several opportunities of combining AC and DC parts within a farm setup. For example the intermediate DC-circuits of the units can be coupled directly before converting to the grid frequency. 3. TECHNICAL REQUIREMENTS FOR DG OPERATION AND PROTECTION 3.1 From Single Units to Micro grids Whereas single DG unit is usually easy to tolerate by the grid, a lot of technical challenges are coming up with an increasing density of DG installations. The transmission grid will develop itself in a balancing or equalizing network. The control centers have to be adapted. Maintenance concepts have to be coordinated. The combination of heat and power with e.g. fuel cells or micro turbines on the low voltage level are leading to new requirements for the operation and energy management. One major recent development is the micro grid or virtual utility combining several DG units with a coordinated and optimized operation. A typical micro grid or distributed power station might contain about 10 MW of aggregated generating capacity produced in a number of small distributed generation resources. These might include any mix of combined heat and power plants, wind turbines, micro turbines, fuel cells and photovoltaic generators. Figure 3: Micro grid station Figure 4: Mircrogrid or Virtual Utility Concept (DG=Distributed Generation, DS=Distributed Storage) The structure of a micro grid is shown in Fig 4. The micro grid is operated by a control center, which optimizes the use of the different distributed generators (DG) and distributed storages (DS). In the control center the demand for and supply of energy is monitored, and power switched from the main grid to the micro grid, as it is needed. This control center is connected to a business center, which acts in the energy market to trade with the capacity of the micro grid or to buy complementary energy for the consumers in the micro grid. This business center takes care of administration, energy trading and sales on behalf of the operator, while another provides dedicated maintenance and services. The advantages of micro grids are: • Synergies for personnel resources, primary energy purchase, maintenance • high availability • energy mix • modularized operation planning • extended service • resource optimization • economic efficiency Many different types of companies and organizations can benefit from a micro grid system. Residential complexes are obvious candidates, as are rural communities beyond the reach of existing power grids. Others include hotel and tourist complexes, hospitals, shopping malls, industrial parks and large industries with many dispersed sites. Utility companies themselves also have a regular need for additional sources of power. They may have to fuel new growth beyond their current grid, for instance, or require temporary or permanent power support to enable the rehabilitation of old infrastructure. Rural utilities with distributed loads, island communities, energy service providers and power providers in developing countries that lack infrastructure could also benefit from micro grids. 3.2 Protection Beside the operation the protection of the grid in presence of DG is an important task. DG affects the protection of the grid in several ways, dependent on the voltage level (LV or MV) where the DG is connected. • Settings in distance relays (impedance measurement) are affected by the in feed effect. The determined impedance is changed due to current input along the protected line. This may shift the selective tripping schedule and selectivity may not be achieved. A resetting of the distance relay parameters is necessary whenever a significant power is introduced into the MV-network. • The selectivity of short circuit current indicators may be compromised. In case of a fault, the indicators may not only trip from the feeder side, but also from the dead end side of the line. This may happen if there is a significant short circuit current contribution by DG (possible with synchronous generators, but not usually with inverter based systems). • The fusing scheme on the LV level may have to be adjusted to prevent overloading of lines. If a DG is introduced along a line, the line may be overloaded between the DG and a load on the same line without blowing the fuses at the end of the line. In this case additional fuses are needed. • Reverse power relays (these relays are located at each transformer in a meshed LV grid and trip in case of a fault in the corresponding MV line) in meshed systems may have to be replaced, as they may trip under normal working conditions when DG on the low voltage level are delivering power to the medium voltage level. An individual analysis has to be made to determine, if the nominal current differs sufficiently from the fault current to achieve selectivity. • Additional protection devices are needed for DG interconnection, as required from regulations (e.g. loss of mains detection). It can be concluded that selectivity can be compromised both on LV and MV level. Accordingly, measures have to be taken (adjustment of protection devices, changes in protection schemes). Both meshed and non-meshed networks have their advantages for the placing of DG. • Meshed distribution networks have a high short circuit power. Their advantage is relatively balanced voltage profile and high reliability through redundancy. These networks can usually handle more aggregated DG power, if the short circuit power stays within allowable limits. • Non-meshed networks have a low short circuit power, are relatively simple to design, but the voltage profile is more vulnerable to load steps. The introduction of DG usually has a higher impact on the voltage profile in these configurations (Depending on the location in the network). Non-meshed networks do not need reverse power relays and therefore do not need any alternative solution for these relays when introducing DG. A meshed grid can usually be converted to a non meshed grid by opening the disconnectors in the according cable distribution cabinets. This could be necessary, when the short circuit current in a meshed system increases above allowable values (according to the installed equipment) by introducing DG. The splitting of a meshed network reduces the short circuit power in any given point in the grid. Furthermore, the splitting of the network removes the need for reverse power relays and can improve the selectivity of short circuit current indicators. Conclusively, there are no non-solvable problems concerning protection of the grid even with high numbers of DG. 4. CONCLUSION Distributed generation (DG) which is always located close to consumption points for rapid electrification depicts a small scale electricity generation. The trend for the use of DG is supported by recent technological developments like fuel cells, micro turbines, wind farm constructions as well as advanced storage technologies. All in all the, the trend in the direction of DG will increase rapidly. It is worth to support this direction for a long term environmental friendly and sustainable energy supply. 5. REFERENCES [1] "Sulzer-Hexis HXS 1000", Sulzer-Hexis, Product Information,, 2002 [2] Product Information, [3] "J48 Innovative Wind Turbine", Jeumont Framatome ANP, Product Information, 2001. [4] "Lagerwey LW 72 Zephyros", Lagerwey, Product Information and Lagerwey News, Sep. 2001. [5] Bauer, P.; De Haan, S.; Damen, M.; Pierik, J.: "Evaluation of Electrical Systems for Offshore Wind farms", IEEE IAS Annual Meeting, Rome, Italy, 2000. [6] Bauer, P.; De Haan, S.; Damen, M.; Pierik, J.: "Tool for Evaluation of Configuration of Offshore Wind parks: Models of the Components", EPE 2001, Graz, Austria, 2001. [7] European Union: "Share of electricity from renewable to total electricity consumption", EUROSTAT, official European statistic, 2001



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