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Distributed utilities (DU), as the name indicates, employ distributed generation (DG) sources and distributed storage (DS) of energy throughout the service area. DG and DS devices can be operating individually or together and in stand-alone mode or in conjunction with an existing electric utility grid.
Increasing interest in the environmental aspects of electric power generation and utilization, the need to maintain a very high level of reliability and power quality to serve the evolving digital society, and the desire to harness renewable energy resources have combined to sharpen the focus on DG and DS in recent years.
Continuing technological advances in materials, system control and operation, and forecasting techniques have enabled DG with stochastic inputs such as insolation and wind to join the ranks of predict able generation technologies involving fuel cells, microturbines, gas turbines, and battery storage. In most cases, outputs of these devices are small in comparison with traditional utility-scale quantities.
However, strategic deployment of DG and DS can significantly improve the overall operation of the electric power system from the economic, reliability, and environmental points of view.
Ratings of DG can range from a few kilowatts to several megawatts as in the case of large wind farms, utility-scale photovoltaic (PV), and solar-thermal systems. Typically they enter at the secondary volt age level, 120V single-phase to 480V three-phase, and at medium voltage levels, 2.4-25 kV, depending on the technology and the geographic parameters involved. Some of the recent developments envisage the entry of DC microgrids as components of DU to effectively harness renewable energy resources at local levels.
TABLE 1 Distributed Generation Technology Chart
In this section, an overview of the different issues associated with DU (DG and DS), including avail able technologies, interfacing, brief discussion on economic and possible regulatory treatment, applications, and some practical examples are included. Emerging technologies discussed will include fuel cells, microturbines, small gas turbines, PV systems, wind farms, and solar-thermal systems. Energy storage technologies are briefly discussed. Interfacing issues include general protection, overcurrent protection, islanding issues, communication and control, voltage regulation, frequency control, fault detection, safety issues, and synchronization. In the section on applications, some economic issues, auxiliary services, peak shaving, reliability, and power quality issues are briefly covered.
1. Available Technologies
Many of the "new" technologies have been around for several years, but the relative cost per kilowatt of these technologies compared to conventional power plants has made their use limited until now. Utility rules and interconnect requirements have also limited the use of small generators and storage devices to mostly emergency, standby, and power quality applications. The scenario of deregulation coupled with environmental consciousness has changed all that. Utilities are no longer assured that they can recover the costs of large base generation plants, and stranded investment of transmission and distribution facilities is constantly undergoing shifting considerations. This, coupled with improvements in the cost and reliability of DU technologies, has opened an ever-increasing market for small and "green" power plants. In the near future (already in some cases), these new technologies should be competitive with conventional plants and provide high reliability with less investment risk. Some of the technologies are listed in the following. Energy storage technologies are enjoying rapid improvements in the small capacity range, specifically for use in portable electronic devices. At the same time, large-scale storage using flow batteries and similar technologies are also rapidly advancing. Power electronic technologies have experienced dramatic improvements to enable effective interfacing with ac grid, starting with dc outputs or rectified variable frequency outputs. Table 1 lists different technologies, their size ranges, input sources, interface types, and most likely applications.
2. Fuel Cells
Fuel cell technology has been around since its invention by William Grove in 1839. From the 1960s to the present, fuel cells have been the power source used for space flight missions. Unlike other generation technologies, fuel cells act like continuously fueled batteries, producing direct current (DC) by using an electrochemical process. The basic design of all fuel cells consists of an anode, electrolyte, and cathode.
Hydrogen or a hydrogen-rich fuel gas is passed over the anode, and oxygen or air is passed over the cathode.
A chemical combination then takes place producing a constant supply of electrons (DC current) with by products of water, carbon dioxide, and heat. The DC power can be used directly or it can be fed to a power conditioner and converted to AC power (see FIG. 1).
Most of the present technologies have a fuel reformer or processor that can take most hydrocarbon based fuels, separate out the hydrogen, and produce high-quality power with negligible emissions. This would include gasoline, natural gas, coal, methanol, light oil, or even landfill gas. In addition, fuel cells can be more efficient than conventional generators since they operate isothermally and, as such, are not subject to Carnot limitations. Theoretically they can obtain efficiencies as high as 85% when the excess heat produced in the reaction is used in a combined cycle mode. These features, along with relative size and weight, have also made the fuel cell attractive to the automotive industry as an alternative to battery power for electric vehicles. The major differences in fuel cell technology concern the electrolyte composition. The major types are the proton exchange membrane fuel cell (PEFC) also called the PEM, the phosphoric acid fuel cell (PAFC), the molten carbonate fuel cell (MCFC), and the solid oxide fuel cell (SOFC) (table 2).
Fuel cell power plants can come in sizes ranging from a few watts to several megawatts with stacking.
The main disadvantage to the fuel cell is the initial high cost of installation. With the interest in efficient and environmentally friendly generation, coupled with the automotive interest in an EV alternative power source, improvements in the technology and lower costs are expected. As with all new technologies, volume of sales should also lower the unit price.
FIG. 1 Basic fuel cell operation
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TABLE 2 Comparison of Fuel Cell Types
Type: PAFC MCFC SOFC PEMFC
Electrolyte: Phosphoric acid Molten carbonate salt Ceramic Polymer
Operating temperature: 375°F (190°C) Reformate 1200°F (650°C) Reformate 1830°F (1000°C) Reformate 175°F (80°C) Reformate
Fuels: Hydrogen (H2), H2/CO, H2/CO2/CH4, H2
Reforming: External External External External
Oxidant: O2/air CO2/O2/air O2/air O2/air
Efficiency: (HHV) 40%-50% 50%-60% 45%-55% 40%-50%
Source: Li, X. Principles of Fuel Cells, Section 1.4.5, Taylor and Francis, Boca Raton, Florida, 2006.
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FIG. 2 Turbine block diagram configuration with recuperator.
3. Microturbines
Experiments with microturbine technology have been around for many decades, with the earliest attempts of wide-scale applications being targeted at the automotive and transportation markets. These experiments later expanded into markets associated with military and commercial aircraft and mobile systems. Microturbines are typically defined as systems with an output power rating of between 10 kW up to a few 100 kW. As shown in FIG. 2, these systems are usually a single-shaft design with compressor, turbine, and generator all on the common shaft, although some companies are engineering dual-shaft systems. Like the large combustion turbines, the microturbines are Brayton cycle systems, and will usually have a recuperator in the system.
The recuperator is incorporated as a means of increasing efficiency by taking the hot turbine exhaust through a heavy (and relatively expensive) metallic heat exchanger and transferring the heat to the input air, which is also passed through parallel ducts of the recuperator. This increase in inlet air temperature helps reduce the amount of fuel needed to raise the temperature of the gaseous mixture during combustion to levels required for total expansion in the turbine. A recuperated Brayton cycle microturbine can operate at efficiencies of approximately 30%, while these aeroderivative systems operating without a recuperator would have efficiencies in the 15-18% range.
Another requirement of microturbine systems is that the shaft must spin at very high speeds, in excess of 50,000 RPM and in some cases doubling that rate, due to the low inertia of the shaft and connected components. This high speed is used to keep the weight of the system low and increase the power density over other generating technologies. Although many of the microturbines are touted as having only a single moving part, there are numerous ancillary devices required that do incorporate moving parts such as cooling fans, fuel compressors, and pumps.
Since the turbine requires extremely high speeds for optimal performance, the generator cannot operate as a synchronous generator. Typical microturbines have a permanent magnet motor/generator incorporated onto the shaft of the system. The high rotational speed gives an AC output in excess of 1000 Hz, depending on the number of poles and actual rotational speed of the microturbine. This high frequency AC source is rectified, forming a common DC bus voltage that is then converted to a 60 Hz AC output by an onboard inverter.
The onboard electronics are also used to start the microturbine, either in a stand-alone mode or in grid parallel applications. Typically, the utility voltage will be rectified and the electronics are used to convert this DC voltage into a variable frequency AC source. This variable frequency drive will power the permanent magnet motor/generator (which is operating as a motor), and will ramp the turbine speed up to a preset RPM, a point where stabile combustion and control can be maintained. Once this preset speed is obtained and stabile combustion is taking place, the drive shuts down and the turbine speed increases until the operating point is maintained and the system operates as a generator. The time from a "shaft stop" to full load condition is anywhere from 30s to 3 min, depending on manufacturer recommendations and experiences.
Although microturbine products are in the early stages of commercialization, there are cost targets that have been announced from all of the major manufacturers of these products. The early market entry price of these systems is in excess of $600 per kilowatt, more than comparably sized units of alternative generation technologies, but all of the major suppliers have indicated that costs will fall as the number of units being put into the field increases.
The microturbine family has a very good environmental rating, due to natural gas being a primary choice for fuel and the inherent operating characteristics, which puts these units at an advantage over diesel generation systems.
4. Combustion Turbines
There are two basic types of combustion turbines (CTs) other than the microturbines: the heavy frame industrial turbines and the aeroderivative turbines. The heavy frame systems are derived from similar models that were steam turbine designs. As can be identified from the name, they are of very heavy construction. The aeroderivative systems have a design history from the air flight industry, and are of a much lighter and higher speed design. These types of turbines, although similar in operation, do have some significant design differences in areas other than physical size. These include areas such as turbine design, combustion areas, rotational speed, and air flows.
Although these units were not originally designed as a "distributed generation" technology, but more so for central station and large co-generation applications, the technology is beginning to economically produce units with ratings in the hundreds of kilowatts and single-digit megawatts. These turbines operate as Brayton cycle systems and are capable of operating with various fuel sources. Most applications of the turbines as DG will operate on either natural gas or fuel oil. The operating characteristics between the two systems can best be described in tabular form as shown in Table 3.
The CT unit consists of three major mechanical components: a compressor, a combustor, and a turbine. The compressor takes the input air and compresses it, which will increase the temperature and decrease the volume per the Brayton cycle. The fuel is then added and the combustion takes place in the combustor, which increases both the temperature and volume of the gaseous mixture, but leaves the pressure as a constant. This gas is then expanded through the turbine where the power is extracted through the decrease in pressure and temperature and the increase in volume.
If efficiency is the driving concern, and the capital required for the increased efficiency is available, the Brayton cycle systems can have either co-generation systems, heat recovery steam generators, or simple recuperators added to the CT unit. Other equipment modifications and improvements can be incorporated into these types of combustion turbines such as multistage turbines with fuel re-injection, inter-cooler between multistage compressors, and steam/water injection.
Typical heat rates for simple cycle combustion turbines vary across manufacturers, but are in a range from 11,000 to 20,000 BTU/kWh. However, these numbers decrease as recuperation and co-generation are added. CTs typically have a starting reliability in the 99% range and operating reliability approaching 98%.
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TABLE 3 Basic Combustion Turbine Operating Characteristics (coming soon)
Heavy Frame Aeroderivative Size (same general rating) Large Compact Shaft speed Synchronous Higher speed (coupled through a gear box) Air flow High (lower compression) Lower (high compression) Start-up time 15 min 2-3 min
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The operating environment has a major effect on the performance of combustion turbines. The elevation at which the CT is operating has a degradation factor of around 3.5% per 1000 ft of increased elevation and the ambient temperature has a similar degradation per 10° increase.
FIG. 3 shows a block diagram of a simple cycle combustion turbine with a recuperator (left) and a combustion turbine with multistage turbine and fuel re-injection (right).
FIG. 3 Basic combustion turbine designs.
5. Photovoltaics
PV devices, commonly known as solar cells, convert incident solar radiation (insolation) directly into dc electrical energy. PV technology is highly flexible and versatile in terms of size (milliwatts to megawatts) and siting (from watches and calculators to utility-scale central station systems). Starting with its discovery in Bell Labs in 1954, PV has progressed from an expensive but critical option for space programs to utility-scale terrestrial use.
Insolation is an abundant resource. It arrives on the earth's surface at a rate of 120 million GW (about 20,000 kW for each and every 1 of the 6 billion humans inhabiting the earth). In comparison, the total rate of energy consumed by humans is estimated to be 15,000 GW, about 8,000 times smaller than insolation.
Among the many attributes of PV are silent, simple, and environmentally benign operation, no moving parts (other than trackers, if used), modular with no serious efficiency and cost penalties, and high power output to weight ratio. However, cost continues to be a barrier for large-scale generation of electricity. Cost of PV modules is in the range of $3-$4 per peak watt, cost of energy output is still about four to six times the generation cost realized with modern large wind-electric conversion systems. Absence of the resource at night times and variability introduced by moving clouds necessitate some form of energy storage and reconversion system or backup generation (including a conventional power grid). Many niche applications have been developed that are cost-effective at present. PV's siting flexibility will allow installation in locations ranging from rooftops and village-level systems all the way to large MW-scale central station configurations.
With an average overall efficiency of 10% and a peak insolation of 1 kW/m2 , an aggregated cell area of 1 ha (10,000 m2 or 2.471 acres) will generate an electrical output of 1 MW. However, the need to have tilted arrays of small modules spaced to avoid inter-array shadowing will result in a land area requirement of approximately 3-4 ha per MW with fixed-tilt flat-plate arrays. If tracking is employed, the land area required will be even larger. Concentrating plants will require about 2-2.5 times the land area to avoid shadowing as compared to fixed-tilt flat-plate arrays. The need to improve overall efficiency is obvious to reduce the land area required.
There are no serious technical problems for large-scale generation of electricity using PV. The first 1 MW plant was built in 9 months and started operation near Hesperia, California in 1982, and generated about 3 million kWh per year. By 2010, large-scale generation of electricity using PV had spread to more than 100 countries. At 97 MW, the Sarnia PV plant in Canada is the largest plant at present.
Other large PV plants are Montalto di Castro plant in Italy rated 84.2 MW, Finsterwalde Solar Park in Germany rated at 80.7 MW, Rovigo plant in Italy rated at 70 MW, Olmedilla PV Park in Spain rated at 60 MW, Strasskirchen Solar Park in Germany rated at 54 MW, and Lieberose PV Park, also in Germany rated at 53 MW. Many more are in the planning and construction stages all around the world.
Some of the other developments include transparent building-integrated PV (BIPV) systems, development of ac modules employing small power electronic systems with each module and improvements in the large-scale manufacturing of PV cells, panels, and arrays. Most recent advances are aimed at capturing a larger fraction of the incident solar spectrum and increasing the efficiencies using titanium based nanotechnologies. All along, technologies based on silicon in its various forms (single crystal, poly crystal, amorphous, etc.) have exhibited dramatic efficiency improvements ranging all the way to just above 24%, a new world record.
6. Solar-Thermal-Electric Systems
Solar-thermal-electric systems harness insolation in the form of thermal energy and utilize the solar heat as the input for a conventional thermal cycle with a suitable working fluid, which could be simply water. The temperatures that can be obtained on the hot side depend on the type of collector employed.
Coupled with a supplementary heater (typically burning natural gas) and/or a thermal energy storage and retrieval system, schedulable power output can be obtained on demand on a 24/7 basis.
There are two basic options available for collecting solar thermal energy:
1. Distributed collector with central or DG
2. Central collector with central generation system Solar thermal collectors are classified based on the collection temperature as low, medium, or high. For conversion to electrical energy, only medium and high temperature collectors are viable and practical.
Medium temperatures at around 400°C are obtained using parabolic trough systems. Parabolic dishes can concentrate insolation with ratios in the range of 600-2000 and temperatures in excess of 1500°C can be obtained at the focal point. Central receiver systems operate at concentration ratios in the range of 300-1500 and collect solar energy in thermal form at temperatures of 500°C-1500°C.
Distributed collectors can be a flat-plate type (not suitable for electric power generation) or a parabolic trough type resulting in a line-focus system or a point-focusing parabolic dish. With parabolic dishes, one generator can be placed in the focus of each dish and aggregation of energy is accomplished on the electrical side. With line-focus systems, an array of collectors (requiring approximately 2 ha per MWe) is employed to collect and transport the medium-grade thermal energy to a central location for use in a thermodynamic cycle.
In a central collector (receiver) plant, heliostats concentrate insolation onto a central receiver (in the 300-1500 ratio) where the energy is transferred to a working fluid (typically water) or to molten salt for storage and retrieval. The thermal energy thus collected is used to generate steam, which drives a turbine/generator system to produce electricity. Central receiver plants must be built in the scale of tens or hundreds of MW to be economically feasible.
Thermal energy storage and retrieval is far easier than storage in electrical form. It can be supplemented by fossil fuel (typically natural gas) burning subsystems to obtain schedulable and economically viable outputs. This is one of the primary reasons for the recent upsurge in construction and operation of solar-thermal-electric systems worldwide.
Solar thermal power industry is growing rapidly around the world with 1.2 GW of capacity under construction as of April 2009 and another 13.9 GW announced through 2014. Spain was the epicenter of solar thermal power development in 2010 with 22 projects aggregating to 1037 MW. In the United States, 5600 MW of solar thermal power projects have been announced. Solar Millennium, LLC and Chevron Energy Solutions, joint developers of a project, propose to construct, own, and operate the Blythe Solar Power Project in Southern California. The project is a concentrated solar thermal electric generating facility with four adjacent, independent, and identical solar plants of 250 MW nominal capacity each for a total capacity of 1000 MW. eSolar and Penglai Electric, a privately owned Chinese electrical power equipment manufacturer, have reached a master licensing agreement to build at least 2 GW of solar thermal power plants in China over the next 10 years. In India, the Jawaharlal Nehru National Solar Mission project is aimed at supplementing up to 20 GW of solar generated energy by 2022 across India.
The prime phase of the project is to establish off-grid solar PV as well as solar thermal by 2013.
7. Wind Electric Conversion Systems
Wind is moving air, resulting from the uneven heating of the earth's atmosphere by the incident solar energy. Thus, wind energy is an indirect form of solar energy, which is fully renewable. Wind energy is abundant (about 1670 trillion kWh/year over the land area of the earth). Including the offshore resources, a comprehensive study undertaken in 2005 estimated the potential of wind power on land and near-shore to be 72 TW, equivalent to 54,000 MTOE (million tons of oil equivalent) per year, which is over five times the world's current energy use in all forms. This abundant resource can be easily converted to rotary mechanical energy for coupling to an electrical generator to generate electricity. Since the collection area is perpendicular to ground surface, they pose minimal burden on land area and can coexist with many farming and other activities. It is estimated that over the land area, large WECS require about 6 ha per MW. Smaller unit sizes will require larger land areas. The blades occupy only a small fraction of the collector area, thus promising cost-effective conversion. Concerns about avian mortality have been largely mitigated by employing large diameter (60-120 m) turbines operating at slow rotational speeds in the range of 15-20 RPM.
Wind turbines have progressed from the early multibladed farm windmills that dotted the Midwest for pumping water for livestock in the United States to technologically complex large units employing sophisticated electromechanical energy converters operating in conjunction with advanced power electronic systems.
After several unsuccessful attempts to operate large wind turbines at constant speeds coupled to synchronous generators, detailed computer models revealed that such operation incurred undue stresses on the blades, tower, etc. and that variable speed operation alleviated this problem and extended the life of the system. Consequently, all the modern large WECS operate in the variable speed mode and employ suitable techniques on the electrical side to obtain constant frequency output. At present, the most widely used approach involves a double output induction generator (DFIG) that feeds the grid directly from the stator and through a power electronic frequency converter from the rotor. With a 20% variation in the range of wind turbine speeds, the rating of the power electronic system need be only 20% of the overall system rating. One of the promising new technologies under development is to eliminate the gear box altogether and employ direct drive large diameter (around 6 m) permanent magnet generators with lightweight rotating magnet systems and fixed armatures. The variable frequency output from the stationary armature is processed through a full power converter and fed to the grid.
Selecting a suitable location to site wind turbines is very critical to the success of the system. In order to aid this process, wind regimes are classified into several classes as listed in Table 4. Even small differences in mean wind speeds can lead to significant changes in the energy generated on an annual basis and capacity factors, which typically range from 20% to 30% or above.
In the year 2000, wind electrical conversion was the fastest growing and least cost electric generation option in the world. Almost 4000 MW of installed capacity was added during 2000, bringing the total to slightly above 17,500 MW globally. At the start of 2009, aggregated worldwide nameplate capacity of wind power plants was approximately 122 GW and energy production was around 260 TWh, which was about 1.5% of the worldwide electricity usage. By May 2009, around 80 countries were using wind power on a commercial basis. Countries such as Denmark and Spain have considerable share of wind in energy generation sector with shares of 19% and 13%, respectively. China had originally set a generation target of 30,000 MW by 2020 from renewable energy sources and reached 22,500 MW by the end of 2009 with a potential to surpass 30,000 MW by the end of 2010.
In recent years, United States has added more wind energy to its grid than any other country by installing 35,159 MW of wind power. According to the U.S. Department of Energy, wind power is capable of becoming a major contributor of electricity to America in three decades. A scenario of 20% of electricity from wind by 2030 is proclaimed as a national goal. The annual energy generated by these installations will depend on their capacity factors, which are strongly dependent on the prevailing wind regimes.
8. Storage Technologies
Storage technologies include batteries, flywheels, ultracapacitors, and to some extent photovoltaics.
Most of these technologies are best suited for power quality and reliability enhancement applications, due to their relative energy storage capabilities and power density characteristics, although some large battery installations could be used for peak shaving. All of the storage technologies have a power electronic converter interface and can be used in conjunction with other DU technologies to provide "seam less" transitions when power quality is a requirement.
TABLE 4 Wind Power Density Classes at 10 and 50 m Heights
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TABLE 5 Interface Issues (coming soon)
Issue - Definition - Concern
Automatic reclosing - Utility circuit breakers can test the line after a fault - If a generator is still connected to the system, it may not be in synchronization, thus damaging the generator or causing another trip
Faults - Short circuit condition on the utility system - Generator may contribute additional current to the fault, causing a miss operation of relay equipment
Islanding - A condition where a portion of the system continues to operate isolated from the utility system - Power quality, safety, and protection may be compromised in addition to possible synchronization problems
Protection - Relays, instrument transformers, circuit breakers - Devices must be utility grade rather than industrial grade for better accuracy; Devices must also be maintained on a regular schedule by trained technicians
Communication - Devices necessary for utility control during emergency conditions - Without control of the devices, islanding and other undesirable operation of devices
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TABLE 6 Operating Limits
1. Voltage-The operating range for voltage must maintain a level of ±15% of nominal for service voltage (ANSI C84.1), and have a means of automatic separation if the level gets out of the acceptable range within a specified time
2. Flicker-Flicker must be within the limits as specified by the connecting utility. Methods of controlling flicker are discussed in IEEE Std. 519-1992, 10.5
3. Frequency-Frequency must be maintained within ±0.5 Hz of 60 Hz and have an automatic means of disconnecting if this is not maintained. If the system is small and isolated, there might be a larger frequency window. Larger units may require an adjustable frequency range to allow for clock synchronization
4. Power factor-The power factor should be within 0.85 lagging or leading for normal operation. Some systems that are designed for compensation may operate outside these limits
5. Harmonics-Both voltage and current harmonics must comply with the values for generators as specified in IEEE Std. 519-1992 for both total and individual harmonics
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9. Interface Issues
A whole chapter could be written just about interface issues, but this discussion will touch on the high lights. Most of the issues revolve around safety and quality of service. We will discuss some general guidelines and the general utility requirements and include examples of different considerations. In addition to the interface issues, the DU installation must also provide self-protection to prevent short circuit or other damage to the unit. Self-protection will not be discussed here. The most important issues are listed in Table 5.
In addition to the interface issues identified in Table 5, there are also operating limits that must be considered. These are listed in Table 6.
Utility requirements vary but generally depend on the application of a distributed source. If the unit is being used strictly for emergency operation, open transition peak shaving, or any other stand-alone type operation, the interface requirements are usually fairly simple, since the units will not be operating in parallel with the utility system. When parallel operation is anticipated or required, the interface requirements become more complex. Protection, safety, power quality, and system coordination become issues that must be addressed. In the case of parallel operation, there are generally three major factors that determine the degree of protection required. These would include the size and type of the generation, the location on the system, and how the installation will operate (one-way vs. two-way). Generator sizes are generally classified as follows:
Large: Greater than 3 MVA or possibility of "islanding" a portion of the system Small: Between large and extremely small Extremely small: Generation less than 100 kVA
Location on the system and individual system characteristics determine impedance of a distribution line, which in turn determines the available fault current and other load characteristics that influence
"islanding" and make circuit protection an issue. This will be discussed in more detail later.
The type of operation is the other main issue and is one of the main determinants in the amount of protection required. One-way power flow where power will not flow back into the utility has a fairly simple interface, but is dependent on the other two factors, while two-way interfaces can be quite complex. An example is shown in FIG. 4. Smaller generators and "line-commutated" units would have less stringent requirements. Commutation methods will be discussed later. Reciprocating engines such as diesel and turbines with mass, and "self-commutating" units, which could include microturbines and fuel cells, would require more stringent control packages due to their islanding and reverse power capabilities.
Most of the new developing technologies are inverter based and there are efforts now in IEEE to revise the old Standard P929 Recommended Practice for Utility Interface of Photovoltaic (PV) Systems to include other inverter-based devices. The standards committee is looking at the issues with inverter based devices in an effort to develop a standard interface design that will simplify and reduce the cost, while not sacrificing the safety and operational concerns. Inverter interfaces generally fall into two classes: line-commutated inverters and self-commutated inverters.
9.1 Line-Commutated Inverters
These inverters require a switching signal from the line voltage in order to operate. Therefore, they will cease operation if the line signal, i.e., utility voltage, is abnormal or interrupted. These are not as popular today for single-phase devices due to the filtering elements required to meet the harmonic distortion requirements, but are appearing in some of the three-phase devices where phase cancellation minimizes the use of the additional components.
9.2 Self-Commutated Inverters
These inverters, as implied by the name, are self-commutating. All stand-alone units are self-commutated, but not all self-commutated inverters are stand-alone. They can be designed as either voltage or current sources and most that are now being designed to be connected to the utility system are designed to be current sources. These units still use the utility voltage signal as a comparison and produce cur rent at that voltage and frequency. A great deal of effort has gone into the development of nonislanding inverters that are of this type.
FIG. 4 Example of large generator interface requirements for distribution.
(From Georgia Power Bulletin, 18-8, generator interface requirements.)
10. Applications
Applications vary and will become more diverse as utilities unbundle. Listed in the following are some examples of the most likely.
10.1 Ancillary Services
Ancillary services support the basic electrical services and are essential for the reliability and operation of the electric power system. The electrical services that are supported include generating capacity, energy supply, and the power delivery system. FERC requires six ancillary services, including system control, regulation (frequency), contingency reserves (both spinning and supplemental), voltage control, and energy imbalance. In addition, load following, backup supply, network stability, system "black start," loss replacement, and dynamic scheduling are necessary for the operation of the system. Utilities have been performing these functions for decades, but as vertically integrated regulated monopoly organizations. As these begin to disappear, and a new structure with multiple competing parties emerges, DU might be able to supply several of these.
The DU providing these services could be owned by the former traditional utility, customers, or third-party brokers, depending on the application. The main obstacles to this approach are aggregation and communication when dealing with many small resources rather than large central station sources.
10.2 "Traditional Utility" Applications
Traditional utilities may find the use of DU a practical way to solve loading and reliability problems if each case is evaluated on a stand-alone individual basis. Deferring investment is one likely way that DU can be applied. In many areas, substations and lines have seasonal peaks that are substantially higher than the rest of the year. In these cases, the traditional approach has been to increase the capacity to meet the demand. Based on the individual situation, delaying the upgrade for 2-5 years with a DU system could be a more economical solution. This would be especially true if different areas had different seasonal peaks and the DU system was portable, thus deferring two upgrades. DU could also be used instead of conventional facilities when backup feeds are required or to improve reliability or power quality.
In addition, peak shaving and generation reserve could be provided with strategically placed DU systems that take advantage of reducing system losses as well as offsetting base generation. Again, these have to be evaluated on an individual case basis and not a system average basis as is done in many economic studies. The type of technology used will depend on the particular requirements. In general, storage devices such as flywheels and batteries are better for power quality applications due to their fast response time, in many cases half a cycle. Generation devices are better suited for applications that require more than 30 min of supply, such as backup systems, alternate feeds, peak shaving, and demand deferrals. Generation sources can also be used instead of conventional facilities in certain cases.
10.3 Customer Applications
Individual customers with special requirements may find DU technologies that meet their needs.
Customers who require "enhanced" power quality and reliability of service already utilize UPS systems with battery backup to condition the power to sensitive equipment, and many hospitals, waste treatment plants, and other emergency services providers have emergency backup systems supplied by standby generator systems. As barriers go down and technologies improve, customer-sited DU facilities could provide many of the ancillary services as well as sell excess power into the grid. Fuel cell and even diesel generators could be especially attractive for customers with requirements of heat and steam. Many of the fuel cell technologies are now looking at the residential market with small units that would be connected to the grid but supply the additional requirements for customers with special power quality needs.
10.4 Third-Party Service Providers
Third-party service providers could provide all the services listed earlier for the utilities and customers, in addition to selling power across the grid. In many cases, an end user does not have the expertise to operate and maintain generation systems and would prefer to purchase the services.
11. Conclusions
Significant portions of the evolving "smart grid" will contain both DG and DS to enable the integration of renewable energy technologies and improve the overall economics, reliability, and power quality. Strong desire to decrease the carbon footprint will accelerate this process. Advances in material technologies (in particular the use of nanotechnologies to improve PV devices), power electronic devices and systems, use of sensors and two-way communication, and the merging of internet and power grid to replace "copper and steel" by "silicon and glass" all point to an expanding role for DU.