Grid Interconnection documentation

GRID INTERCONNECTION OF RENEWABLEENERGY SOURCES AT THE DISTRIBUTION LEVELWITH POWER-QUALITY IMPROVEMENT FEATURESABSTRACTRenewable energy resources (RES) are being increasingly connected in distribution systems utilizing power electronic converters. This paper presents a novel control strategy for achieving maximum benefits from these grid-interfacing inverters when installed in 3-phase 4-wire distribution systems. The inverter is controlled to perform as a multi-function device by incorporating active power filter functionality. The inverter can thus be utilized as: 1) power converter to inject power generated from RES to the grid, and 2) shunt APF to compensate current unbalance, load current harmonics, load reactive power demand and load neutral current. All of these functions may be accomplished either individually or simultaneously. With such a control, the combination of grid-interfacing inverter and the 3-phase 4-wire linear/non-linear unbalanced load at point of common coupling appears as balanced linear load to the grid. This new control concept is demonstrated with extensive MATLAB/Simulink simulation studies and validated through digital signal processor-based laboratory experimental results.

INTRODUCTIONELECTRIC utilities and end users of electric power are becoming increasingly concerned about meeting the growing energy demand. Seventy five percent of total global energy demand is supplied by the burning of fossil fuels. But increasing air pollution, global warming concerns, diminishing fossil fuels and their increasing cost have made it necessary to look towards renewable sources as a future energy solution. Since the past decade, there has been an enormous interest in many countries on renewable energy for power generation. The market liberalization and governments incentives have further accelerated the renewable energy sector growth.Renewable energy source (RES) integrated at distribution level is termed as distributed generation (DG). The utility is concerned due to the high penetration level of intermittent RES in distribution systems as it may pose a threat to network in terms of stability, voltage regulation and power-quality (PQ) issues. Therefore, the DG systems are required to comply with strict technical and regulatory frameworks to ensure safe, reliable and efficient operation of overall network. With the advancement in power electronics and digital control technology, the DG systems can now be actively controlled to enhance the system operation with improved PQ at PCC. However, the extensive use of power electronics based equipment and non-linear loads at PCC generate harmonic currents, which may deteriorate the quality of power. Generally, current controlled voltage source inverters are used to interface the intermittent RES in distributed system. Recently, a few control strategies for grid connected inverters incorporating PQ solution have been proposed. In an inverter operates as active inductor at a certain frequency to absorb the harmonic current. But the exact calculation of network inductance in real-time is difficult and may deteriorate the control performance. A similar approach in which a shunt active filter acts as active conductance to damp out the harmonics in distribution network is proposed. A control strategy for renewable interfacing inverter based on theory is proposed. In this strategy both load and inverter current sensing is required to compensate the load current harmonics. The non-linear load current harmonics may result in voltage harmonics and can create a serious PQ problem in the power system network. Active power filters (APF) are extensively used to compensate the load current harmonics and load unbalance at distribution level. This results in an additional hardware cost. However, in this paper authors have incorporated the features of APF in the, conventional inverter interfacing renewable with the grid, without any additional hardware cost. Here, the main idea is the maximum utilization of inverter rating which is most of the time underutilized due to intermittent nature of RES. It is shown in this paper that the grid-interfacing inverter can effectively be utilized to perform following important functions: 1) transfer of active power harvested from the renewable resources (wind, solar, etc.); 2) load reactive power demand support; 3) current harmonics compensation at PCC; and 4) current unbalance and neutral current compensation in case of 3-phase 4-wire system. Moreover, with adequate control of grid-interfacing inverter, all the four objectives can be accomplished either individually or simultaneously. The PQ constraints at the PCC can therefore be strictly maintained within the utility standards without additional hardware cost.

DISTRIBUTED GENERATIONDistributed generation, also calledon-site generation,dispersed generation,embedded generation, decentralized generation, decentralized energy or distributed energy enerateselectricityfrom many small energy sources. Currently, industrial countries generate most of their electricity in large centralized facilities, such asfossil fuel(coal,gas powered)nuclearor hydropowerplants. These plants have excellent economies of scale, but usually transmit electricity long distances and negatively affect the environment.Most plants are built this way due to a number ofeconomic,health&safety,logistical, environmental,geographicalandgeologicalfactors. For example, coal power plants are built away from cities to prevent their heavy air pollution from affecting the populace. In addition, such plants are often built nearcollieriesto minimize the cost of transporting coal.Hydroelectricplants are by their nature limited to operating at sites with sufficient water flow. Most power plants are often considered to be too far away for their waste heat to be used for heating buildings.Low pollution is a crucial advantage ofcombined cycleplants that burnnatural gas. The low pollution permits the plants to be near enough to a city to be used fordistrict heatingand cooling. Distributed generation is another approach. It reduces the amount of energy lost in transmitting electricity because the electricity is generated very near where it is used, perhaps even in the same building. This also reduces the size and number of power lines that must be constructed. Typical distributed power sources in aFeed-in Tariff(FIT) scheme have low maintenance, low pollution and high efficiencies. In the past, these traits required dedicated operating engineers and large complex plants to reduce pollution. However, modernembedded systemscan provide these traits with automated operation andrenewable, such as sunlight, wind andgeothermal. This reduces the size of power plant that can show a profit.Distributed energy resourceDistributed energy resource (DER) systems are small-scale power generation technologies (typically in the range of 3kW to 10,000kW) used to provide an alternative to or an enhancement of the traditional electric power system. The usual problems with distributed generators are their high costs.One popular source issolar panelson the roofs of buildings. The production cost is $0.99 to 2.00/W (2007) plus installation and supporting equipment unless the installation isDo it yourself(DIY) bringing the cost to $6.50 to 7.50 (2007).This is comparable to coal power plant costs of $0.582 to 0.906/W (1979),adjusting for inflation. Nuclear power is higher at $2.2 to $6.00/W (2007).[4]Some solar cells ("thin-film" type) also have waste disposal issues; since "thin-film" type solar cells often contain heavy-metal electronic wastes, such asCadmium telluride(CdTe) andCopper indium gallium selenide(CuInGaSe), and need to be recycled. As opposed to silicon semi-conductor type solar cells which is made from quartz. The plus side is that unlike coal and nuclear, there are no fuel costs, pollution, mining safety or operating safety issues. Solar also has a lowduty cycle, producing peak power at local noon each day. Average duty cycle is typically 20%.Another source is smallwind turbines. These have low maintenance, and low pollution. Construction costs are higher ($0.80/W, 2007) per watt than large power plants, except in very windy areas. Wind towers and generators have substantial insurable liabilities caused by high winds, but good operating safety. In some areas of the US there may also be Property Tax costs involved with wind turbines that are not offset by incentives oraccelerated depreciation. Wind also tends to be complementary to solar; on days there is no sun there tends to be wind and vice versa.Many distributed generation sites combine wind power and solar power such asSlippery Rock University, which can bemonitored online.Distributedcogenerationsources usenatural gas-firedmicro turbinesorreciprocating enginesto turn generators. The hot exhaust is then used for space or water heating, or to drive anabsorptive chiller[6]forair-conditioning. The clean fuel has only low pollution.Designscurrently have uneven reliability, with some makes having excellent maintenance costs, and others being unacceptable. Co generators are also more expensive per watt than central generators. They find favor because most buildings already burn fuels, and the cogeneration can extract more value from the fuel.Some larger installations utilizecombined cyclegeneration. Usually this consists of agas turbinewhose exhaust boilswaterfor asteam turbinein aRankine cycle. The condenser of the steam cycle provides the heat for space heating or an absorptive chiller. Combined cycle plants with cogeneration have the highest known thermal efficiencies, often exceeding 85%.In countries with high pressure gas distribution, small turbines can be used to bring the gas pressure to domestic levels whilst extracting useful energy. If the UK were to implement this countrywide an additional 2-4GWe would become available. (Note that the energy is already being generated elsewhere to provide the high initial gas pressure - this method simply distributes the energy via a different route.) Future generations of electric vehicles will have the ability to deliver power from the battery into the grid when needed. This could also be an important distributed generation resource.

Recently interest in Distributed Energy Systems (DES) is increasing, particularly onsite generation. This interest is because larger power plants are economically unfeasible in many regions due to increasing system and fuel costs, and more strict environmental regulations. In addition, recent technological advances in small generators, Power Electronics, and energy storage devices have provided a new opportunity for distributed energy resources at the distribution level, and especially, the incentive laws to utilize renewable energies has also encouraged a more decentralized approach to power delivery.There are many generation sources for DES: conventional technologies (diesel or natural gas engines), emerging technologies (micro turbines or fuel cells or energy storage devices), and renewable technologies (small wind turbines or solar/photovoltaics or small hydro turbines). These DES are used for applications to a standalone, a standby, a grid-interconnected, a cogeneration, peak shavings, etc. and have many advantages such as environmental-friendly and modular electric generation, increased reliability, high power quality, uninterruptible service, cost savings, on-site generation, expandability, etc. So many utility companies are trying to construct small distribution stations combined with several DES available at the regions, instead of large power plants. Basically, these technologies are based on notably advanced Power Electronics because all DES require Power Converters, interconnection techniques, and electronic control units. That is, all power genera ed by DES is generated as DC Power, and then all the power fed to the DC distribution bus is again converted into an AC power with fixed magnitude and frequency by control units using Digital Signal Processor (DSP). So improved power electronic technologies that permit grid interconnection of asynchronous generation sources are definitely required to support distributed generation resourcesThe research works in the recent papers about DES focus on being utilized directly to a standalone AC system or fed back to the utility mains. That is, when in normal operation or mainfailures, DES directly supply loads with power (standalone mode or standby mode), while, when DES have surplus power or need more power, this system operates in parallel mode to the mains. Therefore, in order to permit to connect more generators on the network in good conditions, a good technique about interconnection with the grid and voltage regulations should overcome the problems due to parallel operation of Power Converter for applications to DES.

DISTRIBUTED ENERGY SYSTEMSToday, new advances in technology and new directions in electricity regulation encourage a significant increase of distributed generation resources around the world. As shown in Fig. the currently competitive small generation units and the incentive laws to use renewable energies force electric utility companies to construct an increasing number of distributed generation units on its distribution network, instead of large central power plants. Moreover, DES can offer improved service reliability, better economics and a reduced dependence on the local utility. Distributed Generation Systems have mainly been used as a standby power source for critical businesses. For example, most hospitals and office buildings had stand-by diesel generation as an emergency power source for use only during outages. However, the diesel generators were not inherently cost-effective, and produce noise and exhaust that would be objectionable on anything except for an emergency basis.

Fig. A large central power plant and distributed energy systems

Meanwhile, recently, the use of Distributed Energy Systems under the 500 kW level is rapidly increasing due to recent technology improvements in small generators, power electronics, and energy storage devices. Efficient clean fossil fuels technologies such as micro-turbines and fuel cells, and environmentally friendly renewable energy technologies such as solar/photovoltaics, small wind and hydro are increasingly used for new distributed generation systems. These DES are applied to a standalone, a standby, a grid-interconnected, a cogeneration, peak shavings, etc. and have a lot of benefits such as environmental-friendly and modular electric generation, increased reliability, high power quality, uninterruptible service, cost savings, on-site generation, expandability, etc. The major Distributed Generation technologies that will be discussed in this section are as follows: micro-turbines, fuel cells, solar/photovoltaic systems, and energy storage devices.Micro-turbines, especially the small gas fired micro turbines in the 25-100 kW that can be mass-produced at low cost have been more attractive due to the competitive price of natural gas, low installation and maintenance costs. It takes very clever engineering and use of innovative design (e.g. air bearing, recuperation) to achieve reasonable efficiency and costs in machines of lower output, and a big advantage of these systems is small because these mainly use high-speed turbines (50,000-90,000 RPM) with air foil bearings. Therefore, micro turbines hold the most promise of any of the DES technologies today.Fuel cells are also well used for distributed generation applications, and can essentially be described as batteries which never become discharged as long as hydrogen and oxygen are continuously provided. The hydrogen can be supplied directly, or produced from natural gas, or liquid fuels such as alcohols, or gasoline. Each unit ranges in size from 3 250 kW or larger MW size. Even if they offer high efficiency and low emissions, todays costs are high. Phosphoric acid cell are commercially available in the range of the 200 kW, while solid oxide and molten carbonate cell are in a pre-commercial stage of development.The possibility of using gasoline as a fuel for cells has resulted in a major development effort by the automotive companies. The recent research work about fuel cells is focused towards the polymer electrolyte membrane (PEM) fuel cells. Fuel cells in sizes greater than 200 kW, hold promise beyond 2005, but residential size fuel cells are unlikely to have any significant market impact any time soon. Mixed micro-turbine and fuel cell systems will also be available as a distributed generation source. Recently, a solid oxide fuel cell has been combined with a gas micro-turbine creating a combined cycle power plant. It has expected electrical efficiency of greater than 70 %, and the expected power levels range from 250 kW to 2.5 MW. Solar/photovoltaic systems may be used in a variety of sizes, but the installation of large numbers of photovoltaic systems is undesirable due to high land costs and in many geographic areas with poor intensity and reliability of sunlight. In general, almost one acre of land would be needed to provide 150 kW of electricity, so solar/photovoltaic systems will continue to have limited applications in the future. Energy storage devices such as ultra capacitors, batteries, and flywheels are one of the most critical technologies for DES. In general, the electrochemical capacitor has high power density as well as good energy density. In particular, ultra capacitors have several benefits such as high pulse power capacity, long lifetime, high power density, low ESR, and very thin and tight. In contrast, batteries have higher energy density, but lower power density and short lifetime relative to ultra-capacitor. So hybrid Power System, a combination of ultra-capacitor and battery, is strongly recommended to satisfy several requirements and to optimize system performance. Recently storage systems are much more efficient, cheaper, and longer than five years ago. In particular, flywheel systems can generate 700 kW for 5 seconds, while 28-cell ultra capacitors can provide up to 12.5 kW for a few seconds.In the past, the electric utility industry did not offer various options that were suited for a wide range of consumer needs, and most utilities offered at best two or three combinations of reliability-price. However, the types of modern DES give commercial electric consumers various options in a wider range of reliability-price combinations. For these reasons, DES will be very likely to thrive in the next 20 years, and especially, distributed generation technologies will have a much greater market potential in areas with high electricity costs and low reliability such as in developing countries.PROBLEM STATEMENTS

DES technologies have very different issues compared with traditional centralized power sources. For example, they are applied to the mains or the loads with voltage of 480 volts or less; and require power converters and different strategies of control and dispatch. All of these energy technologies provide a DC output which requires power electronic interfaces with the distribution power networks and its loads. In most cases the conversion is performed by using a voltage source inverter (VSI) with a possibility of pulse width modulation (PWM) that provides fast regulation for voltage magnitude. Power electronic interfaces introduce new control issues, but at the same time, new possibilities. For example, a system which consists of micro-generators and storage devices could be designed to operate in both an autonomous mode and connected to the power grid. One large class of problems is related to the fact that the power sources such as micro turbines and fuel cell have slow response and their inertia is much less. It must be remembered that the current power systems have storage in generators inertia, and this may result in a slight reduction in system frequency. As these generators become more compact, the need to link them to lower network voltage is significantly increasing. However, without any medium voltage networks adaptation, this fast expansion can affect the quality of supply as well as the public and equipment safety because distribution networks have not been designed to connect a significant amount of generation. Therefore, a new voltage control system to facilitate the connection of distributed generation resources to distribution networks should be developed. In many cases there are also major technical barriers to operating independently in a standalone AC system, or to connecting small generation systems to the electrical distribution network with lower voltage and the recent research issues includes:1. Control strategy to facilitate the connection of distributed generation resources to distribution networks.2. Efficient battery control.3. Inverter control based on only local information.4. Synchronization with the utility mains.5. Compensation of the reactive power and higher harmonic components.6. Power Factor Correction.7. System protection.8. Load sharing.9. Reliability of communication.10. Requirements of the customer.DES offers significant research and engineering challenges in solving these problems. Moreover, the electrical and economic relationships between customers and the distribution utility and among customers may take forms quite distinct from those we know today. For example, rather than devices being individually interconnected in parallel with the grid, they may be grouped with loads in a semi-autonomous neighborhood that could be termed a micro grid is a cluster of small sources, storage systems, and loads which presents itself to the grid as a legitimate single entity. Hence, future research work will focus on solving the above issues so that DES with more advantages compared with tradition large power plants can thrive in electric power industry.

PROBLEM DESCRIPTION

These new distributed generations interconnected to the low grid voltage or low load voltage cause new problems which require innovative approaches to managing and operating the distributed resources. In the fields of Power Electronics, the recent papers have focused on applications of a standby generation, a standalone AC system, a combined heat and power (cogeneration) system, and interconnection with the grid of distribution generations on the distribution network, and have suggested technical solutions which would permit to connect more generators on the network in good conditions and to perform a good voltage regulation. Depending on the load, generation level, and local connection conditions, each generator can cause the problems described in the previous chapter. The main goals which should be achieved will thus be: to increase the network connection capacity by allowing more consumers and producer customers connection without creating new reinforcement costs, to enhance the reliability of the systems by the protections, to improve the overall quality of supply with a best voltage control.A. Configurations for DES

1) Case I: A Power Converter connected in a Standalone AC System or in Parallel with the Utility Mains Fig. show a distributed power system which is connected to directly load or in parallel with utility mains, according to its mode. This system consists of a generator, an input filter, an AC/AC power converter, an output filter, an isolation transformer, output sensor (V, I, P), and a DSP controller. In the Figures, a distributed generator may operate as one of three modes: a standby, a peak shaving, and a standalone power source. In a standby mode shown in Fig. a generator set serves as a UPS system operating during mains failures. It is used to increase the reliability of the energy supply and to enhance the overall performance of the system. The static switch SW 1 is closed in normal operation and SW 2 is open, while in case of mains failures or excessive voltage drop detection SW 1 is open and SW 2 is simultaneously closed. In this case, control techniques of DES are very similar to those of UPS. If a transient load increases, the output voltage has relatively large drops due to the internal impedance of the inverter and filter stage, which frequently result in malfunction of sensitive load. Fig. can serves as a peak shaving or interconnection with the grid to feed power back to mains. In both modes, the generator is connected in parallel with the main grids. In a peak shaving mode, this generator is running as few as several hundred hours annually because the SW 1 is only closed during the limited periods. Meanwhile, in an interconnection with the grid, SW 1 is always closed and this system provides the grid with continuous electric power. In addition, the converter connected in parallel to the mains can serve also as a source of reactive power and higher harmonic current components.In a standalone AC system shown in Fig. the generator is directly connected to the load lines without being connected to the mains and it will operate independently. In this case, the operations of this system are similar to a standby mode, and it serves continuously unlike a standby mode and a peak shaving mode.

Fig. Block diagram of a standby mode

Fig. Block diagram of a peak shaving mode

Fig. Block diagram of a standalone mode

As shown in Fig. the output voltage of the generator is fed to a DC/AC converter that converts a DC output of the generator to be fixed voltage and frequency for utility mains or loads. The DSP controller monitors multiple system variables on a real time basis and executes control routines to optimize the operation of the individual subsystems in response to measured variables. It also provides all necessary functions to sense output voltages, current, and power, to operate protections, and to give reference signals to regulators. The output power of the converter is controlled according to the reference signal of the control unit. As described above, in order to compensate for reactive power and higher harmonic components or to improve power factor, the active power (P) and reactive power (Q) should be controlled independently. Moreover, the above system needs over-dimensioning some parts of the power converter in order to produce reactive power by the converter at rated active power. Because a power converter dimensioned for rated current can supply reactive power only if the active component is less than rated. Therefore, a control strategy easy to implement is required to ensure closed loop control of the power factor and to provide a good power quality. In case that a generator is used for distributed generation systems, the recent research focuses are summarized as follows:1. Control strategy which permits to connect more generators on the network2. Compensation of the reactive power and higher harmonic components3. An active power (P) and a reactive power control (Q) independently4. Power factor correction5. Synchronization with the utility mains6. System protections2) Case II: Power Converters supplying power in a standalone mode or feeding it back to the utility mains Fig. shows a block diagram of multiple power converters for a standalone AC system or feeding generated powers back to the utility mains. If all generators are directly connected to the loads, the systems operate as a standalone AC system. Meanwhile, if these are connected in parallel to the mains, these provide the utility grids with an electric power. Each system consists of a generator, an input filter, an AC/AC power converter, an output filter, an isolation transformer, a control unit (DSP), a static switch (SW 1) and output sensors (V, I, P). The function of the static switch (SW 1) is to disrupt the energy flow between the generator and mains or loads in the case of disturbances in the mains voltage. As shown in Fig., this configuration is very similar to parallel operation of multiple UPS systems except that the input sources of inverters are independent generation systems such as micro turbines, fuel cells, and photovoltaic, etc. instead of utility mains.In case of parallel operation of UPS systems, a recent critical research issue is to share linear and nonlinear load properly by each unit. In general, the load sharing is mainly influenced by non uniformity of the units, component tolerance, and line impedance mismatches. Another issue is a proper control scheme without any control interconnection wires among inverters because these wires restrict the location of the inverter units as well as these can act as a source of the noise and failure. Moreover, in three-phase systems they could also cause unbalance and draw excessive neutral currents. Even if conventionally passive L-C filters were used to reduce harmonics and capacitors were employed to improve the power factor of the ac loads, passive filters have the demerits of fixed compensation, large size, and resonance. Therefore, the injected harmonic, reactive power burden, unbalance, and excessive neutral currents definitely cause low system efficiency and poor power factor. In particular, a power factor can be improved as AC/AC power converters function a complete active filter for better power quality and the above problems should be overcome by a good control technique to assure the DES to expand increasingly around the world.

Fig. Block diagram of power converters connected in parallel

So the above issues can be applied to distributed power systems similarly, and the recent research focuses are summarized as follows:1. Standardized DES modeling using the software tools2. Equal load sharing such as the real and reactive power, the load harmonic current among the parallel connected inverters.3. Connection capability of more DES to the utility mains in best conditions4. Independent P, Q control of the inverters5. Power factor correction6. Reduction of Total Harmonic Distortion (THD).Distributed Generation (DG) is commonly defined as electric power generation facilities that are not directly connected to a bulk power transmission system. They cover a multitude of energy sources, fuels, and conversion methods to produce electricity through photovoltaic (PV) arrays, wind turbines, fuel cells, micro turbines, liquid and gas-fueled reciprocating engines, etc. Given the wide variety of sources, it is natural that specific impacts associated with DG would vary with type and application. However, there are many common threads on how DG benefits the customers they serve and society at large. This is demonstrated in this paper through several examples, giving testimonials of the positive impact these installations have.

EXAMPLES

Case 1 Emergency/Temporary Power Application

In mid-November 2007, the town of Chester, California, was preparing to undergo a 72-hour power shutdown. This was required to improve the service reliability in Chester after past sustained outages due to circuit configuration, condition and exposure. For that reason, Pacific Gas and Electric (PG&E) would replace 41 power poles and their cross-arms on its Hamilton Branch transmission line. This line serves the Chester community with 1,652 electric meters within, and accounting for a combined electrical demand in the range of 1.9 MW to 2.4 MW. Given PG&Es commitment to customer satisfaction, and to maintaining their service uptime percentage as high as possible, a prolonged outage was really not a viable option. The solution was a temporary DG installation consisting of two 2 MW diesel generator sets operated through a parallel configuration with the PG&E power supply (see Fig. 1). With this configuration, only two brief electrical clearances, totaling less than 30 seconds, were necessary. Over 2,378,880 outage minutes were avoided for PG&E customers. Chesters lights remained on and the aging poles were replaced. PG&E avoided the combination of lost power sales during any outage, and the added expense and safety concerns of having to work crews around the clock for 72 hours to replace all the poles.

Fig. - Cat XQ2000 Power Modules in Chester

Case 2 Open-Market Price Hedging

The volatility in the energy market was greatly affecting the power department budget in the city of Hurricane, Utah. Its population surged from 8,250 in 2000 to 12,084 in 2006 an increase of 46.5 percent. That growth put stress on the municipal power system, operated by Hurricane City Power, especially in the summer when temperatures can exceed 110F. This demand, coupled with high prices on the energy market, forced the city to dip into budgetary reserves to pay for power a few years in a row .In order to diversify its electric supply options, Hurricane turned to natural gas generator sets. The citys new generator sets would have to meet Hurricanes needs load following, summer peaking and open market price-hedging strategies. Hurricane also needed to boost end-of-line voltage and frequency for distribution system enhancement over a period of three years, six Cat G3520C natural gas generator sets with Cat Oxidation Catalysts were installed (see Fig. 2). The catalysts lower emissions of carbon monoxide by 93% and decrease hydrocarbons by more than 40%, greatly reducing the generator sets environmental impact. One of these units serves the nearby city of Washington and is used when supplemental power is needed there. The generator sets are rated at 1,940 ekW at 1800 rpm, in 115F temperature and at an elevation of 3,000 feet. They operate together with paralleling switchgear at 12,470 V. This power is connected directly to the adjacent substations distribution buss. The reliability and cost-effectiveness of this power solution earned Hurricane City Power and Washington City Power a joint award in 2007 for the Most Improved System of the Year from the Utah Associated Municipal Power Systems (UAMPS). The city has been able to save as much as $10,000 to $12,000 a day because of their ability to react to market prices quickly, and run their generators instead of buying power on the market when the cost is high. In addition to cost savings for the city, the gensets provide peak power production support and backup power in case of a citywide blackout. Three area blackouts have occurred in the three years since this system has been in place, and the gensets have provided the power needed to get the city up and running with no outside power available.

Fig. - DG in Hurricane City Power Plant

Case 3 Combined CHP and Standby Power for Hospital

A cost effective combined heat and power (CHP) and standby power generation package was required for the Norfolk and Norwich Hospital being built by Octagon Healthcare in the United Kingdom. In addition, tariff quality metering was necessary to qualify the CHP as eligible for payment under the Climate Change Levy Good Quality CHP scheme. Rather than the usual basement plant room, a stand-alone energy center was built to give better access for servicing and supplies. The CHP system prime mover is a Cat G3516 lean burn gas engine. Heat is recovered from the engine exhaust, jacket water and oil cooler circuits, to provide 1314 kW. It is used heat the returning medium-pressure hot water before it re-enters the boiler, so the CHP acts as lead boiler. When thermal demand is low excess heat is dumped to a remote radiator. A Cat SR4 generator directly linked to the engine provides 400 volts at 50 Hz. This feeds a synchronizing circuit breaker inside a control panel, in turn connected to the hospitals HV line via a step-up transformer. The complete system is displayed on a graphical overview; a simple touch of the screen is all that is required for an operator to interact with the system.The standby generation system comprises four 2250 kVA (1800 kW) Caterpillar 3616B diesel generator sets guarding against utility failures. Generators work in an n+1 configuration so that full coverage of the hospital power requirements continues if one set is unavailable; say due to servicing at the time of power failure. The generators are connected to the hospital heating and ventilation power feed via a HV switchboard. Like the CHP system, the master control panel provides a graphical overview of the system. Restoration to utility power, once service has been deemed to be back to normal, is fully automatic. The system is tested monthly with a real disconnection from the utility supply. Case 4 Landfill ApplicationsLandfill-gas-to-energy embodies the ideal solution to an environmental problem: it turns nuisance waste into a product with a practical use and economic value. It is an important and growing component of North Americas power generation mix. Generation from municipal solid waste and landfill gas is projected to increase to about 31 billion kilowatt-hours by 2025. Although the U.S. leads in this category, landfill gas is globally available as the chart below illustrates.

Fig. - Estimated Global Landfill Methane Emissions

Landfill gas (LFG) is produced naturally as organic waste decomposes in landfills. LFG is composed of about 50 percent methane, about 50 percent carbon dioxide and small amounts of non-methane organic compounds. At most municipal solid-waste landfills, the methane and carbon dioxide are destroyed in a gas collection and control system or utility flare. However, to use LFG as an alternative fuel, the gas is extracted from landfills using a series of wells and a vacuum system. Pipes are inserted deep into the landfill to provide a point of release for the landfill gases. A slight vacuum is then applied in the pipe to draw the gases into and through it to a central point, where it can be processed and treated for use in generating electricity, replacing the need for conventional fossil fuels. Here are a few examples from around the world of how LFG is used to produce electric power through engine generator sets in landfill configurations.

Fig. Landfill Gas Engine Generator Set

Seneca Meadows Landfill, Seneca Falls, New York This energy system, owned by Innovative Energy Systems of Oakfield, NY, began operation in 1996 and has been expanded three times to its current 11.2 MW capacity. The system (see Fig.) uses fourteen Cat G3516 generator sets that have been modified for landfill use. Overall energy plant NOx emissions are compliant with local air-quality standards.

Fig. - Seneca Meadows Landfill, Seneca Falls, NY

Hartland Landfill in Victoria, British Columbia, Canada The landfill receives municipal solid waste from a population of roughly 400,000. Until the power generating system went online, the landfill gas had been flared. Independent power producer Maxim Power Corporation of Calgary, Alberta, installed the land-fill-gas-to-energy system (see Fig. 6). The electric energy output (continuous duty at 1.6 MW) is being sold to BC Hydro for that companys Green Power program.

Fig. Harland Landfill, Victoria, BC, Canada

South East New Territories Landfill, Hong Kong This site operated by Green Valley Landfill Ltd., installed two Cat G3516 landfill generator sets in 1997. Each unit is rated at 970 kW, providing 1.9 MW of continuous power for the landfill infrastructure and an on-site wastewater treatment plant (see Fig. 7). The units operate in parallel with the local utility, exporting excess power to the grid. The generator sets have oversized radiators to compensate for tropical heat and humidity.

Fig. - South East New Territories Landfill, Hong Kong

Case 5 Biogas Applications

Biogas is produced through the natural anaerobic decomposition or fermentation of organic waste, such as manure, municipal solid waste, biodegradable waste or any other biodegradable feedstock within an anaerobic environment. Biogas consists primarily of methane (50-80 percent) and carbon dioxide (20-50 percent). Biogas can be extracted for commercial use from almost any of its sources. For example, some livestock farms or large feeder operations use a lagoon to store the manure generated by their livestock. Instead of releasing the methane and carbon dioxide generated by the decomposition of this manure into the atmosphere, the methane can be extracted and burned at the farm in biogas-fueled boilers, heaters or other gas consuming devices, including gas engines. In addition to livestock farms, other agricultural operations afford opportunities for biogas productions. For example, cassava-processing plants, which produce starch, are common in China, India and Indonesia and may utilize biogas for electric power. By tapping their biogas resources, these plants not only avoid the cost of purchasing heavy fuel oil and electricity but also reclaim valuable land that would otherwise have to be used to purify the factorys wastewater, and virtually eliminating odor and pest issues caused by large-scale decomposition of organic material.As an example of this type of DG application, let us consider the Nong Rai Farm, in Rayong, Thailand. The farm partners with the CP Group, one of the largest food suppliers in Thailand, and runs a feeder operation for more than 30,000 hogs. Nong Rai Farms consumes approximately 200 kW of power for blowers, drying systems and other auxiliary needs associated with its operations. The manure produced by its hogs is piped into a digester pond (see Fig. 8), where it generates biogas that is used to fuel the generator sets, which produce sufficient power for all of Nong Rai Farms electric power requirements.

Fig. - Buffer Tank and Digestion Process in Thailand

Case 6 Coal Mine Methane (CMM) Gas Applications

The anthropogenic release of methane (CH4) into the environment and its global warming potential continues to draw attention globally. Methane can be released into the atmosphere through sources where it naturally occurs: landfill decomposition, agriculture, gas and oil extraction systems, and coal mining activities. About 8% of total anthropogenic methane emissions come from coal mines. Globally, coal mines emit approximately 400 million metric tons or 28 billion cubic meters of carbon dioxide equivalent annually. This amount is equivalent to consumption of 818 million barrels of oil or the carbon dioxide emissions of 64 million passenger cars. Between 1994 and 2005, a U.S. Emission decreased by over 20%, in large part due to the coal mining industrys increased recovery and utilization of drained gas. China leads the world in coal mine methane with about 14 billion cubic meters of CO2 emitted annually a 2004 measurement estimated nearly 200 million metric tons were emitted that year. Aside from the U.S. and China, other leading emitters include Ukraine, Australia, Russia and India. Once methane is released into the atmosphere it remains in it for approximately 15 years. It is a greenhouse gas with an estimated global warming potential of 21. This means that emissions of methane have an estimated effect on global warming equal to 21 times the effect of carbon dioxide. Implementing methods to use CMM instead of emitting it to the atmosphere will help mitigate global warming, improve mine safety, and productivity and generate revenues and cost savings.There are several options currently available for CMM mitigation, including reciprocating gas engines, gas turbines, industrial boilers and furnaces, and chemical processing. Other technologies like catalytic systems and fuel cells are also being developed. Two examples of this type of DG application are described next, where CMM is sequestered and used as an alternative fuel for reciprocating gas engine generator sets. This is a mature and proven technology highly effective for greenhouse gas.

MODELING AND CONTROL OF INVERTER INTERFACED DG UNITS

Basically each DG unit may have DC type or rectified generation unit (Fuel cell, solar cell, wind turbine, micro turbine), storage devices, DC-DC converter, DC-AC inverter, filter, and transformer for connecting to loads or utility in order to exchange power. Model and dynamic of each of this part may have influence in system operation. But here for simplification it is considered that DC side of the units has sufficient storage and considered as a constant DC source. Hence only DC-AC inverter modeling and control investigated in this paper.A circuit model of a three-phase DC to AC inverter with LC output filter is further described in Figure As shown in the figure, the system consists of a DC voltage source (Vdc), a three- phase PWM inverter, an output filter (Lf and C with considering parasitic resistance of filter- Rf). Sometimes a transformer may be used for stepping up the output voltage and hence Lf can be transformer inductance.

Figure PWM inverter diagram

There are two ways for controlling an inverter in a distributed generation system

A. PQ Inverter Control

This type of control is adopted when the DG unit system is connected to an external grid or to an island of loads and more generators. In this situation, the variables controlled by the inverter are the active and reactive power injected into the grid, which have to follow the set points Pref and Qref, respectively. These set points can be chosen by the customer or by a central controller. The PQ control of an inverter can be performed using a current control technique in qd reference frame which the inverter current is controlled in amplitude and phase to meet the desired set-points of active and reactive power.With the aim of Park transform and equations between inverter input and output, the inverter controller block diagram for supplying reference value of Pref and Qref is as figures. For the current controller, two Proportional-Integral (PI) regulators have been chosen in order to meet the requirements of stability of the system and to make the steady state error be zero. With this control scheme, it is possible to control the inverter in such way that injects reference value of Pref, Qref into other part of stand-alone network. When the output voltage is needed to be regulated, the PV control scheme that is similar to PQ mode with feedback of voltage used to adjust Qref.

Figure : PQ control scheme of inverter

B. Vf Inverter ControlThis controller has to act on the inverter whenever the system is in stand-alone mode of operation. In fact in this case it must regulate the voltage value at a reference bus bar and the frequency of the whole grid. A regulators work in order to keep the measured voltages upon the set points. Moreover the frequency is imposed through the modulating signals of the inverter PWM control by mean of an oscillator. A simple PI controller can regulate bus voltage in reference value with getting feedback of real bus voltage. Figure outlines this control strategy. In this case it is obvious that the DG unit should have storage device in order to regulate the power and voltage.

Figure: Vf control scheme of inverter

POWER QUALITYThe contemporary container crane industry, like many other industry segments, is often enamored by the bells and whistles, colorful diagnostic displays, high speed performance, and levels of automation that can be achieved. Although these features and their indirectly related computer based enhancements are key issues to an efficient terminal operation, we must not forget the foundation upon which we are building. Power quality is the mortar which bonds the foundation blocks. Power quality also affects terminal operating economics, crane reliability, our environment, and initial investment in power distribution systems to support new crane installations. To quote the utility company newsletter which accompanied the last monthly issue of my home utility billing: Using electricity wisely is a good environmental and business practice which saves you money, reduces emissions from generating plants, and conserves our natural resources. As we are all aware, container crane performance requirements continue to increase at an astounding rate. Next generation container cranes, already in the bidding process, will require average power demands of 1500 to 2000 kW almost double the total average demand three years ago. The rapid increase in power demand levels, an increase in container crane population, SCR converter crane drive retrofits and the large AC and DC drives needed to power and control these cranes will increase awareness of the power quality issue in the very near future.POWER QUALITY PROBLEMSFor the purpose of this article, we shall define power quality problems as:Any power problem that results in failure or misoperation of customer equipment manifests itself as an economic burden to the user, or produces negative impacts on the environment.When applied to the container crane industry, the power issues which degrade power quality include: Power Factor Harmonic Distortion Voltage Transients Voltage Sags or Dips Voltage SwellsThe AC and DC variable speed drives utilized on board container cranes are significant contributors to total harmonic current and voltage distortion. Whereas SCR phase control creates the desirable average power factor, DC SCR drives operate at less than this. In addition, line notching occurs when SCRs commutate, creating transient peak recovery voltages that can be 3 to 4 times the nominal line voltage depending upon the system impedance and the size of the drives. The frequency and severity of these power system disturbances varies with the speed of the drive. Harmonic current injection by AC and DC drives will be highest when the drives are operating at slow speeds. Power factor will be lowest when DC drives are operating at slow speeds or during initial acceleration and deceleration periods, increasing to its maximum value when the SCRs are phased on to produce rated or base speed. Above base speed, the power factor essentially remains constant. Unfortunately, container cranes can spend considerable time at low speeds as the operator attempts to spot and land containers. Poor power factor places a greater kVA demand burden on the utility or engine-alternator power source. Low power factor loads can also affect the voltage stability which can ultimately result in detrimental effects on the life of sensitive electronic equipment or even intermittent malfunction. Voltage transients created by DC drive SCR line notching, AC drive voltage chopping, and high frequency harmonic voltages and currents are all significant sources of noise and disturbance to sensitive electronic equipmentIt has been our experience that end users often do not associate power quality problems with Container cranes, either because they are totally unaware of such issues or there was no economic Consequence if power quality was not addressed. Before the advent of solid-state power supplies, Power factor was reasonable, and harmonic current injection was minimal. Not until the crane Population multiplied, power demands per crane increased, and static power conversion became the way of life, did power quality issues begin to emerge. Even as harmonic distortion and power Factor issues surfaced, no one was really prepared. Even today, crane builders and electrical drive System vendors avoid the issue during competitive bidding for new cranes. Rather than focus on Awareness and understanding of the potential issues, the power quality issue is intentionally or unintentionally ignored. Power quality problem solutions are available. Although the solutions are not free, in most cases, they do represent a good return on investment. However, if power quality is not specified, it most likely will not be delivered.

Power quality can be improved through: Power factor correction, Harmonic filtering, Special line notch filtering, Transient voltage surge suppression, Proper earthing systems.In most cases, the person specifying and/or buying a container crane may not be fully aware of the potential power quality issues. If this article accomplishes nothing else, we would hope to provide that awareness.In many cases, those involved with specification and procurement of container cranes may not be cognizant of such issues, do not pay the utility billings, or consider it someone elses concern. As a result, container crane specifications may not include definitive power quality criteria such as power factor correction and/or harmonic filtering. Also, many of those specifications which do require power quality equipment do not properly define the criteria. Early in the process of preparing the crane specification: Consult with the utility company to determine regulatory or contract requirements that must be satisfied, if any. Consult with the electrical drive suppliers and determine the power quality profiles that can be expected based on the drive sizes and technologies proposed for the specific project. Evaluate the economics of power quality correction not only on the present situation, but consider the impact of future utility deregulation and the future development plans for the terminalTHE BENEFITS OF POWER QUALITYPower quality in the container terminal environment impacts the economics of the terminal operation, affects reliability of the terminal equipment, and affects other consumers served by the same utility service. Each of these concerns is explored in the following paragraphs.1. Economic ImpactThe economic impact of power quality is the foremost incentive to container terminal operators. Economic impact can be significant and manifest itself in several ways:

a. Power Factor PenaltiesMany utility companies invoke penalties for low power factor on monthly billings. There is no industry standard followed by utility companies. Methods of metering and calculating power factor penalties vary from one utility company to the next. Some utility companies actually meter kVAR usage and establish a fixed rate times the number of kVAR-hours consumed. Other utility companies monitor kVAR demands and calculate power factor. If the power factor falls below a fixed limit value over a demand period, a penalty is billed in the form of an adjustment to the peak demand charges. A number of utility companies servicing container terminal equipment do not yet invoke power factor penalties. However, their service contract with the Port may still require that a minimum power factor over a defined demand period be met. The utility company may not continuously monitor power factor or kVAR usage and reflect them in the monthly utility billings; however, they do reserve the right to monitor the Port service at any time. If the power factor criteria set forth in the service contract are not met, the user may be penalized, or required to take corrective actions at the users expense. One utility company, which supplies power service to several east coast container terminals in the USA, does not reflect power factor penalties in their monthly billings, however, their service contract with the terminal reads as follows:The average power factor under operating conditions of customers load at the point where service is metered shall be not less than 85%. If below 85%, the customer may be required to furnish, install and maintain at its expense corrective apparatus which will increase thePower factor of the entire installation to not less than 85%. The customer shall ensure that no excessive harmonics or transients are introduced on to the [utility] system. This may require special power conditioning equipment or filters.The Port or terminal operations personnel, who are responsible for maintaining container cranes, or specifying new container crane equipment, should be aware of these requirements. Utility deregulation will most likely force utilities to enforce requirements such as the example above.Terminal operators who do not deal with penalty issues today may be faced with some rather severe penalties in the future. A sound, future terminal growth plan should include contingencies for addressing the possible economic impact of utility deregulation.b. System LossesHarmonic currents and low power factor created by nonlinear loads, not only result in possible power factor penalties, but also increase the power losses in the distribution system. These losses are not visible as a separate item on your monthly utility billing, but you pay for them each month. Container cranes are significant contributors to harmonic currents and low power factor. Based on the typical demands of todays high speed container cranes, correction of power factor alone on a typical state of the art quay crane can result in a reduction of system losses that converts to a 6 to 10% reduction in the monthly utility billing. For most of the larger terminals, this is a significant annual saving in the cost of operation.C. Power Service Initial Capital InvestmentsThe power distribution system design and installation for new terminals, as well as modification of systems for terminal capacity upgrades, involves high cost, specialized, high and medium voltage equipment. Transformers, switchgear, feeder cables, cable reel trailing cables, collector bars, etc. must be sized based on the kVA demand. Thus cost of the equipment is directly related to the total kVA demand. As the relationship above indicates, kVA demand is inversely proportional to the overall power factor, i.e. a lower power factor demands higher kVA for the same kW load. Container cranes are one of the most significant users of power in the terminal. Since container cranes with DC, 6 pulse, SCR drives operate at relatively low power factor, the total kVA demand is significantly larger than would be the case if power factor correction equipment were supplied on board each crane or at some common bus location in the terminal. In the absence of power quality corrective equipment, transformers are larger, switchgear current ratings must be higher, feeder cable copper sizes are larger, collector system and cable reel cables must be larger, etc. Consequently, the cost of the initial power distribution system equipment for a system which does not address power quality will most likely be higher than the same system which includes power quality equipment. 2. Equipment ReliabilityPoor power quality can affect machine or equipment reliability and reduce the life of components. Harmonics, voltage transients, and voltage system sags and swells are all power quality problems and are all interdependent. Harmonics affect power factor, voltage transients can induce harmonics, the same phenomena which create harmonic current injection in DC SCR variable speed drives are responsible for poor power factor, and dynamically varying power factor of the same drives can create voltage sags and swells. The effects of harmonic distortion, harmonic currents, and line notch ringing can be mitigated using specially designed filters.3. Power System AdequacyWhen considering the installation of additional cranes to an existing power distribution system, a power system analysis should be completed to determine the adequacy of the system to support additional crane loads. Power quality corrective actions may be dictated due to inadequacy of existing power distribution systems to which new or relocated cranes are to be connected. In other words, addition of power quality equipment may render a workable scenario on an existing power distribution system, which would otherwise be inadequate to support additional cranes without high risk of problems.4. EnvironmentNo issue might be as important as the effect of power quality on our environment. Reduction in system losses and lower demands equate to a reduction in the consumption of our natural nm resources and reduction in power plant emissions. It is our responsibility as occupants of this planet to encourage conservation of our natural resources and support measures which improve our air quality.

ACTIVE POWER FILTERSActive Filters are commonly used for providing harmonic compensation to a system by controlling current harmonics in supply networks at the low to medium voltage distribution level or for reactive power or voltage control at high voltage distribution level. These functions may be combined in a single circuit to achieve the various functions mentioned above or in separate active filters which can attack each aspect individually. The block diagram presented in section shows the basic sequence of operation for the active filter. This diagram shows various sections of the filter each responding to its own classification. Classification of active filters The block diagram shown in figure represents the key components of a typical active power filter along with their interconnections. The reference signal estimator monitors the harmonic current from the nonlinear load along with information about other system variables. The reference signal from the current estimator, as well as other signals, drives the overall system controller. This in turn provides the control for the PWM switching pattern generator. The output of the PWM pattern generator controls the power circuit through a suitable interface. The power circuit in the generalized block diagram can be connected in parallel, series or parallel/series configurations, depending on the transformer used.

Active power filters according to can be classified based on the following criteria: 1. Power rating and speed of response required in compensated systems; 2. Power-circuit configuration and connections; 3. System parameters to be compensated; 4. Control techniques employed; and 5. Technique used for estimating the reference current/voltage. Classification according to power rating and speed of response in compensated system The block diagram shown in figure shows the classification based on this criterion. The size of nonlinear loads play a major role in deciding the way different control methods are implemented. The filter required for compensation must be practical for the load and this decision affects the speed of response. In general a reciprocal relationship exists between the cost of a particular system to the required speed of response.

Low power applications Low power applications govern applications with a power rating below 100kVA. Applications of these sizes are generally associated with residential areas, commercial buildings, hospitals and for a wide range of medium sized factory loads and motor drive systems. Active filters chosen for this power range employ sophisticated techniques catering with high pulse number PWM voltage or current source inverters. The response time for smaller applications is relatively much faster than other sizes ranging from ten microseconds to ten milliseconds. This type comprises the following two categories. Single-phase systems Low power rating loads generally require single phase active filters. They are generally most employed in commercial buildings with a large number of computers. This application means that current harmonics can be treated at the point of common coupling (PCC). It is often economical and practical to install single phase active filters on distribution based sites of reduced size capacity than a larger rated filter installed upstream. This is due to the large number of the single-phase loads within one building and the harmful consequences associated with the presence of large amounts of harmonic in the neutral line. This allows for more selective compensation as the operating conditions vary. Due to the load capacity drawn from residential loads, it is rare for a high concentration of harmonics, and thus the impacts on the neutral lines are not significant. Residential customers tend not invest in purchasing active filters because there are no compulsory harmonic regulations however, the main advantage of such an installation are that operating frequencies can be increased moving to improved performance since only low ratings are employed. Three-phase systems The installation of three-phase filters is used for three-phase applications. Different filter configurations can be tested and installed based upon whether the loads are balanced or unbalanced. At levels below 100kVA, a three phase filter can be reconfigured to compensate for three individual single phases in one unit or for a single three-phase supply. When nonlinear loads are balanced, meaning all three phased impedances are equal, a single three-phase-inverter configuration is employed. This choice of inverter is used when the objective is to eliminate as many current harmonics as possible, assuming that the magnitudes and respective phase angles in each phase are the same. In the situation when nonlinear loads are unbalanced, or supply voltages are unsymmetrical, three single phase inverter circuits are used. Medium power applications Power systems ranging between 100kVA to 10MVA fit the class of a medium power application. Due to the fact that phase unbalances are reduced on this sized system, the major objective is to eliminate current harmonics. In general, capacitive and Inductive static compensators, line-commutated thyristor converters, synchronous condensers and cascaded multilevel-inverter VAR compensators, are often more economic as reactive power compensation using active filters often is not viable. This is due to the high voltage as well as problems with isolation and series/parallel connection of switches. The speed of response expected in this range is of the order of tens of milliseconds.

High power applications At high power ratings, the use of active filters becomes very uneconomical. This is because of the lack of high switching frequency power devices that can control the current flow. Thus, this is a major disadvantage for such systems. In addition, even the latest advances in semiconductor technology still fall short as extra high voltages of a few hundred kilovolts cannot be tolerated. The series-parallel combination is possible however; implementation is difficult and also cost-ineffective. Harmonic pollution upstream affecting high power ranges above 10MVA is not such a problem compared against low power systems. The implementation of single and three phase filters downstream at the low voltage system provides suitable compensation such that significant harmonic pollution upstream is minimal. The static-VAR compensation is then the major concern and is usually compensated for by using traditional static power conditioners/filters as well as several sets of synchronous condensers connected in parallel and cascaded multilevel-inverter VAR compensators. The required response time for such cases is in the range of tens of seconds, which is sufficient for contactors and circuit breakers to operate after taking the optimal-switching decision. Power fluctuations in the range of a few seconds are, on the other hand, treated by the generating stations' ancillary devices. Classification according to power circuit, configurations and connections The choice of power circuit chosen for the active filter greatly influences its efficiency and accuracy in providing true compensation. It is therefore important that the correct circuit configuration is chosen. Figure 3.3 classes three major types of filter structures along with the relevant power circuit.

Shunt active filters Shunt active filters are by far the most widely accept and dominant filter of choice in most industrial processes. Figures show the system configuration of the shunt design. The active filter is connected in parallel at the PCC and is fed from the main power circuit. The objective of the shunt active filter is to supply opposing harmonic current to the nonlinear load effectively resulting in a net harmonic current. This means that the supply signals remain purely fundamental. Shunt filters also have the additional benefit of contributing to reactive power compensation and balancing of three-phase currents. Since the active filter is connected in parallel to the PCC, only the compensation current plus a small amount of active fundamental current is carried in the unit. For an increased range of power ratings, several shunt active filters can be combined together to withstand higher currents. This configuration consists of four distinct categories of circuit, namely inverter configurations, switched-capacitor circuits, lattice-structured filters and voltage-regulator-type

Series active filters The objective of the series active filter is to maintain a pure sinusoidal voltage waveform across the load. This is achieved by producing a PWM voltage waveform which is added or subtracted against the supply voltage waveform. The choice of power circuit used in most cases is the voltage-fed PWM inverter without a current minor loop. The active filter acts as a voltage source and thus it is often a preferred solution of harmonic producing loads such as large capacity diode rectifiers with capacitive loads. In general, series active filters are less commonly used against the shunt design. Unlike the shunt filter which carries mainly compensation current, the series circuit has to handle high load currents. This causes an increased rating of the filter suitable to carry the increased current. Series filters offer the main advantage over the shunt configuration of achieving ac voltage regulation by eliminating voltage-waveform harmonics. This means the load contains a pure sinusoidal waveform.

Other combinations In some cases, the combinations of shunt and series active filters provide a greater effectiveness in eliminating harmonic pollution from the system. Combination of both shunt and series active filters The diagram shown in figure shows the combination of both parallel and series active filters. This system combines both the benefits of the shunt and series and is often used to achieve the demanding power system requirements. The control of active filters can be complex. A combination of the two provides an even greater complexity. The higher cost involved in a more complex design has shown a reduced demand for the combined structure. As a result of the increased cost and complexity, this combination has received less attention than other configurations. Flexible AC transmission systems, commonly abbreviated as FACTS regularly make use of the arrangement. Combination of series active and shunt passive filters The combination of the active parallel and active series filters in 3.4.3.1 was seen to be very complex in control yielding a high cost. One method of reducing these problems was to replace the parallel active filter with a passive structure. The series active filter, which constitutes high impedance for high-frequency harmonics, is accompanied by a parallel passive filter to provide a path for the harmonic currents of the load. This combination, represented by figure, permits an improvement over the characteristics of plain series active filters and the extension of their capabilities to include current- harmonic reduction and voltage- harmonic elimination. Passive filters are often easier and simple to implement and do not require any control circuit. This, this deserves to be most beneficial.

Combination of shunt active and passive filters Shunt active filters are best suitable to compensate for lower order harmonics thus only requiring low power rating which serves most economical. This configuration makes use of a passive filter which serves to compensate for the high order load current harmonics. This combination, represented by figure presents this important configuration. Combinations such as this can be designed to compensate for higher powers without excessive costs for high-power switching. The major disadvantage of this configuration is the fact that passive filters can only be tuned for a specific predefined harmonic and thus cannot be easily changed for loads which have varying harmonics.

Active filter in series with shunt passive filters The combination of an active filter in series with a shunt passive filter is considered a significant design configuration for medium and high voltage applications. The passive filter is designed to reduce the voltage stress applied to the switches in the active filter. This design is in its infancy of development however, further research is still needed to assess the effectiveness of the configuration.

Classification according to compensated variable Active filters are designed to provide suitable compensation for a particular variable or a multiple of sorts in cases of combination structures. Figure shows the variety of compensated variable that active filters can provide for.

Reactive power compensation The shunt active filter does provide reactive power compensation however; they rarely treat the problem of power-factor correction on its own owing to the fact that other quasidynamic, cheaper and slower-in-response reactive-power compensators are available in the market. When this technique is applied, lower power applications are more suited since the currents needed for reactive-power compensation are of the same order of magnitude as the rated current of the load. It would be a waste of sophisticated equipment to tackle them without the use of other power factor-correction devices, such as thyristor-controlled reactors and capacitors; especially in single-phase systems, where in certain specific applications the requirement is for accurate compensation without harmonics generation.Harmonic compensation Within the system, active filters can be used to provide suitable harmonic compensation for voltage harmonics and current harmonics. These harmonic are the most important variable requiring compensation. Compensation of voltage harmonics In general, the concern for compensating voltage harmonics is not high due to the fact that power supplies usually have low impedance. Generally, at the point of common coupling, ridged standards are implemented to ensure a correct level of total harmonic distortion (THD) and voltage regulation is maintained. The problem of compensating for voltage harmonics is to ensure the supply to be purely sinusoidal. This is important for harmonic voltage sensitive devices such as power system protection devices and superconducting magnetic energy storage. Voltage harmonics are related to current harmonics by the impedance of the line. Although compensation of voltage harmonics helps to provide a reduction in current harmonics, this however, does not negate the necessity to current harmonic compensation. Compensation of current harmonics Current harmonic compensation strategies are exceptionally important. Current harmonics are greatly reduced by the compensation of voltage harmonics at the consumers point of common coupling. The reduction in current harmonics is not only important for reasons such as device heating and reduction in life of devices but also in design of power system equipment. One of the major design criteria covers the magnitude of the current and its waveform. This is to reduce cable and feeder losses. Since the root mean square (RMS) of the load current incorporates the sum of squares of individual harmonics, true current harmonic compensation will aid system designers for better approached power rating equipment. Balancing of three phase systems In most low and medium voltage distribution systems, it is frequent to find situations where the currents and voltages in the three phases are not balanced and are not evenly distributed by 120 degrees. Balancing of mains voltage in three phase systems Voltage imbalance is a situation where each phase voltage is unequal in magnitude and is not displaced by 120 degrees. This is a direct result of current imbalances and the severity of the system imbalances is governed by the magnitude of the supply impedance. The solution to this problem is to add or subtract the corresponding amount of instantaneous voltage to force it to follow the reference sinusoidal waveform. On high voltage systems, the supply impedance does not impact severely on system performance and thus the problem of mains voltage unbalances are primarily related to low rating systems. Balancing of mains current in three phase systems In low power applications such as compensating for residential loads, the magnitude of currents supplied to the grid depends entirely upon the level of imbalance in the system. In most cases, the compensator would be forced to supply rated current. This places a limitation on the power handling capability.Multiple compensationTo target a variety of variables requiring compensation, often it is usual to combine different combinations to improve the effectiveness of the filter. The following are the most frequently used combinations. Harmonic current with reactive power compensation One very common filter design makes use of combining aspects of reactive power compensation together with harmonic current elimination. This ensures the supply current remains purely fundamental free from distributing harmonics whilst making certain the current is in phase with the supply voltage. This approach is very cost effective because only one device is used for all aspects rather than including multiple circuits for each individual objective. The active filter used here however, suffers from poor power switching limits and thus can only serve as a compensator for low powered applications. Harmonic voltages with reactive power compensation This combination, however rare, takes place in certain configurations for controlling the voltage harmonics, which would normally affect indirectly (using suitable feedback) the reactive-power compensation. This compensation system is only suitable for low-power applications. Harmonic current and voltages To compensate for both current and voltage system harmonics, a shunt and series active filter configuration must be used respectively. Integrating this filter serves to eliminate load harmonics whilst ensuring the supply remains fundamental. This type of design contains very complex control algorithms and is normally used only for very sensitive devices such as power-system-protection equipment and superconducting magnetic-energy storage systems. Harmonic current and voltages with reactive power compensation This filter design incorporates all three compensating variables into one unit. It controls all harmonics and reactive power within the system. This is achieved by implementing of a parallel/series active filter combination. The control for this design is very complex and difficult to maintain and thus is not often employed.

Classification based upon control technique Figure presents the basic control structure for active power system filters. The two main techniques are open look control and closed loop control.

Open loop systems Open-loop systems sense the load current and the harmonics it contains. They inject a fixed amount of power in the form of current (mainly reactive) into the system, which may compensate for most of the harmonics and/or reactive power available. Since there is no feedback loop on this system, there is no reference to check the performance and accuracy of the filter. This is a traditional technique and in present day is not often used. Closed loop systems Closed loop control systems incorporate a feedback loop providing greater accuracy of current injection for harmonic compensation as well as reactive power reduction well over the open loop design. This feature enables true sensing of the required variables under consideration. Almost all new techniques in use are of this type.

Constant capacitor voltage technique In this technique, the DC link contains a capacitor and once charged, this capacitor voltage is the voltage source which controls the current waveform by PWM techniques. The voltage across the terminals of the capacitor often fluctuates due to the fact that energy is either supplied or expelled. To regulate and maintain terminal voltage levels, a reference voltage is chosen. The difference between the actual capacitor voltage and the predefined reference voltage determines the active component of power required to compensate for losses in the filter. This error difference is added to the current-controller error signal to determine the overall system error to be processed by the current controller. This technique is widely accepted and is very popular. Constant inductor current technique The control replaces the use of the capacitor in the DC link with an inductor. The system operates much the same as mentioned in 3.6.2.1 however; the capacitor voltage is replaced with the inductor current. This is achieved in two ways: (i) current pulse-width modulation where like in the PWM provides the required pulses to represent the average current signal and (ii) current pulse amplitude modulation which is a new control method provides the active filter with a basis for amplitude modulation rather than solely the width. Optimization technique The optimization procedure for switched-capacitor and lattice-filter circuits is the same. The rate of rise of the current and the amplitude depend mainly on the size of the capacitors and the initial voltages on them. These factors are functions of the switching patterns, and they provide considerable flexibility in shaping the waveform of the current drawn by the filter. The key to controlling these filter configurations is to determine the appropriate switching function for the switches. The main task of the system controller is to minimize a predetermined number of individual load-current harmonics, in addition minimizing either the THD or the fundamental component of the filter current. However, this is not performed instantaneously. A time delay exists between the detection of a change in the harmonic current and the application of the new set of switching angles obtained from the optimization procedure. This system is mainly suitable for constant or slowly varying loads. Linear voltage control technique Series active filters incorporating the additional benefit of voltage regulation can be controlled using the linear voltage control technique. Through regularly charging and discharging the capacitor through linear control, the capacitor voltage can be regulated. The reference capacitor voltage can be determined based upon the harmonic reference. The charge in the supply loop of the circuit and thus switching frequency can be controlled by the regular variations of the capacitor voltage in contrast to the abrupt changes in inverter voltage waveforms. This technique ensures that the supply side receives no abrupt variation of voltage and this reduces the amount of high-frequency harmonics injected into the supply due to the presence of the PWM inverter. Other techniques Other control techniques exist that simply provide small changes to the aforementioned techniques, providing simply newer or better performance over their predecessors. These techniques may include the use of state of the art adaptive, predictive and sliding-mode controllers, which are normally difficult to implement without the use of Digital Signal Processing (DSP). These techniques can be implemented in either the time domain or the frequency domain. Active filters harmonic detection and extraction A shunt active filter acts as a controllable harmonic current source. In principle, harmonic compensation is achieved when the current source is commanded to inject harmonic currents of the same magnitude but opposite phase to the load harmonic currents. Before the inverter can subtly inject opposing harmonic currents into the power system, appropriate harmonic detection strategies must be implemented to efficiently sense and determine the harmonic current from the nonlinear load. Types of harmonic detection strategies There are 3 different types of harmonic detection strategies used to determine the current reference for the active filter. These are 1. Measuring the load harmonic current to be compensated and using this as a reference command; 2. Measuring source harmonic current and controlling the filter to minimize it; and 3. Measuring harmonic voltage at the active filter point of common coupling (PCC) and controlling the filter to minimize the voltage distortion.

Load current sensing This method involves measurement of the load current and subsequent extraction of its harmonic content using a high pass filter scheme. The harmonic components, so extracted, are adjusted for polarity and used as reference commands for the current controller. This is explained with the help of equation 3.1 and figure 3.14. Denoting the harmonic components of the load current by, the describing equation for this strategy is lhi

Source current sensing In this strategy, the source current is measured and its harmonic component extracted. This is scaled by a suitable controller, generally of the proportional type. The output of the proportional controller is provided as a reference to the current controller. This is schematically represented in figure and analytically expressed by equation. Denoting the harmonic components of the source current by shi, the describing equation for this strategy is

Point of Common Coupling (PCC) voltage sensing This method requires measurement of the harmonic component of the Point of Common Coupling (PCC) voltage, e(t). The harmonic component is then used to generate the current reference, after passing it through a proportional controller. Schematically, it is represented in figure and analytically expressed by equation Denoting the harmonic components of the PCC voltage by, the describing equation for this strategy is

Load current sensing and supply current sensing are suitable for shunt active filters installed in the vicinity of one or more harmonic producing loads by individual high-powered consumers. PCC voltage sensing is suitable for shunt active filters, which will be installed on distribution systems by utilities. Supply current detection is the most basic harmonic detection method for series active filters acting as a voltage source. Classification based upon current/voltage reference estimation technique There are numerous techniques each sub classified in figure which propose methods to calculate and determine the appropriate compensating reference current used for the active filter to pass to the PWM inverter.

Current/voltage reference synthesis (continuous time-domain) In this method, an analogue signal filter is applied at the supply side to determine the current harmonics from the supply. This technique is very simple and easy to implement however introduces major amounts of magnitude and phase errors. High pass filter method This method uses a high pass filter to pass high ordered frequencies effectively removing low order components in the load current signal. The filtered frequencies constitute the reference portion. This technique however, is susceptible to noise as this is undesired.Low pass filter method This method is favored in terms of reference synthesis because unlike the high pass filter method, the effects of noise in the filtered portion are suppressed. The desired reference value is the harmonic component found in the load current. This is determined by subtracting the low order frequency component found from implementing a low pass filter from the total load current. This presents the harmonic portion from the load current waveform. This technique however, introduces large magnitude and phase errors. Current/voltage reference calculation (discrete time or frequency domain) The techniques mentioned have many disadvantages to their use namely, phase and magnitude errors as well as the effects of noise. The calculation of harmonics therefore provides the most appropriate alternative. Two major techniques are classified in either time domain or frequency domain.

Time domain approaches The following seven subdivisions of time-domain approaches are mainly used for three-phase systems except for the fictitious-power-compensation technique which can be adopted for single- or three-phase systems. The time-domain methods are mainly used to gain more speed or fewer calculations compared to the frequency-domain methods. Instantaneous reactive power algorithm Instantaneous power theory determines the harmonic distortion from the instantaneous power calculation in a three-phase system, which is the multiplication of the instantaneous values of the currents and voltages .

The values of the instantaneous power p and q, which are the real and respective imaginary powers, contain dc and ac components depending on the existing active, reactive and distorted powers in the system. The dc components of p and q represent the active and reactive powers and must be removed with high-pass filters to retain only the ac signals. The ac components converted by an inverse transformation matrix to the abc-frame represent the harmonic distortion, which is given as the reference for the current controller. These processes are depicted in figure.

This operation takes place only under the assumption that the three-phase system is balanced and that the voltage waveforms are purely sinusoidal. If, on the other hand, this technique is applied to contaminated supplies, the resulting performance is proven to be poor. Synchronous detection algorithm This technique relies in the fact that the three phase currents are balanced. The average power is calculated and divided equally between the three phases. The signal is t