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Solar Outshining Wind as the Favored Renewable Energy Source?

Solar conversion systems, which had been lagging behind large scale wind farms in the renewable energy sweepstakes, are now coming on strong. Interest in transformers to be used for solar installations has quadrupled in the past year, now clearly outstripping wind farm equipment. Key reasons for the popularity include the fact that solar systems offer more constant loading, far less harmonics, fewer problems associated with over-voltages caused by unloaded generators, and reduced voltage and load fluctuations. On the minus side, solar systems still have to contend with special issues surrounding the use of inverter technology, including effective size restrictions posed by existing limitations of inverter technology. Photovoltaic (PV) systems are used most frequently in large scale solar installations. With PV, silicon dioxide crystals use energy from solar rays to generate DC current. This can be sent either to a battery for storage or to an inverter for conversion to AC voltage for use on the grid. A specially designed step-up transformer is needed to connect the solar inverter systems. It cannot be the type now used for a wind turbine, because the operating environment in the emerging PV solar conversion process is not the same, and the transformer needs to be very different. Designers should consider a variety of parameters, summarized in Table 1, when designing a transformer for use in a solar system. It is important for designers to understand that renewable energy sources should not use a standard, off the shelf transformer, but neither can every renewable energy source use the same transformer. Understanding how the characteristics of solar power affect transformer requirements can have a huge effect on costs for an installation, as well as reliability. Table 1 Design Parameters for Solar Transformers Parameter Solar Conversion Loading Steady state loading when inverters are operating Fault ride through Not yet defined Harmonics Harmonics content is less than 1% Step-up requirements Step up duty, but without over voltages caused by unloaded generators Voltage Operation at rated voltage controlled by inverters Loading issues Operation at rated load Special design issues Design requires 2 separate inputs Size issues Size limited by inverter technology (currently at 1000 kVA) Loading In contrast to wind-powered transformers, which experience variable loading due to wind gusts, solar power facilities experience a steady state loading when inverters are operating. When the sun comes out, there is a dampened reaction process and loading on the transformer is more constant.

Electrical Substation components

A substation is a high-voltage electric facility containing equipment to regulate and distribute electrical energy. While some substations are small with little more than a transformer and associated switches, other substations are large and complex. Functions of a substation include receiving power from a generating facility, regulating distribution, stepping voltage up and down, limiting power surges, and converting power from direct current to alternating current or vice versa. Components in an electric substation Many electrical components work together in a substation to carry out its functions, these include Lightning Arresters protect a substation from voltage surges and are installed on power poles, towers, transformers and circuit breakers to protect them from damage during electrical storms. Lightning Arresters look similar to standoff insulators and bushings, but their unique characteristics is that they have earthing terminals at the bottom where a large ground cable is connected and runs down the structure that connects to the station ground. Switches - measure, regulate, and switch electrical transmissions within the substation as necessary. Switchers turn circuits on and off the grid. Distribution Bus - is an array of switches that direct power out of the substation. Distribution buses are usually pyramid-shaped or rectangular. Circuit Breakers - there are two forms of open circuit breakers, namely, dead tank and live tank. The form of circuit breaker influences the way in which the circuit breaker is accommodated, this may be as a ground mounting and plinth mounting, retractable circuit breaker and suspended circuit breakers. Current Transformers - may be accommodated in one of six manners, namely Installed over circuit breaker bushings or on pedestals In separate post type housings Installed over moving bushings of some types of insulators Installed over power transformer or reactor bushings Installed over wall or roof bushings Installed over cables In all except in separate post type housing, the current transformer occupies incidental space and does not affect the size of the layout. Installation of current transformers over isolator bushings, or bushings through walls or roofs, is usually confined to indoor substations.

SPSU photovoltaic (PV) solar converter step-up transformer

Difficulty in procuring non renewable fossil fuel, fluctuating global prices and growing environmental consciousness is pushing a growing number of industries in the United States to explore the profitability of alternate energy like wind and solar. Thankfully, the US gets solar energy in abundance and an increasing number of industries are waking up to its advantage. Recently, the country too has taken affirmative steps towards harnessing solar energy. The United States has many utility-scale solar power plants, the largest installation in the world being the Solar Energy Generating Systems facility, located in California. There are other solar power plants of varying sizes scattered over the country such as in Nevada, Florida and south-eastern California. In the Solar Energy Industries Association's "2008 U.S. Solar Industry Year in Review" it was noted that U.S. solar energy capacity increased by 17% in 2007, reaching the total equivalent of 8,775 megawatts (MW). In 2008, the Department of Energy (DOE) announced its decision to invest $17.6 million, in early-stage photovoltaic (PV) projects. More recent reports from the Solar Energy Industries Association (SEIA) and GTM Research show solar installations in 2010 are up more than 100% over 2009, and it's looking to be a very real possibility that when accounting for both solar electric and solar thermal installations, the industry could surpass the 1 GW mark for annual installations in 2010. While these are extremely positive signs that push towards greater reliability and affordability of solar energy, work is still on to increase technological advancement. In the area of power infrastructure, namely in the area of production and distribution of solar energy, it is energy transformers that play a critical role. Thus far energy transformers were built to step up or step down energy from non renewable sources, however transmitting and distributing energy from renewable sources like the sun come with their own challenges to energy transformers. What differentiates solar energy transformers? A company like Pacific Crest Transformers has been engaged in transformer manufacturing for nearly a century and feels that the solar energy sector cannot use an 'off the shelf transformer' as its needs a different and highly specialized, this specialized need in turn has an impact on construction, installation and overall cost of a transformer.

Grounding Transformer FAQs

1. What is the purpose for a grounding transformer Simply put, a grounding transformer is used to provide a ground path to either an ungrounded "Y" or a delta connected system and are used to.... Provide a relatively low impedance path to ground, thereby maintaining the system neutral at or near ground potential. Limit the magnitude of transient over voltages when re-striking ground faults occur. Provide a source of ground fault current during line to ground faults. Permit the connection of phase to neutral loads is desired. If a single line to ground fault occurs on an ungrounded or isolated system, no return path exists for the fault current, thus no current flows. The system will continue to operate but the other two un-faulted lines will rise in voltage by the square root of 3 resulting in overstressing of the transformer insulation, and other associated components on the system, by 173%. MOV lightning arresters are particularly susceptible to damage from heating by leakage across the blocks even if the voltage increase is not sufficient to flash over. A grounding transformer provides a ground path to prevent this. 2. What is the kVA rating for my grounding transformer? Unless you are using the grounding transformer to provide auxiliary power, there is no kVA rating, because the grounding transformer does not function as a power source. During "normal operation" no current flows in the grounding circuit because the system is balanced and no neutral current occurs. During a fault, the duration is limited to seconds in the extreme and a few cycles in most cases. Some designers talk about a "fault power" rating but this is time sensitive and not a true "kVA" power rating. A grounding transformer will be labeled for grounding use and rated by the continuous current and fault current it is designed to carry. 3. What current rating do I need to order my grounding transformer? You need to know the available neutral fault current and duration. This value is needed to calculate the short time heating that results from a fault on the system and should be determined from a engineered system study. Typical values for this range from a few hundred amps to a few thousand amps with duration times expressed in seconds and not cycles. For instance a value of 400 amps for 10 seconds is typical. The fault duration is a critical parameter for the transformer designer. Where protection schemes use the grounding transformer for tripping functions, a relatively short time duration is specified ( 5 -10 seconds). On the other hand, a continuous or extended neutral fault current duration would be required when the grounding transformer is used in a ground fault alarm scheme. 4. What is meant by "continuous current"? Are there guidelines for this? You will also be asked to supply the continuous current. You can choose to provide the continuous neutral current or the continuous phase current. This is usually considered to be zero if the system is balanced, however for the purposes of designing a grounding transformer it is a value that is expected to flow in the neutral circuit without tripping protective circuits (which would force the current to be zero) or the leakage current to ground that is not a symmetrical function. Again this value is needed to design for thermal capacity of the grounding transformer. When the continuous current is not known, ANSI/IEEE Std. 32-Reaffirmed 1990 , provides a guideline based on the fault current magnitude. If the value is not specified, the designer will assume the continuous current to be 3% of the short time fault current (based on a 10 second rating) 5. Why is the neutral current 3 times the phase current. When an ungrounded system experiences a ground fault event, the grounding transformer provides the return path for the fault currents. The transformer, "sees" this fault current as a zero sequence fault current, meaning it occurs on all three phase simultaneously. In a three phase transformer with equal impedances to ground on all three legs, the current will divide and flow equally in all three phases simultaneously with 1/3 the fault current in each. 6. Why is the impedance so critical in my grounding transformer? When current flows in the grounding transformer windings a voltage will be developed by the well known formula ( E = IR) where the resistive component is the impedance. Clearly this is different for every location and application, however we can say that because of the magnitude associated with fault currents, if too high a value is given for the impedance, during the fault the resulting voltage can exceed design limits. It is important to remember that one function of the grounding transformer is to provide voltage support for the faulted leg, thus holding that leg above ground and limiting "neutral shift". In all cases it should be chosen so that the un-faulted phase voltages during a ground fault are within the temporary over-voltage capability of the transformer and associated equipment.

Transformer Protection

Transformers of varied sizes and configurations are at the heart of all power systems. As a critical and an expensive component of the power systems, transformers play an important role in power delivery and the integrity of the power system network as a whole. Transformers, however, have operating limits beyond which the transformer loss of life can occur. If subjected to adverse conditions there can be a heavy damage to the system and system equipment, besides intolerable interruption of service to the customers. Since the lead time for repair and replacement of transformers is usually very long, limiting the damage to faulted transformers is the foremost objective of transformer protection. Economic impact of a transformer failure The direct economic impact of repairing or replacing the transformer. The indirect economic impact due to production loss. Operating conditions like transformer overload, through faults, etc often result in transformer failure, highlighting a need for transformer protection functions, such as over excitation protection and temperature-based protection. Extended functioning of the transformer under abnormal condition such as faults or overloads can compromise the life of the transformer. Adequate protection should be provided for quicker isolation of the transformer under such conditions. The type of protection used should reduce the disconnection time for faults within the transformer and minimize the risk of catastrophic breakdown to simplify eventual repair. Transformer Failure The risk of a transformer failure is two-dimensional: the frequency of failure, and the severity of failure. Most often transformer failures are a result of " insulation failure ". This category includes inadequate or defective installation, insulation deterioration, and short circuits, as opposed to exterior surges such as lightning and line faults. Failures in transformers can be classified into Winding failures resulting from short circuits (turn-turn faults, phase-phase faults, phase-ground, open winding) Core faults (core insulation failure, shorted laminations) Terminal failures (open leads, loose connections, short circuits) On-load tap changer failures (mechanical, electrical, short circuit, overheating) Abnormal operating conditions (overfluxing, overloading, overvoltage) External faults

ONAN or ONAF, What is the difference?

One characteristic of all transformers, regardless of size, style, or construction, is that when energized, they create losses in the circuit. Some of these losses are from energizing the core and creating a magnet field, and some losses are resistive losses (IR) from load currents flowing in the conductors of windings. Both types manifest themselves in the form of heat, and heat is the number one enemy of insulation material. The task for transformer designers is thus to allow transformers to dissipate excess heat and thereby ensure longer insulation life. For air cooled transformers this is accomplished by providing adequate ventilation and cooling ducts in the coils. Where there is not enough air flow, fans are added to increase heat transfer away from magnetic elements and vulnerable dialectic insulating components. For liquid filled transformers the approach is similar. Cooling ducts in the coils must be in sufficient number and size to allow dielectric fluid to flow through the coils to remove heat. This fluid can move by simple convection, or it can be "force cooled" by pumping fluid. Additionally, the tank surface must be large enough to transfer heat away from the fluid by a combination of conduction, convection, and radiation. As transformers get larger, tank surface area becomes a constraint, and external radiators are added to increase the surface area for heat transfer. To maximize this process, cooling fans can be added to expedite the heat removal through radiators. How do transformer manufacturers indicate information on transformer rating plates? For dry type transformers which are air cooled, ANSI/IEEE Standard C57.12.01 provide the following designations: Ventilated self-cooled class : Class AA Ventilated forced-air-cooled class : Class AFA Ventilated self-cooled / forced-air-cooled class : Class AA/FA Non-Ventilated self-cooled class : Class ANV Sealed self-cooled class : Class GA

Modern Transformer Design

Power transformers are at the heart of electrical transmission and distribution systems, and as competition increases within the energy sector, so does the pressure on transformer manufacturing industry to improve reliability and reduce costs of transformers. The power transformer concept was conceived and developed in the late 1800's and since then, the basic concept of transformer has remained the same. However, design and construction techniques have improved to increase both - the overall efficiency and cost effectiveness of manufactured units. Why Modern Transformer Design With superior expertise in designing coupled with extensive R&D efforts, modern transformers are much smaller in size, lower in cost, and are able to promise a remarkable increase in efficiency and reduce lost energy. Especially for countries like the US, modern transformer design can play a significant role in reduction of energy loss. The U.S. has only 4% of the world's population but produces 25% of its greenhouse gases. The country has over 9,200 electricity generating units much of them old, needing replacement and thus largely inefficient. Since 1982, growth in peak demand for electricity in the U.S. has exceeded transmission growth by almost 25% each year, even while a majority of the energy transformers in the country continue to waste away large amounts of energy. Better transformer design and the use of superior grade electrical steel can drastically reduce no-load loss, one of the prime components of loss in an energy transformer. No-load loss can be further reduced in some cases if conventional electrical steel can be replaced with amorphous metal. Types of Transformer Designs Transformer life expectancy is based on a number of factors, the most important being the quality of its insulation system. Two things that damage transformer insulation are moisture and excessive heat. Addressing these two factors, modern transformer designs are developed to preserve overall insulation quality of the transformer. Some of these designs include open style, sealed tank, conservator style, and automatic gas pressure. Open Style Design - is a tank design that has an air or gas space in the main tank above the oil level. The benefit of open style design transformer is in its lower initial cost; however, this is the least effective method of protecting a transformer's insulating system. Sealed Tank Design - In the sealed tank design, the core/coils and oil are completely enclosed in the main tank with no ventilation to the atmosphere. This offers better protection against the ingression of moisture and contaminants into the insulation of the transformer. One drawback of this style of transformers, however, is that if a weld, flange or gasket develops a leak in the gas head space above the oil, there will be a direct exchange of the oil with the outside atmosphere. Conservator Type Design - The conservator design has the main tank completely filled with oil and a smaller expansion tank positioned above the main tank, with about 5 to 10 per cent the volume of the main. This design is good to protect transformer insulation.

Can energy transformers reduce costs for the Aluminum industry in the US

The United States is the largest producer, importer, and recycler of aluminum in the world. Its aluminum industry produces 6 million metric tons of primary aluminum, making U.S. the single largest producer of primary aluminum in the world. The country consumes 12 million metric tons of aluminum each year. Great demand for the lightweight, high-strength, and recyclable structural metal allows aluminum to play a crucial role in the U.S. economy. Aluminum has a vital presence in heavy industries such as infrastructure, transportation, aerospace, packaging, defense and construction. Energy demand in the Aluminum Industry Like many processes of metal production, aluminum production too leans heavily on energy. The production of primary aluminum for example relies heavily on electrolytic process that is highly energy-intensive. According to a Manufacturing Energy Consumption Survey (MECS), the U.S. aluminum industry consumes over 800 trillion Btu of energy every year, including electricity losses. This amount represents slightly more than 1% of all energy and 3% of all manufacturing energy used in the country. Increasing costs and difficulty in procurement of fossil fuel, and the need to keep greenhouse gas emissions in check, have cast serious doubt on the global competitiveness of the aluminum industry in the U.S. The last few years have therefore seen an active move by the industry to keep costs down and improve its overall efficiency of energy consumption. Thankfully, technical progress and the use of recycling have allowed the aluminum industry in the country to reduce its energy demand by over two. In an effort to further reduce the industry's consumption of energy, the Energy Information Administration (EIA) encourages aluminum manufacturers to undertake a number of energy-management initiatives, including energy audits, electricity load control, purchase of power under special rate schedules, and others. Overall, 68% companies in the industry reported engaging in at least one energy-management activity in recent times. But are these energy saving methods adequate? Obviously more can be done by focusing on energy efficient transformers. Can energy-efficient transformers reduce energy demand in the Aluminum Industry? Transformers are the vital link between production and consumption of energy. They step up and step down electricity as it travels hundreds, even thousands of miles. A significant amount of energy lost in transmission is determined by the age and condition of transformers.

How Oil By-Products Degrade the Insulation System of a Transformer

Power transformers are the most expensive and critical pieces of equipment in a power distribution system. Ensuring they are running to optimum capacity and in good condition is thus crucial. Fault detection techniques serve as a warning to developing abnormalities in a transformer; and these techniques have many parameters of measurement and visual inspection. Present-day transformers are operated at rated loads and, thanks to an ever-increasing demand for power, many a time at overload. It therefore becomes necessary that transformers are monitored regularly to assess faults and ensure that preventive or corrective actions are promptly taken. Condition monitoring also helps in the assessment of the remaining life of the transformer, ensuring that utilities are not caught unawares by sudden transformer breakdowns which almost always result in severe losses. Factors that Contribute to Transformer Aging There are numerous factors that are responsible for transformer aging. Controlling these variables can actually augment the lifespan of a transformer: Quality of oil Operating temperature Amount of oxygen present Water content in the insulation which can cause molecular chains to decompose, speed up the cellulose aging process and adversely affect the tensile and dielectric properties of the insulation Contaminants How oil by-products degrade the insulation system of transformers Hydrocarbon or mineral-based oils and silicone's are used as insulation fluids in transformers because of their high dielectric strength, heat transfer properties and chemical stability. Under normal operating conditions very little decomposition of the dielectric fluid occurs. However, when a thermal or electrical fault develops, dielectric fluid and solid insulation will partially decompose. The low molecular weight decomposition gases include hydrogen, methane, ethane, ethane, acetylene, carbon monoxide and carbon dioxide. These fault gases are soluble in the dielectric fluid. Analysis of the quantity of each of the fault gases present in the fluid allows identification of fault processes such as corona, sparking, overheating and arcing.

Is it time to change that transformer?

The USA is the largest power consumer in the world. Unfortunately a large portion of the aging USA power distribution grid was installed prior to 1990. That being said, a large installed base of transformers are nearing the end of their useful life expectancy and change-out or replacement should be part of every owner's strategy for system reliability. Modern liquid filled transformers are a critical link in the energy delivery chain between power producers and end users. These transformers have no moving parts and they convert power with efficiencies that exceed 99%, thereby leading to their useful life being measured in decades not years. Considerable costs, however, can be associated with transformer failures, especially if such failures happen without warning and no action for a planned outage can be taken. How do you know if your transformer is in eminent danger and how did it get there? Let's answer the second question first. Modern liquid filled transformers utilize a combination of oil impregnated, thermally upgraded cellulose for conductor insulation, insulation between layers, insulation between coils, and insulation between current carrying parts and ground within the magnetic circuit. When cellulose is dry, free from gas, and immersed in oil, it's the toughest physical insulation system available. It is, however, the weakest link in the transformer insulation system. This is not a "new" discovery. The Electric Journal of April 1920 states that "the arch enemies of solid insulation are moisture and heat". If asked, a chemist would name moisture as the biggest threat. If you ask an electrical engineers the same question, they would respond that heat is the single largest threat. Both answers would be correct. Moisture in combination with heat will destroy an insulation system. Limiting moisture and excessive heating are the keys to getting the longest service life from your transformer. Moisture in the solid insulation can come from three sources: 1) residual moisture from inadequate drying during manufacture; 2) as a by product of cellulose decomposition; 3) and recombining with latent moisture in the oil. Heat, on the other hand, comes largely from loading the transformer beyond it's designed rating. Other contributors can include debris within the transformer blocking oil cooling ducts, blocked cooling radiator openings which restrict flow or oil leaks which lower oil to a level below the radiator openings thus effectively stopping the normal convective cooling process.

A rule of thumb for transformers

As fuel costs rise and power outages become more prevalent around the country, the power generation and distribution system in the US has come into sharp focus. So too has the need to conserve energy and the need to invest in energy-efficient products of all types. Of particular interest are products like power transformers that remain energized and consume energy 24 hours a day. The transformer is a critical component of the energy grid. If even a single transformer shuts down for a short period of time, a large number of households and commercial establishments are plunged into darkness resulting in a substantial economic loss. Unfortunately, a significant amount of equipment in the public utility grid is over 40 years old and needs to be replaced in the near future. According to the Department of Energy (DOE), distribution transformers which are 30 years old or more can waste between 60 and 80 billion kWh annually. A better designed transformer could yield an annual energy savings of up to $1 billion. Thus, maintenance, retrofitting and purchasing of new transformers are fast becoming imperatives. The Importance of Transformers The transformer - particularly the distribution transformer - is the most important single piece of electrical equipment installed in an electrical distribution network. It also has a large impact on a network's overall cost, efficiency and reliability. Selecting and acquiring energy-efficient distribution transformers which are optimized for - A particular distribution network The utility's investment strategy The network's maintenance policies Local service and loading conditions - will provide definite benefits (improved financial and technical performance) for both utilities and their customers. As climate change looms on the horizon, there is also an increased interest in the protection of the environment from greenhouse gas emissions. The regulatory requirement now is to install high-efficiency distribution transformers that have less energy losses, which eventually results in fewer pollutants being released into the environment.

Are Wind Turbine Step-Up Transformers the Weak Link in the Wind Energy Supply Chain

In the rush to cash-in on wind energy, developers are often trading low first costs for higher total costs of ownership to be shouldered later by the wind farm owners and operators Converting wind energy to electrical power is the fastest growing segment of the US energy sector. Today, wind energy represents less than 5% of the US electrical generation and is targeted to reach 20% in the foreseeable future. For this to happen, new sites need to be developed in spite of a down turning economy. Bolstered by available federal stimulus dollars, we are seeing a virtual modern day 'land-rush'. In the words of one industry leader, 'if there is a site that has a viable wind profile, access to network connections, and access for delivery of materials, and we don't develop it, some one else will.' This head long rush to install more and more wind turbines has outstripped the usual developmental learning curve, where new technologies mature by a process of trial and error, resulting in defining equipment suited for the job at hand. The added economic pressure of today's market has made an already competitive market even more demanding. This has, in the view of many industry insiders, resulted in purchasing decisions for equipment based largely on the lowest initial cost solutions and not solutions that will provide the best choice in terms of total cost of ownership, network stability, less down time and lost revenue from high maintenance issues. This is nowhere as apparent as in the case of Wind Turbine Generator (WTG) transformers. Historically this WTG transformer function has been handled by conventional, 'off the shelf' distribution transformers, but the relatively large numbers of recent failures would strongly suggest that WTG transformer designs need to be made substantially more robust. The practice of using conventional 'off the shelf' distribution transformers as a low cost solution is folly. In some cases site operators are maintaining a quantity of spare transformers to combat the frequent outages caused by standard distribution transformers being used where they are not suitable. The role of the Wind Turbine Generator (WTG) transformer in this process is critical and, as such, its design needs to be carefully and thoughtfully analyzed and reevaluated. Transformer Loading: Wind turbine output voltages range from 480 volts to 690 volts. The turbine output is transformed, by the WTG transformer, to a collector voltage of 13,800 to 46,000 volts. The turbines are highly dependent upon local climatic conditions; and this can result in yearly average load factors as low as 35%. The relatively light loading of WTG transformer has a favorable effect on insulation life but introduces two unique and functionally significant problems.

Aging Transformers: A Matter of Concern

It's a challenge that is now common to many utilities in the US: managing aging substation transformers installed in the 1960s and 1970s and fast approaching the end of their 'life'. These transformers didn't cause a blip in the radar during the last two decades, but with every year of the 21st century, their failure rates have become increasingly difficult to predict. This means that resource allocation and repair/replacement decisions are also becoming more and more exigent. Transformer Aging Factors The main factors responsible for transformer aging are given underneath. Controlling these variables can maximize the life of a transformer: Temperature Oxygen Moisture Other factors can include extreme operational conditions, and adverse conditions within its surroundings (such as a high temperature and humidity index), through faults and electrical surges. Degrading Insulation The cumulative effect of elevated temperature over time will adversely affect the useful life of electrical devices in general, and transformers in particular. For the duration of a transformer ' s life, the combination of elevated operating temperature and high ambient temperatures will have a slow degrading effect on its insulation. Insulation degradation can ultimately lead to catastrophic failures in the transformer. Moisture in a transformer's insulation system can cause molecular chains to decompose, speed up the cellulose aging process and adversely affect the tensile and dielectric properties of the insulation. One source of moisture is from humidity in the ambient air surrounding the transformer. Improper or aged transformer gaskets and seals will allow moisture, present in the atmosphere, to penetrate through to the insulation when the pressure gradient changes. This invading moisture speeds up the transformers aging process. Additionally, water vapor is a by-product of the degradation of cellulose insulation. Aging insulation, itself, contributes moisture to the problem, since dielectric strength diminishes with every increase in moisture level. Moisture and oxygen levels are both temperature dependent, increasing as the temperature rises. High levels of moisture and oxygen can lead to the formation of bubbles, which, when trapped within the insulating materials can cause voids and localized stress, leading to flashovers and failures. Water present in the insulation can also impact the insulation's dielectric properties. Insulation power factor increases with increases in moisture content. In order to function reliably, a transformer must stay within acceptable moisture limits, which vary with load and temperature. The moisture content of an oil sample is normally measured with the Karal Fischer reaction test. This has been adopted by the industry as a standard test due to its high selectivity, sensitivity, repeatability and reliability. The Importance of Constant Monitoring The physical parameters and behavior of an insulation system change as it degrades. The degradation of insulation paper and oil leads to the production of moisture and furan, which can both cause further accelerated aging. Overheating of the insulation system, partial discharge and arcing can all lead to the release of gases. Moisture within the insulation chain can help lead to its degradation and failure. Temperature can have an effect on moisture content, and how it moves between the cellulose and the oil. One way to minimize damage in an aging transformer is through constant monitoring of fault gases, temperature and water content. This data can help in detecting the type of fault, its intensity and, to some extent, its location.

Transformer Faults and Detection

In order to maximize the lifetime and efficacy of a transformer, it is important to be aware of possible faults that may occur and to know how to catch them early. Regular monitoring and maintenance can make it possible to detect new flaws before much damage has been done. The four main types of transformer faults are: Arcing, or high current break down Low energy sparking, or partial discharges Localized overheating, or hot spots General overheating due to inadequate cooling or sustained overloading These faults can all lead to the thermal degradation of the oil and paper insulation within the transformer. One way to detect them is by evaluating the quantities of hydrocarbon gases, hydrogen and oxides of carbon present in the transformer. Different gases can serve as markers for different types of faults. For instance, Large quantities of hydrogen and acetylene (C2H2) can indicate heavy current arcing. Oxides of carbon may also be found if the arcing involves paper insulation. The presence of hydrogen and lower order hydrocarbons can be a sign of partial discharge Significant amounts of methane and ethane may mean localized heating or hot spots. CO and CO2 may evolve if the paper insulation overheats; which can be a result of prolonged overloading or impaired heat transfer. Techniques to Detect Faults Techniques for finding faults: Buchholz Relay safety device Dissolved gas analysis Tests to detect oil contaminants and oil quality Techniques to detect transformer faults include the Buchholz Relay safety device, dissolved gas analysis (DGA) tests and a range of tests for detecting the presence of contaminants in the oil, as well as for measuring indicators of oil quality such as electric strength and resistivity. Buchholz Relay A Buchholz Relay is also called a gas detection relay. It is a safety device generally mounted at the middle of the pipe connecting the transformer tank to the conservator. A Buchholz Relay may be used to detect both minor and major faults in the transformer. This device functions by detecting the volume of gas produced in the transformer tank. Minor faults produce gas that accumulates over time within the relay chamber. Once the volume of gas produced exceeds a certain level, the float will lower and close the contact, setting off an alarm. Major faults can cause the sudden production of a large quantity of gas. In this case, the abrupt rise in pressure within the tank will cause oil to flow into the conservator. Once this is detected the float will lower to close the contact, which causes the circuit breaker to trip or sets off the alarm. Dissolved gas analysis (DGA) Dissolved gas analysis, or DGA, is a test used as a diagnostic and maintenance tool for machinery. Under normal conditions, the dielectric fluid present in a transformer will not decompose at a rapid rate. However, thermal and electrical faults can accelerate the decomposition of dielectric fluid and solid insulation. Gases produced by this process are all of low molecular weight, and include hydrogen, methane, ethane, acetylene, carbon monoxide and carbon dioxide. These gases will dissolve in the dielectric fluid. Analyzing the specific proportions of each gas will help in identifying faults. Faults detected in such a way may include processes such as corona, sparking, overheating and arcing. Abnormal functioning within a transformer can be caught early by studying the gases that accumulate within it. If the right countermeasures are taken early on, damage to equipment can be minimized.

Monitoring of Transformers

As a vital part of transmission and distribution systems, transformers are built and expected to be unfailingly reliable. Nevertheless, internal faults like partial discharges can occur, and the problem with such faults is that if left un-corrected, they can eventually morph into catastrophic faults that can result in power outages and even end-user property damage. Manufacturers and utilities have both been at the receiving end of such calamities. Though rare, they can be caused by: Increase in the age of the transformer, beyond the 'sell-by date' as it were Extreme or arduous operating conditions Negligence on the part of operators and management Considerable costs are associated with transformer failure, especially if such failure happens without warning, and no action for a planned outage can be taken. Without redundancy, there can be huge losses in terms of produced energy, process downtime, penalties, and more. Transformer Monitoring: What's Involved Data acquisition Sensor development Data analysis Development of links between measurements and failures Easily Prevented Preventing disasters of this nature is actually quite simple, and involves transformer monitoring. Monitoring transformers and spotting problems before they turn into unmanageable incidents can prevent faults that are costly to fix and may result in a loss of service. Transformer monitoring mainly involves data acquisition, sensor development, data analysis, and the development of causal links between measured values and failures of transformers. Installing monitoring equipment on transformers is usually done for two reasons: Monitoring important transformer functions can help detect developing faults before they lead to a catastrophic failure Monitoring transformer functions can allow for a change from periodic to condition-based maintenance Monitoring Equipment Monitoring equipment is permanently mounted on the transformer and is online 24/7. Reliable, low-cost monitoring is thus a necessary condition. Failure rates of transformers are usually low (0.2 - 2% per transformer/year), and high-cost failure prevention systems cannot thus be justified, especially when redundancy is available and the consequential costs are thus limited. To keep within this cost barrier, some compromise on the functionality of the monitoring equipment is necessary. Transformer Monitoring: Parameters Oil temperature Moisture levels Operation of cooling fans Electrical load levels In a majority of cases, it is enough to supply a reliable warning signal without online analysis and diagnosis, provided that manual or automatic diagnostic methods are available to follow up the alarm. Specifically with regard to power distribution networks in the US, a majority of the transformer population is aging, and most emerging faults can be expected from these units.Monitoring equipment should thus be designed for field installation on operational transformers that might date back a few decades.

How to Choose a Transformer

When choosing a transformer, there are two primary concerns: the load and the application . Several factors must be evaluated carefully while making the choice, to ensure that the needs of both primary concerns are met. To use a cliche, it is typically a 'no-brainer' to choose smaller transformers. A unit with a kVA rating that is larger from the anticipated load can quickly be picked up. But if you are selecting a large unit for an electrical utility system, to be part of a large distribution network, you are typically making a much larger investment; thus the evaluation process is much more detailed and elaborate. With over 90 years of experience in this industry, Pacific Crest Transformers has put together a quick checklist to help you make your choice judiciously. Top Questions There are three major questions that influence your choice: Does the chosen unit have enough capacity to handle the expected load, as well as a certain amount of overload? Can the capacity of the unit be augmented to keep up with possible increase in load? What is the life expectancy of the unit? What are the initial, installation, operational, and maintenance costs? Evaluation Factors The cost and capacity of the transformer typically relate to a set of evaluation factors: 1. Application of the Unit Transformer requirements clearly change based on the application. For example: in the steel industry, a large amount of uninterrupted power is required for the functioning of metallurgical and other processes.Thus, load losses should be minimized - which means a particular type of transformer construction that minimizes copper losses is better suited. In wind energy applications, output power varies a great extent at different instances; transformers used here should be able to withstand surges without failure. In smelting, power transformers that can supply constant, correct energy are vital; in the automotive industry, good short-term overload capacity is a necessary attribute. To Select the Right Transformer, First Determine: Primary voltage, which is the available voltage Secondary voltage required by load equipment Frequency (in Hz) and phase (single or three-phase ? for the secondary voltage as well) kVA load; with possible future increases factored in Is the transformer to be used indoors or outdoors? Is the transformer to be floor or wall-mounted? Is an auto transformer or a double-wound transformer required? Textile industries, using motors of various voltage specifications, will need intermittent or tap-changing transformers; the horticulture industry requires high-performance units that suit variable loading applications with accurate voltage. These examples serve to underline that type of load (amplitude, duration, and the extent of non-linear and linear loads) and placement are key considerations. If standard parameters do not serve your specific application, then working with a manufacturer that can customize the operating characteristics, size and other attributes to your needs will be necessary. Pacific Crest regularly builds custom transformers for unique applications.

Importance and Types of Transformer Cooling Systems

The load that a transformer carries without heat damage can be increased by using an adequate cooling system. This is due to the fact that a transformer's loading capacity is partly decided by its ability to dissipate heat. If the winding hot spot temperature reaches critical levels, the excess heat can cause the transformer to fail prematurely by accelerating the aging process of the transformer's insulation. A cooling system increases the load capacity of a transformer by improving its ability to dissipate the heat generated by electric current. In other words, good cooling systems allow a transformer to carry more of a load than it otherwise could without reaching critical hot spot temperatures. One of the more common types of transformer cooling equipment is auxiliary fans. These can be used to keep the radiator tubes cool, thereby increasing the transformer's ratings. Fans should not be used constantly, but rather only when temperatures are such that extra cooling is needed. Automatic controls can be set up so that fans are turned on when the transformer's oil or winding temperature grows too high. Maintenance of Cooling Systems Dry-Type Transformers: For dry-type transformers, the area in which the transformer is to be installed should have proper ventilation. This ventilation should be checked prior to installation to make sure it is adequate. Additionally, the transformer's radiator vents should be kept clear of obstructions that could impede heat dissipation. Forced Air: If the transformer's temperature is being kept at acceptable levels by forced air from a fan, the fan's motors should be checked periodically to make sure they are properly lubricated and operate well. The thermostat that ensures the motors are activated within the preset temperature ranges should be tested as well. Water cooled systems: Systems that are cooled by water should be tested periodically to make sure they operate properly and do not leak. Leaks can be checked by raising the pressure within the cooling system, which can be done in various ways. If the cooling coils can be removed from the transformer, internal pressure can be applied by adding water. Otherwise, pressure checks can also be made using air or coolant oil, if the coils need to be checked within the transformer itself. If the cooling coils are taken out of the transformer, the water cooling system as a whole can be tested. Here, the coils are filled up with water until the pressure reaches 80 to 100 psi, and left under that pressure for at least an hour. Any drop in pressure could be a sign of a leak. The other equipment linked to a water-cooled system can be tested at the same time, such as the alarm system, water pump and pressure gauges. Also, the water source should be tested to make sure it has sufficient flow and pressure. Liquid coolants: When oil coolants are prepared they are dehydrated, and processed to be free of acids, alkalis, and sulfur. They should also have a low viscosity if they are to circulate easily. If a transformer is cooled by oil, the dielectric strength of the oil should always be tested before the transformer is put into service.

The Differences between Grounding Transformers and Distribution Transformers

Grounding transformers (GT) differ from "standard distributiontransformers" (DT) because they are used to establish a return path forground fault currents on a system which is otherwise isolated oreffectively un-grounded. This differentiates the construction in acouple of ways. Grounding transformers must be designed to meet two basiccriteria: They must be able to carry the continuous phase and neutralcurrents without exceeding their temperatureratings. They must be able to carry the fault current withoutexcessive heating that deteriorates the conductors or adjacentinsulation. It is in the second parameter which most widely separatesgrounding transformers from distribution transformers. DTs are designedto carry a fault current, which is limited by their impedance, for amaximum duration of 2 seconds per standards. Whereas the GTmust carry a fault current that is not limited by its impedance, fordurations exceeding the 2 second limitation. Often this time is 10seconds or more. The GT design must be such that at the end of thisextended time period, the conductor temperature is below the criticalthermal limit as identified in thestandards. DT: MainConcerns The DT main concern is for heatingcaused by loading. Radiators are added to the transformer to help theinsulating fluid control the steady state temperature rise, but thesedo not help during fault conditions. Heat generated during afault happens in such a short period of time (usually seconds) that thecalculation assumes "all heat is stored" in the conductor because heatdissipation does not occur fast enough to combat the rapidly heatingconductors. The GT takes this into account and is designed such thatthe conductor can handle the fault heating without relying oninsulating oil for heat transfer during thefault. Many GT specifications recognize this andallow the steady state cooling to be calculated using the magnetizingcurrent and HV I 2 R loss resulting from energizing the core only. Thisleads to some misconception that the DT is better cooled, but theopposite is during faults. Anothersubtle difference is the way the two devices "see" faults. The DTtypically sees a line to ground fault or maybe, a line to line fault,but since the GT is providing a return path to the network, ittypically sees a zero sequence fault which impresses the fault currentequally on all three legs simultaneously. To combat the forcesgenerated, GT conductors are always copper for maximum strength tocross section ratio, and because copper has a higher thermal withstandcapability. GT coils are always circular on cruciform cores to gain themaximum form stability. Distribution transformers often utilizerectangular coil construction which does not have the same formstability offered by the circular coiltechnology.

Transformer Installation: Some Best Practices

There are many problems that can be avoided during transformer installation simply by installing the transformer in the correct environment. Many difficulties and safety hazards can be avoided or minimized by keeping certain factors in mind while positioning a transformer, before it is set up and connected. Likewise, the location can have a bearing on how the transformer should be set up, and what precautions should be taken in the future. The Standards - Installing transformers in accordance with the ANSI, NEMA, and IEEE standards is critical to ensuring a safe electrical installation as well as a reliable power supply system - especially for those applications where power quality is an issue. Transformer installation is one of the most common-yet-complicated installation practices that are cause for considerable confusion when sizing Over Current Protection Devices (OCPDs) and bonding and grounding conductors. Many electrical installations can be a challenge in terms of NEC requirements, and transformers can raise that challenge to a new level. A properly designed installation will ensure the conductors and equipment are properly sized, protected and also deal with the overriding issue of grounding. Incorrect installation can lead to fires from improper protection or conductor sizes, as well as electric shock from inadequate grounding. This article will provide a brief overview of important considerations to keep in mind during installation, of transformers located outdoors and indoors, and for dry-type vs. liquid-filled transformers.

Grounding Transformers

Grounding is clearly one of the most important aspects of electrical design, but it steadfastly continues to be misinterpreted and misunderstood. Millions of dollars in liability and loss can be attributed to ground-fault arcing; thus, grounding-related issues should top the checklists of any electrical contractor. Grounding Transformers : Simply put, a grounding transformer is used to provide a ground path to either an ungrounded "Y" or a delta connected system. Grounding transformers are typically used to: Provide a relatively low impedance path to ground, thereby maintaining the system neutral at or near ground potential Limit the magnitude of transient over voltages when re-striking ground faults occur Provide a source of ground fault current during line-to-ground faults Permit the connection of phase to neutral loads when desired If a single line-to-ground fault occurs on an ungrounded or isolated system, no return path exists for the fault current, thus no current flows. The system will continue to operate but the other two un-faulted lines will rise in voltage by the square root of 3, resulting in overstressing of the transformer insulation and other associated components on the system by 173%. MOV lightning arresters are particularly susceptible to damage from heating by leakage across the blocks even if the voltage increase is not sufficient to flash over. A grounding transformer provides a ground path to prevent this. Large multi-turbine wind farms provide an example of the use of grounding transformers for fault protection on ungrounded lines. In many wind farms the substation transformer provides the sole ground source for the distribution system. When a ground fault on a collector cable causes the substation circuit breaker for that cable to open, the wind turbine string becomes isolated from the ground source. The turbines do not always detect this fault or the fact that the string is isolated and ungrounded; thus the generators continue to energize the collector cable, and the voltages between the un-faulted cables and the ground rise far above the normal voltage magnitude as described above. A grounding transformer placed on the turbine string provides a ground path in the event the string becomes isolated from the system ground. Construction: Grounding transformers are normally constructed either with A ZigZag (Zn) connected winding with or without an auxiliary winding or As a Wye (Ynd) connected winding with a delta connected secondary that may or may not be used to supply auxiliary power The geometry of the Zig-Zag connection is useful to limit circulation of third harmonics and can be used without a Delta connected winding or the 4- or 5-leg core design normally used for this purpose in distribution and power transformers. Eliminating the need for a secondary winding can make this option both less expensive and smaller than a comparable two-winding grounding transformer. Furthermore, use of a Zig-Zag transformer provides grounding with a smaller unit than a two-winding Wye-Delta transformer providing the same zero sequence impedance. Wye connected grounding transformers, on the other hand, require either a delta connected secondary or the application of 4 or 5 leg core construction to provide a return flux path for unbalanced loading associated with this primary connection. Since it is often desirable to provide auxiliary power from the grounding transformer secondary winding, this benefit can sway the end user to specify a two-winding grounding transformer in lieu of a Zig-Zag connection. The current trend in wind farm designs is toward the Wye connected primary with a delta secondary. It is important to understand that both Zig-Zag and two-winding grounding transformers can be provided with the ability to provide auxiliary power, and this can be either a Wye or Delta connected load. A solidly grounded system using a grounding transformer offers many safety improvements over an ungrounded system. However, the ground transformer alone lacks the current limiting ability of a resistive grounding system. For this reason, neutral ground resistors are often used in conjunction with the grounding transformer to limit neutral ground fault current magnitude. Their ohm values should be specified to allow high enough ground fault current flow to permit reliable operation of the protective relaying equipment, but low enough to limit thermal damage.

Transformer Reliability

The last ten years have seen growing interest in the evaluation and monitoring of power transformers - mainly because a vast number of transformers worldwide are fast approaching the end of their effective operating life. The reliability of the existing units within global distribution networks must thus be assessed to understand if the transformers are fit for use, or need to be replaced or retrofitted. Within power systems, transformers form a substantial part of the asset costs; financial pressures today persuade most utilities to use existing units for as long as possible, as long as overall system reliability is not at risk. Thus, transformer asset management looks set to play a major part in future planning. Factors Influencing Power Transformer Reliability This article focuses on the factors influencing power transformer reliability, which are: Operating Environment The operating environment is the set of conditions which the transformer will be exposed to during its service life. This includes weather conditions, location, electrical loading, operating methods, system parameters and application. Each element that influences the life of the transformer must be recognized and documented. National standards like ANSI, NEMA and others address many of these elements and provide recommendations or standard values wherever applicable; which may be of benefit to the person or organization responsible for assembling the data. Transformer Specification The specification of a transformer should describe the function it is to perform and the constraints which must be met. It should describe each of the identified operating conditions which affect the transformer. Transformer Design Transformer design ensures the operating conditions of the transformer as mentioned in the specifications are converted to concrete design parameters and into usable information by the manufacturing organization. The design engineer must get all the needed information within the specifications, and any clarification must be obtained from the user or use parameters detailed in the national industry standards. While manufactures continue to advance the state of the art to meet changing operating environments, it is still fundamental to the designer that each known environmental condition be considered and addressed in the design. Any omission could result in decreased service life and premature failure. Manufacturing Process Here the physical product is created from the paper design. The reliability of a transformer when it completes the manufacturing cycle will differ from the reliability as demonstrated in the design, due to the variations in the manufacturing process of materials, processes, handling, machines and personnel. Reliability will also differ between similar products. Quality techniques are aimed at manufacturing the product as close as possible to the designed product. To verify the quality and reliability of the manufactured transformer, two types of tests are performed: Compliance Test: The compliance test ensures that the transformer meets the specified design parameters

Testing of Power Transformers

This article is limited in scope to standard testing procedures for transformers prior to installation, with their general purpose and methodology. Testing of the transformers is done to determine their electrical, thermal and mechanical suitability for the system where they will be applied or used. Most of the tests performed on power transformers are defined in national standards created by IEEE, NEMA and ANSI, whose purpose is to define a uniform set of tests recognized by both the manufacturer and the user. Categories of Field Testing In general, field testing can be divided into three categories Acceptance tests Periodic tests Tests after failure Acceptance tests should be performed immediately after the product arrives at the destination. A few tests can be carried out which are stated below: Turns ratio Insulation resistance(Winding and core) Power factor Resistance (winding) Polarity and phase relation Oil tests (DGA, moisture, dielectrics, etc.) Visual inspection Periodic tests are done after the product is installed in its permanent location. The main purpose of this test is to monitor the condition of the unit so that any potential trouble may be spotted early before a failure occurs. Some of these are listed below: Turns ratio Insulation resistance Power factor Resistance (winding) Oil tests (DGA, moisture, dielectrics, etc.) Excitation current test Visual inspection An unscheduled outage and the potential of outright failure can be prevented by following a periodic test schedule. Failure tests , which can be performed, are: Turns ratio Insulation resistance Power factor Resistance Oil tests Excitation current test Combustible gas/ gas-in-oil analysis Visual inspection (internal)

Dry Type and Liquid-Filled Transformers: A Quick Comparison

Transformers under load generate heat due to winding (copper) and core losses occurring during operation. There is an 'acceptable' temperature rise for transformers used in power applications, and this can even limit their size. This acceptable temperature rise is directly related to the limitations of the transformer materials; safety regulations; or component parts in close proximity that may have high-temperature reliability problems. High temperatures can damage the winding insulation; the heat generated from core and winding losses must thus be dissipated. This dissipation can be achieved with a combination of radiation and convection from the exposed surfaces of the transformer. Dry type power transformers up to several hundred kVA can usually be cooled by convection or even by fans. Power transformers can also be immersed in coolant liquids - which can range from mineral oils to silicone-based oils or ester-based vegetable oils. . Based on the type of cooling used, transformers are thus classified into 'dry type' and 'liquid-filled'. Liquid-Filled Transformers Oil-filled Transformers Oil-filled transformers primarily use mineral-based oil and cellulose paper (Kraft or Aramid) in their insulation systems. This proven combination exhibits outstanding thermal and dielectric properties at a relatively low cost. So popular and effective are these units, that all other transformer designs are judged in relation to them. They are still unparalleled in terms of purchase cost, among all the options available. The inherent weakness of a mineral oil-filled transformer, of course, is flammability; which is why oil-filled transformers are usually restricted to outdoor installations, or indoor installations that have elaborate means of fire protection. Typical Applications: Oil-filled transformers, thanks to their lower purchase costs, find applications in literally every sort of power distribution. Of late, the awareness of the fire risks associated with mineral oil-filled transformers has created a movement towards safer alternatives that use non-flammable, biodegradable liquids, or even dry-type transformers. Non-Flammable Liquid-Filled Transformers Polychlorinated biphenyl (PCBs) were produced in large quantities starting as early as the 1930s, in response to the electrical industry's need for a less flammable substitute for mineral oil as a cooling/insulating fluid for transformers. Several industrial incidents, however, brought the toxicity of PCBs to the fore. As confirmed organic pollutants, PCBs were banned by the late 1970s. A number of alternatives have since surfaced - major ones being silicone, perchloroethylene, high temperature hydrocarbons, and mixtures of oil with perchloroethylene. The first high molecular-weight hydrocarbon-based fluid (HMWH), was introduced in 1975. This fluids possesses similar dielectric properties as mineral oil, provide remarkable levels of fire-resistance, and do not have undesirable environmental fallouts. Typical Applications: Non-flammable liquid-filled transformers can be installed indoors and outdoors, close to buildings, walkways and rooftops. Usually, no additional infrastructure is required to address issues like fire safety.