Failures, monitoring, and new trends of power transformers

©DIGITAL VISION

Failures, monitoring, and new trends of power transformers IBRAHIM A. METWALLY,

Digital Object Identifier 10.1109/MPOT.2011.940233 Date of publication: 4 May 2011

36

T

his article presents a survey on failures, m o n i t o r i n g , and new trends of power transformers. There are three main types of power transformers, namely, oilimmersed, gas-insulated, and dry-type transformers with or without cast coil insulation system. Operating stresses of power transformers have increased due to the load growth and the increased bulk power transactions, where recent and imminent alternating current (AC) systems are rated 1,100 kV and 1,200 kV, respectively. Failures of windings, onload tap changers (OLTC), and bushings are the main defective components as they represent about 84% of the failure statistics. Online and

0278-6648/11/$26.00 © 2011 IEEE

offline diagnostic monitoring of power transformers can be used to detect faults at an early stage, prevent degeneration into catastrophic phenomena, and monitor the aging process of the insulating systems. Many of the well-known preventive maintenance techniques are discussed. New trends of power transformers have recently taken many steps forward in different dimensions, e.g., Powerformer, Dryformer and Windformer utilizing high-voltage (HV) cross-linked polyethylene (XLPE) cables, gas-insulated transformers (GIT), converter transformers rated 6800 kV, and high-temperature superconducting (HTS) transformers. The latter type represents a key technology for future IEEE POTENTIALS

For both types of dry transformers, the voltage and the power ratings are in the range for distribution systems, i.e., few tens of kV and a higher power rating up to a few tens of MVA with fan cooling system. Table 1 introduces a comparison among oil-immersed, drytype and cast-coil distribution transformers, where both types of dry-type transformers can be overloaded higher the oil-immersed one.

power systems engineering as they offer many advantages over the others.

Main types

Another version of the dry-type transformer is a cast coil insulation system. It is used when additional coil strength and protection are advisable. This type of transformer can be used in harsh environments such as cement and chemical plants and outdoor installations, where moisture, salt spray, corrosive fumes, dust, and metal particles can destroy other types of dry-type transformers. These cast coil units have higher ability to withstand heavy power surges, such as frequent but brief overloads experienced by transformers serving transit systems and various industrial machineries. Cast coil units are now being used where previously only oilimmersed units were available for harsh environments. They can have the same high levels of BIL while they are still providing ample protection of the coils and the leads going to the terminals.

Global ratings and market World electricity demand is projected to be doubled between 2000 and 2030 at an annual growth rate of 2.4% as shown in Fig. 2. This is faster than any other final energy source. Electricity’s share of total final energy consumption rises from 18% in 2000 to 22% in 2030. Electricity demand growth has the strongest trend in developing countries, where the demand will climb by over 4% per year over the projection period, i.e. more than tripling by 2030. Consequently, the developing countries’ share of global electricity demand jumps from 27% in 2000 to 43% in 2030. That is why the transformer global market concentration in 2002 and 2007 was high in China and India as shown in Table 2. It is expected that there will be no major change in this global market concentration up to 2012.

3,000 Voltage and Power Ratings (kV and MVA)

Oil-immersed transformers are commonly and economically used for a wide range of voltage and power ratings for many decades as shown in Fig. 1, i.e., from distribution to transmission levels or from MV to UHV applications. They use paper-wrapped windings immersed in mineral oil, which serves as both the insulation and cooling medium. Although these transformers are inexpensive and widely used, they are undesirable in tall buildings and densely populated urban areas because of the high fire risks that accompany the use of transformer oil. After about five years, the 1,200 kV alternating current (AC) system in due course is expected to emerge as main transmission level in India because the peak demand is expected to increase more than 500 GW by 2026 and beyond from the present level of about 105 GW. Gas-insulated transformers use polyethylene terephthalate (PET) film to insulate the windings, which are placed in a sealed tank filled with SF6 gas that cools the windings and protects them from moisture and dust. In contrast with transformer oil, SF6 gas is extremely safe. It is highly inert, and has been given a Group 6 UL safety rating, which puts it in the same high safety class as nitrogen gas. Details about this type of transformers will be discussed later. Dry-type transformers can have their windings insulated in various ways. A basic method is to preheat the conductor coils and then dip them in varnish at a high temperature. The coils are then baked to cure the varnish. This process is an open-wound method and helps ensure penetration of the varnish. Cooling ducts in the windings provide an efficient and an economical way to remove the heat produced by the electrical losses of the transformer by allowing air to flow through the duct openings. This dry-type insulation system operates satisfactorily in most ambient conditions as it is also sealed with an epoxy resin mixture.

2,500

3 × 1-Phase Bank Capacity 1 × 3-Phase Bank Capacity Voltage

2,000 1,500 1,000 500 0 1960

1970

1980

1990

2000

2010

Year Fig. 1 Progress in the oil-immersed transformers’ voltage and the power (bank capacity) ratings over five decades.

Table 1. Comparison among oil-immersed, dry-type, and cast-coil distribution transformers. Standard temperature rise, °C Oil-immersed

Dry/Cast coil

Overload capacity, % Oil immersed

Dry/Cast coil

Oil immersed

Dry/Cast coil

15 (#2.5MVA) 25 ([2.5–10]MVA)

33 (STD) 50 (encapsulated cast coil)

55

80/80

0

30/15

65

115/115

0

15/0

55/65

150/150

12

0/0

MAY/JUNE 2011

Fan overload capacity, %

37

tial costs for repair and financial losses due to inadvertent outages. Therefore, utilities have incentive to 2012 assess the condition of their 26.20% transformers, in particular 16.51% the condition of the HV 8.69% insulation system, with the 4.95% aim to minimize the risk of 5.41% failures and to avoid forced 1.80% outages of strategically important units. 2.14% The growth of loads and 2.04% the increased bulk power 1.29% transactions accelerate trans1.28% formers’ physical ageing 29.69% process as a result of increas100.00% ing the operating stresses. This “aging square” phenomenon has been difficult to measure and analyze, as the effects of operating conditions on a transformer vary according to its family, make, model, age and application. On the other hand, a transformer that is older than the original manufacturer’s normal life estimate may be in good physical condition and can last

Table 2. Transformer global market concentration in 2002, 2007, and 2012. 2002

2007

China

21.82%

26.22%

United States

32.02%

18.47%

India

6.29%

7.65%

Japan

5.45%

5.41%

Russia

2.43%

4.99%

Canada

1.71%

2.04%

Germany

2.28%

1.91%

South Korea

1.18%

1.66%

Brazil

1.40%

1.49%

United Kingdom

1.74%

1.46%

23.68%

28.70%

100.00%

100.00%

Failures of oil-immersed transformers

2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0

2012

2011

2010

2006

2005

2004

2003

Power Copper

2002

Power and Copper Demand (TVA and Tt)

Power transformers are the most expensive and strategic components of any power system. Insulation breakdown is a serious failure of large power transformers that can generate substan-

2009

Total

2008

Rest of world

2007

Country/Year

Year Fig. 2 Global power rating and copper demands in trillion (tera or T) volt-amperes and tera tons for one decade and up to 2012.

4.5 4.0 Failure Rate (%)

3.5

OLTC failures Onload tap changers (OLTC) are used to change the tapping connection of the transformer windings while the transformer is energized. The OLTC can be designed as a single unit for singleand three-phase applications with one common neutral point as shown in Fig. 5. Depending on the three-phase rating, it might require three separate units, each having its own insulated phases. Tap changers can be located either inside the transformer main tank or outside in its own compartment; see Fig. 5. Switching from one position to another has to be performed through an impedance to avoid a short circuit between two steps of the regulating winding. The transition impedance can be either a resistor

Core, 5%

4.15

Network Transformers Generator Transformers

Tank, 6%

Auxiliary, 5%

OLTC, 40%

3.0 2.5 1.8

2.0 1.5

1.55

1.35

0.8

1.0 0.5 0.0

0.05 (60–100)

(100–300) Voltage Level (kV)

(300–700)

Fig. 3 Failure rates of power transformers at different voltage levels.

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for decades more. Therefore, it is important to understand completely the condition of a fleet of transformers and manage their remaining life to avoid unplanned outages and catastrophic failures. Fig. 3 illustrates that the highest failure rates can be observed at power transformers in the upper voltage levels. Network and generator transformers considerably differ due to the different loading nature. Generator transformers are normally loaded according to their rated power, whereas network transformers are loaded to 100% or more in emergency situations only. Therefore, monitoring systems are of particular interest for generator transformers and for network transformers in the upper voltage levels. Fig. 4 shows the failure statistics of the defective components. It can be seen that besides the active part (windings), the OLTC and the bushings should be monitored. The next sections discuss these main three defective components.

Bushing, Winding, 14% 30% Fig. 4 Defective components of power transformers according to a CIGRE international survey.

IEEE POTENTIALS

or a reactor. A typical OLTC of distribution-class transformers provides ⫹5% to ⫺15% taps in steps of 1.25% of the rated voltage, while that of transmission-class transformers, e.g. the 220-kV system, provides ⫾10% taps in steps of 0.625% or 1.25% of the rated voltage. A tap changer is the only movable part of a transformer. Therefore, it suffers from various ageing mechanisms. The insulating oil inside the tap changer compartment becomes dirty due to switching arcs, which leads to weakened insulation properties. Electrical treeing along the supporting resin-bonded paper cylinder and insulating drive shaft can occur. Another effect of the switching arcs is wear of the arcing contacts. Mechanical wear can also occur at all movable parts of the tap changer. Another ageing mechanism is the socalled long-term effect on the changeover selector, which occurs when the tap changer is motionless. The long-term effect starts with the formation of a thin layer of oil. This organic film is a less conductive layer built from polymerized oil, which consists of organic components in the transformer oil bond to silver or copper oxide and sulphide that is formed on the stator blocks. This oil film layer does not cause tap changer failures directly. The increased contact resistance due to the oil film layer can cause coking (creation of hard and porous carbon material) at places where the load current flows. The long-term effect is accelerated by high temperatures, high load current, infrequent movement, low contact pressure and develops faster at copper and brass contacts. Due to its infrequent movement, the changeover selector is prone to the long-term effect. However, the changeover selector is not accessible during normal maintenance, so diagnostic measurements are necessary.

ability to withstand future short-circuit forces. Fig. 6 shows the transformer failure rate, where the corresponding exponential curve for a 50% failure rate is at the age of 50 years. Winding failures are governed by the ageing/weakness of the solid insulation system. Ageing of transformer oil can be affected by the following partial discharge (PD) and thermal degradation: 1) Signs and effects of PD • Gas evolution starts from the oil. • These gasses are H2, Hydrocarbons. • Electric stress of gas pockets will be very high due to the difference of permittivities of the gas and solid. 2) Signs and effects of thermal degradation: • An increase of 7 °C approximately doubles the rate of degradation. • High temperature increases the rate of oxidation • High thermal stresses decompose the oil.

Bushing failures An electrical bushing can be defined according to ANSI/IEEE Std. C57.19.00 as “an insulating structure, including a through conductor or providing a central passage for such a conductor, with provision for mounting a barrier, conducting or otherwise, for the purpose of insulating the conductor from the barrier and conducting current from one side of the barrier to the other.” As a less formal explanation, the purpose of an electrical bushing is simply to transmit electrical power in or out of enclosures, i.e., barriers, of an electrical apparatus such as transformers, circuit breakers, shunt reactors, and power capacitors. The bushing conductor may take the form of a conductor built directly as a part of the bushing or, alternatively, as a separate conductor that is drawn

Fig. 5 OLTC for 220-kV, 250-MVA autotransformer.

through, usually through the center of the bushing. Oil-paper insulation is widely used in power transformer bushings and is one of the best insulations with good electrical and heat transfer properties. However, prolonged exposure to extreme electrical, thermal, mechanical and environmental stresses can deteriorate its important properties and can break the cellulose bonds of the paper. This leads to the following byproducts: water, carbon monoxide, carbon dioxide, smaller values of hydrocarbons and furan. PD can be formed in the presence of gas bubbles and can carbonize the insulation leading to conducting tracks which can eventually short out one or more layers of the bushing. A local lack of the mechanical strength can also cause a lack of continuity of the conducting layers of

Ageing of the insulation system reduces both the mechanical and dielectric-withstand strengths of the transformer. As the transformer ages, it is subjected to faults that result in high radial and compressive forces. When the load increases with system growth, the operating stresses also increase. In an ageing transformer failure, the conductor insulation is typically weakened to the point where it can no longer sustain mechanical stresses of a fault. Turn-to-turn insulation then suffers a dielectric failure. Alternatively, a fault causes a loosening of winding clamping pressure, which reduces the transformer’s MAY/JUNE 2011

Transformer Failure Rate (%)

Winding failures 100 90 80 70 60 50 40 30 20 10 0 0

10

20

30

40

50

60

70

80

90

100

Number of Service Years Fig. 6 Transformer failure rate as a function of the number of service years.

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bushings and can lead to arcing or tracking. With an increase of moisture content in the paper from 0.3% to 2%, the rate of ageing is accelerated by 6–16 times. Operating records show that about 90% of all preventable bushing failures are caused by moisture entering the bushing through leaky gaskets or other openings. Most outages due to bushing failures can be prevented by close periodical inspection to find leaks and make repairs when needed. Such an external inspection requires little time and expense, and it is well worth the effort. HV bushings, if allowed to deteriorate, may explode with considerable violence and cause extensive damages to adjacent equipment. Flashovers may be caused by deposits of dirt on the bushings, particularly in areas where there are contaminants such as salts or conducting dusts in the air. These deposits should be removed by periodic cleaning/washing. All HV bushings should be periodically inspected at intervals of 3–5 years. Bushings showing signs of deterioration should be tested at intervals of six months to one year and removed from service if the tests show a dangerous condition.

Preventive maintenance techniques Power transformers are typically classified as one of the high risk and critical impact components in power systems. Failure of transformers is further compounded by the facts of high replacement costs and lengthy delivery times. With the technological advancements that have taken place over the decades, nowadays there are solutions available to prevent most of failures. The cost savings from being able to proactively schedule and perform proper maintenance of transformers can in itself more than pay for the cost. The savings from a dramatic reduction in failures coupled with the savings from proactive maintenance would help provide the necessary funds for system expansions and upgrades. Condition monitoring of electrical equipment, such as transformers, helps users in many ways such as planning of maintenance schedules, obtaining knowledge of the health of equipment, estimating the remaining service life of equipment, finding areas of further improvement, refining product specifications, etc. Diagnostic monitoring can be divided into online, if performed with the transformer in normal operation and offline, 40

if they require the transformer to be powered down. They can be used to: • detect faults at an early stage and enable corrective measures in order to prevent degeneration into catastrophic phenomena • monitor the ageing process of the insulating systems. The well-known preventive maintenance techniques such as dissolved gas analysis (DGA), thermal monitoring, partial discharge measurement, capacitance and loss tangent “dissipation factor” (C and tan δ), frequency response analysis, etc. are performed on transformers for a specific type of problem.

HV bushings and windings continuous monitor It was illustrated earlier in Fig. 4 that 44% of transformer failures are related to bushings and windings. In addition, about 50% of the HV bushing failures are violent and involve collateral damage. One of the most technologically advanced continuous bushings monitor available represents a means of measuring 1–2% winding distortion. It can also provide connections to perform “online” PD measurements and analysis. Monitoring changes in C and tan d of the HV bushings on a continual basis provides an early warning necessary to avert a catastrophic failure. Bushings typically have a short time-span from inception to failure, and the higher the voltage the shorter is this time-span. C and tan d measurement is one of the accepted techniques for evaluating condition of the insulation dielectric properties. Schering bridge is the famous method for measuring the C and tan d. These measurements need to remove the power transformer from the network, i.e. an offline test. This offline test on transformer bushings is performed at 10 kV and the standard offline test is less sensitive to impending failures. In inclement weather or at times with high humidity, maintenance crews at utilities will not perform power factor tests due to their concerns about inaccurate reading.

OLTC analyzers The most important cause of power transformer failures is the OLTC according to an international survey done by the International Council on Large Electric Systems (CIGRE) and shown in Fig. 4. OLTC are used to keep the secondary voltage at an acceptable level when the load changes or to adjust the transformer phase shift. Tap changer diagnostics are important to determine when and which tap changer

maintenance is necessary. Several tap changer diagnostics are available to assess the OLTC conditions. The drive mechanism of the tap changer can be diagnosed by acoustic and vibration fingerprints, motor power fingerprints or a position measurement of the driving axis. Static and dynamic resistance measurements are also used to assess the condition of the contacts. In addition, a temperature difference measurement can show heat development due to deteriorated contacts. OLTC analyzers can be used for offline transformer diagnosis and offer the following functionalities: • dynamic resistance measurement per tap and continuity of OLTC • check timing of the switching time control contactor • position measurement on the OLTC drive axis • motor power measurement on the OLTC drive.

Dissolved gas analysis (DGA) Early detection and accurate location of gassing sites in power transformers can be instrumental in preventing catastrophic failure. Electrical and thermal faults break the C-H and C-C bonds in the oil molecules resulting in a range of small unstable ions and radicals. The concentrations of the following dissolved gases in oil are measured in parts per million by volume (ppm v/v): hydrogen (H2), oxygen (O2), nitrogen (N2), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), ethylene (C2H4), ethane (C2H6), acetylene (C2H2), and propane (C3H8). DGA is a very efficient and reliable tool for the earliest detection of inception faults in transformer and other electrical equipment using insulating oil. The test results from DGA and moisture are used to assign condition codes for the bushing as shown in Table 3, where the 95% threshold concentration of these gases are taken from IEC 60599 to define this condition. The results are condensed to the following five main categories and their diagnostic significance, these are: • acetylene gas levels S arcing • moisture levels S high moisture • total carbon monoxide and dioxide gas S cellulose breakdown • hydrogen gas levels S corona • high hydrocarbon levels S thermal.

Partial discharge monitoring (PDM) systems Kraft paper (cellulose) immersed in mineral oil is used as the insulation IEEE POTENTIALS

Table 3. IEC DGA threshold levels in ppm and summary of percentiles, conditions, and actions for transformer bushing. Percentile

C2H2

C2H4

C2H6

CH4

H2

H2O

Code

Condition

Action

IEC60599 (95%)

2

30

70

40

140

—

—

Threshold

—

99%

>16

>27

>202

>116

>441

>63

4

Extreme

Immediate sampling

system for the copper windings in large power transformers. As the system ages under loading condition, the paper and oil can degrade, potentially leading to catastrophic failure. PD is an important tool for improving the reliability of HV insulation systems. It is a very sensitive and nondestructive method for evaluating of the health of the insulation of any HV equipment. PD is always associated with the degradation of insulation systems in HV equipment. Therefore, PDs need to be detected, measured, located, and reduced to a safe value so that the quality of the insulation system is not affected. Cost benefits of online PDM system can be achieved by several items: • detection of incipient faults and prevention of downtimes. • extension of service lifetime. • use of condition-based maintenance instead of time-based maintenance. • higher overload capacity of transformers. • avoidance of collateral damages.

Frequency response analysis (FRA) The frequency response analysis (FRA) technique is based on the fact that every transformer winding has a unique signature of its transfer function that is sensitive to changes in the parameters of the winding, namely resistance, inductance, and capacitance. It consists of measuring the impedance of offline transformer’s winding over a wide range of frequencies and comparing the results of these measurements with a reference set taken either during installation or at any other point of time. Difference in signature of the responses may indicate damage to the transformer, which can be investigated further using other techniques or by an internal examination.

nostic tool for condition monitoring of the oil-paper insulation of power transformer. RVM improves the ability to detect the content of water concentration and the ageing process in the oil-paper insulation system and can thus be an indicator of insulation quality and ageing condition.

Acoustic detection of PD The acoustic wave induced by PD can be measured and used for monitoring, diagnosing, and locating potential failures in the transformers. Acoustic detection of PD is based on the detection of the mechanical waves propagated from the discharge site to the surrounding medium. This signal is created because when the current streamer is formed within the void, the material around the hot streamer is vaporized. This vaporization causes an explosion of mechanical energy, which then propagates through the transformer tank in the form of a pressure field. Acoustic detection has been widely used in diagnostics of transformers. The use of piezoelectric transducers attached to the transformer tank wall has been the most favored approach in transformers. In addition, fiber optic sensors have been shown to be attractive devices for PD detection because of a number of inherent advantages including small size, high sensitivity, electrical nonconductivity, and immunity to electromagnetic interference. The primary advantage of using acoustic detection over chemical and electrical methods is that position information is readily available from acoustic systems using sensors at multiple locations. This position information can help to identify the type of PD as well as the location and severity of an insulation fault. Acoustic detection has another advantage that observations can be made in the presence of large electromagnetic disturbances.

New trends Return voltage measurement (RVM) Return voltage measurement (RVM) is one of the methods currently being investigated as a possible, nonintrusive diagMAY/JUNE 2011

Dryformers Recently, the mica/epoxy insulation, which has been used in rotating

machines for over a hundred years, is now being replaced by a new concept using HV cross-linked polyethylene (XLPE) cables. Dryformer is a new oilfree HV transformer based on cable technology first used in revolutionary new generator, Powerformer. Forced-air cooled, it has innovative windings made from XLPE cables with circular conductors. Dryformer is designed at present for primary voltages of 36 kV–145 kV and power ratings up to 150 MVA. The absence of oil means that there is no risk of ground or water pollution in the event of damage and a less risk of fire or explosion. Therefore, Dryformer can be sited closer to the consumer, for example below ground and in urban or ecologically sensitive locations. As the electric field is fully contained within the XLPE cable and the cable surface is at ground potential, Dryformer offers unique opportunities for optimizing power transformer design. By using the state-of-the-art of cable technology, XLPE cable can have electric field strengths up to 15 kV/mm. From a manufacturing perspective, the Dryformer has the considerable advantage of having the insulation system built up at the cable factory (unlike in oil/paper insulation, where a thorough drying out process using a combination of high temperature and vacuum and quick assembly is required).

Gas-insulated transformers (GITs) In GITs, SF6 gas is used as insulation media with relatively low gas pressure (0.14 MPa). The principal solid insulation material for the GIT winding is polyethylene terephtalate (PET) and polyphenylene sulphide (PPS) films which are defined as class E insulating material with a temperature limit of 120 °C. It has been initially specified to operate GITs, especially the gas-natural air-natural (GNAN) distribution GIT, with top gas temperature limit of 110 °C instead of the maximum conventional 95 °C top oil temperature for oil-immersed 41

transformers. However, this additional 15 °C of top gas temperature will • put a higher thermal stress to all auxiliary plants adhering or close to transformer body • affect any contact/anti-oxidation grease for the LV connections, especially if aluminum connection is involved • accelerate aging of O ring, polymeric bushing and may lead to premature failures and gas leaks. There have been a number of such interface problems for those heavily loaded GIT caused by gas temperatures higher than 100 °C. Corresponding counter measures, such as oversizing the first section of LV busbar, adoption of higher temperature class bushing material, and modifying clamping design to absorb higher temperature fluctuation have to be introduced. It is noted that such interface problems can be readily resolved but should have been avoided if special attention is made in the design stage. GIT have the following advantageous features: • Noninflammable and nonexplosive, hence they are usable for multistoried buildings, underground markets and other overpopulated places. • Moisture resistant and dust resistant, therefore, they are unaffected by open air moisture, dust and other ambient conditions since the windings and core of these transformers are fully enclosed in mild steel box and sealed with SF6 gas. In addition, they have easy maintenance and check because these transformers are hermetic sealed with an inert SF6 gas and materials are scarcely deteriorated. • Clean as there are no contaminations to surroundings since these transformers are sealed with nonpoisonous, odorless SF6 gas, even if the SF6 gas leaks unlike mineral oil-immersed transformers. • Higher reliability with simple internal structure. • Better compatibility with gas-insulated switchgear (GIS). GIT with onload tap changer (OLTC) was first introduced in 1987. OLTC used to be the most vulnerable part of any power transformer from electrical and mechanical points of view. In line with the use of SF6 gas as insulation media, vacuum switch type OLTC is installed for transmission GIT at 30 MVA and above. These vacuum switches housed inside the gas chamber are used as diverter switches and no arcing product can be possibly produced. Such OLTC is basi42

cally maintenance free. In the extreme case when OLTC malfunction due to mechanical defect or connection problem, the damage will be minimal. GIT turn out to be cheaper than oilimmersed transformers when maintenance costs are considered. Power-distribution transformers have a high recycling value because they can be easily disassembled and their chief constituents, which are high-purity steel, aluminum and copper, can be recycled indefinitely. GIT are far more easily recycled than oil-immersed types. SF6 gas can be reused and PET film recycled into containers, carpets and other products, allowing fully 96% of the natural resources in GIT to be recycled after decommissioning. The oil and paper insulation of oil-immersed transformers cannot be reused, which lowers the reuse ratio to only 71%. In addition, GIT give even greater cost and energy advantages. Nowadays, Toshiba GIT are commercially available with ratings up to 300 kV and 400 MVA (world’s largest GIT capacity and highest voltage). Here are some measures to improve the cooling capability and hence increasing the ratings: • Raise the SF6 gas pressure to 0.5 MPa. • Produce as large gas flow as possible by optimizing the layout of gas ducts through the windings • Develop high capacity gas blower with high reliability. • Apply high thermal-resistant insulating materials to raise the limit of winding temperature rises.

HVDC transformers In 2008, Siemens has opened up a new dimension of HVDC technology with the construction of ±800 kV system in China which represents the first in the world. As a core component of every HVDC transmission system, the converter transformers must also transform the voltage of the power grid to a voltage of 800 kV. To increase transmission capacity, the operating voltages have been increased to 800 kV, 60 percent higher than the peak DC transmission voltage of 500 kV normal for today’s HVDC transmission systems. Large HVDC transformers are normally single-phase transformers. However, the valve windings for the star and delta connection are configured either for one core with at least two main limbs or separately for two cores with at least one main limb depending on the rated power and the system voltage. Appropriately-sized return limbs ensure good

decoupling for a combined arrangement of windings. The windings of HVDC transformers also look very similar to those of HVAC transformers. On the other hand, the insulation structure between windings and to ground, valve side leads and bushings are different from those of HVAC transformers. HVDC transformers are subject to operating conditions that set them apart from conventional system or power transformers. These conditions include: • combined voltage stresses with both ac and dc • high harmonics content of the operating current • dc premagnetization of the core.

HTS transformers Following the discovery of high-temperature superconducting (HTS) materials in 1986, several studies looked into the feasibility of HTS transformers. Previously developed low-temperature superconductors (LTS) required cooling by liquid helium to about 4.2 K, with advanced cryogenic technology that is expensive both in terms of cost and of refrigeration power expended per unit of heat power removed from the cryostat. The technology for the new materials, which is based on liquid nitrogen (LN2) at temperatures up to about 78 K, is simpler and cheaper, and the ratio refrigeration power used to heat removed is reduced from over 1,000 to about 25. HTS materials, such as Bi-2223 systems in LN2, allow making long wires with critical current density more than 104 A cm⫺2 have been developed. The feasibility of HTS transformers is also considered from the viewpoints of high performance of the windings and lower cost of refrigeration. HTS transformers offer the following economical, operational, and environmental advantages: • higher efficiency • emergency overload capability up to twice the normal rating • lower leakage reactance • improved voltage regulation and reliability • material properties of HTS wire provide fault current limiting capabilities • significant estimated cost saving and less weight/size • relevance to areas with high population density • environmentally friendly • possibility of indoor sitting. The potential market for superconducting transformers worldwide has IEEE POTENTIALS

shown that more than 90% of units are in the 10 MVA ⫺100 MVA range. Recently, a 3-phase, 100 MVA, 154 kV/22.9 kV HTS transformer has been designed on a common magnetic core, and a 1-phase 33 MVA HTS transformer with OLTC.

• S. Tenbohlen, T. Stirl, and G. Bastos, “Experience-based evaluation of benefits of online monitoring systems for power transformers,” in CIGRE Session 2002, Paris, 2002, Paper 12-110. • A. Bossi, “CIGRE-WG 12-05: An international survey on failures in large power transformers in service,” Electra No. 88, 1983, pp. 21–48.

Conclusions • Power transformers are the most expensive and strategic components of any power system. Insulation breakdown is a serious failure of large power transformers that can generate substantial costs for repair and financial losses due to inadvertent outages. There are three main types of power transformers, namely, oil-immersed, gas-insulated, and dry-type transformers with or without cast coil insulation system. • World electricity demand is projected to be doubled between 2000 and 2030 at an annual growth rate of 2.4%, where its largest growth occurs in developing countries. The growth of loads and the increased bulk power transactions accelerate transformers’ physical ageing process as a result of increasing the operating stresses. Recently, 1,100 kV ac system is installed in China, Japan and Russia and the 1,200 kV ac system in due course is expected to emerge as main transmission level in India within five years. • The highest failure rates occur at power transformers in the upper voltage levels. Besides the active part (windings), the onload tap changers (OLTC) and the bushings should be monitored, where these three defective components represent about 84% of the failure statistics of these transformers. There are two ageing mechanisms of OLTC, namely, switching arcs and the so-called long-term effect on the changeover selector when the tap changer is motionless. In 2000, the first high-speed resistor vacuum type OLTC for in-tank installation was commercially available. Winding failures are governed by the ageing/weakness of the solid insulation system. Ageing of transformer oil can be affected by PD and thermal degradation. About 90% of all preventable bushing failures are caused by moisture entering the bushing through leaky gaskets or other openings. In addition, about 50% of the HV bushing failures are violent and involve collateral damage. • Many of the well-known preventive maintenance techniques are dis-

MAY/JUNE 2011

cussed, such as dissolved gas analysis (DGA), thermal monitoring, partial discharge measurement, capacitance and loss tangent “dissipation factor,” frequency response analysis, etc. They are performed on transformers for a specific type of problem. • ABB has been recently launched Powerformer, Dryformer, and Windformer, where all utilize a new concept using HV cross-linked polyethylene (XLPE) cables. • Recently, Toshiba GIT are commercially available with ratings up to 300 kV and 400 MVA (world’s largest GIT capacity and highest voltage). GIT have many advantageous features over the oil-immersed transformers, and its top gas temperature limit is 110 °C, i.e., higher than that of the latter by 15 °C leading to a higher loading capability. • Converter transformers represent core components of every HVDC transmission system. In 2008, Siemens has opened up a new dimension of HVDC technology with the construction of ±800 kV system in China which represents the first in the world. • The discovery of HTS materials in 1986 has opened a new dimension for HTS transformers. Recently, a 3-phase, 100 MVA, 154 kV/22.9 kV HTS transformer has been designed on a common magnetic core, and a 1-phase 33 MVA HTS transformer with OLTC. HTS transformers represent a key technology for future power systems engineering as they offer many economic, operational, and environmental advantages.

Read more about it • J. Singh, Y. R. Sood, and R. K. Jarial, “Condition monitoring of power transformers—Bibliography survey,” IEEE Electr. Insul. Mag., vol. 24, no. 3, pp. 11–25, May–June 2008.

• S. Jeszensky, “History of transformers,” IEEE Power Eng. Rev., pp. 9–12, vol. 16, no. 12, Dec. 1996. • Standard General Requirements for Dry-Type Distribution and Power Transformers Including Those with Solid Cast and/or Resin-Encapsulated Windings, IEEE Standard C57.12.01, 2005. • IEEE Standard Terminology for Power and Distribution Transformers, IEEE Standard C57.12.80-2002, Nov. 13, 2002, pp. 1–44. • IEEE Standard for Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, IEEE Standard C57.12.002006, Feb. 28, 2007, pp. 1–57. • Mineral Oil-Impregnated Electrical Equipment in Service—Guide to the Interpretation of Dissolved and Free Gases Analysis, IEC Standard 60599, Edition 2.1, 2007. • S. W. Lee, S. B. Byun, W. S. Kim, J. K. Lee, and K. D. Choi, “Design of a single phase 33 MVA HTS transformer with OLTC,” IEEE Trans. Appl. Superconduct., vol. 17, no. 2, part 2, pp. 1939–1942, June 2007. • I. A. Metwally, R. M. Radwan, and A. M. Abou-Elyazied, “Powerformers: A breakthrough of HV power generators,” IEEE Potentials, vol. 27, no. 3, pp. 37–44, May/June 2008.

About the author Ibrahim A. Metwally (metwally@squ. edu.om) earned a B.Eng. in electrical engineering and both an M.Eng. and a Ph.D. in high-voltage engineering. He is a permanent professor with the Department of Electrical Engineering at Mansoura University, Egypt, and currently on leave as a professor with the Department of Electrical and Computer Engineering, Sultan Qaboos University, Oman. He is a Senior Member of the IEEE and a fellow of the Alexander von Humboldt Foundation in Bonn, Germany.

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