Reliability in the electric power industry formerly came from the ability to plan power systems with significant redundancy. Increasing competition and limited resources are forcing utilities to reduce capital spending and find cost-effective resources and solutions to optimize their new equipment and apply appropriate preventative maintenance of their system.
It is only fair for utilities to make the most efficient use of their transformers using various approaches. The most common approach includes a desire and need for increased loading. In some cases, it has been shown through an overall life cycle cost analysis that it is more profitable to overload existing transformers and accept the penalty of increased loss of life, than to relieve the loading by installing larger or more transformers. Increased loading appears to be a commonly used mean but it can lead to serious problems if no efficient method is applied to accurately and safely monitor the necessary parameters. The most important parameter in a transformer’s life cycle relies on the certainty of the winding’s hot spot(s) value through the transformer’s lifetime.
Fiber optic sensors are an excellent and increasingly cost effective way to access capacity and gather information on the unit’s health through direct temperature measurement of the hot spot. Fiber optic temperature sensors were previously dedicated for use in large power transformers basically due to the expenditure of the fiber optic systems opposed to the cost of the transformer itself. The use of a direct method allows accurate measurement of the transformers’ hot spot for an increased knowledge of operational condition assessment; load planning, asset management and end of life determination.
Considering their design particularities and the impact of temperature on aging of the insulation and life expectancy, the need for direct temperature measurement of small, medium and large power and distribution transformers is much needed.
Hot Spots and their known consequences
For a long time, transformer load was applied conservatively and it was common to add or replace a transformer when it surpassed the 60%-70% load levels. Such load levels did indeed preserve the transformer’s health over longer periods of time as demonstrated with transformers which operated or have been in operation for over 40 years. Nowadays, it is more common to run the transformer closer to its maximum nameplate capacity, planning less redundancy and causing more frequent overloading of the transformer. This situation has a direct impact on the winding hot spot temperature
According to ANSI/IEEE C57.12.80-1978, the continuous rating of a transformer is “The maximum constant load that can be carried continuously without exceeding established temperature-rise limitations under prescribed conditions”. There is a wide array of temperature limitations such as ambient temperature, top oil temperature, core lamination temperature, average winding temperature rise and the maximum winding temperature rise.
One of the most important parameters in determining the balance between lifetime and load within the liquid filled transformer is the “hot spot” temperature or the maximum winding temperature rise. This represents the temperature of the hottest part within the transformer, typically residing in the windings. Should the hot spot exceeds given limits, the rate of deterioration of the solid and liquid insulation system in the transformer will accelerate rapidly.
Temperature has a major influence on the aging of the insulation and on the transformer’s lifetime. An accurate prediction of the hot spot in the winding is consequently very important for both manufacturers and end-users considering the hot spot is responsible for the degradation of the transformer oil and paper insulation.
Since the temperature distribution is not uniform within a transformer, the insulation paper will ordinarily undergo the greatest deterioration when exposed to high temperatures. When insulation paper loses its properties, it weakens the transformer both dielectrically and mechanically. In addition, higher winding hot spot temperatures not only weaken the winding insulation material but it can ultimately result in the formation of gas bubbles that facilitate the dielectric breakdown of the transformer’s oil. Needless to say that, a similar situation is undesirable and as it is in most cases irreversible.
Transformers
Manufacturers offer various transformer types and designs according to the utility’s requirements and needs. Some transformer designs are more at risk considering their complexity and special characteristics. Fibre optics monitoring system provides added value to these specific types of transformers.
• Autotransformers; these transformers tend to have a higher leakage flux when compared to a typical transformer.
• Converter transformers; these transformers show fundamental differences in the flux pattern.
• Axial split designs; Transformers such as generator step-up (GSU’s) designed with two low voltage windings build one on top of the other. Cooling and flux distribution is not as straightforward as with typical transformers and therefore require close monitoring.
• Phase shifters
• Zigzag
• Stepdown transformers
• Repaired or upgraded transformers; specially if a know weakness of the design have been find out
There are other aspects to be considered when deciding to choose direct fiber optic monitoring versus mechanical or electronic simulated methods. A transformer presenting special characteristics is often subjected to natural but yet undesirable phenomena.
• High impedance; causing more leakage flux, worsening the hot spot
• High harmonics; knowing that the magnetic leakage flux is proportional to the square of the frequency. For example, the third harmonics (180Hz) generates 9 times more flux than the fundamental (60Hz), contributing to the winding Eddy losses.
Lastly, it is important to consider in which condition the transformer will operate regardless of its design and/or size.
• Load situation
• Overload situation
• Critical transformer
• Replacement/Backup transformer
• Physical location; According to According to the IEEE C57.12.90 – 1999 (Revision of IEEE C57.12.90-1993) IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers an altitude correction must be applied to the temperature rises.When a transformer tested at an altitude of less than 1000 m (3300 ft) is to be operated at an altitude above 1000 m.
The Use of Fiber Optics Inside Small, Medium and Distribution Transformers
The use of fiber optics has mainly been applied to large power transformers due to their complex design and the benefits gained from direct temperature monitoring are easily paid back. This same technology is now applicable to small, medium and distribution transformers considering it offers an excellent and increasingly cost-effective solution for direct, accurate and real time hot spot temperature measurement.
The distribution transformers tight insulation design and ongoing utilization along with less cooling justify the significant needs for direct, accurate and real time overall temperature of smaller power and distribution transformers. In small distribution transformers the temperature rise is significantly higher due to the number of turns and layers in the windings, compared to larger transformers which have less turns and more cooling ducts. Outdoor ambient conditions may have an effect on the loading capacity of the distribution transformers according to the IEC 60076-7.
Unlike the large power transformers there is no predetermined top oil and hot temperature for the distribution transformers under short term emergency loading but is however known that here may be gas bubble formation should the hot spot exceed 140 C°. Fiber optics is the most accurate and suitable monitoring method to validate hot spot temperatures within the windings.
Small and medium transformers may also necessitate direct fiber optic temperature monitoring depending on operating conditions as mentioned previously.
Benefits of direct temperature monitoring using fiber optics
There are technical benefits associated to the use of fiber optics for hot spot monitoring of transformers.
• Provides assistance in transformer design and rating verification;
• Safely maximizes loading and overloading;
• Reduces overall physical stress on winding;
• Prevents premature failure;
• Prevents outages and catastrophic failures.
There are also economical benefits such as better knowledge of operational condition assessment, load planning, and asset management. End of life determination becomes easier to plan and consequently to manage considering there are less costly interventions.
• Reduces inspection and maintenance costs;
• Reduces failure-related repair or replacement costs;
• Improves real-time transformer loading capability;
• Defers upgrade capital costs due to load growth;
• Defers replacement capital costs due to equipment age or condition.
Available Technologies
Point sensing fiber optic sensors are the most commonly used for hot spot monitoring. There are a few technologies available such as GaAs (Gallium Arsenide) Absorption and Fluoroptics.
Fiber optic temperature sensors for direct winding temperature measurement in transformers are based on light absorption and transmission of light by a semiconductor (GaAs). The effects of temperature variations using this semiconductor are well know and predictable. A tiny GaAs semiconductor is bonded to one end of a well polished optical fiber using high temperature adhesive.
The sensor is comprised of a multimode optical fiber packaged in two layers of durable bright yellow and royal blue PTFE TeflonTM, which is terminated with a semiconductor (GaAs) crystal and a dielectric reflective coating at the fiber tip. The sensors robust and all-dielectric construction offers excellent thermal (-30 to 225° C largely covers the expected range), high tolerance to multiple connections and bending and chemical resistance to oil and kerosene vapour along with a high tolerance to vibration and installation stresses.
Light that is transmitted through the semiconductor impinges on the dielectric reflective coating at the end of the sensor, and is then reflected back to the spectrum analyzer via the optical coupler. This optical signal is converted into an electrical signal using a CCD (charge-coupled device), and processing electronics then evaluate the cutoff wavelength of absorption within the multi-wavelength spectrum. Consequently, analysis of the optical spectrum by the spectrum analyzer is equivalent to knowing the semiconductor’s temperature within the transformer windings. The algorithm/calculations does not depend on light intensity but its spectral response. Since this communication is performed optically, using highly dielectric components within the transformer itself, the reliability of the signal is never compromised by the electric fields.
Although there are several fiber optic direct winding temperature measurement solutions, product reliability, company knowledge, cost-effectiveness and efficient support should be considered. Well established and knowledgeable companies constantly thrive to offer the best solution possible by innovating and offering instrument grade solution adapted for the electric industry.
Up to this day, they are a certain number of available products on the market. Non- experienced players and OEM type products may not be the most suitable choice as they may not be as reliable and well adapted for the specificity of the application. Their knowledge and experience may be very limited and they are not in a position to offer a successful and proven history regarding the long term reliability of their products. It is important to note that OEM type products’ environmental and shielding (EMI, RF) capabilities may be questioned unless integrated within a thoroughly certified third party unit. ISO 9001 compliant suppliers as well as product certification demonstrates and ensures the company’s genuineness and stringent quality standards in design and manufacturing of the product.
Conclusion
Initial investment, design complexity and the utilities’ monitoring capabilities does indeed play an important role in deciding to opt for the optical method.
There is a real benefit associated to on-line monitoring. According to the article “Profitability Assessment of Transformer On-Line Monitoring and Periodic Monitoring” published by Jacques Aubin, André Bourgault and Claude Rajotte, it is estimated, that the failure resolution can amount to approximately $13,000 USD/year for equipment which do not use monitoring compared to $ 6000 USD/year when using monitoring. If you compare the cost associated for the purchase, installation and annual support based on a 20 year expected life of monitoring system, which grossly amounts to $1025 USD/year, there is an annual benefit from failure resolution cost of approximately $ 6000 USD/year.
For a fraction of the overall transformer cost, utilities can integrate a fiber optic “insurance policy. Choosing to do so does not only provide you with long term thermal information but it also provides you economic benefits such as reduced inspection, maintenance costs, failure and improved real-time transformer loading capability confidently.
About the Author
Sharon Walsh, (B.Sc.A) is the Product Line Manager for the Energy/Oil and Gas Industry for FISO Technologies Inc. She previously acted as the Sales Manager for the transformer market and has been employed by FISO for 5 years. She was formerly employed by Nortech Fibronic which also specialized in sensors for transformer monitoring.
Ms. Walsh earned her Bachelor Degree in Consumer Affairs with a minor in Marketing from Laval University in Quebec, Canada.
It is only fair for utilities to make the most efficient use of their transformers using various approaches. The most common approach includes a desire and need for increased loading. In some cases, it has been shown through an overall life cycle cost analysis that it is more profitable to overload existing transformers and accept the penalty of increased loss of life, than to relieve the loading by installing larger or more transformers. Increased loading appears to be a commonly used mean but it can lead to serious problems if no efficient method is applied to accurately and safely monitor the necessary parameters. The most important parameter in a transformer’s life cycle relies on the certainty of the winding’s hot spot(s) value through the transformer’s lifetime.
Fiber optic sensors are an excellent and increasingly cost effective way to access capacity and gather information on the unit’s health through direct temperature measurement of the hot spot. Fiber optic temperature sensors were previously dedicated for use in large power transformers basically due to the expenditure of the fiber optic systems opposed to the cost of the transformer itself. The use of a direct method allows accurate measurement of the transformers’ hot spot for an increased knowledge of operational condition assessment; load planning, asset management and end of life determination.
Considering their design particularities and the impact of temperature on aging of the insulation and life expectancy, the need for direct temperature measurement of small, medium and large power and distribution transformers is much needed.
Hot Spots and their known consequences
For a long time, transformer load was applied conservatively and it was common to add or replace a transformer when it surpassed the 60%-70% load levels. Such load levels did indeed preserve the transformer’s health over longer periods of time as demonstrated with transformers which operated or have been in operation for over 40 years. Nowadays, it is more common to run the transformer closer to its maximum nameplate capacity, planning less redundancy and causing more frequent overloading of the transformer. This situation has a direct impact on the winding hot spot temperature
According to ANSI/IEEE C57.12.80-1978, the continuous rating of a transformer is “The maximum constant load that can be carried continuously without exceeding established temperature-rise limitations under prescribed conditions”. There is a wide array of temperature limitations such as ambient temperature, top oil temperature, core lamination temperature, average winding temperature rise and the maximum winding temperature rise.
One of the most important parameters in determining the balance between lifetime and load within the liquid filled transformer is the “hot spot” temperature or the maximum winding temperature rise. This represents the temperature of the hottest part within the transformer, typically residing in the windings. Should the hot spot exceeds given limits, the rate of deterioration of the solid and liquid insulation system in the transformer will accelerate rapidly.
Temperature has a major influence on the aging of the insulation and on the transformer’s lifetime. An accurate prediction of the hot spot in the winding is consequently very important for both manufacturers and end-users considering the hot spot is responsible for the degradation of the transformer oil and paper insulation.
Since the temperature distribution is not uniform within a transformer, the insulation paper will ordinarily undergo the greatest deterioration when exposed to high temperatures. When insulation paper loses its properties, it weakens the transformer both dielectrically and mechanically. In addition, higher winding hot spot temperatures not only weaken the winding insulation material but it can ultimately result in the formation of gas bubbles that facilitate the dielectric breakdown of the transformer’s oil. Needless to say that, a similar situation is undesirable and as it is in most cases irreversible.
Transformers
Manufacturers offer various transformer types and designs according to the utility’s requirements and needs. Some transformer designs are more at risk considering their complexity and special characteristics. Fibre optics monitoring system provides added value to these specific types of transformers.
• Autotransformers; these transformers tend to have a higher leakage flux when compared to a typical transformer.
• Converter transformers; these transformers show fundamental differences in the flux pattern.
• Axial split designs; Transformers such as generator step-up (GSU’s) designed with two low voltage windings build one on top of the other. Cooling and flux distribution is not as straightforward as with typical transformers and therefore require close monitoring.
• Phase shifters
• Zigzag
• Stepdown transformers
• Repaired or upgraded transformers; specially if a know weakness of the design have been find out
There are other aspects to be considered when deciding to choose direct fiber optic monitoring versus mechanical or electronic simulated methods. A transformer presenting special characteristics is often subjected to natural but yet undesirable phenomena.
• High impedance; causing more leakage flux, worsening the hot spot
• High harmonics; knowing that the magnetic leakage flux is proportional to the square of the frequency. For example, the third harmonics (180Hz) generates 9 times more flux than the fundamental (60Hz), contributing to the winding Eddy losses.
Lastly, it is important to consider in which condition the transformer will operate regardless of its design and/or size.
• Load situation
• Overload situation
• Critical transformer
• Replacement/Backup transformer
• Physical location; According to According to the IEEE C57.12.90 – 1999 (Revision of IEEE C57.12.90-1993) IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers an altitude correction must be applied to the temperature rises.When a transformer tested at an altitude of less than 1000 m (3300 ft) is to be operated at an altitude above 1000 m.
The Use of Fiber Optics Inside Small, Medium and Distribution Transformers
The use of fiber optics has mainly been applied to large power transformers due to their complex design and the benefits gained from direct temperature monitoring are easily paid back. This same technology is now applicable to small, medium and distribution transformers considering it offers an excellent and increasingly cost-effective solution for direct, accurate and real time hot spot temperature measurement.
The distribution transformers tight insulation design and ongoing utilization along with less cooling justify the significant needs for direct, accurate and real time overall temperature of smaller power and distribution transformers. In small distribution transformers the temperature rise is significantly higher due to the number of turns and layers in the windings, compared to larger transformers which have less turns and more cooling ducts. Outdoor ambient conditions may have an effect on the loading capacity of the distribution transformers according to the IEC 60076-7.
Unlike the large power transformers there is no predetermined top oil and hot temperature for the distribution transformers under short term emergency loading but is however known that here may be gas bubble formation should the hot spot exceed 140 C°. Fiber optics is the most accurate and suitable monitoring method to validate hot spot temperatures within the windings.
Small and medium transformers may also necessitate direct fiber optic temperature monitoring depending on operating conditions as mentioned previously.
Benefits of direct temperature monitoring using fiber optics
There are technical benefits associated to the use of fiber optics for hot spot monitoring of transformers.
• Provides assistance in transformer design and rating verification;
• Safely maximizes loading and overloading;
• Reduces overall physical stress on winding;
• Prevents premature failure;
• Prevents outages and catastrophic failures.
There are also economical benefits such as better knowledge of operational condition assessment, load planning, and asset management. End of life determination becomes easier to plan and consequently to manage considering there are less costly interventions.
• Reduces inspection and maintenance costs;
• Reduces failure-related repair or replacement costs;
• Improves real-time transformer loading capability;
• Defers upgrade capital costs due to load growth;
• Defers replacement capital costs due to equipment age or condition.
Available Technologies
Point sensing fiber optic sensors are the most commonly used for hot spot monitoring. There are a few technologies available such as GaAs (Gallium Arsenide) Absorption and Fluoroptics.
Fiber optic temperature sensors for direct winding temperature measurement in transformers are based on light absorption and transmission of light by a semiconductor (GaAs). The effects of temperature variations using this semiconductor are well know and predictable. A tiny GaAs semiconductor is bonded to one end of a well polished optical fiber using high temperature adhesive.
The sensor is comprised of a multimode optical fiber packaged in two layers of durable bright yellow and royal blue PTFE TeflonTM, which is terminated with a semiconductor (GaAs) crystal and a dielectric reflective coating at the fiber tip. The sensors robust and all-dielectric construction offers excellent thermal (-30 to 225° C largely covers the expected range), high tolerance to multiple connections and bending and chemical resistance to oil and kerosene vapour along with a high tolerance to vibration and installation stresses.
Light that is transmitted through the semiconductor impinges on the dielectric reflective coating at the end of the sensor, and is then reflected back to the spectrum analyzer via the optical coupler. This optical signal is converted into an electrical signal using a CCD (charge-coupled device), and processing electronics then evaluate the cutoff wavelength of absorption within the multi-wavelength spectrum. Consequently, analysis of the optical spectrum by the spectrum analyzer is equivalent to knowing the semiconductor’s temperature within the transformer windings. The algorithm/calculations does not depend on light intensity but its spectral response. Since this communication is performed optically, using highly dielectric components within the transformer itself, the reliability of the signal is never compromised by the electric fields.
Although there are several fiber optic direct winding temperature measurement solutions, product reliability, company knowledge, cost-effectiveness and efficient support should be considered. Well established and knowledgeable companies constantly thrive to offer the best solution possible by innovating and offering instrument grade solution adapted for the electric industry.
Up to this day, they are a certain number of available products on the market. Non- experienced players and OEM type products may not be the most suitable choice as they may not be as reliable and well adapted for the specificity of the application. Their knowledge and experience may be very limited and they are not in a position to offer a successful and proven history regarding the long term reliability of their products. It is important to note that OEM type products’ environmental and shielding (EMI, RF) capabilities may be questioned unless integrated within a thoroughly certified third party unit. ISO 9001 compliant suppliers as well as product certification demonstrates and ensures the company’s genuineness and stringent quality standards in design and manufacturing of the product.
Conclusion
Initial investment, design complexity and the utilities’ monitoring capabilities does indeed play an important role in deciding to opt for the optical method.
There is a real benefit associated to on-line monitoring. According to the article “Profitability Assessment of Transformer On-Line Monitoring and Periodic Monitoring” published by Jacques Aubin, André Bourgault and Claude Rajotte, it is estimated, that the failure resolution can amount to approximately $13,000 USD/year for equipment which do not use monitoring compared to $ 6000 USD/year when using monitoring. If you compare the cost associated for the purchase, installation and annual support based on a 20 year expected life of monitoring system, which grossly amounts to $1025 USD/year, there is an annual benefit from failure resolution cost of approximately $ 6000 USD/year.
For a fraction of the overall transformer cost, utilities can integrate a fiber optic “insurance policy. Choosing to do so does not only provide you with long term thermal information but it also provides you economic benefits such as reduced inspection, maintenance costs, failure and improved real-time transformer loading capability confidently.
About the Author
Sharon Walsh, (B.Sc.A) is the Product Line Manager for the Energy/Oil and Gas Industry for FISO Technologies Inc. She previously acted as the Sales Manager for the transformer market and has been employed by FISO for 5 years. She was formerly employed by Nortech Fibronic which also specialized in sensors for transformer monitoring.
Ms. Walsh earned her Bachelor Degree in Consumer Affairs with a minor in Marketing from Laval University in Quebec, Canada.
