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Low maintenance requirements of vacuum type tap changers – A benefit for ecofriendly green power transformers T. STIRL, C. BAUER, J. HARTHUN, M. SARAVOLAC GE Grid Solutions France, Germany

SUMMARY Utilities on principle seek for low maintenance equipment. A power transformer represents one of the centrepieces of a substation. It is thus very important that power transformers require less frequent maintenance intervals. For this scope of application hermetic power transformers offer various advantages. Compared with conventional power transformers they provide a substantially longer service life as well as reduced maintenance through lower oil aging, no dehydrating breather and no hydro-compensator (rubber bag). An elementary part of a hermetic power transformer is the vacuum type tap changer. This tap changer is fitted with vacuum switching chambers so that no switching arc occurs in the tap changer oil volume. The switching operations of conventional tap changers lead to contact erosion and pollution of the mineral oil resulting in quite short maintenance intervals. The benefit of this vacuum type tap changer is the absence of an oil decomposition initiated by switching operations. As a result, contamination is prevented. This type of tap changer opens up the possibility to connect the oil volume of the main tank and the tap changer. By merging the oil volume of the transformer and the tap changer, the design of the power transformer and the attached parts can be significantly improved. The advantages are: fewer components as well as far less time and expense for maintenance. In addition, the entire structure can be build more compact, which decreases weight and overall dimensions. This new concept can also be applied to conventional free breathing transformers. The “Green Transformer” concept combines all of these benefits with further modern technologies such as alternative environment-friendly insulting liquids, low noise, high efficiency, low maintenance and condition monitoring solutions. This paper reports about experiences with vacuum type tap changers in combination with green transformer technologies.

KEYWORDS Low maintenance, hermetic power transformer, vacuum type tap changer, common oil volume of tap changer and main tank, eco-friendly green power transformer

Tobias Stirl, [email protected]

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Introduction The technology of on-load tap changers has improved significantly in recent years with the advent of vacuum technology. This article expands on a new concept for consistently utilizing further advantages of the vacuum on-load tap changer. The key idea is that of providing a common oil volume for the on-load tap changer and the transformer’s main tank. This innovative solution can be used to significantly improve the design of a power transformer and its add-on parts. The advantage: fewer components mean less time and expense for maintenance and repair. This makes vacuum on-load tap changers the preferred choice for ease of maintenance when used in power transformers.

Low maintenance – The new hermetic transformer generation Besides applying vacuum type tap changers our strategy supports using further modern technologies with the aim of optimized components, optimized installation and less maintenance expenditures. In the following the advantages of a hermetically sealed design are described. During the conventional transformer operation, three mechanisms influence the cellulose ageing process (depolymerisation): a. Thermal ageing – At temperatures of about 105 °C the glucose rings will start splitting (depolymerisation). Typical ageing products are free glucose, water, carbon monoxide and carbon dioxide; b. Oxidative ageing – Oxygen leads to a further polymerization, even already at normal operation temperatures of less than 105 °C. Investigations have shown that the ageing rate is tripled when oxygen is present; c. Hydrolytic ageing – Water is the cause as well as the product of cellulose ageing and splits the oxygen bridges between the glucose rings.

Fig. 1:

Principle of low maintenance hermetically sealed power transformer with expandable radiators

By the application of the hermetic transformer design (Fig. 1) two out of the three described ageing processes are eliminated. Only the thermal ageing is still of interest [1]. The transformer tanks are hermetically sealed to protect the insulation liquid against atmospheric oxygen and therefore offer optimal conditions to reduce insulation ageing and to minimize maintenance rate. Additional equipment, e.g. the conservator, which is normally required to compensate the thermal expansion of the insulation and cooling liquid, could be omitted. The additional oil volume will be absorbed by special expansion radiators. A further benefit is the negligence of an additional air dehumidifier and additional pipework. The 2nd generation of hermetic design (Hermetic 2.0) is characterized by an optimized management of pressure conditions along with improved indicator for monitoring the pressure level in service. Table 1 shows the evolution of GE Grid Solutions hermetic design step by step.

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Table 1: Key development milestones of hermetic transformers [2] Year 2000 2006 2006 2007 2009 2010 2010 2014

Milestone Development of first generation up to 80 MVA/110 KV First transformer up to 230 kV First transformer above 100 MVA First transformer filled with natural ester, 31.5 MVA/110 kV More than 100 units built 10 years of experience in operation Development of common oil volume for on-load tap changer and main tank nd Development of 2 generation – Hermetic 2.0 – Optimized operational behavior

In the following chapter the technology of the “common oil volume for on-load tap changer and main tank” is described in more detail.

Innovative new concept using on-load tap changer with vacuum technology (VACUTAP®) Unlike to the power transformer a conventional on-load tap changer is maintenance intensive. In addition to moving mechanical parts, the electrical switching operations cause contact wear and therefore general signs of wear. The insulating oil also has to be monitored and may also have to be replaced in certain circumstances. Necessary maintenance work is unavoidable. In the 80s, special vacuum interrupters were developed and used as a substitute for the conventional contact materials. In the mid-90s, the control of dry-type transformers, in which this technology was used, became increasingly important. At the beginning of the last decade, vacuum interrupters were fitted even in the on-load tap changers which were used as an in-tank installation in oil-immersed transformers. The vacuum on-load tap changers thus developed are distinguished by significantly lower wear of the switching contacts, practically no oil degradation and the considerably extended maintenance intervals [3]. This ease of maintenance was hugely important to the operators of power transformers with the result that they took the place of conventional arc-breaking-in-oil tap changers. This development step was less important for the manufacturers of power transformers. Because no changes were visible from the outside, the new development was barely noticed during the manufacture of new power transformers and during the final electrical tests in the research centre. Following successful introduction of the vacuum technology, it is obvious to optimize the power transformer’s design to make full use of other advantages offered by the vacuum on-load tap changer. Since the load switch operation is performed inside sealed vacuum chambers and the protective spark gaps inside the diverter switch have been substituted by varistors, it is no longer necessary to isolate the two oil volumes of the diverter switch and main tank from each other. The idea of a common oil volume The new concept is distinguished by the use of on-load tap changers with vacuum technology (VACUTAP®). The basic idea is that of providing a common oil volume for the on-load tap changer and the transformer’s main tank. Due to the use of vacuum technology, the amount of gases caused by the switching operation is very low with the result that the gas-in-oil analysis (DGA) is not measurably affected by it. This is possible because of the lack of switching arcs in the oil due to this operating principle and due to a special design of the on-load tap changer’s transition resistors, in which the excess temperatures of the resistors are so low under all operating conditions to be anticipated that no free gases are formed. Initial experiences with transformers in operation have confirmed this. The

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values of the gas-in-oil analyses show no difference between the common oil volume and separate oil chambers. The design of a power transformer and its add-on parts can be simplified considerably by combining the oil volumes of both transformer and on-load tap changer. This new possibility can be applied to both free-breathing transformers and sealed transformers. An improved protection concept is applied to the on-load tap changer at the same time. The advantage: fewer components mean less time and expense for maintenance and repair. The different variants of a common oil volume are discussed in greater detail in the following chapter. Common oil volume in free-breathing transformers In free-breathing transformers, the temperature-related change in volume of the insulating oil is accommodated by an oil conservator. Typically, both the main tank and the on-load tap changer are each fitted with an oil conservator. Both oil conservators are usually fitted with other components such as an oil level indicator, pipes, gate valves and a dehydrating breather. In the new concept of a common oil volume in free-breathing transformers, the separate oil conservator for the on-load tap changer is omitted and not replaced. The pipe downstream of the on-load tap changer’s protective relay is connected to the pipe that leads into the main oil conservator. As a result, the oil chambers are connected to each. The other components previously necessary on the onload tap changer’s oil conservator, such as pipes, gate valves, dehydrating breather and oil level indicator, are omitted (Figs. 2 and 3).

Fig. 2:

Diagram of the new concept of a common oil volume in free-breathing transformers, green: new connecting pipe, red: saving on oil conservator components for the onload tap changer with pipes, gate valves, oil level indicator and dehydrating breather

1N

1U

1V

C L

1W

C L

Pipe for connecting the oil volumes

3U1 3W2

2N

Protection Relay RS2001 Fig. 3:

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2U

1U

2V

2W

1V

1W

New concept of a common oil volume based on the example of a free-breathing 120 MVA transformer

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Common oil volume in sealed transformers Investigations were carried out at Maschinenfabrik Reinhausen which were intended to highlight the efficiency of the protective relay as well as any differences between free-breathing transformers and sealed transformers. With free-breathing transformers, a surge protector is fitted as standard in the pipe between the onload tap changer head and the oil conservator. This oil flow-controlled relay is provided to protect the on-load tap changer and the transformer in the event of a fault inside the on-load tap changer’s or the selector switch’s oil tank. It trips if the specified oil flow speed is exceeded due to a fault. In sealed transformers, the previous protection concept consisted of a combination of pressure relief device (PRD) and double float Buchholz relay. The function of the Buchholz relay’s top float was to release any accumulations of gas present with the help of a solenoid valve. During the investigations, the operating principle of the conventional oil flowcontrolled relay was tested first of all in combination with an expandable radiator such as is used in a sealed transformer instead of the conventional oil conservator (Fig. 4). The investigations showed that the spring characteristic (stiffness) of an expandable radiator behaves linearly. Flow pulses which would lead to tripping of the protective relay in the event of a fault in the on-load tap changer were simulated with a defined volumetric flow rate. The influence of the oil temperature (viscosity of the oil) was also investigated. The higher the temperature, the Fig. 4: Test setup at Maschinenfabrik Reinhausen more oil has to be transported to cause the for investigations on expandable radiators surge flap in the protective relay drop. At low temperatures, on the other hand, a higher pressure is needed to make the protective relay trip. It was established as a result that, with regard to the protective relay, a correctly filled sealed system with expansion radiator behaves in practically the same way as a “classic” free-breathing system. During the investigations regarding the new concept of a common oil volume in sealed transformers, this knowledge was used to investigate the existing protection concept for the on-load tap changer with regard to the possibility of further optimizations. Approximately 10 years of positive operating experience are evidence that this concept has definitely proven itself. Even if the function of the oil flow-controlled relay in conjunction with expansion radiators is guaranteed in principle, it seems sensible to retain the existing configuration of pressure relief device and Buchholz relay in sealed transformers. This is the only way of implementing a protection concept that is capable of functioning under all operating conditions and independently of the pipe geometries. In this case, the pressure relief device assumes the function of the prescribed protective relay which triggers the circuit breaker in the event of a fault. The pressure relief device guarantees extremely responsive, effective protection of the on-load tap changer which always triggers under the same, precisely defined conditions regardless of the solution chosen for hermetic sealing (e.g. solution with expansion radiators or solution with hydro-compensator). The Buchholz relay assumes the function of the gas alarm with signalling function.

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In the new concept of a common oil volume in sealed transformers, the separate expansion radiator with its pipes to the on-load tap changer is omitted. According to the protection concept described, the oil conservator pipe of the on-load tap changer is connected instead directly to the Buchholz relay of the main tank which therefore also acts in a “dual function” as a gas alarm for the on-load tap changer (Fig. 5). The use of vacuum technology has shown that in practice no free gases were generated. The solenoid valve used in the old concept has never actuated during operation. For this reason, the Buchholz relay with solenoid valve is omitted in the new concept.

Radiator for Tap Changer

Solenoid Valve

PRD

Buchholz Relay Tap Changer

Buchholz Relay Tank

Buchholz Relay Tank

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VACUTAP® Tap Changer

VACUTAP® Tap Changer

Old concept

New concept

Fig. 5:

Diagram of the protection concept of sealed transformers: previous solution (left) and the new simplified concept of a common oil volume (right)

Fig. 6:

New concept of a common oil volume based on the example of a 40 MVA sealed transformer

Fig. 5 illustrates the previous concept and the innovative solution in diagram form. The concept is illustrated in Fig. 6 based on the example of a 40 MVA sealed transformer. Fig. 7 shows a detailed view, the area of the on-load tap changer with its very compact pipe connection to the main tank of the transformer [4].

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Flange to transformer tank

Blind flange

Pipe for connecting the oil volumes Buchholz relay Main tank Sealed transformer

Pipe for draining the on-load tap changer

Pressure relief device PRD (on-load tap changer)

On-load tap changer head Fig. 7:

Detailed view of the new concept of a common oil volume in sealed transformers, area of the on-load tap changer head with compact connecting pipe to the main tank of the transformer

To top the overall concept off in the following chapter a reference project is presented which combines the new concept of common oil volume with additional modern technologies which support low maintenance benefits.

Practical experiences with the new green transformer generation Based upon the performed investigations and the gained experience a new design concept for power transformers has been developed. The new “Green Transformer” concept of GE Grid Solutions combines several modern technologies, such as alternative insulating liquids, low noise, high efficiency, cooling optimization, hermetic sealing of the tank and on-line monitoring solutions. In cooperation with a railway utility in Germany and GE Grid Solutions (Fig. 8), two one-phase transformers were built in 2015 [5]. Natural ester was used as insulation liquid instead of common mineral oil. The conductors of the windings were insulated with aramid polymer, which is extremely temperature resistant and tear-proof. Thus, the units are designed for temporary overload operation and obtain a higher expected lifetime at normal operation. Other insulation components like insulation barriers or winding platforms still consist of pressboard and/or laminated wood. Consequently, the inserted insulation is a semi-hybrid insulation system according to IEC 60076-14 [6]. The special insulation reduces the ageing process of the windings compared to common paper insulation, as the ageing is only relevant for the remaining conventional insulation, e.g. insulation barriers. Fig. 9 shows the high voltage (HV) and low voltage (LV) windings of these transformers.

Fig. 8:

10 MVA/110 kV trackside power transformer, now a green project launched with a German railway utility with ester liquid, hermetically sealed, noiseless design (sound pressure level @ 0.3 m of 35 dB(A)) and high efficient hybrid insulation

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Fig. 9:

Windings insulated with aramid polymer for the new Green Transformers for a German railway utility a) HV hybrid winding b) LV hybrid winding on winding machine

Conclusion The ease of maintenance of vacuum on-load tap changers is hugely important to the operators of power transformers with the result that they have taken the place of conventional oil type circuit breakers. Following successful introduction of the vacuum technology, it was obvious to optimize the transformer’s design to make full use of other advantages of the vacuum on-load tap changer. The basic idea was that of providing a common oil volume for the on-load tap changer and the transformer’s main tank. This innovative solution can be used to significantly improve the design of a power transformer and its add-on parts. This new principle can be applied to both free-breathing transformers and sealed transformers. The advantage: fewer components mean less time and expense for maintenance and repair. Besides this, experiences with a combination of the new generation of green technologies are depicted with the example of the German Railway project. In this project a natural ester is used in combination with a hybrid insulation system using an aramid polymer. Besides the dielectric and thermal performance tests in the transformer factory this solution is proved by performing an additional short circuit test at KEMA Laboratories. Air sealing is achieved by using the newly introduced advanced hermetical sealed system with improved operating conditions (Hermetic 2.0). Besides this, noise and losses are optimised.

BIBLIOGRAPHY [1] [2] [3]

[4] [5] [6]

Fink, H.; Devaux, F.; Dolata, B.; Perrier, C.: “New and innovative smart and green transformer technologies”, CIRED 21st International Conference, Paper 0051, Frankfurt, Germany, 2011 Stirl, T.; et al.: “Eco-friendly Green Power Transformers – Innovative Solutions and New Experiences”, MATPOST 2015, Lyon, France, 2015 Gebauer, J.: “VACUTAP® – Stufenschalter in Vakuumschalttechnologie – Lösungen für spezielle Anwendungen”, Stuttgarter Hochspannungssymposium 2010, Stuttgart, Germany, 2010 Stirl, T.; Harthun, J.; Frotscher, R.: “Offshore-Einsatz – Eine Herausforderung für den Transformator“, Stuttgarter Hochspannungssymposium 2012, Stuttgart, Germany, 2012 Harthun, J.; Breuer, C.: “Setting of new standards by environmentally-friendly power transformers”, eb Elektrische Bahnen, Germany, 2015 IEC 60076-14 Power transformers – Part 14: Liquid-immersed power transformers using hightemperature insulation materials, 2013-09

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