design and test experiences for the development of...

DESIGN AND TEST EXPERIENCES FOR THE DEVELOPMENT OF 15 MVA, 66/11.55 kV POWER TRANSFORMERS FILLED

WITH NATURAL ESTER OIL

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Corporate Office: 35-C, Popular Press Bldg. 2nd Floor, Pt. M. M. Malviya Road, Tardeo, Mumbai 400034, India. Tel: + 91 22 23539180-84 Fax: + 91 22 23539186-87 Email: [email protected]

Silvassa Works: Survey No. 263/3/2/2 Village Sayli, Umerkuin Road, Dadra & Nagar Haveli, Silvassa 396230, India. Tel: + 91 260 2630810 / 11 Fax: + 91 260 2642711 Email: [email protected]

Digpalsinh Parmar Asst. Manager – R&D

http://www.imp-powers.com/

Design And Test Experiences For The Development of 15 MVA, 66/11.55 kV Power Transformers Filled with Natural Ester Oil

Content

1. Introduction

2. Design Consideration

• Dielectric Design

• Thermal Design

3. Manufacturing Challenges

4. Remaining Life Assessment (RLA)

5. Conclusion

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Introduction

• In most of the power system equipment mineral oil is used as an insulating medium for more than a century due to its reliable sources and advantages of ideal insulating and cooling properties.

• But concerning to power transformer overloading beyond its name plate rating as well as environmental impact mineral oil has low biodegradability, toxic contamination and fire hazardous nature because of high flammability with fire point less than 105 ⁰C.

• Mineral oil can be replaced with natural ester oil with its excellent properties of biodegradability, nontoxic and suitability due to renewable resources in power transformers.

• And also, natural ester liquid has a very low flammability with flash point of 330 ⁰C as it is categorised in “K class” Insulating fluid.

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Introduction Design Consideration Manufacturing Challenges Remaining Life Assessment Conclusion



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Introduction

• Use of NE oil in transformer as an insulating medium plays a vital role in reducing negative impacts on the environment with enabling development of transformer with K class insulating medium.

• Because of slow ageing properties of NE it is possible to load transformer continuously up to 1.3 times to rated current as well as schedule loading of 1.5 times higher for some time with acceptable temperature rises inside transformer winding and oil.

• Some of the major design consideration and challenges to develop NE filled transformers are discussed in this research paper including comparative studies of thermal and dielectric parameters of same rated transformers of 15 MVA, 66/11.55 kV filled with Mineral Oil and Natural Ester fluid.



Design Consideration

• At the time of designing a power transformer with NE as an insulating medium designers should account for a numbers of significant differences in the dielectric and thermal properties of both liquid as mentioned in below table.

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Introduction Design Consideration Manufacturing Challenges Remaining Life Assessment Conclusion Dielectric Thermal

Sr Properties Unit Mineral Oil NE Oil Rem

1 Dielectric Breakdown kV 30 - 35 82 - 97

2 Relative Permittivity at 25 °C - 2.2 3.2

3 Viscosity

at 0 °C cSt <76 >143

at 40 °C cSt 9.2 33

at 100 °C cSt 2.3 9.0

4 Flash Point °C 145 330

5 Density at 20 °C kg/ dm3 0.88 0.91

6 Expansion Coefficient 10-4, k-1 7 - 9 5.5 - 5.9

7 Thermal Conductivity, W/mk 0.11 - 0.16 0.16 - 0.17

8 Heat transfer Capacity cal/g °C 0.488 0.50 - 0.57

9 Moisture Content ppm 10 - 25 50 - 100

10 Pour Point °C -30 to -60 -19 to -33

11 Interfacial Tension Dynes/cm 40 - 45 25

12 Breakdown Voltage kV 60 74

13 Biodegradability % 30 97 - 99



Main Advantages of NE fluid are:

1. High Biodegradability (Environmental Aspect)

2. Non Toxic

3. “K” Class Fluid Because of High Flash Point

4. Good Thermal Conductivity and Heat Transfer capacity

5. High Relative permittivity Main Dis-advantage of NE fluid are:

1. High Density

2. High Viscosity

3. High Moisture Content

4. Lower Oxidative Stability

• Hence, mainly thermal design is challenging because of high viscosity and density of NE, which causes

lesser amount of fluid flowability and create high temperature difference between top and bottom oil.

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Dielectric Design (Cont…)

• Points to be taken care while designing NE filled Transformer,

1. Hermetic sealing of transformer tank.

2. Proper vapour phase drying (VPD) to absorb moisture.

3. Hybrid winding insulation system.

4. Increase creepage distance for lightning impulse characteristics by 30%.

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Dielectric Design (Cont…)

• Hybrid winding insulation system,

1. Semi Hybrid 2. Mixed Hybrid 3. Fully Hybrid 4. High Temperature

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1.Conventional Axial Spacer

2.Conventional Radial Spacer

3.Thermally Upgraded Kraft Paper

4.Conventional Static Ring

5.Conventional Angle Ring

6.Conventional Barrier

1.Conventional Axial Spacer

2.Conventional Radial Spacer

3.Kraft paper




7.Thermally Upgraded Radial Spacer

8.Thermally Upgraded Kraft Paper

1.Thermally Upgraded Axial Spacer


3.Thermally Upgraded Kraft paper




1.Thermally Upgraded Axial Spacer


3.Thermally Upgraded Kraft paper

4.Thermally Upgraded Static Ring

5.Thermally Upgraded Angle Ring

6.Thermally Upgraded Barrier

Ref : IEC 60076-14



Dielectric Design (Cont…) • Insulation Resistance (IR) and Polarization Index (PI)

• Comment : NE liquid has high moisture content compared to mineral oil. This may rise polar contamination (water molecule) in NE oil, and lead to significantly lesser PI value below 1.2. This negative impact of NE oil can be compensated by proper VPD process.

• Comparative study of IR and PI value for both the transformers are as stated in above table.

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Test Description Test Mode 15 MVA Power Transformer

Natural Ester Mineral Oil

Insulation Resistance (IR) HV/LV+E (MΩ) 1880 23600

LV/HV+E (MΩ) 2090 37200

HV/LV (MΩ) 2900 42300

Polarization Index (PI) HV/LV+E 2.17 1.92

LV/HV+E 2.14 1.60

HV/LV 2.60 1.80


Actual test result.



Dielectric Design (Cont…) • Capacitance and Tan δ

• Comment : C & Tan δ is also higher due to high moisture level in NE liquid and presence of aging products and soluble polar contaminants observed there is a significant increase in tan δ.

• But proper VPD process and maintaining dust free assembly can help out to compensate this negative impact as stated in above table.

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Test Description Test Mode 15 MVA Power Transformer

Natural Ester Mineral Oil

Capacitance at 10 kV UST (HV/LV) (pF) 6210.71 4703.91

GST g (HV/E) (pF) 4350.29 3191.55

GST g (LV/E) (pF) 9128.42 7562.02

Tan δ at 10 kV

(Dissipation Factor)

UST (HV/LV) 0.0022 0.0023

GST g (HV/E) 0.0032 0.0036

GST g (LV/E) 0.0027 0.0033


Actual test result.



Dielectric Design (Cont…) • Summary

o Electric field stress distribution in solid insulation is higher in NE filled transformer, yet it can be compensated as solid insulations have even higher dielectric strength than liquid insulation.

o So, for MV transformer dielectric design is more or less symmetrical in both NE/Mineral oil filled transformers.

o For stress distribution inside inhomogeneous structures, higher voltage stress may be observed at winding end which may lead to failure of impulse voltage distribution or partial discharge (PD) in insulation systems.

o So, for HV and EHV transformer creepage distance should be higher compare to conventional design and also use of hybrid insulation is preferable.

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Thermal Design (cont…)

• Points to be taken care while designing NE filled Transformer,

1. Fluid flow arrangement is improved to enable smooth oil flow from winding and core by changing plain and T-shaped duct size and arrangement.

2. Enhanced thermal head height by introducing swan neck design of radiator.

3. 25 % more radiator fins are provided.

4. Nominal bore diameter of radiator pipe is increased by 20 mm.

5. Conventional kraft paper on conductors replaced with thermally upgraded kraft paper (TUP).

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• Top Oil Temperature Rise comparison of NE and mineral oil filled transformer,

• Comment : By taking care of above discussed point while designing & manufacturing NE filled transformer, we have observed better result as shown in above figure.

• Even NE filled transformer observed around 9 degree cooler then mineral oil.

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15 MVA Power Transformers filled with NE and Mineral Oil actual heat run test result.




• Winding Temperature comparison of NE and mineral oil filled transformer,

• Comment : As seen in graph temperature in winding area is also observed lesser in NE filled transformer compare to mineral filled transformer because of improved fluid flow design.

• Also witnessed stability advantage of NE filled transformer, NE oil is stabilized early compare to mineral oil filled transformer.

• Over loading test of NE filled transformer was done for loading up to 130 % continuous and even for 150 % loading for some time.

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15 MVA Power Transformers filled with NE and Mineral Oil actual heat run test result.




• Summary

o NE fluid flowability due to high viscosity is the biggest concern in all Power/Distribution Transformer. We may observe high temperature rise inside NE filled transformer.

o But this negative impact can be compensate by proper thermal design. At least we may reach the same temperature rise as of mineral oil filled transformer.

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Manufacturing Challenges

• To fill NE in transformer tank, high rated oil duty pumps are required.

• NE to be filled at higher temperature may be at 70°C.

• Moisture impregnation time with NE should be more i.e. almost double that mineral oil.

• Oil settling time before testing is comparatively longer.

• Winding dissipation factor is comparatively higher than achieved with mineral oil filled transformer.

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RLA Calculation (contd…)

• Hot spot temperature of the transformer calculated from the measured Top, Ambient and Winding Gradient,

• From Equation 3, 4 & above calculated hot spot temperature, Remaining Life Assessment (RLA) of NE

filled and Mineral Oil filled Transformer,

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Transformer Description Calculated HST Remark

HV Winding LV Winding

Mineral oil filled 71.424 ⁰C 87.947 ⁰C

Natural Ester filled 59.774 ⁰C 73.593 ⁰C

Transformer Description Relative ageing rate Remark

HV Winding LV Winding At 98⁰ C HST

Mineral oil filled with

kraft paper

0.046 0.313 1.0

Natural Ester filled with

TU kraft paper

0.003 0.016 0.282



RLA Calculation (contd…)

• Summary

o As measured hot spot temperature is low in NE filled transformer compared to mineral oil filled transformer relative ageing rate is also low in NE filled transformer.

o Even with same hot spot temperature relative ageing rate of semi hybrid insulated NE filled transformer is less as shown in below graph.

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Ref : IEC 60076-7



Conclusion:

• Natural ester has its own pros and cons compared to mineral oil, i.e. high viscosity, high moisture content and even high prices but at the same time NE gives high biodegradability up to 97- 99% with higher flash point of 330 ⁰C. This enable us to manufacture “K Class” GREEN transformers.

• As dielectric design consideration semi hybrid insulation system to be used to withstand high temperature with better electric strength and reduced ageing. Proper moisture removal to be done during VPD process, to maintain better dielectric dissipation factor (Tan δ) and polarization index of insulation system.

• Generally high temperature rise and higher hot spot temperature is observed in NE filled transformer due to high viscosity of NE. But by improving thermal design as proposed and taking advantage of better heat transfer capacity and thermal conductivity this negative impact can be compensated.

• Proper handling of active part assembly is to be done during manufacturing process to avoid exposure and contact with atmosphere to minimize oxidative ageing influence on NE.

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References :

1) Dhorali Gnanasekaran and Venkata Prasad Chavidi, “Vegetable Oil based Biolubricantsand Transformer Fluids”, Springer, 2018, Materials Forming, Machining andTribology.

2) J. Hajek, J. Kranenborg, P. Sundqvist, R. Jonsson, T. Skytt, B. Samuelsson, R. AsanoJR, G. K. Frimpong, R. Girgis, “Considerations for the Design, Manufacture, and Retro– filling of Power Transformers with High Fire Point, Biodegradable Ester Fluids”,CIGRE Paris Session, 2012, A2.203.

3) “Liquid immersed power transformer using high temperature insulation materials”, Edition 1.0, 2013-09, IEC 60076, Part - 14.

4) S.V. Kulkarni and S.A. Khaparde, “Transformer Engineering Design, Technology, and Diagnostics”, CRC Press, 2013, Second Edition.

5) “Temperature rise for liquid-immersed transformers”, Edition 3.0, 2011-02, IEC 60076, Part – 2.

6) R. Girgis, M. Bernesjö and G. K. Frimpong, “Detailed performance of a 50 MVA transformer filled with a natural ester fluid versus mineral oil”, CIGRE Paris Session, 2010, A2.107.

7) “Loading guide for oil-immersed power transformers”, Edition 1.0, 2005-12, IEC60076, Part - 7.

8) R. Frotscher, D. Vuković, M. Jovalekic, J. Harthun, M. Schäfer and C. Perrier, “Behaviour of Ester Liquids under Dielectric and Thermal Stress - From Laboratory Testing to Practical Use”, CIGRE Paris Session, 2012, D1.105.

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