JNTUK2

Design of Corona Mitigation Device and Application of ZnO Microvaristor on 66kV Insulators by Finite Element Method M.NageswaraRao EEE Department UCEK, JNTUK Kakinada, India [email protected]

N.Sumathi EEE Department UCEK, JNTUK Kakinada, India [email protected]

 Abstract—Insulators play a vital role in electrical power system.

Due to continuous energisation and presence of various pollutions pollutions in t he atmosphere, electrical stresses on the s urface of the insulator are becoming high. Insulators will degrade and fail due to presence of these higher electrical stresses on its surface. Electrical stresses can be minimized with the placement of corona rings at end fittings. As per IEC standards corona rings need to be preferred for above 220kV insulators only. But below 220kV transmission line insulators like 66kV and 132kV are also degrading due to high pollution contaminations and high electrical stresses. Most of the utility companies in India are recommending corona rings for 66kV and 132kV insulators. But electrical stresses can also be minimized with usage of microvaristor component on the surface of the insulator. In this paper, with the help of FEMM 2D software package corona rings are designed for 66kV ceramic and polymer insulators and electric field was analyzed by finite element method. Application of ZnO microvaristor on the surface of the polymer insulator was evaluated and results are compared between with and without microvaristor. Results proved that insulators with ZnO microvaristor give better field results. Ceramic Insulators, Polymer Insulators  IndexTerms  IndexTerms— Microvaristor, Microvaristor, Corona ring, Finite Element Method (FEM).

I NTRODUCTION Conventional ceramic insulators in transmission and sub transmission system are being replaced with non-ceramic or  polymer  polymer insulators due their advantages advantages like better pollution pollution withstand capacity, light weight and hydrophobicity property etc., Electric field distribution on the surface of the insulator affects both short term and long term performance. Unequal distributions of electric field, high voltage side insulator disc or shed will experiences higher electrical stresses. Under long run insulator may fail due to these stresses. These types of interruptions will degrade the transmission system  performance.  performance. Hence more concentration concentration is required to minimize these electrical stresses. In case of ceramic insulator first disc at the high voltage end can be replaced but for  polymer  polymer insulators it is not possible. possible. Polymer Polymer insulator insulator is a composite structure, hence sheds cannot be replaced. Frequent changing of discs is also not recommended. [1]. Hence more concentration is required to minimize the electrical stresses.

V.S.N.K.Chaitanya EEE Department UCEK, JNTUK Kakinada, India [email protected]

Abdul Azees EEE Department UCEK, JNTUK Kakinada, India [email protected]

Electric field distribution on the surface of the insulator can  be measured by experimental experimental methods, methods, analytical methods methods and numerical methods. Experimental evaluation is not a cost effective method. Hence numerical analysis methods like Finite element method, Boundary element method and charge simulation methods etc., gives accurate electric field results compared to analytical methods [2-3]. Using boundary element method Electric Field Analysis (EFA) was carried out on 400kV polymer insulators. The field results are very less compared to EPRI standard values [4].Some of the researchers had done electric field analysis for 500 kV HVDC insulator under dry and wet conditions. These results helped in optimizing the corona ring. HVAC insulators are analyzed with the help of charge simulation method [5-6]. For different altitude sites EFA has carried for high voltage ceramic and non-ceramic insulators. Some of the research scholars have studied aging of the insulators based on surface characterization techniques [7-9]. Most of the EFA results are recommending corona ring for minimization of electrical stresses. As per IEC standards corona ring is mandatory for above 220kV insulators. But presence of heavy pollutions in the atmosphere electrical stresses on 66kV and 132kV insulators also increases [10-13]. Most of the electrical utility companies in India are recommending corona ring for 66kV and 132kV insulators. This problem will overcome by, usage of microvaristor component on the surface of the insulator. In this paper, using Finite Element Method (FEM) electric field analysis was carried out on 66kV ceramic and polymer insulators. Corona rings are designed and EFA results are compared. Zinc Oxide (ZnO) microvaristor application on  polymer  polymer insulator insulator is evaluated evaluated and results results are compared. compared. ING DESIGN FOR 66K V I NSULATOR  CORONA R ING

Electric field analysis and corona rings are designed for both ceramic and polymer insulators with the help of FEMM 2D software package. This software package gives field results  based on finite finite element element method. method. a. Ceramic Insulators:

Before designing the insulators geometrical configurations of the insulator are taken and tabulated in Table. I. Electric

Field Analysis (EFA) is carryout for both cases (ie., with and without corona ring).

TABLE. I GEOMETRICAL CONFIGURATIONS-CERAMIC I NSULATOR  S.No

Particulars

Dimension

assigned as a surrounding medium. After assigning boundary elements 2D triangular elements are generated throughout the model. This 2D triangular element gives field results at each every point of the insulator.Insulator model with 2D triangular meshes are shown in Fig.2. By running the model electric field results are taken at critical regions of the insulator. EFA results are tabulated in Table III.

1

Sectional length (mm)

882

2

Dry arcing distance (mm)

795

3

Creepage distance (mm)

1090

4

Disc diameter (mm)

256

5

Number of discs

04

1

Creepage Distance

0.680

6

Distance between two discs (mm)

130

2

Arcing Distance

7.420

3

First disc (HV side)

0.283

TABLE. III EFA R ESULTS-CERAMIC I NSULATOR WI THOUT R ING

i.  EFA -without corona ring:

As per the geometrical configurations, insulator is designed  by using FEMM 2D package. After designing, insulator materials like ceramic, steel and cement are assigned to the model with the help of relative permittivity values. Insulator design and their material properties are given in Fig.1 and Table II.

Materials

Max. Electric field (kV/mm)

ii.  EFA- with corona ring:

Relative Permittivity

TABLE. IV EFA R ESULTS-CERAMIC I NSULATOR WITH R ING

1

Ceramic

5.9

2

Cement

19.6

3

Steel

1

4

Air

1

Fig.1 Ceramic Insulator

Critical Regions

For the above model corona rings are designed and added to the model at the high voltage end. Corona ring was designed for two diameters (ie, 130mm and 90mm) as shown in Fig.3 and Fig.4. Corona ring material was assigned as aluminum with a relative permeability value of 3. Above procedure is repeated for both diameters of the insulator models and EFA results are tabulated at the critical regions of the insulator. Results are tabulated in Table IV.

TABLE. II M ATERIAL P ROPERTIES-CERAMIC I NSULATOR  S.No

S.No

S.No

Particulars

Max. Electric field (kV/mm) 130mm

 90mm

 diameter

 diameter

1

Creepage Distance

0.541

0.511

2

Arcing distance

0.030

0.064

3

First disc (HV Side)

0.081

0.081

Fig.2 2D Triangular meshes

After assigning material properties to the insulator model line to ground voltage (ie. 72.5/ √3=42kV) is applied to HV end of the insulator and zero volts are applied to ground end. Air is

Fig.3 130mm Diameter Corona ring Fig.4 90mm Diameter Corona ring

b. Polymer Insulators:

Before designing the insulators geometrical configurations of the insulator are taken and tabulated in Table. V. Electric Field Analysis (EFA) is carryout for both cases (ie., with without corona ring). TABLE. V GEOMETRICAL CONFIGURATIONS-P OLYMER I NSULATOR  S.No

Particulars

Dimensions

After assigning material properties to the insulator model line to ground voltage (ie. 72.5/ √3=42kV) is applied to HV end of the insulator and zero volts are applied to ground end. Air is assigned as a surrounding medium. After assigning boundary elements 2D triangular elements are generated throughout the model. Insulator model with 2D triangular meshes are shown in Fig.6. By running the model electric field results are taken at critical regions of the insulator. Results are tabulated in Table VII.

1

Section length (mm)

880

2

Arcing Distance (mm)

795

3

Creepage Distance (mm)

3020

4

Number of bigger sheds

15

5

Number of smaller sheds

14

1

Creepage Distance

0.034

6

Core diameter (mm)

26

2

Arcing Distance

0.041

7

Pitch (mm)

52.5

3

First shed

0.035

4

Inside FRP

0.125

5

Inside Silicone rubber

0.036

i.EFA-without corona ring:

As per the geometrical configurations, insulator is designed  by using FEMM 2D package. After designing, insulator materials like polymer, FRP and steel are assigned to the model with the help of relative permittivity values. Insulator design and their material properties are given in Fig.5 and Table VI.

TABLE. VII EFA R ESULTS-POLYMER I NSULATOR WITHO UT R ING S.No

Particulars

Max. Electric field (kV/mm)

ii.EFA- with corona ring:

For the above model corona rings are designed and added to the model at the high voltage end. Corona ring was designed for two diameters (ie, 130mm and 90mm) as shown in Fig.7 and Fig.8.

TABLE.VI MATERIAL PROPERTIES-POLYMER I NSULATOR  S.No

Materials

Relative Permittivity

1

Silicone Rubber

3

2

FRP Rod

5

3

Steel

1

4

Air

1

Fig.7 130mm Diameter Corona ring

Fig.5Poly mer Insulator

Fig.6 2D Triangular meshes

Fig.8 9 0mm Diameter Corona ring

Corona ring material was assigned as aluminum with a relative permeability value of 3. Above procedure is repeated for both diameters of the insulator and EFA results are tabulated at the critical regions of the insulator. Results are tabulated in Table VIII. By comparing EFA results for ceramic and polymer insulators with and without corona rings, polymer insulators has less electrical stresses compared to ceramic insulators and usage of corona rings minimized electric fields stresses.

TABLE. VIII EFA R ESULTS-POLYMER I NSULATOR WI TH R ING

S.No

Particulars

Max. Electric field (kV/mm) 130mm

 90mm

 diameter

 diameter

1

Creepage Distance

0.032

0.033

2

Arcing distance

0.049

0.056

3

First shed

0.032

0.033

4

Inside FRP

0.095

0.034

5

Inside Silicone rubber

0.035

0.036

EFA R ESULTS COMPARISON EFA results are compared between with and without microvaristor. Comparative results are shown in Fig.10.

Z NO MICROVARISTOR APPLICATION ON POLYMER INSULATOR 

Corona rings will minimize the electrical stresses and helps for uniform distribution of electric field. But as per latest IEC standards corona rings are mandatory for above 220kV insulators. But 66kV and 132kV insulators also fail due to the  presence of heavy pollution in the atmosphere. This problem can be overcome by usage of microvaristor component. Microvaristor component is an electrical grading material which minimizes the electrical stresses. In this paper, ZnO microvaristor component is used for minimization of electrical stresses. For the above 66kV polymer insulator model without grading ring a layer of 1mm thickness is designed throughout the insulator rubber housing and weathersheds. ZnO microvaristor material is assigned to the layer with the relative  permeability as 12 [10]. Insulator design with ZnO microvaristor was shown in Fig.9.

(a)

(b)

(c) Fig.9 Polymer Insulator with ZnO microvaristor Component

Electric field analysis is carried out by applying line to ground voltage (72.5/√3=42kV)   and generating 2D triangular elements. EFA results are tabulated in Table IX. TABLE. IX EFA R ESULTS-POLYMER I NSULATOR W ITH MICROVARISTOR  S.No

Particulars

Max. Electric field (kV/mm)

1

Creepage Distance

0.030

2

Arcing distance

0.040

3

First shed

0.030

4

Inside FRP

0.125

5

Inside SiR

0.036

(d)

[3]

M. Abdel-Salam, “Field Optimization of High-Voltage Insulators”, IEEE Trans. Industry Applications, Vol. 22, pp. 594-601, 1986.

[4] M.Nageswararao, “Electric field analysis and experimental evaluation of 400kV Silicone composite insulator,” International Journal Electrical, computer, Energetic, Electronic and Computer Engineering, vol. 10, no.7, pp. 484-487, 2016.

(e) Fig.10 EFA Results Comparison

Maximum electrical stresses will occur on the surface of the polymer insulator housing and weathersheds. Hence ZnO microvaristor is added on the surface of the insulator. By comparative results, insulators with ZnO microvaristor have very less electrical stresses on its surface (creepage distance along the surface of the insulator, leakage distance-shortest distance between two metal fittings and first shed -on HV side). With usage of microvaristor application, electrical stresses at creepage distance, arcing distance and first shed are reduced by 10.3%, 1.2% and 14% respectively. In case of Inside FRP and inside silicone rubber electrical stresses are constant. EFA results proved that microvaristor application is the best solution for minimization of electrical stresses. CONCLUSION Insulators will degrade and fails due to presence of higher electrical stresses on its surface. In this paper electrical stresses are minimized for both ceramic and polymer insulators with the placement of corona rings at the HV end. But as per IEC standard corona ring is mandatory for insulators above 220kV. Due to the presence of heavy pollution in the atmosphere 66kV and 132kV insulators are degrading. This problem is overcome  by microvaristor application on the surface of the insulator. In this paper EFA was carried out on polymer insulator with and without microvaristor application and results are compared. With the application of microvaristor, electrical stresses at creepage distance, arcing distance and first shed are reduced  by 10.3%, 1.2% and 14% respectively. Comparative results  proved that, microvaristor application is the best s olution for minimization of electrical stresses.

 Acknowledgment We acknowledge our gratitude to University College of Engineering, JNTU Kakinada for their support in completion of this work.

References [1] J. F. Hall, “History and Bibliography of Polymeric Insulators,” IEEETransactions on Power Delivery, vol. 8, pp. 376-385, 1993. [2] M. Abdel-Salam, “Optimizing Field Stress on High Voltage Insulators”, IEEE Trans. Electr. Insul., Vol. 22, pp. 47-56, 1987.

[5] Jiahong “Charge Simulation Based Electric Field Analysis of Composite Insulators for HVDC Lines,” IEEE Transactions on Dielectrics and Electrical Insulation., vol. 21, no. 6, pp.25412548, Dec. 2014. [6] Jing Li, “Electric Field Calculation and Grading Ring Optimization of Composite Insulator for 500kV AC Transmission Lines,” International Conference on Solid Dielectrics, Potsdam, Germany., pp.1-4, July. 2010. [7] Rahul Krishnan “Electric Field Analysis of High Voltage Insulators,” International Journal of Computer Science and Informatics., ISSN:2231-5292, vol. 1, Iss.4, pp.31-35, 2012. [8] YujunGuo and Xingliang Jiang, “AC Flashover Performance ofPorcelain, Glass and Polymeric I nsulators at High Altitudes,” IEEE 11thInternational Conference on the Properties and Applications ofDielectric Materials, pp. 376-385, 2015. [9] G.Haddad,“Evaluation of the aging processof composite insulator based on surface characterization techniques an die electrical method,” IEEE Transaction on Dielectrics and electrical Insulation, vol. 23, no.1, pp. 732-737, Feb 2016. [10] R.Add-Rahman, “Stress Control on polymeric outdoor insulators using zinc oxide microvaristor composites,” IEEE Transaction on Dielectrics and electrical Insulation, vol. 19, no.2, pp. 705-713, April 2012. [11] IEC Standard-61009 “Insulators for Overhead Lines CompositeSuspension and Tension Insulators for A.C. Overhead Lines with aNominal Voltage Greater Than 1 000 V Definitions, Test Methods andAcceptance Criteria,” 2008. [12] IS Standard-731 “Specification for Porcelain Insulators for OverheadPower Lines With A Nominal Voltage Greater Than 1 000 V,” 2001. [13] IEC Standard-60383-2 “Insulators for Overhead Lines with a  NominalVoltage Greater Than 1 000 V Definitions, Insulator string and Insulatorsets for A.C system test Methods and acceptance criteria,” 1993.