Three Dimensional Electric Field Analysis in Spacer Cable Systems

Fermín P. Espino-Cortés1, Isaías Ramírez Vázquez2 1SEPI ESIME Zac., Instituto Politécnico Nacional (IPN)

U. P. "Adolfo López Mateos", Edificio Z-4 Primer piso C. P. 07738 México, D. F. Mexico

2Instituto de Investigaciones Eléctricas Reforma 113, Col. Palmira, C.P. 62490

Cuernavaca, Morelos, México

Abstract- In this work, three dimensional finite element modeling is used to compute the electric field distribution on a spacer cable system. The electric field for different compatibility test configurations is computed in order to show the differences observed between the use of single phase and three phase power supply to energize the test setup. According to the simulation results, the electric field enhancement seen around the seat area of the spacer in contact with the cable can be reduced by reducing the dielectric permittivity of the materials; however, materials with low permittivity can be difficult to obtain for this application. It is considered that, instead of trying to reduce and find exact match of materials permittivity, tracking resistance must be improved as much as possible in order to extend the service life of the system.

I. INTRODUCTION

n order to reduce failure rates on distribution feeders installed in wooded zones or lines with tight right of way, several utility companies around the world have installed covered conductors in compact overhead distribution lines [1-4]. The use of systems with covered conductors (CC) can significantly improve the reliability of distribution networks [2-3], with reductions up to one order of magnitude in fault rates. The Spacer Cable System (SCS) is a special design for compact distribution lines with covered conductors. This system was introduced almost four decades ago and its use is increasing as this system has proven to be a long-term benefit for electric utilities [5]. Additionally to the increment in reliability by using SCS, other benefits are: the reduction in right of way, the possibility of installing more than one circuit on a common pole [4], and a reduction in safety hazards [6]. In SCS´s all the components are designed as a coordinated system with dielectric compatibility between materials, this is with the purpose of reducing the probability of surface discharge activity; however, electrical tracking on the insulating materials has been reported once in operation. This phenomenon is associated to the high electric field concentration around the contact area between covered conductor and spacer. The SCS is constituted of phase covered conductors, a high strength messenger/neutral wire, and insulating spacers [1], as shown in Fig. 1. The CC used on SCS, depending of the voltage rating, can consist of one, two or three extruded layers. The first layer can be a semi-conducting strand shield, followed by an insulation layer and a track-resistant covering layer. The semi-conducting strand

shield is intended to reduce voltage stresses during tree contacts. The second layer can be of low-density polyethylene (LDPE), while the outer layer of high-density polyethylene (HDPE) resistant to tracking [2]. Tracking and erosion have still been a problem [7, 8] and it has been considered the main cause of failure in SCS systems [7]. A better understanding of the effect of material properties on the magnitude of the electric field and how these systems can be qualified might be useful to improve the SCS. This paper presents three dimensional electric field simulations on SCS that can help to identify the areas of high electric stress prone to tracking on the surface of the CC and spacer. The effect of changes in permittivity of the materials and how the system is energized on compatibility tests are analyzed.

II. THREE DIMENSIONAL ELECTRIC-FIELD COMPUTATION IN

CABLE SPACERS

In this section the finite element based software COMSOL® is employed to evaluate the electrostatic electric field distribution. Special infinite elements were used to simulate open boundaries. The dimensions of the spacer and the covered conductor considered in the simulations correspond to

Fig. 1. Spacer Cable System (SCS) on overhead compact distribution lines.

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174978-1-4673-1252-3/12/$31.00 ©2012 IEEE

Fig. 2. Spacer (a) and pin post ceramic insulator (b) geometry considered in simulations. Air between the spacer and cable insulation on the seat area of

the spacer in (c).

a typical 25 kV class system; the model of the spacer used in the simulation is shown in Fig. 2(a). In order to compare the results of the electric field in systems with spacers and the electric field in systems with ceramic insulators, additional simulations were performed with the CC and the pin post insulator shown in Fig. 2(b). The maximum size of element on the seat area is 1x10-4 m. For the simulations, the CC is considered as a single layer. The insulating layer of the CC is simulated with an initial relative permittivity (εr) of 3. The instantaneous three phase voltages considered in the simulations presented in this section and in section III, are in the ratio of 1Vp:-0.5Vp:-0.5Vp, with Vp= 18.78 kV (the phase-to ground peak voltage of a 23 kV system). The voltage distribution on the spacer for the instantaneous peak voltage on phase C is presented in Fig. 3(a), while in Fig. 3(b) the electric field on the spacer is plotted. As it can be seen in this figure, the higher electric field appears on the seat area of the conductors; in this case the hook corresponding to phase C has the maximum electric field intensity. Similar condition is found in the seat area of the insulator, in Fig. 4 the voltage distribution is the CC insulator system is presented, considering the peak voltage on the central phase conductor. In Fig. 5 it is clear the electric field enhancement that is present around the surface of contact between CC and spacer. The electric field intensity in the hook is considerable high compared with the electric field on the surface of the cable. This problem is typical of configurations where two or three dielectric materials form a zero contact angle also known as wedge effect [7, 9]. Wherein spacer or insulator and cable

insulation touch, there is air in parallel as it can be seen in Fig. 2 (c). In this type of insulation systems, the weak medium is the air and if the electric field is high enough, the air in the seat area will become ionized producing a partial discharge, and with time, tracking and erosion [10].

Fig. 3. (a) Voltage distribution on the spacer, and (b) electric field on the spacer; instantaneous peak voltage on phase C. The covered conductors are not shown in the figure.

Fig. 4. Voltage distribution on the CC system using pin post ceramic insulators.

Fig. 5. Electric field enhancement on the seat area of the spacer.

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III. EFFECT OF CHANGES IN PERMITTIVITY OF THE SPACER

AND THE CC

The dielectric compatibility of the insulating materials used in systems with CC has been one of the main problems. It is supposed that, by keeping the same relative permittivity on the materials, problems due to the high electric stress are reduced. The maximum electric field for different dielectric permittivity values of the materials in a SCS system is computed. The electric field is register in the line in blue along the surface of the contact area of the spacer; see Fig. 2(a). The results are presented in Table I, where the maximum electric field in the contact area between spacer and insulation of the conductor is listed for different values of relative permittivity.

TABLE I MAXIMUM ELECTRIC FIELD AS FUNCTION OF THE RELATIVE PERMITTIVITY OF

CC AND SPACER MATERIALS Cable

Relative Permittivity

Spacer Relative

Permittivity

Maximum Electric

Field

1.5 1.5 3.3x105

2 2 4.2 x105

3 3 6x105

4 4 7.5 x105

5 5 9 x105

If it is consider that the relative permittivity with values around 3 can be the most common condition for both materials (in the CC and in the spacer), according to the results presented in Table I, as the permittivity increases, the electric stress around the contact area increases. By reducing the permittivity below 3, the maximum electric field is proportionally reduced. This last option can be a possible solution; however, materials for outdoor insulation applications with such permittivity values are not technically available. CCs are been used to replace bare conductors in distribution systems; unfortunately, in many cases the CC is used with ceramic insulators. In the case of the CC insulator system from Fig. 4, for a ceramic insulator with εr_insulator= 6 and for the CC εr_cover= 3, the maximum electric field is 9.5 x105 V/m. If a nonceramic insulator with εr_insulator= 3 is used, the maximum electric field is reduced to 5.7x105 V/m, that is 40% lower.

IV. ELECTRIC FIELD DISTRIBUTION FOR DIFFERENT

COMPATIBILITY TESTS CONFIGURATIONS

The damage of the insulating material produced by the tracking and erosion in areas of high electric field has been used as a reference to compare the performance of different materials and designs in SCS. Compatibility tests in which the resistance to tracking of spacers, insulators, and ties of polymeric material is measured and are now included in standards [11]. The dielectric compatibility test should be performed to the complete system as shown in Fig. 6. In the compatibility test, a three phase supply must be employed, as shown in Fig. 7(a) (Case I); however, sometimes a single phase

Fig. 6. Dielectric compatibility testing of the complete SC system [12].

power supply is used to energize the CCs on the testing chamber. When only one phase is used two options have been used to energize the system, Fig. 7(b) present the option where the phase conductor is in one of the hooks while the rest of the conductors are grounded (Case II), this disposition is intercalated in the next spacers to energize each of the three hooks. The second option (Case III) is to use the same phase (φA) in the three cables as shown in Fig. 7(c). The maximum electric field along the edge of the seat area on the spacer, dotted line in Fig. 2 (c), is computed for all the possibilities of energizing in the compatibility test, and the results are presented in Table II. For these simulations the relative permittivity value is the same for the spacer and conductor cover (εr_spacer= εr_cover= 3). In Case I, when a three phase supply is used, the maximum instantaneous electric field takes place in the hook of the phase with the instantaneous peak voltage. The maximum electric field in the hook of phase C results below the maximum field seen on the contact area of phases A and B, due to the geometry of the spacer. The maximum electric stress on the system is reduced if only one phase conductor is energized and the rest is grounded as shown in Table II for Case II. When the three CC´s are energized with a single phase, Case III, the maximum electric field is moved to the hook of the messenger wire and the phase conductors will not have the maximum electric stress, see Table II.

Fig. 7. Different configurations for energization in the dielectric compatibility test.

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TABLE II LOCALIZATION OF THE INSTANTANEOUS MAXIMUM ELECTRIC FIELD ON THE

CONTACT AREA OF THE CABLE SPACER FOR DIFFERENT CONFIGURATIONS

Case Instantaneous Applied Voltage Emax

[V/m] Localization

Hook Phase A Phase B Phase C

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1Vp -0.5Vp -0.5Vp 6.0x105 Phase A

-0.5Vp 1Vp -0.5Vp 6.0x105 Phase B

-0.5Vp -0.5Vp 1Vp 4.75x105 Phase C

II

1Vp Ground Ground 4.7x105 Phase A

Ground 1Vp Ground 4.5x105 Phase B

Ground Ground 1Vp 3.5x105 Phase C

III 1Vp 1Vp 1Vp 5.0x105 Ground Wire

V. DISCUSSION

In spacer cable systems the compatibility between the insulating materials of conductor and spacer is desirable, but according to the simulation results, even if the materials have the same permittivity the electric field around the points of contact can reach several times the electric field value in the surface of the CC that is not in contact with the spacer. Reducing the permittivity of the material as much as possible can help to reduce the electric stress in the critical areas; however, materials with low permittivity values can be difficult to obtain for this application. If the tracking and erosion resistance of the materials can be improved without increasing significantly the relative permittivity [13], the SCS should have a better performance. In systems where CC’s are used with ceramic insulators, the high permittivity of the insulator will increase the electric field around the contact area with the cable. By using nonceramic insulators with an εr similar in value to the relative permittivity of the insulation of the cable, the electric field can be reduced. Dielectric compatibility tests are intended to verify the performance of the complete SCS. In order to reproduce the electric field distribution that is seen in field, a three phase systems must be used. By using a single phase power supply in the test, the maximum electric stress can be moved to the messenger/ground wire (Case III), or it cannot reproduce the magnitude of the electric field seen on a real situation (Case II).

VI. CONCLUSIONS

According to the simulation results, if the permittivity can be reduced as much as possible, the maximum electric field on the contact area between CC and spacer can be significantly reduced. However, materials with low permittivity for outdoor insulation are not yet available. A more practical solution can be to improve the tracking resistance in order to extend the service life of the system. If ceramic insulators are used with CCs, an intense electric field will appear around the contact area. When a nonceramic insulator with εr similar to the εr of the cable used, the maximum electric field is reduced.

The configuration that is used to energize the compatibility test can considerably change the electric stress distribution. It is necessary energize the test with a three-phase system to reproduce the localization and magnitude of the maximum electric stress that can be seen on field.

VII. ACKNOWLEDGMENT

The authors acknowledge Mr. Daniel Olvera Contreras and Mr. Victor Ortega Romero for their valuable help in this work. The authors would like to express their appreciation to the financial support provided by the National Polytechnic Institute and CONACYT in Mexico City.

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