flehretz@lappinsulators.com

COMPACT OHTL INSULATION– FUTURE TRENDS AND TECHNOLOGIES

LEHRETZ, Fabian SEIFERT, Jens Martin

Lapp Insulators GmbH – BU LIKE

Germany

TROPPAUER, Wolfgang

Mosdorfer – BU MOVE

Austria

SUMMARY

The planning process for new overhead transmission lines (OHTL) in north Europe is more than challenging.

Due to the general shutdown of nuclear power plants in Germany, a reorientation of energy distribution is

necessary. Electrical energy from Off-shore wind parks in the North Sea or solar power plants at the rural

areas have to be transferred over big distances to the heavy industry in the south of the country. Due to the

new situation new transmission lines, such as HVAC and HVDC at 400/420 kV and 500/550 kV, are

necessary. On the other hand, the acceptance of new OHTL constructions by population is presently extreme

low, driven by the environmental and visual impact of large OHTL structures. Possible countermeasures are

the usage of existing line corridors and the application of higher transmission voltages by using up to date

insulating structures, such as:

Insulating cross-arms

Inverted V sets

Semi-anchored sets

Y and T assemblies

Interphase and intersystem spacer

This paper reports on the state of the art of technology, mechanical and electrical design considerations, type

tests and static/dynamic mechanical load performance limits of OHTL insulation. In the most cases

composite insulators are applied, due to their better pollution performance, dynamic impact loads and shorter

insulating length. In addition to the above mentioned solutions, a complete new indea of compacting lines by

means of bundling the 3 phases of a system in a “Diamond” arrangement for 420 kV is presented. The report

closes with case studies of already realized and future planned constructions in Germany, South Africa,

Turkey, Belgium, United States and United Kingdom. An outlook is given to the ideas for standardization,

e.g. as an application guide within the new family of IEC 62609-X for composite line post insulators and

their assemblies.

KEYWORDS Composite Insulators, Compact OHTL, Cross-arm, HVDC

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1.0 Compact Insulation Elements

This chapter gives a survey about insulation elements based on modern composite insulators [1]

applied for compact overhead transmission line design up to system voltages of Um=550 kV.

Composite insulators can be manufactured in single units, possess high mechanical impact strengths,

fail-safe characteristics and are resistant to severe pollution conditions [2].

1.1 Horizontal V Sets

The fixed (non-pivoting) base assembly (Fig. 1) is applied in structures that have to accommodate

longitudinal loads e.g. angle towers. Some national standards request this type of assembly in case of

road crossings. The longitudinal load performance depends strongly on the FRP core diameter and on

the geometry of the assembly. For high longitudinal loads (such as in the case of conductor breakage)

the post insulators are arranged horizontally in V shape (Fig. 1 middle). The fixed base assembly

requires special construction elements at the tower side (base of the assembly). In case of parallel post

arrangements the tower and connection components and fixing devices (screws) have to withstand

significant bending moments. Torsional loads to the tower have to be considered in case of V shaped

post insulators.

Fig. 1: 420 kV Fixed Base Horizontal V Assemblies (left: parallel posts, middle: V setup of post

insulators, right: 420 kV Horizontal (Pivoting) V Assembly)

1.2 Inverted V and T Sets

Inverted V sets are applied as “pollution” sets in cases where the tower to conductor clearance is

limited but the pollution conditions on-site require high insulation length for accommodation of high

creepage distances. This case often occurs in replacement measures where conventional glass or

porcelain disk insulators failed due to pollution flashover and have to be substituted by silicone rubber

composite insulators [4]. Fig. 2 shows how inverted V sets are applied for DC line uprating on existing

towers with remaining clearance from conductor to tower. The application is also increasing the

pollution performance since much more creepage distance can be designed for the insulators. Similar

effects can be achieved by T sets (Fig. 2).

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DC 533kV DC 560 kV DC 600 kV

Fig. 2: Inverted V and T (pollution) sets for the accommodation of very high creepage distance on

“limited” tower clearance for 533, 560 and 600 kV DC applications [4].

1.3 Y Sets

Y sets are applied for line voltage uprating, e.g. from 245 kV to 420 kV on same tower cross-arm

structures. Fig. 3 shows an example for 420 kV in Germany. The Y set limits the swivel of the set and

therefore clearance (conductor to tower) is secured. In some cases, compromises regarding lower BSL

levels (950 kV instead of 1050 kV) have to be accepted.

Fig. 3: Y set for line uprating 245 to 420 kV with BSL of 950 kV

1.4 Semi-Anchored Sets

Semi-anchored sets are used for line uprating. A rigid insulating structure is formed in order to avoid

any kind of swivelling of the insulators (Fig. 4). The mechanical and electrical performance is proven

for more than 30 years in service at Um=420 kV.

Fig. 4: 420 kV semi-anchored set consisting of a compression and a double tension insulator string.

The design has been applied for 30 years in OHL uprating 245 to 420 kV in Germany.

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1.5 “3-Phase” Insulation Assemblies

The idea of 3-phase insulation assemblies was derived from 24 kV medium voltage compact lines

realised in the 1970s in Germany and Denmark. Presently, an innovative solution is developed in the

UK for 420 kV OHTL. The compactness and low space requirement is demonstrated in Fig. 5

compared to the standard solution. Type and design tests of the insulation were passed successfully. In

the next stage, a pilot line with a length of some km will be realised to gather experience.

Fig. 5: “Diamond” T-Pylon Assembly - An innovative 3-phase compact insulation project for 420 kV

in UK. The sketch shows the height of a standard 420 kV lattice steel tower compared to the space

requirements of the T-Pylon.

1.6 Interphase Spacers

Interphase spacers are fundamental elements of compact line insulations. They are applied in order to

avoid conductor galloping and to secure the clearance between conductors. Fig. 6 shows a typical

arrangement realised with flexible composite insulators which also contribute to the damping of

conductor vibrations. Service experience with composite insulators for this kind of application is

available for more than 35 years.

Fig. 6: 420 kV interphase spacers installed in Germany in order to avoid conductor galloping and to

secure clearances between conductors on an uprated OHTL (245 to 420 kV) with low mid-field

clearances by means of a 2-unit flexible composite insulator arrangement.

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2. MECHANICAL CHARACTERISTICS

The mechanical load performance for post assemblies can be calculated with the Euler buckling load

cases according to Tab. 1 [3].

Tab. 1: Critical Buckling Loads

Assembly

Type

Line

End

Structure

End

Critical Buckling

Load

Non-

pivoting V pinned fixed

2

2

)7.0(

EI

pivoting V pinned pivot 2

2

EI

The load performance of post assemblies (pivoting and non-pivoting) is strongly depending on the

compression load performance of the post insulator and therefore on the E (Young’s modulus) and the

core diameter. State of the art is a solid core FRP rods with diameters up to 130 mm. Larger diameters

(170 and 200 mm) are under investigation and development. Porcelain solid core post insulators can

be manufactured with diameters up to 300 mm.

More accurate load performance characteristics can be obtained by FEM calculations considering the

exact degrees of freedom given by the applied hardware components.

Typical E modulus is 35.000 MPa for EP/FRP composite material and 80.000 MPa for C120/C130

porcelain (ceramic) material. These values can be considered for design calculations.

Assemblies for 420 kV can be realised today to accommodate the following load situations:

Pivoting V assembly:

Non-Pivoting V assembly:

The non-pivoting V assembly can carry longitudinal loads and therefore can be applied for angle

tower structures and can also accommodate longitudinal loads e.g. in case of conductor breakage or

uneven wind in neighboured span fields.

All other elements can be realised with high strength composite insulators with SML (Tension Load)

up to 1500 kN per unit.

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3. ELECTRICAL CHARACTERISTICS

Compact OHTL line insulation shall be designed in accordance with IEC 60071-1 [5]. In some cases

lower BIL or BSL levels have to be accepted e.g. in case of Y insulator sets. For V (vertical and

horizontal) assemblies the recommended insulation levels can be met in most cases. For phase-phase

insulation IEC 60071-1 and IEC 60815 [6] shall be applied which leads typically to increased values

of BIL/BSL (factor 1.5) and creepage distance (factor 3).

Beside proper selection of insulation levels, the design of electrical field grading systems is of high

importance for compact insulation in order to avoid overstress by electrical fields. Today, modern 3D

FEM/BEM simulation tools are available to study and optimise electrical field grading on assemblies.

Fig. 7 demonstrates the result of such tools for a 170 kV non-pivoting V assembly. The electrical

stress on polymeric (silicone rubber) housing surfaces shall be limited to 0.45 kV/mm [7].

Fig. 7: FEM/BEM Electrical Field Calculations for optimisation of field control (grading ring) systems (170 kV

non-pivoting V). Red line: stress limit [7].

4. TESTING

For the time being, there is no IEC standard for the testing of complete insulating cross-arm assemblies, but

testing can be coordinated in accordance with IEC 61109 / IEC 61952 for composite and IEC 60383 / IEC 60168

for porcelain insulators. Electrical testing and mechanical type tests are performed typically on the full assembly.

In the mechanical type test, the allocated working loads are verified as combined loads (V: vertical, H:

transversal, L: longitudinal) in a special test arrangement, for example, as shown in Fig. 8. The combined loads

as per load case table are applied to the top end of the cross-arm assembly (yoke plate). Deflections, strains and

elongations are measured by means of strain gauges and displacement sensors. The combined working loads are

verified and typically a 115…125% overloading is simulated. These working load verification tests are non-

destructive and can be repeated several times. An important insight is the condition for buckling of the post

insulator(s). After collection of all of these load-case data points, destructive (ultimate breaking load) test is

applied for the most critical load condition.

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Fig. 8: Mechanical type test setup of 420 kV non-pivoting V assembly. Combined (complex) loads are applied at

the junction (yoke) plate at the top by means of hydraulic cylinders (for L and H) and weight (for V).

5. SERVICE EXPERIENCE

Long-term service experience with existing silicone rubber composite post/tension insulators for

system voltages up to 550 kV is available for more than 15 years in Switzerland, USA and Spain. Fig.

9 shows an installation for 420 kV in South Africa for heavy polluted and severe environmental

conditions. The line “Palmiet-Stikland” has a length of 50 km and the line route is close to the sea

coast. Space requirement of this 420 kV line is the same as for a conventional 110 kV line using lattice

steel towers. The utility decided in favour of a compact line in order to minimise visual and

environmental impact and to gain acceptance of population. The line was installed in 2008, therefore 5

years of positive field experience has been achieved even under severe conditions (pollution, high UV

stress).

Fig. 9: Example of 420 kV compact OHTL with non-pivoting V (silicone rubber) insulator assemblies

operating under severe environmental conditions successfully for more than 5 years.

6. STANDARDISATION

The idea for standardisation of such applications is to establish an application guide. This guide could

be a part of the standard family of IEC 62609-x for composite line post insulators and their assemblies.

Presently, work bodies in IEC SC 36B and CIGRE B2 are dealing with such standard development.

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BIBLIOGRAPHY

[1] J. M. Seifert, “Innovations in the Field of Composite Insulators – Review on 40 Years of Experience“, LAPP

RODURFLEX® 40 Years Convention, 14-16th

February 2008, Tröstau, Germany.

[2] J. M. Seifert, D. Stefanini, “High Pollution Resistant Composite Insulators“, ICHVE 2008, Nov. 9-13th,

2008, Chongqing, China.

[3] IEEE Task Force 15.09.09.07, “IEEE Application Guide for Braced Post High Voltage Insulator

Assemblies”, 2008.

[4] J.M. Seifert, D. Stefanini, H. Janssen, “HTV Silicone Composite Insulators for HVDC Applications – Long-

term Experiences with Material and Design for 500 kV and above”. ISH 2009, Cape Town, South Africa.

[5] IEC 60071-1: Insulation Coordination.

[6] IEC 60815-1, -2, -3: Selection and dimensioning of high-voltage insulators intended for use in polluted

conditions - Parts 1-3.

[7] EPRI Guide on Evaluation of Electric Field Exposure Assessment Methods, Electric Power Research

Institute, USA, 2008.