Transmission Utility Re-Designed Insulator Strings to Upgrade Thermal Rating of Lines

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A decade ago, CEPS, the Transmission System Operator in the Czech Republic, faced the need to increase the longitudinal profiles of several overhead transmission lines. The goal was to increase phase-to-earth clearances at critical locations with the ultimate objective of increasing the ampacity of these lines. The final engineering solution, based on upgrading thermal rating through increasing the conductor attachment height, is reviewed in this past INMR article.

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The term longitudinal profile is used here to describe actual clearances from ground and other objects located beneath the overhead line. It is based on calculation of different possible scenarios defined in the standards (e.g. ice loading, minimum temperature, highest operational temperature, etc.) and also reflects various security as well as public safety criteria. Increasing the transmission capacity of existing lines is now one of the main topics in the European power supply industry given today’s open marketplace and the steady growth in transfers of electrical energy among EU member countries. This goal is all the more pressing since obtaining approvals for constructing new lines is very difficult even as new power sources, such as renewable wind generation, require increasing existing power transfer capacities.

Decision-Making Process

There are basically two mechanisms to increase the capacity of existing power lines, i.e. increasing either their thermal rating or their voltage rating. As voltage upgrading is mainly applicable in the case of long transmission lines, the method of thermal uprating was considered the more suitable by CEPS management. In general, increasing thermal rating of lines can be dealt with as follows:

Re-tensioning the conductor

This method is easily applied and there are low material requirements. However, the main disadvantage is that re-tensioning conductors which have been in operation for 20 years or more can be risky and a ‘memory effect’ of the conductor is highly probable.

Replacing the brackets on lattice steel structures

This method has shown good results and is therefore already widely used on overhead transmission lines in the Czech Republic. However, depending on conductor arrangement, a modification of the higher-positioned insulator string becomes necessary because of the shortening of the phase-to-phase distance (see Fig. 1).

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Fig. 1: 220 kV line with modified brackets and insulator strings. insulator string Transmission Utility Re-Designed Insulator Strings to  Upgrade Thermal Rating of Lines 220 kV line with modified brackets
Fig.1: 220 kV line with modified brackets and insulator strings.
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Fig. 2: 400 kV line with insulator strings modified into ‘upturned T’. insulator string Transmission Utility Re-Designed Insulator Strings to  Upgrade Thermal Rating of Lines 400 kV line with insulator
Fig.2: 400 kV line with insulator strings modified into ‘upturned T’.
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Modification of insulator strings into special types (upturned V, T)

In most cases, this method requires replacing existing insulators. It is particularly applicable for lines with suspension insulator strings and where it has been shown to be highly effective (see Fig. 2).

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All of the above alternatives are concerned with increasing conductor attachment height and that also means increasing electrical clearances as the basic requirement for system security. Before a final decision was made on which of these solutions to adopt, CEPS engineers reviewed the following factors:

• The condition of existing components on the affected line, especially steel structures and conductors;

•The ready availability of suitable components on the market (including conductors, insulators, etc.);

•Time duration of the power cut on the affected line as well as the estimated cost of the work;

•Ease of access to particular points along the line, i.e. terrain profile, character (forests, agricultural areas, meadows) and ownership of land (public versus private).

Other Considerations

1. Deterioration of components, especially steel structures and conductors, would require their refurbishment. In that case, the selected parameters of the newly-replaced components would match the requirements of the new line profile and there would be no need for any special re-engineering.

2. In those cases where there would be a requirement to replace conductors or insulators, new technologies (e.g. conductors with reinforced cores, composite insulators) would make it easier to engineer the best solution.  

3. The Czech power grid, based on a network of 400 kV and 220 kV overhead lines, is quite dense so the duration of any outage becomes an important factor in decision-making.

4. In the case of good terrain, a machine-operated platform could be used. This would not only make the work easier but also save time transporting materials.

The ampacity of the conductors installed according to the original design criteria of the affected line was based on a maximum operating temperature of 40°C. The subsequent EN standard, however, recommended setting the thermal rating of such lines at 70°C, with a maximum long-term operating temperature of the conductor in most cases being 80°C. For this reason, the goal by CEPS was to increase the thermal rating of the line to 80°C, meaning a 40°C thermal uprating.

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After reviewing all of the above factors, engineers decided on achieving this using the method requiring modification of selected insulator string configurations.

Requirements for Insulators

The basic requirements for any new insulator strings were:

1. the insulators had to be commercially-available on market, i.e. without any need for customization;

2. the weight of the new insulator strings could not exceed the weight of the previous strings.

From among the three main types of insulators available on the market from the dielectric material point of view (i.e. porcelain, toughened glass and composite) the composite design was eventually chosen. The basic reasons for this selection were: reduction in weight; superior mechanical resistance in the direction perpendicular to the axis of the insulator; better price; and easier erection procedure.

Testing New Insulator Strings

All electrical tests were carried out in accordance with IEC and EN requirements. There was no need for any mechanical testing. Electrical testing included: a power arc test; a wet switching impulse test; a dry lightning impulse test; and testing for any radio interference voltage (R.I.V.). The result of R.I.V. characteristics for the insulators selected is shown in Fig. 3. The value at 242 kV was lower then 73 dB, in accordance with EN 61000-6-4:2002. Corona testing was then performed to confirm that any corona occurred only at the grading rings.

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Fig. 3: R.I.V. characteristics.
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Corona effect during tests. insulator string Transmission Utility Re-Designed Insulator Strings to  Upgrade Thermal Rating of Lines Corona effect during tests
Fig. 4:Corona effect during tests.
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Application of Methodology

A double circuit 400 kV line, which was one of the recent applications for such a thermal uprating, crossed a dry area with barriers against periodic flooding. Because of this, the longitudinal profile (clearances) had to be increased to fulfil the requirement of system security whenever the maximum level of such flooding would occur. This project dealt with replacing of former ceramic double insulator string having a total length of 4982 mm. The new insulator string, in the shape of an ‘upturned T’, was installed and having a total vertical length of 1927 mm, which resulted in increasing of phase-to-earth distance by 3055 mm. This added clearance then enabled increasing the thermal rating of the line’s conductor to 80°C. Replacing each affected insulator string took about half a day. For an average span of 300 m, the line’s thermal rating depends on clearances according to the following calculations:

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Fig. 5: Assembling new insulator strings. insulator string Transmission Utility Re-Designed Insulator Strings to  Upgrade Thermal Rating of Lines Assembling new insulator strings
Assembling new insulator strings.
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T+1°C = D + 0.03 m

D+1°C = I + 30 A

where, T = thermal rating

D = distance in the middle of the span (i.e. clearance from the ground)

I = current rating of the three-bundled conductors

The first relationship shows that increasing the thermal rating by each 1°C requires shortening conductor attachment height by 3 cm. The second relation demonstrates that thermal uprating by 1°C results in possible higher conductor rating of 30 Amps.

Conclusions

The main advantages of this retro-fit process could be summarized as follows:

• An excellent methodology for thermal uprating of a line (allowing a gain of more than 100°C);

• A relatively short shut-down time for the affected line since only about a half a day was required for each affected insulator string.

On the negative side, however, existing conductors have to be cut and in the event of a failure, the new shorter suspension string represented a higher load for the brackets of the structure in the axial direction.