Composite Insulator Design for New Transmission Lines in the U.K.

Advertisement
PPC Insulators transmission lines Composite Insulator Design for New Transmission Lines in the U.K. banner530x150 modified

Upcoming changes to generation have meant substantial investment in expanding and upgrading the transmission network in the United Kingdom. The TSO, National Grid, embraced the need to harness such innovation to build new and upgrade existing transmission assets so as to operate a more flexible network, facilitating connection of customers – including an increasing proportion of lower carbon generation to meet Government environmental targets. This INMR article from autumn 2015, based on a contribution by Mike Fairhurst of the National Grid, provided insight into ongoing development of unique 400 kV double circuit transmission lines across the U.K.


Lattice steel structures have been deployed in the U.K. for the last 90 years, with the first design selected based on a design competition and evolving over time. In 2011, the Department of Energy & Climate Change as well as National Grid called on architects, designers, engineers and students to come up with innovative transmission tower designs that balance energy needs and visual impact, again carried out on a competitive basis. The competition attracted 250 entries with scale models of the finalists going on public display at the Victoria and Albert Museum as part of the London Design Festival. The winning design – the ‘T’ Pylon – submitted by A Danish company was the overall winner. The basic loading criteria for the design is based on an existing lattice design consisting of a double circuit 400 kV line with a maximum demand transfer of 3800 MVA on a 3-phase, triple bundled, 500 mm spaced, 700 mm2 AAAC conductors (Araucaria) National Grid subsequently engaged this firm to take the design from concept to full engineering compliance, having established that they had the necessary experience and capability in transmission tower design. The design team also engaged a number of composite insulator manufacturers to develop the insulator array. As the ‘T’ Pylon moved from concept to detailed design, National Grid worked closely with the design team to develop a tower family, based on the original concept, so that it could become a viable alternative to traditional lattice designs for transmission routes of the future. In moving from concept to full design, a great deal of effort was put into the mechanical performance and manufacturing processes. This involved producing full-scale prototypes and developing procedures to fully test mechanical performance under a number of extreme conditions. The diamond shaped design has to withstand high compression and torsional forces under broken wire conditions as well as extreme wind and ice.

Advertisement
GIG Advertisement transmission lines Composite Insulator Design for New Transmission Lines in the U.K. GIG Jan 22

In parallel with this developmental work, the electrical performance needed to be fully understood since this was a completely new insulator arrangement that incorporates composite line insulators along with substation composite post insulators – both solid and hollow core. No international standards yet exist for this arrangement and National Grid therefore engaged two domestic universities to carry out electrical performance studies. In addition, European-accredited test institutions were engaged in association with National Grid and the designer in compiling a robust test protocol to prove the electrical performance of the insulator array.

National Grid also has responsibility to ensure that the erection, maintenance and repair of this novel design of transmission line can be carried out safely and efficiently. To that end, a full-scale training facility was constructed that incorporates the entire ‘T’ Pylon suite of towers. The National Training Centre at Eakring in Nottingham allows development of erection and stringing techniques as well as maintenance and repair procedures in advance of any new build. A plan and photo of this facility is shown below in Appendix 1.

Tower Types

At the project’s onset, the team looked at existing tower structures and their capability to determine what tower types would be required to design and construct a new line. In the U.K. a family of towers is typically designed with a selection of standard height suspension towers with an array of extensions, a range of tension towers with various maximum angles of deviation (typically 10°, 30°, 60° and 90°). In addition, there are two types of terminal (end) towers, double and single circuit. In many cases, a double circuit junction tower was also designed to enable existing lines to be tied in to create a 3-ended circuit. Since the winning entry in the competition was a suspension tower, the team had to produce similar tension tower designs. The aim was to keep the same conductor geometry by staying with the diamond shaped configuration while producing a design capable of restraining the mechanical loads and forces imposed when deviating from a straight line. Outline designs were produced for 30°, 60° and 90° deviations; however it soon became apparent that for monopole designs with angles above 30°, towers would have to be extremely strong, large and wide to meet conductor loading criteria. The resulting structures, compared to the suspension tower that has a slender, light and uplifting visual appearance, looked heavy, wide, cumbersome and less aesthetic. In order to retain the original visual concept, further development on early tension tower designs greater than 30° was frozen and the team looked to design a suspension tower with a maximum deviation of 10°. This tower, in combination with the D30 tension tower, allows the line to deviate in sweeping bends rather than the traditional method of turning corners. The deployment of this combination has the effect that the ‘T’ Pylon follows through the landscape rather than crossing it in a number of sharp turns. By adopting this principle, the number of tower types was reduced to just 5, with 360 m span standard and height being 35 m.

‘T’ Pylon Tower Types

Suspension (Straight Line) Tower

Flying Angle F10: Suspension tower with a maximum deviation of 10 degrees

Tension D30: Angle tower with a maximum deviation of 30 degrees

Double Diamond Terminal Tower: Two independent diamond-shaped structures

Single Circuit Gantry – Terminal Tower: Low level ‘goal post’ gantry tower

Outline drawings and images of these are shown in Appendix 2

Insulation Type

Suspension (Straight Line) Tower

Insulation Array consists of two 600 kN standard design suspension long rod composite insulators to form the top section of the array and two 300 kN extra creepage suspension long-rod composite insulators to form the bottom section; composite hollow core or solid core composite compression insulator to provide phase to phase insulation between the points of the diamond.

Flying Angle F10 – Suspension Tower with Maximum Deviation of 10°

Insulation Array consists of two 1100 kN solid core composite standard post insulators to form the top section of the array, two 1100 kN extra creepage solid core composite standard post insulators to form the bottom section; composite hollow core or solid core composite compression insulator to provide phase-to-phase insulation between the points in the diamond.

Tension D30 – Angle Tower with Maximum Deviation of 30°

Insulator Array consists of a standard 300 kN, 400kV composite long rods in a triple formation – one for each sub-conductor with standard sag adjuster fittings. Fitted with solid core composite insulators on the inner and outer phases to ensure jumper connections maintain 400 kV phase-to-earth (ground) clearance.

Double Diamond Terminal Tower – Two Independent Diamond-Shaped Structures

Advertisement
transmission lines Composite Insulator Design for New Transmission Lines in the U.K. CSL Advertisement e1477326535980

Insulator Array – same as D30

Single Circuit Gantry 

Insulator Array – As D30

See Appendix 3 – Outline insulator drawings

Construction

During development of the ‘T’ Pylon, a number of workshops took place with all OHL contractors and National Grid maintenance teams (including Eakring OHL training staff), with respect to build issues and repair techniques. These workshops highlighted the main areas of concern, that in the main were concentrated on the suspension and flying angle towers and associated insulator arrays. Main issues arising from the construction and line repair workshops associated with the Eakring ‘Need Case’ included:

1. Tower construction & assembly;
2. Tower access & egress;
3. Conductor stringing;
4. Performance of suspension tower insulator array imbalance when using traditional, one phase at a time, tension string techniques;
5. Erection of insulator array;
6. Earth wire (ground wire) stringing and induced currents;
7. Conductor running blocks;
8. Clipping in new conductor;
9. Raising and lowering of insulator array;
10. Changing defective suspension insulators.

Working on tension towers and terminal towers will, in principle, use current techniques and practices since tower attachment points are fixed and will carry conventional composite insulators string arrangements. Since there is no requirement to climb ‘T’ Pylon structures, access methods will deploy Mobile Elevated Working Platforms (MEWP’s) and techniques will therefore be developed at the training facility.

Technical Specification Development

Initial electrical specification was developed on the basis of existing National Grid’s technical specifications, which are derived and adhere to IEC. However National Grid sought verification and input from an independent, internationally accredited test institution to carry out a review and make further recommendations as well as to develop a suite of mechanical and electrical test protocols. Three suppliers were engaged to produce insulators and fittings to meet mechanical and electrical performance criteria. All 3 suspension insulator designs were then subjected to the test regime. Once all the results are compiled, the information and findings will then form the basis for the insulator technical specification and the type test protocol. This will enable National Grid go out to the global market on a commercial basis.

Extract ‘T’ Pylon Test Specification

‘T’ Pylon Development Dielectric Testing Program

This document seeks to define the functional electrical testing requirements for the single insulator unit (‘diamond’) that forms a critical part of the suspension design and the F10 ‘flying angle’ deviation. This includes the cast iron components with machined finish that tie the cross-arms to the monopole steel structure and the cross-arm to the insulator array diamond. The ‘T’ Pylon design is optimized for triple Araucaria (3 x 700 mm2, 35 mm diameter) conductor but can also be used with a twin conductor bundle such as Redwood (2 x 850 mm2, 41 mm diameter) as well as other configurations. Fittings are typically cast iron but use of fabricated steel welded fittings is still to be investigated. Conductor bundle spacing for the tests shall be 500 mm; the triple bundle is arranged apex down i.e. in an inverted triangle. Twin earth wires are situated at the top of the diamond, just below the tip of the cross-arm.

Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. T Pylon tower
CLICK TO ENLARGE

The novel diamond assembly is fundamentally a 3-phase configuration employing both phase-to-earth (AB/AC) and phase-to-phase (BC/BD/CD) insulation. This is a new application and there is very little anecdotal evidence against which to compare the design. Therefore, it is necessary to perform basic performance tests to establish confidence in the design. The purpose of these tests is to understand and demonstrate the basic electrical integrity of components when combined to form the 3-phase assembly. This document considers a range of tests on the full-scale diamond in an attempt to replicate ‘as installed’ conditions, recognizing that the interaction of the 3 phases and the electric field arising in the geometry is key to successful operation of the pylon. (Note: Individual single-phase element tests have been demonstrated by the supplier.) The designer and National Grid were to have at least 3 suspension insulator units for testing; and one F10 flying angle insulator array. These tests therefore enabled determining the effectiveness of the different solutions as well as their operating envelopes. The spirit of National Grid Technical Specification (NGTS) 3.04.17 (i.e. insulator sets for overhead lines) & NGTS 3.04.18 (i.e. composite insulators for OHLs) and IEC testing were adopted and applied where practical. However, this was developmental not type testing and, as such, the aim was to establish, where possible, the limit or margin of dielectric insulation such that design optimization can eventually be considered to reduce the diamond’s dimensions without unduly compromising dielectric performance and safety. Recognizing that these were development tests, the output of some might not align with expectations. Therefore, tests would need to be revised or repeated. All tests were performed with arcing horns and corona rings connected to replicate the final configuration of the diamond i.e. no tests to be undertaken without arcing horns.

Advertisement
BPG International Advertisement transmission lines Composite Insulator Design for New Transmission Lines in the U.K. BPG logo

Although the ‘T’ Pylon can accommodate a twin Redwood conductor, it is optimized for triple Araucaria. All tests will therefore be undertaken with triple Araucaria. The conductor system will be represented with appropriately sized tubes connected to the ungalvanized cast iron fittings.

Description of Tests

Corona/RIV testing

• These tests are to establish respective inception & extinction levels to support development of FEA models.
• Noting that the facilities may be limited to 3-phase 430 kV test voltage, this voltage should be maintained and detailed recordings undertaken of any corona/RIV using an image intensifier (can be supplied by National Grid). These tests will also demonstrate that the IU can be energized to nominal voltage.
• 2-phase tests using 2-off 550 kV single-phase transformers that can be independently regulated and can be connected with a 120° phase shift or with 180° shift, resulting in different voltages phase-to-phase. The purpose of this test is to provide further input to the FEA work that will be undertaken.

Dry Lightning Impulse Testing

• Dry lightning impulse withstand level (LIWL) testing to 1425 kV; Arcing horns set to ‘beyond’ value of 2.8 m (as NGTS1 Table 4 at 1.6 km from the substation; NGTS 3.4.17 Section 2.4)

Wet Switching Impulse Testing

• Wet switching impulse withstand level (SIWL) testing to 1575 kV; Arcing horns set to ‘beyond or mid line’ value of 2.8 m (as NGTS1 Table 4; NGTS 3.4.17 Section 2.4)

Further Impulse Testing

• Horizontal compression insulator (HCI) performance envelope (SIWL tests): Two variants to be tested – solid core 2-element unit and single hollow core unit.
• These tests seek to understand what minimum level of insulation is required on the HCI such that the metal element can be extended to improve overall mechanical performance;
• The output of these tests could enable optimization of the diamond and hence overall pylon dimensions;
• These tests follow on from the SIWL that benchmark performance of the diamond. The installation of arcing horns on the HCI is expected to affect electric field but not impact SIWL benchmark tests;
• The arcing horns will be set to replicate HCI insulation length, nominally 5.2 m, with the up/down method and U50 value being re-evaluated to collaborate the benchmark value;
• The arcing horn distance will then be reduced at intervals of at least 10 cm (5 cm for each arcing horn) and the up/down method and U50­ test performed to determine flashover point.
• The arcing horn distance will be reduced to a point at which a flashover to point A begins to occur repeatedly. This, in theory, could be at a distance of 3.9 m, aligning with phase-to-earth insulation length. 

Advertisement
transmission lines Composite Insulator Design for New Transmission Lines in the U.K. INTEGRATED Engineering Software Mar 11

Additional developmental testing on the full insulator array includes ice accretion at full system voltage on all 3 phases.

Salt – Fog pollution tests

Water Droplet corona

Appendix 1

Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. T Pylon Training
T Pylon Training and Test Facility at National Grid Training Centre at Eakring.
CLICK TO ENLARGE

After extended consultation with local planning authorities, plans where put forward to construct a line with reduced span lengths that would incorporate the full suite of towers; 6 towers, 5 spans (see details above). Land adjacent to the existing training facility at Eakring (3 towers 2 spans) was purchased and outline planning permission agreed.

Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. Eakring Training facility
Eakring Training facility under construction
CLICK TO ENLARGE
Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. Insulator Fittings
Insulator fittings.
CLICK TO ENLARGE
Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. Suspension Towers
Suspension towers in Eakring have insulators erected with running blocks ready for stringing.
CLICK TO ENLARGE

Appendix 2: Outline Tower Drawings

Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. D30 Tension Tower
D30 Tension Tower
CLICK TO ENLARGE
Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. Suspension Tower
Suspension Tower
CLICK TO ENLARGE

 

Advertisement
Yizumi Ad transmission lines Composite Insulator Design for New Transmission Lines in the U.K. INMR YIZUMI July edition

 

Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. F10 Flying Angle
F10 Flying Angle
CLICK TO ENLARGE

 

Appendix 3: Examples of ‘T’ Pylon Insulator Arrays

Flying Angle

Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. T Pylon Insulator Array 1
CLICK TO ENLARGE
Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. T Pylon Insulator Array 2
CLICK TO ENLARGE

 

Suspension Insulator Array

Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. Suspension Insulator Array 1
CLICK TO ENLARGE
Composite Insulator Design for New U.K. Transmission Lines transmission lines Composite Insulator Design for New Transmission Lines in the U.K. Suspension Insulator Array 2
CLICK TO ENLARGE

 

Advertisement
Desma Advertisement transmission lines Composite Insulator Design for New Transmission Lines in the U.K. Desma banner Apr 26

Appendix 4

Extract – Insulator Unit (‘Diamond’) Test Program & Test Environment
The diamond assembly is fundamentally a three-phase configuration employing both phase-to-earth and phase-to-phase insulation. As such, there is no standard test program for this type of assembly and the spirit of standards will be followed for tests below:

1.1 Test Environment

• The diamond comprising all 5 (4 diagonal & 1 horizontal) composite insulators and galvanized cast iron metal fittings (insulator end fittings, corona rings, arcing horns) will be tested as they are to be used in the final configuration on a line. All components will be supplied by National Grid.
• The pylon and cross-arm shall be represented with the equivalent earth planes appropriate to the pylon cross section to provide representative field distortion. Where calculation or modelling can demonstrate alternative arrangements, this will be acceptable subject to agreement with National Grid.
• In an attempt to obtain results that can be correlated against similar tests, the conductors shall be represented with an equivalent diameter unit and length in compliance with IEC 6184, Fig. 13, Table 4. The ends of these tubes shall be shielded to minimize risk of unwanted discharges. The tubes will be held in typical suspension shoes i.e. no armour rods.
• Wet testing shall follow guidelines given in IEC 60060-1:2010. The conductivity of the water shall be 100 ± 15 μS/cm and rain intensity, vertically and horizontally, shall be reasonably close to the values given in the standard. Since more than one insulator can be stressed at the same time, a combination of rain angles (e.g. vertical and simultaneous horizontal wetting of AB/AC) shall be considered depending on the facilities available.

1.2 Radio Interference Voltage & Corona Detection (3-phase)

• The objective of the three-phase test on a triple conductor system is to primarily record any corona/RIV events that occur so that the information can be fed into FEA studies on the diamond. The FEA studies will then help define the limits of performance and a future single-phase equivalent corona/RIV test voltage. It may be necessary for corona ring changes to be made during the test. Arcing horns shall be set to the ‘Approach’ value of 2.5 m.
• Radio noise will be measured and calibrated with instruments in accordance with CISPR 18-2:2010 and IEC 60437:1997. Measurement frequency shall be 1 MHz and the result shall be presented in dB above 1 μV across 300 Ω. Maximum RIV level at 40 dB is to be recorded for the phase-to-ground voltage.
• For the purposes of these tests, observations of corona shall be made using image intensifier technology to allow for accurate correlation to voltage test levels e.g. high sensitivity video recording equipment that can capture even faint corona discharges.
• Energize all three phases of the diamond until 430 kV and record any corona/RIV. If any Corona/RIV is recorded below 430 kV, reduce voltage until an extinction value is obtained.
• Note 7: These tests also provide confidence that the diamond can be energized beyond its normal operating voltage.

1.3 Radio Interference Voltage and Corona Detection (2-phase)

• The objective of these 2-phase tests, up to 550 kV, is to identify the inception and extinction voltage, with an aim to define limits of performance. It may be necessary for corona ring changes to be made during the test. Arcing horns shall be set to the ‘Approach’ value of 2.5 m.
• Radio noise will be measured and calibrated with instruments in accordance with CISPR 18-2:2010 and IEC 60437:1997. Measurement frequency shall be 1 MHz and the result shall be presented in dB above 1 μV across 300 Ω. Maximum RIV level at 40 dB is to be recorded for the phase-to-ground voltage.
• For the purposes of these tests, observations of corona shall be made using image intensifier technology (e.g. high sensitivity video recording equipment that can capture even faint corona discharges.
• Voltage applied to terminal B-D. Terminals A and C connected to ground.
• Voltage applied to terminal C-D. Terminals A and B connected to ground.
• Voltage applied to terminal B-C. Terminals A and D connected to ground.

1.4 Dry Lightning Impulse Test

• This test, up to 1425 kV phase-to-ground / phase-to-phase, shall determine actual withstand voltages of the diamond limbs using the Up and Down method described in IEC 60060-1:2010. Arcing horns shall be set to a ‘Beyond’ value of 2.8 m.
• At the flashover voltage 30 positive and 30 negative impulses will be applied to determine the U50 Voltage steps shall be in 3% changes. Note 8: no verification tests are necessary i.e. +/- 15 impulses.
• Voltage applied to terminal B. Terminals A, C and D connected to ground.

Note 9: Only terminal B is tested as with the pylon in place, terminal B is no longer equivalent to terminal C and the expectation is that the ‘outer’ conductor will be most vulnerable to lightning.

Note 10: The inner phase conductor is shielded against direct strikes but could be subject to backflashover if the distance from pylon (~7m) is not sufficient. It is noted that there is no specification for this test.

• Voltage applied to terminal D. Terminals A, B and C connected to ground.

1.5 Wet Switching Impulse Test

• This test, up to 1050 kV phase-to-ground & 1575 kV phase-to-phase shall determine the actual withstand voltages using the Up and Down method described in IEC 60060-1, with 30 positive and 30 negative impulses. Arcing horns set to ‘Beyond or Mid line’ value of 2.8 m
• At the flashover voltage, 30 positive and 30 negative impulses will be applied to determine the U50 Voltage steps shall be in 3% changes.

Note 10: no verification tests are necessary i.e. +/- 15 impulses.          

Note 11: With the pylon in place, terminal B is no longer equivalent to terminal C. Both terminals though are susceptible to switching events.

• Voltage applied to terminal B. Terminals A, C and D connected to ground.
• Voltage applied to terminal C. Terminals A, B and D connected to ground.
• Voltage applied to terminal D. Terminals A, B and C connected to ground.

2.0 Test Sequence

2.1 Suspension  Insulator Unit 1 – Hollow Core Compression

• Radio Noise & Corona measurement – 3 phase
• Radio Noise & Corona measurement – 2 phase
• Lightning impulse, dry
• Switching impulse, wet
• Repeat with compression insulator variant – Solid core

2.3 Flying Angle Insulator Unit 2 – Hollow Core Compression

• Radio Noise & Corona measurement – 3 phase
• Radio Noise & Corona measurement – 2 phase
• Lightning impulse, dry
• Switching impulse, wet
• Repeat with compression insulator variant – Solid core