21, rue d’Artois, F-75008 PARIS B3-206 CIGRE 2006

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Substation Design Using Phase to Phase Insulation

A Renton
Maunsell Ltd
New Zealand

SUMMARY
The use of phase to phase insulation on Transmission Systems appears to have been limited
to use as conductor spacers to prevent line clashing on transmission circuits. Further
applications within transmission systems and in particular switchyards, do not appear to have
been pursued. While investigating expansion options for a 220 kV substation, we found that
by using a higher voltage class composite insulator to separate the main and transverse bus
sections, a number of advantages over current designs are possible. These ideas can be
used for both the redevelopment of existing and the construction of new substations.

Work was carried out to investigate expansion options at a 220 kV substation. The brief for
this review involved examining the existing switchyard designs and suggesting possible
options that would meet the following key design criteria;

• Reduced site visual impact

• Elimination of special components
• Utilisation of standard Air Insulated Switchgear (AIS) and line hardware
• Utilisation of AIS equipment from alternative suppliers
• Compliance with clients seismic and electrical clearance design standards

One design option that evolved made use of a rigid aluminium main busbar separated from
the flexible (stranded conductor) transverse bus by phase to phase insulation, made up of a
standard 400 kV composite line suspension insulator hung from the main bus.
This approach enabled the design criteria objectives to be met primarily by reducing the
number of components used. By changing the design of the main bus supports, savings in
foundations and structures were also realised. With the use of a larger diameter main bus,
the requirement for rigid transverse buswork and associated support posts, foundations and
insulators was removed as the suspended composite insulator strings support the transverse
bus conductor. As an added advantage this arrangement enabled the physical spacing of
the transverse bus and the main bus support structures to be reduced without affecting
electrical clearances, resulting in a shorter and narrower bay size.

andrew.renton@maunsell.com
Taken together this arrangement has enabled the following;

• Fewer components to be utilised, with no special or one supplier only parts being
required as the flexible transverse bus acts as both transverse bus and equipment
connector
• Reduced number of structures to be installed and maintained
• Improved access to the switchyard through the removal of clutter and extraneous
structures
• Reduced construction time & cost savings of approximately 55-80% per bus section
• Improved environmental effects by reducing the station footprint and visual impact
with fewer structures being present
• Improved equipment maintenance access

This paper describes the design of this alternative approach, and compares and contrasts
technical and financial attributes with more traditional designs.

KEYWORDS

Conductor, Busbar, Phase to Phase, Insulation, Rigid-Bus, Flexible-Bus, Substation

andrew.renton@maunsell.com
INTRODUCTION
The owner of the New Zealand National Grid, wished to investigate possible expansion
options at a 220 kV substation. The work involved reviewing the existing switchyard design
and formulating alternative switchyard bus configurations, which complied with both the
National Grid Operators standards for the design and layout of substation and buswork, as
well as meeting a number of key design criteria. These criteria were;

• Reduced site visual impact

• Elimination of special components
• Utilisation of standard Air Insulated Switchgear (AIS) and line hardware
• Utilisation of AIS equipment from alternative suppliers
• Compliance with clients seismic and electrical clearance design standards

Outlined below are the significant electrical, mechanical & environmental design parameters
that formed the basis for the design of the proposed substation expansion.

2 DESIGN REQUIRMENTS

2.1 Critical Design Parameters

The following critical design parameters were evaluated as part of the conceptual design
stage and formed the basis of the electrical and mechanical calculations.

2.1.1 Electrical Ratings & Requirements

• Nominal Operating Voltage 220 kV
• Maximum Operating Voltage 245 kV
• Lightning Impulse Withstand Level 1050 kV
• Maximum Operating Current 1200 A
• Fault Current & Duration 25 kA/3 s

2.1.2 Environmental
• Maximum Ambient Temperature 30°C
• Maximum Continuous Conductor Temperature 80°C
• Maximum Short Time Conductor Temperature 250°C
• Minimum Wind Velocity 0.6 m/s
• Relative Conductor Emissivity 0.5
• Pollution Level Heavy/Very Heavy
• Creepage 25 mm/kV
• Seismic Loads to AS/NZS:1170
• Line Pull <=1 kN

2.1.3 Electrical Clearances, Spacings & Distances

• Minimum Distance Phase – Phase & Phase - Earth 2100 mm
• Minimum Height Live Conductors to Ground Level 4500 mm
• Minimum Vertical Maintenance Distance 4600 mm
• Minimum Horizontal Maintenance Distance 3600 mm
• Minimum Busbar Height Above Ground Level 5500 mm
• Minimum Rigid Busbar Centre Line Spacing 3600 mm
• Minimum Vehicle Busbar Approach Distance 4600 mm
• Minimum Vehicle Height 3400 mm

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3 BUS DESIGN
The following sections demonstrate the proposed designs evolution by contrasting the
existing, an interim and the proposed busbar configuration, before going on to detail a series
of calculations used to verify the design.

3.1 Configurations

3.1.1 Existing Rigid-Rigid, Post Mounted Phase-Earth Insulation

The current preferred 220 kV busbar design utilises a high level rigid, aluminium, tubular
busbar, supported on post insulators and mounted on individual steel posts and concrete pad
foundations. Located at right angles to it, underneath and of a similar construction to the
main busbar is the transverse bus. Jumpers assembled from flexible stranded conductor with
compression fittings are used to make bolted connections between bus sections and from
bus sections to switchgear. Typical examples of this arrangement are shown in Figure 1

Figure 1 Existing Rigid Post Mounted Main - Rigid Post Mounted Transverse Bus

3.1.2 Possible Rigid-Flexible, Post Mounted Phase-Earth Insulation

Due to the additional cost of rigid busbar, and that some bus sections only have one
switchgear bay connected to them, investigations took place into whether the rigid transverse
bay could be replaced with lower cost stranded aluminium conductor, as shown in Figure 2.

Figure 2 Rigid Post Mounted Main - Flexible Post Mounted Transverse Bus

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The use of flexible conductor for the transverse bus has construction, access and cost
advantages when only one switchgear bay is installed, however these benefits reduce when
additional bays are required, because it requires the same number of insulators, steel posts
& foundations as its rigid equivalent. As multiple switchgear bays are a possibility, it was
decided to investigate the feasibility of removing the additional transverse support posts
while retaining the flexible stranded conductor.

3.1.3 Proposed Rigid-Flexible, Suspended Phase to Phase Insulation

Additional posts are required to support and restrain the transverse bus stranded conductor
during fault conditions to prevent it moving and compromising electrical clearances. At the
same time, sufficient movement needs to be retained to accommodate equipment
displacement during earthquakes. The first item considered for removal was the yellow
phase transverse support post. By moving the switchgear closer to the busbar the need for
the yellow phase support would be negated. However, this was unable to be achieved
because the outer phases of the disconnector and the outer main bus support post electrical
clearances were compromised. This conflict was resolved by changing the design of the
main bus support posts from a single post per phase design, to a heavier single column
supporting all three phases. The result was a larger, heavier more expensive bus support,
able to resist the seismic loads applied to it

With these conflicts resolved, the disconnector could be repositioned closer to the main bus,
removing the requirement for any yellow phase bus supports while leaving the challenge of
the outer blue phase flexible connections. The design criteria specified in Section 2.1, limited
the load applied to equipment terminals by flexible connections to less than 1 kN. After
consideration it was proposed that suspension insulators, used in a similar manner to how
transmission line phase spacers are used, could support the flexible transverse bus.
Research aimed at identifying where phase to phase insulation was used on transmission
systems other than line phase spacers and how it had been used within substations met with
only limited success. Therefore a number of arrangements using phase to phase insulation,
some examples are shown in Figure 3, were proposed before the final design of two
insulators arranged in a “V” configuration was settled on.

Figure 3 Alternative Phase to Phase Suspension Insualtor Configurations

The final configuration utilises the standard main bus support post insulators, a larger
diameter rigid tubular aluminium busbar and standard 400 kV composite suspension
insulators. These suspension insulators along with the hardware are hung from the main
bus, supporting the flexible conductor transverse bus. When viewed from above the
suspension insulators are diagonally offset to one another to prevent significant sideways
displacement under fault conditions.

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Having established that this arrangement met the design criteria without major drawbacks,
further detailed design checks where undertaken to confirm the design concept.

3.2 Conductor Selection

3.2.1 Main Bus

To prevent the need for specialised components and hardware and to minimise costs, a
larger sized standard tubular main bus was used. The bus chosen was a 200 mm diameter
aluminium alloy tube with a 10 mm wall thickness and a current rating in excess of 4500 A
and 200 kA/3 s. This tube was approximately 50% more expensive than the normally
specified 140 mm diameter bus, however as less busbar is required, a saving of
approximately 30% resulted for this item.

3.2.2 Transverse Bus

Single “Cicada”, 37/4.65, all aluminium conductor was selected for the transverse bus due to
its fault rating of 36.2 kA/3 s, its normal and emergency current ratings of 1050 A & 1550 A
respectively.

3.3 Conductor Deflection

3.3.1 Main Bus

The static deflection of the standard 140 mm diameter bus bar is approximately 65 mm over
a 13 m span. This compares to a deflection of 31 mm for the 200 mm diameter busbar before
the suspension insulators are added. Each of the 400 kV composite suspension insulators
complete with grading rings weighs approximately 29 kg. The addition of these to the busbar
results in deflection increasing to around 60 mm over a 13 m span, which is comparable to
the already accepted in service design.

3.3.2 Transverse Bus

The proposed use of the transverse bus made from stranded conductor suspended from the
main busbar by composite insulators was checked to ensure that during either a single
phase to earth or three phase fault that the electrical phase to phase and phase to earth
clearances would not be compromised. Using the formula and methodology given in
IEC 60865-1, the ultimate limit state condition during short circuits Fsc (ult) is calculated as
shown in Equation 1 below,

Equation 1 Applied Short Circuit Force

Fsc (ult) = 0.4 K (Icc)2 / d

Where

Fsc (ult) = Short Circuit Load in (N/m)

K =4
Icc 2 = Initial Symmetrical rms Value of Three Phase Short Circuit Current (kA)
D = Conductor Spacing (m)

It should be noted that this short circuit force is twice the value given by the CIGRE Formula,
which specifies a maximum service load.

The calculated results demonstrated that for a conductor length of 3.9 m and a fault level of
25 kA, the maximum short circuit current force experienced by the conductor equated to
277 N/m. As the conductor’s displacement during a fault would result in a tangential force

6
being applied to the insulator, the maximum line pull applied by the conductor to the attached
equipment is less than the specified maximum of 1 kN. During a fault the side ways
deflection of a single phase stranded conductor is less than 0.66 m. With a transverse bus
spacing of 3.6 m this deflection does not compromise either the phase to phase (3.6 m – 2 x
0.66 m = 2.28 m) or phase to earth clearances.

3.4 Further Design Checks

Further design checks were undertaken to confirm serviceability and ultimate limit state
dead, earthquake, short circuit, wind & ice loads, vibration and thermal expansion. All of
these were either satisfactory or could be accommodated by ensuring the bus support
structures were improved.

4 BUS ADVANTAGES & DISADVANTAGES

In addition to the cost effectiveness of this approach the proposed busbar design has a
number of other advantages and few disadvantages as noted below,

4.1 Disadvantages
• Failure of phase to phase insulation results in the loss of a busbar or bus section
• Composite insulators may require earlier replacement than traditional ceramics
• Larger and heavier main bus support posts are not as easy to handle on site

4.2 Advantages
• Use of standard substation & transmission line components
• No reduction in required clearances and spacings
• Better access for maintenance due to fewer obstructions
• Lower visual impact due to fewer structures
• Reduced construction time and cost
• Smaller land area, and therefore lower associated land and civil costs

A comparison between the existing and proposed configurations is shown in Figure 4.

Figure 4 Proposed Versus Existing Bus Configuration

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4.3 Costings
Although the proposed design requires a larger diameter main bus bar and heavier main bus
support posts, it is a more cost effective arrangement than conventional designs as,

• Fewer posts, foundations, and insulators are required

• Fewer bus support and other ancillary fittings are required
• Reduced civil, mechanical and electrical site labour is required

Figure 5 below indicates where the savings have predominately been made.

BUS CONFIGURATION COST COMPARRISION

100%
% Versus Two SwGr Bay Rigid Main Rigid Transverse

Foundations
90%
Insualtors
80%
Support Posts
70% Buswork & Conductors
60%

50%

40%

30%

20%

10%

0%
Rigid Rigid Rigid Rigid Rigid Rigid
Main/Rigid Main/Strung Main/Strung Main/Rigid Main/Strung Main/Strung
Transverse Transverse Transverse Transverse Transverse Transverse
Post Suspension Post Suspension
No Sw Gr Bays Per Bus Tw o Sw Gr Bays Per Bus

Figure 5 Bus Cost Comparison

It should be noted that in preparing these costings, only obvious and direct savings were
accounted for, such as number of insulators, reduced busbar length, bus fittings, and fewer
support posts, foundations and earth connections. No allowance has been made for
additional cost savings resulting from smaller space requirements such as those associated
with earth works, earthgrid, control cables, land purchase and fencing.

The reduction in land area is assumed to be only the area directly under a single bus and a
single switchgear bay as far as the line/transformer CT position. This approach ignores the
benefits of easier vehicle access to the main bus that the increased height of the busbar
enables. Based on the dimensions given in Figure 4 above (Proposed 13 x 17 m =221 m2
Vs Existing12.2 x 23.7 m = 290 m2) it is estimated that a substation’s area may be reduced
by between 15% and 25% depending upon issues such as vehicle access, and number of
switchgear bays connected to a given bus section.

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Overall, conservative estimates of the proposed design suggest that construction costs per
two switchgear bay bus section could be reduced by approximately 55%, when compared to
a more traditional arrangement. This total is likely to be closer to 80% per bus section once
savings related to reduced space, land purchase, civil works, cabling etc are taken into
account.

5 SUMMARY & CONCLUSION

This project’s objective was to identify new busbar configurations that would meet key design
criteria without significant compromises being made. The design selected utilised phase to
phase insulation to reduce the number of structures and components within the switchyard.
While research was undertaken to identify where phase to phase insulation was used on
transmission systems and how it had been used within substations, to date only the use of
phase to phase insulation on transmission circuits as phase spacers has been identified.

This proposed design offers a number of potential benefits in addition to cost savings of
between 55-80% per bus section, without appearing to have any significant disadvantages.

BIBLIOGRAPHY

[1] “The thermal behaviour of overhead conductors Section 1 and 2 Mathematical model for
evaluation of conductor temperature in the steady state and the application thereof” (Working
Group SC 22-12 CIGRE, Electra number 144 October 1992 pages 107-125)
[2] Transmission Line Reference Book, 115-138 kV Compact Line Design, EPRI, 1978, pages 12-
24 & 147-143.
[3] AS/NZS:1170 Parts 1 -5 Structural Design Actions (NZ Loadings Code)
[4] IEC60865-1 Short-circuit currents - Calculation of effects - Part 1: Definitions and calculation
methods