New Concept in Busbar Design Employs Phase-to-Phase Insulation

New Concept in Busbar Design Employs Phase-to-Phase Insulation

UTILITY PRACTICE & EXPERIENCE One of the most commonly used phaseto-phase insulation applications on HV electrical systems today is as transmission line spacers to avoid phase clashing. There are also lesser-used applications within

New Concept in Busbar Design Employs Phase-to-Phase Insulation

switchyards, such as reactor spacers.

However, during an investigation of expansion options at an existing 220 kV substation in New Zealand, engineers at Transpower recognized that employing phase-to-phase insulation between the main and transverse bus sections might offer valuable benefits over traditional ground-mounted designs.

Due to project pressures the idea was not immediately pursued but development of the concept proceeded several years later and resulted in a design and verification process for what came to be known as the High Voltage Underhung Busbar System (HVUBS). Moreover, after being subjected to full scale short circuit testing, it is now installed at two 220 kV substations in that country with plans to incorporate it at other substations as well

This article, contributed by Andrew Renton, Transpower’s Asset Development Engineering Manager and Development Engineer, Dan Tombleson, discusses this concept and compares with previous designs from both the technical and economic points of view.

Photo: INMR ©

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Introduction In 2005, Transpower New Zealand, owner of the country’s transmission grid, investigated expansion options at its Ashburton 220 kV substation. The following design criteria were considered an integral part of the project: • Reducing visual impact; • Utilizing standard air insulated switchgear (AIS) as well as transmission line and substation hardware; • Complying with all seismic & electrical clearance requirements; • Reducing both construction & maintenance costs; • Lowering the risk of failures; • Reducing the components & hardware required for the expansion; • Improving access for subsequent maintenance.

Design Parameters The following tables outline the electrical, mechanical and environmental design parameters that were considered during this process: Conceptual Bus Design Configurations a. Existing Rigid-Rigid & RigidFlexible, Post Mounted Phase-Earth Insulation At present, the preferred 220 kV busbar design at Transpower utilizes a rigid tubular aluminium busbar supported on post insulators as well

Table 1: Electrical Ratings & Requirements Nominal Operating Voltage

220 kV

Maximum Operating Voltage

245 kV

Lightning Impulse Withstand Level

1050 kV

Maximum Operating Current

4500 A

Fault Current

25/31.5/40 kA

Fault Duration

3s

Table 2: Environmental Requirements 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

Pollution Level

Heavy/Very Heavy

Minimum Creepage

25 mm/kV

Seismic Standard

AS/NZS:1170

Table 3: Electrical Clearances, Spacing & Distances Minimum Distance Phase-Phase & Phase-Earth

2100 mm

Minimum Height Live Conductors to Ground Level

4540 mm

Minimum Busbar Height Above Ground Level

5540 mm

Minimum Rigid Busbar Centre Line Spacing

3600 mm

Removal Rigid Transverse Bus

Removal of Bus Support Posts and Foundations

Figure 1: Traditional rigid post-mounted, main-flexible post mounted transverse bus (left) and underhung phase-to-phase insulator configuration concepts.

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as individual steel posts mounted on concrete pad foundations. The rigid transverse bus is located beneath it, at right angles, and is of similar construction.

Jumpers (assembled from flexible stranded conductor equipped with compression fittings) are then used to make bolted connections between the bus sections and the switchgear.

A second design option, also utilized in the past, especially in cases where the bus section is connected to only one switchgear bay, has been to replace the rigid transverse bus with lower cost stranded aluminium conductors mounted on insulators and steel posts (as shown on the left hand side of Figure 1). b. Proposed New Rigid-Flexible, Underhung Phase-to-Phase Insulation To improve access and reduce costs associated with the planned expansion, it was proposed that the steel posts supporting the flexible transverse bus sections be removed and that the conductor be supported instead using phase-to-phase insulators ‘underhung’ from the main bus (see Figure 1, right hand side).

To gain more information and evaluate whether such a design concept had been tried elsewhere, research was undertaken into the application of phase-to-phase insulation on transmission systems and in particular within substations. This work could not find any cases of such a bus design concept. Still, having established that this arrangement met all the required criteria and had no foreseeable drawbacks, detailed checks where undertaken to confirm the design.

1. Conductor Selection Main Bus To reduce the need for specialized components and hardware as well as to minimize costs, a 200 mm diameter, 6 mm wall thickness standard tubular main bus was used, having a current and fault rating of 4500 A and 200 kA/3 s respectively.

This tube was approximately 50% more costly than that normally

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specified in the case of a 140 mm diameter bus.

Transverse Bus ‘Cicada’, 37/4.65, all aluminium conductor (AAC) was selected for the transverse bus due to its fault rating of 36.2 kA/3 s, and its normal and emergency current ratings of 1290 A & 1550 A. 2. Conductor Deflection Main Bus The static deflection of the 200 mm diameter bus is 31 mm before adding any insulators. Each of the 400 kV composite suspension insulators (complete with grading rings) weighs approximately 46 kg and the addition of these to the busbar would result in deflection increasing to around 60 mm over the 13 m span (approximately equal to normally accepted bus work).

Transverse Bus The strength of the proposed transverse bus made from stranded conductor and suspended from the main busbar by the composite insulators was verified and it was also ensured that during a short circuit, the minimum electrical phase-to-phase and phase-to-earth clearances would not be compromised.

Using IEC 865-1, the force under the ultimate limit state condition during short circuits (FSC (ULT)) was calculated:

As the conductor’s displacement during a fault would result in a tangential force applied to the insulators, the maximum line pull was also taken into account and determined to be less than Transpower’s specified maximum of 1 kN. Similarly, sideways deflection was found to be less than 0.66 m. Further design checks were undertaken to confirm that other parameters were also satisfactory, including: serviceability, ultimate limit state, static, seismic, short circuit, wind and ice loads, vibration and thermal expansion.

Advantages & Disadvantages Having verified the new design concept technically, a case had to be made to justify investment that would then be needed to turn the concept into reality.

Disadvantages • Perceived increase in risk by applying phase-to-phase insulation within substations and on busbars; • Composite insulators that might require replacement before 25 years (earlier than traditional porcelain); • Larger heavier main bus and support posts.

Reviewing these possible drawbacks, the greatest objection was the perceived increase in risk of a sustained bus fault due to failure of

Equation 1: Applied Short Circuit Force

It should be noted that this calculationis twice the value of the CIGRE formula (which specifies maximum serviceability limit state load). Initial calculations demonstrated that at a fault level of 25 kA, the maximum short circuit load would amount to 277 N/m.

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the phase-to-phase insulation. The most effective method to validate the design against such a risk was to perform a full-scale short circuit test.

Assuming the concept successfully passed such a test, it would then offer the following advantages:

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Advantages • Use of standard substation and transmission line components; • No reduction in published required clearances or spacings; • Better access for maintenance due to fewer obstructions; • Lower visual impact due to fewer structures; • Lower risk of failure and less maintenance due to fewer components; • Reduced construction time and cost; • Smaller site footprint.

1.Economic Assessment Although the proposed design would require a larger main busbar and heavier main bus support posts, it was still found to be a more cost effective arrangement than conventional designs. Figure 2 indicates where savings could be achieved.

Figure 2: Cost comparison against standard 220 kV buswork.

It should also be noted that, during this economic assessment, only direct cost savings were considered, (e.g. number of insulators, reduced busbar length, bus fittings, as well as fewer support posts, foundations and earth connections). No allowance was made for additional savings that would result from smaller space requirements associated with earth works, earth grid, construction, control cables, land purchase and fencing.

Figure 3: FEA analysis of busbar terminal.

For example, based on the dimensions, it was estimated that a substation’s area could be reduced by from 15 to 25% depending on vehicle access and the number of switchgear bays connected to a given bus section. Overall, conservative estimates of the proposed design suggested that construction costs per two switchgear bay bus section could be reduced by approximately NZ$ 60,000 and this could be closer to NZ$ 100,000 per bay if reduced space requirements such as land purchase, civil works, cabling, etc. were taken into account.

Figure 4: Bus temperature and strain sensor (top); underhung composite strain sensor.

Design & Validation With the economic benefits clearly demonstrated, a detailed design and testing process took place between Photo: INMR ©

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2008 and June 2009. In early 2008, following acceptance of the concept’s commercial and operational benefits, approval to proceed with the first stage of the design and testing programme was granted. Transpower worked closely in this regard with its engineering partner, Electropar, to prove that the proposed HVUBS was technically and physically possible.

The first milestone was comprehensive design and analysis of the fundamental components to ensure that their mechanical and electrical properties where adequate. A detailed design of the proposed HVUBS was prepared that utilized standardized accepted components (i.e. buswork, clamps, fittings, insulators, stands, foundations) and a static analysis of each component was undertaken to ensure that it met all load cases. The design parameters used each case were conservative and assumed a close-in fully asymmetric fault. In total 15 locations within the HVUBS were modelled under 27 load combinations.

This work identified several components that were close to their assumed capacity limits so more detailed finite element analysis (FEA) was undertaken.

adequate resolution, accuracy and ease of installation. Moreover, to ensure proper data collection and processing, a ¼ scale test rig was manufactured assembled and given a trial.

given demonstrating that the project investment would be recovered in savings on the first two substations alone. Based on this, approval was obtained in early 2009.

With the successful completion of the design and testing arrangement, came the final selection of a high power test laboratory with sufficient space to accommodate a full 220 kV bay. Globally, only a handful of high current test laboratories were deemed capable of providing the required asymmetric fault current level at the suitable system impedance and who could accommodate the 15m x 15m x 10m test rig within the necessary time frame. A lab in Canada was ultimately selected and provisional arrangements made for testing in June 2009, subject to the final business case and Stage 2 of the project being approved by Transpower management.

A 40 ft container was shipped from New Zealand to the test laboratory and contained the required test rig and all other equipment including bus support posts, insulators and buswork, clamps, conductors, fittings, measuring equipment, tools and spares. In addition, a local contractor was hired to construct Transpower standard foundations, erect the support post and bus work and to be available during the test for a variety of additional tasks.

At this point, the final design and testing costs were known, the concept proven from a theoretical standpoint and financial justification

Subsequent testing involved two objectives, the first being to ensure the structural integrity of the components and the second to verify that the electrical performance and clearances would not be compromised. This would be achieved by a combination of visual inspection, strain gauge measurement at critical points and displacement measurement using high-speed cameras.

The final test rig was comprised of one high level 220 kV rigid main busbar section complete with 400 kV underhung composite insulators. The rig was designed to accommodate either simplex or quad flexible transverse conductors, as well as a re-locatable shunt (shorting conductor) to enable the simulation of either bus or close-in line/bay faults.

Figure 5: ¼ scale test rig set-up.

Figure 6: Full scale test rig line/bay shunt (bottom).

It was identified early on that due to the high-speed nature of electrical short circuits and the levels of EMF generated during such events, traditional electronic strain gauges and temperature measurement devices would not work properly. As a result, specialized data capture equipment was brought in, including optical strain and temperature transducers and multi-channel signal conditioners.

The transducers were selected based on their immunity to EMI/EMF, Photo: INMR ©

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Two types of conductor (Butterfly AAC and Cicada AAC) were used and fault levels tested at 25, 31.5, 40, 45 and 63 kA. Each test comprised a number of symmetrical calibration shots followed by fully asymmetric 0.5 s, 1 s, 1.5 s and 3 s shots having an X/R ratio of between 27 and 49.

Over the five days of the test programme, data analysis confirmed that the proposed design was feasible and that none of the structural or electrical design limits were exceeded.

The test results also confirmed that the maximum loads on the system were dependant on the peak asymmetrical fault current and not significantly affected by its duration. Both the highspeed camera and strain transducers used to measure deflection provided similar results and were consistently less than the calculated values.

bus system instead of only individual components, a number of items traditionally used by Transpower were also tested. For example, one clamp design was found to have insufficient strength at fault levels > 31.5 kA. Conclusions During the period of investigation, design, development and testing, the following were noted:

As a side benefit of testing the entire

to the high EMF environment.

4. The HVUBS design delivers net cost saving as well as safety, maintenance and environmental benefits.

5. Concept may not be new, but the development of new insulating materials such as composite insulators is the enabler of such designs, where traditional equipment is to heavy.

1. HVUBS does not compromise either phase-phase or phase-earth Transpower is happy to share it's clearances and also meets all findings with other interested TNSP's structural integrity requirements as well as load cases.

Today, work such as this and the 2. The HVUBS system static inertia positive results that come out of it are not possible without the close appears to mitigate the effects involvement and complementary skills of the forces applied by a peak asymmetric fault by up to a of a variety of parties besides power factor of 2. utility engineering staff, including suppliers, contractors and laboratory 3. Careful selection of testing test facilities.  instrumentation is required due

Figure 7: Comparison measured strain values vs. time at one location over three test

Figure 8: Simplex Butterfly conductor, both before fault and at maximum deflection.

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