New Substation Design

Utility Develops Under Hung Busbar Concept As Step Toward New Substation Design

U tility D evelops U nDer H Ung B UsBar C onCept a s s tep t owarD n ew ...

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At first glance, Drury Substation on the outskirts of Auckland appears a typical switching station, in this case bussing circuits from two 220 kV lines to supply 150 MW of power for New Zealand’s largest city. However, closer inspection reveals that this substation is anything but ordinary. In fact, Drury is not only New Zealand’s newest transmission substation but also one where a unique busbar scheme has just been put into operation. Archana Devi was Project Manager for Drury and reports that the substation, built at a cost of about NZ$ 15.6 million, was commissioned at the end of April 2010 – 9 months after construction began. Says Devi, “this substation is state-of-the-art in terms of layout, spacing and safety distances. It’s also unusually open

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and relies on fewer components than similar substations.” Andrew Renton, Transpower’s Asset Development Engineering Manager and Cameron Wallace of Electropar, now a subsidiary of USbased Preformed Line Products, were both active in the project and closely involved in the development and testing of the station’s busbar concept. “If you look at Drury,” observes Wallace, “there are a lot of ‘firsts’ – not only for New Zealand but for the rest of the world as well. These innovations have come from a conscious effort by management to look for improvements based on operating experience. This whole process then culminated in this revolutionary design, which will now become the model on which future substations will be built.”

Utility D evelops U nDer H Ung B UsBar C onCept a s s tep t owarD n ew ...

Wallace and Renton explain that successful implementation of the unique under hung busbar system relied primarily on two factors: the first was an innovative application for composite post insulators while the second was development of a new disconnect clamp. Both were seen as key requirements in eliminating the usual bus side disconnector one would usually expect to find in a switchyard. In regard to insulators, Wallace stresses that the whole concept relied on having a light insulator attached to the bus.

“The key advantages of polymers in this case was reduced weight and also the self-damping effect built into them. Such a design would never have been possible using porcelain since the busbar would then have to be dimensioned much larger in order to withstand normal short circuit forces. If all that additional weight – in this case 120 kg for porcelain per insulator compared to 55 kg for composite – had to be taken into account, everything would start to go against us and the whole concept would quickly become impractical.”

Wallace also points out that engineers considered various alternative configurations for the insulators, including a V string, but were worried about possible swaying. Therefore polymeric posts were selected to support the flat profile conductors. The critical requirement in regard to their performance was that they be sufficiently rigid to assure no secondary faults under a short circuit event. A second requirement, according to Renton, was for a single-piece molded design with as few interfaces as possible and with a fitting designed so that water would run off and not pool at either end. Yet another consideration was keeping the suspended insulators as short as possible while still ensuring good service performance given the station’s high pollution environment. “As you go toward longer insulators,” says Renton, “the whole construction has to be higher and this is detrimental both for visual impact and seismic considerations. These insulators are actually quite special,” he adds. “With a pitch circle diameter of 5 inches (12.7 cm), they offer

View of under hung bus concept during construction.

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Wallace (left) and Renton alongside Drury’s under hung bus.

relatively high stiffness while at the same time sufficient specific leakage distance (31 mm/kV) to meet our minimum requirement for this area. In the end, achieving this came down to finding the optimal shed profile.” In fact, Transpower engineers were so pleased with the composite insulators ultimately selected that they wanted to use the same technology for the posts that support the tubular bus. However, Renton reports that no insulator supplier could be found who would guarantee the required mechanical withstand and therefore there was no choice but to use porcelain. Renton observes that application of such phase-to-phase insulators is quite rare at substations where the more typical requirement is for phase-to-earth insulation. But he adds that Transpower has already had experience with this concept at another switching substation in nearby Huapai. There, land restrictions prevented erection of a typical entrance gantry and led to phase-to-phase insulation being employed to allow conductors to drop vertically into the station. Explains Renton, “for us, the fact that these insulators are being used on a bus versus on an overhead line means that if one fails, it will take down the station through a bus fault.” Concern over such a serious outcome resulted in a substantial testing effort to validate the concept, conducted first at Electropar on models and then on a full-scale set-up at a high power laboratory in Canada. “The final

Specially designed composite post insulator met all the needs of this application, including stiffness and good pollution performance.

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Porcelain insulators used for bus supports at Drury to assure necessary mechanical withstand.

configuration passed testing for 3 seconds at a fault level of 40 kA,” says Wallace, “and then again at 63 kA for one second.” Apart from the custom-designed composite post insulators used at Drury, the second key requirement behind the under hung busbar concept was a means to easily disconnect any circuit for maintenance. The clamp developed at Electropar for this purpose was designed to be operated with a hot stick and to lock onto any flexible conductor, whether single, double or quad. According to Electropar’s Engineering Manager, Greg Barclay, the disconnect clamp quickly accesses bolts from below the bus, utilizing a socket fitting and is so cleverly designed and functional that it has since been patented. “Transpower gave us the basic concept,” he says, “and our task was to turn this into a tangible product to replace the current solution of someone standing on an elevated platform.” Barclay goes on to state that the key to making this solution practical was plenty of available working space for maintenance personnel. Indeed, Wallace and Renton point to the fact that one of the principal benefits of the new bus concept is that it results in an open, uncluttered switchyard. Says Wallace, “in terms of ease of access, it’s perfect since maintenance staff can simply position their vehicles and work directly below the bus or, if necessary, extend a boom

Huapi Substation near Auckland employs unique phase-to-phase insulation arrangement that allows full-scale tower to be sited at center of switchyard.

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Renton demonstrates how specially developed clamp allows maintenance personnel to disconnect any conductor using hot stick.

to work on the breaker.” He also observes that the uncluttered Drury layout contrasts with the situation at most large substations, where conventional bussing usually means a dense arrangement of station posts, foundations and concrete pads. Apart from the convenience that derives from Drury’s open layout, an even more important benefit is prolonging the normal interval between maintenance. “If you look at this HV isolator,” says Wallace pointing to the disconnector on the breaker side, “such equipment must typically be maintained once every 5 years to align and clean the contacts. This means that need for a bus outage is determined solely by the maintenance cycle for this apparatus and this results in less grid availability. By omitting the isolator entirely and relying instead on a disconnecting circuit breaker with a normal 14-year maintenance interval you effectively shift the maintenance cycle from 5 to 14 years. In effect, it’s now the busbar itself that drives the maintenance cycle and no longer the isolator.” Wallace goes on to explain that whenever maintenance is required, all that needs to be done at Drury is to drop the under hung conductors to the ground without need for a bus side disconnector. “This is a key issue for

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U tility D evelops U nDer H Ung B UsBar C onCept a s s tep t owarD n ew ...

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“Drury is the start of an entirely new look for substations driven by a new generation of engineers anxious to take advantage of what the insulator industry offers in terms of better technologies.”

power supply companies worldwide,” he emphasizes, “namely how to optimize maintenance intervals in order to improve availability.” Wallace also sees this as a step to smart design of power grids. “The smart part here is that you no longer need to put out switching, isolating and temporary earthing to make a defined work area. All that’s needed is a hot stick and the new clamp.” As novel as Drury is in terms of design, Renton sees it as only a step forward in an ongoing process that will eventually see less cluttered substations with correspondingly less HV equipment. “The first thing to disappear,” he predicts, “will be the disconnectors. But over time the CTs will go as well, replaced by optical CTs without pedestals. All that will be left will be the disconnecting breakers, both live and dead tank. And everything will feature composite insulation. I see Drury not solely as an innovation but as a start toward this new generation of substation design.” Finally comes the important consideration of economics. Clearly, the engineering and other innovations at Drury carried with them incremental engineering costs. Here, Renton and Wallace point to

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the direct savings, which they claim outweigh the amount invested in this design concept. Says Renton, “while the under hung busbar system cost us about NZ$ 600,000 – of which about $230,00 was spent on testing – it immediately saved us six sets of disconnectors at Drury as well as all the normal bus support posts and foundations of a conventional design. These capital savings alone helped pay for the development program.”

Says Devi, “instead of many pieces of code, we can use only 5 or 6 equations regardless of substation configuration. The interlock scheme is independent of substation bus arrangement whether single, double or any other arrangement of the switchgear. For us, as the operator, it offers the same look and feel of conventional systems but now with the benefit of smart electronics. The logical next step will be to go to optical CTs.”

In fact, he notes that the economics of the solution used at Drury was never the ‘hard sell’ to Transpower management. Rather, it was the technical arguments that needed to be made. But even here he says that the risk was seen as low since there was always the option of a retrofit. “And any concern about this never having been done before,” he points out, “was eliminated by our testing program.”

Devi also points out that the cabinets at Drury are solid state with comparatively little wiring. Moreover, they have been tested for seismic performance and found to be robust enough to stand up to most earthquakes through their special system of bolts and washers.

Reviewing all that has been accomplished here, Electropar’s Wallace observes that this is the start of an entirely new look for substations driven by a new generation of Apart from the changes in terms of switchyard layout, the control room at engineers. “These people are not afraid Drury also features some of the latest to innovate,” he says,” and therefore substation technologies. According to they are anxious to take advantage of what the insulator industry offers in Project Manager Devi, the interlock terms of better technologies. It’s really scheme of the disconnectors is of a ‘smart’ type that uses relays and state a case of the new materials finally catching up with the power industry’s equations in place of mechanical own application goals.”  interlocks.


New Substation Design

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