new substation design

8/12/2019 New Substation Design

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Utility Develops Under HungBusbar Concept As Step Toward

New Substation Design

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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 Zealands largest city.

However, closer inspection reveals

that this substation is anything but

ordinary. In fact, Drury is not only

New Zealands newest transmissionsubstation 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. Its also unusually open

and relies on fewer components than

similar substations.

Andrew Renton, Transpowers

Asset Development Engineering

Manager and Cameron Wallace of

Electropar, now a subsidiary of US-

based Preformed Line Products,

were both active in the project and

closely involved in the developmentand testing of the stations 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.


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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 newdisconnect 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 towithstand 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 thestations 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

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View of under hung bus concept during

construction.


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relatively high stiffness while at the

same time sufficient specific leakagedistance (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 conceptat 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 takedown 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

Wallace (left) and Renton alongside

Drurys under hung bus.

Specially designed composite post

insulator met all the needs of thisapplication, including stiffness and good

pollution performance.




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 Electropars 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 resultsin an open, uncluttered switchyard.

Says Wallace, in terms of ease of

access, its perfect since maintenance

staff can simply position their

vehicles and work directly below the

bus or, if necessary, extend a boom

Porcelain insulators used for bus supports

at Drury to assure necessary mechanical

withstand.

Huapi Substation near Auckland employs

unique phase-to-phase insulationarrangement that allows full-scale tower

to be sited at center of switchyard.


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to work on the breaker. He also

observes that the uncluttered Drurylayout 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 Drurys open layout,

an even more important benefit

is prolonging the normal interval

between maintenance. If you look

at this HV isolator, says Wallacepointing 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,

its 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 sidedisconnector. This is a key issue for


Renton demonstrates how specially

developed clamp allows maintenance

personnel to disconnect any conductor

using hot stick.


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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 thats needed is a hot stick and thenew 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 withthem incremental engineering costs.

Here, Renton and Wallace point to

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.

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.

Apart from the changes in terms of

switchyard layout, the control room at

Drury also features some of the latest

substation technologies. According to

Project Manager Devi, the interlock

scheme of the disconnectors is of a

smart type that uses relays and stateequations in place of mechanical

interlocks.

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 smartelectronics. The logical next step will

be to go to optical CTs.

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, Electropars

Wallace observes that this is the start

of an entirely new look for substations

driven by a new generation of

engineers. These people are not afraid

to innovate, he says, and therefore

they are anxious to take advantage of

what the insulator industry offers in

terms of better technologies. Its really

a case of the new materials finallycatching up with the power industrys

own application goals.

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.

new substation design

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