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I'm linesman and Electrical Fitter in powerline distribution in Australia AMA ?

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tobyrobb:
I've worked in the Australian Power distribution industry for the last 14 years in NSW as a linesman and other roles.

I thought I would gather a few thoughts here and post after a recent episode on the amp hour around the power grid.

A lot of people have no real exposure to the distribution system other than the outlet on the wall and a basic understanding of AC.

But it's actually not that hard or that different. In fact, a lot of the layout is fairly basic compared to your average electronic circuit.

However the distribution system has its own unique challenges such as fault detection and isolation, as well as high voltage conductors and reactive power.

The high voltages also mean that special principles apply to circuit breaker design and  insulation.

In NSW, generators usually producing 11 or 22KV step up to 330 or 500KV and pass this to the transmission operator (transgrid) who marshall the feeds at switching yards before transforming it down to 132KV and sending it out on long span steel towers to our  local companies, essential energy, endeavor energy and ausgrid.

Distribution companies receive the 132kv and generally pass that around their own network on steel towers, although more recently vertically constructed concrete poles are available for 132.

Transmission zone subs pretty much only deal with splitting up the 132 and do not supply any customers, they will generally kick the volts from 132 down to 66kv and send this out to various distribution zone subs.

The general layout for a zone sub is one or two incoming feeders at the highest voltage sit on what we call a primary bus bar. The primary bus bars job is pretty much mechanical with some mechanical switches available. Hanging off the bar will be a couple of transformers that have a 132KV primary and a 66 KV secondary. The output of those transformers will go onto the secondary bar which also contains mechanical switches.

Each connection of a feeder or a transformer to a bar will go via a breaker. There will also be a current sensor ( CT ) and a voltage transformer (VT) which generally transforms the voltage to a standard of 110 volts.

Relays monitor a proportion of the real current via the CT and a proportion of the real voltage via the VT. Parameters can be set in the relay which has control over the breaker.

When high voltage contacts are opened an arc is formed and this can continue to exist as it ionizes the air around it creating a nice path. Arcs are not actually a dead short and therefore will not induce any further tripping as over current.

 Unfortunately, an arc is hotter than the surface of the sun. In fact, the hottest thing in the universe and if it's there for too long, it's going to melt everything including your metal, which by the way is often aluminium as it is almost as good a conductor as copper but much lighter and therefore much more suitable for an aerial conductor.
 
 High voltage breakers rely on different techniques to extinguish the arc . High speed opening, air blasts, arc chutes which are layers of metal that become magnetized during an arc and will suck the arc into them, oil or gas like SF6 sulphur hexafluoride which is an insulating gas meaning the arc has trouble passing through it.

If you look closely, you will see levers that can open mechanical connections across the tubular bar  conductors. These are usually not operated under load. But you can see plenty of YouTube videos of people doing it the wrong way. That's when you see the big arcs. Generally, breakers are used to de energise the bar in a zone sub and then the mechanical switches are opened, locked off and danger tagged.

Sometimes the bar can be split into one or more sections by a breaker to allow isolating for faults or to switch between say Transformer 1 and Transformer 2 . One may be in use and the other is awaiting to step in If the first one has a fault it will automatically step in or one may be in use and the other is currently undergoing maintenance.

After leaving the secondary bar at 66 KV, via transmission breakers it often leaves on poles and wires to the distribution zone subs. These are the buildings and yards you see scattered around your neighbourhood. There is usually an incoming 66 and an outgoing 66 as the network is set up either daisy chained or in a ring configuration. When set up as a ring this means if an incoming feeder needs to go down, the network can be back fed to provide supply from the other direction.

Distributions zone subs take the 66 and split it down via 1,2 or several Transformers and put this onto the 11KV bar (sometimes 22) . Hanging off this bar are all the distribution circuit breakers. This is where the real business begins as the 3 wire 3 phase conductors leave generally on poles and wires or possibly underground out into the neighbourhood.

The three conductors travel on wooden poles with ceramic or poly insulators keeping the conductor from arcing to the pole.
In general, dry wood is a pretty good insulator and helps insulate the system from the ground.

The three phases enter a pole mounted transformer or a ground level padmount transformer primary windings and are connected in Delta. There are always fuses or a breaker on the high voltage side leading into the transformer. (Eg expulsive drop out fuses) (>11kv requires boric acid) these can also be used for isolation to allow working on the transformer.

On the secondary side, we have our four phases derived from a star configuration in the windings. The centre of the star becomes the neutral which in Australia is tied to the earth. The nominal voltage between phases in Australia is 400 volts and from phase to neutral or ground is 230 volts. (Not 240 dave!! ;))

There are fuses on the secondary side but they are fairly slow acting HRC. There is usually a wiring harness on the Transformer that connects to the conductors in both directions and provides fusing and blade switching options.

The lowest set of 4 wires travels from pole to pole and at each premises a single layer XLPE aerial bundle cable (service mains, private service, private mains) is tapped onto the conductors and sent to the point of attachment on the house.

From there we have the consumer mains which run from the point of attachment into the house and down into the switchboard, usually a galvanised box in Australia on the side of the house.

Inside the switchboard is a service fuse which is a HRC for each phase. This then carries through to the meter and onto the customer's main switch and hot water switch which go out to the customers light and power CBs and thence into your GPOs and lights etc.

Some things to note.

We measure our transformers in KVA. A typical pole mounted transformer may be  100 - 300 KVA. A padmount transformer would be 500 KVA. To work out power we multiply kva by power factor.

We measure our ratios beyween capacitive, inductive and resistive loads using power factor.
We would love our power factor to be 1.0 -  inductive loads reduce that and adding capacitors can raise the power factor back up. In Australia, if you are a private householder you do not need to consider power factor and you will not be charged for poor power factor EG. Lots of inductive loads. However, as a large commercial customer, you are expected to have power factor correction, capacitors etc. It's not as hard as you think. In fact, most appliances have power factor correction capacitors installed.

So what is power factor? Well, let's say you have a poor power factor. This means that you will be drawing your current and voltage out of phase because a perfect power factor of one means that the current and voltage are in phase and whatever voltage and current you are taking from us is all we need to supply. However, let's imagine a load that's highly inductive. We know that power is volts times amps so If we think about a voltage sine wave then maximum power will be if both the voltage and current sine waves rise together . Anytime that they are out of sync, the actual true power delivered will be lower.
So the grid is supplying a lot of extra current that isn't really making it into true power. But that extra current still causes losses due to the I squared r rule etc

Why does inductive current lag behind voltage ?
The best way to imagine inductive power is to imagine that as soon as you apply power, the voltage will be instantly measurable on the terminals of the coil. However, due to lenzs law, the current will be restricted because as the magnetic field builds up, it will induce an opposite current to restrict current flow. Therefore the voltage will lead the current..

Why does voltage lag behind current in a capacitor ? The best way to remember this is that as you apply voltage, the current is drawn straight away to begin to fill the capacitor but The voltage only starts to rise as the capacitor fills. Therefore, the current leads the voltage.

A lot of people ask or assume that the distribution system is antiquated. The reality is that simple devices made of steel and filled with oil are robust against lightning strikes, heavy loading, faults and are forgiving and easily maintained, cleaned and repaired.

Generators have governors which adjust the speed of the generator automatically to feed in more fuel as the load makes the 50hz slow down.

Solar has ohms law, eg you need to float your solar voltage higher than the network if you wish to drive current out on to the grid.

We don't have our data and control systems like scada on internet, and the control systems mostly don't make relay decisions eg overcurrent (which is hardwired directly in to the relay) , our control systems mostly report and maybe trigger a reclose or allow the control room to switch brakers and portions of the network.

We do have load shedding, this is so that if a supply feeder is overloading we can remove feeders to reduce the load and keep the network from crashing.  Eg a black start.

Most gennys are between 600 and 1000 Megwatts.

There are some strange gennys like wind turbines and gas powered converted caterpillar engines in shipping containers that can be switched in and out they are about 1mw each

There are also the odd Reactor and capacitor banks spread around, but they are usually to deal with odd feeders that mya supply am aluminium works for example they stirke HV arcs to melt crucibles! gives the network many spikes that have to be smoothed.

There are so many more features, if you have questions , ask away!!

coppercone2:
do you know anything about the superalloys used in switch gear etc contacts, for instance tungsten silver alloys?

This is a part of the magic of how those beastly contacts work. I think its a bit more then just arc management, its also materials choice. I imagine a piece of brass in the tip of the switch would be adios muchachos for the switch yard

Whales:
Wild question: ever envisage a future where local polemount and padmount transformers (11kV->240V) come with automatic on-load tap-changing?  That'd be an interesting way to address voltage rise caused by home solar; and I don't think it'd need as much copper as some other solutions; but I don't know how reliable these mechanisms are.

tobyrobb:
I think for breaker contacts inconel might be the material?

We try to do a lot of off-load switching and use arc suppression strategies for some of the high current interruptions. 132 is almost always SF6 gas filled. But street hv blades eg USL's often lighty coated copper.

Each substation breaker has the fault current recoded if possible and maintenance is carried out and documented, this includes contact cleaning or replacement!

I think the Arc suppresion through various means help a lot with arcing and contact pitting. And the relays try to open on the zero crossing point, eg no current..

tobyrobb:
Yep absolutely, we have autotapping on the HV through Zone transformers and the pole mounted three phase regulators, ie a good solution on long distance feeders where the voltage drops/fluctuates. But yeh we have may have to tap distribution transformers up in the Winter and down in the summer. Atm, requires an outage to do it. The autotappers are complicated and they have multiple coil windings in an Auto Transformer arrangement. Lots of contacts x3 and require to carry full load current.
 and if its field mounted, i guess its more costly again.. High power semiconductors are just not there yet..

The mechanism is reliable, but with hard to change components, it's best to have changeover spares in situ to get the power back up and running asap, something possible in some subs, eg two transformers running in parallel but one can handle both loads of auto switched... but in the field of it could be problematic to fix, replace..

having said that, I've replaced many Transformers on the pole just takes 4-8hrs..

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