Electricity questions

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qwerty7

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#97487 15-Feb-2012 15:43

I was wondering if anybody wanted to answer some questions that come from my natural curiosity about electricity. I feel I have a reasonable understanding but still have some questions.

1)How many phases is standard house power in new zealand?

2)I understand that a ground/earth is in case a hot wire touches a metal cased appliance. But what is the other side of this circuit the electricity goes into the ground through the ground spike or water pipe but then where ?

3)In a power point you have a hot wire a neutral wire and a ground/earth. What is the return path for the neutral wire? Does it travel back through the power lines down the street to the substation where it ground into the earth?

cyril7

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#581911 15-Feb-2012 15:53

Hi, one phase is normal, but 2phases in particular with older homes is not uncommon, three phase for domestic is quite rare.

The neutral and earth are connected together in the switchboard (and street transformer), this means the phase and neutral are referenced to the earth potential (whatever that may be). If the phase touches the case which is connected to earth then the current flows back to the street transformer via the ground current (yes through the soil).

And yes the current normallly flows between phase and neutral, which are referenced to the ground potential, in theory no current flows through the earth. If a fault condition occurs then that fault current will now flow down the earth rather than the netural, this imbalance in phase/neutral current is what will trip an RCD if in circuit.

hope that helps.
Cyril

stevenz

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#581924 15-Feb-2012 16:04

Similar to this, what would be an expected maximum deviation from norm for supplied voltage\frequency?

@KnightNZ

cyril7

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#581928 15-Feb-2012 16:08

Suppliers are required to maintain the voltage to within +-5% and as for frequency, I could not tell you off the top of my head but the infinite bus that it all is tied to and the level of load and generation resource will dictate how far it steps from the nominal 50Hz. A slowing of frequency (therefore a slowing of all rotating machines on the infinite bus) due to a lack of rotating source machines (generators) compared to the load is what caused the network to faulter a few months back.

Cyril

lchiu7

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#581973 15-Feb-2012 17:44

cyril7: Suppliers are required to maintain the voltage to within +-5% and as for frequency, I could not tell you off the top of my head but the infinite bus that it all is tied to and the level of load and generation resource will dictate how far it steps from the nominal 50Hz. A slowing of frequency (therefore a slowing of all rotating machines on the infinite bus) due to a lack of rotating source machines (generators) compared to the load is what caused the network to faulter a few months back.

Cyril

Transpower contracts generators to do frequency keeping. Generators makes offers for this work as for any generation they offer to the market.

I can't remember exactly what the tolerances are but they are something like 49.8 to 50.2 Hz (something like that). If for whatever reason the frequency drops below certain tolerances frequency keeping stations kick in and if more then load shedding can take place,

As an aside in the SI because of the lack of reactive load (a lot of the load in the SI is from the Aluminium smelter which does not place a reactive load on the network), frequency can go up and there is a need to lower it to the accepted level.

A few months ago was a once in a 40 year event. There was a bus failure at Huntley which I understand caused a cascading trip at the station taking the entire station off line, simplistically speaking. That was about 900MW of generation gone. Of course that meant a drastic drop in frequency. The stations that are contracted to do frequency keeping kicked in to help recover the situation but there was too much generation gone off line for that to work. So AUFLS kicked in (which is Automatic Under Frequency Load Shedding, an appropriate acronym to be sure).

This causes stations/distributors to automatically shed load to reduce the load on the network. This means areas in the NI lost power as the load shedding occurred.

Note that this entire event took place in about 6-7s and was handled automatically. If there had to be manual intervention there would not have been enough time to shed the requisite load and the entire network might have collapsed. That would not have been a nice situation!

Staying in Wellington. Check out my AirBnB in the Wellington CBD. https://www.airbnb.co.nz/h/wellycbd PM me and mention GZ to get a 15% discount and no AirBnB charges.

qwerty7

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#582004 15-Feb-2012 19:14

thanks, think i am finally getting my head around this
so i am guessing the power coming into my home is 240 volt single phase.

the lines running down the street have 3 main wires running along the top (3 phase i guess) then there are 6 wires lower down (3 on each side of the post) which the houses seem to latch onto (there are wires attached to them running down the post. So how do i end up with 240 volt single phase, the 3 phase is stepped down from.. ?

cyril7

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#582007 15-Feb-2012 19:25

Hi, in Oz and many other countries its 240V, but in NZ we have 230V.

The poles you see in the street will be carrying the 11kV feed on the 3 top wires, this will be the input to your local street transformer there are only 3wires in delta config, ie just 3 wires with no neutral. The lower 6 wires will be 3phases at LV (400/230V) in Y format, thats three phases plus neutral and the other two are most likely one for street lighting and one for load shed pilot but the exact config may vary from lines co to lines co and area to area.

At the street transformer the delta 11kV is transformed to 400V 3phase (230V/phase) in Y format so 3phases plus neutral and that neutral will be bonded to ground at the street transformer as well as each and every house connection.

Hope that helps

Cyril

Nikoftime

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#582312 16-Feb-2012 12:42

The New Zealand electricty regulations 1997 are available at
http://energysafety.govt.nz/upload/33242/Electricity_Regulations_1997.pdf
for some further information

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Handle9

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#582320 16-Feb-2012 12:54

qwerty7: thanks, think i am finally getting my head around this
so i am guessing the power coming into my home is 240 volt single phase.

the lines running down the street have 3 main wires running along the top (3 phase i guess) then there are 6 wires lower down (3 on each side of the post) which the houses seem to latch onto (there are wires attached to them running down the post. So how do i end up with 240 volt single phase, the 3 phase is stepped down from.. ?

3 Phase is 400V potential phase to phase or 230V phase to neutral. It's not stepped down it's just a different potential from phase to nuetral than phase to phase.

http://en.wikipedia.org/wiki/Delta-wye_transformer

k14

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#582334 16-Feb-2012 13:13

lchiu7: Transpower contracts generators to do frequency keeping. Generators makes offers for this work as for any generation they offer to the market.

I can't remember exactly what the tolerances are but they are something like 49.8 to 50.2 Hz (something like that). If for whatever reason the frequency drops below certain tolerances frequency keeping stations kick in and if more then load shedding can take place,

That is correct, the frequency keeper tries to keep their respective islands frequency (NI and SI frequency are independent because of the DC link between islands) between 49.8 and 50.2Hz. When it falls below this threshold instantaneous reserve kicks in which is referred to as FIR and SIR. Fast Instantaneous Reserve is available within 6 seconds and able to be sustained for 60 seconds. Sustained Instantaneous Reserve is available within 60 seconds and able to be sustained for 15 minutes. For over frequency events generators can be armed to trip off if the frequency reaches a certain level, although that is a much lower risk compared to a low frequency.

FYI I am currently the SI frequency keeper. In general it stays between 49.9 and 50.1 for 98% of the time but when it does drop you have no time to react hence the full automation of the system and the AUFULS for backup. I don't want to have to go through a black start (starting the whole island up from scratch!), would take too much time away from sitting on Geekzone

cyril7

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#582338 16-Feb-2012 13:18

I don't want to have to go through a black start (starting the whole island up from scratch!), would take too much time away from sitting on Geekzone

And I guess months of endless paper work to follow :)

Cyril

lchiu7

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#582356 16-Feb-2012 13:46

From what I have been told when MOW built the power stations they had black start capability. But when the kit was out of maintenance it was deemed not necessary to keep going give the unlikelihood of the event. But the Huntley incident got close!

Now there are two stations (one NI and one SI) that are responsible for black start so that the outage would be minimal (not that it would not a major impact). The spectre of folks driving madly to power stations carrying portable generators to power gates etc. is mind boggling!

Staying in Wellington. Check out my AirBnB in the Wellington CBD. https://www.airbnb.co.nz/h/wellycbd PM me and mention GZ to get a 15% discount and no AirBnB charges.

k14

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#582366 16-Feb-2012 13:54

lchiu7: From what I have been told when MOW built the power stations they had black start capability. But when the kit was out of maintenance it was deemed not necessary to keep going give the unlikelihood of the event. But the Huntley incident got close!

Now there are two stations (one NI and one SI) that are responsible for black start so that the outage would be minimal (not that it would not a major impact). The spectre of folks driving madly to power stations carrying portable generators to power gates etc. is mind boggling!

Yeah the Clyde dam is capable of black starting the SI grid. Whether it actually is successful is another question, you can test as much as you want but you'll never get to do a test on a fully black grid! Not till the time it has to work is upon us.

The whole station will function fine however with power and all generators shut down. Diesel backup generator to power the essential AC systems and UPS's/DC battery backup for control system PC's and generator sensors etc mean that everything is always read to go. It gets quite complex though having Transpower switching in the right transmission lines/busses and getting the right amount of load on a generator (or two) and having a semi stable localised system before the main objective (Tiwai) is bought back on.