MCY Mercury NZ ( MRP Mighty River Power)

[Snoopy]

Snoopy, I don't know for sure, but I suspect after recent upgrade work that the HVDC has the capability to take any available excess.

In the Meridian prospectus, on page 55 it states:

"If the Tiwai Point aluminium smelter were to close, the current transmission system operated by Transpower has the capacity to allow Meridian to supply the majority of Manapōuri’s generation capacity to the national grid and the wholesale electricity market. Transpower has advised that the transmission network between Manapōuri and the rest of the South Island could be upgraded and it expects such an upgrade to take three years to implement from commencement of the project. In Meridian’s opinion, under normal market conditions this upgrade would allow Meridian to supply a significant majority of Manapōuri’s generation capacity to the national grid".

Thanks for that Meridian prospectus quote Cyclist (my bold). I think Tiwai have to give some years notice of a shut down. Three years is probably enough time with notice for Transpower to upgrade the lines north if Tiwai decides to go.

i.e. some constraint in the 220 kV AC network only, but nothing major.

The other thing that is on my mind is that we are transferring kilowatt hours (a unit of energy) not kilovolts (a unit of potential). The higher the volts the lower the amps you need to transport a given amount of power (kilowatts). IIRC that is the reason for a high voltage DC line in the first place. Lower the amperage to transmit a given amount of power and you minimise the energy lost in transportation of the power (the waste heat).

I mention this because that means a 350kV line can be 100% utilised to capacity with only a 220kV input. No doubt some of the power gurus on this forum will put me right if I have got that wrong!

SNOOPY

[mouse]

Thanks for that Meridian prospectus quote Cyclist (my bold). I think Tiwai have to give some years notice of a shut down. Three years is probably enough time with notice for Transpower to upgrade the lines north if Tiwai decides to go.

I mention this because that means a 350kV line can be 100% utilised to capacity with only a 220kV input. No doubt some of the power gurus on this forum will put me right if I have got that wrong!

SNOOPY

I think Snoopy, that a 220kV line can only be used at that voltage, or lower. Since a 350kV line requires much higher insulation design. However, the idea that it would take three years to build another power line is, frankly, laughable. It would take at least eleven years to get past The Greens, The Consents, The Landowners and the other power companies. It would only take six months to actually build the line, but this is NZ. Where things can be 'arranged' if desired.

[Harveyp]

SNOOPY, I would just like to point out, that the reason for high voltage is so the cross sectional area of the transmission line can be smaller, as you've already worked out, higher voltage lower the amperage. The reason you want a smaller cross sectional area of your cabling is.......... To make it cheaper to install. Smaller cables are cheaper.

Otherwise we'd have a bloody greatly expensive transmission system, with cables larger than the harbour bridge powering our little country. Just though i'd point it out, as i've seen it mentioned a few times.

Disc. Registered Electrician

[Harveyp]

Also that would mean, that you could not use 220kv on a 350kv line, as the lower voltage would have increased amperage, resulting in potential insulation breakdown. This is why we have small and large switching stations located around the country to switch from higher to lower and lower to higher. Hope that helps.

[fish]

Also that would mean, that you could not use 220kv on a 350kv line, as the lower voltage would have increased amperage, resulting in potential insulation breakdown. This is why we have small and large switching stations located around the country to switch from higher to lower and lower to higher. Hope that helps.

Big help
thx

[horus1]

The 350KV line is Direct current and the 220KV line is alternating current . You cant interchange them

[Snoopy]

I think Snoopy, that a 220kV line can only be used at that voltage, or lower. Since a 350kV line requires much higher insulation design.

Yes I think your view that a 350kV line requires much higher insulation than a 220KV line is correct mouse. However, I am not sure your assertion that a 220kV line can only be used at that voltage is correct. Why can't it be used at a lower voltage? Of course a lower voltage implies a higher amperage to transmit the same power. That in turn means more losses during transmission. So in practice, there would be no incentive to operate a 220kV line at a lower voltage. Unless that is, a line upstream or downstream in the power path was being repaired, meaning a lower voltage rating was applied to a whole longer power path for a limited period.

But this is getting away from my original point that a 220kV power path could completely fill a 350kV power path if the power is transmitted in the 350kV power path is at a lower amperage. Of course the converse probably does not apply.

A fully utilised 350kV power path could only go wholly into a 220kV power path if the 220kV power path took a much higher current. And that higher current would likely blow the 220kV insulation bringing power transmission to a halt.

However, the idea that it would take three years to build another power line is, frankly, laughable. It would take at least eleven years to get past The Greens, The Consents, The Landowners and the other power companies. It would only take six months to actually build the line, but this is NZ. Where things can be 'arranged' if desired.

I think that view is too cynical. Most of these power paths already have resource consent. Just upgrading existing equipment is unlikely to stretch into an eleven year project IMO.

SNOOPY

[Snoopy]

SNOOPY, I would just like to point out, that the reason for high voltage is so the cross sectional area of the transmission line can be smaller, as you've already worked out, higher voltage lower the amperage. The reason you want a smaller cross sectional area of your cabling is.......... To make it cheaper to install. Smaller cables are cheaper.

Otherwise we'd have a bloody greatly expensive transmission system, with cables larger than the harbour bridge powering our little country. Just though i'd point it out, as i've seen it mentioned a few times.

Disc. Registered Electrician

Thanks for the trade knowledge Harveyp. I agree with every point you have made in your post. But I would also add that a lower current also wastes less power as that power is transmitted. An extra benefit over and above those you mentioned.

SNOOPY

[Snoopy]

Also that would mean, that you could not use 220kv on a 350kv line, as the lower voltage would have increased amperage, resulting in potential insulation breakdown. This is why we have small and large switching stations located around the country to switch from higher to lower and lower to higher. Hope that helps.

Yes thanks for clarifying you can't simply plug one line into another without 'transforming' (sic) the current to match.

SNOOPY

[Snoopy]

The 350KV line is Direct current and the 220KV line is alternating current . You cant interchange them

No 350kV DC and 220kV AC are not interchangable, I agree. But you can use a transformer to adjust the voltage and an AC/DC convertor at each end of the DC line to adjust the form of electricity. That would allow a 220kV AC line from Benmore to feed into a 350kV DC cable, and allow 220kV AC to be pulled out of the other end at Haywards would it not?

SNOOPY