Theory behind cable losses for AC and DC

[Iain123]

Hi Forum

could someone please explain
Advised to keep DC cables runs as short aspossible on Solar PV due to losses,
And the longer run in AC from inverter output to the main distribution board.

AC mains 230volts
DC string in this example has normal opperating voltage of 350volts

Power=Volts*Amps
In a 1000watt example with no inverter losses
DC circuit 1000=350*2.86
AC circuit 1000=230*4.35
As the Voltage increases the current decreases for both AC and DC which is good for losses is it not?

If say a cable run has a resistance of 2ohms
Volt drop=Amps*Resistance
DC Voltdrop=2.86*2=5.72Volts
AC Voltdrop=4.35*2=8.7Volts

Power=Volts*Amps
Therefore DC powerloss=5.72*2.86=16.4Watts
Which is less than the ACpower loss=8.7*4.35=37.8Watts

So why in relation to cable losses is itnot be better to have a longer DC run than AC run if the DC operatingvoltage is greater than 230Volts?


Thanks for replies in advance
Iain

 

[Knobhead]

The higher the voltage the lower the losses (otherwise the CEGB have made a major **** up with all their pylons). Where has this “information” come from?

The only reasoning I can think of is a reluctance to run a higher voltage through a house than is “normal”. 350V is well within the capabilities of most cables, use 600/1000V grade and it’s well on top of it.

 

[Chr!s]

Hi Forum

could someone please explain
Advised to keep DC cables runs as short aspossible on Solar PV due to losses,
And the longer run in AC from inverter output to the main distribution board.

AC mains 230volts
DC string in this example has normal opperating voltage of 350volts

Power=Volts*Amps
In a 1000watt example with no inverter losses
DC circuit 1000=350*2.86
AC circuit 1000=230*4.35
As the Voltage increases the current decreases for both AC and DC which is good for losses is it not?

If say a cable run has a resistance of 2ohms
Volt drop=Amps*Resistance
DC Voltdrop=2.86*2=5.72Volts
AC Voltdrop=4.35*2=8.7Volts

Power=Volts*Amps
Therefore DC powerloss=5.72*2.86=16.4Watts
Which is less than the ACpower loss=8.7*4.35=37.8Watts

So why in relation to cable losses is itnot be better to have a longer DC run than AC run if the DC operatingvoltage is greater than 230Volts?


Thanks for replies in advance
Iain

You need to look at what level of current is being drawn in the two sections, you calculate using I2 x R x L/ 1000.

 

[Iain123]

You need to look at what level of current is being drawn in the two sections, you calculate using I2 x R x L/ 1000.

Hi Chris

So the power loss in Watts for the cable = I x I x R x L
I = Current in cable
R = Resistance of one meter of cable from table
L = Length of cable run in meters

So you could compare the losses for AC and DC

Thanks
Iain

 

[topquark]

Hi Chris

So the power loss in Watts for the cable = I x I x R x L
I = Current in cable
R = Resistance of one meter of cable from table
L = Length of cable run in meters

So you could compare the losses for AC and DC

Thanks
Iain

There are other factors to take into account as well, but for a simplistic comparison, yes you can compare a corresponding run of DC versus the same length of AC driven.

 

[Iain123]

There are other factors to take into account as well, but for a simplistic comparison, yes you can compare a corresponding run of DC versus the same length of AC driven.

Hi Topquark
What other factors are there if its not too dificult to explain.

By the way I had used the SMA sunny design to calculate the losses and this led me to not understanding the reason i had been told to keep the DC run as short as possible.

Thanks
Iain

 

[topquark]

There are a whole host of electromagnetic effects that come into play with power (and signal) transmission. Probably not relevant to the particular instance you are talking about here with respect to a solar installation.

One of particular interest is the way that current "flows" in a core, do some research on the "skin effect" of cables (AC). If you want to delve into some more of the theory then there is a book by William Hayt (Engineering Electromagnetics).

There's a copy here: http://www.ie.itcr.ac.cr/acotoc/Mae..._Comunicacion_II/Material/Biblio2/preface.pdf

 

[Iain123]

Hi Topquark
Yes i can remember the term skin effect from my college days over 20 years ago. Interesting to see it can have an effect even at low frequency like 50hz.

Thanks
Iain

 

[edexlab]

Did they tell you this on a Pv course ?
I would agree that it is better to have a longer DC run as it is cheaper to do so on longer runs ,and as you are aware the volt drop is less due to the higher voltage
however you'll have to decide how to run and protect the cable on the Dc side as there are no protective devices involved in the circuit.

 

[topquark]

Eh? Higher voltage on the DC side than the AC side? Rather depends on how many panels you have in the string doesn't it?

 

[topquark]

Eh? Higher voltage on the DC side than the AC side? Rather depends on how many panels you have in the string doesn't it?

Apologies, that just shows my ignorance, just looked at one of the SB Inverter specs 250-600V DC, didn't think they ran that high, kinda begs the question why the advice is to prefer longer AC runs in that case.