The voltage on the grid varies throughout the day depending on how much power is being drawn from the grid and much solar is being exported. For energy to flow, the voltage at the inverter must be higher than at the grid. Issues can occur when cables haven’t been sized correctly because too much resistance results in a rise in voltage which can result in self-protection methods being instigated.
In AS/NZ 4777.1 volt rise is restricted to a maximum of 2% and this is from the Point of Supply (P.O.S.) to the last inverter with the highest current rating.
As the distributor requires this information, the installer must assess the existing cable and perform the necessary calculations. This determines any new cabling specs.
If we are talking 3 phase supply, we use a nominal voltage of 400V x 2% so we have 8 volts to play with over the entire cable run; both existing and planned.
We will assume a 100 kVA solar system on the AC side using 4 x 25 kVA inverters so will need to check that the cable from POS to MSB ( Main Switch Board) can carry the required current. Then a check of the volt rise along this section of the cable is required.
In this particular example there are three sections of cable involved. The first is from the POS to the MSB; the next section is the run from the MSB to the PVDB secondary protection board; and then finally from the secondary protection board to the inverter with the highest current rating that is furthest away.
So:
We have 100 kVA of inverter capacity, a three phase system so the calculation is as follows:
POS to MSB uses 95mm2 and the distance is 86 metres. Now AS/NZ 4777.1 refers to another standard, AS/NZ 3008 and specifically Table 7, Column 24 for example, see below:
Same size cable can have different CCC* and this can depend on cable type, proximity to other cables, method and location of installation.
*current carrying capacity
Table 7, Column 24 assumes a certain cable type installed in a certain way and this example is not the worst case scenario but is close. Ultimately it’s up to the installer to assess the cable situation themselves!
According to the referenced AS/NZ 3008 table, (Table 7, Column 24) 95mm2 cable can carry 217 amps so all good in this respect but this is not all! We have to now look at the voltage rise so the next step is to go to another table that AS/NZ 4777.1 references, again in AS/NZ 3008, specifically, Table 41, column 6 which gives us a volt drop/rise factor of 0.449 for the existing 95mm2 cable.
See table below:
So the calculation is Vd/r = L x I x Vc/1000 where:
Vd/r = (86m x 144.51A x 0.449) /1000
Vr = 5.58 volts.
Voltage rise is 5.58 volts for POA to MSB and as a %, this is 1.39%, so from the MSB to inverters, the designer has 0.61% to play with or 2.42 volts!
This process has to be repeated on all sections from POS all the way to the inverter(s)
How do you do this?
Let’s assume we have done the calculations from the MSB to PVDB and the cable selected there had a 0.86V rise in voltage so we add this figure to the existing volt rise:
5.58V + 0.86V = 6.44V
So now we have a max of 1.54V left to play with for the PVDB to inverter run.
In our case we have 4 x 25 kVA inverters and they are all around the same distance from the PVDB, give or take a metre or so. Now, the voltage rise calculation is performed on the largest current output with the longest run so in this case we select one of the 25 kVA inverters that is furthest away.
We could use the formula 25kVA / (1.73 x400) = 36 Amps for the 25 kVA inverters but best to check the data sheet for the max AC output of the inverter you are using and in this case we’ll roundup to 40 Amps.
Say we select 10 mm2 and this can cope with up to 59A continuous according to AS3008 Table 7, row 24 and it’s only a 7 metre run.
7m x 40A x (3.86/1000) = 1.08 Volts
Total over the entire run is Vd = 7.52 Volts. A success seeing as we fall under the 8 volt maximum stipulated in the Standards.
Australian Standards stipulate a maximum 2% volt rise from POA to last inverter and your job is to calculate the volt rise and current carrying capacity for every section of cable run and also to take into account possible derating for heat due to installation location and method.
If you need any help with these calculations please contact our grid application service and one of our engineers will be happy to assist you.
