Cable selection and derating

Tuesday, October 19, 2021
Training
by
Veli Markovic

Volt rise and current carrying capacity

When it comes to selecting the right cable for a DC or AC application on your next commercial solar project the standard approach is calculate what current will be carried through the cable, work out the cable run distances, then the volt drop DC and volt rise AC particulars.

For example in AS4777 the  example references Table 7, column 24 in regards to current carry capacity and then another table for the mv/A.m factor used to determine the voltage rise on the AC side. This example isn't the worst case scenario but is close.

But what if we need more specific detailed information that has to be passed onto the distributor and the project? For example not only the cable you are installing but the cable already there. What then?

Case study example 

In this presentation we are going to look at an actual case study of a 300 kW + system.

We are systematically going to go through all the calculations from the inverter via the GridSafe to the MSB to the transformer. 

We look at the current carrying capacity of the cables and how their method of installation affects this current carrying capacity.

Inverters being used

So what inverters are we using on this site? 

Inverter number 1: SUN2000-100KTL-H1, 100 kW inverter

Inverter number 2: SUN2000-100KTL-H1, 100 kW inverter

Inverter number 3: SUN2000-SUN2000-29.9KTL-M3, 29.9 kW inverter

Inverter number 4: SUN2000-SUN2000-40KTL-M3, 40 kW inverter

What current are we talking about?

To determine the actual current outputting from these inverters one calculation that can be used is as follows:

Nameplate rating in kVA  of inverter/(nominal voltage ( 3 phase) x square root of three

  • 100kW but 105 kVA = 105,000/(400V x 1.732) = 151.55A. 2 x 100 kW units so 303.1A
  •  44kW but 44  kVA =  44,000/(400V x 1.732)  =  63.51A
  • 29.9kW and 29.9kVA =  29,900/(400V x 1.732)  = 43.16A

So a grand total of 409.77A output under normal conditions.

But where in the whole system are we seeing 409.77A?

Now this particular configuration involves the following:

  • The four inverters all cabled to 
  • A secondary protection board, in this case, of course, a Greenwood Solutions GridSafe board
  • The secondary protection board acts as a PVDB and connects to the MSB

In regards to the 409.77A we are looking at this only applies from the GridSafe to the MSB and, if all loads are off etc out through to the grid.

This means that all the cabling from the GridSafe main switch ( a motorised MCCB) must be able to cope with this current.

And what cables are we installing?

We will assume that the existing cable between the MSB and POA or Transformer is more than sufficient from a current carrying capacity perspective so we don’t have to worry about it for now. (we will still have to do some voltage rise calculations, but later on).

Let’s have a look at the inverters’ cabling requirements.

How is the cable being installed?

The maximum distance from the last inverter to the PVDB is 15 metres and from the PVDB to the MSB is 20 metres.

As we are using 3 x different capacity inverters the inverters with the highest capacity will be installed closest to the GridSafe. This will play a role when we look at the voltage rise calculations required.

Got it! Now we look at how the cabling is being installed.

We have decided to install a 300 mm cable tray affixed to the wall answer and will be using lid to enclose everything.

So what cable do we use?

How is the cable being installed?

Now the standard for general cabling is AS3008 and in this case will be referencing the latest iteration, 2017.

We have selected the cable support that will be a 300mm cable tray with a lid and for ease of convenience we are using 4 core and earth multicore. 

The insulation type is X-90 XLPE which has a higher current rating than thermoplastic. In this case we look at Table 14, column 11 and we find that 70mm2 cable will be sufficient for us as it has a 173A rating and the 100 kVA output is 151.55A

Is that it?

So we have selected the cable based on the current carrying capacity of the inverter output with the installation method and location taken into account but is there more?

Yes we have to look at the derating factor of proximity to cables or circuits, in this case, in the same tray. We will have 4 x separate multicore cables, one for each inverter all in the same tray.

Have to look at Table 22, column 7, item 2 where we discover a derating factor of 0.65.

So what is the current?

So we have 151.55A/phase max output from inverter but because we are installing the cable between the inverter and the PVDB in tray with lid and there are three other inverters we have to derate by 0.65 which is huge so the cable we select has to be able to cope with 151.55 / 0.65 = 233.2A

So back to Table 14, Column 11 we find that 120mm2 XLPE 90 deg will do the trick.

Note: remember to coordinate the breaker with the cable.


Same thing for the other inverters?

We go through the process for the other inverters:

  • 44 kVA, 63.51A, check Table 14, 35mm2 will do 114A, derate by 0.65= 74.1A so all good
  • 29.9 kW, 43.16A , check Table 14, 16mm2 will do 68A, derate by 0.65, 44.2A so all good


Now from the GridSafe to the MSB

All the cables have been run into the GridSafe and the cumulative current of all the inverters is 409.77A.

We have to go through the same process. We will be using single core XLPE as the multicore option is too ‘cumbersome’ and is hard to handle.

Also we will be installing in ladder, horizontally with no lid so heat dissipation will be less of an issue compared with the run in vertical tray with lid between the inverters and the PVDB

Look at Column 15, Table 8, 300mm2 will do 469A

Now have to look at derating

As we are installing the single core cable in a horizontal ladder in the air with no lid we have to reference a different derating table which in this case is Table 23.

Now there is only one circuit so no derating and in fact as the installer has spaced the cables there would be no derating even if there were other circuits, see Clause 3.5.2.2 (C) and Figure 1a.

Outcome is that we can use the 300mm2 cable selected!

MSB to the Transformer

Now the cable or cables from the MSB to the pad mount transformer is something we don’t change and in this example existing cable arrangement is 8 x single core, 300mm2  Al - 90 degrees rating so no problems with the cable’s ability to handle 409.77A. 

We have the current all sorted but there is more!

Now AS4777 stipulates a max voltage rise of 2% from inverter with largest voltage rise calculation all the way through to the transformer in this case. 

Now, as we are talking three phase the 2% applies to 400v nominal and this equates to 8 volts so the designer has only 8 volts to play and really their choices are limited to the cabling between the inverters and the GridSafe and between the GridSafe and the MSB. 

Don’t really want muck around with the cabling between the MSB and the transformer. Now there are a few ways to calculate voltage drop. AS4777 references the mV.A.m method that produces a conservative result. The other method we will look at in our next presentation uses the circuit impedance method.

Conclusion

The area of correct cable selection to satisfy  CCC capacity, voltage rise and cable costing economics relies on AS3008. The ability to understand this standard is of paramount importance. The correct current must be assessed every step of the way and,  as we have seen,  the method of installation, location and derating factors really affects this initial figure.


If you’d like to see what Greenwood Solutions get up to in the real world of renewable energy, solar, battery storage and grid protection check out our industry and commercial pages:

https://www.greenwoodsolutions.com.au/industry 

https://www.greenwoodsolutions.com.au/commercial

https://www.greenwoodsolutions.com.au/news

https://www.greenwoodsolutions.com.au/commercial/customer-stories


About the author

Veli Markovic

CEC Designer
Veli has nearly two decades of experience in the renewable industry. He is passionate about providing people with valuable education and is highly regarded throughout the industry as an educator and operator.
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