Solar power plant cabling: Allowable Voltage drop from PV string to String Inverter

[RRaghunath]

In AC systems there are norms for allowable voltage drop that we follow while sizing the cables.
Typically, from transformer LV terminals to the switchboard - 2% (max) and from LV switchboard to the equipment connected - 3% (max). Thus, the allowable total voltage drop from source to connected equipment is capped to 5%.
Are there similar norms in Solar power plants?
I am not much familiar with design of solar power plant design and appreciate any guidance.
Thanks & Regards

 

[cuky2000]

Hi RRaghunath,

It is interesting to point out that codes such as the NEC in the US don't require the calculation of voltage drop because it's not a safety issue. On the other hand, solar inverters have a wide range of dc input voltage tolerance while maintaining a fairly constant AC output.
Primarily DC cables are sized for the maximum current (I) carried by the conductor and by a reasonably acceptable joule loss (RI²).

<sub>NOTES:
a) It should be noted that every percentage of voltage drop results in a percentage of power loss from the inverter. While this may not seem like much, the cumulative energy (kWh) lost over the life of the system can be significant.

b) The NEC Section 690.8 states the base wiring ampacity for DC conductors carrying current generated by PV modules be based on 125% of the STC short circuit current capability of the string/array.

c) Although not mandated by code, most inverter manufacturers recommend a maximum of 5% voltage drop for the system— typically 2.5% on either side of the inverter.</sub>

 

[jfpe]

I typically see engineers design for about 3% voltage drop on the DC side.

Like cuky2000 said modern photovoltaic inverters have a wide input window, so DC voltage drop won't push you outside of the allowable input range. DC voltage drop impacts production, so the designer has to decide if they want to spend more to upsize cables or take a hit on kWhs over the life of the system.

AC voltage rise is a bigger issue. Voltage rises at the inverter terminals as the inverter produces power. It is not unusual to see excess voltage rise trip an inverter (or a solar farm) offline. It's particularly common this time of year when the utility voltage tends to run a little high.


-JFPE