Bipolar Array Trenching

[Andrew445]

I am designing my first bipolar PV system, and would like some clarification on trenching requirements. I understand that the monopole circuits should be kept separate in order to prevent jumping to 1200V in failures.

Article 690.4(G) states that "the electrical outputs from each monopole subarray shall be installed in separate raceways" This is all fine and good. I look to the Article 100 definition of a raceway and see that a raceway is "An enclosed channel of metal of nonmetallic materials designed expressly for holding wires, cables, or busbars" Okay, so this leads me to think the monopoles can be run in the same trench, as long as they are in separate conduits.

Then I continue reading 690.4(G), "All conductors from each separate monopole subarray shall be routed in the same raceway." Now I am confused. They are obviously not going to all be in the same conduit. Is a "raceway" in this section a "trench"? If so, do I need separate trenches?

A continuation of this question would be in the case of direct-buried cables. Now there aren't any raceways by the definition of Article 100.

I think I might be thinking too hard about this. Some clarification would be appreciated. Thanks.

 

[SolarPro]

As Greg Ball explains in an article about central inverters for PV power plants in the Oct/Nov 2010 issue of SolarPro magazine:

"Though not a new concept or topology, bipolar inverters are resurgent with the success of the AE Solaron inverter platform. The AE Solaron incorporates a unique bipolar design that allows for transformerless operation with a standard 480 Vac low voltage output. Two separate 600 Vdc arrays feed the inverter: One is referenced positive and the other is referenced negative with respect to ground. The inverter, therefore, operates with a 1,200 Vdc maximum bus, even though equipment in the field does not exceed 600 Vdc relative to ground.

Voltage ratings for cables, combiner boxes and switches do not have to exceed 600 Vdc, as long as the circuits from the positive and negative arrays are run in separate conduits and enclosures up to the interface with the inverter. The neutral cables of both arrays are referenced to ground and should be treated like any other grounded, current-carrying conductor. When the inverters are off, these neutral circuits are hard- connected to ground. During operation, the neutral circuits are referenced via the inverter switching to the ac neutral, which is grounded at the ac service entrance or panel."

In the same article, an applications engineer for AE explains that the dc input to a Solaron inverter (which AE now refers to as "NX" series inverters) is "a 4-wire bipolar configuration with two wires for the positive array and two wires for the negative array." So there are two current-carrying conductors associated with positive monopole of the bipolar array, and two current-carrying conductors associated with negative monopole. Even if you are using a different bipolar inverter, the basic concept is the same.

690.4(G) requires that the current-carrying conductors associated with each monopole are kept together, which is what you would expect since both conductors are part of the same 600 Vdc circuit. If you put the conductors associated with the positive monopole in a separate conduit from the conductors associated with the negative monopole, then you can use 600 V-rated wire for both circuits. In order for all four current-carrying conductors—two each from both monopoles—to share a raceway, you would have to use something like 2,000 V-rated wire, since the sum of the voltage between the two monopoles is 1,200 Vdc.

No doubt someone else on this forum can make quick work of the direct-burial-cable-in-a-single-trench part of your question. (Or just ping an applications engineer for the inverter manufacturer—they must field this question all the time.)

 

[jaggedben]

Then I continue reading 690.4(G), "All conductors from each separate monopole subarray shall be routed in the same raceway." Now I am confused. They are obviously not going to all be in the same conduit. Is a "raceway" in this section a "trench"? If so, do I need separate trenches?

I think you are misreading the sentence you quoted. It does not say "All conductors from all separate monopole subarrays...". It is saying that the conductors from a given monopole subarray must not be separated into different raceways.

A trench is not a raceway, and you can put the multiple raceways from the different monopole subarrays in the same trench.