grounding bushings on TL systems
- Thread starter Zee
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shortcircuit2
Senior Member
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- South of Bawstin
Yes you do. The definition of to "voltage to ground" in an ungrounded system is the potential between conductors of the circuit...
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
Yes and no. If the conductors in an ungrounded bipolar system (there are ungrounded systems which are not bipolar and therefore are not different from conventional systems WRT equipment grounding) from the upper array are kept separate from those of the lower array all the way back to the inverter, then the voltage between the positive conductor of the positive array and the negative conductor of the lower array is not an issue.
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
... positive conductor of the upper array, I meant to say.
BillK-AZ
Senior Member
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- Mesa Arizona
Some of this does not make sense. Please give an example of "ungrounded systems which are not bipolar". One guess is tripolar with an ungrounded center tap (not ungrounded after all).
I do not see how " If the conductors in an ungrounded bipolar system (......) from the upper array are kept separate from those of the lower array all the way back to the inverter, then the voltage between the positive conductor of the positive array and the negative conductor of the lower array is not an issue."
If one end of a high voltage string that is ungrounded becomes grounded due to a fault, the opposite end has full voltage with respect to ground. Cannot use 600V wire in a 1000V bipolar system. It may be good practice to use separate conduits as suggested by several inverter manufacturers as it reduces the voltage stress on wire insulation. After all, we are trying to instal PV systems that will still be safe in 25 years or more. The full voltage is an issue.
I do not see how " If the conductors in an ungrounded bipolar system (......) from the upper array are kept separate from those of the lower array all the way back to the inverter, then the voltage between the positive conductor of the positive array and the negative conductor of the lower array is not an issue."
If one end of a high voltage string that is ungrounded becomes grounded due to a fault, the opposite end has full voltage with respect to ground. Cannot use 600V wire in a 1000V bipolar system. It may be good practice to use separate conduits as suggested by several inverter manufacturers as it reduces the voltage stress on wire insulation. After all, we are trying to instal PV systems that will still be safe in 25 years or more. The full voltage is an issue.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
SMA has a line of inverters which are monopolar transformerless.
http://www.sma-america.com/en_US/pr...000tl-us-9000tl-us-10000tl-us-11000tl-us.html
I have never heard of a tripolar DC system and I don't know what that would look like.
My company has installed many REFUsol (now AE) bipolar transformerless inverters. Their manual directs that the upper and lower (which they call DC1 and DC2) array wiring be kept separate from the arrays all the way back to the inverter (not bundled at the array and in separate conduits all the way back to the inverter), but it does not stipulate 1000V wiring. In the event of a fault, the inverter shuts down and grounds the center point (negative of DC1 and positive of DC2) so that no voltages are more than 500V to ground.
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THanks, Shortcircuit2.
I did not know the potential between conductors is what counts here. (my systems are not bipolar. e.g. SMA SB3000TL)
I will continue to use grounding bushings, the bane of my professional life. :weeping:
Also, thanks all. The bipolar issue is not one i face.
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shortcircuit2
Senior Member
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- South of Bawstin
jaggedben
Senior Member
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- Northern California
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- Solar and Energy Storage Installer
Article 100:
Enough to answer Zee's original question pretty clearly.
As for the bipolar PV circuit with a grounded neutral (the only type of bipolar PV circuit allowed by the NEC), the voltage to ground is the highest potential between either ungrounded conductor and the grounded conductor. (It's all one circuit.) I'd say this is true regardless of whether the positive and negative ungrounded conductors are kept in separate raceways. Some parts of the NEC may require they be kept separate if the voltage to ground is above 600V, but the definition quoted above refers to the circuit not the raceway.
Enough to answer Zee's original question pretty clearly.
As for the bipolar PV circuit with a grounded neutral (the only type of bipolar PV circuit allowed by the NEC), the voltage to ground is the highest potential between either ungrounded conductor and the grounded conductor. (It's all one circuit.) I'd say this is true regardless of whether the positive and negative ungrounded conductors are kept in separate raceways. Some parts of the NEC may require they be kept separate if the voltage to ground is above 600V, but the definition quoted above refers to the circuit not the raceway.
texie
Senior Member
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ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
I don't have my codebook at home, but could you cite that article? I think that in AE bipolar inverters (the former Solaron line), the only time the array is center grounded is when the inverter is off. Caveat: that's from memory; I haven't designed a system around one in a while and the manual is at work.
jaggedben
Senior Member
- Location
- Northern California
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- Solar and Energy Storage Installer
690.41
But now that I look at it again, the exception (systems complying with 690.35) applies to all types of systems, so I was wrong.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Especially on the internet, it takes a Big Man...
Kidding aside, if I had a dollar for every time I've been wrong, I'd be fishing in the Caribbean instead of sitting here at this computer.
So Cal
Member
- Location
- Los Angeles, Ca
Sometimes, when installing a grounding bushing is going to be particularly difficult, I substitute it for an up sized meyers type hub and a reducing bushing (bridging past the concentric KOs).
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
"Missed it by that much...."
I think that only problem with the NEC is the requirement that all circuit conductors in one raceway have insulation rated for the highest voltage differential between any conductors of that circuit.
If your sub-array voltage is 490 volts you cannot do that with 600 volt building wire but can with PV-1000.
If the sub-array voltage is greater than 500, you cannot put them in one raceway using PV-1000, but could using PV-2000.
I suppose that you could also use an MV wiring type, but that would be too expensive.
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