[RE-wrenches] Surge arrestors on strings

bob at midnitesolar.com bob at midnitesolar.com
Tue Oct 9 13:39:03 EDT 2018 

For an inverter with two MPPT inputs, you will need to treat the 
negatives and positive lines separate unless
the manufacturer says something else.  That means one  SPD per array 
unfortunately.  Negative and positive connect
to separate leads of the SPD and then ground in the middle.  As both 
positive and negative PV lines rise together
relative to ground, the SPD is supposed to clamp that rise to below the 
equipment's hi-pot voltage that they test
to at the factory.  (i.e. common mode voltage rise) All wires except for 
ground (chassis) of the equipment are tied
together and the hi-pot machine (sort of like a Megger ?) applies the 
high voltage between all those wires on one
side and ground on the other to make sure the insulation doesn't break 
down inside the equipment.
This hi-pot test goes for AC  utility input/output connections and DC 
from the PV input side.

The SPD  has to clamp plus and minus and AC inputs to a lower voltage 
than this hipot rating but higher voltage
than lines will see differentially in voltage.  Remember that in a 
3-wire SPD, there is two sets of MOVs in series and
the center is tied to ground.  If one MOV starts conducting at say, 500 
volts DC then you can apply up to 1000
volts  between the hot leads.  Each plus and negative line can rise up 
to 500 volts from the lightning before they
start to clamp.  The  bigger the lightning strikes effect, the more 
current will be applied to the MOVs though and
the clamp voltage will rise somewhat.  That's why the SPD arrestor 
voltage should be well below the hi-pot voltage,
whatever that is...  Hi-pot voltages are quite a bit higher than the 
operating voltage.  For example, All Classic CC's
are tested to 2270 volts DC from hot lines to ground.  It's something 
like double the operating voltage plus 1000V
and multiply by 1.414 for DC rating.....  I'm going off memory so don't 
quote me on that but something like that.

I think that Ray has the right idea about short connections like from 
the local generator.  Battery also.
Just apply SPDs to anything that goes outside like an antenna.

boB


On 10/9/2018 8:26 AM, cwarfel wrote:
>
> I think something on the order of 90+% lightning strikes come from the 
> utility feed, not the dc side. That has to be ignored if in a high 
> strike area orgrid isolated.
>
>
> On 10/8/2018 10:37 PM, Ray wrote:
>>
>> I believe you can use the SPD 300 AC on 2 strings, if PV neg is 
>> grounded.  You would connect the green to ground (or PV neg) and then 
>> connect the red to one string pos, and connect the black to the other 
>> string pos.  This would be limited to 300 v also.  I haven't tried 
>> that exact configuration, but I have used the 300 AC to protect both 
>> the AC in and AC out on 120 vac inverters.  I also believe you can 
>> use a 600 DC version, and wire as I mentioned, but the LEDs won't 
>> light. If you are using ungrounded inverters, then I think you would 
>> need an SPD for each string in order to protect both the pos and neg 
>> of each string, like you've been doing.
>>
>> Maybe someone from Midnite can clarify this.
>>
>> As far as it being worth it, I haven't had lightning related damage 
>> since I started using the Midnite SPDs, and I used to have system 
>> damage every year.  If you are not in lightning country, or the 
>> install is in a low spot with less likelihood of taking a hit, you 
>> might be able to skimp.  But for some of these off grid ridge top 
>> installs, I consider it a must. Some times I don't install one on the 
>> gen circuit, if its really close.
>>
>>
>> Ray Walters
>> Remote Solar
>> 303 505-8760
>> On 10/8/18 4:03 PM, Drake wrote:
>>> Hello Wrenches,
>>>
>>> Do you use surge arrestors with multi MPPT input string inverters? 
>>> Do you put a separate arrestor on each MPPT input? I find it sort of 
>>> pricey using a Midnite Solar arrestor on each string, but have been 
>>> doing that. Is there a less expensive approach that is effective?
>>>
>>> I believe the Midnite units are the best. Are they worth the expense?
>>>
>>> Thank you,
>>>
>>> Drake
>>>
>>>
>>> Drake Chamberlin
>>> /Athens Electric LLC
>>> OH License 44810
>>> CO License 3773
>>> NABCEP Certified Solar PV
>>> 740-448-7328
>>> /http://athens-electric.com/
>>>
>>>
>>>
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>
> -- 
>
>          Christopher Warfel, PE
>        ENTECH Engineering, Inc.
> PO Box 871, Block Island, RI 02807
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