Redwood Infrared
Member
- Location
- Arcata, CA, USA
Please bear with me as I'm not an electrician but a certified thermographer with more experience in simply identifying electrical anomalies that represent potential issues than actually understanding or mitigating them. I usually just point and say, “ya know, sooner or later the smoke’s gonna come out of that one. You might want to fix that while the guys are off one weekend soon. Either that or stock up on brooms so you can keep ‘em busy during your impending unscheduled shutdown.”
I was called into a 50,000 sf metal commercial building that had a 125kW rooftop solar array installed about two years ago. Over that time a vibration with an audible hum that corresponds to how heavily the system is loaded has developed and according to building occupants continues to get worse with time. Under full sun it’s loud enough in some office spaces to drive occupants away from their desks for the afternoon.
The installing electrician and local utility have both evaluated the system, installed power monitors, etc., and have no idea what’s causing the issue.
The arrays feed into 11 inverters on the south side of the building. The inverters feed into a combiner box that routes 3 phase power via aluminum conductors 250 ft away to an AC Solar Disconnect (ACSD) next to the Main Service Panel (MSP) at the north side of the building. Due to the long distance and several corners the owner elected to use aluminum wire splitting the load across redundant conductors, with a pair of parallel conduits each carrying all three phases, neutral, and ground.
It is along these conduits through the building that the vibration hum is observed.
Array components, inverters, and combiner box panel are grounded back to the MSP via these parallel grounds. The parallel conduits, which pass through the panel boxes at both ends with metallic hardware, themselves represent yet another parallel ground path. None of the array, inverter, or combiner box components have separate grounds at their locations, only the connection back to the AC Solar Disconnect, which is then bonded to the MSP, which is in turn connected to the building frame, with a separate GEC originating just a few feet away and entering the slab, interrupted by a strap around a water pipe along its path.
Ultrasound indicated only audible frequencies so we can remove corona or other high frequency sources of vibration.
Using FLIR I determined that both the combiner box bus bars and main disconnect near the inverters and the knife switches in the 600A AC Solar Disconnect 250 feet away were over 72°F rise over ambient, which NEMA Thermal Severity Criteria classify as seriously deficient - and this was on a day with a thin cloud layer so we were only seeing about 75% of production capacity, or about 200 amps per phase so it’s unlikely the panel components ever see more than 50% of their rated capacity.
Hot spots observed appeared to originate at points of contact in the switchgear and thermal gradients showed heat dissipating along the length of the conductors. I did not observe significant inductive heating where conductors exit the conduits, which is sometimes the case in certain situations.
Significant vibrations could be tangibly felt by hand along insulated sections of conductor accessible well away from contact points within the AC Solar Main Disconnect.
I theorize there are two possible contributors to the problem vibration and it could be either or both simple induced currents due to inductive effects and/or a ground loop owing to the multiple parallel ground paths connecting the inverter combiner box to the AC Solar Disconnect. At the terminal end it appears the conductors exit the conduit and enter the box under some lateral strain such that some but not all of them are in pretty tight but insulated contact with the edge of the conduit/panel interface. I have seen examples where conductors coming off a high current bus bar and mechanically fastened to the panel using metallic clamps or cleats can buzz like the dickens, and the problem goes away when the cleats are replaced by nonmetallic straps. Could the conduit itself be acting as the secondary in a crude linear transformer and that’s the only thing causing the vibration?
But upon further inspection of the ground system I have trouble not believing it's at least part of the equation.
First off, we have those multiple ground paths between far ends of the building, all running in parallel to current carrying phases and neutral. Again, with no separate ground at the point of the inverters/combiner box. All components within the system are bonded together and tied to a single ground point at the line side of the building’ MPS.
Second, I measured what some would classify as ‘objectionable current’ * in the ground itself, totaling as much as 8amps, split unevenly between the two parallel grounds at about a 2:1 ratio. Note that these ground conductors share the exact same origination and termination lug points at each end of their run but run through separate parallel conduits along with the other parallel redundant conductors only in order to reduce the required wire size for the installation.
(*I did enough searching for answers before posting this that I am aware “objectionable current” is poorly defined and can be a hotly debated topic but objectionable or not, I don’t run across it often and it strikes me as unusual.)
I also measured what I perceive as odd currents in the ground wire running from the MSP to the building frame at about 2 amps, then about the same 2 amps along the next ground wire a few feet away running from the building frame to where it’s interrupted by the water pipe, but it jumps to about 8 amps from the water pipe to the slab. Unfortunately I only had a Fluke T5 on hand so my CT couldn’t wrap around the pipe but I’d have to assume there’s 5 amps in that as well.
The one other ground effect observation is that when we went on the roof (metal roof deck with a recently applied thick elastomeric treatment), every person in our party received a significant static shock upon first contact with any metal component. None of the grounding cables running from the array panels to the rooftop combiner boxes nor the combiner boxes to the inverters appeared to be carrying any measurable current.
From that my non-electrician brain would infer that there is a significant potential difference developing at different grounded points within and on top of the building. Whether this is or is not exclusively or even partly responsible for the audible hum in the conduits remains unknown to me.
I am about to suggest having the electrician carry out a few tests the client might carry out in order to eliminate potential sources of the problem.
I realize any or all of 2 through 4 might be unconventional, non-code compliant, and perhaps flat out unsafe in the long run. But for purposes of figuring this out I believe – IF it is possible to do the tests safely under tightly controlled conditions – we might be able to collect some very helpful observations.
Or better yet, one of you geniuses who has read this far and hasn’t already decided I’m an idiot and not worth the trouble might already know exactly what’s going on here and can just tell me?
I thank you in advance for your indulgence and any thoughtful responses this might elicit.
I was called into a 50,000 sf metal commercial building that had a 125kW rooftop solar array installed about two years ago. Over that time a vibration with an audible hum that corresponds to how heavily the system is loaded has developed and according to building occupants continues to get worse with time. Under full sun it’s loud enough in some office spaces to drive occupants away from their desks for the afternoon.
The installing electrician and local utility have both evaluated the system, installed power monitors, etc., and have no idea what’s causing the issue.
The arrays feed into 11 inverters on the south side of the building. The inverters feed into a combiner box that routes 3 phase power via aluminum conductors 250 ft away to an AC Solar Disconnect (ACSD) next to the Main Service Panel (MSP) at the north side of the building. Due to the long distance and several corners the owner elected to use aluminum wire splitting the load across redundant conductors, with a pair of parallel conduits each carrying all three phases, neutral, and ground.
It is along these conduits through the building that the vibration hum is observed.
Array components, inverters, and combiner box panel are grounded back to the MSP via these parallel grounds. The parallel conduits, which pass through the panel boxes at both ends with metallic hardware, themselves represent yet another parallel ground path. None of the array, inverter, or combiner box components have separate grounds at their locations, only the connection back to the AC Solar Disconnect, which is then bonded to the MSP, which is in turn connected to the building frame, with a separate GEC originating just a few feet away and entering the slab, interrupted by a strap around a water pipe along its path.
Ultrasound indicated only audible frequencies so we can remove corona or other high frequency sources of vibration.
Using FLIR I determined that both the combiner box bus bars and main disconnect near the inverters and the knife switches in the 600A AC Solar Disconnect 250 feet away were over 72°F rise over ambient, which NEMA Thermal Severity Criteria classify as seriously deficient - and this was on a day with a thin cloud layer so we were only seeing about 75% of production capacity, or about 200 amps per phase so it’s unlikely the panel components ever see more than 50% of their rated capacity.
Hot spots observed appeared to originate at points of contact in the switchgear and thermal gradients showed heat dissipating along the length of the conductors. I did not observe significant inductive heating where conductors exit the conduits, which is sometimes the case in certain situations.
Significant vibrations could be tangibly felt by hand along insulated sections of conductor accessible well away from contact points within the AC Solar Main Disconnect.
I theorize there are two possible contributors to the problem vibration and it could be either or both simple induced currents due to inductive effects and/or a ground loop owing to the multiple parallel ground paths connecting the inverter combiner box to the AC Solar Disconnect. At the terminal end it appears the conductors exit the conduit and enter the box under some lateral strain such that some but not all of them are in pretty tight but insulated contact with the edge of the conduit/panel interface. I have seen examples where conductors coming off a high current bus bar and mechanically fastened to the panel using metallic clamps or cleats can buzz like the dickens, and the problem goes away when the cleats are replaced by nonmetallic straps. Could the conduit itself be acting as the secondary in a crude linear transformer and that’s the only thing causing the vibration?
But upon further inspection of the ground system I have trouble not believing it's at least part of the equation.
First off, we have those multiple ground paths between far ends of the building, all running in parallel to current carrying phases and neutral. Again, with no separate ground at the point of the inverters/combiner box. All components within the system are bonded together and tied to a single ground point at the line side of the building’ MPS.
Second, I measured what some would classify as ‘objectionable current’ * in the ground itself, totaling as much as 8amps, split unevenly between the two parallel grounds at about a 2:1 ratio. Note that these ground conductors share the exact same origination and termination lug points at each end of their run but run through separate parallel conduits along with the other parallel redundant conductors only in order to reduce the required wire size for the installation.
(*I did enough searching for answers before posting this that I am aware “objectionable current” is poorly defined and can be a hotly debated topic but objectionable or not, I don’t run across it often and it strikes me as unusual.)
I also measured what I perceive as odd currents in the ground wire running from the MSP to the building frame at about 2 amps, then about the same 2 amps along the next ground wire a few feet away running from the building frame to where it’s interrupted by the water pipe, but it jumps to about 8 amps from the water pipe to the slab. Unfortunately I only had a Fluke T5 on hand so my CT couldn’t wrap around the pipe but I’d have to assume there’s 5 amps in that as well.
The one other ground effect observation is that when we went on the roof (metal roof deck with a recently applied thick elastomeric treatment), every person in our party received a significant static shock upon first contact with any metal component. None of the grounding cables running from the array panels to the rooftop combiner boxes nor the combiner boxes to the inverters appeared to be carrying any measurable current.
From that my non-electrician brain would infer that there is a significant potential difference developing at different grounded points within and on top of the building. Whether this is or is not exclusively or even partly responsible for the audible hum in the conduits remains unknown to me.
I am about to suggest having the electrician carry out a few tests the client might carry out in order to eliminate potential sources of the problem.
- Going for the obvious problem first, I suggest they mitigate the heat issues identified with the FLIR survey. Lug connections had already been re-torqued before my arrival but obviously fixed nothing so I suggest removal and visual inspection with renewal of conductor endpoints with fresh application of Noalox if warranted. If upon re-inspection the combiner box main disconnect is still flagged under NEMA Thermal Severity Criteria I’ll recommend replacing that component. I'll be surprised if this eliminates the hum but it needs to be done regardless.
- Since we have redundant parallel ground conductors between panels, I suggest lifting one of them under controlled conditions to see if its removal has any effect. If it does not, we might need to try to figure out a way to temporarily also remove one or both of the electrical bonds represented by the conduits themselves. Logistically that will be much more challenging but I believe there must be available some sort of insulating bushing that could be installed to isolate the conduit from the panel to which it is mechanically fastened. Unfortunately this will require substantial disassembly of at least one of the panels to achieve.
- We might also try temporarily lifting the ground connection to the water pipe, and/or tying the MSP directly to the ground by removing its connection to the building’s steel frame.
- We could try installing a direct ground at the location of the inverter combiner box.
I realize any or all of 2 through 4 might be unconventional, non-code compliant, and perhaps flat out unsafe in the long run. But for purposes of figuring this out I believe – IF it is possible to do the tests safely under tightly controlled conditions – we might be able to collect some very helpful observations.
Or better yet, one of you geniuses who has read this far and hasn’t already decided I’m an idiot and not worth the trouble might already know exactly what’s going on here and can just tell me?
I thank you in advance for your indulgence and any thoughtful responses this might elicit.
