Harmonics anyone?

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bnewport

Member
Location
Indiana
OK guys- I have a few PQ questions:

I am troubleshooting (3) HVAC control cabinets. Each cabinet is fed by its own 120V circuit, but all 3 share the same neutral. There is one single phase 120/24 VAC control transformer per cabinet. The transformer primary shows high levels of 3rd order harmonics, and at random times of the day, the N-G voltages are peaking at over 5 volts. They have burned up numerous boards.

X2 is grounded, and the customer asked if this is required by the NEC.?

What is an acceptable level of voltage and current harmonics? Can triplen harmonics pass through a single phase transformer, and if so, can they destroy circuit boards?

I am guessing that the harmonics from all 3 cabinets are adding on the neutral, overloading it, and causing the voltage rise on N-G. Am I on the right track? :confused:

Thanks-
Bryan
 

pierre

Senior Member
Re: Harmonics anyone?

"I am troubleshooting (3) HVAC control cabinets. Each cabinet is fed by its own 120V circuit, but all 3 share the same neutral."

Is this a 3 phase system?
Have you tried replacing the 1 neutral with a separate (3) neutral to each cabinet?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Harmonics anyone?

How does a high neutral to ground voltage cause problems on the 24 volt side of the transformer?
Don
 

charlie tuna

Senior Member
Location
Florida
Re: Harmonics anyone?

most large hvac control cabinets require a "dedicated" low voltage circuit. even though there is a control transformer, the neutral is connected to the board through the control transformer. i would (as suggested above) shut down the system and re-pull using the existing common neutral to provide individual neutrals to each cabinet. this may not be the actual problem, but is a big step in the right direction!
 

bnewport

Member
Location
Indiana
Re: Harmonics anyone?

The 120V is fed from a 120/208 3 phase system. I will suggest pulling in separate neutrals. I dont believe the N-G voltage rise is affecting the secondary, but could harmonics pass through this single phase xfmr and affect the controls? The N-G voltage is something the maint crew noticed with a Dranetz meter, and it prompted me to look for overloaded neutral / harmonic problems.

Bryan
 

charlie tuna

Senior Member
Location
Florida
Re: Harmonics anyone?

we have found in some johnson-control equipment that the neutral is connected to the metal housing that holds all the electronics cards. the low voltage secondary transformer is also connected to this metal. any potential produced on the neutral will be seen on this metal housing and does not bother the 120 volt end of it---but remember the secondary of that transformer is only six volts --- a two volt buildup on that plate could reduce the voltage required for operation to some components by 30 per cent!
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Harmonics anyone?

Charlie,
a two volt buildup on that plate could reduce the voltage required for operation to some components by 30 per cent!
The neutral to EGC voltage on the 120 volt side of the transformer does not change the 6 volts on the output side. It is still 6 volts between the two conductors. If they are using a reference to earth in place of one of the secondary conductors there would be a problem, but as long as they are using both of the secondary conductors, the equipment connected to those conductors will only see the secondary voltage.
Don
 

bnewport

Member
Location
Indiana
Re: Harmonics anyone?

There are 3 control cabinets, the middle one is the only one having problems. The conduit containing the branch circuits enters the middle cabinet. There is a convenience outlet in each cabinet, and this circuit also feeds the control xfmr. When I arrived, the EGC was hanging out of the conduit, and was not bonded to anything. They have since corrected this. Even when the EGC was not connected, the H-G voltage was ok, since the conduit is metallic back to the branch panel, and the recept boxes are also metal. X2 of the xfmr runs into the recept box, and is tied to the ground pin of the recept, besides feeding the controls along with X1. In the other 2 cabinets, X2 is bonded to the back plate of the cabinet. I installed a Fluke 43 to the secondary of the 24V control xfmr to get an idea of what might be happening. I will find out tomorrow... The maintenance crew also mentioned a worker feeling a "tingle" on a rooftop unit when it was wet outside. They said this happened a while ago, and they have made changes since then, and they are not sure if this is somehow related.

Bryan
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Re: Harmonics anyone?

Bryan,

After reading your post again, I'd have to ask if the harmonics are coming from your circuits or being induced elswhere. I'm not very astute when it comes to harmonics, but if they are coming from other ciruits ....?
 

bnewport

Member
Location
Indiana
Re: Harmonics anyone?

I not yet sure if the harmonics are being generated in the control cabinets, or if they are coming from the VFD's in the room, since there is no one-line or print to see what panels are feeding what. When I checked the 120V circuits the other day, I remember there being a large percentage of 3rd order current harmonics, but I dont remember what percent of voltage harmonics were present. I am somewhat new to the PQ field, so I am learning as I go along.

Bryan
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Re: Harmonics anyone?

Bryan,

A couple more things. 1st a quote from ECM, "What's the lesson here? Harmonics is not necessarily a problem just because of a customer's nonlinear loads."
http://bg.ecmweb.com/ar/electric_fundamentals_harmonics_testing/

I inferred the numerous boards being burned up are not on just one unit. I also assume you believe it is the harmonics that are causing your problem. While I'm not saying it is not, it just seems that you might have another problem. I'd check out the grounding. Look for poor connections on your neutral. The 5 volts you mention between ground and neutral may indicate a high resistance neutral.

Good luck, and when you find the problem, please share what it was and the solution.

Thanks,
HWS.
 

bnewport

Member
Location
Indiana
Re: Harmonics anyone?

The boards being burned up are only in the middle cabinet and are fed from the 24VAC control xfmr. The N-G voltage spikes only occur intermittently, which makes it tough to trace. I don't know if harmonics are causing the problem, but I have read that the 3rd order harmonics add in the neutral, which could overload the neutral. This in turn could be causing the high N-G voltage. There are 3 phases (1 per cabinet) with one shared neutral. Thanks for the link to EC&M. Coincidentally, I just read that article last night. I also subscribe to EC&M. Great magazine! Anyways- I am trying to keep an open mind concerning this problem. The customer kept burning up controls, and noticed the intermittent N-G voltage rise with a Dranetz meter. The HVAC contractor seems to think this N-G voltage is burning up their boards. I haven't spent much time tracing down the problem yet, only researching and thinking about it before I go back to the site. I tend to agree with Don that the N-G problem probably would not affect the low side of the xfmr. I will let you know what I find...

Bryan
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Re: Harmonics anyone?

The fact that it is isolated to just one unit makes me think the harmonics are not the culprit. It is either specific to the unit or the phase. I would suspect the unit. I'd look for a good ground connection to the unit, plus I'd still hunt for a suspect neutral joint. Wire nut, connection at the panel, etc. Check EVERY connection.

One thing you might consider is changing power supplies between the unit giving you problems and another unit. This way if it is the transformer, it will move with the change, and if it is unit specific, it will stay.

Good luck, let us know what you find please,

HWS
 

charlie tuna

Senior Member
Location
Florida
Re: Harmonics anyone?

port,
the fact that this problem has been "ongoing" and you found the egc common to the three circuits disconnected tells me --- someone doesn't realize just how important this wire is on control equipment. for many years i used to think it was crazy for us to have to run a number six ground back to the panel for phone equipment --- "why not just tie onto the raceway!". well, as more and more computerized equipment entered our field i began to realize that this equipment required a very low resistance ground path back to its power source! many times we have been called due to problems in phone and data systems only to find improperly grounded return paths or people depending on ground rods and in these instances, when we install a dedicated return conductor -- the problems stop????????????????? i have seen many control cabinets where the neutral is connected to the primary and secondary of the control transformer which the transformer's common is connected to the frame where the boards are housed. this tells me that any voltage on the incomming neutral could directly effect the potential of the control transformer's secondary? we did alot of control work for johnson controls and many times the control board failures were caused by improperly grounded equipment!!!!!!!!!!!!
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Harmonics anyone?

Charlie,
. this tells me that any voltage on the incoming neutral could directly effect the potential of the control transformer's secondary?
How? The voltage between the two secondary conductors does not change, even where one of the secondary conductors is bonded to a primary conductor that is not at "ground" potential.
Don
 

charlie tuna

Senior Member
Location
Florida
Re: Harmonics anyone?

from what i read into this is the bond wire was not connected to the cabinet --- there was a potential on the neutral --- as this neutral potential changes it will have some effect on the secondary output voltage of the control transformer since it has one side of the control transformer connected to the neutral. remember, this system is running on pulses......just my $.02
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Harmonics anyone?

A change in potential between the neutral and anything other than the ungrounded conductor does not produce a change between the secondary conductors.
Don
 

charlie tuna

Senior Member
Location
Florida
Re: Harmonics anyone?

it has been my expieriance that as voltage(not harmonic voltage) increases on the neutral that the potential difference between the phase conductor and the neutral decreases.... with this in mind, i'm saying also that the voltage on the secondary of the control transformer will decrease with this decrease in primary voltage. it is not so much the raising and lowering of this voltage as much as the pulse effect on the electronic network.
 

bnewport

Member
Location
Indiana
Re: Harmonics anyone?

I went to the site today... no new events since last weeks changes. (Last Thursday we found a bad crimp terminal on one of the branch circuits feeding one of the panels. It was solid wire and someone used a non-insulated terminal crimper -the terminal showed signs of arcing and burning. One of the techs found it by accident, and it created an event (N-G voltage rise) on the Dranetz meter when he bumped it. They also bonded the EGC in the conduit to the panel ground as it should be.) I checked the harmonics today- the current harmonics are high, but the voltage harmonics are well below 5% THD, so I am not as concerned about that as I was. I am monitoring the secondary voltage and current of the xfmr as well as the 120V circuit H-N-G voltage and neutral current. So far the neutral shows no signs of being overloaded. Hopefully fixing the grounding issues resolved the problems.

Bryan
 
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