High Fundamental Frequency in Voltage Transformer Neutrals

[tivester]

I am a data center technician for a financial company in St. Louis, MO. I have my EIT certification and along with my other duties am involved in various projects throughout the company's St. Louis campuses. I was recently asked to perform some basic harmonic analysis at one of the large office buildings characterized by large lighting loads, personal desktop computers, and VFD controlled mechanical equipment. The initial cause for the study was a pitch from a salesman to buy a 3rd harmonic current filter. When a Fluke 43B power quality analyzer was placed on the secondary side neutrals of 3 different transformers in the building some interesting results were obtained.

Case 1:
XFMR2, 500KVA, 480/208,120V , 3 feed cables per phase, 3 neutral cables
RMS readings on one cable per phase were:
A-90.3A @60Hz, 34.9A @180Hz
B-87.7A @60HZ, 44.5A @180HZ
C-52.5A @60Hz, 27.1A @180Hz
Normal neutral readings were
Cable 1- 26.2A @60HZ, 74.4A @180Hz
Cable 2- 31.6A @60Hz, 69.2A @180Hz
Cable 3- 52.0A @60Hz, 90.7A @180Hz
(Accounting for the difference in the neutral cables to that they were taking individually and not simultaneously with loads fluctuating frequently)
An anomaly occurred on the neutral in Cable 1 only. Every 15-20 for a period of roughly 5 seconds a reading with a fundamental frequency of 175 Hz would occur, for example:
Cable 1- 83.5A @175Hz, 8.3A @526Hz
No 60Hz current was shown, the loads are obviously not balanced, as well as this only occurred on 1 cable of the neutral.

Case 2:
XFMR3, 750KVA, 480/208,120V , 5 feed cables per phase, 10 neutral cables
8 of the 10 cables showed a fundamental current of approximately 180Hz, their averages were as folows:
THD-15.5%
Total rms current-21.5 A
Fundamental Frequency-20A @180Hz
3rd Harmonic-3A @540 Hz
DC component- 5-10% of total or about 1-2A
The other two cables were connected to the same neutral bus, but from an addition to the bottom and therefore are of a slightly longer length. Their averages were:
THD-88.7%
Total rms current-17.9A
Fundamental Frequency-8A @60Hz
3rd Harmonic- 16A @180Hz
DC component-negligible
Once again loads are not balanced among the phases. On one cable of each phase these were the readings:
A-50Arms, 50.5%THD
40A @60Hz, 22A @180Hz
B-47.9Arms, 34.9%THD
43A @60Hz, 12.7A @180Hz
C-34.1Arms, 56.2%THD
27A @60Hz
17.5A @180Hz

There was also a third case with XFMR-1, where one of the neutral of five had a similar reading to those with approximately 180 Hz fundamental reading. We have so far been unable to duplicate these readings on other simpler amp meters. I was wondering if any one has come across this situation before, if possibly the Fluke 43B isn’t accurately depicting what’s happening, or has an idea as to what could possibly account for these readings.

There have been many modifications to this building’s electrical distribution from the utility transformers down to the main distribution switchgear since the building was initially built in the 70’s. They have also experienced heating up of the contacts in the utility to generator ATS’s and recently experienced a demolishing fire in one of the ATS’s. Russelectric’s report on the ATS gave possible causes as follows:
“The emergency contacts showed considerable damage due to arcing. Determining when this damage occurred is almost impossible. We know that during the fire the normal utility source circuit breaker tripped. This began the sequence to transfer to emergency power. When the emergency contacts closed, they once again energized the rear compartment subjecting the device to a second high current fault. This is what we call a ‘Close on, Bolted Fault’ an extremely severe operation. These contacts are designed to withstand this type of fault, without welding shut, but arcing damage is inevitable. The ATS had a history of high temperature readings and a flash over on the normal source. The finger clusters were replaced but not the contacts. Proper inspection of the contacts not completed. Contacts were most likely the cause of the high heat and subsequent flash over. …The heat rise as recorded on site was duplicated here in the factory. The main contacts were confirmed as the source of the heating, not the finger clusters as first believed. Both main and Emergency contacts show considerable damage. Damage can be caused by either load transients during transfers or late out of phase closure during closed transition transfers. Damage to main contacts lead to overheating which will have a detrimental effect on finger cluster springs. Springs can loose tension when subjected to heating considerably beyond spec. Loss of tension creates more heat leading to arcing. Eventually the phases will flash over at the point of minimal electrical clearance which is at the finger clusters. The above items were most likely involved in the ultimate failure of [the ATS].”

The reason I included this with the question is to see if anyone else thinks the two could be related and if the strange readings observed were not due to possible unintentional cross-connecting of transformers or neutrals not being connected in a proper manner due to the many changes the building has had at it’s electrical entrance.

 

[dpc]

I am really not clear on what your questions are.

Third harmonics in the neutral current are extremely common in office and data center environments. The 3rd harmonic currents do not cancel out so having 180 Hz current exceeding 60 Hz current in the neutral is not unusual.

Typical VFDs will produce a fair amount of 5th and 7th harmonics in the phase currents.

I don't see how the failure of the ATS would be related to your harmonics problem. Based on the report, the ATS closed into a fault, it failed and was, I assume, replaced or repaired.

With harmonics, the main question is whether or not they are creating any actual problems.

 

[tivester]

I am not concerned that there is current at 180 Hz. Harmonics, especially 3rd harmonics, are typically found in the neutrals servicing non-linear loads. What does concern me is that what the meter appears to show is a 175 to 180 Hz fundamental freqency in only some of the neutrals on each transformer, which although seperate cables should all have same begining and ending points and therefore the same readings. Also there appears to be no 60 Hz current in what is obviously not a balanced system between the phases, so there should still be a readable amount of 60 Hz current flowing in the neutral. I also am curious as to the distinct diffrence in current draw and THD of the neutrals with 60 hz fundamental as opposed to those with seemingly 180 Hz fundamental.

 

[tivester]

Also the problem with the ATS's is that there are continuing to heat up, even though they have been replaced, so there is still something going on to cause this. Possibly circulating currents or other issue due to improperly connected systems that would also give strange readings at other points in the system, such as the transformers.

 

[dpc]

The fundamental is 60 Hz. If you are reading current in a neutral conductor and it shows only 180 Hz, that is still a third harmonic. Even if the phase currents are perfectly balanced, there can still be 180 Hz current in the neutral.

It's possible one of the neutral conductors has a high contact resistance at one termination, or both. With multiple parallel conductors, a very small difference in contact resistance at the terminations can create a large difference in current. You might want to clean and re-torque your connections.

I'm not sure why the ATS would be getting hot under normal operation. I assume the rms current is within the ATS rating? Is this is three-pole or four-pole ATS? In cases of extremely high 3rd harmonic current, it's necessary to oversize the neutral conductors.

You also have to watch measurement techniques. If you are using a split-core current probe, make sure the surface area where the jaws meet is extremely clean. Also, make sure the jaws are properly mating on each measurement and that the probe is at right angles to the conductor (and the conductor is centered in the probe). It's often difficult to get the probe properly positioned when there are so many parallel conductors.

Also, what you are doing is dangerous. Proper safety methods and PPE should be worn - but you probably already know that.