First, let me start by explaining what I mean by obliquely. I am referring to the shape and size of the voltage and current curves caused by unbalanced voltages and currents, negative phase sequencing, further exacerbated by harmonic disturbances.
In a previous newsletter, I asked the question, who “police” the unbalanced voltages, phase-shift variations, or harmonics disturbances. Let me rather put it a different way. Do the utility companies regularly inspect the power network for voltage magnitude and phase-shift variations, or do they regularly check for harmonic disturbances? The answer is NO, they do not.
In January of 2017, I was asked to attend a meeting with the executive of a mid-sized factory in Gauteng, South Africa. During that meeting, I learnt that their plant occasionally shut down for no apparent reason. This while the lights stayed on. The shutdown of the plant was then traced to the motor controllers that mysteriously “shuts down” for inexplainable reasons.
The above situation reminded me about a similar issue that I became involved in many years ago, in the Western Cape, South Africa. In both cases, the motor-controllers, such as variable-frequency drives, soft-starters, and other speed controllers, are placed in a separate distribution board from where most, if not every, motor is controlled through a computer or programmable controller. The computer or programmable controller is supplied from one of the three phases, which is not uncommon. Not all computer or programmable controller can operate on “dirty power systems” and due to its sensitivity, it shuts down when the supply voltage “floats” outside the allowable voltage range. When the controllers shut down, so does the plant. As I said, this type of situation is not uncommon or restricted to certain industries. If affects a wide variety of industries which include, inter alia, mining.
In the latest blog, I talked about another situation that I was asked to resolve in the mid- to late-eighties. In that situation, my advice to the end-customer was to install a very large motor-generator set with a massive flywheel to keep the momentum going during the power distortions. Quite a few times, they suffered great financial losses while spending months to repair their plant.
In a recent blog, I alluded to an incident that I accidently became involved in. In late August, a consulting engineer approached me to measure and record the loading on a 6.6kV cable feeder. Shortly after he received the data, he asked me why the current on one of the feeders was so much lower than the other two. Because I was not asked to do a full-scaled Power Quality analysis, I did not pay attention to anything other than to look for the maximum current over the week-long period. When I re-examined the data, I found that the voltage on one of the phases was almost negligible causing the high current in the other two phases plus a “very high current” in the neutral conductor. I suspected that one of the contacts of the circuit breaker did not make. It was then that supplied him with a lot more analytical data. He passed that, from what I was told, on to the utility company, but until today, no further action appears to have taken place as far as I am aware of.
I have no intention of circumventing the consulting engineer and going directly to the utility company with the information. I therefore suppose, because of my stance, end-customers fed from that substation are being negatively affected. I do, however, hope that that representatives of utility companies read this newsletter, wake-up and realize that many more have similar situations are taking place right now and contact me directly to have a Load Profiling exercise done for them to find out whether they are having similar issues as I have explained above and as I have also alluded to in my blogs. This kind of situation may be more prominent at medium- to large-power users.
On the Agulhas Utilities Corporation Negative Phase Sequencing web page, I have several images and videos to explain the entire concept. In one of the videos, the disformed three-phase waves are shown. In the video below, a voltage magnitude variation in one of the phases plus harmonic disturbances on all three phases are shown.
In this video I am showing what a three-phase voltage wave may look like with a 6% voltage reduction in one of the phases and a 10% 3rd Harmonic Disturbance in all three phases. Pay attention to the Yellow and Blue Phases and how it rises above the “standard” voltage levels. I also want to draw your attention to the shifting in the crossing points of the waves of the different phases. This results in a phase shift in the vectors, meaning that the phase shift is no longer 120-degrees between the phases.
Some may say that this is extreme. NO, it is not. I have seen worse situations.
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