Technical Articles
In the series of Case Studies on Power Quality Solutions, unique solution for systems with High Voltage Harmonics always takes special interest because of the complexity it brings in. In this case study, we will demonstrate such unique, yet very effective PQ solution in a Spinning Mill. This specific Spinning Mill, located near Erode, Tamil Nadu, employs high concentration of VFD (non-linear) loads, close to 90%. This is the typical scenario in most of the spinning mills as well, because they are more concerned in Energy Management & to maximize savings. But as a side effect, it impacts the Power Quality in the worst way – in the form of Voltage Harmonics. Such High V-THD not only affects their entire system, but also the grid & near-by industries. We can notice this in many textile belts across the country.
Unlike Current Harmonics, Voltage Harmonics are not confined to specific feeder. High V-THD can be observed at all points in the electrical system. Hence it can result in premature equipment failures – electronics card failure, capacitor / reactor failure, transformer & bus bar humming noise, etc. With high V-THD (> 5%) Active Harmonic Filters alone cannot be the ideal solution, as the rating required will be very high due to resonance/amplification. It requires careful selection of detuned reactors that reduces the amplification at the same time they can withstand such high V-THD. This also ensures significant reduction of AHF ratings, by more than 150%.
The Spinning Mill that we are talking about was also in similar fix. They had installed LTXL Ultra Heavy Duty Capacitors for PF correction without detuned reactors. With very high level of harmonics (as shown below), they were discussing about installing 800 A of AHF:
Above readings are taken at the LT main incomer with around 500 kVAr capacitors in ON condition.
Maximum V-THD at the incomer: 10.5%
I-THD at the incomer during full load: 33%
Harmonics measured at the APFC incomer are:
V-THD at APFC incomer: 11.1%
I-THD at APFC incomer: 70%
Current drawn by 25 kVAr 440 V capacitor: 42 A (against the rated value of 32 A). Since LTXL capacitors are installed, they are working for past 2½ years without any issue.
Due to such high voltage harmonics, some of frequent problems include high temperature in the transformer (60⁰ to 65⁰ C), busbar humming noise, over-heating of servo stabilizers and electronic card failures in VFD
Solution proposed:
Harmonics measured at the APFC incomer are:
We proposed to install Special High V-THD withstand detuned reactors along with the LTXL Ultra Heavy Duty capacitors. After that we estimated only 300 A AHF would be sufficient to bring down the harmonics to IEEE 519 limits, against the 800 A AHF as proposed by another 3rd party solution provider.
After installing High V-THD withstand reactors:
Below are the actual readings recorded after installing High V-THD detuned reactors in APFC panels:
APFC panels are without any detuned reactors. These capacitors were amplifying harmonics and getting overloaded.
Above readings are taken at the LT main incomer with around 500 kVAr capacitors in ON condition, after detuning with High V-THD reactors.
Maximum V-THD at the incomer: 5.5%
I-THD at the incomer during full load: 16.6%
Below readings are taken at the APFC incomer, after detuning with High V-THD reactors:
Harmonics measured at the APFC incomer are:
V-THD at APFC incomer: 7%
I-THD at APFC incomer: 8.9%
Below is the summary of readings before & after implementing the solution:
As depicted in the above table, by avoiding resonance / harmonic amplification, overall current & voltage harmonics came down significantly. At the APFC incomer, there is significant reduction in current harmonics. Also even at 7% voltage harmonics, the high-VTHD reactors are functioning normally without undergoing any humming noise or overheating. That’s the speciality of these unique reactors.
Customer feedback after this solution are reduced transformer temperature by 5⁰ C, bus bar humming noise completely gone and no electronic card failures. Another advantage of detuning with these reactors is the rating optimization of Active Harmonic Filter. For the same site, another firm offered 800 A AHF without detuning. But now, with our high-VTHD reactors, 300 A AHF only would suffice to contain harmonics under limit. The same satisfied customer went for the same solution in their other Unit of Spinning Mill as well.
It is now evident that only one set of product is not sufficient to give best solution. Combination of solutions along with better application knowledge is the need of the hour for many such industries. This case is an apt example to the combination of proper application knowledge & unique superior products. There are many other similar cases, were optimum techno-commercial solutions were given to ensure minimum life cycle cost of the solution.
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