Technical Articles
Electrical networks often contain significant levels of harmonic distortion, which has led the large majority of manufacturers of automatic capacitor banks to unanimously include detuned filter units in their offerings. In this article, we will try to explain the impact of different detuned filters and the consequences of a poor choice, as well as the recommendation for avoiding these possible risks.
However, there is no such unanimity in the choice of the tuning frequency of the detuned filter offered as standard.
In case the network has predominant 3rd order harmonics (150 Hz in 50 Hz networks), the use of detuned filters tuned at 134 Hz is more common (overvoltage factor of p = 14%).
However, a large majority of installations require capacitor bank fitted with detuned filters appropriate for 5th order harmonics (250 Hz in 50 Hz networks) or higher, which are normally produced by the more usual harmonic current sources, such as, drives, AC/DC rectifiers, induction ovens, etc. In such cases, there are two options available, that corresponding to an overvoltage factor of p = 7% (tuning frequency of 189 Hz in 50 Hz networks) and p = 5.67% (tuning frequency of 210 Hz in 50 Hz networks).
It may seem from the above that the choice of a value of p = 7% or p = 5.67% might be indifferent and that both should give the same result when they are connected to the electrical network, but this is not strictly true.
To follow the arguments of this, we will briefly go through the operating principle of detuned filters.
Observing the impedance-frequency graph at rated current of a standard reactor-capacitor unit with p = 7% (green line in Fig. 1), we see that it offers least impedance at 189 Hz, whereas that corresponding to p = 5.67% (red line in Fig. 1) offers the least impedance at 210 Hz. In both cases, the impedance gradually increases on either side of it. The impedance is capacitive at frequencies under 189 Hz for 7% and 210 Hz for 5.67% respectively, and inductive at higher frequencies. It is this inductive character with harmonic frequencies of the 5th order or higher that prevents the possibility of a resonance phenomenon being produced at any of those frequencies. However, another key parameter for the correct operation of the detuned filter is the value of impedance at the different harmonic frequencies. Therefore, at said impedance-frequency in Fig. 1 the impedance difference of each tuning can clearly be seen at a harmonic frequency of 250 Hz
One basic point is the fact that if, to start with, a reactor with p = 5.67% is going to have a larger harmonic current consumption, then the reactor and the associated capacitor must be designed to withstand the overload to which they are to be subjected on the level of intensity and temperature. However, in the particular case of a similarly-designed reactor with p = 7%, the result is a smaller and lighter reactor, and a lower cost.
In short, in case of reactor with p=5.67%, there is a risk that the capacitor bank might have to withstand higher levels of harmonic overloading, which would inevitably cause faster wear than in case of a capacitor with filter of p = 7%.
The other essential point to be considered, which is the most important, is the influence of the capacitor capacity in tuning the reactor-capacitor series group according to the formula for tuning frequency:
A decrease in the capacitor capacity will result in an increase in the unit's resonance frequency. Capacitors are elements that lose capacity with time either due to their conditions of use {voltage, temperature, connection operation rate, etc.), or due to the natural deterioration of the polypropylene of their dielectrics. A same loss of capacity in a p = 5.67% filter and in one of p = 7%, means that the first will come much closer to the 5th order frequency than the second, and the closer it comes, the greater harmonic current absorption it will present, the greater overloading it will suffer, leading to greater deterioration.
In other words, the safety margin given with this loss of capacity is considerably higher in a filter with p = 7%
The conclusion in this case is clear, and is the unequivocal recommendation of the use of filters with p = 7% instead of p = 5.67% in all installations where they have to be applied due to the level of harmonic distortion.
The purpose of this recommendation is none other than to reduce the obvious risk that a loss of capacitor capacity could cause as a result of overcurrent in the capacitor bank much earlier. Thus, 7% detuned reactors allow a longer reaction time through pertinent maintenance actions and the application of corrective measures before the damage is definitive and, therefore, avert worse economic conditions.
Sourav Dasmodak,
Product Management & Marketing (Powergear - ACB)Product Owner of Air Circuit Breaker (ACB) of Lauritz Knudsen for Domestic & International Market. I can talk to you about Electrical Products' Sales, Business Development, Market Expansion, Cracking Critical Strategic Account, handling Key Account & of course how to develop & motivate Channels along with the organizational growth. Having near about one and a half decade of experience across the country with major electrical manufacturers (Top 4).
Assistance Required?
Select an option to Contact Us
Contact Us 
