De-Tuned Reactors
 

Power Factor Correction without the risk of harmonic resonance:

 

The anti-resonant (de-tuned) harmonic filtering of capacitor banks are specifically designed for networks containing harmonic energies which would otherwise damage standard fixed or automatic capacitor banks. The De-tuned Power Factor Correction systems are designed to provide power factor correction in today’s industrial networks.

The De-tuned assemblies include custom designed iron-core reactors in series with three-phase heavy duty capacitor modules. The series capacitor/reactor combination is tuned well below the first dominant harmonic (usually the 5th) thus preventing resonance and harmonic magnification. In addition to providing power factor correction without the risk of resonance, the De-tuned filter may absorb up to 50% of the 5th harmonic (depending upon network characteristics).

The tuned harmonic filter systems are specifically designed for harmonic filtering with power factor correction as a secondary benefit. The series capacitor/reactor combination is tuned close to the 5th harmonic (4.7x50 Hz). Such close tuning to the target harmonic increases the effectiveness of harmonic energy absorption of the capacitor/reactor stage. Due to the specific nature of the tuned filter systems, application issues must be examined prior to system installation.

 
  The solution:
 

To accomplish power factor correction while maintaining harmonic control, add de-tuned or tuned reactor to the P.F. capacitor bank. The series capacitor/reactor combination tunes the network below the first dominant harmonic (usually the 5th). Since most three-phase networks contain little or no harmonic current below the 5th, there is no energy available to resonate.

Figure 4 illustrates the effect of the de-tuned bank on the harmonic currents. Note that there is a reduction in harmonic current amplification. In addition, because the impedance of the de-tuned filter is low near the fifth harmonic, it may actually absorb up to 50% of that current depending on the exact network characteristics. This has the desirable effect of reducing overall voltage distortion. The tuned filter is designed to absorb even higher amounts of the dominant harmonic – typically up to 80%.

 
  Harmonics and Standard Capacitors don’t mix:
 

Harmonics are a natural by-product of non-linear loads such as drives (AC and DC), motor soft starters, welders, UPS systems, and other electronic loads. Harmonics are multiples of 50 Hz currents and voltages which are produced by these devices in response to the manner in which they draw current (see Figure 1). As more non-linear loads are added to the electrical distribution system, the amount of harmonic current rises. Harmonics can cause serious problems when standard power factor correction capacitors are added to the network.

 
  The problem:
 

There are two main considerations when applying power factor correction capacitors. First, capacitors are a natural low impedance path for harmonic currents and will therefore absorb these energies. This increase in capacitor current results in higher element temperature which reduces the life of the capacitor. Also, because capacitors reduce the network impedance, capacitors can actually increase the level of harmonic current on the network. It is important to remember that while capacitors do not produce harmonic currents, they can magnify their effects. Furthermore, harmonic voltages present on the network create voltage stresses on the capacitor. The second and potentially more serious concern is network resonance. When capacitors are added to the network, they set up a parallel resonance circuit between the capacitors and the network inductance (see Figure 2). Harmonic current components that are close to the parallel resonance point are magnified (see Figure 3).

The magnified current can cause serious problems such as excessive voltage distortion, nuisance fuse and breaker operation, over voltage tripping of drives and insulation breakdown within motors, transformers and conductors.

 

Figure 4 illustrates the effect of the de-tuned bank on the harmonic currents. Note that there is a reduction in harmonic current amplification. In addition, because the impedance of the de-tuned filter is low near the fifth harmonic, it may actually absorb up to 50% of that current depending on the exact network characteristics. This has the desirable effect of reducing overall voltage distortion. The tuned filter is designed to absorb even higher amounts of the dominant harmonic – typically up to 80%.

 
  When to apply a de-tuned or tuned filtered capacitor bank:
 

In general, if less than 15% of the facility load is nonlinear (i.e. harmonic generating) and power factor correction is desired, a standard capacitor bank can be applied. If non-linear load content is higher, a de-tuned or tuned filtered bank should be applied. For example, if a facility has 2000 kW of connected load, of which 400 kW is variable speed drives (AC or DC), then the non-linear load percentage is 20% (400/2000). In this instance, a de-tuned bank is recommended as a minimum solution. If harmonic filtering is required, then tuned harmonic filter is better suited. Applying capacitors to a network containing highly cyclical loads or harmonic producing loads will warrant special considerations. It is highly recommended that you contact us for application assistance.

 
  Tuning Reactor construction requirement:
 

Fine tuning of a harmonic rector is done by adjusting the air gap. With a three phase reactor the fringing characteristics are different for the centre leg than they are for outer legs, which will affect the flux density and inductance of that phase. This is a tough physical characteristic to overcome, being all three gaps are usually the same for a three phase core.

They are techniques that allow three-phase reactors to be tuned reasonably balanced, but it takes additional care to do. By using a distributed gap, instead of single gap as with an E-I lamination, this effect can be minimized. On the other hand, with single phase reactors each phase may be individually tuned and adjusted. The cost is higher by 20% compare to three-phase reactor.

 
 
 
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