Think The Only Reason To Install a Harmonic Filter Is To Filter? Think Again!

Contrary to popular belief, it’s not. And whenever I say this, I see looks of confusion from those around me. However, as our conversation progresses, what started as confusion changes to understanding, and eventually to full comprehension. The fact is the majority of drive systems applied nowadays do not require harmonic filtering, as they are "IEEE 519 compliant." That is, they meet the harmonic current distortion limits put forth in the IEEE-519-2014 - Recommended Practice and Requirements for Harmonic Control in Electric Power Systems. Even so, harmonic filters are still required for other purposes. Read this article to understand why.

Reactive Compensation

Plants need reactive compensation to correct power factor, to avoid KVA demand charges and power factor penalties. The first inclination, then, is to go for a standard shunt power capacitor bank, the lowest cost and preferred option for reactive compensation. It is low cost and simple, and the most sought out solution. However, an unavoidable consequence of using capacitor banks is uncontrolled resonance. Resonance caused by capacitor banks on medium-voltage power systems can magnify harmonic currents by over 100 times, and so even small amounts of harmonic currents present in the network like those put out by "IEEE compliant" drives can quickly become an issue.

So, in effect, the application of capacitor banks cause resonance and the magnification of harmonic current where there was little-to-none before. Capacitor banks even with the “IEEE 519 compliant” drives, cannot be utilized for their intended purpose without introducing these additional resonance complications. Harmonic filters, on the other hand, can provide the needed reactive compensation without the pitfalls of resonance. These harmonic filters are also known as “tuned capacitor banks” and "detuned capacitor banks". They all are essentially harmonic filters.

Prevention of Resonance

As mentioned above, and stated another way, installing a harmonic filter bank (a tuned capacitor bank) is an effective way to provide reactive compensation on a system without resonance. It is often thought that “IEEE 519” compliant drives allow for the use of standard shunt power capacitor banks, the fact is they do not. For this reason, tuned capacitor banks, (aka harmonic filter banks, detuned capacitor banks) are very often applied for reactive power compensation and the avoidance of harmonic resonance.

Harmonic Filtering

In some cases, especially in large industrial plants, filtering harmonics is a primary objective. Such applications usually involve plants with large drive systems and power rectifiers. In these systems, harmonic filtering and reactive compensation are the primary objectives and without them the system cannot operate. The filters are designed with a key objective of filtering, IEEE 519 compliance, and reactive compensation. They are often multi-tuned to maximize filtering effectiveness and capability. 

Preclusion of Resonance

Preclusion of resonance is often another reason to install harmonic filters. In such applications, there may not necessarily be a need for harmonic filtering, but there is a desire to prevent harmonic resonance associated with unrelated capacitor banks installed elsewhere on the system.

A useful rule-of-thumb to remember is that tuning a harmonic filter to a given frequency will prevent harmonic resonance from occuring at that frequency. For example, putting a filter on a system tuned to the 5th harmonic will prevent resonance from occurring at the 5th harmonic. Such applications are commonly used within multi-tuned harmonic filter systems on large high-pulse drive systems and on utility general distribution systems where there are resonance concerns from remotely connected capacitor banks. 

Contact NEPSI or myself ([email protected]) for procurement and application support on your next medium voltage metal enclosed capacitor bank or harmonic filter bank project. 

About NEPSI

NEPSI is the world's leading supplier of medium-voltage metal-enclosed capacitor banks and harmonic filter banks. NEPSI’s equipment is used by industrial, commercial, and utility power system owners and operators with operating voltages from 2.4kV through 38kV to correct power factor and improve power quality. For more information on NEPSI, see our website at: www.nepsi.com