High Frequency Noise & Isolation Transformers

What is "High Frequency Noise" and where does it originate? Why should there be concern about having it in an electrical distribution system and what should you do about it?

Most of what is defined as High Frequency Noise typically falls into the 100KHz to 100MHz frequency range. The source of this noise in most facilities is switch mode power supplies, electronic lighting ballasts, photo copiers, and laser printers. In industrial and medical facilities, the additional sources would include medical equipment, variable speed drives, and computerized industrial loads.

Little attention is paid to noise generated by all these devices until computer and other systems experience problems. The normal sequence of events will be intermittent lock-up or malfunction. The staff will blame the software and the department or individuals in charge of software will blame the hardware. Equipment will be sent out for repair and nothing will be found wrong. Blame will be heaped upon the repair service organization because the same problems are still being experienced. Meetings will be held and fingers will be pointed. The solution will not be found as the culprit is still not known. Does any of this sound like a meeting you have attended?

If someone does not figure out the problem they will learn to live with it or replace their equipment to get rid of the gremlins. The sad part is that newer equipment is generally faster and more sensitive to high frequency noise and the problems will be compounded in the new equipment. The supplier of the new equipment will be called on the carpet and told his systems are junk and he had better fix them...... or.... Do you get the picture?

There is a great deal of distance between fact and assumption when dealing with power related problems and high frequency noise can be among the most difficult problem to identify and solve. Many who specify protection equipment deny the need for TVSS filter systems when isolation transformers are in use. This can be a mistake if protection system using an isolation transformer system does not include a TVSS with high frequency noise filtering. To understand why TVSS with noise filtration is such an important part of any power protection system, consider the following facts about isolation transformers. Keep in mind, many think an isolation transformer will be a "solve all power related problems" solution.

Isolation Transformers
Facts: I.T. attenuate some conducted noise. (<10%)
I.T. re-derive the neutral to ground bond.
I.T. capable of stepping up or down voltage.
I.T. not filter load generated noise.
I.T. not suppress load generated electrical transients.

The impact of high frequency noise will grow as systems become more sensitive and the impact of higher signal / system speed will make the problems geometric. When you add in the fact most higher speed digital signals are smaller in amplitude you have an equation for disaster in many systems.

 High frequency noise on the power distribution system can be filtered from the line to eliminate system upset. There are two approaches to consider and both have both benefits and drawbacks. The parallel approach has a line-to-neutral capacitor as its main element. This method allows the high frequency noise to be shunted away from the load. It also has an advantage, it is bi-directional. Series filtering is most normally achieved by using a large inductor in series with the power line. The inductor represents a high impedance to the high frequency noise and will reduce conducted noise. A series unit must be load rated and can become expensive and very large.

Few isolation transformers are designed to block high frequency noise. The ones that are designed to do so are series rated and normally do not provide bi-directional filtration. There are isolation transformers that have high quality, low pass surge suppression and good parallel filtering. These units are not the norm and cost much more than the common isolation transformers. To protect branch circuits or load from another circuit or load you must provide for each circuit or load. It is impractical in most situations to do this and more common to see capacitors placed parallel across the line.

To understand why a capacitor or inductor will act as a filter, all you need to understand is impedance. Impedance is a measure expressed in Ohms of the total opposition (resistance, capacitance and inductance) offered to the flow of alternating current. High frequency noise is high frequency alternating current to the capacitor and inductor.

Normal AC is 60Hz and considered a very low frequency. A capacitor represents high impedance to a 60Hz line frequency and draws very little power. This bi-directional filter works for the high frequency noise coming from the line or the load.

Due to the inductive characteristics of a wiring distribution system, the parallel filter is most effective when placed close to the protected load. If you install filters at the branch panel, you protect optimum noise reduction while preventing one load from affecting another.

Using a combination of parallel and series filtration provides the best option. This method will yield the benefit of both. This is more expensive, but best if the load is critical.

PSI 1995

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