Harmonics vs. Transients

Harmonic distortion is blamed for many power quality disturbances that are actually transients. A measurement of the event may show a distorted waveform with obvious high frequency components. Although transient disturbances contain high frequency components, transients and harmonics are distinctly different phenomena and are analyzed differently.

1. Definition: Transients - An event that is undesirable but momentary in nature. Pertaining to or designating a phenomenon or a quantity which varies between two consecutive steady states during a time interval that is short compared to the time scale of interest. A transient can be a unidirectional impulse of either polarity or a damped oscillatory wave with the first peak occurring in either polarity. Commonly referenced as a surge. Reflecting the waveshape of a current will determine if the transient is labeled impulsive or oscillatory.
2. Definition: Harmonics - Periodic distortion of the sine wave. Total Harmonic Distortion - The Ratio of the root-mean-square of the harmonic content to the root-mean-square of the fundamental quantity, expressed as a percent of the fundamental.

One must be careful when describing harmonic phenomena to understand that here are distinct differences between the causes and effects of harmonic voltages and currents. The use of the term harmonics should be qualified accordingly. By popular convention in the power industry, the majority of times the term is used by itself when referring to load apparatus, the speaker is referring to the harmonic currents. When referring to the utility system, the voltages are generally the subject. To be safe, make a habit of asking for clarification.

Transient waveforms exhibit the high frequencies only briefly after there has been an abrupt change in the power system. The frequencies are not necessarily harmonics; they are whatever the natural frequencies of the system are at the time of the switching operation. These frequencies have no relation to the system fundamental frequency.

Harmonics, by definition, occur in the steady state, and are integer multiples of the fundamental frequency. The waveform distortion that produces the harmonics is present continually, or a least for several seconds. Transients are usually dissipated within a few cycles. Transients are associated with changes in the system such as switching a capacitor bank. Harmonics are associated with the continuing operation of a load.

Harmonics and Single Phase Power Supplies

A major harmonics concern in commercial buildings is that power supplies for single phase electronic equipment will produce too much distortion for the wiring. Direct current power for modern electronic and microprocessor based office equipment is commonly derived from single phase full wave diode bridge rectifiers. The percentage of load which contains electronic power supplies is increasing at a dramatic pace, with the increased utilization of personal computers in every commercial sector.

Two major types of single phase power supplies are common. Older technologies use ac-side voltage control methods, such as transformers, to reduce voltages to the level required for the dc bus. The inductance of the transformer provides a beneficial side effect by smoothing the input current waveform, reducing harmonic content. Newer technology, switch-mode power supplies, use dc/dc conversion techniques to achieve a smooth dc output with small, lightweight components. The input diode bridge is directly connected to the ac line, eliminating the transformer. This results in a coarsely regulated dc voltage on the capacitor. This dc is then converted back to ac at a very high frequency by the switcher and subsequently rectified again. Personal computers, printers, copiers, and most other single phase electronics equipment now almost universally employ switch-mode power supplies. The key advantages are the light weight, compact size, efficient operation, and lack of need for a transformer. They can usually tolerate large variations in input voltage.

Because there is no large ac-side inductance, input current to the power supply comes in very short pulses as the capacitor regains its charge on each half cycle. A distinctive characteristic of switch-mode power supplies is a very high third harmonic content in the current. Since third harmonic current components are additive in the neutral of a three-phase system, the increasing application of switch-mode power supplies causes concern for overloading of neutral conductors, especially in older buildings where an undersized neutral may have been installed. Concern for transformer heating is also important when the load includes a significant amount of switch-mode power supplies.

Switch-mode power supplies are also beginning to find application in electronic ballasts for fluorescent lighting systems. The high frequency, controlled-output voltage that is possible with transistorized inverters increases fluorescent tube efficiency, and permits more sophisticated control, such as dimming. The harmonic current injected by many electronic ballasts looks very similar to power supplies used in computers and other electronic equipment. Increased harmonic generation from fluorescent lighting can be very important because lighting typically accounts for 40-60 percent of a commercial building load. Some vendors have responded with designs that produce a much cleaner waveform.

Principles for Controlling Harmonics

1. The source of harmonic currents is too great.
2. The path in which the currents flow is too long (electrically), resulting in either high voltage distortion or telephone interference.
3. The response of the system accentuates one or more harmonics.

1. Reduce the harmonic currents produced by the load.
2. Add filters to siphon the harmonic currents off the system or block the currents from entering the system, or supply the harmonic currents locally.
3. Alter the frequency response of the system by filters, inductors, and capacitors.

Telecommunications Interference from Harmonics

Harmonic currents on the power system are coupled into communication circuits by either induction or direct conduction. Coupling from the neutral of an overhead distribution line by induction was a severe problem in the days of open wire telephone circuits. Now, with the prevalent use of shielded, twisted-pair conductors for telephone circuits, this mode of coupling is less significant. The direct inductive coupling is equal in both conductors, resulting in zero net voltage in the loop formed by the conductors.

Inductive coupling can still be a problem if high currents are induced in the shield surrounding the telephone conductors. Current flowing in the shield causes an IR drop, which results in a potential difference in the ground references at the ends of the telephone cable.

Shield currents can also be caused by direct conduction when the shield is in parallel with the power system ground path. If local ground conditions are such that a relatively large amount of current flows in the shield, high shield IR drop will again cause a potential difference in the ground references at the ends of the telephone cable.

Additional information and site survey information can be obtained by contacting

POC: Mike Helms mike@lightningmike.com