What Are Power Harmonics, and Where Do They Come From?

Harmonics are high frequency current and voltage distortions within your power system. Their frequencies are multiples of the fundamental system frequency; e.g., if the fundamental frequency (the first harmonic) is 50 Hz, the second harmonic is 100 Hz; the third, 150 Hz; the fourth, 200 Hz; and so on.

Power harmonics have become much more widespread with the development of high-efficiency electronic equipment. Personal computers, medical test equipment, solid-state motor drives, uninterruptible power supplies (UPS), etc., are designed to draw current only in pulses, during the peak of the incoming voltage wave. This results in a non-linear load, which, in turn, creates the distorted (non-sinusoidal) waveforms that cause harmonics to flow back into the power system (see Figure 1).

Harmonics can be present in both single- and three-phase non-linear loads.

To quantify the distortion, the term total harmonic distortion (THD) is used. The term expresses the distortion as a percentage of the fundamental (pure sine) of voltage and current waveforms.

Figure 1: Distorted Waveform Composed of Fundamental and 3rd Harmonic.

Why are voltage and current harmonics a problem?

• Transformer overheating. Current harmonics are a problem because they cause increased losses in the customer's and utility's power system components. Transformers are especially sensitive to this problem and may need to be de-rated to as much as 50% capacity when feeding loads with extremely distorted current waveforms (current total harmonic distortion above 100%). Another option is to use special type of transformer, also called a "K-factor" transformer.

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Low power factor

. Loads with highly distorted current waveforms also have a very poor power factor; because of this, they use excessive power system capacity and could be a cause of overloading. Voltage source electronic adjustable speed drives (ASD) often have a total power factor of approximately 65% because of the highly distorted current. This total power factor could be corrected to approximately 85% using line-side chokes (reactors) on the drive. The chokes limit the rate of rise and the peak value of the line current, dramatically reducing the current THD.

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Premature motor burnout

. In addition, current harmonics can distort the voltage waveform and cause voltage harmonics. Voltage distortion affects not only sensitive electronic loads but also electric motors and capacitor banks. In electric motors, negative sequence harmonics (i.e. 5th, 11th, 17th), so called because their sequence (ABC or ACB) is opposite of the fundamental sequence (see figure 2), produce rotating magnetic fields. These fields rotate in the opposite direction of the fundamental magnetic field and could cause not only over-heating but also mechanical oscillations in the motor-load system.

Figure 2: Balanced 3-Phase voltage with 5th harmonic. Note the inverted sequence of the 5th harmonic.

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Capacitor bank failure

. The problem with capacitor banks, on the other hand, is that the reactance (impedance) of a capacitor bank decreases as the frequency increases. This causes the bank to act as a sink or trap for higher harmonic currents from the surrounding customer and/or utility system. The effect is increased current, increased heating and dielectric stresses that could lead to capacitor bank failure.

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Neutral conductor overheating

Single-phase non-linear loads, like personal computers, electronic ballasts and other electronic equipment, generate odd harmonics (i.e. 3rd, 5th, 7th, 9th, etc.). The troublesome harmonics for single- phase loads are the 3rd and odd multiples of the 3rd (9th, 15th, etc.). These harmonics are called "triplens" and they will add rather than cancel on the neutral conductor of a 3-phase 4-wire system. This can overload the neutral if it is not sized to handle this type of load.
On the other hand, 3-phase non-linear loads like 3-phase ASDs, 3-phase DC drives, 3-phase rectifiers, etc., for not generate current triplen harmonics (3rd, 9th, 15th, etc.). These types of loads generate primarily 5th and 7th current harmonics and a lesser amount of 11th, 13th, and higher order.

How do you check for harmonic currents?

When non-linear loads are a considerable part of the total load in the facility (more than 20%), there is a chance of a harmonics problem. Another consideration is the amount of current distortion produced by the non-linear loads. Electronic ballasts, for example, come with current THD ranging from 6% to 100%. It is important to avoid electronic ballasts with more than 20% current THD. PWM ASDs typically produce close to 100% current THD, which can be reduced to less than half by installing inexpensive 3% impedance line-side reactors (chokes).

When current THD exceeds 15%, a professional should evaluate the service transformer capability.

Another important way to check for harmonic currents is to measure the current in the neutral of a 3-phase 4-wire system. If the neutral current is considerably higher than the value predicted from the imbalance in the phase currents, there is a good possibility of heavy presence of triplen harmonics.

Other signs of current harmonics include inexplicable higher-than-normal temperatures in the transformer, voltage distortion and high crest factor.

Does having harmonics in the power system always mean trouble?

Harmonics only mean trouble if the power system is not designed to handle them. High harmonic neutral currents are a problem only if the neutral is not properly sized. Current harmonics are not a problem to a transformer if it is de-rated appropriately. Even some voltage distortion below 8% THD at the point of utilization) is acceptable as long as sensitive equipment is not affected. However, it is always important to be aware of the presence of harmonics and to try to minimize them by purchasing low distortion electronic ballasts and reactors for PWM ASDs. This will not only keep the harmonics in check and improve the power factor in the facility, but will also save energy by reducing losses on power system components. In addition, any time there is a considerable increase of non-linear loads, it is important to check power system components to prevent problems.