Technical Brief Harmonics

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Powel l North Canton8967 Pleasantwood Avenue NW North Canton, OH 44720 (330) 966-1750 Sales Fax (330) 966-1787

Technical Brief Harmonics

Origin of Harmonics

Harmonic distortion originates with non-linear devices on the power system. Suchdevices produce non-sinusoidal current wave forms when energized with a sinusoidalvoltage. The majority of this type of load is of the shunt type and falls primarily underthe following classifications:

1. Ferromagnetic (transformer and saturated reactor magnetizing)

2. Arcing (furnaces and many types of non-incandescent lighting)3. Electronic (rectifiers and inverters)

The use of 2 and 3 above are cause for concern, particularly with the addition of powerfactor correction capacitors.

Power systems can absorb a high level of harmonic current. Problems usually occurwhen capacitors cause the system to be in resonance at a particular frequency.

In radial distribution feeders, the harmonic currents have been seen to flow from thesource of harmonics toward the utility source. The presence of power factor capacitors

can modify that current path for one or two harmonics.

Industrial power systems are like short distribution feeders with the majority of the seriesimpedances coming from transformers. Therefore, the tuning of the circuit can be quitesharp, resulting in failure of the weakest link. This is generally the capacitor (also seeEffects of Harmonic Distortion).

Effects of Harmonic Distortion

A harmonic producing load can affect other loads if significant voltage distortion is

present. The voltage distortion is a function of the systems impedances and currentinjected. If the system impedance is low, or the harmonic current is low, the voltagedistortion consequently is of low value.

The primary effect of voltage distortion is capacitor and rotating machinery failure. Insome cases, when harmonic currents travel in a path with exposure to parallel circuits,telephone interference may result.


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It should be noted that harmonics are a steady-state condition as opposed to transients.It may be suggested that certain types of welding produce transients. In most cases,these are fluctuating harmonic values due to a cyclical load. In general a transient is anevent that is damped out within a few cycles.

Flow of Harmonic Current

Harmonic currents due to non-linear loads generally flow from the load to the utilitysource. The impedances seen by the harmonic current source are the loads in parallelwith the utility source impedance.

The figures show a one-line diagram with normal current flow and an equivalent circuitas seen by the harmonic currents.

One-Line

Capacitors can change the flow path of the harmonic currents. Without capacitors, thesystem looks primarily inductive at 60 Hz and at harmonic frequencies. Whencapacitors are connected into such a circuit, the circuit will resonate at the naturalfrequency if excitation exists. If this natural frequency is at or near an existing harmonicfrequency parallel resonance may occur.

At the resonant frequency, the parallel combination of the capacitor bank and thesource reactance appears as a large impedance. As harmonic currents flow, voltagedistortion will result due to the large impedance. The high harmonic voltage will result inhigh harmonic currents in the capacitor bank and the source reactance. Thus, theharmonic currents injected by the non-linear load will be amplified.

The effect of capacitors depends to a large extent of the sharpness of the resonance.System load plays a major role in attenuating harmonic distortion caused by resonant

Loads

HarmonicCurrent Path

HarmonicSource


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conditions. As the load increases the harmonic magnification occurring at theresonance decreased due to lower impedance paths for the current. Thus resonance ismore likely to occur on lightly loaded circuits or when a large part of the load is made upof motors.

The natural frequency noted above may be calculated as follows:

H =

H = multiple of fundamental frequencyKVA = substation ratingZt = substation transformer impedanceKVAR = capacitor(s) rating

The figures below show a one-line diagram and equivalent circuit, both with a harmonicsource and a capacitor.

One-Line

Series resonance may occur when capacitors are located primarily at the ends offeeders, i.e. at the load, and when harmonic generators are present.

The low impedance is in series with the capacitors looking from the harmonic source.Thus a low impedance is presented to one of the harmonic currents.

The following one-line diagram and equivalent circuit demonstrates series resonance.

CapacitorHarmonicSource

KVA x 100KVAR x Zt


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One-Line

Concerns for Voltage Distortion

All loads on a power system have in common a voltage being impressed. When thevoltage is distorted all loads are affected (a distorted current may affect those loadsproducing or amplifying the distorted current).

Resistive loads are little affected by the presence of distorted voltage. These loads willconsume a little more power because the RMS voltage is higher than normal.

Induction motors respond to harmonic voltages by producing harmonic fluxes andconsequently harmonic torques. Thus losses are increased and heating is the result.

Electronic controls are very susceptible to harmonic voltage. These are, in industrialplants often used in converters and inverters. It is interesting to note that these areusually responsible for the distortion.

Many such devices depend on zero crossing signals to time the firing of the thyristor.When the voltage wave form is distorted the timing is inaccurate and poor speed controlwill result.

Capacitors endure the greatest effect of voltage distortion. With even a small amount ofvoltage distortion they can overheat due to high RMS current. This is primarily due to a

capacitors impedance decreasing with frequency.

Capacitors will also suffer insulation failure because of excessive peak voltages. Thispeak voltage can be as high as the arithmetic sum of the harmonics.

Communications interferences may occur but this is a special condition with equallyspecial solutions. They are deemed to be outside the scope of this document unlessdealt with in another section.

CapacitorHarmonicSource

Line Impedance


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As a general comment, voltage distortion in excess of 2% and current distortion inexcess of 5% are considered to be cause for concern if power factor correction iscontemplated.

At these values fuse elements and thermal trip mechanisms in circuit breakers may beartificially heated resulting in operation at levels well below their ratings. This is due toconjunction with the fundamental current.

Switchgear bus work and cables, feeders and other system conductors will be heatedas well.

Higher utility costs will result from the utility meters recognizing the harmonic power andrecording it as KVA.

Measuring devices, including ground fault detectors, and equipment with digital displayswill all respond to harmonic voltages and currents with varying degrees of inaccuracy.

Powell Harmonic Analysis

Powell will determine the presence and magnitude of current and voltage harmonics

within the facility network. These harmonics will be recorded under normal loadedconditions. Powell will recommend solutions to such problems as may be diagnosed orperceived and suggest methods to correct existing and future problems.

Powell will specify filter equipment to be applied to this system. The harmonic study willinclude the following items:

1. Two weeks of field data collection at the end users facility. A harmonic analyzerwill be used to measure harmonic current and voltage levels at each substation.Nameplate data will be collected from pertinent system components (SCRdrives/rectifiers, transformers, etc.).

2. Data will be input to a computer for system modeling. What if scenarios can beanalyzed. The effects of various capacitor connections can be investigated. Ifproblems are discovered recommendations for correction will be made. Thesewill be detailed specifications, suitable for bid release. Specifications will bemade for any power factor correction/filter equipment.

3. The final report shall be bound and submitted.

Technical Brief Harmonics

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