Herb Prychodko 
Kentucky Service Co., Inc.

Lexington, Kentucky The advent of Variable Frequency Drives (VFDs) has allowed many applications to take advantage of the increased controllability of AC motors. This has resulted in a large increase in the use of these devices. However, an unwanted consequence of using VFDs is the distortion of the supply’s fundamental sine wave, commonly referred to as line harmon­ics. There are two types of harmonics, current and voltage. Remedial methods for both will be covered in this article.

Typical issues associated with line harmonics are:

• Incorrect meter readings of voltage and current devices. 
• Premature fuse failure, intermittent tripping of relays and protective devices. 
• Interference with telephone and other communication devices. 
• Motor bearing failure. 
• Heating transformers, conduc­tors and motors. 
• Causeof logic faults in digital devices.

These issues contribute to expensive equipment malfunctions and/or failures and repairs in the industrial community. 

What are harmonics? 
Before measures can be applied to reduce line harmonics, such as the addition of line reactors and RFI (radio frequency interference) filters, it helps to have a better understanding of what harmonics are and how they are generated. Harmonics are created by non-linear loads being applied to the source sinusoidal waveform. VFDs, by the nature of their design (see Figure 1), create a non-linear load by drawing current only when the line voltage exceeds the DC bus voltage within the VFD, by 0.7 volts, the typical drop across a diode. This current pulse then distorts the voltage waveform by “flattening” it during the duration of conduction. Thus, a voltage harmonic has been created. In contrast, motors connected across the line may phase shift the voltage and current wave­forms, but the waveforms remain nearly identical sine waves. 

How are harmonics measured? 
Harmonics are multiples of a fundamental frequency, in our case, the 60 Hz line frequency. Harmonics are measured as a percentage of the fundamental, with the frequencies nearest the fundamental having the highest magnitudes. The percent of distortion then decays for each subsequent harmonic. Total harmonic distortion (THD) is used to quantify the combined magnitudes of all of the harmonics. Even and triplen harmonics (divisible by three) are typically of no concern in a balanced 3-phase supply. Harmonics are categorized by their frequency as:

• Low Order – 300 Hz to 1000 Hz 
• Low Order – 300 Hz to 1000 Hz 
• HighOrder – 1000 Hz to 150 kHz 
• Radio Frequency – 150 kHz to 30 MHz

VFDs typically generate harmon­ics, that can affect other equipment, in the low order and radio frequency categories.

The harmonics of primary concern are the 5th (300 Hz), 7th (420 Hz), 11th (660 Hz), and 13th (780 Hz). Subsequent multiples, except for those in the radio frequency category, are typically of no concern due to the lower magnitudes associated with them. VFDs will impress these harmonics and their associated voltages onto the AC line as a distorted fundamental sine wave. Figure 2 is a waveform displaying this distortion. 

Corrective actions 
Since line harmonics are the source of many issues associated with the utility supplying the plant power, measures have been taken to attempt to regulate these line disturbances. IEEE has published a standard (Standard 519) that attempts to define the acceptable levels of current distortion, thus limiting voltage distortion. Many utilities regulate harmonics and assess stiff penalties for generators of severe distortion. Thus, it is extremely important for plant engineers to apply corrective measures to devices that are known generators of line harmonics. 

Current harmonic reduction can be accomplished in a number of ways. The methods used are:

• Increasing the reactive inductance in front of the drive. 
• Phase shifting the supply for multiple drive units, utilizing a delta configuration for one leg and a wye configuration for the other. 
• Increasing the current rating of the source transformer, thus reducing the percentage of distortion. Caution must be exercised due to the increased fault currents

The most common method is to increase the input reactance. This can be accomplished by moving the drive further from the source transformer, installing a line reactor, or by the addition of an isolation transformer. Facilities with numerous VFDs will find that the most cost effective route will be to add line reactors ahead of each unit. Placement of the reactor must be as near the VFD as possible. Many drive manufacturers offer line reactors, designed with a specific impedance and current rating, for each of their models. Typically, a total reactance of 5% or less is sufficient to reduce current harmonics substan­tially. Additional reactance will not provide further reduction of the current harmonics. In fact it may cause excessive voltage drop, requiring the VFD to draw higher currents, thus causing a more severe distortion level. 

Reduce source impedance 
In a properly constructed distribu­tion system, reducing the current harmonics will also reduce the voltage harmonics. Many plants have been constructed prior to the installation of VFDs, therefore adding source impedance by the incorrect placement of a line reactor. This may actually aggravate the voltage harmonics. Since voltage harmonics are due to the flow of current through the source imped­ance, additional reactance (increasing source impedance) will increase the voltage distortion. To minimize voltage distortion, the source imped­ance must be reduced. This seems to contradict the remedial recommenda­tions for current harmonics. Reducing the source impedance to the point of common coupling (where all electrical branches meet the incoming supply bus) is the solution to reducing the voltage distortion. This can be accomplished by: 

• Increasing the size of the source transformer. 
• Increasing the current capacity of the cabling. 
• Keeping the point of common coupling as far upstream from the drives as possible. 
• Insuring that added reactance is as far downstream as possible.

A survey of the facility distribu­tion will dictate which method(s) would be the most beneficial. 

Another voltage harmonic concern generated by VFDs, are the frequencies classified within the radio frequency range. Electronic equipment, such as meters, transducers, and computerized controls, are sensitive to these frequen­cies. The magnitude of these harmonics are such that inexpensive low pass filters, again typically supplied by the VFD manufacturers and third parties, can be installed ahead of the unit to shunt high frequencies to ground while allowing the lower frequencies to pass through. If a specific frequency is isolated as being of prime concern, specific tuned filters can be designed to shunt it to ground. 

Conclusion 
For facilities that are exhibiting excessive electronic problems, recruiting an engineering firm or the utility to perform a facility power audit will allow them to review the primary causes for their issues. If it is found that harmonics are a substantial cause of their issues, proper integration of line reactors and low pass filters may prove quite beneficial. Any new installation of a VFD should be reviewed to insure that harmonic issues are dealt with from the onset. 

In closing, proper utilization of these devices may reduce overall plant maintenance costs thereby increasing profitability. 

Categories: Design/theory/application, Drives/controls

Tags: Variable frequency drives

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